Current differential relay

Abstract

A current differential relay apparatus according to the present embodiment detects an accident in an electric power system and, upon detecting an accident, outputs a trip command to a breaker, the current differential relay apparatus having a first means, a second means, a third means, and a fourth means. The first means periodically samples a host terminal current in an electric power system and all other terminal currents for terminals other than the host terminal at the same time, computes a difference current from the sampled data, and detects a system accident according to the magnitude of the difference current. The second means detects an overcurrent and a variation current for the current of the host terminal. The third means detects overcurrents of the other terminals. The fourth means generates a trip command output according to action conditions for all of the first means, the second means, and the third means.

Description

[Technical Field]

[0001] SUMMARY OF THE INVENTION An embodiment of the present invention relates to a current differential relay. [Background technology]

[0002] The transmission lines of a power system are equipped with a current differential relay device that detects a system fault (hereinafter simply referred to as an "fault") that occurs on the transmission line and, when a fault is detected, outputs a tripping command to the circuit breaker belonging to that transmission line to isolate the fault from the power system and prevent the fault from spreading (maintaining stability).

[0003] One type of current differential relay detects the occurrence of a fault based on the differential current calculated from the currents at both ends of a power system transmission line. A current differential relay is installed at each terminal of a power system transmission line, inputs its own terminal current, converts it from analog to digital, and transmits the current data via a transmission line to a current differential relay installed at each terminal. In a current differential relay, the current differential relay calculates the difference current between the current data of the power system transmission line at the terminal where it is installed and the current data of the opposite terminal received via a transmission line from a current differential relay installed at the opposite terminal of the same transmission line, and detects the occurrence of a fault based on the magnitude of the differential current resulting from the calculation.

[0004] Some current differential relays have a main and fail-safe configuration, which improves the reliability of fault detection by including multiple relays operating on different principles. For example, if the main relay is a current relay, the fail-safe relay may be an undervoltage relay for detecting short-circuit faults, an overvoltage relay for detecting ground faults, an overcurrent relay, or a differential current relay. A current differential relay with a main and fail-safe configuration may be configured by using a current differential relay as the main relay and an overcurrent relay or a differential current relay as the fail-safe relay. This is because, since the current differential relay, which is the main relay, operates based on current data, using an overcurrent relay or a differential current relay, which operates based on current data like the main relay, as the fail-safe relay is advantageous in terms of compactness and economy, even in a main and fail-safe configuration.

[0005] However, in a power system, even if no fault has actually occurred, the current data may change due to some factor, such as a system arc or a wiring cable fault. In this case, even if the current differential relay is configured as a main and fail-safe relay, if the fail-safe relay operates based on the current data, the change in current data may cause the fail-safe relay to operate. As a result, both the current differential relay (main relay) and the fail-safe relay may operate as if a fault has occurred, potentially outputting a trip command. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] "Electric Cooperative Research, Vol. 41, No. 4, Digital Relay...Digital Relay Specialist Committee", Chapter 9, pp. 130-147, Electric Cooperative Research Association, published January 21, 1986 Summary of the Invention [Problem to be solved by the invention]

[0007] The problem to be solved by the present invention is to provide a current differential relay device that can improve the reliability in detecting a system fault. [Means for solving the problem]

[0008] A current differential relay according to an embodiment detects a fault in a power system and outputs a trip command to a circuit breaker when the fault is detected, and includes first, second, third, and fourth means. The first means periodically samples the current at its own terminal and all other terminal currents of the power system at the same time, calculates a difference current from the sampled data, and detects a system fault based on the magnitude of this difference current. The second means detects an overcurrent or a current change in the current at the own terminal. The third means detects an overcurrent at the other other terminal. The fourth means generates a trip command output based on all operating conditions of the first, second, and third means. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram showing an example of the configuration of a power system to which a current differential relay device according to an embodiment is applied; [Figure 2] 1 is a diagram showing an example of the configuration of a current differential relay device according to a first embodiment; [Figure 3] FIG. 3 is a diagram for explaining a first operation of the current differential relay device of the first embodiment. [Figure 4] FIG. 4 is a diagram for explaining a second operation of the current differential relay device of the first embodiment. [Figure 5] FIG. 10 is a diagram showing an example of the configuration of a current differential relay device according to a second embodiment. [Figure 6] FIG. 10 is a diagram showing an example of the configuration of a current differential relay device according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, a current differential relay device according to an embodiment will be described with reference to the drawings.

