Electric power conversion apparatus

Abstract

An electric power conversion apparatus (100) comprises an electric power converter (2) and a control device (3). The electric power converter (2) is connected to an AC system (12) via a transformer (13) and an AC circuit breaker (15), and includes a first DC terminal (Np) and a second DC terminal (Nn) which are connected to a DC circuit (14), and a plurality of converter cells (7). The electric power conversion apparatus (100) further comprises a resistor (17) provided between the first DC terminal (Np) and ground, or between the second DC terminal (Nn) and ground. When a ground fault occurs in an AC line connecting the transformer (13) and the electric power converter (2), the control device (3) controls all switching elements included in the electric power converter (2) to an OFF state. The resistor (17) has a resistance value that reduces fault current occurring in response to a ground fault, thereby causing alternating current flowing through the AC circuit breaker (15) within a specified time to cross zero.

Description

Power conversion device

[0001] The present disclosure relates to a power conversion device.

[0002] High-voltage direct current (HVDC: High Voltage Direct Current) power transmission is a power transmission method suitable for long-distance and large-power transmission. As a self-excited power converter suitable for HVDC applications, a modular multilevel converter (MMC: Modular Multilevel Converter) is known. The MMC is composed of arms in which a plurality of unit converters called converter cells are cascade-connected. The converter cell includes a plurality of switching elements and energy storage elements (for example, capacitors).

[0003] In an HVDC system using an MMC, when a single-phase ground fault occurs between the power converter and the coupling transformer, the fault current flows through the power converter and the AC system, so that the AC current flowing through the primary-side AC circuit breaker of the coupling transformer may not become zero (that is, there is no zero-crossing point). When an opening command is given to the AC circuit breaker in such a state, the arc between the poles cannot be extinguished, and the AC circuit breaker may be damaged.

[0004] For example, European Patent No. 3639353 (Patent Document 1) discloses a method for handling faults in a converter unit for HVDC. Patent Document 1 discloses that an impedance unit in which an inductor component and a resistor component are connected in series between the neutral point of a bipolar configuration DC power transmission system and the ground attenuates the fault current caused by a fault between the power converter and the transformer, and as a result, the AC current flowing through the primary side of the transformer has a zero-crossing point.

[0005] European Patent No. 3639353

[0006] However, in Patent Document 1, since it is necessary to adopt an inductor component in addition to the resistor component as the impedance unit, the power conversion device becomes large-sized and the cost increases.

[0007] One objective of this disclosure is to provide a power converter that can appropriately open an AC circuit breaker with a simple configuration in the event of a ground fault between a power converter and a transformer.

[0008] A power converter according to one embodiment comprises a power converter that performs power conversion between an AC system and a DC circuit, and a control device that controls the power converter. The power converter is connected to the AC system via a transformer and an AC circuit breaker and includes a first DC terminal and a second DC terminal connected to the DC circuit, and a plurality of converter cells connected in series between the first DC terminal and the second DC terminal. Each of the plurality of converter cells has a plurality of switching elements and a capacitor connected to the plurality of switching elements. The power converter further comprises a resistor provided between the first DC terminal and ground, or between the second DC terminal and ground. The control device controls all switching elements in the power converter to an off state when a ground fault occurs in the AC line connecting the transformer and the power converter. The resistor has a resistance value such that the AC current flowing to the AC circuit breaker crosses zero within a specified time by suppressing the fault current generated in response to the ground fault.

[0009] According to this disclosure, in the event of a ground fault between a power converter and a transformer, the AC circuit breaker can be properly opened with a simple configuration.

[0010] This figure shows an example of the configuration of a power control system according to Embodiment 1. This figure shows an example of the configuration of a power converter. This is a circuit diagram showing an example of a converter cell. This is a block diagram showing an example of the hardware configuration of a control device. This figure shows the path of fault current that flows when a ground fault occurs according to Embodiment 1. This figure shows the time variation of arm current and AC current in a configuration without resistors. This figure shows the time variation of AC current in a configuration with resistors. This is an equivalent circuit for explaining the method of determining the resistance value of the resistor. This is a diagram for explaining the time from the output of the open command to the AC circuit breaker to the interruption of current. This figure shows another example of the path of fault current that flows when a ground fault occurs. This figure shows an example of the configuration of a power control system according to Embodiment 2. This figure shows the path of fault current that flows when a ground fault occurs according to Embodiment 2.

[0011] This embodiment will be described below with reference to the drawings. In the following description, identical parts are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed descriptions of them will not be repeated.

[0012] Embodiment 1. <System Configuration> Figure 1 is a diagram showing an example of the configuration of a power control system according to Embodiment 1. Referring to Figure 1, the power control system is a system for controlling the power of a unipolar DC transmission system. Typically, the DC circuit 14 is a unipolar DC transmission system, and power is transmitted between two AC systems 12 and AC systems 120 via the positive DC line 14P and the negative DC line 14N included in the DC circuit 14. Typically, AC systems 12 and AC systems 120 are three-phase AC systems.

[0013] AC power line 12 is connected to the AC terminals of power converter 2 via AC circuit breaker 15 and transformer 13. AC power line 120 is connected to the AC terminals of power converter 20 via AC circuit breaker 150 and transformer 130.

