Power conversion device and control method for power conversion device

Abstract

A power conversion device (1) comprises: a power converter (4) that is connected to a three-phase AC power system (2) and has a self-extinguishing type semiconductor switching element (Q); and a converter control unit (7) that controls the power converter (4). The converter control unit (7) includes a reverse-phase reduction control unit (8). When an unbalanced fault in which the amplitude of the voltage of at least one phase decreases occurs in the AC power system (2), the reverse-phase reduction control unit (8) generates a reverse-phase current command value for adjusting the phase of the reverse-phase voltage of the AC power system (2) to reduce the maximum value of the amplitude of the voltage of a sound phase in which the unbalanced fault has not occurred. The converter control unit (7) controls the on / off of the semiconductor switching element (Q) on the basis of the reverse-phase current command value.

Description

Power conversion device and control method for power conversion device

[0001] The present disclosure relates to a power conversion device and a control method for a power conversion device.

[0002] In recent years, the introduction of renewable energy sources such as solar and wind power has expanded, and the proportion of thermal power generation, which has the ability to adjust power supply, in the power system has decreased. As a result, stabilizing the power system during large-capacity power transmission has become important. For this reason, static synchronous compensators (STATCOMs) have been introduced to stabilize the power system. STATCOMs contribute to stabilizing the system voltage by compensating for reactive power. STATCOMs are required to stabilize the power system not only during steady-state operation but also in transient situations such as when a system fault occurs.

[0003] STATCOMs may be connected to power systems with small short-circuit capacity and large system voltage fluctuations. In such power systems, when an unbalanced fault occurs, such as a single-phase ground fault, in which the voltage amplitude of only one phase drops, the STATCOM outputs reactive power to increase the amplitude of the positive-sequence voltage of the power system. Meanwhile, the reactive power output by the STATCOM further increases the voltage amplitude of phases (healthy phases) where the fault has not occurred and the voltage amplitude has not dropped. As a result, the voltage amplitude of the healthy phases may exceed the voltage amplitude within the operating range of the grid-connected equipment, which may shut down the equipment to protect it from overvoltage and overcurrent.

[0004] When an unbalanced fault occurs in a power system, a STATCOM is required to output reactive power to increase the positive-sequence voltage of the power system while suppressing an excessive rise in healthy phase voltage. To address this issue, the converter control unit that controls the power converter described in Patent Document 1 includes an AC voltage detection unit and an output limit unit. The AC voltage detection unit detects voltage information of the power system to which the power converter is connected, and the output limit unit determines whether or not to limit the output reactive power of the power converter based on the voltage information detected by the AC voltage detection unit, and limits the output reactive power of the power converter if necessary. This solves the excessive rise in healthy phase voltage.

[0005] International Publication No. 2022 / 085101

[0006] However, in conventional methods, the output limiter limits the output of reactive power, which results in a decrease in the positive-sequence voltage of the power system. In other words, when an unbalance fault occurs in the power system, there is a trade-off between suppressing the healthy phase voltage and increasing the positive-sequence voltage. The present disclosure discloses a technology for alleviating the above-mentioned trade-off issue, and aims to suppress the amplitude of the healthy phase voltage while maintaining a high amplitude of the positive-sequence voltage during an unbalance fault in the power system.

[0007] A power conversion device according to the present disclosure is provided which is connected to a three-phase AC power system and includes a power converter having self-extinguishing semiconductor switching elements, and a converter control unit which controls the power converter, wherein the converter control unit includes a negative-phase sequence reduction control unit which, when an unbalanced fault occurs in the AC power system in which the amplitude of the voltage of at least one phase decreases, generates a negative-phase sequence current command value which adjusts the phase of the negative-phase sequence voltage of the AC power system so as to reduce the maximum value of the amplitude of the voltage of a healthy phase in which no unbalanced fault has occurred, and the converter control unit controls the on / off of the semiconductor switching elements based on the negative-phase sequence current command value.

[0008] According to the power conversion device of the present disclosure, when an unbalanced fault occurs in a three-phase power system and the healthy phase voltage excessively increases, the negative-phase sequence reduction control unit generates a negative-phase sequence current command value that adjusts the phase of the negative-phase sequence voltage of the power system so as to reduce the maximum amplitude of the healthy phase voltage, and the converter control unit controls the negative-phase sequence reactive power based on the negative-phase sequence current command value generated by the negative-phase sequence reduction control unit. This eliminates the need to reduce the voltage more than necessary, making it possible to suppress an excessive increase in the healthy phase voltage while maintaining a high positive-phase sequence voltage of the power system.

[0009] 1 is a schematic configuration diagram of a power conversion device; FIG. 2 is a circuit diagram showing an example configuration of a power converter; FIG. 3 is a circuit diagram showing another example configuration of a power converter; FIG. 4 is a diagram showing an example configuration of an arm; FIG. 5 is a circuit diagram showing the internal configuration of a unit cell configured as a full bridge; FIG. 6 is a circuit diagram showing the internal configuration of a unit cell configured as a half bridge; FIG. 7 is an overall block diagram of a converter control unit; FIG. 8 is a block diagram of a DC voltage control unit of a gate signal calculation unit; FIG. 9 is a block diagram of a circulating current control unit; FIG. 10 is a diagram showing a negative-sequence voltage when the amplitudes of two healthy phase voltages become equal when a V-phase ground fault occurs; FIG. 11 is a diagram showing a negative-sequence voltage when the amplitudes of two healthy phase voltages become equal when a W-phase ground fault occurs; FIG. 12 is a diagram showing a negative-sequence voltage when the amplitudes of two healthy phase voltages become equal when a U-phase ground fault occurs; FIG. 13 is a diagram showing an example of a negative-sequence voltage when a V-phase ground fault occurs in an AC power system; FIG. 14 is a diagram showing voltage waveforms of each phase when a V-phase ground fault occurs in an AC power system; FIG. 15 is a diagram showing another example of a negative-sequence voltage when a V-phase ground fault occurs in an AC power system; FIG. 1 is a diagram showing an example of a negative-sequence voltage when a UV-phase ground fault occurs in an AC power system. FIG. 2 is a diagram showing another example of a negative-sequence voltage when a UV-phase ground fault occurs in an AC power system. FIG. 3 is a block diagram of a negative-sequence reduction control unit. FIG. 4 is a block diagram of a case where the negative-sequence reduction control unit receives an unbalance fault detection signal from the outside. FIG. 5 is a block diagram of an output current control unit. FIG. 6 is a block diagram showing the internal configuration of a voltage command value calculation unit. FIG. 7 is a block diagram showing the internal configuration of a cell individual DC voltage control unit. FIG. 8 is a flowchart of negative-sequence voltage control when an unbalance fault occurs.