[0011] Fig. 1 is a diagram showing an example of the configuration of a power system to which a current differential relay according to an embodiment is applied. Fig. 1 shows an example of a power system 1 that transmits power between generators AC (generators AC-1 and AC-2), for example.

[0012] The power system 1 is provided with, for example, two current transformers CT (current transformers CT-1 and CT-2), two circuit breakers CB (circuit breakers CB-1 and CB-2), and two current differential relays 100 (current differential relays 100-1 and 100-2) between terminal T-1 of generator AC-1 and terminal T-2 of generator AC-2. In the power system 1, the current transformer CT-1 and circuit breaker CB-1 are installed at terminal T-1 to which power is supplied from generator AC-1, and the current transformer CT-2 and circuit breaker CB-2 are installed at terminal T-2 to which power is supplied from generator AC-2. For this reason, in the power system 1, a current transformer CT-1, a circuit breaker CB-1, and a current differential relay 100-1 are arranged near terminal T-1 of generator AC-1, and a current transformer CT-2, a circuit breaker CB-2, and a current differential relay 100-2 are arranged near terminal T-2 of generator AC-2, which is the opposite terminal of terminal T-1 on the transmission line PL. Terminal T-1 is an example of a "first terminal," and terminal T-2 is an example of a "second terminal." When focusing on current differential relay 100-1, terminal T-1 is the "first terminal," and terminal T-2 is the "second terminal," but when focusing on current differential relay 100-2, terminal T-2 is the "first terminal," and terminal T-1 is the "second terminal."

[0013] Each of the current transformers CT converts the current on the system primary side flowing in the transmission line PL into a current on the system secondary side that is input to the current differential relay 100. Current transformer CT-1 converts the current on the system primary side flowing in the transmission line PL at terminal T-1 into a current on the system secondary side that is input to the current differential relay 100. Current transformer CT-2 converts the current on the system primary side flowing in the transmission line PL at terminal T-2 into a current on the system secondary side that is input to the current differential relay 100. Each of the current differential relays 100 receives as input the current on the system secondary side (hereinafter simply referred to as "current") in the corresponding current transformer CT. Each of the current differential relays 100 transmits current data (hereinafter simply referred to as "current data") obtained by converting the current obtained from the current transformer CT to the current differential relay 100 that is the opposite terminal. The current transformer CT-1, which inputs the current on the system secondary side to the current differential relay 100-1, is an example of a "first current transformer," and the current on the system secondary side input to the current differential relay 100-1 is an example of a "first current." The current transformer CT-2, which inputs the current on the system secondary side to the current differential relay 100-2, is an example of a "second current." The current transmitted from the current differential relay 100-1 via the transmission line is an example of a "first current data," and the current transmitted from the current differential relay 100-2 via the transmission line is an example of a "second current data."

[0014] When a system fault X (hereinafter simply referred to as "fault X") occurs due to, for example, a lightning strike, each of the circuit breakers CB interrupts the connected power transmission line PL in response to an interruption command (so-called trip command) from the corresponding current differential relay 100. The circuit breaker CB-1 interrupts the connection between the terminal T-1 and the terminal T-2 on the power transmission line PL in response to the interruption command from the current differential relay 100-1. The circuit breaker CB-2 interrupts the connection between the terminal T-1 and the terminal T-2 on the power transmission line PL in response to the interruption command from the current differential relay 100-2. When focusing on the current differential relay 100-1, the circuit breaker CB-1 is an example of a "circuit breaker," and when focusing on the current differential relay 100-2, the circuit breaker CB-2 is an example of a "circuit breaker."

[0015] Each current differential relay 100 detects a fault on the transmission line PL. Each current differential relay 100 is connected to other current differential relays 100 corresponding to the same transmission line PL by a transmission line CC. The transmission line CC is, for example, a communication cable such as an optical fiber cable. Each current differential relay 100 converts the current obtained from the current transformer CT at the terminal T of the current differential relay 100 and transmits the converted current data to the other current differential relays 100 via the transmission line CC. Each current differential relay 100 detects a fault on the transmission line PL based on the current data of the corresponding current transformer CT and the current data of the current transformer CT corresponding to the other current differential relay 100 transmitted by the other current differential relay 100. When each current differential relay 100 detects that a fault X has occurred on the transmission line PL, it outputs a trip command TC to the corresponding circuit breaker CB to prevent the effects of the transmission line PL (fault line) where the fault X has occurred from affecting the entire power system.

