DC Circuit Breaker

Abstract

The DC circuit breaker (100) connected in series with the DC reactor (25) to the electric line (30) includes N (N is an integer of 2 or more) circuit breaker units (101 to 10N) connected in series, and a controller (105). Each circuit breaker unit (101 to 10N) includes a circuit breaker and a lightning arrester connected in parallel. When the controller (105) detects a break command (Strp) for the DC circuit breaker (100), it generates opening commands (S1 to SN) for M (M is an integer of 1 to N) circuit breaker units out of the N circuit breaker units (101 to 10N). The value of M is variably set based on at least one of the voltages on the electric line side and the bus bar side of the series circuit (150) of the DC reactor (25) and the N circuit breaker units (101 to 10N).

Description

[Technical field]

[0001] The present disclosure relates to a direct current circuit breaker. [Background technology]

[0002] Multi-terminal high-voltage direct current (HVDC) transmission is attracting attention due to the rapid expansion of offshore wind power generation worldwide and the need for long-distance power transportation from generation areas to demand areas. In multi-terminal HVDC transmission, in the event of a system fault, the faulted line must be separated from healthy lines by a direct current circuit breaker (DCCB) to prevent a large-scale blackout.

[0003] For example, JP 62-113326 A (Patent Document 1) describes the configuration of a DC circuit breaker in which multiple commutation switches (circuit breakers) are connected in series and a nonlinear resistor whose main component is zinc oxide or the like is connected in parallel with each commutation switch.

[0004] According to the DC circuit breaker of Patent Document 1, by connecting a plurality of commutation switches in series, it is possible to increase the interruptable voltage (withstand voltage) and improve suitability for high voltage applications such as HVDC power transmission. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 113326 / 1983 Summary of the Invention [Problem to be solved by the invention]

[0006] Patent Document 1 describes that, when the DC circuit breaker performs a breaking operation, each of a plurality of commutation switches is opened almost simultaneously, and that a nonlinear resistor connected in parallel limits the overvoltage generated between the poles of each open commutation switch, thereby configuring a DC circuit breaker that can break without causing insulation breakdown.

[0007] However, in multi-terminal HVDC systems, a configuration in which a DC reactor is connected in series with a DC circuit breaker is sometimes adopted in order to mitigate the slope of the fault current that rises suddenly in the event of a ground fault or short circuit. When the DC circuit breaker performs an interruption operation, there is a risk that a high voltage will be generated in the peripheral devices, including the DC circuit breaker and the DC reactor, due to the voltage suppressed by the lightning arrester, which is a component of the DC circuit breaker. This raises concerns that the equipment will become larger in size to ensure the ground insulation distance, and that costs will increase due to the need to make the component devices withstand higher voltages.

[0008] The present disclosure has been made to solve such problems, and the purpose of the present disclosure is to suppress the voltage to ground of a DC circuit breaker and overvoltages generated in peripheral equipment during circuit breaking operation in a DC circuit breaker having a configuration in which multiple circuit breaker units are connected in series. [Means for solving the problem]

[0009] In one aspect of the present invention, a DC circuit breaker is provided. The DC circuit breaker is connected in series with a DC reactor to an electric line and includes N circuit breaker units (N is an integer of 2 or more) connected in series, first and second voltage detectors, and a controller. Each circuit breaker unit includes a circuit breaker and a lightning arrester connected in parallel. The first and second voltage detectors measure the voltages on the bus side and the electric line side of a series circuit of the DC reactor and the N circuit breaker units. The controller generates an opening command for the circuit breakers of the N circuit breaker units. When the controller detects a breaking command for the DC circuit breaker, the controller opens the circuit breakers of M (M is an integer of 1 to N) circuit breaker units that are variably set among the N circuit breaker units, based on at least one of the bus side voltage and the electric line side voltage of the series circuit. Effect of the Invention

[0010] According to the present disclosure, in a DC circuit breaker having a configuration in which multiple circuit breaker units are connected in series, the number of the multiple circuit breaker units that are opened can be variably controlled, thereby suppressing the ground voltage of the DC circuit breaker and overvoltages generated in its peripheral devices during circuit breaking operation. [Brief description of the drawings]

[0011] [Figure 1] 1 is a conceptual diagram illustrating a configuration example of a multi-terminal HVDC power transmission system shown as an application example of a DC circuit breaker according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a conceptual diagram illustrating a configuration example of an HVDC power transmission system to be compared with FIG. 1. [Diagram 3] FIG. 1 is a schematic circuit diagram showing a basic configuration of a DC circuit breaker. [Figure 4] 4 is an example of an operation waveform diagram of the DC circuit breaker shown in FIG. 3. [Diagram 5] FIG. 3 is a conceptual diagram illustrating an example of the current-voltage characteristics of a lightning arrester. [Figure 6] FIG. 1 is a schematic circuit diagram showing the configuration of a high-voltage DC circuit breaker. [Figure 7A] FIG. 7 is a circuit diagram of the DC circuit breaker shown in FIG. 6 in a closed state. [Figure 7B] FIG. 7 is a circuit diagram of the DC circuit breakers shown in FIG. 6 in a fully open state. [Figure 7C] FIG. 7 is a circuit diagram of the DC circuit breaker shown in FIG. 6 in a partially open state. [Figure 8] FIG. 13 is an example of an operational waveform diagram of a cutoff operation according to a comparative example. [Figure 9] 6 is an example of an operational waveform diagram of a cutoff operation according to the present embodiment; [Figure 10] 1 is a block diagram illustrating a configuration of a DC circuit breaker according to an embodiment of the present invention. [Figure 11] 4 is a flowchart illustrating a breaking operation of the DC circuit breaker according to the present embodiment. [Figure 12] 11 is a first conceptual diagram illustrating an example of setting the number of DC circuit breakers to be opened when cutting off a positive electrode of the DC circuit breaker shown in FIG. 10. FIG. [Figure 13] 11 is a second conceptual diagram illustrating an example of setting the number of DC circuit breakers to be opened when cutting off a positive pole of the DC circuit breaker shown in FIG. 10. FIG. [Figure 14] 11 is a conceptual diagram for explaining an example of setting the number of open circuit breakers when the DC circuit breaker shown in FIG. 10 cuts off a negative pole. FIG. [Figure 15A] FIG. 1 is a first diagram illustrating a variation of the DC circuit breaker according to the present embodiment. [Figure 15B] FIG. 2 is a second diagram illustrating a variation of the DC circuit breaker according to the present embodiment. [Figure 15C] FIG. 11 is a third diagram illustrating a variation of the DC circuit breaker according to the present embodiment. [Figure 15D] FIG. 4 is a fourth diagram illustrating a variation of the DC circuit breaker according to the present embodiment. [Figure 16] FIG. 15D is a conceptual diagram for explaining an example of setting the number of open circuit breakers in FIG. 15C. [Figure 17] FIG. 15E is a first conceptual diagram illustrating an example of setting the number of open circuit breakers in FIG. 15D. [Figure 18] FIG. 15E is a second conceptual diagram illustrating an example of setting the number of open circuit breakers in FIG. 15D. [Figure 19] 11 is a conceptual diagram illustrating details of a process for detecting a current flow direction during an interruption operation. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the following, the same or corresponding parts in the drawings are denoted by the same reference characters, and their description will not be repeated in principle.

[0013] (Example of application of DC circuit breaker) FIG. 1 is a conceptual diagram illustrating a configuration example of a multi-terminal power transmission system shown as an application example of a DC circuit breaker according to the present embodiment.