[0010] Hereinafter, a power conversion device according to an embodiment of the present disclosure will be described with reference to the drawings.

[0011] (Overall Configuration of Power Conversion Apparatus) FIG. 1 is a schematic configuration diagram of a power conversion apparatus 1 according to this embodiment. As shown in FIG. 1, the power conversion apparatus 1 includes a power converter 4 and a converter control unit 7 that controls the power converter 4. In the example of FIG. 1, some components of the converter control unit 7 are omitted. To simplify the diagram, multiple measurement lines of the same type are represented by a single line, and the number of lines is indicated by a numerical value. Note that the number N in 3N, which indicates the number of lines, is the number of unit cells 10, which will be described later. The same notation may be used in subsequent drawings. To simplify the diagram, the voltage Vdccell and the valve branch currents Iuv, Ivw, and Iwu of the DC capacitor 11 are depicted as if they were input from outside the power conversion apparatus 1. However, the voltage Vdccell and the valve branch currents Iuv, Ivw, and Iwu of the DC capacitor 11 are values ​​detected by the power converter 4, as will be described later.

[0012] The power converter 4 has a self-extinguishing switching element and is connected to the three-phase AC power system 2 via an interconnection transformer 3. Although the power converter 4 is connected to the three-phase AC power system 2 via the interconnection transformer 3 in Fig. 1, it may be configured to be connected to the three-phase AC power system 2 via an interconnection reactor instead of the interconnection transformer 3. In the following description, the three-phase AC power system will be simply referred to as an AC power system.

[0013] FIG. 2 is a circuit diagram showing an example of the configuration of the power converter 4 according to this embodiment. Only one power line on the AC power system side of the interconnection transformer 3 is shown. As shown in FIG. 2 , the power converter 4 includes a plurality of arms 41 (first arm, second arm, third arm) corresponding to each phase (U phase, V phase, W phase) of the AC power system 2. The first to third arms 41 are provided with arm current detectors 42 for detecting arm currents Iuv, Ivw, and Iwu flowing through the first to third arms 41. In the example shown in FIG. 2 , the first arm 41 is connected between the U phase and the V phase of the AC power system 2, the second arm 41 is connected between the V phase and the W phase of the AC power system 2, and the third arm 41 is connected between the W phase and the U phase of the AC power system 2, forming a so-called delta connection. The connection ends of the first to third arms 41 are connected to the AC power system 2 via the interconnection transformer 3. The connection of the first to third arms 41 is not limited to the delta connection, and as shown in FIG. 3 , the first arm 41 may be connected between the U-phase and the neutral point of the AC power system 2, the second arm 41 may be connected between the V-phase and the neutral point of the AC power system 2, and the third arm 41 may be connected between the W-phase and the neutral point of the AC power system 2, which is a so-called star connection.

[0014] FIG. 4 is a diagram illustrating an example of the configuration of the arm 41. The configuration of the arms 41 of each phase is the same. As illustrated in FIG. 4, the arm 41 of each phase includes a plurality of unit cells 10 cascaded to each other and a reactor 43. The reactor 43 may be inserted at any position in the arm 41. Furthermore, a plurality of reactors 43 may be included in each arm 41 of each phase. The number of unit cells 10 connected to one arm 41 is N, where N is a natural number, and the total number of unit cells 10 included in all arms 41 is 3N. The number of unit cells 10 included in each arm of each phase does not need to be N. It is sufficient that the number is equal to or greater than the minimum number required for operation of the power conversion device 1, and the total number of all unit cells 10 is not limited to 3N. Furthermore, the number of unit cells 10 included in one arm 41 may be one. Such a power converter 4 is called a modular multilevel converter (MMC), and is capable of achieving large capacity and high voltage resistance, making it suitable for use as a power converter connected to an AC power system. Although the circuit configuration of the power converter 4 is not limited to an MMC, the present embodiment will be described using an MMC as an example.

[0015] (Configuration Example of Unit Cell) Next, a configuration example of the unit cell 10 will be described with reference to FIGS. 5 and 6. FIG.

[0016] FIG. 5 is a circuit diagram showing the internal configuration of a unit cell 10 configured as a full bridge. The unit cell 10 includes four switching elements Q1, Q2, Q3, and Q4; four diodes D1, D2, D3, and D4 connected in anti-parallel to the four switching elements Q1, Q2, Q3, and Q4, respectively; a DC capacitor 11; and a DC capacitor voltage detector 12 that detects the voltage Vdccell of the DC capacitor 11. The switching elements Q1, Q2, Q3, and Q4 may be self-extinguishing switching elements such as insulated-gate bipolar transistors (IGBTs). The unit cell 10 shown in FIG. 4 outputs, as the voltage Vcell between its output terminals, a voltage of the same polarity as the voltage Vdccell of the DC capacitor 11, a voltage of the opposite polarity, or zero voltage, by turning on or off the switching elements Q1, Q2, Q3, and Q4 in response to an input gate signal Sgate.

[0017] FIG. 6 is a circuit diagram showing the internal configuration of a unit cell 10 configured as a half-bridge. The configuration example in FIG. 6 is a configuration in which the switching elements Q3 and Q4 and the diodes D3 and D4 are removed from the configuration example of the unit cell 10 shown in FIG. 5. The unit cell 10 shown in FIG. 6 outputs a voltage Vcell between its output terminals, which is either a voltage of the same polarity as the voltage of the DC capacitor 11 or zero voltage, by turning on or off the switching elements Q1 and Q2 in response to the input gate signal Sgate. When referring to any of the multiple switching elements Q1 to Q4 or Q1 to Q2 of the unit cell 10, or when referring collectively, the switching elements will be referred to as switching element Q.

[0018] 5 and 6, any configuration is possible as long as the unit cell 10 has a DC capacitor 11, a switching element Q, and an output terminal, and selectively outputs a DC capacitor voltage Vdccell as the voltage Vcell between the output terminals in response to an input gate signal. In the following description, the "unit cell" will be referred to as a "cell" where appropriate.

[0019] 1 , the power conversion device 1 further includes an AC voltage detector 20 for detecting the AC voltage of each phase at an interconnection point between the power conversion device 1 and the AC power grid 2, and an AC current detector 30 for detecting the AC current flowing in the power line of each phase. The signals detected by these detectors are input to the converter control unit 7.

[0020] The AC voltage detector 20 detects system AC voltages Vu, Vv, and Vw (hereinafter also collectively referred to as "system voltages V") at the interconnection point between the power conversion device 1 and the AC power system 2. The AC current detector 30 detects system AC currents Iu, Iv, and Iw at the interconnection point between the power conversion device 1 and the AC power system 2.

[0021] (Control Device) Next, the configuration and operation of the converter control unit 7 will be described with reference to Fig. 1 and Fig. 7. Fig. 7 is an overall block diagram of the converter control unit 7.