[0016] 1, current differential relaying devices 100-1 and 100-2 have the same configuration. Current differential relaying device 100-1 transmits current data I-1 of corresponding current transformer CT-1 to current differential relaying device 100-2 via transmission line CC. Current differential relaying device 100-2 transmits current data I-2 of corresponding current transformer CT-2 to current differential relaying device 100-1 via transmission line CC. Current differential relaying device 100-1 detects a fault in transmission line PL based on the current data I-1 and the current data I-2 transmitted by current differential relaying device 100-2 via transmission line CC, and when it detects that fault X has occurred on transmission line PL, it outputs a tripping command TC-1 to circuit breaker CB-1. The current differential relay 100-2 detects a fault in the transmission line PL based on the current data I-2 and the current data I-1 transmitted by the current differential relay 100-1 via the transmission line CC, and if it detects that a fault X has occurred in the transmission line PL, it outputs a tripping command TC-2 to the circuit breaker CB-2.

[0017] Each current differential relay 100 includes, for example, a main relay unit 110, a failsafe relay unit 120, a failsafe relay unit 130, and an output unit 140. The main relay unit 110 is a main detection unit that detects a fault X that has occurred in the power transmission line PL. The failsafe relay unit 120 and the failsafe relay unit 130 improve the reliability of the detection result of the fault X by the main relay unit 110. The output unit 140 outputs a trip command TC to the corresponding circuit breaker CB in accordance with the detection results by the main relay unit 110, the failsafe relay unit 120, and the failsafe relay unit 130. The main relay unit 110 is an example of a "main relay unit," the failsafe relay unit 120 is an example of a "first failsafe relay unit," and the failsafe relay unit 130 is an example of a "second failsafe relay unit." The main relay unit 110 is an example of a “first means,” the failsafe relay unit 120 is an example of a “second means,” the failsafe relay unit 130 is an example of a “third means,” and the output unit 140 is an example of a “fourth means.” The current data I-1 is an example of a “first current data,” and the current data I-2 is an example of a “second current data.”

[0018] [Configuration of current differential relay] Next, an example of the detailed configuration of the current differential relay 100 will be described. As described above, the current differential relay 100-1 and the current differential relay 100-2 have the same configuration. In the following explanation, the current differential relay 100-1 (hereinafter simply referred to as "current differential relay 100") located at terminal T-1 will be described as a representative example.

[0019] (First embodiment) FIG. 2 is a diagram illustrating an example of the configuration of a current differential relay 100 according to the first embodiment. The current differential relay 100 includes a main relay unit 110 having a current differential relay 112, a fail-safe relay unit 120 having an overcurrent relay 122, a variation current relay 124, and an OR circuit 126, a fail-safe relay unit 130 having an overcurrent relay 132, and an output unit 140 having an AND circuit 142 and an AND circuit 144. The current differential relay 112 is an example of a "first means." The configuration of the overcurrent relay 122, the variation current relay 124, and the OR circuit 126 is an example of a "second means." The overcurrent relay 132 is an example of a "third means." The configuration of the AND circuit 142 and the AND circuit 144 is an example of a "fourth means."

[0020] The current differential relay 112 is a current differential relay (DIF) that detects a fault in the power transmission line PL based on two input current data I, current data I-1 and current data I-2, and issues a tripping command to the circuit breaker CB-1 if a fault is detected. The current differential relay 112 performs a differential current calculation using the current data I-1 of the current transformer CT-1 and the current data I-2 of the current transformer CT-2 transmitted by the current differential relay 100-2, and determines that a fault has occurred in the power transmission line PL based on the magnitude of the differential current resulting from the calculation. The current differential relay 112 then outputs an operation command (hereinafter referred to as a "detection signal DIF"). More specifically, the current differential relay 112 performs a differential current calculation by adding the current data I-1 and I-2. The current differential relay 112 then determines that a fault has occurred in the power transmission line PL when the calculation result IDIF of the differential current calculation satisfies the following equation (1), and outputs a detection signal DIF indicating this determination result.

[0021] IDIF = |(I-1) + (I-2)| ≧ predetermined current value (1)

[0022] The main relay unit 110 outputs the detection signal DIF of the current differential relay 112 to the AND circuit 144 included in the output unit 140 as the detection signal in the main relay unit 110. The current differential relay 112 is an example of a "current differential relay." The detection signal DIF is an example of a "first detection signal." However, if a change in current data occurs due to some cause, such as a system arc or a wiring cable fault, even though no accident has actually occurred, the change in current data will be used to make a decision and an unnecessary detection signal will be output.

