COMMON MODE CURRENT CONTROLLER OF INVERTER

TECHNICAL FIELD

The embodiments may relate to the field of power electronics technologies and to a common mode current controller of an inverter.

BACKGROUND

With increasing depletion of conventional energy sources such as oil and natural gas, new energy technologies such as photovoltaic (PV) power generation and wind power generation have been rapidly developed. The photovoltaic power generation is to convert solar radiation energy into electric energy by using a photovoltaic effect of a semiconductor material. For example, a direct current is generated under illumination by using a photovoltaic module. A photovoltaic power generation system and a wind turbine generator are usually direct current voltage sources. A generated direct current is converted into an alternating current by using an inverter, and then the alternating current is tied to a power grid for transmission. An alternating current output by a grid-tied inverter (IGT) needs to be synchronized with the mains in the power grid in terms of a frequency and a phase, so that the alternating current is transmitted by using the power grid.

In a current photovoltaic power generation system, a positive direct current busbar and a negative direct current busbar of an inverter usually float. Affected by factors such as a temperature, humidity, device aging, and improper operation, the positive direct current busbar and the negative direct current busbar may have asymmetric impedance to ground, leading to different potentials to ground, and further leading to a potential difference between a midpoint of a direct current busbar capacitor and a protective earthing (PE) end, also referred to as a grounding end. The potential difference is essentially a common mode voltage. When the inverter is started and is tied to the power grid, in other words, at an instant of closing a grid-tied switch, a parasitic capacitor to ground of the photovoltaic power generation system, the photovoltaic power generation system, and the power grid form a loop, and a large common mode current is formed based on the common mode voltage, and is also referred to as a leakage or residual current (RC). The common mode current formed at a grid-tied instant is usually large. A related potential difference may be as high as several hundred volts in a severe case, to interfere with detection and control of the system and cause an operation fault, for example, trigger an action of a leakage current protector and even lead to a disconnection of the grid-tied inverter.

In the conventional technology, to suppress impact and interference exerted, on a system, by a common mode current generated at an instant of closing a grid-tied switch, a manner is to modulate a common mode voltage by using a modulation algorithm such as pulse width modulation (PWM). However, phase detection and compensation may be used in this manner, causing a complex structure. Another manner is to dispose a compensation power supply between a main circuit of an inverter and the ground, to compensate for an inverter-to-ground common mode voltage to obtain a target value. However, in this manner, the target value that is to be obtained through compensation needs to be calculated based on an alternating current port-to-ground voltage collected in real time, increasing a degree of complexity of a system design and costs of the system design. Still another manner is to dispose, for a relay between an AC end of a grid-tied inverter and a power grid, an impulse current suppression branch connected to the relay in parallel, to suppress an impulse leakage current generated at an instant of closing the relay. However, in this manner, whether to apply control cannot be determined before an alternating current switch of the inverter is closed. Consequently, a magnitude of a common mode current generated at a grid-tied instant cannot be effectively pre-controlled.

SUMMARY

The embodiments may aim to provide a common mode current controller of an inverter. An application scenario of the common mode current controller includes, but is not limited to, a grid-tied photovoltaic inverter, a photovoltaic system connected to a power grid, and another scenario in which impact of a common mode current generated at a grid-tied instant needs to be controlled. The common mode current controller includes a switch and an impedance correction circuit. One end of the impedance correction circuit is grounded, and the other end of the impedance correction circuit is connected to one end of the switch. The other end of the switch is connected to one terminal in a positive terminal and a negative terminal of a direct current busbar capacitor, so that the common mode current controller and at least a part of an intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the one terminal in the positive terminal and the negative terminal of the direct current busbar capacitor and the ground. The common mode current controller is configured to: when a midpoint-to-ground voltage of the direct current busbar capacitor exceeds a threshold range, close the switch to adjust impedance matching between positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the inverter can pre-control a magnitude of a common mode current generated when the inverter is grid-tied. Therefore, the common mode current controller implements introduction of the impedance correction circuit and adjustment of impedance matching based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before a grid-tied switch of the inverter is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

According to a first aspect, an embodiment may provide a common mode current controller of an inverter. The common mode current controller is connected between a direct current busbar capacitor of the inverter and a direct current input voltage source of the inverter, a positive terminal and a negative terminal of the direct current busbar capacitor are respectively connected to a positive terminal and a negative terminal of the direct current input voltage source, and the common mode current controller includes a switch and an impedance correction circuit. One end of the impedance correction circuit is grounded, the other end of the impedance correction circuit is connected to one end of the switch, and the other end of the switch is connected to one terminal in the positive terminal and the negative terminal of the direct current busbar capacitor, so that the common mode current controller and at least a part of an intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the one terminal in the positive terminal and the negative terminal of the direct current busbar capacitor and the ground. The common mode current controller is configured to: when a midpoint-to-ground voltage of the direct current busbar capacitor exceeds a threshold range, close the switch to adjust impedance matching between positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the inverter can pre-control a magnitude of a common mode current generated when the inverter is grid-tied.

In the first aspect, introduction of the impedance correction circuit and adjustment of impedance matching may be implemented based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before a grid-tied switch of the inverter is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

According to the first aspect, in a possible implementation, the midpoint-to-ground voltage of the direct current busbar capacitor is determined based on a positive terminal voltage of the direct current busbar capacitor and a negative terminal voltage of the direct current busbar capacitor.

Therefore, the midpoint-to-ground voltage is indirectly determined based on the positive terminal voltage and the negative terminal voltage of the direct current busbar capacitor, to implement introduction of the impedance correction circuit and adjustment of impedance matching based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before the grid-tied switch of the inverter is closed, to help reduce the degree of complexity of the system design and reduce the costs of the system design.

According to the first aspect, in a possible implementation, the other end of the switch is connected to the positive terminal of the direct current busbar capacitor, and when the midpoint-to-ground voltage of the direct current busbar capacitor is greater than a first threshold, the switch is closed, so that the impedance correction circuit and a positive terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the positive terminal of the direct current busbar capacitor and the ground, to reduce a magnitude of the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor.

Therefore, the switch is closed, so that the impedance correction circuit and the positive terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel, to implement introduction of the impedance correction circuit and adjustment of impedance matching based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before the grid-tied switch of the inverter is closed, to help reduce the degree of complexity of the system design and reduce the costs of the system design.

According to the first aspect, in a possible implementation, the other end of the switch is connected to the negative terminal of the direct current busbar capacitor, and when the midpoint-to-ground voltage of the direct current busbar capacitor is less than a first threshold, the switch is closed, so that the impedance correction circuit and a negative terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and the ground, to reduce a magnitude of the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor.

Therefore, the switch is closed, so that the impedance correction circuit and the negative terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel, to implement introduction of the impedance correction circuit and adjustment of impedance matching based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before the grid-tied switch of the inverter is closed, to help reduce the degree of complexity of the system design and reduce the costs of the system design.

According to the first aspect, in a possible implementation, an impedance value of the impedance correction circuit is adjustable, and the impedance value of the impedance correction circuit is adjusted, so that the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor matches the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor.

Therefore, the impedance value of the impedance correction circuit is adjusted, so that the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor matches the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, to implement introduction of the impedance correction circuit and adjustment of impedance matching based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before the grid-tied switch of the inverter is closed, to help reduce the degree of complexity of the system design and reduce the costs of the system design.

According to the first aspect, in a possible implementation, the inverter includes a direct current-direct current (DC-DC) converter and a direct current-alternating current (DC-AC) converter, and the common mode current controller is between the DC-DC converter and the DC-AC converter.

Therefore, the common mode current controller is disposed between the DC-DC converter and the DC-AC converter, to perform control on a DC side of the inverter and help simplify the system design and reduce the costs.

According to the first aspect, in a possible implementation, the common mode current controller is further configured to control release of a residual charge of a Y capacitor at a terminal of the inverter.

Therefore, the residual charge of the Y capacitor at the terminal of the inverter is released, which is conducive to system stability.

According to the first aspect, in a possible implementation, the inverter is a grid-tied inverter of a photovoltaic power generation system, and the common mode current controller is configured to control a magnitude of a common mode current generated when the photovoltaic power generation system is grid-tied.

Therefore, introduction of the impedance correction circuit and adjustment of impedance matching are implemented based on the midpoint ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the photovoltaic power generation system is grid-tied can be pre-controlled before a grid-tied switch of the photovoltaic power generation system is closed, to help reduce the degree of complexity of the system design and reduce the costs of the system design.

According to a second aspect, an embodiment may provide a common mode current controller of an inverter. The common mode current controller is connected between a direct current busbar capacitor of the inverter and a direct current input voltage source of the inverter, a positive terminal and a negative terminal of the direct current busbar capacitor are respectively connected to a positive terminal and a negative terminal of the direct current input voltage source, and the common mode current controller includes: a first switch, where one end of the first switch is connected to the positive terminal of the direct current busbar capacitor; a second switch, where one end of the second switch is connected to the negative terminal of the direct current busbar capacitor; a first impedance correction circuit, where one end of the first impedance correction circuit is grounded, and the other end of the first impedance correction circuit is connected to the other end of the first switch; and a second impedance correction circuit, where one end of the second impedance correction circuit is grounded, and the other end of the second impedance correction circuit is connected to the other end of the second switch. The common mode current controller is configured to: when a midpoint-to-ground voltage of the direct current busbar capacitor is greater than a first threshold, close the first switch, and open the second switch, so that the first impedance correction circuit and a positive terminal part of an intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the positive terminal of the direct current busbar capacitor and the ground, to reduce a magnitude of positive terminal-to-ground equivalent impedance of the direct current busbar capacitor. The common mode current controller is configured to: when the midpoint-to-ground voltage of the direct current busbar capacitor is less than a second threshold, close the second switch, and open the first switch, so that the second impedance correction circuit and a negative terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and the ground, to reduce a magnitude of negative terminal-to-ground equivalent impedance of the direct current busbar capacitor. The common mode current controller adjusts impedance matching between the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the inverter can pre-control a magnitude of a common mode current generated when the inverter is grid-tied.

In the second aspect, introduction of the first impedance correction circuit or the second impedance correction circuit and adjustment of impedance matching may be implemented based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before a grid-tied switch of the inverter is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

According to the second aspect, in a possible implementation, the midpoint-to-ground voltage of the direct current busbar capacitor is determined based on a positive terminal voltage of the direct current busbar capacitor and a negative terminal voltage of the direct current busbar capacitor.

According to the second aspect, in a possible implementation, an impedance value of the first impedance correction circuit and an impedance value of the second impedance correction circuit are both adjustable, and the impedance value of the first impedance correction circuit and the impedance value of the second impedance correction circuit are adjusted, so that the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor matches the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor.