[0014] 1, a multi-terminal HVDC power transmission system 1A includes HVAC systems 10A-10D for transmitting and receiving power, AC / DC converters 20A-20D for converting power, and electric power lines 31-34 for transmitting DC power. Each of the electric power lines 31-34 may be formed of a cable or an overhead power line.

[0015] Electric line 31 connects AC / DC converters 20A and 20B, and electric line 32 connects AC / DC converters 20A and 20C. Similarly, electric line 33 connects AC / DC converters 20C and 20D, and electric line 34 connects AC / DC converters 20B and 20D.

[0016] The multi-terminal HVDC power transmission system 1A is provided with AC circuit breakers (ACCBs) 15A-15D and DC circuit breakers 100A-100H for cutting off power when an accident occurs. The AC circuit breakers 15A-15D are connected between the HVAC systems 10A-10D and the AC / DC converters 20A-20D, respectively.

[0017] The DC circuit breakers 100A and 100B are arranged at both ends of the electric line 31. Specifically, the DC circuit breaker 100A is arranged between the AC / DC converter 20A and the electric line 31, and the DC circuit breaker 100B is arranged between the AC / DC converter 20B and the electric line 31. Similarly, the DC circuit breakers 100C and 100D are arranged at both ends of the electric line 32, the DC circuit breakers 100E and 100F are arranged at both ends of the electric line 33, and the DC circuit breakers 100G and 100H are arranged at both ends of the electric line 34.

[0018] In this way, the multi-terminal HVDC power transmission system 1A forms a power transmission network by connecting three or more power converters 20 (collectively referring to the AC / DC converters 20A to 20D) with multiple electric lines. In addition, in many cases, DC reactors (not shown) are interposed and connected to each of the electric lines 31 to 34 that transmit DC power in order to mitigate a sudden rise in fault current when a ground fault or short circuit occurs in the system. In this case, the DC reactors are connected in series to each of the DC circuit breakers 100 (collectively referring to 100A to 100G).

[0019] FIG. 2 shows an example of the configuration of a two-terminal HVDC power transmission system 1B, which is compared with the multi-terminal HVDC power transmission system 1A of FIG.

[0020] 2, the two-terminal HVDC power transmission system 1B forms a power transmission network by connecting two AC / DC converters 20A and 20B with an electric line 30X. When transmitting AC power from the HVAC system 10A to the HVAC system 10B, the AC / DC converter 20A converts the DC power output to the electric line 30X into AC power and outputs the AC power to the HVAC system 10B.

[0021] In the two-terminal HVDC power transmission system 1B, AC circuit breakers 15A and 15B are disposed between the HVAC systems 10A and 10B and the AC / DC converters 20A and 20B, respectively. When a fault current is interrupted in the event of an accident or a load current is interrupted during maintenance, the AC circuit breakers 15A and 15B interrupt the current and also stop the AC / DC converters 20A and 20B. For example, in the two-terminal HVDC power transmission system 1B, if an accident such as a ground fault occurs in the electric line 30X, the AC circuit breakers 15A and 15B are opened and power transmission through the electric line 30X is stopped, resulting in a complete power outage.

[0022] Referring again to FIG. 1, in the multi-terminal HVDC power transmission system 1A, since DC circuit breakers 100 are arranged at both ends of each of the electric lines 31 to 34, it is possible to disconnect only the electric line in which a fault has occurred from the power transmission network.

[0023] For example, if an accident occurs in the power line 32, the power line 32 can be disconnected from the power grid by opening the DC circuit breakers 100C and 100D. This allows the power lines 31, 33, and 34 to transmit power normally, making it possible to avoid a large-scale power outage that would affect the entire system as shown in Fig. 2. On the other hand, the DC circuit breaker 100 is required to have a high-voltage interruption capability compatible with HVDC transmission.

[0024] (Basic configuration and operation of DC circuit breaker) FIG. 3 shows a schematic circuit diagram illustrating a basic configuration of a DC circuit breaker, and FIG. 4 shows an example of an operating waveform diagram of the DC circuit breaker shown in FIG.

[0025] The DC circuit breaker 100 has as its basic element a circuit breaker unit 100U shown in Fig. 3. The circuit breaker unit 100U includes a circuit breaker 110 and a lightning arrester 120 connected in parallel.

[0026] 3, during steady state when the circuit breaker 110 is in a closed state, the line current Iline to be cut off flows through a path P1. Unlike an AC current, a DC current does not form periodic current zero points according to the passage of time, so it is necessary to form current zero points in the cutoff section by a method such as a separate excitation method or a semiconductor method.

[0027] In the example of Fig. 4, when a break command is issued at time ta, the breaker 110 is opened at time tb when a current zero point is formed by any of the methods. Before time ta and between times ta and tb, the line current Iline flows through the path P1 in Fig. 3.

[0028] After the circuit breaker 110 is opened to interrupt the current, the electromagnetic energy remaining in the circuit is applied as a transient interruption voltage (TIV) between the poles of the open circuit breaker 110. Fig. 4 shows schematic waveform diagrams of Vunit, which corresponds to the voltage between the poles of the circuit breaker 110, and Wmr, the energy dissipated by the lightning arrester 120, on the same time axis.

[0029] In the following, the closed state of the circuit breaker 110 will also be referred to as the closed state of the circuit breaker unit 100U, and the open state and opening of the circuit breaker 110 will also be referred to as the open state and opening of the circuit breaker unit 100U.

[0030] 5 shows an example of the current-voltage characteristic of the lightning arrester 120. The lightning arrester 120 corresponds to the nonlinear resistor in Patent Document 1. Vunit in FIG. 4 is applied between the poles of the circuit breaker 110 in the open state according to the current-voltage characteristic of the lightning arrester 120 shown in FIG.

[0031] Then, when the voltage applied between the poles of the circuit breaker 110 reaches a predetermined limit voltage Vlim, the circuit current is commutated to path P2 in Fig. 3. The limit voltage Vlim, which corresponds to the maximum value of the TIV after the circuit breaker 110 is opened, is generally set to about 1.5 times the multi-terminal HVDC system voltage. For example, if the multi-terminal HVDC system voltage (grid voltage) is 1.0 [pu] = 320 "kV", the limit voltage Vlim is generally set to about 1.5 [pu] = 480 "kV".

[0032] As a result, by making the inter-contact voltage of the circuit breaker 110 higher than the system voltage, the line current Iline flowing through the lightning arrester 120 is limited, and the current interruption by opening the DC circuit breaker 100 is completed.

[0033] Representative cases in which the DC circuit breaker 100 is opened include fault current interruption during an accident and load current interruption during maintenance. Fig. 5 also shows an operating point A of the lightning arrester 120 when interrupting a load current and an operating point B of the lightning arrester 120 when interrupting a fault current. It can be seen that there is no significant difference in the TIV generated in the lightning arrester 120 between when interrupting a load current and when interrupting a fault current, although the interrupting current differs.

[0034] FIG. 6 shows an example of a configuration for increasing the voltage of a DC circuit breaker. For example, the DC circuit breaker 100 can be configured to operate at a higher voltage by including N (N: an integer of 2 or more) circuit breaker units 101-10N connected in series. Each of the circuit breaker units 101-10N is configured in the same manner as the circuit breaker unit 100U shown in FIG.

[0035] For example, by connecting four 80 kV rated circuit breaker units in series (N=4), a DC circuit breaker 100 applicable to a 320 kV rated HVDC system can be configured. In the present embodiment, the DC circuit breaker 100 includes N circuit breaker units 101-10N connected in series. Furthermore, the following description will be given assuming that N times the rated voltage of each of the circuit breaker units 101-10N corresponds to the system voltage, i.e., 1.0 pu, and that N times the limit voltage Vlim of each of the circuit breaker units 101-10N corresponds to 1.5 pu of the system voltage.