[0022] As shown in Fig. 1 , the converter control unit 7 includes a negative-phase-sequence reduction control unit 8 and a gate signal calculation unit 9. The configuration and operation of the negative-phase-sequence reduction control unit 8 will be described later. As shown in Fig. 7 , the gate signal calculation unit 9 includes a DC voltage control unit 70, a circulating current control unit 90, an output current control unit 100, a voltage command value calculation unit 110, and a gate signal generation unit 130.

[0023] 8 is a block diagram of the DC voltage control unit 70 of the gate signal calculation unit 9 according to this embodiment. The DC voltage control unit 70 includes a total DC voltage representative value calculation unit 71, a subtractor 72, and a controller 73. In all of the unit cells 10 (3N in this case), 3N DC capacitor voltages Vdccell detected by the respective DC capacitor voltage detectors 12 and an externally input DC voltage command value Vdc* are input to the DC voltage control unit 70. The DC voltage command value Vdc* is the same for all of the unit cells 10.

[0024] The DC capacitor voltages Vdccell detected by the DC capacitor voltage detectors 12 of all unit cells 10 (3N in this case) are input to an all DC voltage representative value calculation unit 71. The all DC voltage representative value calculation unit 71 calculates a DC voltage representative value Vdc that represents all DC capacitor voltages Vdccell. The representative value may be, for example, an average value, a maximum value, or a minimum value.

[0025] The subtractor 72 calculates the deviation between the DC voltage command value Vdc* and the DC voltage representative value Vdc calculated by the total DC voltage representative value calculation unit 71 , and outputs the deviation to the controller 73 .

[0026] The controller 73 calculates the active current command value Iqp* so that the deviation between the input DC voltage command value Vdc* and the DC voltage representative value Vdc becomes zero, i.e., so that the DC voltage representative value Vdc follows the DC voltage command value Vdc*. The controller 73 is, for example, a PI controller.

[0027] 9 is a block diagram of a circulating current control unit 90 according to this embodiment. The circulating current control unit 90 includes an adder 91, a subtractor 92, a multiplier 93, and a controller 94. The valve branch currents Iuv, Ivw, Iwu detected by valve branch current detectors 42 provided in the first to third valve branches 41 and a circulating current command value Iz* input from the outside are input to the circulating current control unit 90.

[0028] The adder 91 calculates the sum of the input valve branch currents Iuv, Ivw, and Iwu and outputs the sum to the multiplier 93. The multiplier 93 calculates ⅓ (Iuv + Ivw + Iwu), which is the product of ⅓ of the sum (Iuv + Ivw + Iwu) of the valve branch currents Iuv, Ivw, and Iwu calculated by the adder 91. The product of ⅓ of the sum of the valve branch currents Iuv, Ivw, and Iwu (⅓ (Iuv + Ivw + Iwu)) is the circulating current Iz. The calculated circulating current Iz is output to the subtractor 92. The subtractor 92 calculates the deviation between the input circulating current command value Iz* and the circulating current Iz and outputs the deviation to the controller 94. The circulating current command value Iz* may be a fixed value or may be a value whose amplitude or phase varies in order to balance the DC capacitor voltages between the phases.

[0029] The controller 94 calculates the zero-phase-sequence voltage command value Vz* so that the deviation between the circulating current command value Iz* and the circulating current Iz becomes zero, i.e., so that the circulating current Iz follows the circulating current command value Iz*. The controller 94 is, for example, a PI controller.

[0030] Next, the negative-phase sequence reduction control unit 8 will be described. When an unbalance fault occurs in the AC power system 2 and the amplitude of the healthy phase voltage excessively increases, the negative-phase sequence reduction control unit 8 generates a negative-phase sequence reactive current command value Idn* and a negative-phase sequence active current command value Iqn* so as to reduce the amplitude of the healthy phase voltage. Here, a method for reducing the amplitude of the healthy phase voltage will be described. Since the method for generating the negative-phase sequence current command value differs depending on the type of unbalance fault, a description will be given for each type of unbalance fault. Here, the negative-phase sequence current command value is a general term that collectively refers to the negative-phase sequence reactive current command value Idn* and the negative-phase sequence active current command value Iqn*.

[0031] <One-Phase Ground Fault> First, we will explain the case of an unbalanced fault in which the voltage amplitude of only one phase decreases, such as a one-phase ground fault (1LG), by taking a V-phase ground fault (V-phase 1LG) as an example. When a one-phase ground fault occurs, the remaining two phases without a ground fault remain healthy. FIG. 10 is a diagram showing a vector diagram of the negative-sequence voltage when the amplitudes of the two healthy phase voltages become equal in the event of a V-phase ground fault. The negative-sequence voltage is shown on a negative-sequence dq coordinate. When a one-phase ground fault occurs in the V-phase, if the phase of the negative-sequence voltage of the AC power system 2 matches the phase of the negative-sequence voltage shown in FIG. 10, the voltage amplitudes of the healthy phases, U-phase and W-phase, will match. This phase is referred to as a healthy phase balanced phase. If the phase of the negative-sequence voltage of the AC power system 2 differs from the phase of the negative-sequence voltage during a V-phase ground fault, the voltage amplitudes of the U-phase and W-phase will not match, resulting in a state where one of the voltage amplitudes is higher. Fig. 10 also shows which healthy phase has a higher voltage amplitude. Fig. 11 shows the negative-sequence voltage when the amplitudes of the two healthy phase voltages become equal when a W-phase ground fault occurs, and Fig. 12 shows the negative-sequence voltage when the amplitudes of the two healthy phase voltages become equal when a U-phase ground fault occurs.

[0032] FIG. 13 illustrates an example of a negative-sequence voltage when a V-phase ground fault occurs in the AC power system 2. In the example of FIG. 13, the negative-sequence voltage is in a region where the W-phase voltage amplitude is higher than the U-phase voltage amplitude. FIG. 14(a) illustrates voltage waveforms when the negative-sequence voltage is in a region where the W-phase voltage amplitude is higher. As shown in FIG. 14(a), the V-phase voltage is constant at zero, the U-phase voltage waveform and the W-phase voltage waveform have the same frequency, and the W-phase voltage amplitude is higher than the U-phase voltage amplitude. As shown in FIG. 13, it is preferable to control the negative-sequence voltage by reducing Vdn while keeping Vqn constant so that the phase of the negative-sequence voltage coincides with the healthy phase balanced phase during a V-phase ground fault. As a result, as shown in FIG. 14(b), the W-phase voltage amplitude and the U-phase voltage amplitude become equal (the healthy phase voltages are balanced), and the maximum amplitude of the healthy phase voltage decreases. In the case of an unbalanced fault in which the amplitude of one phase voltage decreases, such as a one-phase ground fault, the healthy phase voltage can be balanced to reduce the maximum amplitude of the healthy phase voltage while suppressing a decrease in the positive-phase voltage. When the amplitude of the healthy phase voltage is greater than a predetermined threshold Vth1, the phase of the negative-phase voltage may be controlled to further reduce its absolute value while maintaining its phase. This further reduces the amplitude of the healthy phase voltage. Adjusting the negative-phase voltage in this manner effectively reduces the maximum amplitude of the healthy phase voltage while minimizing its impact on the positive-phase voltage. In the example of FIG. 13 , the phase of the negative-phase voltage is adjusted to reduce the magnitude of the negative-phase voltage. However, it is also possible to adjust only the phase while keeping the magnitude of the negative-phase voltage constant.

