Patent Application
20240243681
The present invention relates to a modular multilevel converter (hereinafter, “MMC converter” in the present invention). The present invention relates more particularly to a modular multilevel converter system preferable to constitute a frequency conversion device by connecting two MMC converters back to back on their DC sides, and to constitute a variable-speed generator-motor device by connecting one of the MMC converters on its AC side to a power system, and connecting the other MMC converter on its AC side to an AC electric rotating machine.
A circuit of an MMC converter is formed of a unit converter that generates a required voltage by controlling the modulation factor of a PWM converter using an energy storage element with voltage source properties, such as a capacitor, as a voltage source. The voltage of the energy storage element in the unit converter fluctuates by charging/discharging in a cycle determined by an AC frequency. Six units of two-terminal arms are provided, in each of which the unit converters are connected in series. Three of the two-terminal arms are used as a positive-side arm. First terminals of the positive-side arm are connected to terminals for respective phases of an AC power supply, while second terminals in star connection are connected to a positive-side terminal of a DC power supply. The other three of the two-terminal arms are used as a negative-side arm. Second terminals of the negative-side arm are connected to the terminals for respective phases of the AC power supply, while first terminals in star connection are connected to a negative-side terminal of the DC power supply.
Control by the MMC converter includes a current control to adjust an arm current to an AC current command and a DC current command issued externally (hereinafter, “converter current control” in the present invention), a function of maintaining a balanced average voltage of the energy storage elements between the unit converters by simultaneously adjusting the modulation factors of the PWM converters provided in the unit converters in the arm (hereinafter, “interstage control” in the present invention), a function of maintaining a balanced average voltage of the energy storage elements in the arm between the arms (hereinafter, “interphase balance control” in the present invention), and a function of maintaining an average voltage of all the energy storage elements at a set value (hereinafter, “DC voltage control” in the present invention). In order to implement the interphase balance control and the DC voltage control described above, an inductive element is needed. The inductive element reduces a current flowing from the negative-side arm to the positive-side arm for each phase (hereinafter, “flow-through current” in the present invention).
Patent Literature 1 discloses a system in which, in order to reduce the above flow-through current, three units of two-terminal reactors are provided between a positive-side terminal of a DC power supply and AC power supply terminals for respective phases, and three units of two-terminal reactors are provided between a negative-side terminal of the DC power supply and the AC power supply terminals for respective phases.
Patent Literature 2 discloses a circuit system in which one unit of three-terminal reactor is provided instead of the two units of the two-terminal reactors connected to each of the AC power supply terminals for respective phases in Patent Literature 1, and magnetomotive forces of two windings are subtracted from each other for an AC current, while magnetomotive forces of the two windings are added together for a flow-through current.
A configuration of an MMC converter 28 using two-terminal reactors in Patent Literature 1 is illustrated in
In the configuration described above, currents detected by the current transformers 10 (IP_U, IP_V, IP_W, IN_U, IN_V, and IN_W) are input to a converter-current control device 11, and the converter-current control device 11 computes AC currents (IAC_U, IAC_V, and IAC_W) and flow-through currents for respective phases (IPN_U, IPN_V, and IPN_W) flowing from the negative-side arms 7N to the positive-side arms 7P by using the following equations.
These AC currents and flow-through currents are input to output a gate pulse to the two-terminal arms, the positive-side arms 7P and the negative-side arms 7N.
Conventionally, an air-core reactor has been employed as the two-terminal reactors 29P and 29N of the MMC converter 28. An iron-core reactor is effective to downsize the device. However, the iron-core reactor has not been employed because, as the inductance decreases due to iron-core saturation, the AC current and the flow-through current both increase in inverse proportion to the inductance. The air-core reactor has a larger-size body compared to the iron-core reactor. In addition, there is a possibility for the air-core reactor to cause electromagnetic interference with the surroundings due to flux leakage. For these reasons, the reactors are often installed in a shield chamber. This leads to a problem with a further increase in the size of the facilities.
As the inductance value is increased to reduce the flow-through current, this increases an AC voltage drop which is determined by the product of this inductance value and the AC current. Particularly when reactive power is supplied to the AC-side, it is necessary to increase an output voltage of the arms (the positive-side arms 7P and the negative-side arms 7N) in order to compensate for the AC voltage drop. This leads to a problem with a further increase in the device size.