Therefore, the impedance value of the first impedance correction circuit and the impedance value of the second impedance correction circuit are adjusted, so that the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor matches the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, to implement introduction of an impedance correction circuit and adjustment of impedance matching based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before the grid-tied switch of the inverter is closed, to help reduce the degree of complexity of the system design and reduce the costs of the system design.

According to the second aspect, in a possible implementation, the inverter includes a direct current-direct current (DC-DC) converter and a direct current-alternating current (DC-AC) converter, and the common mode current controller is between the DC-DC converter and the DC-AC converter.

According to the second aspect, in a possible implementation, the common mode current controller is further configured to control release of a residual charge of a Y capacitor at a terminal of the inverter.

Therefore, the residual charge of the Y capacitor at the terminal of the inverter is released, which is conducive to system stability.

According to the second aspect, in a possible implementation, the inverter is a grid-tied inverter of a photovoltaic power generation system, and the common mode current controller is configured to control a magnitude of a common mode current generated when the photovoltaic power generation system is grid-tied.

Therefore, introduction of the impedance correction circuit and adjustment of impedance matching are implemented based on the midpoint ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the photovoltaic power generation system is grid-tied can be pre-controlled before the grid-tied switch of the photovoltaic power generation system is closed, to help reduce the degree of complexity of the system design and reduce the costs of the system design.

According to a third aspect, an embodiment may provide a common mode current controller of an inverter. The common mode current controller is connected between a direct current busbar capacitor of the inverter and a direct current input voltage source of the inverter, a positive terminal and a negative terminal of the direct current busbar capacitor are respectively connected to a positive terminal and a negative terminal of the direct current input voltage source, and the common mode current controller includes: a single-pole triple-throw switch, where a non-movable end of the single-pole triple-throw switch is grounded, and a movable end of the single-pole triple-throw switch includes a first movable end, a second movable end, and a third movable end; a first impedance correction circuit, where one end of the first impedance correction circuit is connected to the first movable end of the single-pole triple-throw switch, and the other end of the first impedance correction circuit is connected to the positive terminal of the direct current busbar capacitor; and a second impedance correction circuit, where one end of the second impedance correction circuit is connected to the second movable end of the single-pole triple-throw switch, and the other end of the second impedance correction circuit is connected to the negative terminal of the direct current busbar capacitor. The common mode current controller is configured to: when a midpoint-to-ground voltage of the direct current busbar capacitor is greater than a first threshold, switch the single-pole triple-throw switch to the first movable end, so that the first impedance correction circuit and a positive terminal part of an intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the positive terminal of the direct current busbar capacitor and the ground, to reduce a magnitude of positive terminal-to-ground equivalent impedance of the direct current busbar capacitor. The common mode current controller is configured to: when the midpoint-to-ground voltage of the direct current busbar capacitor is less than a second threshold, switch the single-pole triple-throw switch to the second movable end, so that the second impedance correction circuit and a negative terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and the ground, to reduce a magnitude of negative terminal-to-ground equivalent impedance of the direct current busbar capacitor. The common mode current controller adjusts impedance matching between the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the inverter can pre-control a magnitude of a common mode current generated when the inverter is grid-tied.

In the third aspect, introduction of the first impedance correction circuit or the second impedance correction circuit and adjustment of impedance matching may be implemented based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before a grid-tied switch of the inverter is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

According to the third aspect, in a possible implementation, the midpoint-to-ground voltage of the direct current busbar capacitor is determined based on a positive terminal voltage of the direct current busbar capacitor and a negative terminal voltage of the direct current busbar capacitor.

Therefore, the midpoint-to-ground voltage is indirectly determined based on the positive terminal voltage and the negative terminal voltage of the direct current busbar capacitor, to implement introduction of an impedance correction circuit and adjustment of impedance matching based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before the grid-tied switch of the inverter is closed, to help reduce the degree of complexity of the system design and reduce the costs of the system design.

According to the third aspect, in a possible implementation, an impedance value of the first impedance correction circuit and an impedance value of the second impedance correction circuit are both adjustable, and the impedance value of the first impedance correction circuit and the impedance value of the second impedance correction circuit are adjusted, so that the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor matches the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor.

According to the third aspect, in a possible implementation, the inverter includes a direct current-direct current (DC-DC) converter and a direct current-alternating current (DC-AC) converter, and the common mode current controller is between the DC-DC converter and the DC-AC converter.

Therefore, the common mode current controller is disposed between the DC-DC converter and the DC-AC converter, to perform control on a DC side of the inverter, and help simplify the system design and reduce the costs.

According to the third aspect, in a possible implementation, the common mode current controller is further configured to control release of a residual charge of a Y capacitor at a terminal of the inverter.

Therefore, the residual charge of the Y capacitor at the terminal of the inverter is released, which is conducive to system stability.

According to the third aspect, in a possible implementation, the inverter is a grid-tied inverter of a photovoltaic power generation system, and the common mode current controller is configured to control a magnitude of a common mode current generated when the photovoltaic power generation system is grid-tied.

According to a fourth aspect, an embodiment may provide a common mode current controller of an inverter. The common mode current controller is connected between a direct current busbar capacitor of the inverter and a direct current input voltage source of the inverter, a positive terminal and a negative terminal of the direct current busbar capacitor are respectively connected to a positive terminal and a negative terminal of the direct current input voltage source, and the common mode current controller includes: a first switch, where one end of the first switch is connected to the positive terminal of the direct current busbar capacitor; a second switch, where one end of the second switch is connected to the negative terminal of the direct current busbar capacitor; and an impedance correction circuit, where one end of the impedance correction circuit is grounded, and the other end of the impedance correction circuit is connected to both the other end of the first switch and the other end of the second switch. The common mode current controller is configured to: when a midpoint-to-ground voltage of the direct current busbar capacitor is greater than a first threshold, close the first switch, and open the second switch, so that the impedance correction circuit and a positive terminal part of an intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the positive terminal of the direct current busbar capacitor and the ground, to reduce a magnitude of positive terminal-to-ground equivalent impedance of the direct current busbar capacitor. The common mode current controller is configured to: when the midpoint-to-ground voltage of the direct current busbar capacitor is less than a second threshold, close the second switch, and open the first switch, so that the impedance correction circuit and a negative terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and the ground, to reduce a magnitude of negative terminal-to-ground equivalent impedance of the direct current busbar capacitor. The common mode current controller adjusts impedance matching between the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the inverter can pre-control a magnitude of a common mode current generated when the inverter is grid-tied.

In the fourth aspect, introduction of the impedance correction circuit and adjustment of impedance matching may be implemented based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before a grid-tied switch of the inverter is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

According to the fourth aspect, in a possible implementation, the midpoint-to-ground voltage of the direct current busbar capacitor is determined based on a positive terminal voltage of the direct current busbar capacitor and a negative terminal voltage of the direct current busbar capacitor.

According to the fourth aspect, in a possible implementation, an impedance value of the impedance correction circuit is adjustable, and the impedance value of the impedance correction circuit is adjusted, so that the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor matches the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor.

According to the fourth aspect, in a possible implementation, the inverter includes a direct current-direct current (DC-DC) converter and a direct current-alternating current (DC-AC) converter, and the common mode current controller is between the DC-DC converter and the DC-AC converter.

According to the fourth aspect, in a possible implementation, the common mode current controller is further configured to control release of a residual charge of a Y capacitor at a terminal of the inverter.

Therefore, the residual charge of the Y capacitor at the terminal of the inverter is released, which is conducive to system stability.

According to the fourth aspect, in a possible implementation, the inverter is a grid-tied inverter of a photovoltaic power generation system, and the common mode current controller is configured to control a magnitude of a common mode current generated when the photovoltaic power generation system is grid-tied.

According to a fifth aspect, an embodiment may provide a common mode current control method for an inverter. A positive terminal and a negative terminal of a direct current busbar capacitor of the inverter are respectively connected to a positive terminal and a negative terminal of a direct current input voltage source of the inverter, and the common mode current control method includes: obtaining a midpoint-to-ground voltage of the direct current busbar capacitor; introducing an impedance correction circuit when the midpoint-to-ground voltage exceeds a threshold range, where the impedance correction circuit and at least a part of an intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between one terminal in the positive terminal and the negative terminal of the direct current busbar capacitor and the ground, to reduce a magnitude of positive terminal-to-ground equivalent impedance of the direct current busbar capacitor or a magnitude of negative terminal-to-ground equivalent impedance of the direct current busbar capacitor; and adjusting impedance matching between the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the inverter can pre-control a magnitude of a common mode current generated when the inverter is grid-tied.

In the fifth aspect, introduction of the impedance correction circuit and adjustment of impedance matching may be implemented based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before a grid-tied switch of the inverter is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

According to the fifth aspect, in a possible implementation, the common mode current control method further includes: separately sampling a positive terminal voltage and a negative terminal voltage of the direct current busbar capacitor; and obtaining the midpoint-to-ground voltage based on the positive terminal voltage and the negative terminal voltage of the direct current busbar capacitor.

According to the fifth aspect, in a possible implementation, the introducing an impedance correction circuit when the midpoint-to-ground voltage exceeds a threshold range includes: introducing the impedance correction circuit when the midpoint-to-ground voltage is greater than a first threshold, where the impedance correction circuit and a positive terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the positive terminal of the direct current busbar capacitor and the ground, to reduce the magnitude of the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor.

According to the fifth aspect, in a possible implementation, the introducing the impedance correction circuit when the midpoint-to-ground voltage exceeds the threshold range includes: introducing the impedance correction circuit when the midpoint-to-ground voltage is less than a first threshold, where the impedance correction circuit and a negative terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and the ground, to reduce the magnitude of the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor.

According to the fifth aspect, in a possible implementation, an impedance value of the impedance correction circuit is adjustable, and the impedance value of the impedance correction circuit is adjusted, so that the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor matches the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor.

According to the fifth aspect, in a possible implementation, the inverter includes a direct current-direct current (DC-DC) converter and a direct current-alternating current (DC-AC) converter, and the impedance correction circuit is between the DC-DC converter and the DC-AC converter.

According to the fifth aspect, in a possible implementation, the inverter is a grid-tied inverter of a photovoltaic power generation system, and the common mode current control method is used to control a magnitude of a common mode current generated when the photovoltaic power generation system is grid-tied.

According to a sixth aspect, an embodiment may provide a photovoltaic power generation system. The photovoltaic power generation system includes a photovoltaic module, a direct current-alternating current inverter circuit, and a grid-tied filter. The direct current-alternating current inverter circuit is coupled between the photovoltaic module and the grid-tied inverter. The photovoltaic power generation system further includes the common mode current controller in any one of the foregoing aspects, configured to control the direct current-alternating current inverter circuit.