[0033] FIG. 15 is a diagram showing another example of the negative-sequence voltage when a V-phase ground fault occurs in the AC power system 2. Only differences from the example in FIG. 13 will be described. In the example in FIG. 15 , the negative-sequence voltage is in a region where the amplitude of the U-phase voltage is higher than that of the W-phase voltage. In this case, it is advisable to control the negative-sequence voltage by reducing Vqn while keeping Vdn constant so that the phase of the negative-sequence voltage coincides with the healthy phase balanced phase during the V-phase ground fault. This makes the amplitudes of the W-phase voltage and the U-phase voltage equal, thereby reducing the maximum amplitude of the healthy phase voltage. Similarly, when a U-phase or W-phase ground fault occurs in the AC power system 2, the phase of the negative-sequence voltage is detected and the phase of the negative-sequence voltage is adjusted to reduce the maximum healthy phase voltage.

[0034] <Two-Phase Ground Fault> Next, a case where an unbalanced fault occurs, in which the two-phase voltage amplitude decreases, such as a two-phase ground fault (2LG), will be described. FIG. 16 is a diagram showing the negative-sequence voltage when a two-phase ground fault occurs in the AC power system 2. The negative-sequence voltage is shown on a negative-sequence dq coordinate. The phase of the negative-sequence voltage shown in FIG. 16 is the phase when the amplitude of the healthy phase voltage without a fault is greatest. This phase is referred to as the healthy phase maximizing phase. In the case of a two-phase ground fault, the amplitude of the healthy phase voltage can be reduced by moving the phase of the negative-sequence voltage away from the healthy phase maximizing phase shown in FIG. 16.

[0035] A method for reducing the amplitude of the healthy phase voltage will be described using an example in which a UV-phase ground fault occurs (UV phase 2LG). Figures 17 and 18 show examples of the negative-phase voltage when a UV-phase ground fault occurs in the AC power system 2. In the example of Figure 17, the amplitude of the healthy phase voltage can be reduced by keeping Vdn constant and reducing the absolute value of Vqn to move it away from the healthy phase maximization phase. When Vqn = 0, the amplitude of the healthy phase voltage is greater than a predetermined threshold, the absolute value of Vdn is further reduced. In this way, the amplitude of the healthy phase voltage can be effectively reduced by further reducing the magnitude of the negative-phase voltage when the phase of the negative-phase voltage is farthest from the healthy phase maximization phase. Although the phase and magnitude of the negative-phase voltage are simultaneously changed from the healthy phase maximization phase, only the phase may be changed while keeping the absolute value constant. This allows the amplitude of the healthy phase voltage to be reduced while suppressing a reduction in the amplitude of the positive-phase voltage. 18, the amplitude of the healthy phase voltage can be reduced by keeping Vqn constant and decreasing the absolute value of Vdn to move it away from the healthy phase maximum phase. When Vdn=0 and the amplitude of the healthy phase voltage is greater than a predetermined threshold, the absolute value of Vqn is further reduced.

[0036] Specific operation of the negative-sequence reduction control unit 8 will be described with reference to Fig. 19 . Fig. 19 is a block diagram of the negative-sequence reduction control unit 8 according to this embodiment. As shown in Fig. 19 , the negative-sequence reduction control unit 8 includes a negative-sequence coordinate conversion unit (UVW / negative-sequence dq) 81, a filter 82, an unbalanced fault detection unit 83, a negative-sequence current command value generation unit 84, and a coordinate conversion unit (negative-sequence dq / positive-sequence dq) 85. System voltages Vu, Vv, and Vw detected by the AC voltage detection unit 20 are input to the negative-sequence reduction control unit 8.

[0037] The negative-phase-sequence coordinate transformation unit 81 calculates the negative-phase-sequence voltages Vdn and Vqn in the rotating coordinate system by the following calculation. First, the negative-phase-sequence coordinate transformation unit 81 substitutes the system voltages Vu, Vv, and Vw into equation (1A) and performs a three-phase / two-phase transformation (UVW / αβ transformation) from the UVW coordinate to the αβ coordinate to calculate the system voltages Vα and Vβ in the αβ coordinate. Next, the negative-phase-sequence coordinate transformation unit 81 substitutes the system voltages Vα and Vβ in the αβ coordinate into equation (1B) and performs a rotating coordinate transformation (αβ / negative-phase dq transformation) from the αβ coordinate to the negative-phase dq coordinate. Note that θ in equation (1B) is a reference phase detected by a PLL unit (not shown), and (-θ) is the opposite phase of the reference phase θ. In this way, the negative-phase-sequence voltages Vdn and Vqn are calculated. <Equation (1A)> <Formula (1B)>

[0038] The negative-phase-sequence voltages Vdn, Vqn calculated by the negative-phase-sequence coordinate transformation unit 81 are input to a filter 82. The negative-phase-sequence voltages Vdn, Vqn have a frequency component of 2f (f is the frequency of the AC power system). The filter 82 removes the 2f frequency component of the negative-phase-sequence voltages Vdn, Vqn. The filter 82 may be a low-pass filter capable of removing the 2f frequency component, or may use a data processing method such as a moving average.

[0039] The unbalance fault detection unit 83 detects whether an unbalance fault has occurred in the AC power system 2, and if an unbalance fault has occurred, detects the type of unbalance fault (one-phase ground fault / two-phase ground fault) and the phase in which the fault occurred. The system voltages Vu, Vv, and Vw detected by the AC voltage detection unit 20 are input to the unbalance fault detection unit 83. The unbalance fault detection unit 83 calculates the system voltage amplitudes Vur, Vvr, and Vwr of each phase by substituting the input system voltages Vu, Vv, and Vw into equation (2). If the calculated system voltage amplitudes Vur, Vvr, and Vwr are smaller than a predetermined threshold, it is determined that a fault has occurred in that phase, and the unbalance fault detection unit 83 outputs an unbalance fault detection signal. <Equation (2)> The method of detecting an unbalance fault may be such that the unbalance fault detector 83 calculates the positive-sequence voltage and the negative-sequence voltage from the system voltages Vu, Vv, and Vw and makes a judgment based on the amplitude values ​​of the positive-sequence voltage and the negative-sequence voltage, or may be such that an unbalance fault detection signal is received from another device such as a protective relay.The method is not limited to the above examples as long as the type of unbalance fault and the fault phase are known.A block diagram of when an unbalance fault detection signal is received is shown in Figure 20.

