Power Conversion Equipment

Abstract

The power converter (100) includes a power converter (2) and a control device (3). The 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) connected to a DC circuit (14), and a plurality of converter cells (7). The power converter (100) further includes 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 power converter (2), the control device (3) controls all switching elements included in the power converter (2) to an off state. The resistor (17) has a resistance value that suppresses a fault current generated in response to a ground fault, so that an AC current flowing through the AC circuit breaker (15) crosses zero within a specified time.

Description

[Technical field]

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

[0002] High voltage direct current (HVDC) is a power transmission method suitable for long-distance large-volume power transmission. The modular multilevel converter (MMC) is known as a self-commutated power converter suitable for HVDC applications. The MMC is composed of arms in which multiple unit converters called converter cells are cascaded. The converter cell includes multiple switching elements and storage elements (e.g., capacitors).

[0003] In an HVDC system using MMC, if a single-phase ground fault occurs between a power converter and an interconnection transformer, the fault current flows through the power converter and the AC system, which may cause the AC current flowing through the AC circuit breaker on the primary side of the interconnection transformer to not become zero (i.e., there is no zero-cross point). If an opening command is given to the AC circuit breaker in this 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 a fault in a converter unit for HVDC. Patent Document 1 discloses that a fault current caused by a fault between a power converter and a transformer is attenuated by an impedance unit having an inductor component and a resistive component connected in series between the neutral point and ground of a bipolar DC transmission system, so that an AC current flowing through the primary side of the transformer has a zero crossing point. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] European Patent No. 3639353 Summary of the Invention [Problem to be solved by the invention]

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

[0007] An object of one aspect of the present disclosure is to provide a power conversion device that is capable of appropriately opening an AC circuit breaker with a simple configuration when a ground fault occurs between a power converter and a transformer. [Means for solving the problem]

[0008] A power converter according to an embodiment includes 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 that are connected to the DC circuit, and a plurality of converter cells that are 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 that is connected to the plurality of switching elements. The power converter further includes a resistor provided between the first DC terminal and ground or between the second DC terminal and ground. When a ground fault occurs in an AC line that connects the transformer and the power converter, the control device controls all switching elements included in the power converter to an off state. The resistor has a resistance value that suppresses a fault current that occurs in response to a ground fault, thereby causing an AC current flowing through the AC circuit breaker to cross zero within a specified time. Effect of the Invention

[0009] According to the present disclosure, when a ground fault occurs between a power converter and a transformer, an AC circuit breaker can be appropriately opened with a simple configuration. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 shows an example of a configuration of a power control system according to a first embodiment. [Diagram 2] FIG. 1 is a diagram illustrating a configuration example of a power conversion device. [Diagram 3] FIG. 2 is a circuit diagram illustrating an example of a converter cell. [Figure 4] FIG. 2 is a block diagram showing an example of a hardware configuration of a control device. [Diagram 5] 4 is a diagram showing a path of a fault current that flows when a ground fault occurs according to the first embodiment. FIG. [Figure 6] FIG. 13 is a diagram showing changes over time in arm current and AC current in a configuration in which a resistor is not provided. [Figure 7] FIG. 13 is a diagram showing a change in AC current over time in a configuration in which a resistor is provided. [Figure 8] 1 is an equivalent circuit for explaining a method for determining the resistance value of a resistor. [Figure 9] 10 is a diagram for explaining the time from output of an opening command to an AC circuit breaker to current interruption. FIG. [Figure 10] FIG. 11 is a diagram showing another example of a path of a fault current that flows when a ground fault occurs. [Figure 11] FIG. 11 shows an example of a configuration of a power control system according to a second embodiment. [Figure 12] FIG. 11 is a diagram showing a path of a fault current that flows when a ground fault occurs according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the present embodiment will be described with reference to the drawings. In the following description, the same components are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed description thereof will not be repeated.

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

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

[0014] The power conversion device 100 includes a power converter 2, a control device 3 that controls the power converter 2, and a resistor 17. The power conversion device 110 includes a power converter 20 and a control device 30 that controls the power converter 20. In the example of Fig. 1, the power conversion device 110 does not include a resistor. However, the power conversion device 110 may be configured to include a resistor having a function similar to that of the resistor 17.