In the sixth aspect, a magnitude of a common mode current generated when the direct current-alternating current inverter circuit is grid-tied can be pre-controlled before a grid-tied switch of the photovoltaic power generation system is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

BRIEF DESCRIPTION OF THE DRAWINGS

To describe the embodiments or the background, the following describes accompanying drawings used in the embodiments or in the background.

FIG. 1 is a schematic diagram of a structure of a photovoltaic power generation system including a common mode current controller of an inverter according to an embodiment;

FIG. 2 is a schematic diagram of a circuit structure of a common mode current controller of an inverter in an implementation according to an embodiment;

FIG. 3 is a schematic diagram of a circuit structure of a common mode current controller of an inverter in another implementation according to an embodiment;

FIG. 4 is a schematic diagram of a circuit structure of a common mode current controller of an inverter in another implementation according to an embodiment;

FIG. 5 is a schematic diagram of a circuit structure of a common mode current controller of an inverter in another implementation according to an embodiment;

FIG. 6 is a schematic diagram of a circuit structure of a common mode current controller of an inverter in another implementation according to an embodiment; and

FIG. 7 is a schematic flowchart of a common mode current control method for an inverter according to an embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments provide a common mode current controller of an inverter. An application scenario of the common mode current controller includes, but is not limited to, a grid-tied photovoltaic inverter, a photovoltaic system connected to a power grid, and another scenario in which impact of a common mode current generated at a grid-tied instant needs to be controlled. The common mode current controller includes a switch and an impedance correction circuit. One end of the impedance correction circuit is grounded, and the other end of the impedance correction circuit is connected to one end of the switch. The other end of the switch is connected to one terminal in a positive terminal and a negative terminal of a direct current busbar capacitor, so that the common mode current controller and at least a part of an intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the one terminal in the positive terminal and the negative terminal of the direct current busbar capacitor and the ground. The common mode current controller is configured to: when a midpoint-to-ground voltage of the direct current busbar capacitor exceeds a threshold range, close the switch to adjust impedance matching between positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the inverter can pre-control a magnitude of a common mode current generated when the inverter is grid-tied. Therefore, the common mode current controller implements introduction of the impedance correction circuit and adjustment of impedance matching based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before a grid-tied switch of the inverter is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

The embodiments may be adjusted and improved based on an environment. This is not limited herein.

The following describes the embodiments with reference to the accompanying drawings.

FIG. 1 is a schematic diagram of a structure of a photovoltaic power generation system including a common mode current controller of an inverter according to an embodiment. As shown in FIG. 1, a photovoltaic power generation system 100 includes a photovoltaic module 102, a common mode current controller 104, a direct current busbar capacitor 106, a direct current-alternating current inverter circuit 108, a grid-tied filter 110, and an alternating current output terminal 112.

The photovoltaic module 102 converts solar radiation energy into a direct current based on a photovoltaic power generation effect, for example, by using a solar cell panel or a photovoltaic power generator set. The photovoltaic module 102 may be considered as an equivalent direct current voltage source 102 on a circuit. A positive electrode PV_positiveand a negative electrode PV_negativeof the direct current voltage source 102 are respectively connected to a positive terminal and a negative terminal of the common mode current controller 104, and the positive terminal and the negative terminal of the common mode current controller 104 are respectively connected to a positive terminal and a negative terminal of the direct current busbar capacitor 106. In other words, the direct current voltage source 102, the common mode current controller 104, and the direct current busbar capacitor 106 are connected in parallel between two busbars. It should be understood that a direct current busbar is a connecting line of a positive electrode or a negative electrode on a direct current side of the inverter; and the direct current busbar capacitor is a capacitor between direct current busbars. The positive terminal and the negative terminal of the direct current busbar capacitor 106 may be respectively connected to a positive terminal and a negative terminal of an input end of the direct current-alternating current inverter circuit 108. The positive terminal of the direct current busbar capacitor 106 and the positive terminal of the input end of the direct current-alternating current inverter circuit 108 are connected to be used as a positive direct current busbar, and the negative terminal of the direct current busbar capacitor 106 and the negative terminal of the input end of the direct current-alternating current inverter circuit 108 are connected to be used as a negative direct current busbar. In a possible implementation, a midpoint terminal of the direct current-alternating current inverter circuit 108 is connected to a negative electrode of a positive busbar capacitor and a positive electrode of a negative busbar capacitor, to be used as a neutral wire. One end of the grid-tied filter 110 is connected to an output end of the direct current-alternating current inverter circuit 108, and the other end is connected to the alternating current output terminal 112. The grid-tied filter 110 is configured to: filter out a high frequency component and control an output voltage/current required for connecting the inverter and a power grid. The alternating current output terminal 112 serves as an external output interface of the photovoltaic power generation system 100, and electric energy generated by the photovoltaic power generation system 100 is output to the power grid by using the alternating current output terminal 112. To be tied to the power grid, an alternating current output by the direct current-alternating current inverter circuit 108 needs to be synchronized with the mains in the power grid in terms of a frequency and a phase. The photovoltaic power generation system 100 may further include a direct current-direct current conversion part (not shown). The direct current-direct current conversion part is disposed between the photovoltaic module 102 and the common mode current controller 104, or between the common mode current controller 104 and the direct current busbar capacitor 106, and is configured to adjust the direct current output by the photovoltaic module 102, to match the direct current-alternating current inverter circuit 108.

The common mode current controller 104 is disposed on a direct current input side of the direct current-alternating current inverter circuit 108, in other words, between the photovoltaic module 102 and the direct current busbar capacitor 106, to improve a capability of the photovoltaic power generation system 100 to resist impact of a common mode current generated at a grid-tied instant, and further help reduce a risk that the inverter is disconnected from the power grid when a common mode voltage suddenly increases and there are asymmetric impedance. The common mode current controller 104 collects a midpoint-to-ground voltage BUSN (not shown) of the direct current busbar capacitor 106, or indirectly obtains a midpoint-to-ground voltage BUSN of the direct current busbar capacitor 106 by collecting a positive terminal-to-ground voltage BUS_positiveand a negative terminal-to-ground voltage BUS_negativeof the direct current busbar capacitor 106. When BUSN exceeds a threshold range, a magnitude of positive terminal-to-ground equivalent impedance of the direct current busbar capacitor 106 or a magnitude of negative terminal-to-ground equivalent impedance of the direct current busbar capacitor 106 is selectively reduced, to adjust impedance matching between the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor 106 and the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor 106, and further pre-control a magnitude of a common mode current generated when the inverter is grid-tied, in other words, pre-control a magnitude of a common mode impulse current (leakage current).

In some example embodiments, the photovoltaic module 102 may be replaced with another new energy power generation device that generates a direct current, for example, a wind turbine generator or a hydroelectric generator, or a direct current energy storage apparatus, for example, a dry cell or a storage battery.

In some example embodiments, the direct current-alternating current inverter circuit 108 may be a three-phase three-level inverter, or may be another three-phase multi-level inverter, or may be another type of inverter that can implement direct current-to-alternating current conversion.

In some example embodiments, the midpoint-to-ground voltage BUSN (not shown) of the direct current busbar capacitor 106, and the positive terminal-to-ground voltage BUS_positiveand the negative terminal-to-ground voltage BUS_negativeof the direct current busbar capacitor 106 may be collected in a direct current voltage measurement method. The midpoint-to-ground voltage BUSN of the direct current busbar capacitor 106 may be obtained by using an indirect manner in an actual application. For example, the positive terminal-to-ground voltage BUS_positiveand the negative terminal-to-ground voltage BUS_negativeof the direct current busbar capacitor 106 are collected, and then a sum of BUS_positiveand BUS_negativeis divided by two.

In some example embodiments, the photovoltaic power generation system 100 may further include a direct current-direct current DC-DC converter (not shown). The DC-DC converter is disposed between the photovoltaic module 102 and the direct current-alternating current inverter circuit 108, and is configured to convert a fixed direct current output by the photovoltaic module 102 into a changeable direct current voltage, to adapt to a corresponding voltage type required by the direct current-alternating current inverter circuit 108. The DC-DC converter may be in a pulse width modulation manner or a frequency modulation manner, may include necessary elements such as a control chip, an inductor, and a capacitor, and may be a boost converter, a buck converter, or a buck-boost converter. When the photovoltaic power generation system 100 includes the DC-DC converter, the DC-DC converter receives an input voltage from the photovoltaic module 102, converts the input voltage, and effectively outputs a fixed voltage. A positive electrode and a negative electrode of an output end of the DC-DC converter are respectively connected to the positive terminal and the negative terminal of the common mode current controller 104. In other words, the DC-DC converter, the common mode current controller 104, and the direct current busbar capacitor 106 are connected in parallel. These may be adjusted and improved based on an environment, and are not limited herein.

In some example embodiments, the common mode current controller 104 is further configured to control release of a residual charge of a Y capacitor at a terminal of the inverter. In a possible implementation, the Y capacitor at the terminal is a capacitor on an AC side of the inverter.

In some example embodiments, the photovoltaic power generation system 100 may include a plurality of parallel grid-tied inverters, and each grid-tied inverter and the common mode current controller 104 include a same device and similar structures, to have the improved capability of resisting the impact of the common mode current generated at the grid-tied instant of the photovoltaic power generation system 100.

FIG. 2 is a schematic diagram of a circuit structure of a common mode current controller of an inverter in an implementation according to an embodiment. As shown in FIG. 2, a positive electrode of a direct current voltage source PV is denoted as PV_positive, a negative electrode is denoted as PV_negative, and there is a positive terminal-to-ground impedance circuit R1 and a negative terminal-to-ground impedance circuit R2. One end of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV is grounded or protecting earthing (PE), and the other end is connected to the positive electrode PV_positiveof the direct current voltage source PV. One end of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV is grounded (PE), and the other end is connected to the negative electrode PV_negativeof the direct current voltage source PV. A circuit equivalent component of a direct current busbar capacitor includes a positive terminal part C_positiveof the direct current busbar capacitor, a negative terminal part C_negativeof the direct current busbar capacitor, a positive terminal part R3 of an intrinsic ground impedance part of the direct current busbar capacitor, and a negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor. One end of the positive terminal part C_positiveof the direct current busbar capacitor is a positive terminal of the direct current busbar capacitor, the positive terminal of the direct current busbar capacitor is connected to the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor, the other end is a midpoint of the direct current busbar capacitor, and a midpoint-to-ground voltage of the direct current busbar capacitor is denoted as BUSN. One end of the negative terminal part C_negativeof the direct current busbar capacitor is a negative terminal of the direct current busbar capacitor, the negative terminal of the direct current busbar capacitor is connected to the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor, and the other end is the midpoint of the direct current busbar capacitor. One end of each of the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded. The positive electrode and the negative electrode of the direct current voltage source PV are respectively connected to the positive terminal part C_positiveof the direct current busbar capacitor and the negative terminal part C_negativeof the direct current busbar capacitor. Therefore, one end of each of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded, and the other end is connected to the positive electrode of the direct current voltage source PV, to form a parallel connection relationship. One end of each of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded, and the other end is connected to the negative electrode of the direct current voltage source PV, to form a parallel connection relationship.

