Protective control device and protective control method

Abstract

This technology protects the power distribution system by simultaneously tripping three-phase power distribution circuit breakers at the appropriate time during a ground fault, while suppressing increased costs and damage to distribution circuit breakers. [Solution] The protective control device for protecting a power distribution system according to the present disclosure comprises a ground fault detection unit, a current detection unit, a determination unit, and a command output unit. The ground fault detection unit detects ground faults in three-phase cable distribution lines. The current detection unit detects the amplitude of the current flowing through at least two phases of a three-phase distribution circuit breaker. The determination unit determines that a heavy load condition, in which the amplitude of the current flowing through at least two phases detected by the current detection unit is greater than or equal to a threshold, has continued for a first hour. The command output unit outputs a tripping command to the three-phase distribution circuit breaker when a ground fault is detected by the ground fault detection unit and the determination unit determines that a heavy load condition has continued for a first hour.

Description

Technical Field

[0005] ,

[0004] ,

[0001] The present disclosure relates to a technique for protecting a power distribution system when a ground fault occurs in the power distribution system.

Background Art

[0002] In a power system that distributes three-phase alternating current power through a power cable, due to the large capacitance to ground of the power cable, the power cable consumes leading reactive power. Therefore, a shunt reactor is connected between each phase of the power cable and the ground, and leading reactive power is supplied from the shunt reactor to the power cable.

[0003] When a ground fault occurs in any phase of such a power system, the electrodes of the distribution breaker installed between the transformer and each phase of the power cable are opened to attempt to interrupt the current. However, an arc discharge may occur and the current may continue to flow. Since this current is an alternating current, the current is interrupted at the timing when it becomes zero. However, due to the connection of the shunt reactor, a current in which a transient DC component current caused by the shunt reactor is superimposed on the alternating current flows through the sound-phase power cable. The transient DC component current decays due to resistance and approaches zero. However, until the transient DC component current sufficiently decays, the alternating current may be offset in the positive or negative direction and the moment when it becomes zero may not arrive. As a result, current may continue to flow through the distribution breaker, and the distribution breaker may be damaged.

[0004] Therefore, the protective relay device described in Patent Document 1 includes a reactor breaker connected between each phase of the power cable and the shunt reactor. After interrupting the distribution breaker of the ground fault phase, the above protective relay device interrupts the reactor breaker of each phase, and then interrupts the distribution breaker of the sound phase.

[0005] Furthermore, the zero-point transition current interruption fault prevention system described in Patent Document 2 comprises a resistor connected in series with the shunt reactor of each phase and a switch connected in parallel with the resistor. The above-mentioned zero-point transition current interruption fault prevention system opens the switch in the event of a ground fault, allowing current to flow through the resistor, thereby quickly attenuating the transient DC current, and then simultaneously trips the distribution circuit breakers for each phase.

[0006] Furthermore, the control device described in Patent Document 3 pre-calculates the time required for the transient DC current to decay to a certain extent, and after a ground fault, trips the distribution circuit breaker after the calculated time has elapsed. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2003-52121 [Patent Document 2] Japanese Patent Publication No. 2003-209925 [Patent Document 3] Japanese Patent Publication No. 2001-224134 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Patent Document 1 has the problem of increasing costs because it requires a three-phase distribution circuit breaker and reactor circuit breaker that can be interrupted for each phase, as well as control for them. Patent Document 2 also has the problem of increasing costs because it requires resistors and switches for each phase, as well as control for them. Furthermore, Patent Document 3 has the problem that the timing of interrupting the distribution circuit breaker may be unnecessarily delayed because the time it takes for the transient DC current to decay to a certain extent varies depending on the situation.

[0009] This disclosure provides a technology to protect the distribution system by simultaneously tripping three-phase distribution circuit breakers at an appropriate time in the event of a ground fault, while suppressing increased costs and damage to distribution circuit breakers. [Means for solving the problem]

[0010] One aspect of the present disclosure is a protection control device for protecting a distribution system comprising a three-phase distribution circuit breaker connected to a three-phase AC power supply, three-phase cable distribution lines connected to the three-phase distribution circuit breaker, and shunt reactors connected to each of the three-phase cable distribution lines, the device comprising a ground fault detection unit, a current detection unit, a determination unit, and a command output unit. The ground fault detection unit detects a ground fault in the three-phase cable distribution line. The current detection unit detects the amplitude of the current flowing through at least two phases of the three-phase distribution circuit breaker. The determination unit determines that a heavy load condition, in which the amplitude of the current flowing through at least two phases detected by the current detection unit is greater than or equal to a threshold, has continued for a first hour. The command output unit outputs a tripping command to the three-phase distribution circuit breaker when a ground fault is detected by the ground fault detection unit and the determination unit determines that a heavy load condition has continued for a first hour.