[0015] The power converter 2 is provided between the AC system 12 and the DC lines 14P, 14N. The power converter 20 is provided between the DC lines 14P, 14N and the AC system 120. Specifically, a DC line 14P, which is a positive bus of the DC transmission system, is connected to DC terminals on the positive pole side of the power converters 2, 20. A DC line 14N, which is a negative bus of the DC transmission system, is connected to DC terminals on the negative pole side of the power converters 2, 20.

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

[0017] The power converter 20 is configured as a self-excited power converter. For example, the power converter 20 is configured as a modular multilevel converter including a plurality of converter cells connected in series with each other. A "converter cell" is also called a "submodule" or a "unit converter." The power converter 20 is also configured as a self-excited power converter.

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

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

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

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

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

[0023] In the example of Fig. 1, the power converter 2 includes a plurality of leg circuits 4u, 4v, and 4w corresponding 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, collectively referred to as "leg circuits 4" when referring to any one of them) connected in parallel to each other 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] AC terminals Nu, Nv, and Nw provided in the leg circuits 4u, 4v, and 4w, respectively, are connected to an AC system 12 via a transformer 13. In order to facilitate illustration, the connection between the AC terminals Nv, Nw and the transformer 13 is not shown in FIG.

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

[0026] In this manner, the leg circuit 4 is composed of the positive arm 5 and the negative arm 6 connected in series. The connection points (e.g., AC terminals Nu, Nv, Nw) of the positive arm 5 and the negative arm 6 are connected to the AC line of the corresponding phase of the AC system 12. Both ends (i.e., the positive DC terminal Np and the negative DC terminal Nn) of the positive arm 5 and the negative arm 6 connected in series 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. The leg circuits 4v and 4w have the same configuration as the leg circuit 4u, and therefore, hereinafter, the leg circuit 4u will be described as a representative example.

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

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

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

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

[0031] 2, the resistor 17 is connected between the DC line 14N serving as a negative bus connected to the negative DC terminal Nn and the ground. However, the resistor 17 may be provided between the DC line 14P serving as a positive bus connected to the positive DC terminal Np and the ground.

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

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

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

[0035] In the following description, switching elements 31p, 31n, 31p1, 31n1, 31p2, and 31n2 are also collectively referred to as switching element 31. Furthermore, the on / off of the semiconductor switching element in switching element 31 is simply described as "on / off of switching element 31."

[0036] 3(a), both terminals of switching element 31n are input / output terminals P1, P2. The voltage across capacitor 32 and zero voltage are output by the switching operations of switching elements 31p and 31n. For example, when switching element 31p is on and switching element 31n is off, the voltage across capacitor 32 is output. When switching element 31p is off and switching element 31n is on, zero voltage is output.

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

[0038] In the following description, an example will be described in which the converter cell 7 is configured as a half-bridge cell shown in Fig. 3(a) and uses a semiconductor switching element and a capacitor as a storage element. However, the converter cell 7 may also be configured as a full-bridge cell shown in Fig. 3(b). Also, a converter cell with a configuration other than those shown above, for example, a converter cell that applies a circuit configuration also called a 1.5 half-bridge configuration in which the switching element 31p2 in Fig. 3(b) is replaced with only a diode, may also be used.

[0039] <Control device hardware configuration> FIG. 4 is a block diagram showing an example of a hardware configuration of a control device. The control device 3 in FIG. 4 is configured based on a computer. Referring to FIG. 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. Furthermore, the control device 3 includes one or more CPUs (Central Processing Units) 74, a RAM (Random Access Memory) 75, and a ROM (Read Only Memory) 76. Furthermore, the control device 3 includes one or more input / output interfaces 77, an auxiliary storage device 78, and a bus 79 that connects the above-mentioned components to each other.

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

[0041] A 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 the signals held in the multiple 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 multiple A / D converters 73, A / D conversion may be performed in parallel on detection signals of multiple 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 memory of the CPU 74. The ROM 76 stores programs and setting values ​​for signal processing. The auxiliary storage device 78 is a non-volatile memory with a larger capacity than the ROM 76, and stores programs and data on detected electric quantity values. The input / output interface 77 is an interface circuit for communication between the CPU 74 and an external device.

[0044] 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 (e.g., ROM 76) of the control device 3. When the processing circuit is dedicated hardware, the processing circuit is configured, for example, by an FPGA, an ASIC, or a combination of these.

