reactive power compensator

Abstract

A reactive power supplementing device (1) is provided with a power converter (6) and a converter control unit (7), and supplements reactive power of an AC power system (2) with output reactive power of the power converter (6). The converter control unit (7) is provided with an AC voltage detection unit (21) and an output limit unit (22). The AC voltage detection unit (21) detects voltage information about the AC power system (2) to which the power converter (6) is connected. The output limit unit (22) determines whether or not restriction on the output reactive power of the power converter (6) is necessary, on the basis of the voltage information detected by the AC voltage detection unit (21), and restricts the output reactive power of the power converter (6) when restriction on the output reactive power is necessary.

Description

[Technical Field]

[0001] The present application relates to a reactive power compensator. [Background technology]

[0002] In recent years, the introduction of new energy sources such as solar power generation and wind power generation has been increasing, making the stabilization and reliability of power systems important issues. To stabilize power systems, reactive power compensators have been introduced. In particular, static reactive power compensators are required not only to stabilize power systems during steady-state operation, but also to compensate for reactive power during and after a system fault, and are required to stabilize power systems even in transient situations (see, for example, Patent Document 1 below). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5134691 Summary of the Invention [Problem to be solved by the invention]

[0004] Due to their intended use, reactive power compensators are often connected to power systems with small short-circuit capacity and prone to system voltage fluctuations. Even when connected to such power systems, they are required to continue operation and stabilize the power system by compensating for reactive power even in the event of a system fault such as a ground fault.

[0005] When a reactive power compensator is connected to a power system with a small short-circuit capacity, an unbalance fault, such as a single-phase-to-ground fault, may occur, reducing the voltage amplitude of only one phase. If reactive power is output during such an unbalance fault, the voltage of the phase where the voltage remains unchanged (hereinafter referred to as the healthy phase) may rise further. In other words, even though the positive-sequence voltage of the system voltage drops, the voltage of the healthy phase may exceed the voltage within the operating range of the reactive power compensator. In such cases, the control margin for the voltage and current control of the reactive power compensator may be insufficient, resulting in a deterioration in control performance and the possibility of the reactive power compensator shutting down due to overvoltage or overcurrent.

[0006] In particular, if a system fault occurs and sufficient control margin is not secured, and the system fault is then cleared, even if an attempt is made to control the large voltage fluctuations that occur when the system fault is cleared, there will be no control margin, and the reactive power compensation device will not be able to operate as intended.

[0007] The present application discloses a technique for solving the above-described problems, and aims to ensure a control margin of a reactive power compensator and prevent the reactive power compensator from being shut down for protection due to an overvoltage or overcurrent. [Means for solving the problem]

[0008] The reactive power compensation device disclosed in the present application comprises: A reactive power compensation device comprising: a power converter connected to a multi-phase AC power system and having a self-extinguishing switching element; and a converter control unit that controls the switching element of the power converter, the reactive power compensation device compensating for reactive power of the AC power system by output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit detects voltage information of the AC power system to which the power converter is connected; the output limiting unit determines whether or not it is necessary to limit the output reactive power of the power converter based on the voltage information detected by the AC voltage detecting unit, and limits the output reactive current of the power converter if it is necessary to limit the output reactive power; the AC voltage detection unit includes a positive-sequence voltage detection unit that detects a positive-sequence voltage of a system voltage of the AC power system, and calculates a reactive current reference value based on the positive-sequence voltage detected by the positive-sequence voltage detection unit and a voltage command value of the power converter; The output limiting unit calculates a reactive current command value by subtracting a reactive current subtraction amount from the reactive current reference value, and limits the output reactive current of the power converter. Further, a reactive power compensator disclosed in the present application is a reactive power compensator that is connected to a multi-phase AC power system and includes a power converter having a self-extinguishing switching element, and a converter control unit that controls the switching element of the power converter, and compensates for reactive power of the AC power system with output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit detects the magnitude of a voltage of each phase of a system voltage of the AC power system; The output limiting unit calculates the maximum value of the magnitude of the voltage of each phase detected by the AC voltage detecting unit, and determines whether or not it is necessary to limit the output reactive power of the power converter based on the maximum value of the magnitude of the voltage of each phase. Further, a reactive power compensator disclosed in the present application is a reactive power compensator that is connected to a multi-phase AC power system and includes a power converter having a self-extinguishing switching element, and a converter control unit that controls the switching element of the power converter, and compensates for reactive power of the AC power system with output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit includes a positive-sequence voltage detection unit that detects a positive-sequence voltage of a system voltage of the AC power system, a negative-sequence voltage detection unit that detects a negative-sequence voltage of the system voltage of the AC power system, and an unbalance rate detection unit that calculates an unbalance rate that is a ratio of the negative-sequence voltage to the positive-sequence voltage, The output limiter determines whether or not it is necessary to limit the output reactive power of the power converter based on the unbalance factor calculated by the unbalance factor detector. Further, a reactive power compensator disclosed in the present application is a reactive power compensator that is connected to a multi-phase AC power system and includes a power converter having a self-extinguishing switching element, and a converter control unit that controls the switching element of the power converter, and compensates for reactive power of the AC power system with output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit detects the magnitude of a voltage of each phase of a system voltage of the AC power system; The output limiting unit limits the output power of the power converter when it is necessary to limit the output reactive power so that the output voltage of the power converter does not exceed a maximum allowable voltage value, which is the maximum voltage that the power converter can output. Further, a reactive power compensator disclosed in the present application is a reactive power compensator that is connected to a multi-phase AC power system and includes a power converter having a self-extinguishing switching element, and a converter control unit that controls the switching element of the power converter, and compensates for reactive power of the AC power system with output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit calculates a voltage amplitude value of each phase of the system voltage by a phase voltage amplitude detection unit based on the magnitude of the voltage of each phase of the system voltage of the AC power system; The output limiter calculates a maximum value of the voltage amplitude of each phase from the reactive current reference value based on the voltage amplitude value of each phase calculated by the each-phase voltage amplitude detector of the AC voltage detector, and calculates a deviation by subtracting the voltage amplitude value of each phase from a predetermined maximum allowable voltage value, and if the deviation is a negative value, The aforementioned Impedance of AC power system The voltage increase amount for the reactive current output is calculated according to the above, and the voltage increase amount is determined so as to satisfy the output voltage range of the reactive power compensator. A reactive current command value is calculated by subtracting the reactive current subtraction amount, and the output reactive current of the power converter is limited. Further, a reactive power compensator disclosed in the present application is a reactive power compensator that is connected to a multi-phase AC power system and includes a power converter having a self-extinguishing switching element, and a converter control unit that controls the switching element of the power converter, and compensates for reactive power of the AC power system with output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, The AC voltage detection unit detects the magnitude of the voltage of each phase of the system voltage of the AC power system, and the output limit unit calculates the maximum value of the magnitude of the voltage of each phase detected by the AC voltage detection unit, calculates the deviation between the maximum value and a predetermined maximum allowable voltage value, and calculates a reactive current subtraction amount to reduce the deviation, thereby limiting the output reactive current of the power converter. Further, a reactive power compensator disclosed in the present application is a reactive power compensator that is connected to a multi-phase AC power system and includes a power converter having a self-extinguishing switching element, and a converter control unit that controls the switching element of the power converter, and compensates for reactive power of the AC power system with output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit detects the magnitude of a voltage of each phase of a system voltage of the AC power system; The output limiting unit calculates the maximum value of the voltage magnitudes of each phase detected by the AC voltage detecting unit, calculates the deviation between the maximum value and a predetermined maximum allowable voltage value, and calculates a reactive power subtraction amount to reduce the deviation, thereby limiting the output reactive current of the power converter. [Effects of the Invention]