[0014] The power converter 100 includes a power converter 2, a control device 3 for controlling the power converter 2, and a resistor 17. The power converter 110 includes a power converter 20 and a control device 30 for controlling the power converter 20. In the example shown in Figure 1, the power converter 110 does not include a resistor. However, the power converter 110 may be configured to include a resistor having the same function as the resistor 17.

[0015] Power converter 2 is installed between the AC system 12 and the DC lines 14P and 14N. Power converter 20 is installed between the DC lines 14P and 14N and the AC system 120. Specifically, the DC line 14P, which is the positive busbar of the DC power transmission system, is connected to the positive DC terminals of power converters 2 and 20. The DC line 14N, which is the negative busbar of the DC power transmission system, is connected to the negative DC terminals of power converters 2 and 20.

[0016] When power is transmitted from AC system 12 to AC system 120, power converter 2 operates as a forward converter (REC), and power converter 20 operates as an inverse converter (INV).

[0017] Power converter 2 is composed of a self-commutated power converter. For example, power converter 20 is composed of a modular multilevel converter that includes multiple converter cells connected in series with each other. A "converter cell" is also called a "submodule" or "unit converter." Power converter 20 is also composed of a self-commutated power converter.

[0018] The control device 3 acquires electrical quantities (e.g., current, voltage, etc.) used for control from multiple detectors. The control device 3 controls the operation of the power converter 2 based on the electrical quantities acquired from the multiple detectors. Similarly, the control device 30 controls the operation of the power converter 20 based on the electrical quantities acquired from the multiple detectors. Typically, the control device 30 has a similar configuration to the control device 3. The control device 3 may be configured to communicate with the control device 30.

[0019] The resistor 17 is installed, for example, between the connection point between the negative DC terminal of the power converter 2 and the DC line 14N, and ground. Details of the function of the resistor 17 will be described later.

[0020] <Configuration of Power Converter> Figure 2 shows an example of the configuration of a power converter. Referring to Figure 2, the power converter 100 is connected between the AC system 12 and the DC circuit 14. The DC circuit 14 includes DC lines 14P and 14N. Typically, the DC circuit 14 is a single-pole DC power transmission system as shown in Figure 1. Note that the DC circuit 14 may be the DC terminal of another power converter. The DC circuit 14 may also include a power storage device connected to the DC terminal of the power converter 2. The power storage device may include, for example, an electric double-layer capacitor or a battery such as a lithium-ion battery.

[0021] The power converter 2 included in the power conversion device 100 is composed of a double-star type modular multilevel converter that includes a plurality of converter cells (corresponding to "cells" in Figure 1) 7 connected in series between a positive DC terminal (i.e., a high-potential DC terminal) Np and a negative DC terminal (i.e., a low-potential DC terminal) Nn.

[0022] The power converter 2 is connected to the AC system 12 via a transformer 13 and an AC circuit breaker 15. The power converter 2 is also connected to a DC circuit 14 and performs power conversion between the DC circuit 14 and the AC system 12. Specifically, the power converter 2 converts the DC power output from the DC circuit 14 into AC power and outputs this AC power to the AC system 12 via the transformer 13. The power converter 2 also converts the AC power from the AC system 12 into DC power and outputs this DC power to the DC circuit 14.

[0023] In the example shown in Figure 1, the power converter 2 includes a plurality of leg circuits 4u, 4v, and 4w, which correspond to the U phase, V phase, and W phase of the AC system 12, respectively. Specifically, the power converter 2 includes a positive DC terminal Np, a negative DC terminal Nn, and a plurality of leg circuits 4u, 4v, and 4w (hereinafter referred to collectively as "leg circuit 4" when referring to them individually or any of them) connected in parallel between the positive DC terminal Np and the negative DC terminal Nn. The leg circuits 4 are connected between the AC system 12 and the DC circuit 14 and perform power conversion between the two circuits.

[0024] The AC terminals Nu, Nv, and Nw, provided on the leg circuits 4u, 4v, and 4w respectively, are connected to the AC system 12 via the transformer 13. In Figure 1, for the sake of illustration, the connection between the AC terminals Nv and Nw and the transformer 13 is not shown.

[0025] The leg circuit 4u includes a positive arm 5u from the positive DC terminal Np to the AC terminal Nu, and a negative arm 6u from the negative DC terminal Nn to the AC terminal Nu. The connection point between the positive arm 5u and the negative arm 6u is connected to the transformer 13 as the AC terminal Nu. The positive DC terminal Np and the negative DC terminal Nn are connected to the DC circuit 14. The leg circuit 4v includes a positive arm 5v and a negative arm 6v, and the leg circuit 4w includes a positive arm 5w and a negative arm 6w. The positive arms 5u, 5v, and 5w are collectively referred to as "positive arm 5," and the negative arms 6u, 6v, and 6w are collectively referred to as "negative arm 6."

