Protection system

Abstract

This prevents malfunctions of circuit breakers in the event of a ground fault on another line. [Solution] The protection system (101) comprises a first circuit breaker (6), a second circuit breaker (43) that short-circuits and opens the terminals of a capacitor (42), and a control device (5) that controls the first circuit breaker (6) and the second circuit breaker (43). The control device (5) includes a first circuit breaker relay (52) that detects a fault in the busbar (100a) of the power system (100) and trips the first circuit breaker (6), a zero-sequence voltage detector (51) that detects the zero-sequence voltage of the transformer (3), and a second circuit breaker relay (53) that controls the second circuit breaker (43) to short-circuit when at least one of the following occurs: a fault non-detection event in which no fault in the busbar (100a) is detected, and a voltage non-detection event in which a non-zero zero-sequence voltage is not detected.

Description

[Technical Field]

[0001] This disclosure relates to a protection system for protecting a three-phase AC power grid. [Background technology]

[0002] In three-phase AC power systems, faults such as ground faults and short circuits can occur. Conventionally, measures have been taken to protect power systems from such faults.

[0003] For example, Patent Document 1 discloses a protection system in which the neutral point in a power system is grounded via a protection unit in order to prevent a zero current error in the event of a single-phase ground fault. The protection unit comprises a neutral point compensating reactor and a neutral point cathode connected in series with the neutral point compensating reactor.

[0004] When a ground fault occurs inside or outside the electrical equipment where the protection unit is installed, a resonant current generated by the resonance phenomenon of the neutral point compensation reactor and neutral point cabaricator flows from the protection unit to the fault point. The neutral point compensation reactor is installed to compensate for the current flowing through the transmission cable that is branched off from the electrical equipment to the power source. Therefore, the only current flowing to the fault point is the resonant current. If the zero crossing point of this resonant current exists within the current zero miss evaluation time, the circuit breaker can trip, and a current zero miss is avoided. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 7538457 [Overview of the project] [Problems that the invention aims to solve]

[0006] As shown in Figure 5, in a power system equipped with the above protection system, if a single-line-to-ground fault occurs in another circuit 10 connected to the same busbar as the electrical equipment 9 on which the protection unit 8 is installed, the interconnection circuit breaker 11 of circuit 10 is tripped by a relay (not shown) to clear the fault. Also, in the event of a ground fault, as shown by the dashed line in Figure 5, a low-frequency resonant current flows from the electrical equipment 9 towards the fault point P through the closed circuit breaker 6. Since this resonant current has a longer period and larger amplitude than the fundamental wave of AC, it may cause the relay to operate unstably.

[0007] One aspect of this disclosure aims to prevent malfunction of circuit breakers in the event of a ground fault on another line. [Means for solving the problem]

[0008] To solve the above problems, a protection system according to one aspect of the present disclosure is a protection system for protecting a power system that supplies three-phase AC power, comprising: a transformer for transforming the voltage of three-phase AC power; a neutral point compensation reactor connected to the neutral point of the transformer; and a capacitor connected in series with the neutral point compensation reactor, the protection system comprising: a circuit breaker disposed between the power system and the transformer; a switch for short-circuiting and opening both ends of the capacitor; and a control device for controlling the circuit breaker and the switch, wherein the control device includes: a fault detection unit that trips the circuit breaker by detecting a bus fault in the power system; a zero-sequence voltage detection unit that detects the zero-sequence voltage of the transformer; and an on / off control unit that controls the switch to short-circuit when at least one of the following occurs: a fault non-detection event in which the fault detection unit does not detect a bus fault, and a voltage non-detection event in which the zero-sequence voltage detection unit does not detect a non-zero zero-sequence voltage. [Effects of the Invention]

[0009] According to one aspect of this disclosure, it is possible to prevent malfunction of circuit breakers in the event of a ground fault on another line. [Brief explanation of the drawing]

[0010] [Figure 1] This figure shows a power transmission system equipped with a protection system according to one embodiment of the present disclosure. [Figure 2] This is a circuit diagram showing the configuration of the transformer and the neutral point compensation reactor and capacitor connected to the transformer in the above power system. [Figure 3] This flowchart shows the procedure for processing related to the protection operation of the power grid by the above protection system. [Figure 4] This waveform diagram shows the current flowing through each part of the above protection system when a malfunction occurs. [Figure 5] This diagram illustrates the malfunctions of conventional protection systems. [Modes for carrying out the invention]

[0011] [Embodiment] <Configuration of the power system> One embodiment of this disclosure will be described in detail below.