[0011] According to the protection control device of this disclosure, it is confirmed that the heavy load condition has persisted for one hour. When the load receiving power from the distribution system is heavy, the amplitude of the load current flowing from the load to the distribution circuit breaker during a ground fault is large. Therefore, even if a current with a transient DC component superimposed flows to the distribution circuit breaker, the moment when the current becomes zero arrives immediately, allowing the three-phase distribution circuit breaker to be tripped immediately. However, immediately after the distribution circuit breaker is energized, even with a light load, the amplitude of the current flowing to the distribution circuit breaker becomes large due to a transient disturbance. By confirming that the heavy load condition has persisted for one hour, the condition where a light load appears heavy due to a transient disturbance is excluded. Therefore, when a ground fault occurs, the distribution system can be protected by tripping the three-phase distribution circuit breaker all at once at an appropriate timing while suppressing increased costs and damage to the distribution circuit breaker.

[0012] Another aspect of the present disclosure is a protection and control method for a power distribution system comprising a three-phase distribution circuit breaker connected to a three-phase AC power supply, three-phase cable distribution lines connected to the three-phase distribution circuit breaker, and shunt reactors connected to each of the three-phase cable distribution lines, comprising: detecting a ground fault in the three-phase cable distribution line; detecting the amplitude of the current flowing through at least two of the three-phase distribution circuit breakers; determining that a heavy load condition in which the detected amplitude of the current flowing through at least two phases is greater than or equal to a threshold has continued for a first hour; and, if a ground fault is detected and it is determined that a heavy load condition has continued for a first hour, outputting a tripping command to the three-phase distribution circuit breaker.

[0013] By implementing the above protection control method, the same effect as the above protection control device is achieved. [Brief explanation of the drawing]

[0014] [Figure 1] This is a schematic diagram showing the configuration of the power distribution system according to this embodiment. [Figure 2] This figure shows the configuration of the protective control device according to this embodiment. [Figure 3] This figure shows an overview of the confirmation timer and delay timer according to this embodiment. [Figure 4] This figure shows the current distribution of the power distribution system according to this embodiment during a ground fault under no-load conditions. [Figure 5] This figure shows the voltage amplitude fluctuations in each phase and the neutral point during a ground fault. [Figure 6] This figure shows the current waveforms flowing through the healthy phase shunt reactor before and after a ground fault. [Figure 7] This diagram shows the cancellation between the current flowing through the healthy phase shunt reactor and the current flowing through the capacitance to ground. [Figure 8] This figure shows the current waveforms flowing through the healthy phase distribution circuit breaker before and after a ground fault under light load conditions. [Figure 9]It is a diagram showing the current waveform flowing through the distribution breaker of a healthy phase when a ground fault occurs in a phase where no transient DC component occurs in a light load state. [Figure 10] It is a diagram showing the current waveform flowing through the distribution breaker of a healthy phase before and after a ground fault in a light load state. [Figure 11] It is a diagram showing the current waveform flowing through the distribution breaker of a ground fault phase before and after a ground fault in a light load state. [Figure 12] Figure 12A shows the current waveform flowing through the distribution breaker of the ground fault phase during a ground fault in a no-load state. Figure 12B shows the current waveform flowing through the distribution breaker of a healthy phase during a ground fault in a no-load state. [Figure 13] Figure 13A shows the current waveform flowing through the shunt reactor of the ground fault phase during a ground fault in a no-load state. Figure 13B shows the current waveform flowing through the shunt reactor of a healthy phase during a ground fault in a no-load state. [Figure 14] It is a diagram showing the current distribution of the power distribution system according to the present embodiment during a ground fault in a heavy load state. [Figure 15] It is a diagram showing the current waveform flowing through the distribution breaker of a healthy phase before and after a ground fault in a heavy load state. [Figure 16] Figure 16A is a diagram showing the current waveform flowing through the distribution breaker of the ground fault phase during a ground fault in a heavy load state. Figure 16B is a diagram showing the current waveform flowing through the distribution breaker of a healthy phase during a ground fault in a heavy load state. [Figure 17] Figure 17A shows the current waveform flowing through the shunt reactor of the ground fault phase during a ground fault in a heavy load state. Figure 17B shows the current waveform flowing through the shunt reactor of a healthy phase during a ground fault in a no-load state. [Embodiments for Carrying Out the Invention]

[0015] (Embodiment) <1. Power Distribution System> Referring to Figure 1, the overall configuration of the power distribution system 100 according to this embodiment will be described. The power distribution system 100 distributes three-phase AC power to loads. The three-phase AC power to be distributed is power obtained by transforming the three-phase AC power supplied by the power company. The power distribution system 100 includes three-phase cable distribution lines 56a, 56b, 56c, three-phase distribution circuit breakers 53a, 53b, 53c, three-phase current sensors 52a, 52b, 52c, three-phase shunt reactors 54a, 54b, 54c, and a protection control device 10.

[0016] The three-phase distribution circuit breakers 53a, 53b, and 53c are connected to the three-phase AC power supplies 51a, 51b, and 51c. The three-phase AC power supplies 51a, 51b, and 51c include transformers that step down the three-phase AC power received from the power company. The neutral points of the three-phase AC power supplies 51a, 51b, and 51c are grounded via grounding resistors 50. The a-phase distribution circuit breaker 53a is connected to the a-phase AC power supply 51a, the b-phase distribution circuit breaker 53b is connected to the b-phase AC power supply 51b, and the c-phase distribution circuit breaker 53c is connected to the c-phase AC power supply 51c.