[0036] FIG. 7A shows a circuit diagram of the DC circuit breaker 100 in FIG. 6 in a closed state, that is, in a steady state before a break command is generated.

[0037] 7A, a DC reactor 25 is connected between N circuit breaker units 101-10N and power converter 20. That is, DC circuit breaker 100 is connected in series to DC reactor 25 by being inserted into electric lines 30 (collectively referring to electric lines 31-34 in FIG. 1) that connect power converters 20 together.

[0038] In a steady state, each of the breaker units 101 to 10N is in a closed state. Therefore, the current in the electric line 30 is not interrupted, and a line current Iline flows through the electric line 30. In the example of Fig. 7A, the direction of the current flowing from the power converter 20 to the electric line 30 is set to the positive direction (Iline>0).

[0039] At this time, the voltage Vcb between contacts of the entire DC circuit breaker 100 is zero. The voltage Vcb between contacts corresponds to the sum of the voltages between contacts in each of the circuit breakers 110 of the N circuit breaker units 101 to 10N.

[0040] As will be described in detail later, in the DC circuit breaker 100 of this embodiment, in a circuit breaking operation in response to generation of a circuit breaking command, M (M: an integer 1≦M≦N) circuit breaker units, which are variably set, out of the N circuit breaker units 101 to 10N are opened.

[0041] Fig. 7B shows a circuit state indicating a total cut-off state due to a cut-off operation in response to a cut-off command of DC circuit breaker 100. That is, Fig. 7B shows a state where M=N.

[0042] In a state where M=N, all of the N circuit breaker units 101 to 10N are opened in response to a circuit breaker command of the DC circuit breaker 100. As a result, the inter-contact voltage Vcb of the DC circuit breaker 100 becomes N times the limiting voltage Vlim (FIG. 4) of the circuit breaker unit 100U (FIG. 3).

[0043] Fig. 7C shows a circuit state illustrating a partial cutoff state due to the cutting operation of DC circuit breaker 100. Fig. 7C particularly shows a state where M=1.

[0044] In the state of M=1, in response to a break command of the DC circuit breaker 100, only one circuit breaker unit 101 is opened, and the circuit breaker units 102 to 10N are maintained in a closed state. As a result, the inter-contact voltage Vcb of the DC circuit breaker 100 becomes 1 time the limit voltage Vlim (FIG. 4) of the circuit breaker unit 100U (FIG. 3).

[0045] As commonly shown in Figures 7A to 7C, in the following, with regard to the voltages of each part, for both ends of the series circuit 150 of the DC circuit breaker 100 (N circuit breaker units 101 to 10N) and the DC reactor 25, the voltage to ground at a first end on the power line 30 side will be referred to as line voltage Vl, and the voltage to ground at a second end on the power converter 20 side, i.e., the bus side, will be referred to as bus voltage Vb.

[0046] Furthermore, among both ends of the DC circuit breaker 100 (N circuit breaker units 101 to 10N), the voltage to ground at a first end (node ​​Nx) connected to the DC reactor 25 is referred to as a voltage Vx.

[0047] (Comparison example of DC circuit breaker interruption operation) Next, a comparative example of the interruption operation of the DC circuit breaker 100 will be described. In this comparative example, all of the N circuit breaker units 101 to 10N are simultaneously opened in response to a circuit breaker command as described in Patent Document 1. That is, when the DC circuit breaker 100 detects a circuit breaker command, it executes a circuit breaker operation so that the all-unit open state shown in Fig. 7B is formed in any case.

[0048] An example of an operational waveform diagram of an interruption operation according to a comparative example is shown in Fig. 8. Fig. 8(a) shows an example of an operational waveform diagram when a fault current is interrupted.

[0049] 8(a), at time t0 in the steady state, the bus voltage Vb and the line voltage Vl are 1.0 [pu] equivalent to the rated value of the system voltage transmitted by the power line 30, and the DC circuit breaker 100 is in the steady state shown in FIG. 7A, so that all of the N circuit breaker units 101 to 10N are in the closed state. Therefore, the inter-contact voltage Vcb of the DC circuit breaker 100 is 0, and the voltage Vx is 1.0 [pu]. Furthermore, the line current Iline has converged to a steady value, and the reactor voltage Vdcr generated across the DC reactor 25 is 0.

[0050] At time tx after time t0, a ground fault occurs in the power line 30. In response to this, the bus voltage Vb on the non-fault side is maintained at 1.0 [pu], while the line voltage Vl on the fault side decreases. In addition, the line current Iline increases from the steady state.

[0051] At this time, since the DC circuit breaker 100 remains in the state shown in Fig. 7A, the voltage Vx also drops similarly to the line voltage Vl. In response to this, the voltage difference between the bus voltage Vb and the line voltage Vl (voltage Vx) is applied across the DC reactor 25, so the reactor voltage Vdcr rises to 1.0 [pu].

[0052] When a ground fault that occurs at time tx is detected by an overcurrent detection relay (not shown) or the like, a tripping command is generated for the DC circuit breaker 100. In response to the tripping command at time t1, the DC circuit breaker 100 creates the all-open state shown in Fig. 7B. That is, all of the N circuit breaker units 101 to 10N are opened.

[0053] In response to this, the inter-contact voltage Vcb of the DC circuit breaker 100 rises to about 1.5 [pu], which is the sum of the limit voltages Vlim (about 1.5 times the rated value) of the circuit breaker units 101 to 10N, and the circuit voltage becomes higher than the system voltage. In other words, Vlim·N is about 1.5 [pu]. As a result, the line current Iline is commutated to the lightning arrester 120 and gradually decreases, and finally the fault current is interrupted (Iline=0). At this time, Vx rises to about 1.5 [pu] (Vx=Vl+Vcb) with respect to Vl=0, and the reactor voltage Vdcr becomes about -0.5 [pu].

[0054] 8(b) shows an example of an operational waveform diagram when a load current is interrupted by an interruption operation according to a comparative example. For example, in a situation where no accident has occurred, a case where the load current is interrupted occurs when an interruption command for maintenance is generated for the DC circuit breaker 100.

[0055] 8(b), when the load current is interrupted, since no ground fault has occurred, both the bus voltage Vb and the line voltage Vl are about 1.0 [pu], and a disconnection command is input to the DC circuit breaker 100 from a state in which the voltage difference between the two is small. In the comparative example, the DC circuit breaker 100 performs a disconnection operation in which the fully open state shown in FIG. 7B is formed even when the load current is interrupted.

[0056] Therefore, at time t1, all of the N circuit breaker units 101 to 10N are opened in response to the tripping command, as in Fig. 8(a) (Fig. 7B). As a result, the inter-contact voltage Vcb of the DC circuit breaker 100 rises to approximately Vlim·N=1.5 [pu], as in Fig. 8(a), and therefore rises to approximately Vx=2.5 [pu] (Vx=Vl+Vcb) while the line voltage Vl is approximately 1.0 [pu]. As a result, the reactor voltage Vdcr becomes approximately -1.5 [pu] in accordance with the voltage difference between the voltage Vx and the bus voltage Vb.

[0057] In this way, in the comparative example, by forming a fully open state (FIG. 7B) when the load current is interrupted, the voltage Vx rises to about 2.5 [pu], which is higher than 1.5 [pu] when the fault current is interrupted. Furthermore, the voltage difference applied across the DC reactor 25 also rises from 0.5 [pu] to 1.5 [pu].