A configuration of an MMC converter 30 using three-terminal reactors in Patent Literature 2 is illustrated in
Three-terminal reactors 31U, 31V, and 31W are provided between AC terminals (U, V, and W) for respective phases, first terminals of the positive-side arms 7P, and second terminals of the negative-side arms 7N. The AC terminals (U, V, and W) for respective phases are connected to intermediate terminals (UC, VC, and WC) of three-terminal reactors 31 (31U, 31V, and 31W). First terminals of three units of positive-side arms 7P are connected to positive-side terminals (UP, VP, and WP) of the three-terminal reactors 31. Second terminals of three units of negative-side arms 7N are connected to negative-side terminals (UN, VN, and WN) of the three-terminal reactors 31.
A configuration of the three-terminal reactor 31 is illustrated in
The winding 33 and the winding 34 are of a series two-row coil configuration that is the same as that of a core-type two-terminal DC reactor between a positive-side P terminal and a negative-side N terminal. A connection point of the two windings 33 and 34 is led out as an intermediate terminal C to serve as a third terminal.
In view of this configuration, the conventional two-terminal DC reactor has been expected to be also used as a three-terminal reactor with a minimum change. Use of the three-terminal reactor 31 with an iron core has been expected to downsize the device relative to use of the two-terminal reactors 29 (29P and 29N) that are air-core reactors. A voltage drop due to an AC current is equivalent to the leakage inductance between two windings. Thus, it has been expected that an increase in the arm output voltage can be prevented.
However, when an MMC converter employs the three-terminal reactor 31 in which the intermediate terminal is led out from the two-terminal DC reactor, the following problems arise.
An AC current divided from the terminal C of the three-terminal reactor 31 illustrated in
In a case where the three-terminal reactor 31 is employed in an MMC converter, an absolute value of a peak value of alternating magnetomotive force produced by an AC current is always larger than a magnetomotive force with a uniform polarity by a flow-through current flowing through each winding. The reasons for this are described hereinafter with reference to
A circuit of a unit converter that constitutes each of the arms 7 (the positive-side arms 7P and the negative-side arms 7N) is illustrated in
A PWM control method for a unit converter is described below with reference to
“a” in the upper diagram in
In a case where an AC current becomes minimized and a DC current becomes maximized, the magnitude of current is compared between the DC current and the AC current in the unit converter. As illustrated in the bottom diagram in
Where the number of unit converters connected in series to constitute the arm 7 is represented as N, an AC output phase-voltage peak value Vac_phase and an AC output current peak value IAC of the MMC converter are calculated by the following equation (2). The coefficient “2” in the equation to calculate the AC output current peak value IAC is derived from the sum of the arms, that is, the positive-side arm 7 and the negative-side arm 7N.
A DC voltage VDC of the MMC converter is equal to a total DC voltage of the positive-side arm 7P and the negative-side arm 7N. A DC current IDC of the MMC converter is equal to a total DC current of three units of positive-side arms 7P or a total DC current of three units of negative-side arms 7N. Thus, the following equation (3) holds.
For simplicity, when loss of the MMC converter is ignored, and the balance of active power on the condition that the power factor is 1 at which an AC current is minimized, is considered, then the following equation (4) holds.
In view of the above, the relation between the peak value Ib of an AC current and the DC current Id flowing through the arms 7 is expressed as the following equation (5).
As described above, an AC-component peak value of a magnetomotive force is twice or more as large as a DC-component magnetomotive force. Therefore, a peak value of a magnetomotive force of the three-terminal reactor 31 is one-third or less of a peak value of a magnetomotive force of the two-terminal reactor 29. It has been found that in contrast, when the three-terminal reactor 31 is employed in the MMC converter 30, the three-terminal reactor 31 being formed of the windings 33 and 34 and a core-type iron core 35, and being designed on the assumption that an AC-current magnetomotive force is smaller than a magnetomotive force produced by a DC current, then there is a problem that excessive iron loss due to a stray flux, and excessive copper loss due to an eddy current (coil current loss) both occur.
A three-terminal reactor 37 with a structure in which an intermediate terminal of a two-terminal reactor using a shell-type iron core is led out is illustrated in
It has been found that even when the three-terminal reactor 37 with the shell-type iron core described above is applied to the MMC converter, the positive-side winding and the negative-side winding are located in a distributed manner on the magnetic flux path, and this still leads to a problem that excessive iron loss due to a stray flux and excessive copper loss due to an eddy current (coil current loss) both occur similarly to the core-type three-terminal reactor.
The present invention has been achieved to solve the above problems, and an object of the present invention is to provide a small-sized low-loss modular multilevel converter.
In order to achieve the above object, there is provided a reactor preferable for downsizing the modular multilevel converter by using an iron-core reactor instead of an air-core reactor, for reducing the loss generated in the modular multilevel converter, and for preventing an overcurrent by reducing iron-core saturation when a system is being affected by a ground fault at AC side.