[0040] The negative-sequence voltages Vdn and Vqn, from which the 2f frequency component has been removed by the filter 82, and the unbalance fault detection signal output by the unbalance fault detector 83 are input to the negative-sequence current command value generator 84. When an unbalance fault occurs in the AC power system 2 and the healthy phase voltage excessively increases, the negative-sequence current command value generator 84 generates a negative-sequence reactive current command value Idn1* and a negative-sequence active current command value Iqn1* so as to reduce the healthy phase voltage, depending on the type of the unbalance fault and the fault phase. In the case of an unbalance fault in which the amplitude of one phase voltage decreases, the negative-sequence reactive current command value Idn1* and the negative-sequence active current command value Iqn1* are first generated so that the phase of the negative-sequence voltage becomes the healthy phase balanced phase. When the amplitude of the healthy phase voltage is greater than a predetermined threshold value Vth1, the negative-sequence reactive current command value Idn1* and the negative-sequence active current command value Iqn1* are generated so as to further reduce the amplitude of the negative-sequence voltage. In the case of an unbalanced fault in which the amplitudes of the two-phase voltages decrease, a negative-sequence reactive current command value Idn1* and a negative-sequence active current command value Iqn1* are first generated so that the phase of the negative-sequence voltage moves away from the healthy-sequence maximizing phase. If the amplitude of the healthy-sequence voltage is greater than a predetermined threshold Vth1, a negative-sequence reactive current command value Idn1* and a negative-sequence active current command value Iqn1* are generated so that the amplitude of the negative-sequence voltage further decreases.

[0041] The negative-phase-sequence reactive current command value Idn1* and the negative-phase-sequence active current command value Iqn1* generated by the negative-phase-sequence current command value generating unit 84 are input to the negative-phase-sequence-positive coordinate transformer 85. The negative-phase-sequence-positive coordinate transformer 85 calculates the negative-phase-sequence reactive current command value Idn* and the negative-phase-sequence active current command value Iqn* by substituting the negative-phase-sequence reactive current command value Idn1* and the negative-phase-sequence active current command value Iqn1* into equation (3). θ is a reference phase detected by a PLL unit (not shown). Here, the negative-phase-sequence reactive current command value Idn1* and the negative-phase-sequence active current command value Iqn1* are values ​​on the negative-phase-sequence dq coordinate, and the negative-phase-sequence reactive current command value Idn* and the negative-phase-sequence active current command value Iqn* are values ​​on the dq coordinate. <Equation (3)>

[0042] When no unbalance fault occurs in the AC power system 2, the unbalance fault detection unit 83 stops the operation of the negative-phase-sequence current command value generation unit 84 and disables the negative-phase-sequence reduction control. In addition, when an unbalance fault occurs, if the maximum value of the system voltage amplitude calculated by equation (2) is less than the threshold value Vth1, the unbalance fault detection unit 83 disables the negative-phase-sequence reduction control. The threshold value Vth1 is determined, for example, based on the voltage in the operating range of the power converter 4.

[0043] 21 is a block diagram of the output current control unit 100. As shown in FIG. 21 , the output current control unit 100 includes adders 100a, 100b, 102a, and 102b, subtractors 101a and 101b, three-phase / two-phase coordinate conversion units 103a and 103b, and controllers 104a and 104b. The output current control unit 100 receives the negative-phase reactive current command value Idn* and the negative-phase active current command value Iqn* generated by the negative-phase-sequence reduction control unit 8, the system voltages Vu, Vv, and Vw detected by the AC voltage detection unit 20, the system AC currents Iu, Iv, and Iw detected by the AC current detector 30, the reactive current command value Idp* input from outside to control the system voltages Vu, Vv, and Vw of the AC power system 2, and the active current command value Iqp* calculated by the DC voltage control unit 70. The reactive current command value Idp* is used to control the positive-sequence voltage when an unbalanced fault occurs.

[0044] The adder 100a calculates the sum of the reactive current command value Idp* and the negative-phase-sequence reactive current command value Idn* generated by the negative-phase-sequence reduction control unit 8, and outputs the result to the subtracter 101a. The sum of the reactive current command value Idp* and the negative-phase-sequence reactive current command value Idn* is the d-axis current command value Id*. The adder 100b calculates the sum of the active current command value Iqp* generated by the DC voltage control unit 70 and the negative-phase-sequence active current command value Iqn* generated by the negative-phase-sequence reduction control unit 8, and outputs the result to the subtracter 101b. The sum of the active current command value Iqp* and the negative-phase-sequence active current command value Iqn* is the q-axis current command value Iq*.

[0045] The three-phase / two-phase coordinate converter 103a calculates a d-axis current Id and a q-axis current Iq by performing three-phase / two-phase conversion on the valve branch currents Iuv, Ivw, and Iwu detected by the valve branch current detector 42 in a positive phase coordinate system. The three-phase / two-phase coordinate converter 103b calculates a d-axis voltage Vd and a q-axis voltage Vq by performing three-phase / two-phase conversion on the system voltages Vu, Vv, and Vw detected by the AC voltage detector 20 in a positive phase coordinate system.

[0046] The subtractor 101a calculates the deviation between the d-axis current command value Id* and the d-axis current Id and outputs it to the controller 104a. The subtractor 101b calculates the deviation between the q-axis current command value Iq* and the q-axis current Iq and outputs it to the controller 104b. The controllers 104a and 104b are, for example, PI controllers. The controller 104a calculates a d-axis reference voltage Vdref so that the deviation between the d-axis current command value Id* and the d-axis current Id becomes zero, i.e., so that the d-axis current Id follows the d-axis current command value Id*. The controller 104b calculates a q-axis reference voltage Vqref so that the deviation between the q-axis current command value Iq* and the q-axis current Iq becomes zero, i.e., so that the q-axis current Iq follows the q-axis current command value Iq*.

[0047] The adder 102a calculates the sum of the d-axis reference voltage Vdref and the d-axis voltage Vd, and the adder 102b calculates the sum of the q-axis reference voltage Vqref and the q-axis voltage Vq. The sum of the d-axis reference voltage Vdref and the d-axis voltage Vd is the d-axis voltage command value Vd*, and the sum of the q-axis reference voltage Vqref and the q-axis voltage Vq is the q-axis voltage command value Vq*, which are output from the output current control unit 100.