[0017] The three-phase cable distribution lines 56a, 56b, and 56c are connected to three-phase distribution circuit breakers 53a, 53b, and 53c, respectively. The a-phase cable distribution line 56a is connected to the a-phase distribution circuit breaker 53a, the b-phase cable distribution line 56b is connected to the b-phase distribution circuit breaker 53b, and the c-phase cable distribution line 56c is connected to the c-phase distribution circuit breaker 53c.

[0018] The three-phase distribution circuit breakers 53a, 53b, and 53c are a type of switch that closes (i.e., energizes) or opens (i.e., disconnects) the connection between the three-phase AC power sources 51a, 51b, and 51c and the three-phase cable distribution lines 56a, 56b, and 56c. The three-phase distribution circuit breakers 53a, 53b, and 53c are controlled to be energized or disconnected as a whole for all three phases. When the three-phase distribution circuit breakers 53a, 53b, and 53c are energized, the three-phase cable distribution lines 56a, 56b, and 56c distribute the power received from the three-phase AC power sources 51a, 51b, and 51c to the load. For example, the load may be ancillary equipment for a train.

[0019] The three-phase cable distribution lines 56a, 56b, and 56c each have three relative ground capacitances 55a, 55b, and 55c, respectively. The three-phase shunt reactors 54a, 54b, and 54c are connected between the three-phase cable distribution lines 56a, 56b, and 56c and the ground, respectively. Specifically, the a-phase shunt reactor 54a is connected in parallel to the a-relative ground capacitance 55a, the b-phase shunt reactor 54b is connected in parallel to the b-relative ground capacitance 55b, and the c-phase shunt reactor 54c is connected in parallel to the c-relative ground capacitance 55c.

[0020] The three-phase relative ground capacitances 55a, 55b, and 55c consume and absorb leading reactive power (supplying lagging reactive power) according to their charging capacity. As a result, there is a possibility that the system voltage of the distribution system 100 will be higher than that of the three-phase AC power sources 51a, 51b, and 51c, a decrease in the transmission capacity of the active power distributed by the distribution system 100, and an increase in voltage fluctuations. The three-phase shunt reactors 54a, 54b, and 54c mitigate the problems caused by the three-phase relative ground capacitances 55a, 55b, and 55c by supplying leading reactive power (consuming lagging reactive power).

[0021] The three-phase current sensors 52a, 52b, and 52c are connected to the three-phase cable distribution lines 56a, 56b, and 56c, respectively, and detect the magnitude of the current flowing through the three-phase distribution circuit breakers 53a, 53b, and 53c. The a-phase current sensor 52a detects the magnitude of the current flowing through the a-phase distribution circuit breaker 53a, the b-phase current sensor 52b detects the magnitude of the current flowing through the b-phase distribution circuit breaker 53b, and the c-phase current sensor 52c detects the magnitude of the current flowing through the c-phase distribution circuit breaker 53c. In this embodiment, the three-phase current sensors 52a, 52b, and 52c are current transformers that transform the large current flowing through the three-phase distribution circuit breakers 53a, 53b, and 53c, which are input to the primary side, into proportionally smaller currents and output them to the protection control device 10.

[0022] When the protection control device 10 detects a ground fault in any of the phases of the three-phase cable distribution lines 56a, 56b, or 56c, it controls the three-phase distribution circuit breakers 53a, 53b, and 53c to an interrupted state in order to limit the effects of the ground fault, thereby interrupting the current flowing through the three-phase cable distribution lines 56a, 56b, and 56c. However, when the three-phase distribution circuit breakers 53a, 53b, and 53c are interrupted, an arc discharge may occur between the electrodes, causing current to continue flowing through the three-phase distribution circuit breakers 53a, 53b, and 53c. Consequently, the three-phase distribution circuit breakers 53a, 53b, and 53c may be damaged. However, since the current flowing through the three-phase distribution circuit breakers 53a, 53b, and 53c is alternating current, there will eventually be a point at which it becomes zero, at which point the current is interrupted. The timing of this zero point varies depending on the magnitude of the load connected to the distribution system 100. When the protection control device 10 detects a ground fault, it trips the three-phase distribution circuit breakers 53a, 53b, and 53c at an appropriate timing according to the load. Details of the protection control device 10 will be described later.

[0023] <2. Ground faults under no-load and light-load conditions> Figure 4 shows the current distribution in the power distribution system 100 when phase a is grounded under no-load conditions. In Figure 4, solid arrows indicate the forward direction of the current. Dashed arrows indicate transient DC current. Dotted arrows indicate leading AC current, and dashed arrows indicate lagging AC current. Another dashed arrow indicates the resistor current flowing through the grounding resistor 50.

[0024] During a phase a ground fault, the current flowing through the grounding resistor 50, the leading AC current flowing through the phase b and c relative ground capacitances 55b and 55c, and the lagging AC current and transient DC current flowing through the phase b and c shunt reactors 54b and 54c, all combine to form a ground fault current at the fault point. The resistive current is the AC current that flows from the earth to the grounding resistor 50 and flows through the neutral point to the phase a distribution circuit breaker 53a. The leading AC current flows from the earth to the phase b and c relative ground capacitances 55b and 55c and then to the phase b and c distribution circuit breakers 53b and 53c. The leading AC current corresponds to the charging current that charges the phase b and c relative ground capacitances 55b and 55c. The lagging AC current flows from the ground to the b-phase and c-phase shunt reactors 54b and 54c, and then to the b-phase and c-phase distribution circuit breakers 53b and 53c. The transient DC current is explained below.