[0023] The overcurrent relay 122 is an overcurrent relay (OCR) that detects that an overcurrent equal to or greater than a predetermined current value is flowing in the power transmission line PL based on the input current data I, and outputs a detection signal indicating this. The overcurrent relay 122 outputs a detection signal (hereinafter referred to as "detection signal OC-1") indicating that it has detected that an overcurrent is flowing in the power transmission line PL from the current data I-1 of the current transformer CT-1, to the OR circuit 126. The overcurrent relay 122 is an example of a "first overcurrent relay."

[0024] The variation over current relay 124 is a relay (ΔOCR) that detects that the current flowing through the transmission line PL has changed by a predetermined current value or more based on the input current data I, and outputs a detection signal indicating this. The variation over current relay 124 outputs a detection signal (hereinafter referred to as "detection signal ΔOC-1") indicating that it has detected that the current flowing through the transmission line PL has changed by a predetermined current value or more based on the current data I-1 of the current transformer CT-1, to the OR circuit 126. The variation over current relay 124 is an example of a "first variation over current relay."

[0025] The OR circuit 126 performs a logical OR on the detection signal OC-1 of the overcurrent relay 122 and the detection signal ΔOC-1 of the change current relay 124. The OR circuit 126 outputs a detection signal (hereinafter referred to as "detection signal FS-1") indicating the result of the logical OR. The OR circuit 126 outputs the detection signal FS-1 indicating that a fault has occurred on the power transmission line PL to the AND circuit 142 provided in the output unit 140. The detection signal FS-1 is an example of a "second detection signal."

[0026] Similar to the overcurrent relay 122 included in the failsafe relay unit 120, the overcurrent relay 132 is a relay that detects that an overcurrent of a predetermined current value or more is flowing in the power transmission line PL based on the input current data I, and outputs a detection signal indicating this. However, the overcurrent relay 132 outputs a detection signal (hereinafter referred to as "detection signal OC-2") based on the current data I-2 of the current transformer CT-2 transmitted by the current differential relay device 100-2.

[0027] The failsafe relay unit 130 outputs the detection signal OC-2 of the overcurrent relay 132 to the AND circuit 142 provided in the output unit 140 as the detection signal in the failsafe relay unit 130. The overcurrent relay 132 is an example of a "second overcurrent relay." The detection signal OC-2 is an example of a "third detection signal."

[0028] The AND circuit 142 takes a logical AND of the detection signal FS-1 of the failsafe relay unit 120 and the detection signal OC-2 of the failsafe relay unit 130. The AND circuit 142 outputs a detection signal indicating the result of the logical AND to the AND circuit 144.

[0029] The AND circuit 144 takes a logical AND of the detection signal DIF of the main relay unit 110 and the detection signal of the AND circuit 142. The AND circuit 144 outputs a detection signal indicating the result of the logical AND as a cutoff command TC.

[0030] The output unit 140, configured with the AND circuit 142 and the AND circuit 144, outputs a cutoff command TC-1 to the corresponding circuit breaker CB-1 to instruct the cutoff of the transmission line PL when an accident is detected in all of the main relay unit 110, the failsafe relay unit 120, and the failsafe relay unit 130.

[0031] With this configuration, the current differential relay 100 detects a fault in the transmission line PL, the main relay unit 110 detects a fault X that has occurred in the transmission line PL, and the failsafe relay unit 120 and the failsafe relay unit 130 improve the reliability of the detection result of the main relay unit 110. Then, when a change in the current data in the transmission line PL is caused by the occurrence of the fault X, the current differential relay 100 outputs a tripping command TC-1 to the corresponding circuit breaker CB-1 to instruct the tripping of the transmission line PL.

[0032] [First operation of current differential relay] Next, the operation of the current differential relay 100 will be described. In the following description, the current differential relay 112 included in the main relay unit 110 outputs a “High” level detection signal DIF when it is determined that a fault has occurred in the power transmission line PL. The overcurrent relay 122 included in the failsafe relay unit 120 outputs a “High” level detection signal OC-1 when it is determined that an overcurrent is flowing in the power transmission line PL. The variable current relay 124 included in the failsafe relay unit 120 outputs a “High” level detection signal ΔOC-1 when it is determined that the current flowing in the power transmission line PL has changed by more than a predetermined current value. The overcurrent relay 132 included in the failsafe relay unit 130 outputs a “High” level detection signal OC-2 when it is determined that an overcurrent is flowing in the power transmission line PL. The output unit 140 outputs a “High” level shutdown command TC when instructing the shutdown of the power transmission line PL.