[0026] Thus, the leg circuit 4 is composed of a positive arm 5 and a negative arm 6 connected in series. The connection points of the positive arm 5 and the negative arm 6 (for example, AC terminals Nu, Nv, Nw) are connected to the AC lines of the corresponding phases of the AC system 12. Both ends of the series-connected positive arm 5 and negative arm 6 (i.e., the positive DC terminal Np and the negative DC terminal Nn) are connected to the DC circuit 14. Specifically, the positive DC terminal Np is connected to the DC line 14P, and the negative DC terminal Nn is connected to the DC line 14N. Since the leg circuits 4v and 4w have the same configuration as the leg circuit 4u, the leg circuit 4u will be described below as a representative example.

[0027] In the leg circuit 4u, the positive arm 5u includes a plurality of transducer cells 7 connected in series with each other, and a reactor 9a. The plurality of transducer cells 7 and the reactor 9a are connected in series with each other. The negative arm 6u includes a plurality of transducer cells 7 connected in cascade with each other, and a reactor 9b. The plurality of transducer cells 7 and the reactor 9b are connected in series with each other.

[0028] The reactor 9a may be inserted at any position on the positive arm 5u, and the reactor 9b may be inserted at any position on the negative arm 6u. There may be multiple reactors 9a and 9b. The inductance values ​​of each reactor may be different from each other. Furthermore, only the reactor 9a on the positive arm 5u or only the reactor 9b on the negative arm 6u may be provided.

[0029] The power converter 100 has various detectors (not shown) such as an AC voltage detector, an AC current detector, a DC voltage detector, an AC current detector, etc. These detectors measure the electrical quantities (i.e., current and voltage) used to control the power converter 2. The signals detected by these detectors are input to the control device 3.

[0030] Specifically, the AC voltage detector detects the AC voltages of the U-phase, V-phase, and W-phase of the AC system 12. The AC current detector detects the AC currents flowing through the AC lines of the U-phase, V-phase, and W-phase of the AC system 12. The DC voltage detector detects the DC voltage Vdc of the DC circuit 14. The power converter 100 also has current detectors for detecting the primary current I1, which is the current on the primary side of the transformer 13 (i.e., the AC circuit breaker 15 side), and the secondary current I2, which is the current on the secondary side of the transformer 13 (i.e., the power converter 2 side).

[0031] In the example shown in Figure 2, the resistor 17 is connected between the DC line 14N, which is a negative bus connected to the negative DC terminal Nn, and ground. However, the resistor 17 may also be provided between the DC line 14P, which is a positive bus connected to the positive DC terminal Np, and ground.

[0032] <Configuration of the Converter Cell> Figure 3 is a circuit diagram showing an example of a converter cell. The converter cell 7 shown in Figure 3(a) has a circuit configuration called a half-bridge configuration. This converter cell 7 includes a series unit formed by connecting two switching elements 31p and 31n in series, a capacitor 32 as an energy storage element, and a voltage detector 33. The series unit and the capacitor 32 are connected in parallel. The voltage detector 33 detects the voltage Vc, which is the voltage across the capacitor 32.

[0033] The converter cell 7 shown in Figure 3(b) has a circuit configuration called a full-bridge configuration. This converter cell 7 includes a first series unit formed by connecting two switching elements 31p1 and 31n1 in series, a second series unit formed by connecting two switching elements 31p2 and 31n2 in series, a capacitor 32, and a voltage detector 33. The first series unit, the second series unit, and the capacitor 32 are connected in parallel. The voltage detector 33 detects the voltage Vc.

[0034] The two switching elements 31p and 31n in Figure 3(a) and the four switching elements 31p1, 31n1, 31p2, and 31n2 in Figure 3(b) are configured by connecting a freewheeling diode in antiparallel to a semiconductor switching element such as an IGBT, GCT thyristor, or MOSFET (Metal Oxide Semiconductor Field-Effect Transistor). In Figures 3(a) and 3(b), the capacitor 32 is mainly a film capacitor or the like.

[0035] In the following explanation, the switching elements 31p, 31n, 31p1, 31n1, 31p2, and 31n2 will also be collectively referred to as switching element 31. Furthermore, the on / off state of the semiconductor switching elements within switching element 31 will simply be described as "on / off state of switching element 31".

[0036] Referring to Figure 3(a), both terminals of the switching element 31n are designated as input / output terminals P1 and P2. The switching operation of the switching elements 31p and 31n outputs the voltage across the capacitor 32 and a zero voltage. For example, when switching element 31p is ON and switching element 31n is OFF, the voltage across the capacitor 32 is output. When switching element 31p is OFF and switching element 31n is ON, a zero voltage is output.

[0037] Next, referring to Figure 3(b), the midpoints of switching element 31p1 and switching element 31n1, and the midpoints of switching element 31p2 and switching element 31n2, are designated as input / output terminals P1 and P2 of the converter cell 7, respectively. The converter cell 7 shown in Figure 3(b) outputs a positive voltage or zero voltage by turning on switching element 31n2, turning off switching element 31p2, and alternately turning on switching elements 31p1 and 31n1. Furthermore, the converter cell 7 shown in Figure 3(b) can output a zero voltage or a negative voltage by turning off switching element 31n2, turning on switching element 31p2, and alternately turning on switching elements 31p1 and 31n1.