[0048] Next, the configuration and operation of the voltage command value calculation unit 110 will be described with reference to FIGS. 22 and 23 . FIG. 22 is a block diagram showing the internal configuration of the voltage command value calculation unit 110 according to this embodiment. As shown in FIG. 22 , the voltage command value calculation unit 110 includes a dq / three-phase coordinate conversion unit 111, adders 112a, 112b, and 112c, and an individual cell DC voltage control unit 120. The internal configuration of the individual cell DC voltage control unit 120 will be described later. The d-axis voltage command value Vd* and the q-axis voltage command value Vq* generated by the output current control unit 100, the zero-phase voltage command value Vz* generated by the circulating current control unit 90, the valve branch currents Iuv, Ivw, and Iwu detected by the valve branch current detector 42, and the voltage Vdccell of the DC capacitor 11 detected by the DC capacitor voltage detector 12 are input to the voltage command value calculation unit 110.

[0049] A dq / three-phase coordinate converter 111 calculates three-phase voltage command values ​​by performing dq / three-phase coordinate conversion on the d-axis voltage command value Vd* and the q-axis voltage command value Vq* in a positive phase coordinate system. Adders 112a, 112b, and 112c add the zero-phase voltage command value Vz* to the voltage command values ​​of each phase calculated by the dq / three-phase coordinate converter 111 to calculate arm voltage command values ​​Vuv*, Vvw*, and Vwu* for each phase.

[0050] 23 is a block diagram showing the internal configuration of the individual cell DC voltage control unit 120 according to this embodiment. The individual cell DC voltage control unit 120 includes individual cell control units 128, the number of which is the same as the number of unit cells 10 (3N in this embodiment). The individual cell DC voltage control unit 120 receives the arm voltage command values ​​Vuv*, Vvw*, Vwu* for each phase, first to third arm currents Iuv, Ivw, Iwu detected by arm current detectors 42 provided in the first to third arms 41, and DC capacitor voltages Vdccell of all unit cells 10.

[0051] The individual cell control unit 128 includes a phase representative value calculation unit 124, a cell voltage extraction unit 125, a filter 126, a subtractor 121, an adder 122, a controller 127, and a multiplier 123. An individual cell control unit 128 is provided corresponding to each unit cell 10. The m-th individual cell control unit 128 of the first arm 41 will be described as an example, where m is a natural number equal to or less than N.

[0052] The arm voltage command value Vuv*, the arm current Iuv of the first arm 41, and the DC capacitor voltages Vdccell of all the unit cells 10 in the first arm 41 are input to the m-th cell individual control unit 128 of the first arm 41. The DC capacitor voltages Vdccell of all the unit cells 10 in the first arm 41 are input to a phase representative value calculation unit 124 and a cell voltage extraction unit 125. The phase representative value calculation unit 124 calculates an arm voltage representative value Vdcuav of the first arm 41. The arm voltage representative value Vdcuav is, for example, the average, maximum, or minimum value of the DC capacitor voltages Vdccell of all the unit cells 10 in the first arm 41. The cell voltage extraction unit 125 extracts the DC capacitor voltage Vdcuvm of the m-th unit cell from the DC capacitor voltages Vdccell of all the unit cells 10 in the first arm 41. The DC capacitor voltage Vdcuvm of the mth unit cell extracted by the cell voltage extraction unit is input to the filter 126. The filter 126 removes the 2f frequency component from the DC capacitor voltage Vdcuvm and outputs the cell DC voltage Vdcuvmf. The filter 126 is, for example, a low-pass filter configured to remove the 2f frequency component of the DC capacitor voltage Vdcuvm.

[0053] The subtractor 121 calculates the deviation between the valve branch voltage representative value Vdcuav and the cell DC voltage Vdcuvmf and outputs it to the controller 127. The controller 127 calculates a control output so that the deviation between the valve branch voltage representative value Vdcuav and the cell DC voltage Vdcuvmf becomes zero. The multiplier 123 calculates the product of the control output calculated by the controller 127 and the first valve branch current Iuv.

[0054] The adder 122 calculates the sum of the products of the arm voltage command value Vuv* of the first arm 41, the first arm current Iuv calculated by the multiplier 123, and the control output calculated by the controller 127. The sum of the products of the arm voltage command value Vuv* of the first arm 41, the first arm current Iuv, and the control output calculated by the controller 127 is the cell DC voltage command value Vdcuvm* of the m-th unit cell 10 of the first arm 41. The cell DC voltage command values ​​Vdcuvm* of all the unit cells 10 in the first to third arms 41 are collected together to form the output voltage command value Vcell* in FIGS.

[0055] The operation of the gate signal generating unit 130 will be described using Figure 7. The switching elements Q1 to Q4 or the switching elements Q1 and Q2 of each unit cell 10 are driven on / off by PWM control to output the output voltage command value Vcell* for each unit cell. The gate signal generating unit 130 generates a gate signal Sgate for driving the switching elements Q1 to Q4 or the switching elements Q1 and Q2 of each unit cell 10 on / off based on the output voltage command value Vcell* for each unit cell.

[0056] <Control> A control method according to this embodiment will be described with reference to Fig. 24. Fig. 24 is a flowchart of negative-phase-sequence voltage control when an unbalanced fault occurs.

[0057] In step ST1, the unbalance fault detection unit 83 determines whether or not an unbalance fault has occurred in the AC power system 2. If an unbalance fault has not occurred (NO in step ST1), the unbalance fault detection unit 83 disables the function of the negative-phase-sequence current command value generation unit 84. If an unbalance fault has occurred (YES in step ST1), the unbalance fault detection unit 83 proceeds to step ST2, where it determines whether or not the maximum value of the amplitude of the healthy phase voltage is equal to or greater than a predetermined threshold value Vth1.

[0058] If the maximum value of the amplitude of the healthy phase voltage is less than the predetermined threshold value Vth1 (NO in ST2), the unbalance fault detection unit 83 disables the function of the negative-phase-sequence current command value generation unit 84. If the maximum value of the amplitude of the healthy phase voltage is equal to or greater than the predetermined threshold value Vth1 (YES in step ST2), the unbalance fault detection unit 83 proceeds to step ST3 and determines whether the unbalance fault is an accident in which the amplitude of one phase voltage decreases.

[0059] If the accident is one in which the amplitude of the voltage of one phase decreases (YES in step ST3), the negative-sequence current command value generating unit 84 proceeds to step ST4, and controls the negative-sequence voltage of the AC power system 2 so that the phase of the negative-sequence voltage of the AC power system 2 coincides with the healthy phase balanced phase.