[0012] Figure 1 shows a power system 100 equipped with a protection system 101 according to Embodiment 1. Figure 2 is a circuit diagram showing the configuration of a transformer 3 and a neutral point compensation reactor 41 and capacitor 42 connected to the transformer 3 in the power system 100.

[0013] The protection system 101 is a system that protects a three-phase power system 100 and transmits three-phase AC power through a three-phase circuit. As shown in Figure 1, the power system 100 includes a power transmission cable 2, a transformer 3, electrical equipment 4, and a control device 5. The power system 100 also includes a first circuit breaker 6, a second circuit breaker 43 (switch) in the electrical equipment 4, and a third circuit breaker 7 as circuit breakers.

[0014] In the power system 100, a busbar 100a is formed by overhead lines and cables between the power source 1 and the interconnection point P2. The busbar 100a has a back impedance Z.

[0015] The power transmission cable 2 is provided on one of the electric circuits branched from the electric circuit on the downstream side of the connection point P2 and is connected to power generation equipment (not shown) and the like that is interconnected with the power system 100. That is, the power system 100 can be connected to power generation equipment and the like via the connection point P2 (the first circuit breaker 6) shown in FIG. 1.

[0016] The transformer 3 transforms the voltage of the three-phase alternating current power supplied from the power source 1. The transformer 3 is provided on the other electric circuit branched from the electric circuit on the downstream side of the connection point P2. The transformer 3 may be any type of transformer as long as it is a three-phase transformer. The primary side of the transformer 3 is connected to the power source 1. The secondary side of the transformer 3 is connected to a filter, a phase modulation facility, and the like.

[0017] As shown in FIG. 2, the primary side winding of the transformer 3 is Y-connected, and the secondary side winding of the transformer 3 is Δ-connected. The neutral point NP is located at the junction point of the Y-connection of the transformer 3. However, the position of the neutral point NP is not limited to this example. For example, when the transformer 3 is a grounding transformer, the neutral point NP may be located at the junction point of the staggered winding of the grounding transformer. The neutral point NP is grounded via a neutral point compensation reactor 41 and a capacitor 42, which will be described later. In FIG. 2, for the sake of convenience, the second circuit breaker 43, which will be described later, is omitted.

[0018] The first circuit breaker 6 is arranged at the connection point P2 and interrupts the current at the connection point P2. The first circuit breaker 6 is arranged between the power source 1 and the transformer 3. FIG. 1 illustrates a case where a single-phase-to-ground fault occurs at a fault point P1 on the power source 1 side of the connection point P2. When a single-phase-to-ground fault occurs in the power system 100, a fault current flows from the power transmission cable 2 toward the first circuit breaker 6.

[0019] The third circuit breaker 7 is located between the power transmission cable 2 and the first circuit breaker 6. Specifically, the third circuit breaker 7 is located on the power transmission cable 2 side of the power transmission cable 2 and transformer 3, which are connected to branch off from the circuit from the first circuit breaker 6. Therefore, the tripping operation of the third circuit breaker 7 does not affect the current path from the power source 1 to the transformer 3.

[0020] Electrical equipment 4 is equipment that supplies lagging reactive power to compensate for the charging capacity of the power transmission cable 2. Electrical equipment 4 includes a neutral point compensating reactor 41, a capacitor 42, and a second circuit breaker 43 (switch).

[0021] The neutral point compensation reactor 41 is provided for the purpose of compensating for the charging capacity of the power transmission cable 2. One end of the neutral point compensation reactor 41 is connected to the neutral point NP of the transformer 3, and the other end of the neutral point compensation reactor 41 is connected to one end of the capacitor 42. The other end of the capacitor 42 is grounded by being connected to the grounding terminal. In this way, the capacitor 42 is connected in series with the neutral point compensation reactor.

