Protective relay and distribution board

Abstract

To solve the problem that a phase of a zero-phase current cannot be accurately detected because magnetic saturation of a current transformer and saturation of a BPF occur due to the zero-phase current.SOLUTION: A protective relay according to the present disclosure includes: a current transformer in which a zero-phase current is input to a primary side; a transformer in which a zero-phase voltage is input to a primary side; a filter circuit including a low-pass filter that extracts a basic frequency component of the zero-phase current and the zero-phase voltage and a high frequency component of the zero-phase current and the zero-phase voltage, and a band-pass filter that extracts a signal of a basic frequency of the zero-phase current and the zero-phase voltage; a measurement unit that measures magnitudes and phases of the zero-phase current and the zero-phase voltage extracted by the filter circuit; a control unit that calculates the phase difference from a phase when a magnitude of the zero-phase current and a magnitude of the zero-phase voltage exceed a threshold value, determines an accident system and a healthy system of a power system, and operates a circuit breaker of the power system; and a shunt resistor that is connected in parallel with both ends of the primary side of the current transformer and shunts a current flowing on the primary side of the current transformer so as to be smaller than a current magnetically saturated by the current transformer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a protective relay used for earth fault directional protection of a power system and a switchboard equipped with the same. [Background technology]

[0002] When a ground fault occurs and each phase of a polyphase AC circuit becomes unbalanced, zero-sequence current and zero-sequence voltage are generated. The technique of detecting these zero-sequence current and zero-sequence voltage to isolate only the faulted system in a power system and protect the remaining system is called earth fault protection. Earth fault directional protective relays identify the faulted section of a power system by detecting the magnitude of the zero-sequence current and zero-sequence voltage and the phase difference between the zero-sequence current and zero-sequence voltage via a ZCT (Zero Phase Sequence Current Transfer), EVT (Earthing Voltage Transfer), or ZPD (Zero Phase Voltage Detector). Accurately identifying the faulted section of the line requires accurate detection of the magnitude and phase difference between the zero-sequence current and zero-sequence voltage.

[0003] When an intermittent ground fault occurs in a distribution line, a ground fault voltage and a pulsed zero-phase current of several tens to several hundreds of amperes are generated. As in Patent Document 1, a protective relay with directional ground fault protection is sometimes provided with a band pass filter (BPF) to distinguish between a healthy system and a faulted system. However, when the zero-phase current becomes large, the output of the BPF saturates, making it difficult to correctly detect the phase difference of the zero-phase current. To prevent this, Patent Document 1 discloses a technique that combines a BPF with a varistor element. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 60-204218 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in conventional protective relays with directional earth fault protection, when a large current flows on the secondary side of the ZCT, there was a problem that the phase of the zero-phase current could not be accurately detected due to magnetic saturation of the current transformer used to measure the zero-phase current and saturation of the BPF output.

[0006] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a protective relay that can suppress magnetic saturation of a current transformer through which a zero-phase current flows and prevent saturation of the BPF output. [Means for solving the problem]

[0007] The protective relay according to the present disclosure includes a current transformer, the primary side of which receives a zero-phase current flowing through the power system when a fault occurs in the power system, via a zero-phase current transformer; a transformer, the primary side of which receives a zero-phase voltage applied to the power system when a fault occurs in the power system; low-pass filters, which are provided on the secondary side of the current transformer and the secondary side of the transformer, respectively, and which extract fundamental frequency components of the zero-phase current and the zero-phase voltage and high-frequency components of the zero-phase current and the zero-phase voltage; and low-pass filters, which are provided on the secondary side of the current transformer and the secondary side of the transformer, respectively, and which extract fundamental frequency signals of the zero-phase current and the zero-phase voltage. a measurement unit that measures the magnitude of the zero-phase current and zero-phase voltage extracted by the filter circuit and measures the phase of the zero-phase current and zero-phase voltage; a control unit that monitors the power system, etc., and, when the magnitude of the zero-phase current and zero-phase voltage exceeds a threshold, calculates a phase difference from the phase to distinguish between a faulted system and a healthy system in the power system and activates a circuit breaker in the power system; and a shunt resistor that is connected in parallel to both ends of the primary side of the current transformer and shunts the current flowing in the primary side of the current transformer so that it is smaller than the current at which the current transformer magnetically saturates. [Effects of the Invention]

[0008] According to the protective relay of the present disclosure, magnetic saturation of the current transformer used to detect the zero-phase current can be suppressed and saturation of the BPF output can be prevented, thereby enabling accurate detection of the phase of the zero-phase current. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic diagram illustrating a configuration of a protective relay according to a first embodiment of the present disclosure. [Figure 2] 4 is a graph showing an operating range of the protective relay according to the first embodiment of the present disclosure. [Figure 3] 1 is a circuit diagram showing an equivalent circuit of a current transformer of a protective relay according to a first embodiment of the present disclosure. [Figure 4] 1 is a schematic diagram illustrating a configuration of a filter circuit of a protective relay according to a first embodiment of the present disclosure. [Figure 5] FIG. 10 is a schematic diagram illustrating a configuration of a protective relay according to a second embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, a protective relay according to a first embodiment of the present disclosure will be described with reference to the drawings.