[0038] In the following explanation, the converter cell 7 is assumed to be a half-bridge cell configuration as shown in Figure 3(a), and the example will be given using a semiconductor switching element and a capacitor as an energy storage element. However, the converter cell 7 may also be a full-bridge configuration as shown in Figure 3(b). Furthermore, a converter cell with a circuit configuration other than those shown above may be used, for example, a converter cell with a circuit configuration also called a 1.5 half-bridge configuration in which the switching element 31p2 in Figure 3(b) is replaced only with a diode.

[0039] <Hardware Configuration of Control Device>Figure 4 is a block diagram showing an example of the hardware configuration of the control device. In the case of Figure 4, the control device 3 is configured based on a computer. Referring to Figure 4, the control device 3 includes one or more input converters 70, one or more sample-and-hold circuits (S / H circuits) 71, a multiplexer (MUX: multiplexer) 72, and an A / D converter 73. Further, the control device 3 includes one or more CPUs (Central Processing Unit) 74, a RAM (Random Access Memory) 75, and a ROM (Read Only Memory) 76. Further, the control device 3 includes one or more input / output interfaces 77, an auxiliary storage device 78, and a bus 79 that interconnects the above components with each other.

[0040] The input converter 70 includes an auxiliary transformer for each input channel. Each auxiliary transformer converts a detection signal from various detectors provided in the power conversion device 100 into a signal with a voltage level suitable for subsequent signal processing.

[0041] The sample-and-hold circuit 71 is provided for each input converter 70. The sample-and-hold circuit 71 samples and holds a signal representing an electrical quantity received from the corresponding input converter 70 at a specified sampling frequency.

[0042] The multiplexer 72 sequentially selects signals held in a plurality of sample-and-hold circuits 71. The A / D converter 73 converts the signal selected by the multiplexer 72 into a digital value. Note that by providing a plurality of A / D converters 73, A / D conversion may be performed in parallel for detection signals of a plurality of input channels.

[0043] The CPU 74 controls the entire control device 3 and executes arithmetic processing according to a program. The RAM 75 as a volatile memory and the ROM 76 as a non-volatile memory are used as the main memories of the CPU 74. The ROM 76 stores programs, set values for signal processing, and the like. The auxiliary storage device 78 is a non-volatile memory with a larger capacity than the ROM 76 and stores programs, data of electrical quantity detection values, and the like. The input / output interface 77 is an interface circuit for communicating between the CPU 74 and an external device.

[0044] Note that the functions of the control device 3 are realized by a processing circuit included in the control device 3. The processing circuit may be dedicated hardware or may be the CPU 74 that executes a program stored in an internal memory (for example, the ROM 76) of the control device 3. When the processing circuit is dedicated hardware, the processing circuit is configured by, for example, an FPGA, an ASIC, or a combination thereof.

[0045] <Operation at the time of a ground fault accident> Fig. 5 is a diagram showing the path of an accident current flowing at the time of a ground fault accident according to Embodiment 1. In the example of Fig. 5, current paths 210 and 220 that flow when a single-phase ground fault (hereinafter, also simply referred to as "U-phase ground fault") occurs in the U phase at the accident point F between the transformer 13 and the power converter 2 are shown. Specifically, the accident point F indicates a ground fault point on the AC line connecting the secondary side of the transformer 13 and the AC terminal Nu.

[0046] The control device 3 determines that a U-phase ground fault has occurred in the AC line connecting the transformer 13 and the power converter 2 (that is, at the accident point F) based on the electrical quantities detected by various detectors. For example, the control device 3 includes a ratio differential relay unit for protecting the transformer 13.

[0047] The ratio differential relay unit receives inputs of the primary current I1 and the secondary current I2 of the transformer 13. Each of the primary current I1 and the secondary current I2 is a current after gain matching based on the CT winding ratio and the voltage ratio of the transformer 13 and phase matching based on the winding configuration is performed.

[0048] The ratio differential relay unit performs ratio differential relay calculations based on the suppression current and differential current calculated from the primary current I1 and secondary current I2. If the suppression current and differential current satisfy a specified relationship, the ratio differential relay unit determines that a ground fault has occurred at fault point F.

[0049] The control device 3 may also use other known fault detection methods to determine a one-phase ground fault in the AC line connecting the transformer 13 and the power converter 2.

[0050] When the control device 3 determines that a U-phase ground fault has occurred, it controls all switching elements 31 included in the power converter 2 to the OFF state. For example, the control device 3 outputs a gate block command to gate block all the switching elements 31 of the converter cells 7 included in the power converter 2. After outputting the gate block command, the control device 3 outputs an open command (for example, a trip command) to the AC circuit breaker 15. In other words, when a U-phase ground fault occurs, the control device 3 controls all the switching elements 31 included in the power converter 2 to the OFF state and then outputs the open command.

[0051] Here, the time from when the gate block command is output until the switching element 31 turns off is overwhelmingly shorter than the time from when the open command is output to the AC circuit breaker 15 until the AC circuit breaker 15 opens. Therefore, after a U-phase ground fault occurs, the switching element 31 is turned off, but the AC circuit breaker 15 is not completely open, and the fault current flows as shown in the current paths 210 and 220 in Figure 5.

