Patent Application
20160306372
The disclosed invention is in the field of power inverter control for microgrids.
A microgrid is a local energy grid with control capability. It can disconnect from the traditional grid and operate autonomously. Due to increased power outages, microgrids are becoming more and more important. However, it is not very easy to set up a microgrid as it requires complicated microgrid management and communication systems with a centralized control. These communication systems require to coordinate between all sources and loads for the stable operation of the microgrid. If the central controller fails, the whole microgrid will fail. Thus, there is a need for systems and methods that efficiently coordinate between all sources and loads for the stable operation of the microgrid.
The present invention provides control systems for power inverters. For example, a control system comprises a plurality of sensors and a controller. The plurality of sensors can be configured to measure electrical signals indicative of output voltages and output currents of the power inverter. The controller, coupled to the power inverter, can be configured to: if the power inverter is in a voltage source mode, determine a target power based on real power frequency droop information and a first frequency; if the power inverter is in a current source mode, determine a target power based on a power limit and a predetermined power command; and generate a second frequency based on the target power, a measured power, and a latency estimate of a simulated generator.
The present invention provides control methods for power inverters. For example, a control method comprises: receiving an operation mode of the power inverter; if the operation mode of the power inverter is a voltage source mode, determining a target power based on real power frequency droop information and a first frequency; if the operation mode of the power inverter is a current source mode, determining a target power based on a power limit and a predetermined power command; and generating a second frequency based on the target power, a measured power, and a latency estimate of a simulated generator.
The general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as defined in the appended claims. Other aspects of the present invention will be apparent to those skilled in the art in view of the detailed description of the invention as provided herein.
The summary, as well as the following detailed description, is further understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings exemplary embodiments of the invention; however, the invention is not limited to the specific methods, compositions, and devices disclosed. In addition, the drawings are not necessarily drawn to scale. In the drawings:
The present invention may be understood more readily by reference to the following detailed description taken in connection with the accompanying figures and examples, which form a part of this disclosure. It is to be understood that this invention is not limited to the specific devices, methods, applications, conditions or parameters described and/or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of the claimed invention. Also, as used in the specification including the appended claims, the singular forms “a,” “an,” and “the” include the plural, and reference to a particular numerical value includes at least that particular value, unless the context clearly dictates otherwise. The term “plurality”, as used herein, means more than one. When a range of values is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. All ranges are inclusive and combinable.
It is to be appreciated that certain features of the invention which are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. Further, references to values stated in ranges include each and every value within that range.
The power inverters 101 can be connected to various power sources 102 such as batteries, solar arrays, fuel cells, micro turbines, wind turbines, or the like. Each power inverter 101 can operate in one of two modes: (1) grid-forming mode or voltage source mode; and (2) grid-following mode or current source mode. When the microgrid 100 is connected to the main grid 107, the power inverters 101 can operate in current source mode importing power from or exporting power to the main grid 107. When the microgrid 100 is isolated from the main grid 107, the power inverters 101 can operate in either current source mode or voltage source mode. For example, power inverters 101 that are connected to renewable energy sources such as solar arrays or wind turbines usually operate as a current source. Power inverters 101 that are connected to batteries usually operate as a voltage source when they form the microgrid 100, but can operate as a current source when they recharge the batteries using the generator 105. By changing a setting using an external controller or based on the status of the contactor 106 or 108, the power inverters 101 can dynamically set their operations to voltage source mode or current source mode. This can make the power inverters 101 very flexible and adaptable to different microgrid configurations.
The controller 210 can read the electrical signals from the plurality of sensors 220 and determine a switching pattern for the power transistors to control the output power of the power inverter 230. For example, the controller 210 performs real power calculation 211 and reactive power calculation 212 using the electrical signals measured at the sensor 220. Once the real (Pmeasured) and reactive powers (Qmeasured) are calculated, the controller 210 can perform real power control 213 to determine the frequency command (second frequency). Specifically, in the real power control 213, the controller 210 can first determine a target power based on frequency droop information. And then the controller 210 can calculate, based on the target power, the frequency command (FreqCmd). If the power inverter 230 is operating in voltage source mode, the controller 210 can determine the target power based on real power frequency droop information and a previous frequency (first frequency). If the power inverter 230 is operating in current source mode, the controller 210 can determine the target power based on power limits and a predetermined power command. In addition, the controller 210 can perform reactive power control 214 to determine the voltage magnitude command (Vcmd) after the reactive power is calculated.
After the real power control 213 and the reactive power control 214 are performed, the controller 210 can perform voltage calculation 215 based on the frequency command and the voltage magnitude command. For example, the controller 210 can calculate instantaneous 3-phase voltages at the voltage calculation 215. The instantaneous 3-phase voltages can be used to generate the power transistor control signals by means of pulse-width modulation (PWM) 216. With the power transistors control signals, the power inverter 230 can generate required currents and voltages. The controller 210 can comprise at least one of a processor, a microprocessor, a digital signal processor (DSP), or the like.