[0060] In step ST5, the unbalanced fault detection unit 83 determines whether the maximum value of the amplitude of the healthy phase voltage is equal to or greater than a predetermined threshold value Vth1. If the maximum value of the amplitude of the healthy phase voltage is less than the predetermined threshold value Vth1 (NO in ST5), the unbalanced fault detection unit 83 disables the function of the negative-sequence current command value generation unit 84. If the maximum value of the amplitude of the healthy phase voltage is equal to or greater than the predetermined threshold value Vth1 (YES in step ST5), the negative-sequence current command value generation unit 84 proceeds to step ST6, where it controls the negative-sequence voltage of the AC power system 2 so as to reduce the amplitude of the negative-sequence voltage of the AC power system 2.

[0061] In step ST7, the unbalanced fault detection unit 83 determines whether the maximum value of the amplitude of the healthy phase voltage is equal to or greater than a predetermined threshold value Vth1. If the maximum value of the amplitude of the healthy phase voltage is less than the predetermined threshold value Vth1 (ST7 NO), the unbalanced fault detection unit 83 disables the function of the negative-phase-sequence current command value generation unit 84. If the maximum value of the amplitude of the healthy phase voltage is equal to or greater than the predetermined threshold value Vth1 (ST7 YES), the unbalanced fault detection unit 83 repeats step ST6 until the maximum value of the amplitude of the healthy phase voltage becomes less than the predetermined threshold value Vth1.

[0062] If it is determined in step ST3 that the accident is one in which the amplitudes of the two-phase voltages decrease (NO in step ST3), the unbalanced fault detection unit 83 proceeds to step ST8, and controls the negative-sequence voltage of the AC power system 2 so that the phase of the negative-sequence voltage of the AC power system 2 moves away from the healthy phase maximization phase.

[0063] In step ST9, the unbalanced fault detection unit 83 determines whether the maximum value of the amplitude of the healthy phase voltage is equal to or greater than a predetermined threshold value Vth1. If the maximum value of the amplitude of the healthy phase voltage is less than the predetermined threshold value Vth1 (NO in ST9), the unbalanced fault detection unit 83 disables the function of the negative-sequence current command value generation unit 84. If the maximum value of the amplitude of the healthy phase voltage is equal to or greater than the predetermined threshold value Vth1 (YES in step ST9), the negative-sequence current command value generation unit 84 proceeds to step ST10, where it controls the negative-sequence voltage of the AC power system 2 so that the amplitude of the negative-sequence voltage of the AC power system 2 is reduced.

[0064] In step ST11, the unbalanced fault detection unit 83 determines whether the maximum value of the amplitude of the healthy phase voltage is equal to or greater than a predetermined threshold value Vth1. If the maximum value of the amplitude of the healthy phase voltage is less than the predetermined threshold value Vth1 (ST11 NO), the unbalanced fault detection unit 83 disables the function of the negative-phase-sequence current command value generation unit 84. If the maximum value of the amplitude of the healthy phase voltage is equal to or greater than the predetermined threshold value Vth1 (ST11 YES), the unbalanced fault detection unit 83 repeats step ST10 until the maximum value of the amplitude of the healthy phase voltage becomes less than the predetermined threshold value Vth1.

[0065] <Effects> The power conversion device 1 according to this embodiment is connected to the AC power system 2 and includes a power converter 4 having a self-extinguishing semiconductor switching element Q, and a converter control unit 7 that controls the power converter 4. The converter control unit 7 includes a negative-phase-sequence reduction control unit 8 that, when an unbalance fault occurs in the AC power system 2, causing a decrease in the amplitude of the voltage of at least one phase, generates a negative-phase-sequence current command value that adjusts the phase of the negative-phase-sequence voltage of the AC power system 2 so as to reduce the maximum value of the amplitude of the voltage of a healthy phase that is not affected by the unbalance fault. The converter control unit 7 controls the on / off of the semiconductor switching element Q based on the negative-phase-sequence current command value.

[0066] With this configuration, it is possible to reduce the maximum value of the amplitude of the healthy phase voltage while suppressing a reduction in the amplitude of the positive phase voltage.

[0067] When an unbalance fault that occurs in the AC power system 2 to which the power conversion device 1 according to this embodiment is connected is an fault in which the amplitude of the voltage of one phase decreases, the negative-sequence reduction control unit 8 of the power conversion device 1 may generate a negative-sequence current command value so that the phase of the negative-sequence voltage becomes a phase in which the imbalance in the amplitude of the voltage between healthy phases is eliminated and the maximum value of the amplitude of the voltage of the healthy phase is reduced.

[0068] This configuration makes the amplitudes of the voltages between the two healthy phases equal, reducing the maximum amplitude of the healthy phase voltage, thereby eliminating the need to excessively reduce the amplitude of the healthy phase voltage and suppressing the reduction in the positive-sequence voltage.

[0069] Furthermore, if an unbalanced fault that occurs in the AC power system 2 to which the power conversion device 1 according to this embodiment is connected is an fault in which the amplitudes of the two-phase voltages decrease, the negative-phase sequence reduction control unit 8 of the power conversion device 1 may generate a negative-phase sequence current command value so that the phase of the negative-phase sequence voltage becomes a phase in which the amplitude of the voltage of the healthy phase decreases.

[0070] With this configuration, it is possible to effectively reduce the maximum value of the amplitude of the healthy phase voltage while suppressing a reduction in the amplitude of the positive phase voltage.

[0071] In addition, if the maximum value of the amplitude of the voltage of the healthy phase after adjusting the phase of the negative-phase voltage is equal to or greater than a predetermined threshold, the negative-phase reduction control unit 8 may generate a negative-phase current command value so as to reduce the amplitude of the negative-phase voltage.

[0072] With this configuration, the phase of the negative-phase voltage is in a phase suitable for reducing the amplitude of the healthy phase voltage, so that the amplitude of the healthy phase voltage can be more effectively reduced by reducing the amplitude of the negative-phase voltage.

[0073] The negative-sequence reduction control unit 8 may further include a negative-sequence coordinate conversion unit 81 that converts the voltage of each phase of the AC power system 2 into a negative-sequence dq coordinate, calculates the phase and amplitude of the negative-sequence voltage on the negative-sequence dq coordinate, and generates a negative-sequence current command value based on the calculated phase and amplitude.

[0074] With this configuration, it is possible to control either the d-axis voltage or the q-axis voltage of the negative-phase-sequence voltage with priority, and the control load can be reduced compared to controlling both simultaneously.

[0075] The power converter 4 also has a plurality of arms 41 corresponding to each phase of the AC power system 2, and the arm 41 of each phase has a plurality of cascaded unit cells 10 and at least one reactor 43 connected in series, and the unit cell 10 may include a plurality of self-extinguishing semiconductor switching elements Q and a DC capacitor 11.

[0076] With this configuration, even when the voltage of the AC power system 2 is high, the power conversion device of the present disclosure reduces the amplitude of the healthy phase voltage when the healthy phase voltage rises excessively during an unbalanced fault.