[0022] The second circuit breaker 43 is a device that short-circuits and opens the terminals of the capacitor 42. As will be described later, the second circuit breaker 43 opens when a fault occurs in the busbar 100a and a ground fault occurs in the power system 100, electrically connecting the capacitor 42 to the neutral point compensation reactor 41. The second circuit breaker 43 also closes when there is no fault in the busbar 100a and no ground fault occurs in the power system 100, bypassing the other end of the neutral point compensation reactor 41 from the ground terminal. As a result, the second circuit breaker 43 electrically disconnects the capacitor 42 from the neutral point compensation reactor 41.

[0023] The second circuit breaker 43 does not need to be able to handle large currents like the first circuit breaker 6, but only needs to be able to interrupt the current that flows when the rated voltage is applied to the neutral point compensating reactor 41. Therefore, a switch may be used instead of the second circuit breaker 43 to short-circuit and open the capacitor 42. The opening and closing of the second circuit breaker 43 is controlled by a control signal SG3 output from the second circuit breaker relay 53, which will be described later.

[0024] The control device 5 controls the first circuit breaker 6 and the second circuit breaker 43. The control device 5 includes a zero-sequence voltage detector 51 (zero-sequence voltage detection unit), a first circuit breaker relay 52 (fault detection unit), and a second circuit breaker relay 53 (switching control unit).

[0025] The zero-sequence voltage detector 51 detects the zero-sequence voltage V0 of the transformer 3. The zero-sequence voltage detector 51 is composed of, for example, an Earthed Voltage Transformer (EVT), but may be composed of other instruments. The zero-sequence voltage V0 is 0V when there is no ground fault in the power system 100, and is a voltage other than 0V when there is a ground fault in the power system 100 because the three-phase voltages become unbalanced.

[0026] The first circuit breaker relay 52 is a relay that trips the first circuit breaker 6 by detecting a fault (short circuit fault and ground fault) in the busbar 100a. The first circuit breaker relay 52 detects a fault in the busbar 100a by receiving an external tripping signal SG1 supplied from an external source, such as the power company, and trips the first circuit breaker 6. The first circuit breaker relay 52 also detects the occurrence of a fault in the busbar 100a based on at least one of the voltage and current of the busbar 100a.

[0027] The first circuit breaker relay 52 outputs a detection signal SG2 that becomes active (e.g., high) when it receives an external tripping signal SG1 or detects a fault in busbar 100a. Conversely, the first circuit breaker relay 52 outputs a detection signal SG2 that becomes inactive (e.g., low) when it does not receive an external tripping signal S1 or detect a fault in busbar 100a.

[0028] The second circuit breaker relay 53 controls the opening and closing of the second circuit breaker 43 based on fault detection events and fault non-detection events by the zero-sequence voltage detector 51, and voltage detection events and voltage non-detection events by the zero-sequence voltage detector 51. A fault detection event is an event in which a fault in busbar 100a is detected. A fault non-detection event is an event in which a fault in busbar 100a is not detected. A voltage detection event is an event in which a non-zero (not 0) zero-sequence voltage V0 is detected. A voltage non-detection event is an event in which a non-zero zero-sequence voltage is not detected.

[0029] The second circuit breaker relay 53 opens the second circuit breaker 43 when a fault detection event or a voltage detection event occurs while the second circuit breaker 43 is in a steady state with the capacitor 42 short-circuited. The second circuit breaker relay 53 also controls the second circuit breaker 43 to short-circuit when at least one of a fault non-detection event or a voltage non-detection event occurs.

[0030] In other words, the second circuit breaker relay 53 outputs a control signal SG3 to the electrical equipment 4, which is the logical AND of the detection signal SG2 from the first circuit breaker relay 52 and the zero-sequence voltage v0. Specifically, the second circuit breaker relay 53 outputs an active control signal SG3 when it receives an active detection signal and a non-zero zero-sequence voltage V0. Also, the second circuit breaker relay 53 outputs an inactive control signal SG3 when it receives at least one of an inactive detection signal and a zero zero-sequence voltage V0.