The real power frequency droop information used at the real power control 213 can represent correlation between frequency and real power associated with each of the operation mode of the power inverter 230. As illustrated in
When the power inverter 230 is operating in current source mode, the controller 210 can determine the target power based on the power limits and a predetermined power command. The power limits can comprise a minimum power and a maximum power at a certain frequency. The minimum and maximum power can be determined based on the real power frequency droop information illustrated in
The maximum power can be determined in accordance with the high limit line 430. The minimum power can be determined in accordance with the low limit line 420. For example, if a previous frequency (first frequency) is 62.5 Hz, the maximum power at the first frequency is 0 kW in accordance with the high limit line 430. At the previous frequency, the minimum power is −100 kW in accordance with the low limit line 420. If the previous frequency is 58.3 Hz, the maximum power is 100 kW in accordance with the high limit line 430. At the previous frequency, the minimum power is 0 kW in accordance with the low limit line 420. When the power inverter 230 is being limited by the low 420 and high limit lines 430, it can effectively operate in voltage source mode because its behavior is similar to that of the voltage source mode.
The predetermined power command can be transmitted from various sources such as a utility grid, an external controller, the power inverter 230, or the like. If it comes from a utility grid, the power inverter 230 can receive it by means of a communication interface. The communication interface can be established when the power inverter 230 provides ancillary serveries to the utility, for example, frequency regulation. If the predetermined power command comes from an external controller, the power inverter 230 can receive it by means of a communication interface between the power inverter 230 and the external controller. The external controller may calculate the predetermined power command using energy management techniques, for example Peak Shaving, based on measurements from a power meter or time of the day.
In an embodiment, the predetermined power command can be set by a user via the front panel interface on the power inverter 230 while commissioning the system. In another embodiment, the predetermined power command can be internally calculated by the power inverter 230 when it is configured to operate as a PV inverter. For example, the power inverter 230 can use the max power point tracking (MPPT) technique to calculate the maximum available power at the PV array and set the power command to that value.
The target power in current source mode can be determined by comparing the predetermined power command to the power limits. Specifically, if the predetermined power command is lower than the minimum power, the controller 210 can set the target power to the minimum power. If the predetermined power command is higher than the maximum power, the controller 210 can set the target power to the maximum power. Otherwise, the target power can be set to the predetermined power command.
After the target power is determined, the controller 210 can calculate a frequency command (second frequency) using the target power, a measured power, and a latency estimate of a simulated generator. The measured power can be an output power at the power inverter 230. It can be calculated based on the electrical signals received from the plurality of sensors 220.
The latency estimate of a simulated generator can represent rotor inertia of the simulated generator. The rotor inertia of the simulated generator can be determined based on at least one of mass of the simulated rotor, shape of the simulated rotor, or power of the simulated generator. In general, mass and shape of a rotor of a real generator are used to calculate the rotor inertia. This means that if the rotor of the real generator is comparable to the rotor of the simulated generator, the mass and shape of the rotor of the real generator can be used to determine the rotor inertia of the simulated rotor. Moreover, if a real generator has comparable power to the simulated generator, the rotor inertia of the real generator can be selected for that of the simulated generator. For example, the inertia of a 100 kW generator can be used for 100 kW power inverter. The rotor inertia of the simulated generator can be adjusted by performing transient response tests to ensure stability of the microgrid 100.
In an embodiment, the controller 210 can adjust the latency estimate depending on the mode of operation in the power inverter 230. For example, if the power inverter 230 is in voltage source mode, the controller 210 can select a first latency estimate from a plurality of preset latency estimates. If the power inverter 230 is in current source mode, the controller 210 can also select a second latency estimate from the plurality of preset latency estimates. The first and second latency estimates can be the same or different. By adjusting the latency estimate, the power inverter 230 can stabilize the microgrid 100 when it changes the mode of operation. For example, when the power inverters 230 are connecting to the generators, the power inverters 230 may response too fast, thereby resulting in excessive power generation. This may cause unstable status of the microgrid 100. However, since the controller 210 can simulate the behavior of the generators with the latency estimate, the power inverters 230 can be easily connected to the generators without causing excessive power generation.
Once the frequency command (second frequency) and the voltage magnitude command are determined at the real power control 213 and the reactive power control at 214 respectively, they can be used to calculate the instantaneous 3-phase voltages at the voltage calculation 215. After that, these voltages can be used to calculate the switching signals for the power transistors using the pulse-width modulation (PWM) 216.
At step 320, if the received operation mode is not voltage source mode, this means that the power inverter operates in current source mode and the target power can be determined based on power limits and a predetermined power command at step 340. The power limits can comprise a minimum power and a maximum power. They can be determined based on the real power frequency droop information and the first frequency of the power inverter in accordance with the low 420 and high limit lines 430 in
At step 350, a second frequency can be generated based on the target power, a measured power, and a latency estimate of a simulated generator. The measured power can be calculated based on the electrical signals measured at the sensors. It can represent an output power of the power inverter. The latency estimate of the simulated generator can represent rotor inertia of the simulated generator. As explained above, the rotor inertia of the simulated generator can be determined based on mass of the simulated rotor, shape of the simulated rotor, or power of the simulated generator.