[0009] According to the reactive power compensator disclosed in the present application, a control margin for the reactive power compensator is ensured, and protective shutdown of the reactive power compensator due to overvoltage or overcurrent is prevented. [Brief explanation of the drawings]

[0010] [Figure 1]1 is a schematic configuration diagram showing a reactive power compensation device according to a first embodiment. [Figure 2] 1 is a circuit diagram showing a configuration example of a power converter according to a first embodiment. [Figure 3] 1 is a circuit diagram showing an internal configuration of a unit cell according to a first embodiment. [Figure 4] FIG. 4 is a circuit diagram showing the internal configuration of another unit cell according to the first embodiment. [Figure 5] 1 is an overall block diagram showing a converter control section for controlling a power converter according to a first embodiment. [Figure 6] 2 is a block diagram showing the internal configuration of a DC voltage control unit according to the first embodiment. FIG. [Figure 7] 3 is a block diagram showing the internal configuration of a circulating current control unit according to the first embodiment. FIG. [Figure 8] 3 is a block diagram showing the internal configuration of an AC voltage detection unit according to the first embodiment. FIG. [Figure 9] 3 is a block diagram showing the internal configuration of an output limiter according to the first embodiment. FIG. [Figure 10] 3 is a block diagram showing an internal configuration of an output current control unit according to the first embodiment. FIG. [Figure 11] 3 is a block diagram showing the internal configuration of a voltage command value calculation unit according to the first embodiment. FIG. [Figure 12] 3 is a block diagram showing the internal configuration of an individual cell DC voltage control unit according to the first embodiment. FIG. [Figure 13] FIG. 10 is a diagram showing the internal configuration of an output limiter according to a second embodiment. [Figure 14] FIG. 11 is a block diagram showing the internal configuration of an AC voltage detection unit according to a third embodiment. [Figure 15] FIG. 11 is a block diagram showing the internal configuration of an output limiter according to a third embodiment. [Figure 16] FIG. 10 is a block diagram showing the internal configuration of an output limiter according to a fourth embodiment. [Figure 17] FIG. 2 is a block diagram illustrating an example of a hardware configuration of a converter control unit according to an embodiment of the present application. DETAILED DESCRIPTION OF THE INVENTION

[0011] Embodiment 1 A reactive power compensator 1 according to a first embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing a reactive power compensator 1 according to the first embodiment. As shown in Fig. 1, the reactive power compensator 1 includes a power converter 6, which is a main circuit, and a converter control unit 7 that controls the power converter 6. The power converter 6 is connected to a multi-phase (three-phase in this embodiment) AC power system 2 via an interconnection transformer 3, and has a self-extinguishing switching element, as will be described later. Note that, although the power converter 6 is connected to the AC power system 2 via the interconnection transformer 3 in Fig. 1, it may also be configured to be connected to the AC power system 2 via an interconnection reactor.

[0012] The configuration of the converter control unit 7 shown in FIG. 1 is mainly described with respect to the exchange of signals between the AC power system 2 and the power converter 6, and therefore some components are omitted. The converter control unit 7 includes an AC voltage detection unit 21 that detects voltage information based on a system voltage detected by a voltage detector 20 that detects a system voltage at the interconnection point between the power converter 6 and the AC power system 2, an output limit unit 22 that calculates a limit value for reactive current or reactive power based on the voltage information detected by the AC voltage detection unit 21, and a gate signal calculation unit 23. The gate signal calculation unit 23 performs PWM (Pulse Width Modulation) control based on the AC currents iu, iv, iw detected by the current detector 30, the system voltages vu, vv, vw detected by the voltage detector 20, the limit values ​​calculated by the output limit unit 22, and the like, to calculate a gate signal G that controls the switching elements of the power converter 6. The detailed configuration and functions of the converter control unit 7 will be described later with reference to FIG.

[0013] FIG. 2 is a circuit diagram showing an example of the configuration of the power converter 6 according to the first embodiment. Since the power converter 6 is connected to the AC power system 2, it needs to be a device with large capacity and high voltage resistance, and multiple converters need to be multiplexed in series or parallel. As a direct connection type power conversion device, multilevel converters in which the outputs of multiple converters are cascade-connected have been proposed, one of which is the MMC (Modular Multilevel Converter). The power converter 6 according to the first embodiment is a type of MMC.

[0014] 2, the power converter 6 includes an arm 4 for each phase (U phase, V phase, W phase), and a plurality of unit cells 10 are connected in series to the arm 4 of each phase, and an arm reactor 5 is also connected in series to the arm 4 of each phase. The arms 4 of each phase are connected in a delta connection, and the connection ends of the arms 4 of each phase are connected to the AC power grid 2 via an interconnection transformer 3. The power converter 6 includes a valve branch current detector 40 that detects the currents iuv, ivw, and iwu of the valve branches 4 of each phase. Furthermore, the power converter 6 includes a DC capacitor voltage detector 50 that detects the voltage vdccell of the DC capacitor 15 in the unit cell 10, which will be described later.