A common mode current controller 200 includes an impedance correction circuit 202 and a switch 204. One end of the impedance correction circuit 202 is connected to the positive electrode PV_positiveof the direct current voltage source PV, the other end is connected to one end of the switch 204, and the other end of the switch 204 is grounded. Therefore, when the switch 204 is closed, one end of the impedance correction circuit 202 is grounded, and the other end is connected to the positive electrode PV_positiveof the direct current voltage source PV, so that the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the positive terminal of the direct current busbar capacitor and a grounding (PE) node. It should be understood that, when the switch 204 is closed, positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor needs to be considered as equivalent impedance between the positive terminal of the direct current busbar capacitor and the ground. In other words, RES_positivemeets Formula (1):

$\begin{matrix} \frac{1}{{RES}_{positive}} = \frac{1}{R_{1}} + \frac{1}{R_{3}} + \frac{1}{R_{202}} & (1) \end{matrix}$

Herein, R₁represents a magnitude of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV, R₃represents a magnitude of the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor, and R₂₀₂represents a magnitude of the impedance correction circuit 202. Therefore, the switch 204 is closed, to introduce the impedance correction circuit 202 and reduce a magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor, and the magnitude of the impedance correction circuit 202 is adjusted, to adjust the magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor. Further, the magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor is adjusted, to adjust impedance matching between the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor and negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor can match the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, to pre-control a magnitude of a common mode current generated when the inverter is grid-tied, in other words, pre-control a magnitude of a common mode impulse current (leakage current). Herein, the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor needs to be understood as a magnitude of equivalent impedance of a parallel connection of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor.

When the switch 204 is opened, the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor is equivalent to a magnitude of equivalent impedance of a parallel connection of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor. Affected by a change in an environmental factor such as a temperature and component aging, the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor or the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor changes, resulting in impedance mismatching. Consequently, a positive terminal voltage BUS_positiveof the direct current busbar capacitor or a negative terminal voltage BUS_negativeof the direct current busbar capacitor fluctuate greatly, and a BUSN potential is not zero. In this case, if a grid-tied switch of the inverter is closed, large impact of the common mode current is generated. In the circuit structure of the common mode current controller of the inverter shown in FIG. 2, when it is detected that the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is greater than a specific threshold (which may be identified as a first threshold), it indicates that the BUSN potential floats upward and goes beyond a normal range and a large common mode impulse current at a grid-tied instant may be generated. In this case, the switch 204 is closed, to reduce the magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor, so as to effectively reduce an upward floating degree of the BUSN potential, and help restore impedance matching and control a magnitude of the common mode impulse current.

In the circuit structure of the common mode current controller of the inverter shown in FIG. 2, the common mode current controller 200 includes the switch 204 and the impedance correction circuit 202. One end of the impedance correction circuit 202 is grounded, the other end of the impedance correction circuit 202 is connected to one end of the switch 204, and the other end of the switch 204 is connected to the positive terminal of the direct current busbar capacitor, so that the common mode current controller 200 and the positive terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the positive terminal of the direct current busbar capacitor and the ground. The common mode current controller 200 is configured to: when the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is greater than the first threshold, close the switch 204 to adjust impedance matching between the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the inverter can pre-control the magnitude of the common mode current generated when the inverter is grid-tied.

Therefore, in the circuit structure of the common mode current controller of the inverter shown in FIG. 2, introduction of the impedance correction circuit 202 and adjustment of impedance matching are implemented based on the midpoint-to-ground voltage BUSN of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before the grid-tied switch of the inverter is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

In some example embodiments, the threshold or the first threshold may be preset, or may be set based on a corresponding inverter type and a parameter, or may be adjusted based on a corresponding state of a power grid and a use scenario. These may be adjusted and improved based on an environment and are not limited herein.

In some example embodiments, the magnitude of the impedance correction circuit 202 may be preset, or may be set based on a corresponding inverter type and a parameter, or may be adjusted based on a corresponding state of a power grid and a use scenario. These may be adjusted and improved based on an environment and are not limited herein.

Various embodiments may be used for a closing/opening mechanism of the switch 204 and a structure of the switch 204. For example, conduction or breaking of a circuit may be controlled by using a controllable electronic component, including, but not limited to, a thyristor, a transistor, a field effect transistor, a silicon controlled thyristor, and a relay. These may be adjusted and improved based on an environment and are not=limited herein.

In some example embodiments, the midpoint-to-ground voltage BUSN of the direct current busbar capacitor may be obtained through direct measurement, for example, may be obtained by using a voltage sensor; or may be obtained through indirect measurement, for example, may be determined based on the positive terminal voltage BUS_positiveand a negative terminal voltage BUS_negativeof the direct current busbar capacitor of the inverter. For example, the positive terminal-to-ground voltage BUS_positiveand the negative terminal-to-ground voltage BUS_negativeof the direct current busbar capacitor 106 are measured, and then a sum of BUS_positiveand BUS_negativeis divided by two.

In some example embodiments, an impedance value of the impedance correction circuit 202 is adjustable, and the impedance value R₂₀₂of the impedance correction circuit 202 is adjusted, so that a positive busbar and a negative busbar of the inverter have symmetric impedance to ground.

In some example embodiments, the common mode current controller 200 is a part of a grid-tied inverter, and the grid-tied inverter includes a direct current-direct current (DC-DC) converter and a direct current-alternating current (DC-AC) converter. The common mode current controller 200 is between the DC-DC converter and the DC-AC converter.

In some example embodiments, the common mode current controller 200 is further configured to control release of a residual charge of a Y capacitor at a terminal of the inverter. In a possible implementation, the Y capacitor at the terminal is a capacitor on an AC side of the inverter.

In some example embodiments, in a photovoltaic power generation system including a plurality of parallel grid-tied inverters, each grid-tied inverter and the common mode current controller 200 include a same device and similar structures, to have an improved capability of resisting impact of a common mode current generated at a grid-tied instant of the photovoltaic power generation system.

In some example embodiments, the common mode current controller 200 is part of the photovoltaic power generation system, the photovoltaic power generation system includes the grid-tied inverter, the grid-tied inverter includes the common mode current controller 200, and the first threshold is set based on a state of a power grid to which the grid-tied inverter is tied.

FIG. 3 is a schematic diagram of a circuit structure of a common mode current controller of an inverter in another implementation according to an embodiment. As shown in FIG. 3, a positive electrode of a direct current voltage source PV is denoted as PV_positive, a negative electrode is denoted as PV_negative, and there is a positive terminal-to-ground impedance circuit R1 and a negative terminal-to-ground impedance circuit R2. One end of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV is grounded (PE), and the other end is connected to the positive electrode PV_positiveof the direct current voltage source PV. One end of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV is grounded (PE), and the other end is connected to the negative electrode PV_negativeof the direct current voltage source PV. A circuit equivalent component of a direct current busbar capacitor includes a positive terminal part C_positiveof the direct current busbar capacitor, a negative terminal part C_negativeof the direct current busbar capacitor, a positive terminal part R3 of an intrinsic ground impedance part of the direct current busbar capacitor, and a negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor. One end of the positive terminal part C_positiveof the direct current busbar capacitor is a positive terminal of the direct current busbar capacitor, the positive terminal of the direct current busbar capacitor is connected to the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor, the other end is a midpoint of the direct current busbar capacitor, and a midpoint-to-ground voltage of the direct current busbar capacitor is denoted as BUSN. One end of the negative terminal part C_negativeof the direct current busbar capacitor is a negative terminal of the direct current busbar capacitor, the negative terminal of the direct current busbar capacitor is connected to the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor, and the other end is the midpoint of the direct current busbar capacitor. One end of each of the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded. The positive electrode and the negative electrode of the direct current voltage source PV are respectively connected to the positive terminal part C_positiveof the direct current busbar capacitor and the negative terminal part C_negativeof the direct current busbar capacitor. Therefore, one end of each of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded, and the other end is connected to the positive electrode of the direct current voltage source PV, to form a parallel connection relationship. One end of each of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded, and the other end is connected to the negative electrode of the direct current voltage source PV, to form a parallel connection relationship.

A common mode current controller 300 includes an impedance correction circuit 302 and a switch 304. One end of the impedance correction circuit 302 is connected to the negative electrode PV_negativeof the direct current voltage source PV, the other end is connected to one end of the switch 304, and the other end of the switch 304 is grounded. Therefore, when the switch 304 is closed, one end of the impedance correction circuit 302 is grounded, and the other end is connected to the negative electrode PV_negativeof the direct current voltage source PV, so that the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and a grounding (PE) node. It should be understood that, when the switch 304 is closed, negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor needs to be considered as equivalent impedance between the negative terminal of the direct current busbar capacitor and the ground. In other words, RES_negativemeets Formula (2):

$\begin{matrix} \frac{1}{R E S_{negative}} = \frac{1}{R_{2}} + \frac{1}{R_{4}} + \frac{1}{R_{3 0 2}} & (2) \end{matrix}$

Herein, R₂represents a magnitude of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV, R₄represents a magnitude of the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor, and R₃₀₂represents a magnitude of the impedance correction circuit 302. Therefore, the switch 304 is closed, to introduce the impedance correction circuit 302 and reduce a magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, and the magnitude of the impedance correction circuit 302 is adjusted, to adjust the magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor. Further, the magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor is adjusted, to adjust impedance matching between positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, so that the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor can match the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, to pre-control a magnitude of a common mode current generated when the inverter is grid-tied, in other words, pre-control a magnitude of a common mode impulse current (leakage current). Herein, the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor needs to be understood as a magnitude of equivalent impedance of a parallel connection of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor.

When the switch 304 is opened, the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor is equivalent to a magnitude of equivalent impedance of a parallel connection of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor. Affected by a change in an environmental factor such as a temperature and component aging, the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor or the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor changes, resulting in impedance mismatching. Consequently, a positive terminal voltage BUS_positiveof the direct current busbar capacitor or a negative terminal voltage BUS_negativeof the direct current busbar capacitor fluctuate greatly, and a BUSN potential is not zero. In this case, if a grid-tied switch of the inverter is closed, large impact of the common mode current is generated. In the circuit structure of the common mode current controller of the inverter shown in FIG. 3, when it is detected that the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is less than a specific threshold (which may be identified as a first threshold), it indicates that the BUSN potential floats downward and goes beyond a normal range and a large common mode impulse current at a grid-tied instant may be generated. In this case, the switch 304 is closed, to reduce the magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, so as to effectively reduce a downward floating degree of the BUSN potential, and help restore impedance matching and control a magnitude of the common mode impulse current.