[0031] The second circuit breaker relay 53 is preferably composed of, for example, a ground fault overvoltage relay. Since the ground fault overvoltage relay detects a ground fault in the circuit, it can also function as the zero-sequence voltage detector 51. Therefore, when a ground fault overvoltage relay is used as the second circuit breaker relay 53, the zero-sequence voltage detector 51 can be omitted. The second circuit breaker relay 53 may also be composed of a relay other than a ground fault overvoltage relay.

[0032] <Protection System Operation> Figure 3 is a flowchart showing the procedure for the protective operation of the power system 100 by the protection system 101. Figure 4 is a waveform diagram showing the current flowing through each part of the protection system 101 when a fault occurs.

[0033] As shown in Figure 3, first, the second circuit breaker relay 53 determines whether the first circuit breaker relay 52 has received the external tripping signal SG1 or detected a fault in the busbar 100a (step S1). The second circuit breaker relay 53 makes the above determination based on the detection signal SG2 from the first circuit breaker relay 52. ​​Specifically, when the detection signal SG2 is active, the second circuit breaker relay 53 determines that the external tripping signal SG1 has been received or a busbar fault has been detected. Also, when the detection signal SG2 is inactive, the second circuit breaker relay 53 determines that the external tripping signal SG1 has not been received or a busbar fault has not been detected.

[0034] In step S1, if the second circuit breaker relay 53 determines that the first circuit breaker relay 52 has received an external tripping signal SG1 or has detected a fault in the busbar 100a (YES), it determines whether the zero-sequence voltage V0 output from the zero-sequence voltage detector 51 is non-zero (step S2).

[0035] In step S2, if the second circuit breaker relay 53 determines that the zero-sequence voltage V0 is non-zero (YES), it opens the second circuit breaker 43 (step S3) and finishes the process. In step S2, the second circuit breaker relay 53 determines that a fault detection event and a voltage detection event have occurred and outputs an active control signal SG3 to open the second circuit breaker 43.

[0036] As a result, when the second circuit breaker 43 opens the terminals of capacitor 42, capacitor 42 is connected to the neutral point compensation reactor 41. Then, due to the resonance of the series circuit consisting of the neutral point compensation reactor 41 and capacitor 42, an oscillating current i n (The resonance signal mentioned above) is generated. Therefore, the first circuit breaker 6 generates an oscillating current i with a zero crossing point. nThis makes it possible to interrupt the 100a busbar. Therefore, it is possible to avoid a current zero miss, where a zero crossover point does not occur for a long period of time when a single-line-to-ground fault occurs.

[0037] Furthermore, in this case, as shown in Figure 5, even if a single-line-to-ground fault occurs in the other line 10 mentioned above, the power system 100 is interrupted at the interconnection point P2 by the first circuit breaker 6, so the oscillating current i n No current flows into the line 10 via the first circuit breaker 6.

[0038] Furthermore, in step S1, if the second circuit breaker relay 53 determines that the first circuit breaker relay 52 has not received the external tripping signal SG1 or detected a fault in the busbar 100a (NO), it closes the second circuit breaker 43 (step S4) and ends the process. In step S1, the second circuit breaker relay 53 determines that a fault non-detection event has occurred and outputs an inactive control signal SG3 to close the second circuit breaker 43.

[0039] As a result, when the second circuit breaker 43 short-circuits the terminals of the capacitor 42, the neutral point compensation reactor 41 is grounded via the second circuit breaker 43 without the capacitor 42 being connected. In this case, an oscillating current i is introduced into the neutral point compensation reactor 41. n Since no oscillation current i occurs, n The current does not flow into busbar 100a. Therefore, as shown in Figure 5, in order to eliminate the single-line-to-ground fault that occurred in the other line 10 mentioned above, the oscillating current i is sent towards the fault point P that trips the interconnection circuit breaker 11 via circuit breaker 6 (equivalent to the first circuit breaker 6 in Figure 1). n No current will flow. Therefore, it is possible to prevent the relay that trips the interconnection circuit breaker 11 from becoming unstable and to avoid malfunction of the interconnection circuit breaker 11.