[0011] Embodiment 1 FIG. 1 is a schematic diagram illustrating a configuration of a protective relay according to a first embodiment of the present disclosure. As shown in FIG. 1, the protective relay 100 includes a current transformer 1, a transformer 2, a filter circuit 3, an amplifier circuit 4, a measurement unit 7, a control unit 8, and a shunt resistor 9. The current transformer 1 receives, at its primary side, a zero-phase current I0 that flows through the power system when a fault occurs in the power system. The transformer 2 receives, at its primary side, a zero-phase voltage V0 that is applied to the power system when a fault occurs in the power system. Here, the zero-phase current I0 and the zero-phase voltage V0 are current and voltage that are not detected when the power system is normal. When a ground fault occurs in the power system, the phases of the polyphase AC become unbalanced, generating the zero-phase current I0 and the zero-phase voltage V0. The protective relay 100 according to the first embodiment of the present disclosure protects the power system using the zero-phase current I0 and the zero-phase voltage V0 detected by a zero-phase current transformer (ZCT) and a zero-phase voltage transformer (ZPD) provided in the power system.

[0012] The filter circuit 3 includes a low-pass filter and a band-pass filter 11. The low-pass filters are provided on the secondary side of the current transformer and the secondary side of the transformer, respectively, and extract fundamental frequency components of the zero-phase current I0 and the zero-phase voltage V0 and high-frequency components of the zero-phase current I0 and the zero-phase voltage V0. The band-pass filters 11 are provided on the secondary side of the current transformer and the secondary side of the transformer, respectively, and extract fundamental frequency components of the zero-phase current and the zero-phase voltage. In the present disclosure, the fundamental frequency component refers to the commercial frequency (50 or 60 Hz) of the power system. The filter circuit 3 is configured, for example, with resistors, capacitors, inductors, etc. The amplifier circuit 4 is provided downstream of the filter circuit and amplifies the signals of the zero-phase current I0 and the zero-phase voltage V0 extracted by the filter circuit 3. By providing the amplifier circuit 4, the protective relay 100 according to the first embodiment of the present disclosure can amplify even weak signals, thereby improving the accuracy of detecting ground faults. By providing the filter circuit 3 and the amplifier circuit 4 separately, the protective relay 100 according to the first embodiment of the present disclosure can accurately detect the zero-phase current I0 and the zero-phase voltage V0.

[0013] The measurement unit 7 includes an A / D converter 5 and a phase detector 6, and measures the magnitudes of the zero-phase current I0 and zero-phase voltage V0 extracted by the filter circuit 3, and measures the phases of the zero-phase current I0 and zero-phase voltage V0. The A / D converter 5 converts the signals amplified by the amplifier circuit 4, which indicate the magnitudes of the zero-phase current I0 and zero-phase voltage V0, from analog signals to digital signals. The phase detector 6 extracts only the phase component from the signals amplified by the amplifier circuit 4, and outputs a signal indicating the phase difference.

[0014] The control unit 8 operates a circuit breaker of a line of the power system corresponding to the phase difference between the zero-phase current I0 and the zero-phase voltage V0 when the magnitudes of the zero-phase current I0 and the zero-phase voltage V0 exceed a threshold value. The control unit 8 is, for example, a CPU (Central Processing Unit) such as a microcomputer.

[0015] FIG. 2 is a graph showing the operating range of the protective relay according to the first embodiment of the present disclosure. FIG. 2 shows the phase of the zero-phase current I0 relative to the zero-phase voltage V0. For example, if the magnitudes of the zero-phase voltage V0 and the zero-phase current I0 exceed a threshold and the phase difference between the zero-phase current I0 and the zero-phase voltage V0 is zero, the control unit 8 determines that the operating range is in FIG. 2 and that the corresponding system is a fault system. In this case, the control unit 8 outputs a signal to operate a circuit breaker provided at a location corresponding to the fault system.