[0025] As shown in Figure 5, when a ground fault occurs in phase a, the amplitude of the phase a voltage to ground becomes zero, and the amplitude of the neutral point voltage to ground increases. Then, the amplitudes of the phase b and c voltages to ground, which are healthy phases, increase. Due to the fluctuation in the phase b and c voltages to ground, the current flowing through the phase b and c shunt reactors 54b and 54c also changes. However, since the phase b and c shunt reactors 54b and 54c are coils, they act to prevent abrupt changes in current. That is, if there is a difference between the AC current corresponding to the phase b voltage before the ground fault and the AC current corresponding to the phase c voltage after the ground fault, the AC current does not change immediately after the ground fault, and a transient DC component current with the same magnitude as the difference but in the opposite direction is generated.

[0026] Therefore, as shown in Figure 4, after a ground fault, transient DC current flows through the b-phase and c-phase shunt reactors 54b and 54c. The transient DC current is a DC component that decays over time. As shown in Figure 6, after a ground fault, a current flows through the healthy b-phase and c-phase shunt reactors 54b and 54c, which is a lagging AC current superimposed with the transient DC current. In other words, a lagging AC current offset by the magnitude of the transient DC current flows through the b-phase and c-phase shunt reactors 54b and 54c.

[0027] Therefore, a combined current consisting of transient DC current, lagging AC current, and leading AC current flows through the b-phase and c-phase distribution circuit breakers 53b and 53c. As shown in Figure 7, the lagging AC current, which is 90° behind in phase, and the leading AC current, which is 90° ahead in phase, have a phase difference of 180° and are diametrically opposed. Therefore, the lagging AC current and the leading AC current cancel each other out. Furthermore, in the no-load state, the current flowing from the load to the b-phase and c-phase distribution circuit breakers 53b and 53c after a ground fault is zero. Also, in the light-load state, the amplitude of the AC current flowing from the load to the b-phase and c-phase distribution circuit breakers 53b and 53c after a ground fault is small. As a result, in the no-load or light-load state, as shown in Figure 8, the current flowing through the b-phase and c-phase distribution circuit breakers 53b and 53c is dominated by transient DC current, and the amplitude of the AC current is small.

[0028] Therefore, in no-load or light-load conditions, the amplitude of the AC current flowing through the b-phase and c-phase distribution circuit breakers 53b and 53c is small compared to the magnitude of the transient DC current, and the current flows through the b-phase and c-phase distribution circuit breakers 53b and 53c until the transient DC current has sufficiently decayed, but it does not cross zero. In other words, for a while immediately after a ground fault, a zero-miss current flows through the b-phase and c-phase distribution circuit breakers 53b and 53c. For this reason, when in no-load or light-load conditions, the protection control device 10 issues a tripping command to the three-phase distribution circuit breakers 53a, 53b, and 53c after a period of time has elapsed since ground fault detection, during which it is estimated that the transient DC current has sufficiently decayed.

[0029] Furthermore, if the moment of the ground fault coincides with the moment when the AC currents flowing through the b-phase and c-phase shunt reactors 54b and 54c immediately before and after the ground fault become zero, no transient DC current will be generated. In that case, as shown in Figure 9, the current flowing through the b-phase and c-phase distribution circuit breakers 53b and 53c will be the AC current after the leading AC current and lagging AC current have canceled each other out, and the moment when it becomes zero immediately after the ground fault arrives. However, since the above-mentioned moment of ground fault occurs only twice in one cycle of the AC current, the probability of a transient DC current being generated is very high. Therefore, the protection control device 10 issues tripping commands to the three-phase distribution circuit breakers 53a, 53b, and 53c after a time has elapsed after ground fault detection that is estimated to allow sufficient decay of the transient DC current.

[0030] Furthermore, if a ground fault occurs at a timing 180° different in phase from the case shown in Figure 8, the sign of the transient DC component current will be reversed compared to the transient DC component current shown in Figure 8, as shown in Figure 10. In Figure 8, a positive transient DC component current is generated, but in Figure 10, a negative transient DC component current is generated, and the AC component current flowing through the b-phase and c-phase distribution circuit breakers 53b and 53c is offset to the negative side.

[0031] Furthermore, as shown in Figure 4, the a-phase distribution circuit breaker 53a, which is the ground fault phase, receives a resistive current flowing from the grounding resistor 50, as well as transient DC current, lagging AC current, and leading AC current flowing from the b-phase and c-phase. Among these currents, the resistive current has a large amplitude and is dominant. Therefore, as shown in Figure 11, the current flowing through the a-phase distribution circuit breaker 53a becomes zero immediately after the ground fault, even if the transient DC current is superimposed. However, since the protection control device 10 controls the three-phase distribution circuit breakers 53a, 53b, and 53c collectively, it outputs a tripping command to the three-phase distribution circuit breakers 53a, 53b, and 53c only after the transient DC current has sufficiently decayed.