In an embodiment, the latency estimate can be adjusted depending on the mode of operation in the power inverter. For example, if the power inverter is in voltage source mode, a first latency estimate can be selected from a plurality of preset latency estimates. If the power inverter is in current source mode, a second latency estimate can be selected from the plurality of preset latency estimates. The second latency estimate can be the same as or different from the first latency estimate. By adjusting the latency estimate, the microgrid can be stabilized when the power inverter changes the mode of operation.
In current source mode, power inverters can have a low limit line 420 based on frequency and a high limit line 430 based on frequency. These limit lines can ensure the stability of microgrid by limiting excess export or import of power by the current source inverters. For example, if a solar array generates more power than what the loads and battery charging inverters can consume, the frequency will go up. The high limit line 430 can force the reduction of power generated from the solar array, so that the frequency of microgrid does not rise indefinitely. Outside the low 420 and high limit lines 430, the power inverter can effectively operate in voltage source mode because its behavior is similar to that of the voltage source mode. Within the low 420 and high limit lines 430, the power inverter can operate in current source mode.
In an embodiment, the simulated generator-based controller illustrated in
where Ptarget is the target power, Pm is the simulated mechanical input power, Pmeasured is the measure AC power, τ is the net torque, fprev is previous generator frequency command (first frequency), Δf is generator frequency command change, Tsw is the switching period of the inverter, Jr is the generator's rotor moment of inertia (rotor inertia), and fcmd is the new generator frequency command which becomes the inverter frequency command (second frequency).
The frequency of simulated generator can be measured in Hz. It can be updated as the integral of the net power flow into the rotor of simulated generator divided by the simulated generator's rotor moment of inertia (Jr). The units of moment of inertia are kg×m2 and it can be configured, for example, in a range between 0.01 and 300 kg×m2. The net power flow into the rotor can be defined as the difference between the measured actual instantaneous AC output power (Pmeasured) as measured at the AC output and the simulated mechanical input power (Pm).
Pm can effectively become the power target (Ptarget) for the AC port and can be controlled by two control schemes depending on the mode of operation in a power inverter. For example, if the power inverter is operating in current source mode, Pm can be used as a power command for the AC port. This means that setting Pm to a particular value may result in the measured AC output power being equal to the constant value of the simulated Pm. Therefore, Pm can become the operative variable for executing real power import or export commands (i.e. positive sign can indicate export and negative sign can indicate import). The AC power can follow the variable Pm because the rotor of simulated generator can settle to a frequency equal to that of the grid/microgrid and to a phase angle offset from the grid phase angle that can result in the measured AC power being equal to Pm. If the measured AC power were greater than Pm, for instance, the simulated generator would see that the net power flow to/from the rotor was negative and would slow the rotor down. This can reduce the phase angle offset between the rotor and the grid which reduces real power flow. The converse can be the same when the measured AC power is less than Pm.
If the power inverter is operating in voltage source mode either on its own or in parallel with other inverters, Pm can behave like a typical generator throttle control including droop law functionality based on
In an embodiment, real power vs frequency droop offsets based on battery state of charge can be included in order to encourage multiple batteries connected to a microgrid to stay evenly charged with each other. As illustrated in
In another embodiment, the simulated generator's moment of inertia Jr can be dynamically adjustable. For example, Jr can be changed when the power inverters are switching between current source mode and voltage source mode. This means that Jr can be optimized independently for each of the operating modes. Even if Jr is changed suddenly, the output of the power inverter is not affected by it if the power inverter is in steady state because it can only affect the dynamic behavior. Therefore, changing Jr suddenly will have no risk of causing a transient response.
For stability, a frequency oscillation damper can be added to the control system. This damper can apply torque to the rotor of simulated generator that opposes a changing frequency. This damper may help damp out the frequency oscillations that naturally occur in this type of system after any step change in power. In an embodiment, a torque is applied in proportion to the difference between the present generator frequency and a calculated average generator frequency, as if the engine throttle control were intentionally damping oscillations in generator frequency.
If the controller is in manual mode, at step 1120, the power inverter can check the mode setting. If the mode setting is set to grid-following (i.e. the power inverter is in current source mode), the controller can control the real power at step 1140 and the reactive power at step 1150. If the mode setting is set to grid-forming (i.e. the power inverter is in voltage source mode), the controller can control the real power at step 1160 and the reactive power at step 1170. The mode setting can be preset or adjustable. For example, the mode setting for PV inverters is grid-following because PV inverters usually operate in current source mode. The mode setting can also be changed on the fly by an external controller by means of a communication protocol such as Modbus.
The grid contactor that connects the power inverter to the grid can be internal or external to the power inverter.
While the control systems and methods for power inverters has been described in connection with the various embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiments without deviating therefrom. For example, one skilled in the art will recognize that the simulated generator-based control scheme as described in the instant application may apply to any electrical grid, and any power electric devices to control power in the electrical grid. Therefore, the control systems and methods described herein should not be limited to any single embodiment, but rather should be constructed in breadth and scope in accordance with the appended claims.