[0016] FIG. 3 is a circuit diagram showing an equivalent circuit of the current transformer of the protective relay according to the first embodiment of the present disclosure. As shown in FIG. 3, the equivalent circuit of the current transformer 1 is approximated by a primary-side internal resistance R1, an exciting inductance L, and a secondary-side internal resistance R2. Note that the internal inductance of the current transformer 1 and other components are omitted from the equivalent circuit of the current transformer 1 in FIG. 3. When the zero-phase current I0 is detected by the ZCT and a primary current I1 flows in the primary side of the current transformer 1, a magnetic field is generated, and the magnetic flux density inside the current transformer 1 increases. In the current transformer 1, a secondary current I2 corresponding to the number of windings (not shown) wound around the iron core of the current transformer 1 flows in the secondary side of the current transformer 1. However, as the primary current I1 increases, the magnetic material used in the current transformer 1 becomes magnetically saturated, and therefore, an exciting current IL flows in the exciting inductance L, and the secondary current I2 decreases. For this reason, when an intermittent ground fault occurs, for example, the zero-phase current is on the order of several tens to several hundreds [A], so the primary current I1 becomes large and the magnetic material of the current transformer 1 quickly becomes magnetically saturated.

[0017] As shown in FIG. 3 , the protective relay 100 according to the first embodiment of the present disclosure is provided with a shunt resistor 9 to prevent magnetic saturation of the current transformer 1. The shunt resistor 9 is connected in parallel with both ends of the primary side of the current transformer 1 and shunts the current flowing through the primary side of the current transformer 1 so that the current is smaller than the current at which the current transformer 1 becomes magnetically saturated. By providing the shunt resistor 9, the protective relay 100 according to the first embodiment of the present disclosure can adjust the value of the primary current I1 using a shunt ratio determined by the resistance value Rs of the shunt resistor 9 and the internal resistance R1 of the primary side. The shunt ratio increases as the resistance value Rs of the shunt resistor decreases. By adjusting the shunt ratio so that the primary current R1 is smaller than the current at which the current transformer 1 becomes magnetically saturated, magnetic saturation of the current transformer 1 can be prevented. The current at which the current transformer 1 becomes magnetically saturated varies depending on the current transformer 1 used and the current input to the current transformer 1 from the ZCT. Therefore, when using the protective relay 100 according to embodiment 1 of the present disclosure, the manufacturer must appropriately set the resistance value Rs of the shunt resistor 9 based on the output current of the ZCT and the LI characteristics (reactance-current characteristics) of the current transformer 1.

[0018] Furthermore, in the protective relay 100 according to the first embodiment of the present disclosure, the resistance value Rs of the shunt resistor 9 may be set smaller than the resistance value (corresponding to R1 in FIG. 3) across the primary side of the current transformer 1. With this configuration, the primary side current I1 can be suppressed while preventing magnetic saturation of the current transformer 1. Therefore, the protective relay 100 according to the first embodiment of the present disclosure can detect a ground fault in the power system without applying an unnecessarily large load to the current transformer 1.

[0019] FIG. 4 is a schematic diagram showing the configuration of a filter circuit of a protective relay according to the first embodiment of the present disclosure. As shown in FIG. 4, a filter circuit 3 in a protective relay 100 according to the first embodiment of the present disclosure includes a band-pass filter 11 (BPF) following a low-pass filter 10 (LPF). The low-pass filter 10 is a filter that passes signals with frequencies lower than a threshold. For example, the low-pass filter 10 is configured using a resistor or capacitor. In a ground fault (particularly an intermittent ground fault), as described above, the zero-phase current I0 has a high frequency and a large peak value. Therefore, by setting the cutoff frequency of the low-pass filter 10 to, for example, about 1 kHz, it becomes easier to obtain the high-frequency components of the zero-phase current and zero-phase voltage required to detect a ground fault. Furthermore, in the protective relay according to the present disclosure, the cutoff frequency may be set to, for example, up to the 12th harmonic. Even with such a value, the harmonics required to detect a ground fault can be obtained. Here, the cutoff frequency is set to 1 kHz and up to the 12th harmonic, but the low-pass filter 10 only needs to obtain the fundamental frequency component and harmonic components necessary to detect a ground fault. The protective relay 100 according to the first embodiment of the present disclosure can improve the accuracy of ground fault directional protection by extracting some of the harmonic components using the low-pass filter 10. Therefore, in the protective relay 100 of the present disclosure, the voltage input from the low-pass filter 10 to the band-pass filter 11 can suppress output saturation due to unnecessary high-frequency components, and the band-pass filter 11 can accurately extract the phase and output.