[0032] Figure 12A shows the waveform of the current flowing through the a-phase distribution circuit breaker 53a during a ground fault under no-load conditions, and Figure 12B shows the waveform of the current flowing through the b-phase and c-phase distribution circuit breakers 53b and 53c during a ground fault under no-load conditions. Furthermore, Figure 13A shows the waveform of the current flowing through the a-phase shunt reactor 54a during a ground fault under no-load conditions, and Figure 13B shows the waveform of the current flowing through the b-phase and c-phase shunt reactors 54b and 54c during a ground fault under no-load conditions. After the ground fault, no zero-miss current flows through the a-phase distribution circuit breaker 53a, but zero-miss current flows through the b-phase and c-phase distribution circuit breakers 53b and 53c for a predetermined period of time.

[0033] <3. Ground fault under heavy load conditions> Figure 14 shows the current distribution of the power distribution system 100 when phase a is grounded under heavy load conditions. In Figure 14, the dashed arrows indicate the load current flowing from the load. In Figure 14, in addition to the current distribution shown in Figure 4, the load current, which is the AC component, flows from the load to the b-phase and c-phase distribution circuit breakers 53b and 53c. Therefore, the combined current of the load current, transient DC component, lagging AC component, and leading AC component flows through the b-phase and c-phase distribution circuit breakers 53b and 53c.

[0034] Under heavy load conditions, the amplitude of the load current is larger than the magnitude of the transient DC current. Therefore, as shown in Figure 15, the current flowing through the b-phase and c-phase distribution circuit breakers 53b and 53c will become zero immediately after the ground fault, even if the transient DC current is superimposed. Consequently, under heavy load conditions, the protection control device 10 outputs a tripping command to the three-phase distribution circuit breakers 53a, 53b, and 53c all at once without waiting for the transient DC current to decay.

[0035] Figure 16A shows the waveform of the current flowing through the a-phase distribution circuit breaker 53a during a ground fault under heavy load conditions, and Figure 16B shows the waveform of the current flowing through the b-phase and c-phase distribution circuit breakers 53b and 53c during a ground fault under heavy load conditions. Furthermore, Figure 17A shows the waveform of the current flowing through the a-phase shunt reactor 54a during a ground fault under heavy load conditions, and Figure 17B shows the waveform of the current flowing through the b-phase and c-phase shunt reactors 54b and 54c during a ground fault under heavy load conditions.

[0036] The waveform of the current flowing through the b-phase and c-phase shunt reactors 54b and 54c is the same as the waveform of the current flowing through the b-phase and c-phase shunt reactors 54b and 54c during a ground fault under no-load conditions. However, the waveform of the current flowing through the b-phase and c-phase distribution circuit breakers 53b and 53c differs from the waveform of the current flowing through the b-phase and c-phase distribution circuit breakers 53b and 53c during a ground fault under no-load conditions because the load current is superimposed. In this case, there is no zero-miss current flowing, and the current crosses zero immediately after the ground fault.

[0037] <4. Protection and Control Devices> Referring to Figure 2, the configuration of the protection control device 10 according to this embodiment will be described. The protection control device 10 comprises a ground fault detection unit 20, a load current detection unit 30, and a control circuit 40. The ground fault detection unit 20 detects that a ground fault has occurred in the three-phase cable distribution lines 56a, 56b, and 56c based on the sum of the currents flowing through the three-phase cable distribution lines 56a, 56b, and 56c. The load current detection unit 30 detects the amplitude of the current flowing through the three-phase distribution circuit breakers 53a, 53b, and 53c. The control circuit 40 outputs a tripping command to the three-phase distribution circuit breakers 53a, 53b, and 53c when either the first or second condition is met. The first condition is met when a ground fault is detected and the amplitude of the detected three-phase current remains above an overcurrent threshold for a period of time. The second condition is met when two hours have elapsed since the ground fault was detected. The second hour is longer than the first hour.

[0038] More specifically, the ground fault detection unit 20 includes a zero-sequence current differential relay 21. The zero-sequence current differential relay 21 is a ground fault protection relay for detecting ground faults with high accuracy. The zero-sequence current differential relay 21 receives the currents flowing from the three-phase current sensors 52a, 52b, and 52c to the three-phase distribution circuit breakers 53a, 53b, and 53c, respectively. The zero-sequence current differential relay 21 turns on when the sum of the currents flowing through the three-phase distribution circuit breakers 53a, 53b, and 53c is equal to or greater than the ground fault threshold, and outputs "1" to the AND circuit 43 and delay timer 44, which will be described later. The zero-sequence current differential relay 21 turns off when the sum of the three-phase currents is less than the ground fault threshold, and outputs "0" to the AND circuit 43 and delay timer 44.

[0039] The sum of the three phase currents equals the current flowing through the neutral point. When no phases are ground faulted, the current flowing through the neutral point is zero. When any phase is ground faulted, the current flowing through the neutral point increases. Therefore, the zero-sequence current differential relay 21 turns on when the sum of the three phase currents is greater than or equal to the ground fault threshold, that is, when any phase is ground faulted. The ground fault threshold is, for example, a value close to zero.

[0040] The load current detection unit 30 includes an a-phase overcurrent relay 31, a b-phase overcurrent relay 32, and a c-phase overcurrent relay 33. The a-phase overcurrent relay 31, the b-phase overcurrent relay 32, and the c-phase overcurrent relay 33 are overcurrent protection relays.