In the circuit structure of the common mode current controller of the inverter shown in FIG. 3, the common mode current controller 300 includes the switch 304 and the impedance correction circuit 302. One end of the impedance correction circuit 302 is grounded, the other end of the impedance correction circuit 302 is connected to one end of the switch 304, and the other end of the switch 304 is connected to the negative terminal of the direct current busbar capacitor, so that the common mode current controller 300 and the negative terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and the ground. The common mode current controller 300 is configured to: when the midpoint-to-ground voltage of the direct current busbar capacitor is less than the first threshold, close the switch to adjust impedance matching between the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the inverter can pre-control the magnitude of the common mode current generated when the inverter is grid-tied.

Therefore, in the circuit structure of the common mode current controller of the inverter shown in FIG. 3, introduction of the impedance correction circuit 302 and adjustment of impedance matching are implemented based on the midpoint-to-ground voltage BUSN of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before the grid-tied switch of the inverter is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

In some example embodiments, the threshold or the first threshold may be preset, or may be set based on a corresponding inverter type and a parameter or may be adjusted based on a corresponding state of a power grid and a use scenario. These may be adjusted and improved based on an environment and are not limited herein.

In some example embodiments, the magnitude of the impedance correction circuit 302 may be preset, or may be set based on a corresponding inverter type and a parameter, or may be adjusted based on a corresponding state of a power grid and a use scenario. These may be adjusted and improved based on an environment and are not limited herein.

Various embodiments may be used for a closing/opening mechanism of the switch 304 and a structure of the switch 304. For example, conduction or breaking of a circuit may be controlled by using a controllable electronic component, including, but not limited to, a thyristor, a transistor, a field effect transistor, a silicon controlled thyristor, and a relay. These may be adjusted and improved based on an environment and are not limited herein.

In some example embodiments, an impedance value of the impedance correction circuit 302 is adjustable, and the impedance value R₂₁₂of the impedance correction circuit 302 is adjusted, so that a positive busbar and a negative busbar of the inverter have symmetric impedance to ground.

In some example embodiments, the common mode current controller 300 is a part of the grid-tied inverter, and the grid-tied inverter includes a direct current-direct current DC-DC converter and a direct current-alternating current DC-AC converter. The common mode current controller 300 is between the DC-DC converter and the DC-AC converter.

In some example embodiments, the common mode current controller 300 is further configured to control release of a residual charge of a Y capacitor at a terminal of the inverter. In a possible implementation, the Y capacitor at the terminal is a capacitor on an AC side of the inverter.

In some example embodiments, in a photovoltaic power generation system including a plurality of parallel grid-tied inverters, each grid-tied inverter and the common mode current controller 300 include a same device and similar structures, to have an improved capability of resisting impact of a common mode current generated at a grid-tied instant of the photovoltaic power generation system.

In some example embodiments, the common mode current controller 300 is part of the photovoltaic power generation system, the photovoltaic power generation system includes the grid-tied inverter, the grid-tied inverter includes the common mode current controller 300, and the first threshold is set based on a state of a power grid to which the grid-tied inverter is tied.

FIG. 4 is a schematic diagram of a circuit structure of a common mode current controller of an inverter in another implementation according to an embodiment. As shown in FIG. 4, a positive electrode of a direct current voltage source PV is denoted as PV_positive, a negative electrode is denoted as PV_negative, and there is a positive terminal-to-ground impedance circuit R1 and a negative terminal-to-ground impedance circuit R2. One end of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV is grounded (PE), and the other end is connected to the positive electrode PV_positiveof the direct current voltage source PV. One end of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV is grounded (PE), and the other end is connected to the negative electrode PV_negativeof the direct current voltage source PV. A circuit equivalent component of a direct current busbar capacitor includes a positive terminal part C_positiveof the direct current busbar capacitor, a negative terminal part C_negativeof the direct current busbar capacitor, a positive terminal part R3 of an intrinsic ground impedance part of the direct current busbar capacitor, and a negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor. One end of the positive terminal part C_positiveof the direct current busbar capacitor is a positive terminal of the direct current busbar capacitor, the positive terminal of the direct current busbar capacitor is connected to the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor, the other end is a midpoint of the direct current busbar capacitor, and a midpoint-to-ground voltage of the direct current busbar capacitor is denoted as BUSN. One end of the negative terminal part C_negativeof the direct current busbar capacitor is a negative terminal of the direct current busbar capacitor, the negative terminal of the direct current busbar capacitor is connected to the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor, and the other end is the midpoint of the direct current busbar capacitor. One end of each of the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded. The positive electrode and the negative electrode of the direct current voltage source PV are respectively connected to the positive terminal part C_positiveof the direct current busbar capacitor and the negative terminal part C_negativeof the direct current busbar capacitor. Therefore, one end of each of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded, and the other end is connected to the positive electrode of the direct current voltage source PV, to form a parallel connection relationship. One end of each of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded, and the other end is connected to the negative electrode of the direct current voltage source PV, to form a parallel connection relationship.

A common mode current controller 400 includes a first impedance correction circuit 402, a first switch 404, a second impedance correction circuit 406, and a second switch 408. One end of the first impedance correction circuit 402 is connected to the positive electrode PV_positiveof the direct current voltage source PV, the other end is connected to one end of the first switch 404, and the other end of the first switch 404 is grounded. Therefore, when the first switch 404 is closed, one end of the first impedance correction circuit 402 is grounded, and the other end is connected to the positive electrode PV_positiveof the direct current voltage source PV, so that the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the positive terminal of the direct current busbar capacitor and a grounding (PE) node. It should be understood that, when the first switch 404 is closed, positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor needs to be considered as equivalent impedance between the positive terminal of the direct current busbar capacitor and the ground. In other words, RES_positivemeets Formula (3):

$\begin{matrix} \frac{1}{R E S_{positive}} = \frac{1}{R_{1}} + \frac{1}{R_{3}} + \frac{1}{R_{4 0 2}} & (3) \end{matrix}$

Herein, R₁represents a magnitude of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV, R₃represents a magnitude of the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor, and R₄₀₂represents a magnitude of the first impedance correction circuit 402. Therefore, the first switch 404 is closed, to introduce the first impedance correction circuit 402 and reduce a magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor, and the magnitude of the first impedance correction circuit 402 is adjusted, to adjust the magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor. Further, the magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor is adjusted, to adjust impedance matching between the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor and negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor can match the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, to pre-control a magnitude of a common mode current generated when the inverter is grid-tied, in other words, pre-control a magnitude of a common mode impulse current (leakage current). Herein, the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor needs to be understood as a magnitude of equivalent impedance of a parallel connection of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor.

When the first switch 404 is opened, the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor is equivalent to a magnitude of equivalent impedance of a parallel connection of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor. Affected by a change in an environmental factor such as a temperature and component aging, the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor or the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor changes, resulting in impedance mismatching. Consequently, a positive terminal voltage BUS_positiveof the direct current busbar capacitor or a negative terminal voltage BUS_negativeof the direct current busbar capacitor fluctuate greatly, and a BUSN potential is not zero. In this case, if a grid-tied switch of the inverter is closed, large impact of the common mode current is generated. In the circuit structure of the common mode current controller of the inverter shown in FIG. 4, when it is detected that the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is greater than a threshold (which may be identified as a first threshold), it indicates that the BUSN potential floats upward and goes beyond a normal range and a large common mode impulse current at a grid-tied instant may be generated. In this case, the first switch 404 is closed, to reduce the magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor, so as to effectively reduce an upward floating degree of the BUSN potential, and help restore impedance matching and control a magnitude of the common mode impulse current.

One end of the second impedance correction circuit 406 is connected to the negative electrode PV_negativeof the direct current voltage source PV, the other end is connected to one end of the second switch 408, and the other end of the second switch 408 is grounded. Therefore, when the second switch 408 is closed, one end of the second impedance correction circuit 406 is grounded, and the other end is connected to the negative electrode PV_negativeof the direct current voltage source PV, so that the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and a grounding (PE) node. It should be understood that, when the second switch 408 is closed, negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor needs to be considered as equivalent impedance between the negative terminal of the direct current busbar capacitor and the ground. In other words, RES_negativemeets Formula (4):

$\begin{matrix} \frac{1}{R E S_{negative}} = \frac{1}{R_{2}} + \frac{1}{R_{4}} + \frac{1}{R_{4 0 6}} & (4) \end{matrix}$

Herein, R₂represents a magnitude of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV, R₄represents a magnitude of the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor, and R₄₀₆represents a magnitude of the second impedance correction circuit 406. Therefore, the second switch 408 is closed, to introduce the second impedance correction circuit 406 and reduce a magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, and the magnitude of the second impedance correction circuit 406 is adjusted, to adjust the magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor. Further, the magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor is adjusted, to adjust impedance matching between the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, so that the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor can match the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, to pre-control a magnitude of a common mode current generated when the inverter is grid-tied, in other words, pre-control a magnitude of a common mode impulse current (leakage current). Herein, the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor needs to be understood as a magnitude of equivalent impedance of a parallel connection of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor.

When the second switch 408 is opened, the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor is equivalent to a magnitude of equivalent impedance of a parallel connection of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor. Affected by a change in the environmental factor such as the temperature and component aging, the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor or the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor changes, resulting in impedance mismatching. Consequently, the positive terminal voltage BUS_positiveof the direct current busbar capacitor or the negative terminal voltage BUS_negativeof the direct current busbar capacitor fluctuate greatly, and the BUSN potential is not zero. In this case, if the grid-tied switch of the inverter is closed, large impact of the common mode current is generated. In the circuit structure of the common mode current controller of the inverter shown in FIG. 4, when it is detected that the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is less than a threshold (which may be identified as a second threshold), it indicates that the BUSN potential floats downward and goes beyond the normal range and a large common mode impulse current at the grid-tied instant may be generated. In this case, the second switch 408 is closed, to reduce the magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, so as to effectively reduce the downward floating degree of the BUSN potential and help restore impedance matching and control the magnitude of the common mode impulse current.