[0058] Since voltage Vx corresponds to the voltage to ground of DC circuit breaker 100 when current is interrupted, there are concerns that the equipment will become larger and more expensive in order to ensure the insulation distance to ground to accommodate an increase in voltage Vx (voltage to ground) and to accommodate an increase in the withstand voltage of DC reactor 25.

[0059] (Interruption Operation of DC Circuit Breaker According to the Present Embodiment) In order to solve the above problem, in the interruption operation of the DC circuit breaker according to this embodiment, the number of circuit breaker units to be opened (the number of openings M) among the N circuit breaker units 101 to 10N is variably controlled. Specifically, when a circuit breaker command is generated, when the voltage applied across the series circuit 150 is large, M is controlled to be large, whereas when the voltage is small, M is controlled to be small.

[0060] FIG. 9 shows an example of the operation waveform diagram of the interruption operation according to the present embodiment. FIG. 9(a) shows the operation waveform diagram at the time of interrupting the fault current similar to FIG. 8(a). At the time of fault interruption, since the line voltage Vl is decreasing at the time of generating the interruption command in response to the occurrence of the fault at time tx, the voltage across the series circuit 150, that is, the voltage difference between the bus voltage Vb and the line voltage Vl is large.

[0061] Therefore, at the time of fault interruption, control according to the interruption command is executed so that M = N, that is, all of the N circuit breaker units 101 to 10N are opened. Therefore, the waveform diagram after time tx in FIG. 9(a) is the same as that in FIG. 8(a). As described with reference to FIG. 8(a), even if all of the N circuit breaker units 101 to 10N are opened, the maximum value of Vx is about 1.5 [pu], and the absolute value of the reactor voltage Vdcr does not exceed 1.0 [pu].

[0062] FIG. 9(b) shows the operation waveform diagram at the time of interrupting the load current similar to FIG. 8(b). At the time of load current interruption, since the line voltage Vl hardly decreases at the time of generating the interruption command for maintenance or the like, the voltage across the series circuit 150, that is, the voltage difference between the bus voltage Vb and the line voltage Vl is small. In such a case, if control is performed with M = N, as shown in FIG. 8(b), the voltage Vx increases and the absolute value of the reactor voltage Vdcr increases as compared with the case of fault interruption.

[0063] Therefore, at the time of load current interruption, the interruption operation according to the interruption command is executed so that M < N and only a part of the N circuit breaker units 101 to 10N are opened. FIG. 9(b) shows the waveform diagram when M = 1, that is, when the circuit state shown in FIG. 7C is formed in response to the interruption command.

[0064] 9(b), at time t1, only one breaker unit 101 is opened (M=1), so that the inter-contact voltage Vcb of the DC breaker 100 becomes the limit voltage Vlim for one breaker unit 100U, and Vcb in Figures 8(a) and 9(a) becomes Vlim·N. That is, Vcb=Vlim in Figure 9(b) is suppressed to (1 / N) times Vlim·N, which corresponds to 1.5 [pu".

[0065] Therefore, at time t1, the voltage Vx increases only by about Vlim·M (here, M=1). Also, the reactor voltage Vdcr decreases only by about Vlim·M (here, M=1).

[0066] As a result, it can be seen that the increase in voltage Vx and the increase in the voltage difference applied to both ends of DC reactor 25 are suppressed in Fig. 9(b) compared to Fig. 8(b). This makes it possible to ensure the ground insulation distance and suppress the increase in size and cost of the equipment required to deal with the increase in the withstand voltage of DC reactor 25.

[0067] FIG. 10 is a block diagram illustrating a configuration example of a DC circuit breaker according to this embodiment.

[0068] 10, a DC circuit breaker 100 according to the present embodiment includes N circuit breaker units 101 to 10N connected in series, a controller 105, voltage detectors 116b, 116x, 116l, and a current detector 117.

[0069] As described above, the N breaker units 101-10N are connected in series with the DC reactor 25 to form the series circuit 150. The line voltage Vl and the bus voltage Vb at both ends of the series circuit 150 are measured by the voltage detectors 116l and 116b, respectively. Furthermore, the voltage Vx at the connection node between the N breaker units 101-10N and the DC reactor 25 is measured by the voltage detector 116x. By measuring the voltage Vx, the reactor voltage Vdcr can be measured from the voltage difference with the bus voltage Vb.

[0070] Each of the voltage detectors 116b, 116x, 116l is configured, for example, by a voltage dividing circuit with a known voltage division ratio. The controller 105 can obtain the bus voltage Vb, the line voltage Vl, and the voltage Vx by using the divided voltages (measured values) Vb#, Vx#, Vl# by the voltage detectors 116b, 116x, 116l and the voltage division ratios. That is, the voltage detectors 116b, 116l correspond to an embodiment of the "first voltage detector" and the "second voltage detector".

[0071] The current detector 117 can detect the magnitude and direction of the line current Iline. For example, the current detector 117 can be configured with a shunt resistor or the like.

[0072] The controller 105 can generate opening commands S1 to SN for the circuit breaker units 101 to 10N. The circuit breaker units 101 to 10N are maintained in a closed state when the opening commands S1 to SN are not generated, and are opened in response to the generation of the opening commands S1 to SN. When a break command Strp for the DC circuit breaker 100 is received, the generation of the opening commands S1 to SN is controlled using voltage and current information at the time of detection of the break command. That is, the number of openings M can be variably controlled depending on how many of the opening commands S1 to SN are generated. Since the contact voltage Vcb when the DC circuit breaker 100 is broken is expressed by Vcb=M·Vlim, the contact voltage Vcb is variably controlled depending on the number of openings M.

[0073] 11 is a flow chart for explaining the interruption operation of the DC circuit breaker according to the present embodiment. The control process shown in FIG.

[0074] In step (hereinafter simply referred to as "S") 100, the controller 105 judges whether or not a shutoff command has been detected. For example, when an external overcurrent relay (not shown) detects the occurrence of an accident, or when a shutoff command for maintenance is manually input, a shutoff command Strp is input to the controller 105, and a YES judgment is made in S100. Alternatively, the controller 105 may generate the shutoff command itself based on the line current Iline, etc. If a shutoff command is not detected, the processing from S110 onwards is not started.

[0075] When a trip command is detected (YES in S100), the controller 105 acquires current and voltage information of each part in S110. For example, the bus voltage Vb, the line voltage Vl, the voltage Vx, and the line current Iline are acquired from the measured values ​​of the voltage detectors 116b, 116x, and 116l and the current detector 117 shown in FIG.

[0076] In addition, in S110, filtered smoothed values ​​may be obtained for each of the bus voltage Vb, the line voltage Vl, the voltage Vx, and the line current Iline.

[0077] In S120, the controller 105 detects the current direction to be interrupted (i.e., the current flow direction during the interruption operation) based on the direction of the line current Iline acquired in S110. In step S120, if the direction of the line current Iline is from the power converter 20 to the electric power line 30 (Iline>0), the current flow direction is detected as the line direction. For example, if an accident occurs in the electric power line 30, the current flow direction is detected as the line direction.

[0078] Conversely, when the direction of the line current Iline is from the electric power line 30 to the power converter 20 (Iline<0), the current flow direction is detected as the bus direction. For example, when an accident occurs on the bus side (the power converter 20 side), the current flow direction is detected as the bus direction.

[0079] 8(b) and 9(b), the direction of the line current Iline changes depending on whether a forward current or a reverse current is occurring. Therefore, S120 detects whether the current is flowing in the line direction or in the bus direction depending on the direction of the line current Iline.

[0080] Furthermore, the controller 105 sets the number of openings N in the breaking operation based on the bus voltage Vb and the line voltage Vl in S200. At this time, the logic for determining the number of openings M is switched depending on the current direction in the breaking operation. S200 has S210, S220, and S230.