[0040] Furthermore, in step S2, if the second circuit breaker relay 53 determines that the zero-sequence voltage V0 is not non-zero (NO), it proceeds to step S4 and terminates the process. In step S2, the second circuit breaker relay 53 determines that a voltage non-detection event has occurred and outputs an inactive control signal SG3 to close the second circuit breaker 43.

[0041] Next, we will explain the currents that flow through each part of the protection system 101 when a malfunction occurs.

[0042] As shown in Figure 4, the second circuit breaker relay 53, after a period of approximately 50 ms from the time the fault occurs (0.195 s), recognizes both the fault detection event and the voltage detection event, and then opens (trips) the second circuit breaker 43. During the above period, the current flowing through the neutral point compensation reactor 41 flows through the second circuit breaker 43.

[0043] Furthermore, when the second circuit breaker 43 trips, the capacitor 42 is connected to the neutral point compensation reactor 41. As a result, a current flows through the neutral point compensation reactor 41 and the capacitor 42, which is, for example, a 50Hz fundamental wave superimposed with a low-frequency (e.g., 10Hz) oscillating waveform. Then, the first circuit breaker 6 receives a low-frequency oscillating current i with a zero crossing point. n The current flows. Furthermore, the above elapsed time is shorter than the zero current miss evaluation time. As a result, zero current miss is avoided as described above.

[0044] <Effects of the protection system> As described above, the protection system 101 according to this embodiment includes a control device 5 having a zero-sequence voltage detector 51, a first circuit breaker relay 52, and a second circuit breaker relay 53. The zero-sequence voltage detector 51 detects the zero-sequence voltage V0 of the transformer 3. The first circuit breaker relay 52 trips the first circuit breaker 6 by detecting a fault in the busbar 100a. The second circuit breaker relay 53 outputs a control signal SG3 by logical AND of the detection signal SG2 from the first circuit breaker relay 52 and the zero-sequence voltage v0, and opens and closes the second circuit breaker relay 53 with the control signal SG3.

[0045] Thus, when no fault occurs in the bus 100a, when no ground fault occurs in the power system 100, or when no fault occurs in both, the second breaker relay 53 closes the second breaker 43, so that the second breaker 43 shorts both ends of the capacitor 42. As a result, the capacitor 42 is not electrically connected to the neutral point compensating reactor 41. Therefore, no oscillating current i n is generated by the neutral point compensating reactor 41 and the capacitor 42, and the oscillating current i n does not flow into the other line 10 described above. Therefore, the relay that operates the tie breaker 11 will not become unstable in operation due to the oscillating current i n when a single-line-to-ground fault occurs in the line 10. Thus, malfunction of the tie breaker 11 can be prevented.

[0046] Also, when a fault occurs in the bus 100a and a ground fault occurs in the power system 100, the second breaker relay 53 opens the second breaker 43, so that the second breaker 43 releases both ends of the capacitor 42. As a result, a series circuit is formed when the capacitor 42 is electrically connected to the neutral point compensating reactor 41. Therefore, the low-frequency oscillating current i n generated by the series circuit can be made to flow through the first breaker 6. Thus, the first breaker 6 can perform a correct interruption operation without causing a current zero miss due to the oscillating current i n having a zero crossing point.

[0047] Also, since the second breaker 43 is a breaker with excellent interruption performance, when a fault occurs in the bus 100a and a ground fault occurs in the power system 100, both ends of the capacitor 42 can be surely interrupted.