In the circuit structure of the common mode current controller of the inverter shown in FIG. 4, the common mode current controller 400 includes: the first switch 404, where one end of the first switch 404 is connected to the positive terminal of the direct current busbar capacitor; the second switch 408, where one end of the second switch 408 is connected to the negative terminal of the direct current busbar capacitor; the first impedance correction circuit 402, where one end of the first impedance correction circuit 402 is grounded, and the other end of the first impedance correction circuit 402 is connected to the other end of the first switch 404; and the second impedance correction circuit 406, where one end of the second impedance correction circuit 406 is grounded, and the other end of the second impedance correction circuit 406 is connected to the other end of the second switch 408. The common mode current controller 400 is configured to: when the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is greater than the first threshold, close the first switch 404, and open the second switch 408, so that the first impedance correction circuit 402 and the positive terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the positive terminal of the direct current busbar capacitor and the ground, to reduce the magnitude of the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor. The common mode current controller 400 is configured to: when the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is less than the second threshold, close the second switch 408, and open the first switch 404, so that the second impedance correction circuit 406 and the negative terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and the ground, to reduce the magnitude of the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor. The common mode current controller 400 adjusts impedance matching between the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the inverter can pre-control the magnitude of the common mode current generated when the inverter is grid-tied.

Therefore, in the circuit structure of the common mode current controller of the inverter shown in FIG. 4, introduction of the first impedance correction circuit 402 or the second impedance correction circuit 406 and adjustment of impedance matching are implemented based on the midpoint-to-ground voltage BUSN of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before the grid-tied switch of the inverter is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

In some example embodiments, the first threshold or the second threshold may be preset, or may be set based on a corresponding inverter type and a parameter, or may be adjusted based on a corresponding state of a power grid and a use scenario. These may be adjusted and improved based on an environment and are not limited herein.

In some example embodiments, the magnitude of the first impedance correction circuit 402 or the second impedance correction circuit 406 may be preset, or may be set based on a corresponding inverter type and a parameter, or may be adjusted based on a corresponding state of a power grid and a use scenario. These may be adjusted and improved based on an environment and are not limited herein.

Various embodiments may be used for a closing/opening mechanism of the first switch 404 or the second switch 408 and a structure of the first switch 404 or the second switch 408. For example, conduction or breaking of a circuit may be controlled by using a controllable electronic component, including, but not limited to, a thyristor, a transistor, a field effect transistor, a silicon controlled thyristor, and a relay. These may be adjusted and improved based on an environment and are not limited herein.

In some example embodiments, the midpoint-to-ground voltage (BUSN) of the direct current busbar capacitor may be obtained through direct measurement, for example, may be obtained by using a voltage sensor; or may be obtained through indirect measurement, for example, may be determined based on the positive terminal voltage BUS_positiveand a negative terminal voltage BUS_negativeof the direct current busbar capacitor of the inverter. For example, the positive terminal-to-ground voltage BUS_positiveand the negative terminal-to-ground voltage BUS_negativeof the direct current busbar capacitor 106 are measured, and then a sum of BUS_positiveand BUS_negativeis divided by two.

In some example embodiments, an impedance value of the first impedance correction circuit 402 and an impedance value of the second impedance correction circuit 406 are both adjustable, and the impedance value of the first impedance correction circuit 402 and the impedance value of the second impedance correction circuit 406 are adjusted, so that a positive busbar and a negative busbar of the inverter have symmetric impedance to ground.

In some example embodiments, the common mode current controller 400 is a part of the grid-tied inverter, and the grid-tied inverter includes a direct current-direct current DC-DC converter and a direct current-alternating current DC-AC converter. The common mode current controller 400 is between the DC-DC converter and the DC-AC converter.

In some example embodiments, the common mode current controller 400 is further configured to control release of a residual charge of a Y capacitor at a terminal of the inverter. In a possible implementation, the Y capacitor at the terminal is a capacitor on an AC side of the inverter.

In some example embodiments, in a photovoltaic power generation system including a plurality of parallel grid-tied inverters, each grid-tied inverter and the common mode current controller 400 include a same device and similar structures, to have an improved capability of resisting impact of a common mode current generated at a grid-tied instant of the photovoltaic power generation system.

In some example embodiments, the common mode current controller 400 is part of the photovoltaic power generation system, the photovoltaic power generation system includes the grid-tied inverter, the grid-tied inverter includes the common mode current controller 400, and the first threshold and the second threshold are set based on a state of a power grid to which the grid-tied inverter is tied.

FIG. 5 is a schematic diagram of a circuit structure of a common mode current controller of an inverter in another implementation according to an embodiment. As shown in FIG. 5, a positive electrode of a direct current voltage source PV is denoted as PV_positive, a negative electrode is denoted as PV_negative, and there is a positive terminal-to-ground impedance circuit R1 and a negative terminal-to-ground impedance circuit R2. One end of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV is grounded (PE), and the other end is connected to the positive electrode PV_positiveof the direct current voltage source PV. One end of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV is grounded (PE), and the other end is connected to the negative electrode PV_negativeof the direct current voltage source PV. A circuit equivalent component of a direct current busbar capacitor includes a positive terminal part C_positiveof the direct current busbar capacitor, a negative terminal part C_negativeof the direct current busbar capacitor, a positive terminal part R3 of an intrinsic ground impedance part of the direct current busbar capacitor, and a negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor. One end of the positive terminal part C_positiveof the direct current busbar capacitor is a positive terminal of the direct current busbar capacitor, the positive terminal of the direct current busbar capacitor is connected to the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor, the other end is a midpoint of the direct current busbar capacitor, and a midpoint-to-ground voltage of the direct current busbar capacitor is denoted as BUSN. One end of the negative terminal part C_negativeof the direct current busbar capacitor is a negative terminal of the direct current busbar capacitor, the negative terminal of the direct current busbar capacitor is connected to the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor, and the other end is the midpoint of the direct current busbar capacitor. One end of each of the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded. The positive electrode and the negative electrode of the direct current voltage source PV are respectively connected to the positive terminal part C_positiveof the direct current busbar capacitor and the negative terminal part C_negativeof the direct current busbar capacitor. Therefore, one end of each of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded, and the other end is connected to the positive electrode of the direct current voltage source PV, to form a parallel connection relationship. One end of each of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded, and the other end is connected to the negative electrode of the direct current voltage source PV, to form a parallel connection relationship.

A common mode current controller 500 includes a first impedance correction circuit 502, a second impedance correction circuit 504, and a single-pole triple-throw switch (SP3T) 506. The single-pole triple-throw switch 506 includes one non-movable end and three movable ends, the non-movable end is grounded, and the three movable ends respectively correspond to three connection states. One end of the first impedance correction circuit 502 is connected to the positive electrode PV_positiveof the direct current voltage source PV, and the other end is connected to the first movable end of the single-pole triple-throw switch 506. Therefore, when the single-pole triple-throw switch 506 is switched to a connection state of the first movable end, one end of the first impedance correction circuit 502 is grounded, and the other end is connected to the positive electrode PV_positiveof the direct current voltage source PV, so that the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the positive terminal of the direct current busbar capacitor and a grounding (PE) node. In this case, positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor may be considered as equivalent impedance between the positive terminal of the direct current busbar capacitor and the ground. In other words, RES_positivemeets Formula (5):

$\begin{matrix} \frac{1}{R E S_{positive}} = \frac{1}{R_{1}} + \frac{1}{R_{3}} + \frac{1}{R_{5 0 2}} & (5) \end{matrix}$

Herein, R₁represents a magnitude of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV, R₃represents a magnitude of the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor, and R₅₀₂represents a magnitude of the first impedance correction circuit 502. Therefore, the single-pole triple-throw switch 506 is switched to the connection state of the first movable end, to introduce the first impedance correction circuit 502 and reduce a magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor, and the magnitude of the first impedance correction circuit 502 is adjusted, to adjust the magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor. Further, the magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor is adjusted, to adjust impedance matching between the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor and negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor can match the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, to pre-control a magnitude of a common mode current generated when the inverter is grid-tied, in other words, pre-control a magnitude of a common mode impulse current (leakage current). Herein, the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor needs to be understood as a magnitude of equivalent impedance of a parallel connection of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor.

One end of the second impedance correction circuit 504 is connected to the negative electrode PV_negativeof the direct current voltage source PV, and the other end is connected to the second movable end of the single-pole triple-throw switch 506. The third movable end of the single-pole triple-throw switch 506 may correspond to a case in which the single-pole triple-throw switch 506 is connected to neither the first impedance correction circuit 502 nor the second impedance correction circuit 504, for example, is grounded. Therefore, when the single-pole triple-throw switch 506 is switched to a connection state of the second movable end, one end of the second impedance correction circuit 504 is grounded, and the other end is connected to the negative electrode PV_negativeof the direct current voltage source PV, so that the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and a grounding (PE) node. In this case, it should be understood that, the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor needs to be considered as equivalent impedance between the negative terminal of the direct current busbar capacitor and the ground. In other words, RES_negativemeets Formula (6):

$\begin{matrix} \frac{1}{R E S_{negative}} = \frac{1}{R_{2}} + \frac{1}{R_{4}} + \frac{1}{R_{5 0 4}} & (6) \end{matrix}$

Herein, R₂represents a magnitude of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV, R₄represents a magnitude of the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor, and R₅₀₄represents a magnitude of the second impedance correction circuit 504. Therefore, when the single-pole triple-throw switch 506 is switched to the connection state of the second movable end, the second impedance correction circuit 504 may be introduced, and the magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor may be reduced, and the magnitude of the second impedance correction circuit 504 is adjusted, to adjust the magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor. Further, the magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor is adjusted, to adjust impedance matching between the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, so that the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor can match the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, to pre-control the magnitude of the common mode current generated when the inverter is grid-tied, in other words, pre-control a magnitude of a common mode impulse current (leakage current). Herein, the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor needs to be understood as a magnitude of equivalent impedance of a parallel connection of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor.

Affected by a change in an environmental factor such as a temperature and component aging, the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor or the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor changes, resulting in impedance mismatching. Consequently, a positive terminal voltage BUS_positiveof the direct current busbar capacitor or a negative terminal voltage BUS_negativeof the direct current busbar capacitor fluctuate greatly, and a BUSN potential is not zero. In this case, if a grid-tied switch of the inverter is closed, large impact of the common mode current is generated. In the circuit structure of the common mode current controller of the inverter shown in FIG. 5, when it is detected that the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is greater than the first threshold, it indicates that the BUSN potential floats upward and goes beyond a normal range and a large common mode impulse current at a grid-tied instant may be generated. In this case, the single-pole triple-throw switch 506 is switched to the connection state of the first movable end, to reduce the magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor, so as to effectively reduce an upward floating degree of the BUSN potential, and help restore impedance matching and control a magnitude of the common mode impulse current. In addition, when it is detected that the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is less than a second threshold, it indicates that the BUSN potential floats downward and goes beyond the normal range and a large common mode impulse current at the grid-tied instant may be generated. In this case, the single-pole triple-throw switch 506 is switched to the connection state of the second movable end, to reduce the magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, so as to effectively reduce a downward floating degree of the BUSN potential and control a magnitude of the common mode impulse current.