[0045] <Operation when a ground fault occurs> Fig. 5 is a diagram showing a path of a fault current that flows when a ground fault occurs according to the first embodiment. The example of Fig. 5 shows current paths 210, 220 that flow when a one-phase ground fault in the U phase (hereinafter also simply referred to as a "U-phase ground fault") occurs at fault point F between transformer 13 and power converter 2. Specifically, fault point F indicates a ground fault point in the AC line connecting the secondary side of transformer 13 and 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 (i.e., at the fault point F) based on the electrical quantities detected by the 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 as input the primary current I1 and secondary current I2 of the transformer 13. Each of the primary current I1 and secondary current I2 is a current that has been subjected to gain matching based on the CT winding ratio and the transformation ratio of the transformer 13, and phase matching based on the winding configuration.

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

[0049] The control device 3 may determine a single-phase ground fault in the AC line connecting the transformer 13 and the power converter 2 using another known fault determination method.

[0050] When it is determined that a U-phase ground fault has occurred, the control device 3 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 for gate-blocking the switching elements 31 of all converter cells 7 included in the power converter 2. After outputting the gate block command, the control device 3 outputs an opening command (e.g., a trip command) to the AC circuit breaker 15. That is, when a U-phase ground fault has occurred, the control device 3 controls all switching elements 31 included in the power converter 2 to the OFF state, and then outputs the opening command.

[0051] Here, the time from when the gate block command is output until the switching element 31 is turned 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 is opened. Therefore, after the occurrence of a U-phase ground fault, in a state in which the switching element 31 is turned off but the AC circuit breaker 15 is not completely opened, a fault current flows as shown by current paths 210 and 220 in FIG. 5.

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

[0053] Here, in a configuration in which resistor 17 is not provided, if a U-phase ground fault occurs at fault point F, the fault current flows through a current path of V-phase negative arm 6v, transformer 13, AC circuit breaker 15, AC system 12, and ground, and a current path of W-phase negative arm 6w, transformer 13, AC circuit breaker 15, AC system 12, and ground. In other words, the fault current cannot be subjected to the current suppression effect of resistor 17, and takes a long time to attenuate.

[0054] Fig. 6 is a diagram showing changes over time in arm current and AC current in a configuration in which a resistor is not provided. Referring to Fig. 6, the vertical axis of graph 500 indicates arm current Ivn of V-phase negative arm 6v and arm current Iwn of W-phase negative arm 6w, and the horizontal axis indicates time. The vertical axis of graph 510 indicates AC currents (i.e., U-phase AC current, V-phase AC current, W-phase AC current) flowing through each of the AC lines of U-phase, V-phase, and W-phase of AC system 12, and the horizontal axis indicates time.

[0055] With reference to graphs 500 and 510, for example, the arm current Ivn and the V-phase AC current are substantially zero in the period "0.109 to 0.112 sec", and the arm current Iwn and the W-phase AC current are substantially zero in the period "0.115 to 0.118 sec".

[0056] On the other hand, the U-phase AC current is not zero in any period, and there is no zero cross 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 opening command is output to the AC circuit breaker 15 to start the opening operation, the current cannot be completely interrupted because the arc between the poles is not extinguished, and the AC circuit breaker 15 may be damaged.

[0057] Fig. 7 is a diagram showing the change over time of AC current in a configuration with resistors. The vertical axis of Fig. 7 shows the AC current flowing through each of the U-phase, V-phase, and W-phase AC lines, and the horizontal axis shows time. The V-phase AC current becomes zero after about 1.073 sec, and the W-phase AC current becomes zero after about 1.100 sec.

[0058] The U-phase AC current also gradually decays and becomes zero at about 1.100 sec. This is because the fault current is suppressed by resistor 17. In the example of FIG. 7, it can be seen that the U-phase AC current crosses zero about 1.5 cycles after the opening command is output to AC circuit breaker 15. This causes the arc between the poles of AC circuit breaker 15 to be extinguished, completing the current interruption.

[0059] <Resistance value determination method> Fig. 8 is an equivalent circuit for explaining a method for determining the resistance value of a resistor. Specifically, the equivalent circuit shown in Fig. 8 is an equivalent circuit based on the current paths 210 and 220 at the time of a U-phase ground fault occurring at the fault point F in Fig. 5.

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

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

[0062] The line voltage Vbvu between the V phase and the U phase is expressed by the following formula (1). Here, "Vs" is the effective value of the line voltage between the V phase and the U phase of the AC system 12, and "θ" is the phase of the line voltage. In the following formula (1), V bvu For example, "V in a formula" should be written in subscript form. bvu " is the same as "Vbvu in the text." This also applies to the formulas described later.