[0048] 〔Summary〕 A protection system according to a first aspect of the present disclosure is a protection system for a power system that supplies three-phase AC power, comprising: a transformer for transforming the voltage of three-phase AC power; a neutral point compensation reactor connected to the neutral point of the transformer; and a capacitor connected in series with the neutral point compensation reactor, the protection system for protecting a power system that supplies three-phase AC power, comprising: a circuit breaker disposed between the power system and the transformer; a switch for short-circuiting and opening the ends of the capacitor; and a control device for controlling the circuit breaker and the switch, wherein the control device includes: a fault detection unit that trips the circuit breaker when it detects a fault in the busbar in the power system; a zero-sequence voltage detection unit that detects the zero-sequence voltage of the transformer; and an on / off control unit that controls the switch to short-circuit when at least one of the following occurs: a fault non-detection event in which the fault detection unit does not detect a fault in the busbar, and a voltage non-detection event in which the zero-sequence voltage detection unit does not detect a non-zero zero-sequence voltage.

[0049] According to the above configuration, if no fault occurs on the busbar, and if no ground fault occurs, the switch short-circuits the capacitor. As a result, the capacitor is bypassed and not connected to the neutral point compensation reactor. Therefore, no oscillating current is generated by the neutral point compensation reactor and the capacitor, and this oscillating current does not flow to other circuits on the busbar shared with the transformer.

[0050] Therefore, when a single-line-to-ground fault occurs on another line, the relay that operates the circuit breaker to interrupt that line can be prevented from becoming unstable due to oscillating currents. Thus, malfunctions of the circuit breaker can be prevented.

[0051] In the protection system according to a second aspect of the present disclosure, in the first aspect, the switching control unit may open the switch when the switch is short-circuiting the capacitor and a fault detection event occurs in which the fault detection unit detects a fault in the busbar, and a voltage detection event occurs in which the zero-sequence voltage detection unit detects a non-zero zero-sequence voltage.

[0052] According to the above configuration, if a fault occurs in the busbar and a ground fault occurs, the switch opens both ends of the capacitor. As a result, the oscillating current generated by the neutral point compensation reactor and capacitor flows to the circuit breaker. Therefore, the circuit breaker operates normally, and a zero current error can be avoided.

[0053] In the third aspect of this disclosure, the protection system may, in the first or second aspect, have the switch be a circuit breaker.

[0054] According to the above configuration, a circuit breaker with superior interruption performance is used as the switch. This ensures that the switch can be reliably opened in the event of a fault in the busbar and a ground fault in the power system.

[0055] [Additional Notes] This disclosure is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Furthermore, embodiments obtained by appropriately combining the technical means disclosed in each embodiment are also included in the technical scope of this disclosure. [Explanation of symbols]

[0056] 3. Transformer 5 Control device 6. First circuit breaker (circuit breaker) 41 Neutral point compensating reactor 42 Capacitors 43. Second circuit breaker (switch) 51 Zero-sequence voltage detector (zero-sequence voltage detection unit) 52. First circuit breaker relay (fault detection unit) 53. Second circuit breaker relay (opening / closing control unit) 100 Power system 101 Protection System NP neutral point

Claims

1. A protection system for protecting a power system that supplies three-phase AC power, comprising a transformer for transforming the voltage of three-phase AC power, a neutral point compensation reactor connected to the neutral point of the transformer, and a capacitor connected in series with the neutral point compensation reactor, A circuit breaker is placed between the power system and the transformer, A switch that shorts and opens the terminals of the capacitor, The circuit breaker and the switch are controlled by a control device, The control device is A fault detection unit that trips the circuit breaker by detecting a fault in the busbar in the power system, A zero-sequence voltage detection unit for detecting the zero-sequence voltage of the transformer, A protection system comprising: an on / off control unit that controls the switch to short-circuit when at least one of the following occurs: a fault non-detection event in which the fault detection unit does not detect a fault in the busbar, and a voltage non-detection event in which the zero-sequence voltage detection unit does not detect a non-zero zero-sequence voltage.

2. The protection system according to claim 1, wherein the switching control unit opens the switch when the switch is short-circuiting the capacitor and a fault detection event occurs in which the fault detection unit detects a fault in the busbar, and a voltage detection event occurs in which the zero-sequence voltage detection unit detects a non-zero zero-sequence voltage.

3. The protection system according to claim 1 or 2, wherein the switch is a circuit breaker.

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