In the circuit structure of the common mode current controller of the inverter shown in FIG. 5, the common mode current controller 500 includes: the single-pole triple-throw switch 506, where the non-movable end of the single-pole triple-throw switch 506 is grounded, and the movable ends of the single-pole triple-throw switch 506 include the first movable end, the second movable end, and the third movable end; the first impedance correction circuit 502, where one end of the first impedance correction circuit 502 is connected to the first movable end of the single-pole triple-throw switch 506, and the other end of the first impedance correction circuit 502 is connected to the positive terminal of the direct current busbar capacitor; and the second impedance correction circuit 504, where one end of the second impedance correction circuit 504 is connected to the second movable end of the single-pole triple-throw switch 506, and the other end of the second impedance correction circuit 504 is connected to the negative terminal of the direct current busbar capacitor. The common mode current controller 500 is configured to: when the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is greater than the first threshold, switch the single-pole triple-throw switch 506 to the first movable end, so that the first impedance correction circuit 502 and the positive terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the positive terminal of the direct current busbar capacitor and the ground, to reduce a magnitude of the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor. The common mode current controller 500 is configured to: when the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is less than the second threshold, switch the single-pole triple-throw switch 506 to the second movable end, so that the second impedance correction circuit 504 and a negative terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and the ground, to reduce a magnitude of the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor. The common mode current controller 500 adjusts impedance matching between the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the inverter can pre-control the magnitude of the common mode current generated when the inverter is grid-tied.

Therefore, in the circuit structure of the common mode current controller of the inverter shown in FIG. 5, introduction of the first impedance correction circuit 502 or the second impedance correction circuit 504 and adjustment of impedance matching are implemented based on the midpoint-to-ground voltage BUSN of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before the grid-tied switch of the inverter is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

In some example embodiments, the first threshold or the second threshold may be preset, may be set based on a corresponding inverter type and a parameter, or may be adjusted based on a corresponding state of a power grid and a use scenario. These may be adjusted and improved based on an environment and are not limited herein.

In some example embodiments, the magnitude of the first impedance correction circuit 502 or the second impedance correction circuit 504 may be preset, or may be set based on a corresponding inverter type and a parameter, or may be adjusted based on a corresponding state of a power grid and a use scenario. These may be adjusted and improved based on an environment and are not limited herein.

Various embodiments may be used for a connection state switching mechanism of the single-pole triple-throw switch 506 and a structure of the single-pole triple-throw switch 506. For example, conduction or breaking of a circuit may be controlled by using a controllable electronic component, including, but not limited to, a thyristor, a transistor, a field effect transistor, a silicon controlled thyristor, and a relay. These may be adjusted and improved based on an environment and are not limited herein.

In some example embodiments, an impedance value of the first impedance correction circuit 502 and an impedance value of the second impedance correction circuit 504 are both adjustable, and the impedance value of the first impedance correction circuit 502 and the impedance value of the second impedance correction circuit 504 are adjusted, so that a positive busbar and a negative busbar of the inverter have symmetric impedance to ground.

In some example embodiments, the common mode current controller 500 is a part of the grid-tied inverter, and the grid-tied inverter includes a direct current-direct current (DC-DC) converter and a direct current-alternating current (DC-AC) converter. The common mode current controller 500 is between the DC-DC converter and the DC-AC converter.

In some example embodiments, the common mode current controller 500 is further configured to control release of a residual charge of a Y capacitor at a terminal of the inverter. In a possible implementation, the Y capacitor at the terminal is a capacitor on an AC side of the inverter.

In some example embodiments, in a photovoltaic power generation system including a plurality of parallel grid-tied inverters, each grid-tied inverter and the common mode current controller 500 include a same device and similar structures, to have an improved capability of resisting impact of a common mode current generated at a grid-tied instant of the photovoltaic power generation system.

In some example embodiments, the common mode current controller 500 is part of the photovoltaic power generation system, the photovoltaic power generation system includes the grid-tied inverter, the grid-tied inverter includes the common mode current controller 500, and the first threshold and the second threshold are set based on a state of a power grid to which the grid-tied inverter is tied.

FIG. 6 is a schematic diagram of a circuit structure of a common mode current controller of an inverter in another implementation according to an embodiment. As shown in FIG. 6, a positive electrode of a direct current voltage source PV is denoted as PV_positive, a negative electrode is denoted as PV_negative, and there is a positive terminal-to-ground impedance circuit R1 and a negative terminal-to-ground impedance circuit R2. One end of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV is grounded (PE), and the other end is connected to the positive electrode PV_positiveof the direct current voltage source PV. One end of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV is grounded (PE), and the other end is connected to the negative electrode PV_negativeof the direct current voltage source PV. A circuit equivalent component of a direct current busbar capacitor includes a positive terminal part C_positiveof the direct current busbar capacitor, a negative terminal part C_negativeof the direct current busbar capacitor, a positive terminal part R3 of an intrinsic ground impedance part of the direct current busbar capacitor, and a negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor. One end of the positive terminal part C_positiveof the direct current busbar capacitor is a positive terminal of the direct current busbar capacitor, the positive terminal of the direct current busbar capacitor is connected to the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor, the other end is a midpoint of the direct current busbar capacitor, and a midpoint-to-ground voltage of the direct current busbar capacitor is denoted as BUSN. One end of the negative terminal part C_negativeof the direct current busbar capacitor is a negative terminal of the direct current busbar capacitor, the negative terminal of the direct current busbar capacitor is connected to the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor, and the other end is the midpoint of the direct current busbar capacitor. One end of each of the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded. The positive electrode and the negative electrode of the direct current voltage source PV are respectively connected to the positive terminal part C_positiveof the direct current busbar capacitor and the negative terminal part C_negativeof the direct current busbar capacitor. Therefore, one end of each of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded, and the other end is connected to the positive electrode of the direct current voltage source PV, to form a parallel connection relationship. One end of each of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor is grounded, and the other end is connected to the negative electrode of the direct current voltage source PV, to form a parallel connection relationship.

A common mode current controller 600 includes an impedance correction circuit 602, a first switch 604, and a second switch 606. One end of the impedance correction circuit 602 is grounded, and the other end is connected to both the first switch 604 and the second switch 606. One end of the first switch 604 is connected to the impedance correction circuit 602, and the other end is connected to the positive electrode PV_positiveof the direct current voltage source PV. One end of the second switch 606 is connected to the impedance correction circuit 602, and the other end is connected to the negative electrode PV_negativeof the direct current voltage source PV. Therefore, when the first switch 604 is closed and the second switch 606 is opened, one end of the impedance correction circuit 602 is grounded, and the other end is connected to the positive electrode of the direct current voltage source PV, so that the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the positive terminal of the direct current busbar capacitor and a grounding (PE) node. In this case, positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor may be considered as equivalent impedance between the positive terminal of the direct current busbar capacitor and the ground. In other words, RES_positivemeets Formula (7):

$\begin{matrix} \frac{1}{R E S_{positive}} = \frac{1}{R_{1}} + \frac{1}{R_{3}} + \frac{1}{R_{6 0 2}} & (7) \end{matrix}$

Herein, R₁represents a magnitude of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV, R₃represents a magnitude of the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor, and R₆₀₂represents a magnitude of the impedance correction circuit 602. Therefore, the first switch 604 is closed and the second switch 606 is opened, to introduce the impedance correction circuit 602 and reduce a magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor, and the magnitude of the impedance correction circuit 602 is adjusted, to adjust the magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor. Further, the magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor is adjusted, to adjust impedance matching between the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor and negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor can match the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, to pre-control a magnitude of a common mode current generated when the inverter is grid-tied, in other words, pre-control a magnitude of a common mode impulse current (leakage current). Herein, the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor needs to be understood as a magnitude of equivalent impedance of a parallel connection of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor.

When the first switch 604 is opened and the second switch 606 is closed, one end of the impedance correction circuit 602 is grounded, and the other end is connected to the negative electrode of the direct current voltage source PV, so that the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV and the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and a grounding (PE) node. In this case, the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor may be considered as equivalent impedance between the negative terminal of the direct current busbar capacitor and the ground. In other words, RES_negativemeets Formula (8):

$\begin{matrix} \frac{1}{R E S_{negative}} = \frac{1}{R_{2}} + \frac{1}{R_{4}} + \frac{1}{R_{6 0 2}} & (8) \end{matrix}$

Herein, R₂represents a magnitude of the negative terminal-to-ground impedance circuit R2 of the direct current voltage source PV, R₄represents a magnitude of the negative terminal part R4 of the intrinsic ground impedance part of the direct current busbar capacitor, and R₆₀₂represents the magnitude of the impedance correction circuit 602. Therefore, the first switch 604 is opened and the second switch 606 is closed, to introduce the impedance correction circuit 602 and reduce a magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, and the magnitude of the impedance correction circuit 602 is adjusted, to adjust the magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor. Further, the magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor is adjusted, to adjust impedance matching between the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, so that the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor can match the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, to pre-control the magnitude of the common mode current generated when the inverter is grid-tied, in other words, pre-control the magnitude of the common mode impulse current (leakage current). Herein, the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor needs to be understood as a magnitude of equivalent impedance of a parallel connection of the positive terminal-to-ground impedance circuit R1 of the direct current voltage source PV and the positive terminal part R3 of the intrinsic ground impedance part of the direct current busbar capacitor.

Affected by a change in an environmental factor such as a temperature and component aging, the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor or the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor changes, resulting in impedance mismatching. Consequently, a positive terminal voltage BUS_positiveof the direct current busbar capacitor or a negative terminal voltage BUS_negativeof the direct current busbar capacitor fluctuate greatly, and a BUSN potential is not zero. In this case, if a grid-tied switch of the inverter is closed, large impact of the common mode current is generated. In the circuit structure of the common mode current controller of the inverter shown in FIG. 6, when it is detected that the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is greater than a first threshold, it indicates that the BUSN potential floats upward and goes beyond a normal range and a large common mode impulse current at a grid-tied instant may be generated. In this case, the first switch 604 is closed and the second switch 606 is opened, to reduce the magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor, so as to effectively reduce an upward floating degree of the BUSN potential and help restore impedance matching and control a magnitude of the common mode impulse current. In addition, when it is detected that the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is less than a second threshold, it indicates that the BUSN potential floats downward and goes beyond the normal range and a large common mode impulse current at the grid-tied instant may be generated. In this case, the first switch 604 is opened and the second switch 606 is closed, to reduce the magnitude of the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor, so as to effectively reduce a downward floating degree of the BUSN potential and control the magnitude of the common mode impulse current.