[0015] Next, the internal structure of the unit cell 10 will be described with reference to FIGS. FIG. 3 is a circuit diagram showing the internal configuration of a full-bridge unit cell 10. As shown in FIG. 3, unit cell 10 is configured by connecting two series bodies 101 in parallel, each of which is made up of self-extinguishing switching elements 103 such as IGBTs (Insulated-Gate Bipolar Transistors), and further connecting a DC capacitor 15 in parallel with series body 101. Series body 101 is configured by connecting multiple (here, two) semiconductor switches 102 in series, each of which is made up of diodes 104 connected in anti-parallel to switching elements 103. As shown in FIG. 3, unit cell 10 uses the terminals of semiconductor switches 102, which are intermediate connections between series bodies 101, as output terminals. By turning switching elements 103 on and off, unit cell 10 outputs a same-polarity voltage, a reverse-polarity voltage, or zero voltage of DC capacitor 15 from this output terminal.

[0016] Another example of the configuration of the unit cell 10 will be described with reference to Fig. 4. Fig. 4 is a circuit diagram showing the internal configuration of the unit cell 10 configured as a half-bridge. In Fig. 4, the unit cell 10 is composed of a series body 101 composed of switching elements 103, and a DC capacitor 15 connected in parallel to the series body 101. The series body 101 is made up of a plurality (two in this case) of semiconductor switches 102 connected in series, each of which is composed of a diode 104 connected in anti-parallel to the switching element 103. As shown in Fig. 4, the unit cell 10 uses both terminals of one of the semiconductor switches 102 as output terminals, and outputs a voltage of the same polarity as the DC capacitor 15 or zero voltage from these output terminals by turning the switching element 103 on and off.

[0017] The configuration of the unit cell 10 is not limited to that shown in Figures 3 and 4, as long as it is configured to include a series body made up of semiconductor switches and a DC capacitor connected in parallel to this series body, and the semiconductor switch in the series body selectively outputs a DC capacitor voltage to the output terminal. Hereinafter, "unit cell" will be referred to as "cell" where appropriate.

[0018] Next, the control of the reactive power compensator 1 of this embodiment will be described. FIG. 5 is an overall block diagram of converter control section 7 for controlling power converter 6 according to the first embodiment. The converter control unit 7 mainly comprises an AC voltage detection unit 21, an output limit unit 22, a DC voltage control unit 60, a circulating current control unit 70, an output current control unit 90, a voltage command value calculation unit 130, and a gate signal generation unit 120. The DC voltage control unit 60, the circulating current control unit 70, the output current control unit 90, the voltage command value calculation unit 130, and the gate signal generation unit 120 are collectively combined into a functional block called a gate signal calculation unit 23.

[0019] FIG. 6 is a block diagram showing the internal configuration of the DC voltage control unit 60 in the gate signal calculation unit 23 according to the first embodiment. As shown in FIG. 6, the DC voltage control unit 60 includes a DC voltage representative value calculation unit 61, a subtractor 62, and a controller 63. The DC voltage control unit 60 receives as input the all-cell DC capacitor voltages vdccell detected by the DC capacitor voltage detectors 50 of all (N in this case) unit cells 10. In the DC voltage control unit 60, a DC voltage representative value calculation unit 61 calculates a voltage representative value vdc such as the average value, maximum value, or minimum value of the all-cell DC capacitor voltages from the all-cell DC capacitor voltages vdccell. A subtractor 62 calculates a deviation Δvdc between the DC total voltage command value vdc* and the voltage representative value vdc calculated by the DC voltage representative value calculation unit. The controller 63 calculates the active current command value iq* so that the calculated deviation Δvdc becomes zero, that is, so that the voltage representative value vdc of the DC capacitors of all the cells follows the DC total voltage command value vdc*. By using the average value, maximum value, or minimum value of the DC capacitor voltages of all the cells as the representative voltage value vdc, the DC capacitor voltages of all the cells are controlled to be constant.

[0020] FIG. 7 is a block diagram showing the internal configuration of the circulating current control unit 70 according to the first embodiment. The circulating current control unit 70 controls the current circulating through the arms 4 of each phase of the power converter 6, thereby balancing the DC capacitor voltages between the phases. As shown in FIG. 7, the circulating current control unit 70 includes a subtractor 71, an adder 72, a multiplier 73, and a controller 75. In the circulating current control unit 70, the adder 72 adds up the arm current values ​​iuv, ivv, iwv detected by the arm current detector 40, and the multiplier 73 multiplies the sum by 1 / 3 to calculate the circulating current iz. A subtractor 71 calculates the deviation Δiz between the circulating current command value iz* and the circulating current iz. The circulating current command value iz* is a fixed value or a value for balancing the DC capacitor voltages between the phases. The controller 75 calculates a zero-phase sequence voltage command value vz* so that the deviation Δiz becomes zero, that is, so that the circulating current iz follows the circulating current command value iz*.

[0021] Next, the AC voltage detection unit 21 and the output limit unit 22, which are essential parts of the converter control unit 7 in the first embodiment, will be described.

[0022] FIG. 8 is a block diagram showing the internal configuration of AC voltage detection unit 21 according to the first embodiment. As shown in FIG. 8, the AC voltage detection unit 21 includes a positive-phase voltage detection unit 11, an individual-phase voltage amplitude detection unit 12, a voltage control unit 13, and a subtractor 300. The positive-sequence voltage detection unit 11 calculates a positive-sequence voltage from the system voltage detected by the voltage detector 20. The AC power system 2 consists of three phases, u, v, and w, and if the instantaneous voltages are vu, vv, and vw, the positive-sequence voltage detection unit 11 calculates the positive-sequence voltage vs based on the following equation (1):

[0023] vs=(vu 2 +vv 2 +vw 2 ) 1 / 2 ...Equation (1)

[0024] The AC voltage detection unit 21 receives the voltage command value vref (command value of the effective voltage) of the power converter 6, and calculates a deviation Δv by subtracting the positive-phase voltage vs calculated by the positive-phase voltage detection unit 11 from the voltage command value vref using a subtractor 300. The deviation Δv is then input to the voltage control unit 13. The voltage control unit 13 is configured by, for example, a PI (Proportional Integral) controller, and calculates a reactive current reference value idref for reducing the input deviation Δv.