[0052] Current path 210 shows the path of the fault current that flows through the negative arm 6v of the V phase, the transformer 13, the AC circuit breaker 15, the AC system 12, the ground, and the resistor 17 when a U-phase ground fault occurs. Current path 220 shows the path of the fault current that flows through the negative arm 6w of the W phase, the transformer 13, the AC circuit breaker 15, the AC system 12, the ground, and the resistor 17 when a U-phase ground fault occurs. In this way, the fault current flows through the resistor 17 and gradually attenuates.

[0053] Here, assuming a configuration without resistor 17, if a U-phase ground fault occurs at fault point F, the fault current flows through the current path of the V-phase negative arm 6v, transformer 13, AC circuit breaker 15, AC system 12, and ground, and the current path of the W-phase negative arm 6w, transformer 13, AC circuit breaker 15, AC system 12, and ground. In other words, the fault current cannot be affected by the current suppression effect of resistor 17 and takes a long time to decay.

[0054] Figure 6 shows the time variation of arm current and AC current in a configuration without resistors. Referring to Figure 6, the vertical axis of graph 500 shows the arm current Ivn of the negative arm 6v of the V phase and the arm current Iwn of the negative arm 6w of the W phase, and the horizontal axis shows time. The vertical axis of graph 510 shows the AC currents flowing through the U phase, V phase, and W phase AC lines of the AC system 12 (i.e., U phase AC current, V phase AC current, and W phase AC current), and the horizontal axis shows time.

[0055] Referring to graphs 500 and 510, for example, the arm current Ivn and the V-phase AC current are approximately zero during the period of “0.109 to 0.112 sec”, and the arm current Iwn and the W-phase AC current are approximately zero during the period of “0.115 to 0.118 sec”.

[0056] On the other hand, the U-phase AC current is never zero during any period, and there is no zero-crossing point. This is because the DC component of the fault current is superimposed on the AC current of the AC system 12. In such a case, even if an open command is output to the AC circuit breaker 15 and the opening operation is initiated, the arc between the poles is not extinguished, so the current cannot be completely interrupted, and there is a possibility that the AC circuit breaker 15 will be damaged.

[0057] Figure 7 shows the time variation of the alternating current in a configuration with resistors. The vertical axis of Figure 7 represents the alternating current flowing through the U-phase, V-phase, and W-phase alternating current lines, and the horizontal axis represents time. The V-phase alternating current becomes zero after approximately 1.073 seconds, and the W-phase alternating current becomes zero after approximately 1.100 seconds.

[0058] Furthermore, the U-phase AC current also gradually decreases, becoming zero at approximately 1.100 seconds. This is because the fault current is suppressed by the resistor 17. In the example shown in Figure 7, it can be seen that the U-phase AC current crosses zero approximately 1.5 cycles after the open command is output to the AC circuit breaker 15. As a result, the arc between the poles of the AC circuit breaker 15 is extinguished, and the current interruption is completed.

[0059] <Method for Determining Resistance Value> Figure 8 is an equivalent circuit for explaining the method for determining the resistance value of a resistor. Specifically, the equivalent circuit shown in Figure 8 is based on the current paths 210 and 220 during a U-phase ground fault that occurred at fault point F in Figure 5.

[0060] In the equivalent circuit in Figure 8, "La" is the inductance component of each arm (for example, corresponding to reactors 9a and 9b in Figure 2), and "Lst" is the inductance component of the transformer 13 and AC system 12 in each phase. "Ivn" is the arm current of the negative arm 6v of the V phase, and "Iwn" is the arm current of the negative arm 6w of the W phase.

[0061] "Vdc" is the DC voltage between DC line 14P and DC line 14N. "Vbu" is the U-phase background voltage (e.g., the U-phase AC voltage of AC system 12), "Vbv" is the V-phase background voltage (e.g., the V-phase AC voltage of AC system 12), and "Vbw" is the W-phase background voltage (e.g., the W-phase AC voltage of AC system 12). "Vcapvp" is the sum of the capacitor voltages of all capacitors 32 included in the positive arm 5v of the V-phase, and "Vcapwp" is the sum of the capacitor voltages of all capacitors 32 included in the positive arm 5w of the W-phase. "Vnu" is the voltage applied to the inductance component Lst on the transformer 13 and AC system 12 side and the resistor 17 in the U-phase.

[0062] The line voltage Vbvu between the V-phase and U-phase is expressed by the following equation (1). Here, "Vs" is the effective value of the line voltage between the V-phase and U-phase of the AC system 12, and "θ" is the phase of the line voltage. Note that in the following equation (1), for the sake of simplicity of representation, V ｂｖｕ It should be indicated in the form of a subscript, as shown above. For example, "V in mathematical formulas"ｂｖｕ This is the same as "Vbvu in the text". This is also true for the formula described later.

[0063]

[0064] Furthermore, the line voltage Vbwu between the W phase and the U phase is expressed by the following equation (2).

[0065]

[0066] When current flows only between the W and U phases, the following equation (3) holds true, where Rg is the resistance of resistor 17. The period during which current flows only between the W and U phases corresponds, for example, to the period of 0.109 to 0.112 sec in Figure 6. In equation (3), X is the reactance expressed in equation (4). In equation (4), Xs is the reactance of the AC system 12, Xtr is the reactance of the transformer 13, and Xa is the reactance of the reactors of each arm of the power converter 2 (for example, reactors 9a and 9b in Figure 2).