[0041] The a-phase overcurrent relay 31 receives the current flowing through the a-phase distribution circuit breaker 53a. The b-phase overcurrent relay 32 receives the current flowing through the b-phase distribution circuit breaker 53b. The c-phase overcurrent relay 33 receives the current flowing through the c-phase distribution circuit breaker 53c. The a-phase overcurrent relay 31 turns on and outputs "1" to the AND circuit 41 described later when the amplitude of the AC component of the input current, excluding the DC component, is greater than or equal to the overcurrent threshold. The a-phase overcurrent relay 31 turns off and outputs "0" to the AND circuit 41 when the amplitude of the AC component of the input is less than the overcurrent threshold. The overcurrent threshold is set to be greater than or equal to the maximum value of the transient DC current flowing through the shunt reactor 54a, based on the reactance and resistance values ​​of the shunt reactor 54a.

[0042] Similarly, the b-phase overcurrent relay 32 turns on and outputs "1" to the AND circuit 41 when the amplitude of the input AC component is greater than or equal to the overcurrent threshold, and turns off and outputs "0" to the AND circuit 41 when the amplitude of the input AC component is less than the overcurrent threshold. The c-phase overcurrent relay 33 turns on and outputs "1" to the AND circuit 41 when the amplitude of the input AC component is greater than or equal to the overcurrent threshold, and turns off and outputs "0" to the AND circuit 41 when the amplitude of the input AC component is less than the overcurrent threshold.

[0043] The control circuit 40 includes an AND circuit 41, a confirmation timer 42, an AND circuit 43, a delay timer 44, and an OR circuit 45. The AND circuit 41 outputs "1" to the confirmation timer 42 when "1" is input from all of the a-phase, b-phase, and c-phase overcurrent relays 31, 32, and 33. The AND circuit 41 outputs "0" to the confirmation timer 42 when "0" is input from at least one of the a-phase, b-phase, and c-phase overcurrent relays 31, 32, and 33. In other words, the AND circuit 41 outputs "1" when the amplitude of the current flowing through each of the three-phase distribution circuit breakers 53a, 53b, and 53c is all greater than or equal to the overcurrent threshold. The AND circuit 41 outputting "1" corresponds to a heavy load being connected to the distribution system 100.

[0044] The confirmation timer 42 outputs "1" to the AND circuit 43 if "1" is continuously input from the AND circuit 41 for the first time, and outputs "0" to the AND circuit 43 if "1" is not continuously input from the AND circuit 41 for the first time.

[0045] The first time is set to be longer than the time from when the three-phase distribution circuit breakers 53a, 53b, and 53c are energized until the current disturbance is suppressed. As shown in Figure 3, immediately after energizing the three-phase distribution circuit breakers 53a, 53b, and 53c, a disturbance occurs in the current flowing through the three-phase distribution circuit breakers 53a, 53b, and 53c, and the amplitude of the current becomes large even when a light load is connected to the distribution system 100. In other words, immediately after energizing the three-phase distribution circuit breakers 53a, 53b, and 53c, even when a light load is connected to the distribution system 100, it appears as if a heavy load is connected. Therefore, the confirmation timer 42 outputs "1" if "1" is continuously input from the AND circuit 41 beyond the time of current disturbance occurrence.

[0046] The first time is set by pre-analyzing the current waveform using XTAP (an instantaneous value analysis program that analyzes transient phenomena in electrical circuits at the waveform level). Specifically, the current that flows when the three-phase distribution circuit breakers 53a, 53b, 53c are energized is analyzed from the resistance and reactance values ​​of the power supply system connected to the three-phase AC power supplies 51a, 51b, 51c, the resistance and reactance values ​​of the three-phase shunt reactors 54a, 54b, 54c, and the resistance, reactance values ​​and ground capacitance values ​​of the three-phase cable distribution lines 56a, 56b, 56c, and the current waveform of each phase is obtained. Then, the time until the current disturbance of each phase disappears is calculated from the obtained three-phase current waveforms, and the first time is set to be longer than the longest time among the three phases.

[0047] The AND circuit 43 outputs "1" to the OR circuit 45 when it receives a "1" from both the zero-sequence current differential relay 21 and the confirmation timer 42. The AND circuit 43 outputs "0" to the OR circuit 45 when it receives a "0" from at least one of the zero-sequence current differential relay 21 and the confirmation timer 42. In other words, the AND circuit 43 outputs "1" when the first condition described above is met, and outputs "0" when the first condition is not met.

[0048] The delay timer 44 outputs the input value ("1" or "0") received from the zero-sequence current differential relay 21 to the OR circuit 45 after two time intervals have elapsed since the input. In other words, the delay timer 44 outputs "1" to the OR circuit 45 if the second condition described above is met, and outputs "0" if the second condition is not met.