[0025] The phase voltage amplitude detector 12 of the AC voltage detector 21 receives the voltages vu, vv, and vw of the AC power system 2 and detects the voltage amplitude values ​​vur, vvr, and vwr of the phases based on the following equation (2).

[0026]

number

[0027] FIG. 9 is a block diagram showing the internal configuration of the output limiter 22 according to the first embodiment. The output limiter 22 includes a maximum value calculator 16, a subtraction value calculator 17, and subtractors 400 and 410. The maximum value calculation unit 16 calculates the maximum value vrmax of the voltage amplitude values ​​of each phase based on the voltage amplitude values ​​vur, vvr, and vwr of each phase detected by the phase voltage amplitude detection unit 12 of the AC voltage detection unit 21. Then, a subtractor 410 subtracts the maximum value vrmax of the phase voltage amplitude calculated by the maximum value calculation unit 16 from a predetermined maximum allowable voltage value Vth to calculate a deviation Δvr. Here, the maximum allowable voltage value Vth is determined by the specifications of the reactive power compensator 1 and is the maximum voltage that the power converter 6 can output. Next, the reactive current subtraction amount Δid for reducing the deviation Δvr is calculated in the subtraction value calculation unit 17. When the deviation Δvr is a positive value, the reactive current subtraction amount Δid is set to zero. Next, the subtractor 400 subtracts the reactive current subtraction amount Δid from the reactive current reference value idref calculated by the voltage control unit 13 of the AC voltage detection unit 21 to calculate the reactive current command value id*. As described above, the output limiter 22 calculates the maximum value vrmax of each phase voltage amplitude value based on the phase voltage amplitude value detected by the phase voltage amplitude detector 12, and determines whether or not it is necessary to limit the output reactive power of the power converter 6 based on the maximum value vrmax of each phase voltage amplitude value.

[0028] Next, the configuration and operation of the output current control unit 90 of the converter control unit 7 will be described with reference to FIG. FIG. 10 is a block diagram showing the internal configuration of the output current control unit 90 according to the first embodiment. The output current control unit 90 controls the active current iq and reactive current id to control the power of the power converter 6. An active current command value iq* and a reactive current command value id* are input to the output current control unit 90. The active current command value iq* is calculated by the DC voltage control unit 60 as described above, and the reactive current command value id* is calculated by the output limit unit 22 as described above.

[0029] 10, the output current control unit 90 includes a reference voltage calculation unit 91, a three-phase / two-phase coordinate conversion unit 92, and adders 93a and 93b. The reference voltage calculation unit 91 includes a three-phase / two-phase coordinate conversion unit 94, subtractors 96a and 96b, and a controller 97. The three-phase / two-phase coordinate conversion unit 94 performs three-phase / two-phase conversion on the AC currents iu, iv, and iw detected by the current detector 30 in a positive phase coordinate system to calculate a reactive current id and an active current iq. The subtractor 96a calculates the deviation Δid between the reactive current command value id* and the reactive current id. The subtractor 96b calculates the deviation Δiq between the active current command value iq* and the active current iq. The controller 97 is configured, for example, by a PI controller, and calculates reference voltages vdref and vqref so that the deviation Δid and deviation Δiq become zero, i.e., so that the reactive current id follows the reactive current command value id* and the active current iq follows the active current command value iq*. The three-phase / two-phase coordinate conversion unit 92 performs three-phase / two-phase conversion on the system voltages vu, vv, vw detected by the voltage detector 20 in the positive phase coordinate system to calculate positive sequence voltages vd, vq. The output current control unit 90 then adds the positive-sequence voltages vd, vq to the reference voltages vdref, vqref, which are the outputs of the controller 97 in a feedforward manner using adders 93a, 93b, to calculate voltage command values ​​vd*, vq*.

[0030] Next, the configuration and operation of the voltage command value calculation unit 130 of the converter control unit 7 will be described with reference to FIGS. FIG. 11 is a block diagram showing the internal configuration of the voltage command value calculation unit 130 according to the first embodiment. The voltage command value calculation unit 130 includes a two-phase / three-phase coordinate conversion unit 131, an adder 132, and an individual cell DC voltage control unit 140. The internal configuration of the individual cell DC voltage control unit 140 will be described later. First, the voltage command values ​​vd* and vq* are converted into three phases in a positive phase coordinate system by a two-phase / three-phase coordinate conversion unit 131. An adder 132 adds the zero-phase sequence voltage command value vz* to each voltage command value converted into three phases by the two-phase / three-phase coordinate conversion unit 131, and calculates arm voltage command values ​​vuv*, vvw*, and vwv* for each phase. The individual cell DC voltage control unit 140 calculates the output voltage command value vcell* for each of all unit cells 10 based on these phase arm voltage command values ​​vuv*, vvw*, vwu*, arm currents iuv, ivw, iwu, and each cell DC voltage vdccell.

[0031] FIG. 12 is a block diagram showing the internal configuration of the individual cell DC voltage control unit according to the first embodiment. The individual cell DC voltage control unit 140 includes an individual cell control unit 141 for N cells. The individual cell control unit 141 includes a phase representative value calculation unit 142, a cell voltage extraction unit 143, a filter 144, subtractors 145 and 148, a controller 146, and a multiplier 147. In the individual cell control unit 141, the phase representative value calculation unit 142 calculates a phase voltage representative value vdcuvav, such as the average, maximum, or minimum value of the DC capacitor voltage of each phase, from each cell DC voltage vdccell. Furthermore, the cell voltage extraction unit 143 calculates an individual cell DC voltage vdcuv1 from each cell DC voltage vdccell. The filter 144 calculates vdcuvf1 by removing the AC component of frequency 2f from the individual cell DC voltage vdcuv1. The subtractor 145 calculates the deviation Δvdcuv1 by subtracting vdcuvf1 from the phase voltage representative value vdcuvav. The controller 146 calculates the control output vdcuv1ref so that the calculated deviation Δvdcuv1 becomes zero. Furthermore, the control output vdcuv1ref is multiplied by the arm current iuv, which is in phase with the control output vdcuv1ref, in a multiplier 147. The output of the multiplier 147 is subtracted from the arm voltage command value vuv* of each phase in a subtractor 148 to calculate the DC voltage command value vdcuv1* of each cell. Note that the DC voltage command values ​​vdcuv1* to vdcwuN* of each cell in FIG. 12 become the output voltage command value vcell* of each cell in FIGS. 5 and 11.