[0081] In S210, the controller 105 branches the process depending on the current flow direction at the time of the breaking operation detected in S120. Specifically, when the current flow direction is the line direction, a YES determination is made in S210, and the process proceeds to S220 for positive electrode breaking to break the current toward the electric power line 30. On the other hand, when the current flow direction is the bus direction, a YES determination is made in S210, and the process proceeds to S230 for negative electrode breaking to break the current toward the bus.

[0082] In S220, the controller 105 determines the number of openings M according to logic for positive electrode interruption. An example of setting the number of openings (M) for positive electrode interruption will be described with reference to Fig. 12 and Fig. 13. In the positive electrode interruption of the DC circuit breaker in Fig. 10, a current (Iline) in a direction from the DC reactor 25 to the DC circuit breaker, with the DC reactor 25 on the upstream side, is interrupted, so it is preferable to determine the number of openings M taking into consideration not only the rise in voltage Vx but also the rise in reactor voltage Vdcr (Vdcr = Vb - Vx) applied between the terminals of the DC reactor 25.

[0083] Fig. 12 is a first conceptual diagram for explaining an example of setting the number (M) of openings when interrupting the positive pole of the DC circuit breaker shown in Fig. 10. Fig. 12 shows an example of setting the number (M) of openings to suppress an excessive rise in voltage Vx.

[0084] On the horizontal axis of FIG. 12, the line voltage Vl is shown, and on the vertical axis, the value after the rise of the voltage Vx after the opening of M circuit breaker units is shown. The voltage Vx on the vertical axis of FIG. 12 corresponds to the voltage Vx (maximum value) at time t1 in FIGS. 8(a), 8(b), 9(a), and 9(b).

[0085] In the DC circuit breaker 100, considering the current direction during positive pole interruption, there is a relationship of Vx = Vl + Vcb between the reactor voltage Vdcr of the DC reactor 25, the line voltage Vl, the bus voltage Vb, and the pole-to-pole voltage Vcb (absolute value) generated during the interruption operation.

[0086] In FIG. 12, when N = 4, characteristic lines CLA1 to CLA4 showing the voltage Vx when the number of opened units M = 1 to 4 are plotted. Each of the characteristic lines CLA1 to CLA4 is represented by a linear function of the line voltage Vl with the product of the limiting voltage Vlim (FIG. 4) for one circuit breaker unit and the number of opened units M as the intercept. That is, the voltage Vx(M) when the number of opened units is M is expressed by the following formula (1), considering that the magnitude of Vcb is M·Vlim.

[0087] Vx(M)=Vl+Vcb=Vl+M·Vlim …(1) Therefore, according to the intersection points of the design upper limit voltage Vmax of the voltage Vx and the characteristic lines CLA1 to CLA4, within the range where Vx(M) < Vmax is satisfied, the number of opened units M can be determined so that M becomes the maximum value. The design upper limit voltage Vmax is, for example, about 1.5 [pu]. Therefore, in the following, a schematic characteristic line is shown with Vlim = (1.5 / 4) [pu]. The design upper limit voltage Vmax corresponds to an embodiment of the "first allowable upper limit voltage".

[0088] As shown in Fig. 12, when Vx = Vmax and the x - coordinates of the intersections of the characteristic lines CLA4 to CLA1 according to Equation (1) with the line voltage Vl, that is, the values of the line voltage Vl are V0, V1, V2, and V3 respectively, M can be set to 4 in the voltage range of 0 ≦ Vres ≦ V0, and M can be set to 3 in the voltage range of V0 < Vl ≦ V1. Similarly, M can be set to 2 in the voltage range of V1 < Vl ≦ V2, and M can be set to 1 in the voltage range of V2 < Vres.

[0089] Fig. 13 is a second conceptual diagram for explaining an example of setting the number of openings (M) during anode interruption of the DC circuit breaker shown in Fig. 10. Fig. 13 shows an example of setting the number of openings (M) to suppress excessive rise of the voltage Vdcr.

[0090] On the horizontal axis of Fig. 13, the line voltage Vl is shown as in Fig. 12, and on the vertical axis, the value after rise after opening of M breaker units of the reactor 2Vdcr is shown. The voltage Vx on the vertical axis of Fig. 13 corresponds to the voltage Vdcr (maximum value) at time t1 in Figs. 8(a), 8(b), 9(a), and 9(b). In Fig. 10, when Vdcr = Vb - Vx and substituting Vx = Vl + Vcb described above, the following Equation (2) holds for the reactor voltage Vdcr.

[0091] Vdcr = Vb - Vx = Vb - Vl - Vcb …(2) During anode interruption in Fig. 10, since the current direction is with the power converter 20 (bus side) as the upstream side, the bus voltage Vb during the interruption operation can be regarded as the rated value Vb* (for example, 1.0 [pu]).

[0092] Therefore, as shown in Fig. 13, in the case of N = 4 similar to Fig. 12, the characteristic lines CLB1 to CLB4 showing the reactor voltage Vdcr when the number of openings M = 1 to 4 are plotted.

[0093] Each of the characteristic lines CLB1 to CLB4 is represented by a linear function of the line voltage Vl having an intercept determined by a constant Vb* (for example, 1.0 [pu]) corresponding to the rated value of the bus voltage Vb and Vcb = M·Vlim in the above formula (2). That is, the reactor voltage Vdcr(M) at the number of open switches M is represented by the following formula (3).

[0094] Vdcr(M)=-Vl+(Vb*-M·Vlim) …(3) Therefore, according to the intersection points of the design limit voltages ±VXmax of the reactor voltage Vdcr and the characteristic lines CLC1 to CLC4, the number of open switches M can be determined so that M becomes the maximum value within the range satisfying -VXmax < Vdcr(M) < VXmax. VXmax is, for example, about 1.0 [pu]. VXmax corresponds to an embodiment of the "allowable upper limit voltage" or the "second allowable upper limit voltage".

[0095] For example, according to the intersection points of the design limit voltages ±VXmax of the reactor voltage Vdcr and the characteristic lines CLB1 to CLB4, the number of open switches M can be determined so that M becomes the maximum value within the range satisfying -VXmax < Vdcr(M) < VXmax. Let the values of the x-coordinates (i.e., the line voltage Vl) of the intersection points of the characteristic lines CLC1 and CLC2 shown in FIG. 13 and the design limit voltage -VXmax be Vx and Vy, respectively. Then, M can be set to 4 in the voltage range of Vl ≦ Vx, M can be set to 3 in the voltage range of Vx < Vl ≦ Vy, and M can be set to 2 (or M = 1) in the voltage range of Vl > Vy.

[0096] In this way, when the positive electrode is interrupted, the number of opened units M is determined based on the line voltage Vl, which is the voltage information on the downstream side of the interrupted current. Generally, as common to FIGS. 12 and 13, when interrupting from a state where the line voltage Vl is low and the voltage difference across the series circuit 150 is large, the number of opened units M can be increased to ensure the voltage interruption ability (FIG. 9(a)). On the other hand, when interrupting from a state where the line voltage Vl is high and the voltage difference across the series circuit 150 is small, the number of opened units M can be decreased to suppress the voltage Vx at the connection node between the DC reactor and the N breaker units 101 to 10N, and the rise of the applied voltage of the DC reactor 25 (FIG. 9(b)).

[0097] However, since the positive electrode interruption of the DC circuit breaker 100 in FIG. 10 interrupts the current with the DC reactor 25 on the upstream side, it is a current interruption in the direction in which the reactor voltage Vdcr rises. Therefore, in light of both the relationship between the line voltage Vl and the voltage Vx (FIG. 12) and the relationship between the line voltage Vl and the reactor Vdcr (FIG. 13), the number of opened units M is set based on the line voltage Vl.