[0067]

[0068] Here, from the equivalent circuit, when the voltage Vdvn becomes negative (i.e., Vdvn < 0), the arm current Ivn of the V phase begins to flow (i.e., current begins to flow in the V phase). Here, "Vdvn = Vbv - Vnu". Therefore, if the arm current Iwn of the W phase becomes zero before "Vbv - Vnu < 0" occurs (i.e., before current begins to flow in the V phase), a zero-crossing point will also occur in the U phase AC current. This is because, in the case of a U phase ground fault, considering the current paths 210 and 220 in Figure 5, the arm current Ivn flows through the V phase and the U phase, and the arm current Iwn flows through the W phase and the U phase. Therefore, when both the arm current Ivn and the arm current Iwn are 0, no current flows in the U phase.

[0069] The W-phase arm current Iwn is expressed by the following equation (5), where "k = Rg / X" and "γ = tan －１(k) is the arm current Iwn when the arm current Ivn becomes zero, and θw is the phase of the arm current Iwn when the arm current Ivn becomes zero.

[0070]

[0071] By appropriately setting the constant k from equation (5), the arm current Iwn can be made zero before the arm current Ivn begins to flow.

[0072] Figure 9 is a diagram illustrating the time from the output of the open command to the AC circuit breaker to the interruption of current. The vertical axis of Figure 9 represents the time t from the output of the open command to the AC circuit breaker 15 to the interruption of current, and the horizontal axis represents the constant k. Figure 9 also shows the relationship between the constant k and time t when the short circuit ratio (SCR) of the AC system 12 is changed. For example, "SCR = (short circuit capacity of AC system 12) / (rated converter capacity of power converter 2)". Figure 9 shows the cases where SCR = 2, 5, 10, and 20.

[0073] As shown in Figure 9, when the constant k is set to 0.05 or greater, it can be seen that regardless of whether the SCR is 2, 5, 10, or 20, the AC circuit breaker 15 is completely open (i.e., the current is interrupted) within one cycle of the AC system 12's system cycle (for example, 0.02 seconds when the system frequency is 50 Hz) from the output of the open command to the AC circuit breaker 15. Here, from equation (4) and "k = Rg / X", the relationship P "Rg = kX = k{2(Xs + Xtr) + Xa}" holds.

[0074] Therefore, by determining the smallest Rg for resistor 17 such that k ≥ 0.05 and relation P is satisfied, the U-phase AC current crosses zero within a specified time (for example, within the time from when the open command to the AC circuit breaker 15 is output until one cycle of the system cycle has elapsed).

[0075] When expressed in units of the PU (Per Unit) method, there is a relationship between SCR and reactance Xs as "Xs[pu] = 1 / SCR". Here, let Srate be the rated capacity of AC system 12 and Vacrate be the rated line voltage of AC system 12. In this case, reactance Xs is "Xs[Ω] = (1 / SCR) * (Vacrate) ２ It is expressed as "SCR = Vacrate". ２ The relationship " / (Srate * Xs)" holds true. "*" represents the multiplication symbol.

[0076] In summary, the resistor 17 has a resistance value Rg such that the AC current flowing through the AC circuit breaker 15 crosses zero within a specified time by suppressing the fault current generated in response to a ground fault (for example, a U-phase ground fault). Specifically, when a single-phase ground fault occurs in the U-phase of the AC line connecting the transformer 13 and the power converter 2, the resistor 17 has a resistance value Rg such that the AC current of the U-phase flowing through the AC circuit breaker 15 crosses zero within a specified time.

[0077] Furthermore, the resistance value Rg is determined based on the reactance Xs, which is derived from the short-circuit capacity ratio of the AC system 12, the reactance Xtr of the transformer 13, and the reactance Xa of the arm included in the power converter 2. Specifically, the resistance value Rg is determined to be the minimum value that satisfies k ≥ 0.05 and relation P.

[0078] <Modification> In Figure 5, when the resistor 17 is installed between the DC line 14N connected to the negative DC terminal Nn and the ground, the fault current during a ground fault flows through the resistor 17 and is gradually attenuated. Here, we will explain that even when the resistor 17 is connected between the DC line 14P connected to the positive DC terminal Np and the ground, the fault current is also gradually attenuated.

[0079] Figure 10 shows another example of the path of fault current that flows when a ground fault occurs. In the example in Figure 10, the current paths 230 and 240 that flow when a one-phase ground fault occurs in the U phase at fault point F between the transformer 13 and the power converter 2 are shown. The location of fault point F is the same as in the example in Figure 5.

[0080] When the control device 3 determines that a U-phase ground fault has occurred, it controls all switching elements 31 included in the power converter 2 to the OFF state (for example, it outputs a gate block command). After outputting the gate block command, the control device 3 outputs an open command to the AC circuit breaker 15. After the U-phase ground fault occurs, the switching elements 31 are turned OFF, but the AC circuit breaker 15 is not completely open, and the fault current flows as shown in the current paths 230 and 240 in Figure 10.