[0032] Returning to Figure 5, the gate signal generation unit 120 calculates a gate signal G that controls the on and off driving of the switching element 103 of each cell so as to perform PWM (Pulse Width Modulation) control based on the output voltage command value vcell* of each cell, which is the output of the voltage command value calculation unit 130.

[0033] In the above description of the embodiment, the AC voltage detection unit 21 includes the phase voltage amplitude detection unit 12 that detects the phase voltage amplitude values ​​vur, vvr, and vwr of the system voltage of the AC power system 2, and the output limit unit 22 calculates the maximum value vrmax of the phase voltage amplitude values ​​based on the phase voltage amplitude values ​​detected by the phase voltage amplitude detection unit 12, and determines whether or not it is necessary to limit the output reactive power of the power converter 6 based on the maximum value vrmax of the phase voltage amplitude values. However, the AC voltage detection unit 21 may detect the effective value of the voltage of each phase of the system voltage of the AC power system 2, and the output limit unit 22 may calculate the maximum value of the effective values ​​of the voltage of each phase detected by the AC voltage detection unit 21, and determine whether or not it is necessary to limit the output reactive power of the power converter 6 based on the maximum effective value of the voltage of each phase.

[0034] As described above, this embodiment is a reactive power compensator that is connected to a multi-phase AC power system and includes a power converter having a self-extinguishing switching element, and a converter control unit that controls the switching element of the power converter, and compensates for reactive power of the AC power system using output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit detects voltage information of the AC power system to which the power converter is connected; The output limiting unit determines whether or not it is necessary to limit the output reactive power of the power converter based on the voltage information detected by the AC voltage detecting unit, and if it is necessary to limit the output reactive power, it limits the output reactive power of the power converter. Therefore, when the reactive power compensator of this embodiment is connected to an AC power system with a small short-circuit capacity, for example, and an unbalanced fault occurs in the power system, causing reactive power to be output, which increases the voltage amplitude of the healthy phase of the system voltage, the reactive power compensator limits the reactive power to be output, thereby ensuring a control margin for the reactive power compensator and making it possible to prevent the reactive power compensator from being shut down for protection purposes due to overvoltage or overcurrent.

[0035] Furthermore, when it is necessary to limit the output reactive power, the output limit unit limits the output reactive power of the power converter so that the output voltage of the power converter is within an output possible range of the power converter. Therefore, it is possible to ensure a control margin for the reactive power compensator.

[0036] Furthermore, the output limiter limits the output reactive current of the power converter when it is necessary to limit the output reactive power of the power converter, which makes it easier to control the reactive power compensator.

[0037] Further, the AC voltage detection unit includes a positive-sequence voltage detection unit that detects a positive-sequence voltage of a system voltage of the AC power system, and calculates a reactive current reference value based on the positive-sequence voltage detected by the positive-sequence voltage detection unit and a voltage command value of the power converter; The output limiting unit calculates a reactive current command value by subtracting a reactive current subtraction amount from the reactive current reference value, and limits the output reactive current of the power converter, thereby facilitating control of the reactive power compensation device.

[0038] Furthermore, the AC voltage detection unit detects the magnitude of the voltage of each phase of the system voltage of the AC power system, for example, the voltage amplitude value and the voltage effective value of each phase, and the output limit unit calculates the maximum value of the magnitude of the voltage of each phase detected by the AC voltage detection unit, and determines whether or not it is necessary to limit the output reactive power of the power converter based on the maximum value of the magnitude of the voltage of each phase. Therefore, it is possible to easily determine whether or not it is necessary to limit the output reactive power of the power converter, and to ensure a control margin for the reactive power compensator.

[0039] The output limiting unit calculates the maximum value of the voltage magnitudes of the phases detected by the AC voltage detecting unit, calculates the deviation between the maximum value and a predetermined maximum allowable voltage value, and calculates a reactive current subtraction amount to reduce the deviation, thereby limiting the output reactive current of the power converter. This makes it easier to control the reactive power compensator, and ensures a control margin for the reactive power compensator.

[0040] Furthermore, the power converter includes three arms connected to the AC power grid; Each arm has a configuration in which one or more cascaded unit cells and a reactor are connected in series, The unit cell includes a series body of a plurality of the switching elements connected in series with each other, and a DC capacitor connected in parallel to the series body. The three arms are also delta connected. Furthermore, for example, the unit cell has a full-bridge configuration in which a plurality of the series bodies of the switching elements are connected in parallel. Therefore, it is possible to configure a power converter that is interconnected to an AC power system and has a large capacity and a high withstand voltage.

[0041] Embodiment 2 The reactive power compensator of the second embodiment is obtained by modifying the subtraction value calculation unit in the output limit unit of the reactive power compensator of the first embodiment.

[0042] The configuration and operation of the reactive power compensator of the second embodiment will be described below, focusing on the differences from the first embodiment, and descriptions of the same parts as in the first embodiment will be omitted as appropriate.

[0043] FIG. 13 is a block diagram showing the internal configuration of the output limiter 22B according to the second embodiment. The output limiter 22B of the second embodiment includes a maximum value calculator 16B, a subtraction value calculator 17B, a subtractor 400B, and a subtractor 410B.

[0044] As in the first embodiment, maximum value calculation unit 16B calculates the maximum value vrmax of the voltage amplitude values ​​of each phase based on the voltage amplitude values ​​vur, vvr, and vwr of each phase calculated by each phase voltage amplitude detection unit 12 of AC voltage detection unit 21. Then, subtractor 410B subtracts the maximum value vrmax of the voltage amplitude of each phase calculated by maximum value calculation unit 16B from a predetermined maximum allowable voltage value Vth to calculate deviation Δvr.