[0098] Specifically, in S220, based on the line voltage Vl, using FIG. 12, N1 satisfying Vx < Vmax is obtained, and using FIG. 13, N2 satisfying -VXmax < Vdcr < VXmax is obtained. Further, the number of opened units N at the time of positive electrode interruption can be set by the operation of N = min(N1, N2) for obtaining the minimum value of these.

[0099] In S230, the controller 105 determines the number of opened units M according to the logic at the time of negative electrode interruption.

[0100] FIG. 14 shows a conceptual diagram for explaining an example of setting the number of opened units (M) at the time of negative electrode interruption of the DC circuit breaker shown in FIG. 10.

[0101] 10, the current (Iline) from the DC circuit breaker 100 toward the DC reactor 25, with the DC reactor 25 downstream, is interrupted, so the voltage Vx decreases. Therefore, when interrupting the negative pole, the number of openings M may be determined by taking into consideration only the increase in the reactor voltage Vdcr, without considering the increase in the voltage Vx.

[0102] 14, the horizontal axis represents the bus voltage Vb, and the vertical axis represents the reactor voltage Vdcr generated across the DC reactor 25.

[0103] When the negative pole is cut off, the direction of the current (Ilne) to be cut off is opposite to that when the positive pole is cut off, so the polarity of the inter-pole voltage Vcb generated when the DC circuit breaker 100 is cut off is reversed from that when the positive pole is cut off. Therefore, when the negative pole is cut off, Vx=Vl-Vcb, and by substituting this into the above-mentioned Vdcr=Vb-Vx, the following equation (4) is established for the reactor voltage Vdcr.

[0104] Vdcr = Vb + Vcb - Vl ... (4) Here, as described above, Vcb = M Vlim. Furthermore, since the current direction is such that the power line 30 is the upstream side, the line voltage Vl at the time of the breaking operation can be regarded as the rated value Vl* (for example, 1.0 [pu]).

[0105] Therefore, as shown in FIG. 14, in the case of N=4 as in FIGS. 12 and 13, characteristic lines CLC1 to CLC4 indicating the reactor voltage Vdcr when the number of open circuits M=1 to 4 are plotted.

[0106] Each of the characteristic lines CLC1 to CLC4 is expressed by a linear function of the bus voltage Vb, with an intercept determined by a constant Vl* that corresponds to the rated value of the line voltage Vl and Vcb = M Vlim in the above formula (4). That is, the reactor voltage Vdcr(M) when the number of open circuits is M is expressed by the following formula (5).

[0107] Vdcr(M)=Vb+(M·Vlim-Vl*) …(5) Therefore, according to the design limit voltage ±VXmax of the reactor voltage Vdcr described above and the intersections of the characteristic lines CLC1 to CLC4, the number of open switches M can be determined such that M reaches the maximum value within the range satisfying -VXmax < Vdcr(M) < VXmax.

[0108] Assuming that the x - coordinates (i.e., the bus voltage Vb) of the intersections of the characteristic lines CLC4 and CLC3 according to Equation (5) and the design upper - limit voltage VXmax, as shown in FIG. 14, are Vx′ and Vy′ respectively, M can be set to 4 in the voltage range of Vb ≤ Vx′, set to 3 in the voltage range of Vx′ < Vb ≤ Vy′, and can be set to 2 (or M = 1) in the voltage range of Vb > Vy′.

[0109] Therefore, even during negative - pole interruption, the number of open switches M can be obtained based on the bus voltage Vb, which is the voltage information on the downstream side of the interrupted current. Generally, as shown in FIG. 14, when interrupting from a state where the bus voltage Vb is low and the voltage difference across the series circuit 150 is large, the number of open switches M can be increased to ensure the voltage interruption ability. On the other hand, when interrupting from a state where the bus voltage Vb is high and the voltage difference across the series circuit 150 is small, the number of open switches M can be decreased to suppress the rise of the reactor voltage Vdcr. In this way, during negative - pole interruption, the number of open switches M is set based on the bus voltage Vb in light of the relationship between the bus voltage Vb and the reactor voltage Vdcr.

[0110] Referring to FIG. 11 again, when the controller 105 determines the number of open switches M of the circuit breaker unit that responds to the interruption command according to S220 or S230 as described in FIGS. 12 and 13, or FIG. 14, in response to this determination, it generates some or all of the opening commands S1 to SN (i.e., for M switches). In this way, the lower the bus voltage Vb (during positive - pole interruption) or the line voltage Vl (during negative - pole interruption) on the downstream side of the interrupted current, which is selected according to the energization direction during the interruption operation, the larger the number of open switches M is set, while the higher the bus voltage Vb or the line voltage Vl, the smaller the number of open switches M is set.

[0111] As a result, according to the DC circuit breaker of this embodiment, when a plurality (N) of circuit breakers 110 (FIG. 3) are connected in series and used in series with the DC reactor 25, the number (M) of circuit breakers to be opened can be variably controlled to suppress increases in voltage Vx (voltage to ground of the DC circuit breaker 100) and reactor voltage Vdcr during interruption operation. As a result, the voltage to ground of the DC circuit breaker 100 during interruption operation and overvoltages occurring in its peripheral devices can be suppressed.

[0112] In this way, by using the measured values ​​of the actual line voltage Vl and bus voltage Vb at the time of detecting the cutoff command, the number of openings N in the DC circuit breaker 100 can be appropriately set according to the actual situation, in response to the accident occurrence situation, for example, the voltage distribution state on the power line 30 which changes depending on the positional relationship between the accident point and the location of the DC circuit breaker 100.

[0113] Next, variations of the DC circuit breaker according to the present embodiment will be described with reference to Figs. 15A to 15D.

[0114] 8(a), 9(a), and 10, FIG. 15A shows a circuit diagram when an accident occurs in the electric line 30 in a configuration in which the DC reactor 25 is connected to the power converter 20 side of N breaker units 101 to 10N. At this time, the current flowing toward the electric line 30 is interrupted, so that the current flow direction at the time of interruption is detected as the line direction from the direction of the line current Iline. As a result, the logic at the time of positive pole interruption (S220) (FIGS. 12 and 13) sets the number of openings M based on the line voltage Vl in light of both the relationship between the line voltage Vl and the voltage Vx and the relationship between the line voltage Vl and the reactor Vdcr.

[0115] Fig. 15B shows a case where an accident occurs on the power converter 20 side in a configuration similar to that of Fig. 15A. In the case of Fig. 15B, the current flowing toward the power converter 20 side is interrupted, so the current flow direction at the time of interruption is detected as the line direction from the direction of the line current Iline. As a result, the logic (Fig. 14) at the time of negative pole interruption (S230) sets the number of openings M based on the bus voltage Vb in light of the relationship between the bus voltage Vb and the reactor voltage Vdcr.

[0116] 15C and 15D show a configuration in which, contrary to FIGS. 15A and 15B, the DC reactor 25 is connected to the electric line 30 side of the N breaker units 101 to 10N.

[0117] Fig. 15C shows a circuit diagram when an accident occurs on the electric line 30. In Fig. 15C, similar to Fig. 15A, the current flowing toward the electric line 30 is interrupted. Therefore, the current direction at the time of interruption is detected as the line direction, and the number of openings M is set by the logic at the time of positive pole interruption (S220).