In the circuit structure of the common mode current controller of the inverter shown in FIG. 6, the common mode current controller 600 includes: the first switch 604, where one end of the first switch 604 is connected to the positive terminal of the direct current busbar capacitor; the second switch 606, where one end of the second switch 606 is connected to the negative terminal of the direct current busbar capacitor; and the impedance correction circuit 602, where one end of the impedance correction circuit 602 is grounded, and the other end of the impedance correction circuit 602 is connected to both the other end of the first switch 604 and the other end of the second switch 606. The common mode current controller 600 is configured to: when the midpoint-to-ground voltage BUSN of the direct current busbar capacitor is greater than the first threshold, close the first switch 604, and open the second switch 606, so that the impedance correction circuit 602 and the positive terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the positive terminal of the direct current busbar capacitor and the ground, to reduce the magnitude of the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor. The common mode current controller 600 is configured to: when the midpoint-to-ground voltage of the direct current busbar capacitor is less than the second threshold, close the second switch 606, and open the first switch 604, so that the impedance correction circuit and the negative terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and the ground, to reduce the magnitude of the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor. The common mode current controller 600 adjusts impedance matching between the positive terminal-to-ground equivalent impedance of the direct current busbar capacitor and the negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so that the inverter can pre-control the magnitude of the common mode current generated when the inverter is grid-tied.

Therefore, in the circuit structure of the common mode current controller of the inverter shown in FIG. 6, introduction of the impedance correction circuit 602 and adjustment of impedance matching are implemented based on the midpoint-to-ground voltage BUSN of the direct current busbar capacitor, so that the magnitude of the common mode current generated when the inverter is grid-tied can be pre-controlled before the grid-tied switch of the inverter is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

In some example embodiments, the magnitude of the impedance correction circuit 602 may be preset, may be set based on a corresponding inverter type and a parameter, or may be adjusted based on a corresponding state of a power grid and a use scenario. In addition, the magnitude of the impedance correction circuit 602 may be separately set for adjusting the magnitude of the positive terminal-to-ground equivalent impedance RES_positiveof the direct current busbar capacitor or the negative terminal-to-ground equivalent impedance RES_negativeof the direct current busbar capacitor. These may be adjusted and improved based on an environment and are not limited herein.

Various embodiments may be used for a closing/opening mechanism of the first switch 604 and the second switch 606 and a structure of the first switch 604 and the second switch 606. For example, conduction or breaking of a circuit may be controlled by using a controllable electronic component, including, but not limited to, a thyristor, a transistor, a field effect transistor, a silicon controlled thyristor, and a relay. These may be adjusted and improved based on an environment and are not limited herein.

In some example embodiment, an impedance value of the impedance correction circuit 602 is adjustable, the impedance value of the impedance correction circuit is set to a first impedance value when the midpoint-to-ground voltage is greater than the first threshold, the impedance value of the impedance correction circuit is set to a second impedance value when the midpoint-to-ground voltage is less than the second threshold, and the first impedance value and the second impedance value are set, so that a positive busbar and a negative busbar of the inverter have symmetric impedance to ground.

In some example embodiments, the common mode current controller 600 is a part of the grid-tied inverter, and the grid-tied inverter includes a direct current-direct current DC-DC converter and a direct current-alternating current DC-AC converter. The common mode current controller 600 is between the DC-DC converter and the DC-AC converter.

In some example embodiments, the common mode current controller 600 is further configured to control release of a residual charge of a Y capacitor at a terminal of the inverter. In a possible implementation, the Y capacitor at the terminal is a capacitor on an AC side of the inverter.

In some example embodiments, in a photovoltaic power generation system including a plurality of parallel grid-tied inverters, each grid-tied inverter and the common mode current controller 600 include a same device and similar structures, to have an improved capability of resisting impact of a common mode current generated at a grid-tied instant of the photovoltaic power generation system.

In some example embodiments, the common mode current controller 600 is part of the photovoltaic power generation system, the photovoltaic power generation system includes the grid-tied inverter, the grid-tied inverter includes the common mode current controller 600, and the first threshold and the second threshold are set based on a state of a power grid to which the grid-tied inverter is tied.

FIG. 7 is a schematic flowchart of a common mode current control method for an inverter according to an embodiment. As shown in FIG. 7, the control method includes the following steps.

Step S700: Obtain a midpoint-to-ground voltage of a direct current busbar capacitor of the inverter.

The midpoint-to-ground voltage of the direct current busbar capacitor may be obtained through direct measurement, for example, may be obtained by using a voltage sensor; or may be obtained through indirect measurement, for example, may be determined based on a positive terminal voltage and a negative terminal voltage of the direct current busbar capacitor of the inverter. The positive terminal voltage and the negative terminal voltage of the direct current busbar capacitor of the inverter may be separately sampled and the midpoint-to-ground voltage may be obtained based on the positive terminal voltage and the negative terminal voltage of the direct current busbar capacitor of the inverter.

Step S702: Compare the midpoint-to-ground voltage and each of a first threshold and a second threshold.

The first threshold or the second threshold may be preset, or may be set based on a corresponding inverter type and a parameter, or may be adjusted based on a corresponding state of a power grid and a use scenario.

Step S704: Introduce a first impedance correction circuit when the midpoint-to-ground voltage is greater than the first threshold.

One end of the first impedance correction circuit is grounded, and the first impedance correction circuit and a positive terminal part of an intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel.

In a possible implementation, the inverter receives a direct current input from a direct current voltage source. One end of the first impedance correction circuit is connected to a positive electrode of the direct current voltage source, the other end is connected to one end of a first switch, and the other end of the first switch is grounded. Therefore, when the first switch is closed, one end of the first impedance correction circuit is grounded, and the other end is connected to the positive electrode of the direct current voltage source, so that a positive terminal-to-ground impedance circuit of the direct current voltage source PV and the positive terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between a positive terminal of the direct current busbar capacitor and a grounding (PE) node. When it is detected that the midpoint-to-ground voltage of the direct current busbar capacitor is greater than the first threshold, it indicates that a potential floats upward and goes beyond a normal range and a large common mode impulse current at a grid-tied instant may be generated. In this case, the first switch is closed, to reduce a magnitude of positive terminal-to-ground equivalent impedance of the direct current busbar capacitor, so as to effectively reduce an upward floating degree of the potential and control a magnitude of the common mode impulse current.

Step S706: Introduce a second impedance correction circuit when the midpoint-to-ground voltage is less than the second threshold.

One end of the second impedance correction circuit is grounded, and the second impedance correction circuit and a negative terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel.

In a possible implementation, the inverter receives the direct current input from the direct current voltage source. One end of the second impedance correction circuit is connected to a negative electrode of the direct current voltage source, the other end is connected to one end of a second switch, and the other end of the second switch is grounded. Therefore, when the second switch is closed, one end of the second impedance correction circuit is grounded, and the other end is connected to the negative electrode of the direct current voltage source, so that a negative terminal-to-ground impedance circuit of the direct current voltage source PV and the negative terminal part of the intrinsic ground impedance part of the direct current busbar capacitor are connected in parallel between the negative terminal of the direct current busbar capacitor and the grounding (PE) node. When it is detected that the midpoint-to-ground voltage of the direct current busbar capacitor is less than the second threshold, it indicates that the potential floats downward and goes beyond the normal range and a large common mode impulse current at the grid-tied instant may be generated. In this case, the second switch is closed, to reduce a magnitude of negative terminal-to-ground equivalent impedance of the direct current busbar capacitor, so as to effectively reduce a downward floating degree of the potential and control a magnitude of the common mode impulse current.

Therefore, in the common mode current control method for the inverter shown in FIG. 7, introduction of the impedance correction circuit and adjustment of impedance matching are implemented based on the midpoint-to-ground voltage of the direct current busbar capacitor, so that a magnitude of a common mode current generated when the inverter is grid-tied can be pre-controlled before a grid-tied switch of the inverter is closed, to help reduce a degree of complexity of a system design and reduce costs of the system design.

the embodiments may be implemented by using any one or a combination of hardware, software, firmware, or a solid-state logic circuit, and may be implemented in combination with signal processing, control, and/or a dedicated circuit. A device or an apparatus provided in the embodiments may include one or more processors (for example, a microprocessor, a controller, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), or a field programmable gate array (FPGA)), and the processors process various computer-executable instructions, to control operation of the device or the apparatus. The device or the apparatus provided in the embodiments may include a system bus or a data transmission system in which various components are coupled together. The system bus may include any one of different bus structures or a combination of different bus structures, for example, a memory bus or a memory controller, a peripheral bus, a universal serial bus, and/or a processor or a local bus for which any of a plurality of bus architectures is used. The device or the apparatus provided in the embodiments may be independently provided, or may be a part of a system, or may be a part of another device or apparatus.

The embodiments may include a non-transitory computer-readable storage medium or a combination with the non-transitory computer-readable storage medium, for example, one or more storage devices that can provide non-transitory data storage. The non-transitory computer-readable storage medium/storage device may be configured to store data, a program, and/or instructions, and when the data, the program, and/or the instructions are executed by a processor of the device or the apparatus provided in the embodiments, the device or the apparatus performs a related operation. The non-transitory computer-readable storage medium/storage device may include one or more of the following features: a volatile feature, a non-volatile feature, a dynamic feature, a static feature, a read/write feature, a read-only feature, a random access feature, a sequential access feature, location addressability, file addressability, and content addressability. In one or more example embodiments, the non-transitory computer-readable storage medium/storage device may be integrated into the device or the apparatus provided in the embodiments, or belong to a common system. The non-transitory computer-readable storage medium/storage device may include an optical storage device, a semiconductor storage device, a magnetic storage device, or the like, and may further include a random access memory (RAM), a flash memory, a read-only memory (ROM), an erasable programmable read only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), a register, a hard disk, a removable disk, a recordable and/or rewritable compact disc (CD), a digital versatile disc (DVD), a large-capacity storage medium device, or any other form of suitable storage medium.

The foregoing is an implementation of the embodiments. It should be noted that steps in the method described in the embodiments may be adjusted, combined, and deleted based on an actual requirement. In the foregoing embodiments, the description of each embodiment has respective focuses. For a part that is not described in detail in an embodiment, refer to related descriptions in other embodiments. It may be understood that a structure shown in the embodiments and the accompanying drawings do not constitute a limitation on a related apparatus or system. In some other embodiments, the related apparatus or system may include more or fewer components than those shown in the embodiments and the accompanying drawings, or combine some components, or split some components, or have different component arrangements. A person skilled in the art can understand that various modifications or changes may be made to the arrangements, operations, and details of the method and device recorded in the embodiments without departing from the scope of the embodiments. Several improvements and modifications may be further made without departing from the embodiments.

	Number	Date	Country
Parent	PCT/CN2021/106045	Jul 2021	US
Child	18154274		US

COMMON MODE CURRENT CONTROLLER OF INVERTER

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