[0045] In the second embodiment, the compensator 18 of the subtraction value calculation unit 17B inputs a deviation Δvr obtained by subtracting the maximum value vrmax of each phase voltage amplitude from the maximum allowable voltage value Vth, and calculates a reactive power subtraction amount ΔQ for reducing the deviation Δvr. Note that, when the reactive power subtraction amount ΔQ is a reactive power amount that results in capacitive operation (capacitive reactive power), the reactive power subtraction amount ΔQ is set to a subtraction amount that makes the output reactive power zero. Here, reactive power has two polarities: capacitive reactive power (capacitive operation) and inductive reactive power (inductive operation). When the AC system voltage is lower than the reference value, the reactive power compensator 1 works to increase the voltage and outputs capacitive reactive power. On the other hand, when the AC system voltage is higher than the reference value, the reactive power compensator 1 works to decrease the voltage and outputs inductive reactive power. Therefore, when an unbalanced fault occurs in the power system and the voltage of the healthy phase of the system voltage rises, the voltage rises too much in the case of capacitive reactive power, so that the reactive power subtraction amount ΔQ is set to a subtraction amount that makes the output reactive power zero so that reactive power is not output.

[0046] Next, a divider 420 divides the reactive power subtraction amount ΔQ calculated by the compensator 18 by the positive-sequence voltage vs calculated by the AC voltage detection unit 21 to calculate a reactive current subtraction amount Δid2.

[0047] Finally, the subtractor 400B subtracts the reactive current subtraction amount Δid2 from the reactive current reference value idref to calculate the reactive current command value id*. Other configurations and operations are the same as those in the first embodiment.

[0048] As described above, according to this embodiment, similar to the first embodiment, when an unbalance fault occurs in the power system and reactive power is output, causing an increase in the voltage amplitude of a healthy phase of the system voltage, for example, in a case where the power system is connected to an AC power system with a small short-circuit capacity, the reactive power to be output can be limited, thereby ensuring a control margin for the reactive power compensator and preventing the protective shutdown of the reactive power compensator due to an overvoltage or overcurrent.

[0049] The output limiting unit calculates the maximum value of the voltage magnitudes of the respective phases detected by the AC voltage detecting unit, calculates the deviation between the maximum value and a predetermined maximum allowable voltage value, and calculates the amount of reactive power subtraction to reduce the deviation, thereby limiting the output reactive current of the power converter. This makes it easier to control the reactive power compensator, and ensures a control margin for the reactive power compensator.

[0050] Embodiment 3 Next, the reactive power compensator 1 according to the third embodiment will be described. The reactive power compensator 1 in the third embodiment has the same configuration and operation as those in the first embodiment, except for the AC voltage detection unit 21C and the output limit unit 22C. Note that the description of the same parts as those in the first and second embodiments will be omitted as appropriate. Here, a description will be given of AC voltage detection unit 21C and output limit unit 22C in embodiment 3. Fig. 14 is a block diagram showing the internal configuration of AC voltage detection unit 21C according to embodiment 3, and Fig. 15 is a block diagram showing the internal configuration of output limit unit 22C according to embodiment 3.

[0051] First, an AC voltage detection unit 21C according to the third embodiment will be described with reference to FIG. The AC voltage detection unit 21C of the third embodiment includes a positive-phase voltage detection unit 11C, a negative-phase voltage detection unit 31, a voltage control unit 13, an unbalance factor detection unit 32, and a subtractor 300C. The positive-sequence voltage detection unit 11C calculates the positive-sequence voltage vs based on the instantaneous voltages vu, vv, and vw detected by the voltage detector 20, as in the first embodiment. The negative-phase-sequence voltage detection unit 31 calculates the negative-phase-sequence voltage vneg based on the instantaneous voltages vu, vv, and vw detected by the voltage detector 20 using a method such as rotational coordinate transformation. The positive-sequence voltage vs calculated by the positive-sequence voltage detection unit 11C and the negative-sequence voltage vneg calculated by the negative-sequence voltage detection unit 31 are input to the unbalance factor detection unit 32. The unbalance rate detector 32 calculates the unbalance rate Rpn (=vneg / vs), which is the ratio of the negative-sequence voltage vneg to the positive-sequence voltage vs. The operation of AC voltage detection unit 21C other than that described above is the same as that of embodiment 1. That is, AC voltage detection unit 21C receives as input a voltage command value vref (command value for effective voltage) of power converter 6, and calculates deviation Δv by subtracting the positive-sequence voltage vs calculated by positive-sequence voltage detection unit 11 from the voltage command value vref using subtractor 300C. The voltage control unit 13 is configured by, for example, a PI (Proportional Integral) controller, and calculates a reactive current reference value idref for reducing the input deviation Δv.

[0052] Next, an output limiter 22C according to the third embodiment will be described with reference to FIG. The output limiter 22C of the third embodiment includes a subtraction value calculator 17C, a subtractor 400C, and a subtractor 410C.

[0053] First, the unbalance factor Rpn calculated by the unbalance factor detection unit 32 of the AC voltage detection unit 21C is input to the output limit unit 22C. A subtractor 410C subtracts the unbalance factor Rpn input to the output limit unit 22C from a threshold value Rpnth that is preset based on the impedance of the AC power system 2 to calculate a deviation ΔRpn. The threshold value Rpnth will now be explained. The greater the ratio of negative-sequence voltage to positive-sequence voltage, the greater the unbalance. For example, in the case of a near-end single-phase-to-ground fault event of an infinite bus (when the AC power system is strong), the ratio of positive-sequence voltage to negative-sequence voltage is 2 / 3 and 1 / 3, respectively, resulting in an unbalance factor Rpn of 1 / 2. Therefore, for example, this unbalance factor (1 / 2) can be used as the threshold value Rpnth.

[0054] Next, the subtraction value calculation unit 17C calculates a reactive current subtraction amount Δid3 for reducing the deviation ΔRpn. Note that if the deviation ΔRpn is a positive value, the reactive current subtraction amount Δid3 is set to zero. Then, a subtractor 400C subtracts the reactive current subtraction amount Δid3 from the reactive current reference value idref to calculate the reactive current command value id*. Note that the reactive current subtraction amount Δid3 may be calculated by calculating a reactive power subtraction amount ΔQ3 for reducing the deviation ΔRpn and dividing it by the positive-sequence voltage vs, as shown in the second embodiment.