[0049] As shown in Figure 3, the second time is analyzed using XTAP or similar software and is preset to be longer than the time from immediately after the ground fault is detected until the current flowing through the healthy three-phase distribution circuit breakers 53a, 53b, and 53c first crosses zero. Specifically, the current flowing through the three-phase distribution circuit breakers 53a, 53b, and 53c when a ground fault occurs is analyzed from the resistance and reactance values ​​of the power supply system connected to the three-phase AC power supplies 51a, 51b, and 51c, the resistance and reactance values ​​of the three-phase shunt reactors 54a, 54b, and 54c, and the resistance, reactance values ​​and capacitance to ground values ​​of the three-phase cable distribution lines 56a, 56b, and 56c, and the current waveform of each phase is obtained. Then, from the obtained current waveforms of the two healthy phases, the time from immediately after the ground fault occurs until the current first crosses zero is calculated, and the second time is set to be longer than the longest time among the two healthy phases.

[0050] The OR circuit 45 outputs a tripping command to the three-phase distribution circuit breakers 53a, 53b, and 53c when it receives a "1" input from the AND circuit 43 or the delay timer 44. In other words, the OR circuit 45 outputs a tripping command to the three-phase distribution circuit breakers 53a, 53b, and 53c when either the first or second condition described above is met. As a result, the three-phase distribution circuit breakers 53a, 53b, and 53c are tripped when either the first or second condition is met. This ensures that the three-phase distribution circuit breakers 53a, 53b, and 53c are tripped at the appropriate timing depending on whether or not a heavy load is connected to the distribution system 100.

[0051] In this embodiment, the confirmation timer 42 corresponds to an example of the determination unit of this disclosure, and the OR circuit 45 corresponds to an example of the command output unit of this disclosure. Furthermore, the AND circuit 43 corresponds to an example of the first output unit of this disclosure, and the delay timer 44 corresponds to an example of the second output unit of this disclosure.

[0052] <3. Effects> According to the embodiment described in detail above, the following effects are achieved. (1) When the first condition is met, the protection control device 10 outputs a tripping command to the three-phase distribution circuit breakers 53a, 53b, and 53c. This allows the protection control device 10 to exclude situations where a light load appears heavy due to a transient disturbance, and when a heavy load is connected to the distribution system 100, it can trip all three-phase distribution circuit breakers 53a, 53b, and 53c at the appropriate time in the event of a ground fault, while suppressing increased costs and damage to the three-phase distribution circuit breakers 53a, 53b, and 53c. Ultimately, this allows the distribution system 100 to be protected at the appropriate time.

[0053] (2) When the second condition is met, the protection control device 10 outputs a tripping command to the three-phase distribution circuit breakers 53a, 53b, and 53c. As a result, even when the load connected to the distribution system 100 is light, the protection control device 10 can trip the three-phase distribution circuit breakers 53a, 53b, and 53c all at once when the transient DC current has decayed, thereby protecting the distribution system 100.

[0054] (3) In the control circuit 40, an AND circuit 43 that takes the output from the confirmation timer 42 as input and a delay timer 44 that does not depend on the output from the confirmation timer 42 are arranged in parallel. As a result, even if a heavy load is connected to the power distribution system 100 and a failure occurs in the load current detection unit 30, resulting in no "1" being output from the confirmation timer 42, the three-phase power distribution circuit breakers 53a, 53b, and 53c can be tripped if the second condition is met.

[0055] (4) The first time is set to be longer than the time from the start of energization to the three-phase distribution circuit breakers 53a, 53b, and 53c until the current disturbance is suppressed, thereby eliminating the effects of temporary current disturbances. This prevents the three-phase distribution circuit breakers 53a, 53b, and 53c from being immediately tripped when a light load is connected to the distribution system 100, due to a false determination that a heavy load is connected, before the current flowing through the healthy phases approaches zero. Consequently, it is possible to suppress the induction of failures in the three-phase distribution circuit breakers 53a, 53b, and 53c due to arc discharge.

[0056] (5) The second time is set to be longer than the time from the occurrence of the ground fault until the current flowing through the healthy phase crosses zero, so that when a light load is connected to the distribution system 100, the three-phase distribution circuit breakers 53a, 53b, and 53c can be tripped all at once after the transient DC current has sufficiently decayed.

[0057] (Other embodiments) Although embodiments of the present disclosure have been described above, the present disclosure is not limited to the embodiments described above and can be implemented in various modified forms.

[0058] (a) In the above embodiment, the load current detection unit 30 was equipped with a-phase, b-phase, and c-phase overcurrent relays 31, 32, and 33. However, it may be equipped with two of these overcurrent relays, and the AND circuit 41 may output "1" when the amplitude of the current flowing through two of the three-phase distribution circuit breakers 53a, 53b, and 53c is all greater than or equal to the overcurrent threshold. That is, the above first condition may be met when a ground fault is detected and the state in which the amplitude of the detected two-phase current is greater than or equal to the overcurrent threshold continues for a first time. It is possible to determine whether or not a heavy load is connected to the distribution system 100 based on the amplitude of the two-phase current alone. Note that it is not possible to determine whether or not a heavy load is connected to the distribution system 100 based on the amplitude of the current of one phase alone. If the phase used for determination is the ground fault phase, the amplitude of the current in the ground fault phase will be large regardless of the magnitude of the load connected to the distribution system 100. Therefore, to ensure that the amplitude of the current in a healthy phase is always used for the determination, it is determined whether a heavy load is connected to the distribution system 100 based on the amplitude of the currents in two or more phases.