[0077] Alternatively, the AC power system 2 may be a three-phase AC power system having a first phase, a second phase, and a third phase, and the power converter 4 may have a first arm 41, a second arm 41, and a third arm 41, the first arm 41 being connected between the first and second phases of the AC power system 2, the second arm 41 being connected between the second and third phases of the AC power system 2, and the third arm 41 being connected between the third phase and the first phase of the AC power system 2.

[0078] With this configuration, when the delta-connected static var compensator causes an excessive increase in the healthy phase voltage due to an unbalanced fault, the amplitude of the healthy phase voltage is reduced.

[0079] Furthermore, the AC power system 2 may be a three-phase AC power system having a first phase, a second phase, and a third phase, and the power converter 4 may have a first arm 41, a second arm 41, and a third arm 41, with the first arm 41 connected between the first phase and a neutral point of the AC power system 2, the second arm 41 connected between the second phase and the neutral point of the AC power system 2, and the third arm 41 connected between the third phase and the neutral point of the AC power system 2.

[0080] With this configuration, when the healthy phase voltage excessively rises due to the occurrence of an unbalanced fault, the amplitude of the healthy phase voltage is reduced by the star-connected static var compensator.

[0081] REFERENCE SIGNS LIST 1 power conversion device, 2 AC power system, 3 interconnection transformer, 4 power converter, 41 arm, 42 arm current detector, 43 reactor, 7 converter control unit, 70 DC voltage control unit, 71 total DC voltage representative value calculation unit, 72 subtractor, 73 controller, 8 negative sequence reduction control unit, 81 negative sequence coordinate conversion unit, 82 filter, 83 unbalanced fault detection unit, 84 negative sequence current command value generation unit, 85 negative sequence to positive sequence coordinate conversion unit, 9 gate signal calculation unit, 90 circulating current control unit, 91 adder, 92 subtractor, 93 multiplier, 94 controller, 10 unit cell, 11 DC capacitor, 12 DC capacitor voltage detector, 20 AC voltage detection unit, 30 AC current detector, 100 Output current control unit, 100a, b, 102a, b Adder, 101a, b Subtractor, 103a, b Three-phase / two-phase coordinate conversion unit, 104a, b Controller, 110 Voltage command value calculation unit, 111 dq / three-phase coordinate conversion unit, 112a, b, c Adder, 120 Cell individual DC current control unit, 121 Subtractor, 122 Adder, 123 Multiplier, 124 Phase representative value calculation unit, 125 Cell voltage extraction unit, 126 Filter, 127 Controller, 128 Cell individual control unit, 130 Gate signal generation unit, D1 to D4 Diodes, Q1 to Q4 Switching elements

Claims

1. A power conversion device comprising: a power converter connected to a three-phase AC power system and having self-extinguishing semiconductor switching elements; and a converter control unit that controls the power converter, wherein the converter control unit includes a negative-phase sequence reduction control unit, and when an unbalanced fault occurs in the AC power system, causing a decrease in the amplitude of the voltage of at least one phase, the negative-phase sequence reduction control unit generates a negative-phase sequence current command value that adjusts the phase of the negative-phase sequence voltage of the AC power system so as to reduce the maximum value of the amplitude of the voltage of a healthy phase where no unbalanced fault has occurred, and the converter control unit controls the on / off of the semiconductor switching elements based on the negative-phase sequence current command value.

2. The power conversion device according to claim 1, wherein, when the unbalanced fault is an fault in which the amplitude of the voltage of one phase of the AC power system decreases, the negative-sequence reduction control unit generates the negative-sequence current command value so that the phase of the negative-sequence voltage becomes a phase in which the difference in voltage amplitude between the healthy phases becomes small and the maximum value of the voltage amplitude of the healthy phase decreases.

3. The power conversion device according to claim 2, wherein the negative-phase-sequence reduction control unit generates the negative-phase-sequence current command value so that the phase of the negative-phase-sequence voltage coincides with the amplitude of the voltage between the healthy phases.

4. A power conversion device as described in claim 1, wherein, when the unbalanced fault is an fault in which the amplitude of two phases of voltage in the AC power system decreases, the negative-phase reduction control unit generates the negative-phase current command value so that the phase of the negative-phase voltage becomes the phase in which the amplitude of the voltage of the healthy phase decreases.

5. A power conversion device as described in any one of claims 1 to 4, wherein when the maximum value of the amplitude of the voltage of the healthy phase after adjusting the phase of the negative-phase voltage is equal to or greater than a predetermined threshold, the negative-phase reduction control unit generates the negative-phase current command value so as to reduce the amplitude of the negative-phase voltage.

6. A power conversion device according to any one of claims 1 to 5, wherein the negative-sequence reduction control unit further has a negative-sequence coordinate conversion unit that converts the voltage of each phase of the AC power system into negative-sequence dq coordinates, calculates the phase and amplitude of the negative-sequence voltage on the negative-sequence dq coordinates, and generates the negative-sequence current command value based on the calculated phase and amplitude.

7. A power conversion device according to any one of claims 1 to 6, wherein the power converter has a plurality of arms corresponding to each phase of the AC power system, and the arms of each phase are configured such that a plurality of cascaded unit cells and at least one reactor are connected in series, and the unit cells are provided with a plurality of the self-arc-suppressing semiconductor switching elements and a DC capacitor.

8. The power conversion device according to any one of claims 1 to 6, wherein the phases of the three-phase AC power system are a first phase, a second phase, and a third phase, the power converter has a first arm, a second arm, and a third arm, the first arm is connected between the first phase and the second phase of the AC power system, the second arm is connected between the second phase and the third phase of the AC power system, and the third arm is connected between the third phase and the first phase of the AC power system.

9. The power conversion device according to any one of claims 1 to 7, wherein the phases of the three-phase AC power system are a first phase, a second phase, and a third phase, the power converter has a first arm, a second arm, and a third arm, the first arm is connected between the first phase and a neutral point of the AC power system, the second arm is connected between the second phase and a neutral point of the AC power system, and the third arm is connected between the third phase and a neutral point of the AC power system.

10. A control method for a power conversion device connected to a three-phase AC power system and equipped with a power converter having self-extinguishing semiconductor switching elements, the control method for a power conversion device comprising: a step of determining whether an unbalance fault has occurred in the AC power system, in which the amplitude of the voltage of at least one phase decreases; a step of a negative-sequence current command value generated by a negative-sequence reduction control unit of a converter control unit to adjust the phase of the negative-sequence voltage of the AC power system so as to reduce the maximum value of the amplitude of the voltage of a healthy phase in which the unbalance fault has not occurred; and a step of the converter control unit controlling the on / off of the semiconductor switching elements based on the negative-sequence current command value.

Images