[0118] However, in the positive pole interruption in Fig. 15C, unlike the positive pole interruption in Fig. 15A, the current (Iline) downstream of the DC reactor 25 is interrupted. Therefore, similar to the negative pole interruption in Fig. 15B, the number of openings M may be determined by taking into consideration only the increase in the reactor voltage Vdcr, without considering the increase in the voltage Vx.

[0119] FIG. 16 is a conceptual diagram for explaining an example of setting the number of open circuit breakers in FIG. 15C which is a positive pole interruption.

[0120] 15C, the relationship between the reactor voltage Vdcr and the line voltage Vl, the bus voltage Vb, and the inter-contact voltage Vcb generated during the interruption operation is expressed as Vx=Vl+Vdcr and Vb=Vx+Vcb, depending on the current flow direction. Therefore, the following equation (6) is established for the reactor voltage Vdcr.

[0121] Vdcr=Vx-Vl=(Vb-Vcb)-Vl=-Vl+Vb-Vcb …(6) Therefore, as shown in FIG. 16, in the case of N = 4 similar to FIGS. 12 to 14, characteristic lines CLD1 to CLD4 indicating the reactor voltage Vdcr when the number of open switches M = 1 to 4 are plotted.

[0122] Each of the characteristic lines CLD1 to CLD4 is represented by a linear function of the line voltage Vl having an intercept determined by the constant Vb* corresponding to the rated value of the bus voltage Vb and Vcb = M·Vlim in the above formula (6). That is, the reactor voltage Vdcr(M) at the number of open switches M is represented by the following formula (7).

[0123] Vdcr(M)=-Vl+(Vb*-M·Vlim) …(7) Since formula (7) is the same as formula (3), the characteristic lines CLD1 to CLD4 in FIG. 16 are the same as the characteristic lines CLB1 to CLB4 in FIG. 13. Thus, in FIG. 15C (at the time of positive pole interruption), in S220 (FIG. 11), based on the relationship between the line voltage Vl and the reactor voltage Vdcr in FIG. 15A (at the time of positive pole interruption), the number of open switches M can be set based on the line voltage Vl.

[0124] Therefore, in FIG. 15C, in S220 (FIG. 11), based on the line voltage Vl, by using FIG. 16 to obtain N that satisfies -VXmax < Vdcr < VXmax, the number of open switches N at the time of positive pole interruption can be set.

[0125] FIG. 15D shows a circuit diagram when an accident occurs on the power converter 20 side, contrary to FIG. 15C. In FIG. 15D, similar to FIG. 15B, the current flowing toward the power converter 20 side is interrupted. Therefore, when the energization direction during the interruption operation is detected as the bus direction, the number of open switches M is set according to the logic at the time of negative pole interruption (S230).

[0126] However, in the negative pole interruption in Fig. 15D, unlike the negative pole interruption in Fig. 15B, the current (Iline) with DC reactor 25 on the upstream side is interrupted. Therefore, similar to the positive pole interruption in Fig. 15A, it is preferable to determine the number of openings M by taking into consideration not only the increase in voltage Vx but also the increase in reactor voltage Vdcr applied between the terminals of DC reactor 25.

[0127] Fig. 17 is a first conceptual diagram for explaining an example of setting the number of open circuit breakers in Fig. 15D. Fig. 17 shows an example of setting the number (M) of open circuits for suppressing an excessive rise in voltage Vx.

[0128] In Fig. 15D, when the current direction is taken into consideration, the direction of the inter-contact voltage Vcb generated during the interruption operation is reversed from that in Fig. 15C, and is therefore expressed as Vdcr = Vb - Vl + Vcb. By substituting this into Vx = Vl + Vdcr, the voltage Vx can be expressed by the following equation (8).

[0129] Vx = Vl + Vdcr = Vb + Vcb … (8) 17 plots characteristic lines CLE1 to CLE4 indicating the voltage Vx when the number of openings M=1 to 4 when N=4. Each of the characteristic lines CLE1 to CLE4 is represented by a linear function of the line voltage Vl, with the intercept being the product of the limit voltage Vlim (FIG. 4) of one breaker unit and the number of openings M. That is, the voltage Vx(M) when the number of openings is M is represented by the following equation (9).

[0130] Vx(M)=Vb+M·Vlim …(9) Comparing equation (9) with equation (1), it can be seen that the characteristic lines CLE1 to CLE4 in FIG. 17 correspond to the characteristic lines CLA1 to CLA4 in FIG. 12, with the horizontal axis changed from the line voltage Vl to the bus voltage Vb.

[0131] FIG. 18 is a second conceptual diagram for explaining an example of setting the number of opened circuit breakers in FIG. 15D. FIG. 18 shows an example of setting the number of opened circuit breakers (M) for suppressing an excessive rise in the voltage Vdcr. In particular, in FIG. 18, when N = 4, characteristic lines CLF1 to CLF4 showing the reactor voltage Vdcr when the number of opened circuit breakers M = 1 to 4 are plotted.

[0132] As described above, in FIG. 15D, since there is a relationship of Vdcr = Vb - Vl + Vcb, each of the characteristic lines CLE1 to CLE4 is represented by a linear function of the bus voltage Vb with the product of the limit voltage Vlim (FIG. 4) for one circuit breaker unit and the number of opened circuit breakers M as the intercept. That is, the reactor voltage Vdcr when the number of opened circuit breakers is M is represented by the following equation (10).

[0133] Vdcr(M)=Vb+(M·Vlim-Vl*) …(10) Since Equation (9) is the same as Equation (5), the characteristic lines CLF1 to CLF4 in FIG. 18 are the same as the characteristic lines CLC1 to CLC4 in FIG. 14.

[0134] Therefore, in FIG. 15D, in S230 (FIG. 11), based on the bus voltage Vb, using FIG. 17, N1 that satisfies Vx < Vmax is obtained, and using FIG. 18, N2 that satisfies -VXmax < Vdcr < VXmax is obtained. Further, by the operation of N = min(N1, N2) for obtaining these minimum values, the number of opened circuit breakers N at the time of negative pole interruption can be set.

[0135] In this way, also in FIGS. 15C and 15D, based on the direction of the current to be interrupted, using the line voltage Vl (at the time of positive pole interruption) or the bus voltage Vb (at the time of negative pole interruption), which is the voltage information on the downstream side of the current, the number of opened circuit breakers M (M ≦ N) among the N circuit breaker units can be appropriately set. Further, depending on whether the current to be interrupted is in the direction with the DC reactor 25 on either the upstream side or the downstream side, an excessive rise in the reactor voltage Vdcr and the ground voltage (voltage Vx) of the DC circuit breaker 100 can be suppressed.

[0136] In this manner, in this embodiment, the logic for determining the value of M from the voltage downstream of the current flow direction is switched depending on a combination of first information indicating whether the downstream side of the current direction to be cut off is the line direction (positive pole cutting with the power line 30 side downstream) or the bus direction (negative pole cutting with the power converter 20 side downstream) and second information indicating whether the DC reactor 25 is upstream or downstream, thereby making it possible to suppress the increase in the voltage Vx (the ground voltage of the DC circuit breaker 100) and the reactor voltage Vdcr during the cutting operation.

[0137] The setting of the number of opens M is not limited to the examples in Figures 12 to 14 and 16 to 18. For example, based on the line voltage Vl and the bus voltage Vb acquired in S110, a correspondence relationship between the voltage difference and the number of opens M may be predefined such that M is increased when the voltage difference between both ends of the series circuit 150 is large and decreased when the voltage difference is small.