[0063]

number

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

[0065]

number

[0066] When a current flows only between the W phase and the U phase, the following formula (3) is established, assuming that the resistance value of resistor 17 is "Rg". The period during which a current flows only between the W phase and the U phase corresponds to, for example, the period corresponding to "0.109 to 0.112 sec" in FIG. 6. Note that "X" in formula (3) is the reactance represented by formula (4). In formula (4), "Xs" is the reactance of AC system 12, "Xtr" is the reactance of transformer 13, and "Xa" is the reactance of the reactor of each arm of power converter 2 (for example, reactors 9a and 9b in FIG. 2).

[0067]

number

[0068] Here, according to the equivalent circuit, when the voltage Vdvn becomes negative (i.e., Vdvn<0), the V-phase arm current Ivn starts to flow (i.e., current starts to flow in the V-phase). Here, "Vdvn=Vbv-Vnu". Therefore, if the W-phase arm current Iwn becomes zero before "Vbv-Vnu<0" occurs (i.e., before current starts to flow in the V-phase), a zero-cross point also occurs in the U-phase AC current. This is because, in the event of a U-phase earth fault, in consideration of the current paths 210 and 220 in FIG. 5, the arm current Ivn flows in the V-phase and U-phase, and the arm current Iwn flows in the W-phase and 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 arm current Iwn of the W phase is expressed by the following equation (5). Here, “k=Rg / X” and “γ=tan -1 (k)”, “Iwnini” 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]

number

[0071] By appropriately setting the constant k in equation (5), it is possible to make the arm current Iwn zero before the arm current Ivn starts to flow.

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

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

[0074] Therefore, by determining the minimum resistance value Rg of resistor 17 such that k≧0.05 and the relational expression P are satisfied, the U-phase AC current crosses zero within a specified time (for example, within the time from when an opening command is output to AC circuit breaker 15 to when one system cycle has elapsed).

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

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

[0077] Moreover, the resistance value Rg is determined based on the reactance Xs based on 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. In detail, the resistance value Rg is determined to be the minimum value that satisfies k≧0.05 and the relational expression P.

[0078] <Modification> 5, in the case where the resistor 17 is provided between the DC line 14N connected to the negative DC terminal Nn and ground, the fault current at the time of the occurrence of a ground fault flows through the resistor 17 and therefore gradually attenuates. Here, it will be described that the fault current gradually attenuates even in the case where the resistor 17 is connected between the DC line 14P connected to the positive DC terminal Np and ground.

[0079] Fig. 10 is a diagram showing another example of the path of the fault current that flows when a ground fault occurs. The example of Fig. 10 shows current paths 230, 240 that flow when a one-phase ground fault occurs in the U phase at the fault point F between the transformer 13 and the power converter 2. The position of the fault point F is the same as in the example of Fig. 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, 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 occurrence of a U-phase ground fault, in a state in which the switching elements 31 are turned off but the AC circuit breaker 15 is not completely opened, a fault current flows as shown by current paths 230 and 240 in FIG. 10.

[0081] Current path 230 shows the path of the fault current flowing through V-phase positive arm 5v, 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 flowing through W-phase positive arm 5w, 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 therefore gradually attenuated.

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

[0083] <Advantages> According to the first embodiment, in a DC transmission system having a single pole configuration, a fault current that flows when a one-phase ground fault occurs between the transformer 13 and the power converter 2 can be suppressed by providing the 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. This allows the AC current of the AC system 12 to cross zero within a specified time. In addition, since the resistance value of the resistor 17 only needs to be determined to be the minimum resistance value required to satisfy the relational expression P, and no other components (e.g., inductor components) other than the resistor 17 are required to suppress the fault current, the power conversion device 100 can be made smaller and less expensive.

[0084] Embodiment 2 Fig. 11 is a diagram showing an example of a configuration of a power control system according to Embodiment 2. Referring to Fig. 11, the power control system is a system for controlling power in a DC power transmission system with a bipolar configuration.

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

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

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

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

[0089] Fig. 12 is a diagram showing paths of fault currents that flow when a ground fault occurs according to embodiment 2. The example of Fig. 12 shows current paths 310, 320 that flow when a U-phase ground fault occurs at fault point F between transformer 13 and power converter 2 (i.e., the fault point in the AC line connecting the secondary side of transformer 13 and AC terminal Nu).