In order to provide this reactor, a winding of a shell-type two-terminal reactor is divided concentrically into two to constitute a three-terminal reactor. A P terminal-side coil is wound around an iron core on the inner side, while an N terminal-side coil is wound around the iron core on the outer side. Conversely, the P terminal-side coil may be wound around the iron core on the outer side, and the N terminal-side coil may be wound around the iron core on the inner side. In a configuration of the three-terminal reactor, magnetomotive forces are subtracted from each other, which are produced by an AC current divided into two windings from the intermediate terminal C, and magnetomotive forces of the two windings are added together, which are produced by a flow-through current from the N-terminal to the P-terminal or the other way around.
When the winding configuration in which a winding is divided concentrically into two is employed, this results in an imbalanced leakage inductance between the outer and inner coils wound around the same iron core because a leakage inductance of the outer coil is greater than a leakage inductance of the inner coil. However, it is possible to reduce a total leakage inductance of the two windings to one-tenth or less of the effective inductance. It has been also found that because the converter-current control device 11 can control the fundamental current value to a command value, even though the imbalance rate of leakage inductance between the outer coil and the inner coil increases approximately ten-fold, an increase in the uncharacteristic harmonics due to the imbalanced leakage inductance can be suppressed to only 11 or lower.
In view of the above, abnormal loss can be prevented by providing the shell-type iron core with the inner coil and the outer coil concentrically, so that the MMC converter can be operated with a little increase in the harmonics thereof.
A flow-through current for each phase has the largest AC component at the second harmonic, and the second largest AC component at the fourth harmonic. Over half of these components are generated by charging/discharging that is essential to maintain a constant voltage of a capacitor that constitutes a unit converter. The value at the second harmonic is a value equivalent to approximately 5′% of the fundamental of the arm current.
However, it has been found that zero-phase components at the second harmonic and the fourth harmonic due to this charging/discharging cancel each other out. It has been found that the other even-order harmonics can be reduced to one-third of the algebraic sum of magnetomotive forces for respective phases. It has also been found that the zero-phase component is reduced to one-half of the algebraic sum of odd-order harmonics of flow-through currents for respective phases.
In view of the above, in the MMC converter, a three-phase five-leg six-winding reactor is employed in which a fourth leg and a fifth leg are provided to form a return path for a zero-phase flux. Therefore, a further downsized, lower-loss MMC converter can be obtained as compared to the case where three units of three-terminal reactors of a shell-type two-winding configuration are provided for respective phases.
The modular multilevel converter according to the present invention has an effect where abnormal loss, generated when two windings are located in a direction of an iron-core magnetic path, is prevented by installing a three-terminal reactor for each phase in which two windings are located concentrically around a shell-type iron core.
The modular multilevel converter according to the present invention has a further effect where the length of a connecting wire between the intermediate terminal C and two windings can be minimized by winding one of the coils in a clockwise direction and winding the other coil in a counterclockwise direction, and thus a flux leakage can be reduced, which is caused by the connecting wire extending outside the iron core parallel to the magnetic path.
The modular multilevel converter according to the present invention has a still further effect where the loss due to harmonics generated in a return path of a shell-type reactor provided for each phase is decreased by using a three-phase five-leg six-winding reactor provided with a fourth leg and a fifth leg serving as a return path for a zero-phase flux.
A modular multilevel converter and a modular multilevel converter system according to embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The present invention is not limited to the embodiments.
In an MMC converter 1, three-terminal iron-core reactors (hereinafter, “three-terminal reactors”) 2U, 2V, and 2W are provided for respective AC phases.
A U-phase configuration is described below. A P-side coil 3U is wound around an iron core 5U on the inner side in a clockwise direction, while an N-side coil 4U is wound around the iron core 5U on the outer side in a counterclockwise direction. The iron core 5U is provided with a plurality of gaps 6U of a non-magnetic material.
In a V-phase configuration, a P-side coil 3V, an N-side coil 4V, an iron core 5V, and gaps 6V are identical to those in the U-phase configuration. In a W-phase configuration, a P-side coil 3W, an N-side coil 4W, an iron core 5W, and gaps 6W are identical to those in the U-phase configuration.
A UC terminal, a VC terminal, and a WC terminal of the three-terminal reactors 2U, 2V, and 2W are connected to AC terminals U, V, and W.