[0020] As described above, the protective relay 100 according to the present disclosure includes a current transformer 1 to whose primary side a zero-phase current flowing in the power system when a fault occurs in the power system is input, a transformer 2 to whose primary side a zero-phase voltage to be applied to the power system when a fault occurs in the power system is input, low-pass filters provided on the secondary side of the current transformer 1 and the secondary side of the transformer, respectively, for extracting fundamental frequency components of the zero-phase current I0 and the zero-phase voltage V0 and high-frequency components of the zero-phase current I0 and the zero-phase voltage V0, and a low-pass filter provided on the secondary side of the current transformer 1 and the secondary side of the transformer 2, respectively, for extracting fundamental frequency components of the zero-phase current I0 and the zero-phase voltage V0. The power system includes a filter circuit 3 including a bandpass filter 11 that extracts components; a measurement unit 7 that measures the magnitude of the zero-phase current I0 and zero-phase voltage V0 extracted by the filter circuit 3 and measures the phase of the zero-phase current I0 and zero-phase voltage V0; a control unit 8 that, when the magnitude of the zero-phase current I0 and zero-phase voltage V0 exceeds a threshold, distinguishes between a faulty system and a healthy system in the power system from the phase difference and operates a circuit breaker in the power system; and a shunt resistor 9 that is connected in parallel with both ends of the primary side of the current transformer 1 and shunts the current flowing in the primary side of the current transformer 1 so that it is smaller than the current at which the current transformer 1 saturates.

[0021] Therefore, in the protective relay 100 according to embodiment 1 of the present disclosure, by connecting the shunt resistor 9 in parallel with both ends of the primary side of the current transformer 1, magnetic saturation of the current transformer used to detect the zero-phase current can be suppressed and saturation of the output of the BPF can be prevented, thereby enabling accurate detection of the phase of the zero-phase current.

[0022] Furthermore, the protective relay 100 according to the first embodiment of the present disclosure can improve the detection accuracy of ground fault directional protection without changing the hardware configuration, compared to conventional protective relays.

[0023] Embodiment 2 In the second embodiment, the same components as those in the first embodiment of the present disclosure are designated by the same reference numerals, and descriptions of the same or corresponding parts will be omitted. Hereinafter, a protective relay 101 according to the second embodiment will be described with reference to the drawings.

[0024] 5 is a schematic diagram showing the configuration of a protective relay according to a second embodiment of the present disclosure. A protective relay 101 according to the second embodiment of the present disclosure uses an active filter 12 including an amplifier that amplifies current and voltage as the filter circuit 3 in the first embodiment. In this case, an LPF or a BPF is used in the filter circuit of the active filter 12. Note that in the present disclosure, the filter circuit 3 is considered to include the active filter 12.

[0025] In earth fault directional protection, the protective relay must operate even with small currents and voltages in order to immediately isolate the faulted section. To enable the measurement unit 7 to handle small currents and voltages, it is preferable to amplify the extracted signals simultaneously with the signal extraction by the filter circuit 3. The protective relay 101 according to the second embodiment of the present disclosure uses the active filter 12, so that even if the zero-phase current I0 and the zero-phase voltage V0 have small values, they can be filtered and amplified by the amplifier at the same time. The protective relay 101 according to the second embodiment of the present disclosure can further amplify the signals by the amplifier circuit 4 after amplifying the signals by the active filter 12, so that the measurement unit 7 can detect even small values ​​of the zero-phase current I0 and the zero-phase voltage V0.

[0026] Furthermore, although the active filter 12 is capable of filtering and amplifying the zero-phase current I0, if the zero-phase current I0 amplified by the active filter 12 becomes too large, the output will saturate and phase detection will become impossible. For this reason, the amplification factor of the active filter 12 is set taking into consideration the output on the secondary side of the current transformer 1.

[0027] As described above, the protective relay 101 according to the second embodiment of the present disclosure is characterized in that, in addition to the configuration of the first embodiment, the filter circuit 3 is an active filter 12 including an amplifier that amplifies the current and voltage. Since the zero-phase current I0 and the zero-phase voltage V0 input to the current transformer 1 and the transformer 2 can be detected by the measuring unit 7 even if they are small values, the protective relay 101 according to the second embodiment of the present disclosure can improve the accuracy of detecting ground faults.

[0028] It should be noted that since the present invention relates to a protective relay with earth fault directional protection, only the inputs to the relevant internal circuitry are described.

[0029] Furthermore, the protective relay of the present disclosure is installed in a power receiving and distribution facility such as a switchboard, and measures, controls, protects, etc. the current and voltage of the power system.