[0081] Current path 230 shows the path of the fault current that flows through the positive arm 5v of the V phase, resistor 17, ground, transformer 13, AC circuit breaker 15, and AC system 12 when a U-phase ground fault occurs. Current path 240 shows the path of the fault current that flows through the positive arm 5w of the W phase, resistor 17, ground, transformer 13, AC circuit breaker 15, and AC system 12 when a U-phase ground fault occurs. In this way, the fault current flows through resistor 17 and is gradually attenuated.

[0082] As shown in the example in Figure 10, when the resistor 17 is placed between the DC line 14P and ground, the terminal voltage of the diode is 0V when no current is flowing through the diode. However, since an AC voltage is applied to the diode, when a forward voltage is applied to the lower diode of the converter cell 7 of the positive arm 5, the lower diode conducts and current flows. Since the voltage of the upper diode of the converter cell 7 of the positive arm 5 is smaller than the capacitor voltage, no current flows through the upper diode. Therefore, as shown in the current paths 230 and 240, the fault current is reduced through the resistor 17.

[0083] <Advantages> According to Embodiment 1, in a single-pole DC power transmission system, by providing a resistor 17 between the positive DC terminal of the power converter 2 and the ground, or between the negative DC terminal of the power converter 2 and the ground, the fault current that flows when a single-phase ground fault occurs between the transformer 13 and the power converter 2 can be suppressed. This makes it possible to bring the AC current of the AC system 12 to zero-cross within a specified time. Furthermore, the resistance value of the resistor 17 only needs to be determined to the minimum resistance value necessary to satisfy the relational expression P, and no other components (for example, inductor components) other than the resistor 17 are required to suppress the fault current, so the power converter 100 can be made smaller and less expensive.

[0084] Embodiment 2. Figure 11 shows an example of the configuration of a power control system according to Embodiment 2. Referring to Figure 11, the power control system is a system for controlling the power of a bipolar DC power transmission system.

[0085] The DC circuit 14 is a bipolar DC power transmission system, and power is transmitted between the two AC systems 12 and 120 via the positive DC line 14P and the negative DC line 14N included in the DC circuit 14. The DC circuit 14 also includes a neutral point 19 for the two DC power transmission systems, which are divided into positive and negative poles.

[0086] AC system 12 is connected to AC bus 51. AC bus 51 is connected to power converter 2 via AC circuit breaker 15 and transformer 13, and to power converter 21 via AC circuit breaker 151 and transformer 131. AC system 120 is connected to AC bus 52. AC bus 52 is connected to power converter 20 via AC circuit breaker 150 and transformer 130, and to power converter 22 via AC circuit breaker 152 and transformer 132.

[0087] Power converters 2 and 21 are connected to the neutral point 19, and power converters 20 and 22 are connected to the neutral point 191. Power converters 2 and 20 are connected to the DC line 14P, and power converters 21 and 22 are connected to the DC line 14N. When power is transmitted from the AC system 12 to the AC system 120, power converters 2 and 21 operate as forward converters, and power converters 20 and 22 operate as reverse converters.

[0088] In Embodiment 2, the positive DC terminal Np of the power converter 2 is connected to the DC line 14P (i.e., the positive busbar of a bipolar DC power transmission system), and the negative DC terminal Nn of the power converter 2 is connected to the neutral point 19 (i.e., the neutral point of a bipolar DC power transmission system). The resistor 17 is provided between the neutral point 19, which is connected to the negative DC terminal Nn, and ground.

[0089] Figure 12 is a diagram showing the path of fault current that flows when a ground fault occurs according to Embodiment 2. In the example of Figure 12, the current paths 310 and 320 that flow when a U-phase ground fault occurs at fault point F between the transformer 13 and the power converter 2 (i.e., the fault point on the AC line connecting the secondary side of the transformer 13 and the AC terminal Nu) are shown.

[0090] Similar to Embodiment 1, when the control device 3 determines that a U-phase ground fault has occurred, it controls all switching elements 31 included in the power converter 2 to the OFF state, and then outputs an open command to the AC circuit breaker 15. After the U-phase ground fault occurs, the switching elements 31 are turned OFF, but the AC circuit breaker 15 is not completely open, and the fault current flows as shown in the current paths 310 and 320 in Figure 12.

[0091] Current path 310 shows the path of the fault current that flows through the negative arm 6v of the V phase, the transformer 13, the AC circuit breaker 15, the AC system 12, the ground, and the resistor 17 when a U-phase ground fault occurs. Current path 320 shows the path of the fault current that flows through the negative arm 6w of the W phase, the transformer 13, the AC circuit breaker 15, the AC system 12, the ground, and the resistor 17 when a U-phase ground fault occurs. In this way, the fault current flows through the resistor 17 and gradually attenuates.

[0092] Therefore, similar to Embodiment 1, in Embodiment 2 as well, by determining the resistance value of resistor 17 to be the smallest resistance value Rg that satisfies k ≥ 0.05 and relation P, the U-phase AC current crosses zero within the specified time.