[0138] (Current flow direction detection process) 3 and 4, a time delay occurs between the generation of a trip command and the actual opening of the circuit breaker 110 (FIG. 2), while in this embodiment, the logic (S220, S230) for determining the number of openings M is switched depending on the direction of the current to be tripped. For this reason, if the current flow direction (current direction) is detected in S120 of FIG. 12 using the line current Iline as it is at the time the trip command is detected, there is a concern that the current flow direction detected may be opposite to that when the circuit breaker 110 is actually opened.

[0139] Therefore, in S120, as will be described with reference to FIG. 19, it is preferable to detect the current flow direction using a current prediction value at the time of actual interruption.

[0140] FIG. 19 is a conceptual diagram for explaining details of the process of detecting the current flow direction during an interruption operation. 19, before time ta, a negative line current Iline, i.e., a steady-state current from the power line 30 side to the power converter 20 side, is generated due to power supply and demand, etc. In this state, in response to the occurrence of an accident on the power line 30 side, the interruption command Strp input to the controller 105 changes from a low level (L level) to a high level (H level). As a result, the controller 105 detects the interruption command at time ta.

[0141] In this case, since the current flowing toward the electric line 30 is interrupted, the line current Iline changes toward the positive direction after time ta, and reverses direction at time tc. As a result, since the current flow direction is the line direction at time td when the circuit breaker 110 is actually opened, it is preferable to determine the number of openings M according to the logic for positive electrode interruption (S220) by determining YES in S210 in FIG. 12.

[0142] However, if the detection process of S120 (FIG. 12) is performed using the line current Iline at the time ta when the interruption command Strp is generated, there is a concern that the current flow direction will be detected as the bus direction, and the number of openings M will be determined according to the negative pole interruption logic (S230).

[0143] Therefore, it is preferable that the current flow direction detection process in S120 is performed by calculating the rate of change of the line current Iline after the detection of the interruption command Strp, and based on the positive / negative state of the current prediction value Isht corresponding to the line current Iline at time td.

[0144] For example, the current change rate Kiln can be calculated from the detection values ​​Iline(ta) and Iline(tb) by the current detector 117 at times ta and tb by the following formula (11), and the current prediction value Isht can be obtained by formula (12). Td in formula (12) is the time delay from the detection of the breaker command Strp to the actual opening of the breaker 110, and can be determined in advance as a specification value of the breaker unit.

[0145] Kiln=Iline(tb)-Iline(ta) / (tb-ta) …(11) Isht=Td·Kiln+Iline(ta) …(12) In S120, when the current prediction value Isht obtained by equation (5) is positive (Isht>0), the current direction is detected to be the line direction, whereas when the current prediction value Isht is negative (Isht<0), the current direction is detected to be the bus direction.

[0146] 19, the process of S200 is executed, so that the number of openings M can be determined according to the direction of current flow at the timing when the circuit breaker 110 is actually opened. This makes it possible to suppress the overvoltage more reliably.

[0147] 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 meaning and scope of the claims. [Explanation of symbols]

[0148] 1A,1B Power transmission system, 10A~10D HVAC system, 100U,101~10N Circuit breaker unit, 15A~15D AC circuit breaker, 20 Power converter, 20A~20D AC / DC converter, 25 DC reactor, 30,30X,31~34 Power line, 100,100A~100H DC circuit breaker, 105 Controller, 110 Circuit breaker, 116b,116l,116x Voltage detector, 117 Current detector, 120 Lightning arrester, 150 Series circuit, CL1~CL4,CLX1~CLX4 Characteristic line, Iline Line current, P1,P2 Path, S1~SN Open command (circuit breaker), Strp Interrupt command (DC circuit breaker), Vb Bus voltage, Vcb Inter-pole voltage (DC circuit breaker), Vdcr Reactor voltage, Vl line voltage, Vlim limit voltage, Vmax design upper limit voltage.

Claims

1. A DC circuit breaker connected in series with a DC reactor to an electric line, N circuit breaker units connected in series, The N is an integer of 2 or more, Each of the N circuit breaker units includes a circuit breaker and a lightning arrester connected in parallel, The DC circuit breaker comprises: a first voltage detector and a second voltage detector for measuring voltages on a bus bar side and a power line side of a series circuit of the DC reactor and the N breaker units; A controller that generates an opening command for the circuit breakers of the N circuit breaker units, When the controller detects a break command for the DC breaker, the controller opens the breakers of M breaker units that are variably set among the N breaker units, based on at least one of a bus side voltage and a power line side voltage of the series circuit; A DC circuit breaker, wherein M is determined to be an integer between 1 and N inclusive depending on at least one of the bus side voltage and the power line side voltage when each of the N circuit breaker units is in a closed state when a circuit breaker command for the DC circuit breaker is detected.

2. A current detector for measuring a current flowing through the electric line is further provided, 2. The DC circuit breaker according to claim 1, wherein, when the controller detects the interruption command, the controller detects a current flow direction during an interruption operation based on the line current detected by the current detector, and based on one of the bus side voltage and the power line side voltage that is downstream of the detected current flow direction, the controller sets the value of M to a larger value as the one of the bus side voltage and the power line side voltage is lower, and sets the value of M to a smaller value as the one of the voltages is higher.

3. 3. The DC circuit breaker according to claim 2, wherein, when the current flow direction is from the DC reactor to the DC circuit breaker, the controller determines the value of M based on the voltage of one of the terminals so that, when the M circuit breaker units are opened, a voltage of a connection node between the DC reactor and the N circuit breaker units becomes equal to or lower than a first allowable upper limit voltage, and M becomes a maximum value within a range in which an absolute value of a reactor voltage between both ends of the DC reactor when the M circuit breaker units are opened is equal to or lower than a second allowable upper limit voltage.

4. 4. The DC circuit breaker according to claim 3, wherein, when the current flow direction is from the DC reactor to the DC circuit breaker, the controller determines the value of M based on the one of the voltages to be the minimum of a first number that is a maximum value of M within a range in which the voltage of the connection node is equal to or less than the first allowable upper limit voltage when the M circuit breaker units are opened, and a second number that is a maximum value of M within a range in which the absolute value of the reactor voltage is equal to or less than the second allowable upper limit voltage when the M circuit breaker units are opened.

5. 3. The DC circuit breaker according to claim 2, wherein, when the current flow direction is from the DC circuit breaker to the DC reactor, the controller determines the value of M based on the voltage of one of the terminals so that M is a maximum value within a range in which an absolute value of a reactor voltage between both ends of the DC reactor when the M circuit breaker units are opened is equal to or less than an allowable upper limit voltage.

6. 3. The DC circuit breaker according to claim 2, wherein, when the controller detects the interruption command, it detects a current flow direction during an interruption operation based on a direction of the line current detected by the current detector, and switches a logic for determining the value of M from the one voltage in accordance with a combination of first information indicating whether the current flow direction is a direction from the DC circuit breaker to the DC reactor or a direction from the DC reactor to the DC circuit breaker.

7. The DC circuit breaker according to any one of claims 2 to 6, wherein the controller calculates a rate of change of the line current using a first detection value by the current detector at the time of detection of the shutoff command and a second detection value by the current detector after the detection time, calculates a current prediction value at the time when the circuit breaker is actually opened in response to the shutoff command based on the first detection value and the rate of change, and detects the current flow direction according to the calculated current prediction value.

8. 2. The DC circuit breaker according to claim 1, wherein the controller sets M to a larger value as the voltage difference between the bus bar side and the power line side of the series circuit detected by the first and second voltage detectors at the time of detecting the interruption command is larger, and sets M to a smaller value as the voltage difference is smaller.

9. The DC circuit breaker according to any one of claims 1 to 6 and 8, wherein when the interruption command is generated to interrupt a fault current, the controller sets M to a larger value compared to when the interruption command is generated to interrupt a current when no fault occurs.

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