[0059] (b) Multiple functions of one component in the above embodiment may be realized by multiple components, or one function of one component may be realized by multiple components. Also, multiple functions of multiple components may be realized by one component, or one function realized by multiple components may be realized by one component. Furthermore, some of the configurations of the above embodiment may be omitted. Furthermore, at least some of the configurations of the above embodiment may be added to or replaced with the configurations of other above embodiments. [Explanation of Symbols]

[0060] 10...Protection control device, 20...Ground fault detection unit, 21...Zero-phase current differential relay, 30...Load current detection unit, 31...a-phase overcurrent relay, 32...b-phase overcurrent relay, 33...c-phase overcurrent relay, 40...Control circuit, 41,43...AND circuit, 42...Confirmation timer, 44...Delay timer, 45...OR circuit, 50...Grounding resistor, 51a,51b,51c...Three-phase AC power supply, 52a,52b,52c...Three-phase current sensor, 53a,53b,53c...Three-phase distribution circuit breaker, 54a,54b,54c...Three-phase shunt reactor, 55a,55b,55c...Three-phase relative ground capacitance, 56a,56b,56c...Three-phase cable distribution line, 100...Distribution system.

Claims

1. A protection control device for protecting a power distribution system comprising a three-phase distribution circuit breaker connected to a three-phase AC power supply, a three-phase cable distribution line connected to the three-phase distribution circuit breaker, and a shunt reactor connected to each of the three-phase cable distribution lines, A ground fault detection unit for detecting ground faults in the three-phase cable distribution line, A current detection unit for detecting the amplitude of the current flowing through at least two phases of the three-phase distribution circuit breaker, A determination unit determines that a heavy load condition in which the amplitude of the current flowing through at least two phases detected by the current detection unit is greater than or equal to a threshold has continued for a first time, The system includes a command output unit that outputs a tripping command to the three-phase distribution circuit breaker when the ground fault detection unit detects the ground fault and the determination unit determines that the heavy load condition has continued for the first time. Protection control device.

2. The command output unit further outputs the tripping command to the three-phase distribution circuit breaker when a second time has elapsed since the ground fault was detected by the ground fault detection unit. The second time is longer than the first time. The protective control device according to claim 1.

3. The current detection unit detects the magnitude of the amplitude of the current flowing through each of the three-phase power distribution circuit breakers. The determination unit determines that a heavy load condition, in which the amplitude of the current flowing through each of the three-phase power distribution circuit breakers detected by the current detection unit is greater than or equal to the threshold, has continued for the first time. The protective control device according to claim 1 or 2.

4. A first output unit, which takes the detection result from the ground fault detection unit and the determination result from the determination unit as inputs, outputs a first output value when it is determined that a ground fault has been detected and the heavy load condition has continued for the first time, The system further includes a second output unit that takes the detection result from the ground fault detection unit as input and outputs a second output value when the second time has elapsed since the ground fault was detected, The command output unit outputs the cutoff command when the first output value is output from the first output unit, or when the second output value is output from the second output unit. The protective control device according to claim 2.

5. The first time is set to be longer than the time from the start of energization to the three-phase distribution circuit breaker until current disturbance is suppressed, which is estimated based on the resistance and reactance values ​​of the power supply system connected to the three-phase AC power supply, the resistance and reactance values ​​of the shunt reactor, and the resistance, reactance and capacitance to ground values ​​of each of the three-phase cable distribution lines. A protective control device according to any one of claims 1, 2, or 4.

6. The second time is set to be longer than the time from the occurrence of the ground fault until the current flowing through the healthy phase of the three-phase distribution circuit breaker is zero-loss, which is estimated based on the resistance and reactance values ​​of the power supply system connected to the three-phase AC power supply, the resistance and reactance values ​​of the shunt reactor, and the resistance, reactance and capacitance values ​​to ground of each of the three-phase cable distribution lines. A protective control device according to any one of claims 1, 2, or 4.

7. A protection and control method for a power distribution system comprising: a three-phase distribution circuit breaker connected to a three-phase AC power supply; a three-phase cable distribution line connected to the three-phase distribution circuit breaker; and a shunt reactor connected to each of the three-phase cable distribution lines, To detect a ground fault in the aforementioned three-phase cable distribution line, The magnitude of the amplitude of the current flowing through at least two phases of the three-phase distribution circuit breaker is detected, It is determined that a heavy load condition in which the amplitude of the detected current flowing through at least two phases is greater than or equal to a threshold has continued for a first time, The system includes the following: when the ground fault is detected and it is determined that the heavy load condition has continued for the first time, a tripping command is output to the three-phase distribution circuit breaker. Protection and control method.

8. Outputting a tripping command to the three-phase distribution circuit breaker includes outputting the tripping command when two hours have elapsed since the ground fault was detected. The protection control method according to claim 7.

9. Based on the ground fault detection result and the heavy load condition determination result, if it is determined that the ground fault has been detected and the heavy load condition has continued for the first time, a first output value is output. The system further includes, upon receiving the ground fault determination result, outputting a second output value when the second time has elapsed since the ground fault was detected, Outputting a tripping command to the three-phase distribution circuit breaker includes outputting the tripping command when the first output value is output or when the second output value is output. The protection control method according to claim 8.

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