[0093] This section describes the case where a ground fault occurs at fault point F between transformer 13 and the upper power converter (for example, power converter 2). In this case, as described above, the fault current is reduced by the grounding resistance of the neutral point 19 to which resistor 17 is connected (i.e., the grounding resistance of the negative terminal of the upper power converter from the perspective of the upper power converter). Note that if a ground fault occurs at the fault point between transformer 131 and the lower power converter (for example, power converter 21), the fault current flows through the path described in Figure 10. Therefore, the fault current is reduced by the grounding resistance of the neutral point 19 (i.e., the grounding resistance of the positive terminal of the lower power converter from the perspective of the lower power converter).

[0094] <Advantages> According to Embodiment 2, the same effects as in Embodiment 1 can be obtained in a bipolar DC power transmission system. Specifically, by providing a resistor 17 between the neutral point 19 connected to the negative DC terminal of the power converter 2 and the ground, the fault current that flows when a one-phase ground fault occurs between the transformer 13 and the power converter 2 can be suppressed, and the AC current of the AC system 12 can be brought to zero within a specified time.

[0095] Other embodiments. (1) In the embodiments described above, an example was described in which the secondary side of the transformer 13 (i.e., the power converter 2 side) is configured in a star connection, but the secondary side of the transformer 13 may also be configured in a delta connection.

[0096] (2) The configurations illustrated above as embodiments are examples of the configurations of the present disclosure and can be combined with other known technologies, and can be modified, such as by omitting parts, without departing from the gist of the present disclosure. Furthermore, in the embodiments described above, processes and configurations described in other embodiments may be appropriately adopted and implemented.

[0097] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of this disclosure is indicated by the claims rather than by the foregoing description, and all modifications within the meaning and scope of the claims are intended to be included.

[0098] 2, 20, 21, 22 Power converters, 3, 30 Control devices, 4u, 4v, 4w Reg circuits, 5u, 5v, 5w Positive arm, 6u, 6v, 6w Negative arm, 7 Converter cells, 9a, 9b Reactors, 12, 120 AC systems, 13, 130, 131, 132 Transformers, 14 DC circuits, 14N, 14P DC lines, 15, 150, 151, 152 AC circuit breakers, 17 Resistors, 19, 191 Neutral points, 31n, 31p Switching elements, 32 Capacitors, 33 Voltage detectors, 51, 52 AC buses, 70 Input converters, 71 Sample-and-hold circuits, 72 Multiplexers, 73 A / D converters, 74 CPU, 75 RAM, 76 ROM, 77 Input / Output interfaces, 78 Auxiliary storage device, 79 bus, 100, 110 power converter, 210, 220, 310, 320 current path.

Claims

1. A power converter comprising: a power converter that performs power conversion between an AC system and a DC circuit; and a control device that controls the power converter, wherein the power converter is connected to the AC system via a transformer and an AC circuit breaker and includes a first DC terminal and a second DC terminal connected to the DC circuit, and a plurality of converter cells connected in series between the first DC terminal and the second DC terminal, each of the plurality of converter cells having a plurality of switching elements and a capacitor connected to the plurality of switching elements, and further comprising a resistor provided between the first DC terminal and ground, or between the second DC terminal and ground, wherein the control device controls all switching elements included in the power converter to an OFF state when a ground fault occurs in the AC line connecting the transformer and the power converter, and the resistor has a resistance value such that the AC current flowing to the AC circuit breaker crosses zero within a specified time by suppressing the fault current generated in response to the ground fault.

2. The power conversion device according to claim 1, wherein, in the event of a one-phase ground fault in the first phase of the AC line, the resistor has a resistance value such that the AC current of the first phase flowing to the AC circuit breaker crosses zero within the specified time.

3. The power converter according to claim 1 or 2, wherein the control device controls all switching elements included in the power converter to an off state when a ground fault occurs, and then outputs an open command to open the AC circuit breaker.

4. The power converter according to claim 3, wherein the specified time is the time from when the open command is output until one cycle of the AC power system cycle has elapsed.

5. The power conversion device according to any one of claims 1 to 4, wherein the DC circuit is a single-pole DC power transmission system, the first DC terminal is connected to the positive bus of the DC power transmission system, the second DC terminal is connected to the negative bus of the DC power transmission system, and the resistor is provided between the positive bus connected to the first DC terminal and the ground, or between the negative bus connected to the second DC terminal and the ground.

6. The power conversion device according to any one of claims 1 to 4, wherein the DC circuit is a bipolar DC power transmission system, the first DC terminal is connected to the positive busbar of the DC power transmission system, the second DC terminal is connected to the neutral point of the DC power transmission system, and the resistor is provided between the neutral point connected to the second DC terminal and the ground.

7. The power converter according to any one of claims 1 to 6, wherein the resistance value is determined based on a first reactance based on the short-circuit capacity ratio of the AC system, a second reactance of the transformer, and a third reactance of an arm included in the power converter.

8. When the resistance value is Rg, the first reactance is Xs, the second reactance is Xtr, the third reactance is Xa, and k is a constant of 0.05 or more, the resistance value is determined to be the minimum value that satisfies the following equation (1): Rg = k{2(Xs + Xtr) + Xa} ... (1) The power conversion device according to claim 7.

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