Each of an arm 7P and an arm 7N has two terminals, in which unit converters are connected in series. First terminals A of three units of positive-side arms 7P are connected to a UP terminal, a VP terminal, and a WP terminal of the three-terminal reactors 2U, 2V, and 2W. Second terminals B of the positive-side arms 7P are connected in a star configuration to a positive-side terminal P of the MMC converter 1. Second terminals B of the remaining three units of negative-side arms 7N are connected to a UN terminal, a VN terminal, and a WN terminal of the three-terminal reactors 2U, 2V, and 2W. First terminals A of the negative-side arms 7N are connected in a star configuration to a negative-side terminal N of the MMC converter 1.
At the positive-side terminal P of the MMC converter 1, the potential is fixed by an earthed circuit formed of a high resistance 8P and a current transformer 9P and serving also as a DC voltage transformer. At the negative-side terminal N of the MMC converter 1, the potential is fixed by an earthed circuit formed of a high resistance 8N and a current transformer 9N and serving also as a DC voltage transformer. Due to this configuration, there are effects where a potential to ground of the neutral point of the AC terminals (U, V, and W) is reduced, and where a potential to ground of the iron cores 5U, 5V, and 5W of the three-terminal reactors 2U, 2V, and 2W is reduced.
Current transformers 10 detect currents (IP_U, IP_V, IP_W, IN_U, IN_V, and IN_W) of six coils forming the three-terminal reactors 2U, 2V, and 2W, and output the detected currents to a converter-current control device 11.
The arm 7 is of a configuration in which N (N is a natural number) half-bridge circuits 12, each of which constitutes a unit converter, are connected in series between the first terminal A and the second terminal B.
The half-bridge circuit 12 includes two terminals, a positive-side terminal Y and a negative-side terminal X, in which self-arc-extinguishing elements 13H and 13L and inverse parallel diodes 14H and 14L, forming a bidirectional chopper circuit, are connected to a capacitor 15.
The half-bridge circuit 12 executes PWM control on the basis of a command from the converter-current control device 11 so as to output a target voltage between the X and Y terminals in accordance with an arc ignition/extinction command from gate drive units (GDU) 16H and 16L to the self-arc-extinguishing elements 13H and 13L.
A voltage detector 17 inputs a voltage of the capacitor 15 to the converter-current control device 11 via a signal converter (CONV) 18.
In the MMC converter 1 according to the first embodiment described above, the three-terminal reactors 2U, 2V, and 2W can be located in a distributed manner, so that layout flexibility of the MMC converter 1 can be ensured.
An MMC converter 19 includes a three-phase five-leg iron-core reactor 51.
A U-phase P-side coil 53U is wound around a U-phase iron core 55U serving as a first leg on the inner side in a clockwise direction, while a U-phase N-side coil 54U is wound around the U-phase iron core 55U on the outer side in a counterclockwise direction. The U-phase iron core 55U is provided with a plurality of gaps 56U of a non-magnetic material.
In the V-phase configuration, a P-side coil 53V, an N-side coil 54V, an iron core 55V serving as a second leg, and gaps 56V are identical to those in the U-phase configuration. In the W-phase configuration, a P-side coil 53W, an N-side coil 54W, an iron core 55W serving as a third leg, and gaps 56W are identical to those in the U-phase configuration.
An iron core 20 serves as a fourth leg. An iron core 21 serves as a fifth leg. A zero-phase flux is generated by a magnetomotive force produced by a current passing through coils wound around the first leg to the third leg. The zero-phase flux is divided equally into two and flows through the iron core 20 as the fourth leg and the iron core 21 as the fifth leg.
A UC terminal, a VC terminal, and a WC terminal of the three-phase five-leg iron-core reactor 51 are connected to the AC terminals U, V, and W.
Each of an arm 7P and an arm 7N has two terminals, in which unit converters are connected in series. First terminals A of three units of positive-side arms 7P are connected to a UP terminal, a VP terminal, and a WP terminal of the three-phase five-leg iron-core reactor 51. Second terminals B of the positive-side arms 7P are connected in a star configuration to a positive-side terminal P of the MMC converter 19. Second terminals B of the remaining three units of negative-side arms 7N are connected to a UN terminal, a VN terminal, and a WN terminal of the three-phase five-leg iron-core reactor 51. First terminals A of the negative-side arms 7N are connected in a star configuration to a negative-side terminal N of the MMC converter 19.
In the MMC converter 19 according to the second embodiment described above, reactors for respective phases can be combined into a single unit that is the three-phase five-leg iron-core reactor 51, so that the MMC converter 19 can be downsized.
A first MMC converter 1T and a second MMC converter 1G are connected back to back on the DC side of these two units. AC terminals (U1, V1, and W1) of the first MMC converter 1T are connected to three terminals (U, V, and W) of delta-connected windings of a unit transformer 22. A neutral terminal N of star-connected windings of the unit transformer 22 is directly earthed, and three terminals (A, B, and C) of the star-connected windings are connected to an AC system 23.