[0090] As in the first embodiment, 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 opening command to the AC circuit breaker 15. After the occurrence of a U-phase ground fault, in a state in which the switching elements 31 are turned off but the AC circuit breaker 15 is not completely opened, a fault current flows as shown by the current paths 310 and 320 in FIG.

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

[0092] Therefore, similarly to the first embodiment, in the second embodiment, by determining the minimum resistance value Rg of resistor 17 such that k≧0.05 and the relational expression P are satisfied, the U-phase AC current crosses zero within a specified time.

[0093] Here, a case has been described where a ground fault occurs at fault point F between transformer 13 and an upper power converter (for example, power converter 2). In this case, as described above, the current is reduced by the ground resistance of neutral point 19 to which resistor 17 is connected (i.e., the ground resistance of the negative pole of the upper power converter when viewed from the upper power converter). If a ground fault occurs at a fault point between transformer 131 and a lower power converter (for example, power converter 21), the fault current flows through the path described in FIG. 10. Therefore, the fault current is reduced by the ground resistance of neutral point 19 (i.e., the ground resistance of the positive pole of the lower power converter when viewed from the lower power converter).

[0094] <Advantages> According to the second embodiment, in a DC transmission system having a bipolar configuration, it is possible to obtain the same effects as those of the first embodiment. Specifically, by providing a resistor 17 between a neutral point 19 connected to the negative DC terminal of the power converter 2 and the ground, it is possible to suppress a fault current that flows when a one-phase ground fault occurs between the transformer 13 and the power converter 2, and to make the AC current of the AC system 12 cross zero within a specified time.

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

[0096] (2) The configurations exemplified as the above-mentioned embodiments are examples of the configurations of the present disclosure, and may be combined with other known technologies, or may be modified, such as by omitting some parts, without departing from the scope of the present disclosure. In addition, the above-mentioned embodiments may be implemented by appropriately adopting the processes and configurations described in other embodiments.

[0097] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is defined by the claims, not the above description, and is intended to include all modifications within the scope and meaning equivalent to the claims. [Explanation of symbols]

[0098] 2,20,21,22 Power converter, 3,30 Control device, 4u,4v,4w Leg circuit, 5u,5v,5w Positive arm, 6u,6v,6w Negative arm, 7 Converter cell, 9a,9b Reactor, 12,120 AC system, 13,130,131,132 Transformer, 14 DC circuit, 14N,14P DC line, 15,150,151,152 AC circuit breaker, 17 Resistor, 19,191 Neutral point, 31n,31p Switching element, 32 Capacitor, 33 Voltage detector, 51,52 AC bus, 70 Input converter, 71 Sample and hold circuit, 72 Multiplexer, 73 A / D converter, 74 CPU, 75 RAM, 76 ROM, 77 Input / output interface, 78 Auxiliary storage device, 79 Bus, 100, 110 Power converter, 210, 220, 310, 320 Current path.

Claims

1. a power converter that performs power conversion between an AC system and a DC circuit; A control device for controlling the power converter, The power converter includes: connected to the AC system via a transformer and an AC breaker; 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 includes a plurality of switching elements and a capacitor connected to the plurality of switching elements, Further comprising a resistor provided between the first DC terminal and ground or between the second DC terminal and the ground, the control device controls all switching elements included in the power converter to an off state when a ground fault occurs in an AC line connecting the transformer and the power converter; The resistor has a resistance value that suppresses a fault current generated in response to the ground fault, thereby causing the AC current flowing through the AC circuit breaker to cross zero within a specified time.

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

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

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

5. The DC circuit is a DC transmission system having a unipolar configuration, the first DC terminal is connected to a positive busbar of the DC transmission system, and the second DC terminal is connected to a negative busbar of the DC transmission system; 3. The power conversion device according to claim 1, wherein the resistor is provided between the positive bus bar connected to the first DC terminal and the ground, or between the negative bus bar connected to the second DC terminal and the ground.

6. The DC circuit is a bipolar DC transmission system, the first DC terminal is connected to a positive busbar of the DC transmission system, and the second DC terminal is connected to a neutral point of the DC transmission system; 3. The power conversion device according to claim 1, wherein the resistor is provided between the neutral point connected to the second DC terminal and the ground.

7. 3. The power conversion device according to claim 1, wherein the resistance value is determined based on a first reactance based on a 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 a minimum value that satisfies the following formula (1): Rg=k{2(Xs+Xtr)+Xa}...(1) The power converter according to claim 7.

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