[0055] As described above, according to this embodiment, similarly to the first and second embodiments, when an unbalance fault occurs in the power system and reactive power is output, causing an increase in the voltage amplitude of a healthy phase of the system voltage, for example, in a case where the power system is connected to an AC power system with a small short-circuit capacity, the reactive power to be output can be limited, thereby ensuring a control margin for the reactive power compensator and preventing the protective shutdown of the reactive power compensator due to an overvoltage or overcurrent.

[0056] The AC voltage detection unit includes a positive-sequence voltage detection unit that detects a positive-sequence voltage of a system voltage of the AC power system, a negative-sequence voltage detection unit that detects a negative-sequence voltage of the system voltage of the AC power system, and an unbalance rate detection unit that calculates an unbalance rate that is a ratio of the negative-sequence voltage to the positive-sequence voltage, The output limiter determines whether or not it is necessary to limit the output reactive power of the power converter based on the unbalance factor calculated by the unbalance factor detector, which facilitates control of the reactive power compensator.

[0057] Furthermore, the output limiting unit calculates the deviation between the unbalance ratio calculated by the unbalance ratio detection unit and a predetermined threshold value, and calculates the amount of reactive current subtraction or the amount of reactive power subtraction to reduce the deviation, thereby limiting the output reactive current of the power converter, which makes it easier to control the reactive power compensation device.

[0058] Embodiment 4 Next, the reactive power compensator 1 according to the fourth embodiment will be described. The reactive power compensator 1 in the fourth embodiment has the same configuration as that in the first embodiment, except for a subtraction value calculation unit 17D provided in an output limit unit 22D. Note that the description of the same parts as those in the first to third embodiments will be omitted as appropriate.

[0059] FIG. 16 is a block diagram showing the configuration of an output limiter 22D according to the fourth embodiment. The output limiter 22D includes a maximum value calculator 16D, a subtraction value calculator 17D, a subtractor 400D, and a subtractor 410D.

[0060] 16 performs the same function as in Embodiment 1. That is, maximum value calculation unit 16D calculates the maximum value vrmax of the voltage amplitude values ​​of each phase based on the voltage amplitude values ​​vur, vvr, and vwr of each phase calculated by each phase voltage amplitude detection unit 12 of AC voltage detection unit 21. Then, a subtractor 410D subtracts the maximum value vrmax of the voltage amplitude of each phase calculated by maximum value calculation unit 16D from a predetermined maximum allowable voltage value Vth to calculate a deviation Δvr.

[0061] Then, the subtraction value calculation unit 17D calculates a reactive current subtraction amount Δid4 for reducing the deviation Δvr. If the deviation Δvr is a positive value, the reactive current subtraction amount Δid4 is set to zero, and if the deviation Δvr is a negative value, a reactive current subtraction amount Δid4 determined in advance based on the impedance of the AC power grid 2 is output. Here, the reactive current subtraction amount Δid4 can be set as follows. That is, a voltage increase amount relative to the reactive current output amount can be calculated according to the impedance of the AC power grid 2. The reactive current subtraction amount Δid4 is determined so that the voltage increase amount satisfies the outputtable voltage range of the reactive power compensator 1.

[0062] The reactive current subtraction amount Δid4 may be calculated by calculating the reactive power subtraction amount ΔQ3 for reducing the deviation Δvr and dividing it by the positive sequence voltage vs, as shown in the second embodiment. Furthermore, as in the third embodiment, a deviation ΔRpn may be calculated by subtracting a preset threshold value Rpnth and the unbalance factor Rpn input to the output limit unit 22C from the impedance of the AC power system 2, and the reactive current subtraction amount Δid4 may be calculated from the deviation ΔRpn.

[0063] Then, the subtractor 400D calculates the reactive current command value id* by subtracting the reactive current subtraction amount Δid4 from the reactive current reference value idref.

[0064] As described above, according to this embodiment, similar to the previous embodiment, when an unbalance fault occurs in the power system and reactive power is output, causing an increase in the voltage amplitude of a healthy phase of the system voltage, for example, in a case where the power system is connected to an AC power system with a small short-circuit capacity, the reactive power to be output can be limited, thereby ensuring a control margin for the reactive power compensator and preventing the protective shutdown of the reactive power compensator due to an overvoltage or overcurrent.

[0065] Furthermore, the output limiting unit calculates a reactive current command value by subtracting a reactive current subtraction amount predetermined based on the impedance of the AC power system from the reactive current reference value, thereby limiting the output reactive current of the power converter, which makes it easier to control the reactive power compensation device.

[0066] In the above embodiment, the converter control unit 7 is configured with a processor 200 and a storage device 210, as shown in an example of the hardware configuration in Fig. 17. Although not shown, the storage device 210 includes a volatile storage device such as a random access memory and a non-volatile auxiliary storage device such as a flash memory. Furthermore, an auxiliary storage device such as a hard disk may be provided instead of flash memory. Processor 200 executes a program input from storage device 210. In this case, the program is input from the auxiliary storage device to processor 200 via a volatile storage device. Processor 200 may output data such as calculation results to the volatile storage device of storage device 210, or may store data in the auxiliary storage device via the volatile storage device.

[0067] Although the present application describes various exemplary embodiments and examples, the various features, aspects, and functions described in one or more embodiments are not limited to application to a particular embodiment, but may be applied to the embodiments alone or in various combinations. Therefore, countless variations not illustrated are conceivable within the scope of the technology disclosed in this application, including, for example, cases where at least one component is modified, added, or omitted, and cases where at least one component is extracted and combined with a component of another embodiment. [Explanation of symbols]

[0068] 1 reactive power compensator, 2 AC power system, 3 interconnection transformer, 4 arm, 5 arm reactor, 6 power converter, 7 converter control unit, 10 unit cell, 11 positive sequence voltage detection unit, 12 each phase voltage amplitude detection unit, 13 voltage control unit, 15 DC capacitor, 16 maximum value calculation unit, 17 subtraction value calculation unit, 20 voltage detector, 21 AC voltage detection unit, 22 output limit unit, 23 gate signal calculation unit, 30 current detector, 40 arm current detector, 50 DC capacitor voltage detector, 60 DC voltage control unit, 70 circulating current control unit, 90 output current control unit, 101 series body, 102 semiconductor switch, 103 switching element, 104 diode, 120 gate signal generation unit, 130 voltage command value calculation unit, 140 individual cell DC voltage control unit, 200 processor, 210 storage device.