AC terminals (U2, V2, and W2) of the second MMC converter 1G are connected to three-phase AC terminals (U, V, and W) of an AC electric rotating machine 24. An armature neutral point of the AC electric rotating machine 24 is earthed through a high resistance 25.
In the third embodiment in
The first diagram from the top in
When the positive-side arm currents (IP_U, IP_V, and IP_W) and the negative-side arm currents (IN_U, IN_V, and IN_W) are compared with the AC currents (IAC_U, IAC_V, and IAC_W), the fundamental arm currents flowing through the upper and lower arms merge into an AC current, and in addition, over half of the odd-order-harmonic arm currents become an AC current. In contrast, the even-order-harmonic arm currents, considered to be harmful particularly for the system, do not flow out toward the AC-current side. Although it is harmful if even-order-harmonic currents flow out to the system, the second-order harmonic current is still essential to maintain an average voltage by charging/discharging the capacitor 15 that constitutes the unit converter.
When the flow-through currents (IPN_U, IPN_V, and IPN_W) are compared with the DC current (IDC), a total of DC components of the flow-through currents for three phases is equal to a DC component of the DC current (IDC). The amplitude (root-mean-square value) of the DC current (IDC) is three times as large as that of the flow-through current.
In contrast, the amplitude of odd-order harmonics of the DC current (IDC) is 1.5 times as large as that of the flow-through current. The ratio of amplitude of the odd-order harmonics to the flow-through current is reduced to one-half of the ratio of amplitude of the DC component to the flow-through current.
Zero-phase magnetomotive forces of the second harmonic and the fourth harmonic, which are the largest among the even-order harmonics of the DC current (IDC), cancel each other out to zero. The amplitude of the other even-order harmonics becomes equivalent (1.0 times) to that of the flow-through current. The ratio of amplitude of the other even-order harmonics to the flow-through current is reduced to one-third of the ratio of the amplitude of the DC component to the flow-through current.
In the MMC converter 19 illustrated in
In view of the above, the thickness of a laminated electromagnetic steel plate of the iron core 20 as the fourth leg and the iron core 21 as the fifth leg of the three-phase five-leg iron-core reactor 51 in the MMC converter 19 can be increased. This increases the effective cross-sectional area of the magnetic path. Thus, as compared to the case where three units of three-terminal reactors are provided for respective phases, the three-phase five-leg iron-core reactor 51 has an effect where the loss is decreased because a total cross-sectional area of the iron core 55U as the first leg, the iron core 55V as the second leg, and the iron core 55W as the third leg is equal to a total cross-sectional area of the iron core 20 as the fourth leg and the iron core 21 as the fifth leg, and also the same materials are used.
As illustrated in
The steady operation state has been evaluated by means of harmonics analyzation in the manner described above. In a case where a variable-speed generator-motor device is connected to a trunk power system, it is important to evaluate the operational state during the transient period. Especially, it is important to ensure the operational reliability in the transient state when the system is being affected by a ground fault. In a case where the AC electric rotating machine 24 is directly connected to a variable-speed pump turbine, the transient phenomenon differs significantly between a pumping mode in which active power is supplied from the AC system 23 and a power generation mode in which active power is supplied to the AC system 23. A ground fault mode that occurs in the AC system differs between a case of a symmetric accident and a case of an asymmetric accident.
A case where a symmetric accident has occurred in a power generation mode, and a case where an asymmetric accident has occurred in a pumping mode are selected as a typical example. The operational state in either selected case is described. Hereinafter, the operational state is described using an example with numerical values in which the MMC converter 1T with 60 MVA-15.4 kV is connected to the AC system 23 with 500 kV through the unit transformer 22.
In a case illustrated in
The upper diagram in
In a case illustrated in
The upper diagram in
As described above, when three units of three-terminal reactors 2 are used, AC components of the magnetomotive forces cancel each other out, thereby resulting in a peak value of the magnetomotive force being equal to or smaller than one-third relative to the case where six units of the two-terminal reactors 29 are used, in each of which an arm current produces a magnetomotive force. Therefore, downsizing of the device can be achieved. Because magnification of the peak value of the magnetomotive force when the system is being affected by an accident can be significantly decreased, iron-core saturation can be more easily reduced. Therefore, further downsizing of the device can be achieved.
Furthermore, zero-phase harmonics can be reduced by employing the three-phase five-leg iron-core reactor 51, as compared to the case where three units of the three-terminal reactors 2 are used, so that a further downsized, lower-loss device can be obtained.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/031899 | 8/24/2020 | WO |