[0030] The configurations described in the above embodiments are merely examples of the contents of the present disclosure, and may be combined with other known techniques. Furthermore, parts of the configurations may be omitted or modified without departing from the scope of the present disclosure.

[0031] Various aspects of the present disclosure are summarized below as appendices. (Appendix 1) a current transformer, the primary side of which receives a zero-phase current flowing through the power system when a fault occurs in the power system; a transformer to whose primary side a zero-phase sequence voltage applied to the power system when a fault occurs in the power system is input; a filter circuit including: a low-pass filter provided on the secondary side of the current transformer and the secondary side of the transformer, respectively, for extracting fundamental frequency components of the zero-phase current and the zero-phase voltage and high-frequency components of the zero-phase current and the zero-phase voltage; and a band-pass filter provided on the secondary side of the current transformer and the secondary side of the transformer, respectively, for extracting signals of the fundamental frequencies of the zero-phase current and the zero-phase voltage; a measurement unit that measures the magnitudes of the zero-phase-sequence current and the zero-phase-sequence voltage extracted by the filter circuit and measures the phases of the zero-phase-sequence current and the zero-phase-sequence voltage; a control unit that monitors the electric power system and the like, and when the magnitudes of the zero-phase current and the zero-phase voltage exceed thresholds, calculates a phase difference from the phase, distinguishes between a fault system and a healthy system of the electric power system, and operates a circuit breaker of the electric power system; a shunt resistor connected in parallel with both ends of the primary side of the current transformer and shunting the current flowing through the primary side of the current transformer so that it is smaller than the current at which the current transformer becomes magnetically saturated. (Appendix 2) 2. A protective relay according to claim 1, further comprising an amplifier circuit provided downstream of the filter circuit, which amplifies the signals of the zero-phase current and the zero-phase voltage extracted by the filter circuit. (Appendix 3) 3. A protective relay according to claim 1 or 2, wherein the resistance value of the shunt resistor is smaller than the resistance value across the primary side of the current transformer. (Appendix 4) 4. The protective relay according to claim 1, wherein the filter circuit includes a low-pass filter and the band-pass filter is provided in a subsequent stage thereof. (Appendix 5) 5. A protective relay according to any one of claims 1 to 4, wherein the filter circuit is an active filter including an amplifier that amplifies current and voltage. (Appendix 6) A distribution board characterized by being provided with a protective relay according to any one of appendices 1 to 5. [Explanation of symbols]

[0032] 1 current transformer, 2 transformer, 3 filter circuit, 4 amplifier circuit, 5 A / D converter, 6 phase detector, 7 measurement section, 8 control section, 9 shunt resistor, 10 low-pass filter, 11 band-pass filter, 12 active filter, 100 101 protective relay

Claims

1. a current transformer, the primary side of which receives a zero-phase current flowing through the power system when a fault occurs in the power system; a transformer to whose primary side a zero-phase sequence voltage applied to the power system when a fault occurs in the power system is input; a filter circuit including: a low-pass filter provided on the secondary side of the current transformer and the secondary side of the transformer, respectively, for extracting fundamental frequency components of the zero-phase current and the zero-phase voltage and high-frequency components of the zero-phase current and the zero-phase voltage; and a band-pass filter provided on the secondary side of the current transformer and the secondary side of the transformer, respectively, for extracting signals of the fundamental frequency; a measurement unit that measures the magnitudes of the zero-phase-sequence current and the zero-phase-sequence voltage extracted by the filter circuit and measures the phases of the zero-phase-sequence current and the zero-phase-sequence voltage; a control unit that monitors the electric power system and the like, and when the magnitudes of the zero-phase current and the zero-phase voltage exceed thresholds, calculates a phase difference from the phase, distinguishes between a fault system and a healthy system of the electric power system, and operates a circuit breaker of the electric power system; a shunt resistor connected in parallel with both ends of the primary side of the current transformer and shunting the current flowing through the primary side of the current transformer so that the current is smaller than the current at which the current transformer becomes magnetically saturated.

2. 2. The protective relay according to claim 1, further comprising an amplifier circuit provided downstream of the filter circuit for amplifying the signals of the zero-phase sequence current and the zero-phase sequence voltage extracted by the filter circuit.

3. 3. A protective relay according to claim 1, wherein the resistance value of the shunt resistor is smaller than the resistance value across the primary side of the current transformer.

4. 3. The protective relay according to claim 1, wherein the filter circuit includes the band-pass filter disposed after the low-pass filter.

5. 3. A protective relay according to claim 1, wherein the filter circuit is an active filter including an amplifier for amplifying current and voltage.

6. A switchboard comprising the protective relay according to claim 1 or 2.

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