Claims

1. A reactive power compensation device comprising: a power converter connected to a multi-phase AC power system and having a self-extinguishing switching element; and a converter control unit that controls the switching element of the power converter, the reactive power compensation device compensating for reactive power of the AC power system by output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit detects voltage information of the AC power system to which the power converter is connected; the output limiting unit determines whether or not it is necessary to limit the output reactive power of the power converter based on the voltage information detected by the AC voltage detecting unit, and limits the output reactive current of the power converter if it is necessary to limit the output reactive power; the AC voltage detection unit includes a positive-sequence voltage detection unit that detects a positive-sequence voltage of a system voltage of the AC power system, and calculates a reactive current reference value based on the positive-sequence voltage detected by the positive-sequence voltage detection unit and a voltage command value of the power converter; The output limiting unit calculates a reactive current command value by subtracting a reactive current subtraction amount from the reactive current reference value, and limits the output reactive current of the power converter.

2. A reactive power compensation device comprising: a power converter connected to a multi-phase AC power system and having a self-extinguishing switching element; and a converter control unit that controls the switching element of the power converter, the reactive power compensation device compensating for reactive power of the AC power system by output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit detects the magnitude of a voltage of each phase of a system voltage of the AC power system; The output limiting unit calculates a maximum value among the magnitudes of the voltages of each phase detected by the AC voltage detecting unit, and determines whether or not it is necessary to limit the output reactive power of the power converter based on the maximum value of the magnitude of the voltages of each phase.

3. A reactive power compensation device comprising: a power converter connected to a multi-phase AC power system and having a self-extinguishing switching element; and a converter control unit that controls the switching element of the power converter, the reactive power compensation device compensating for reactive power of the AC power system by output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit includes a positive-sequence voltage detection unit that detects a positive-sequence voltage of a system voltage of the AC power system, a negative-sequence voltage detection unit that detects a negative-sequence voltage of the system voltage of the AC power system, and an unbalance rate detection unit that calculates an unbalance rate that is a ratio of the negative-sequence voltage to the positive-sequence voltage, The output limiter determines whether or not it is necessary to limit the output reactive power of the power converter based on the unbalance factor calculated by the unbalance factor detector.

4. A reactive power compensation device comprising: a power converter connected to a multi-phase AC power system and having a self-extinguishing switching element; and a converter control unit that controls the switching element of the power converter, the reactive power compensation device compensating for reactive power of the AC power system by output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit detects the magnitude of a voltage of each phase of a system voltage of the AC power system; The output limit unit limits the output power of the power converter when it is necessary to limit the output reactive power so that the output voltage of the power converter does not exceed a maximum allowable voltage value, which is the maximum voltage that the power converter can output.

5. A reactive power compensation device comprising: a power converter connected to a multi-phase AC power system and having a self-extinguishing switching element; and a converter control unit that controls the switching element of the power converter, the reactive power compensation device compensating for reactive power of the AC power system by output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit calculates a voltage amplitude value of each phase of the system voltage by a phase voltage amplitude detection unit based on the magnitude of the voltage of each phase of the system voltage of the AC power system; the output limiting unit calculates a maximum value of the voltage amplitude of each phase based on the voltage amplitude value of each phase calculated by the each-phase voltage amplitude detection unit of the AC voltage detection unit from a reactive current reference value, calculates a deviation by subtracting the maximum value of the voltage amplitude of each phase from a predetermined maximum allowable voltage value, and if the deviation is a negative value, calculates a voltage rise with respect to the reactive current output in accordance with the impedance of the AC power system, and calculates a reactive current command value by subtracting a reactive current subtraction amount determined so that the voltage rise satisfies an output voltage range of the reactive power compensation device, thereby limiting the output reactive current of the power converter.

6. A reactive power compensation device comprising: a power converter connected to a multi-phase AC power system and having a self-extinguishing switching element; and a converter control unit that controls the switching element of the power converter, the reactive power compensation device compensating for reactive power of the AC power system by output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit detects the magnitude of a voltage of each phase of a system voltage of the AC power system; The output limiting unit calculates a maximum value among the magnitudes of the voltages of the phases detected by the AC voltage detecting unit, calculates a deviation between the maximum value and a predetermined maximum allowable voltage value, and calculates a reactive current subtraction amount to reduce the deviation, thereby limiting the output reactive current of the power converter.

7. A reactive power compensation device comprising: a power converter connected to a multi-phase AC power system and having a self-extinguishing switching element; and a converter control unit that controls the switching element of the power converter, the reactive power compensation device compensating for reactive power of the AC power system by output reactive power of the power converter, the converter control unit includes an AC voltage detection unit and an output limit unit, the AC voltage detection unit detects the magnitude of a voltage of each phase of a system voltage of the AC power system; The output limiting unit calculates a maximum value among the magnitudes of the voltages of the phases detected by the AC voltage detecting unit, calculates a deviation between the maximum value and a predetermined maximum allowable voltage value, and calculates a reactive power subtraction amount to reduce the deviation, thereby limiting the output reactive current of the power converter.

8. 4. The reactive power compensation device according to claim 3, wherein the output limiting unit calculates a deviation between the unbalance factor calculated by the unbalance factor detection unit and a predetermined threshold value, and calculates a reactive current subtraction amount or a reactive power subtraction amount for reducing the deviation, thereby limiting the output reactive current of the power converter.

9. the power converter includes three arms connected to the AC power grid; Each arm has one or more cascaded unit cells and a reactor connected in series, The unit cell includes a series body of a plurality of the switching elements connected in series with each other and a DC capacitor connected in parallel with the series body. The reactive power compensator according to any one of claims 1 to 8.

10. 10. The reactive power compensator according to claim 9, wherein the three arms are delta connected.

11. 11. The reactive power compensator according to claim 9, wherein the unit cell has a full-bridge configuration in which a plurality of the series bodies of the switching elements are connected in parallel.

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