[0033] FIG. 3 is a diagram illustrating a first operation of the current differential relay 100 according to the first embodiment. The first operation is the operation of the current differential relay 100-1 when an actual fault X occurs on the power transmission line PL. In the following description, it is assumed that the fault X occurs on the c-phase among the three phases, i.e., the a-phase, the b-phase, and the c-phase. In FIG. 3, the current data I, the detection signal DIF output by the current differential relay 112, the detection signal OC-1 output by the overcurrent relay 122, the detection signal ΔOC-1 output by the change current relay 124, the detection signal OC-2 output by the overcurrent relay 132, and the tripping command TC-1 output by the current differential relay 100-1 are represented by adding a letter to identify the corresponding phase after the symbol of each signal. More specifically, the symbol of the signal corresponding to the a-phase is represented by adding "a" after the symbol of the signal corresponding to the b-phase, "b" after the symbol of the signal corresponding to the c-phase, and "c" after the symbol of the signal corresponding to the c-phase.

[0034] When power is being transmitted in a steady state in the power system 1, the detection signal DIF (detection signals DIFa to DIFc) of the current differential relay 112 is at a "Low" level. In this case, the detection signal OC-1 (detection signals OC-1a to OC-1c) of the overcurrent relay 122 and the detection signal ΔOC-1 (detection signals ΔOC-1a to ΔOC-1c) of the variable current relay 124 are also at a "Low" level. Furthermore, the detection signal OC-2 (detection signals OC-2a to OC-2c) of the overcurrent relay 132 is also at a "Low" level. Therefore, the tripping command TC-1 (tripping commands TC-1a to TC-1c) of the output unit 140 is at a "Low" level.

[0035] Here, it is assumed that a fault X occurs on the c-phase transmission line PL at time t1. When the fault X occurs, the current changes significantly at both terminals (terminal T-1 side and terminal T-2 side) of the transmission line PL. As a result, current differential relaying 100-1 inputs current data I-1 (current data I-1a to I-1c) via the corresponding current transformer CT-1. Furthermore, current differential relaying 100-2 inputs current data I-2 (current data I-2a to I-2c) via the corresponding current transformer CT-2. Then, current differential relaying 100-2 transmits the current data I-2 of each current transformer CT-2 to current differential relaying 100-1 via transmission line CC.

[0036] As a result, in current differential relaying device 100-1, current differential relay 112 corresponding to phase c detects a fault in the c-phase transmission line PL based on current data I-1c and current data I-2c and sets detection signal DIFc to a "High" level. Then, overcurrent relay 122 corresponding to phase c detects from the current data I-1c that an overcurrent is flowing in the c-phase transmission line PL and sets detection signal OC-1c to a "High" level. Also, change current relay 124 corresponding to phase c detects from the current data I-1c that the current flowing in the c-phase transmission line PL has changed by more than a predetermined current value and sets detection signal ΔOC-1c to a "High" level. Furthermore, overcurrent relay 132 corresponding to phase c detects from the current data I-2c that an overcurrent is flowing in the c-phase transmission line PL and sets detection signal OC-2c to a "High" level.

[0037] As a result, the output unit 140 sets the interruption commands TC-1a to TC-1c to a "High" level because it has detected that an accident has occurred on the power transmission line PL in all of the main relay unit 110 corresponding to phase c, the fail-safe relay unit 120 corresponding to phase c, and the fail-safe relay unit 130 corresponding to phase c.

[0038] In this way, in the current differential relay device 100, when it is detected that the current flowing through the transmission line PL has changed by more than a predetermined current value in all of the main relay unit 110, the failsafe relay unit 120, and the failsafe relay unit 130, it determines that this change in current data is due to the occurrence of accident X, and outputs a shutoff command TC-1 to the circuit breaker CB-1 to shut off the transmission line PL.

[0039] This operation is similar to that of the current differential relay 100-2. The operation of the current differential relay 100-2 can be achieved by reversing the terminals T-1 and T-2 in the operation of the current differential relay 100-1 shown in FIG.

[0040] [Second operation of current differential relay] Next, another operation of the current differential relay 100 will be described. Fig. 4 is a diagram for explaining a second operation of the current differential relay 100 of the first embodiment. The second operation is an operation of the current differential relay 100-1 when a change in the current data I-1 appears due to some factor even though no fault X has actually occurred on the transmission line PL. In the following description, it is assumed that a change has appeared in the current data I-1c corresponding to the c-phase.

[0041] The operation of the power system 1 when a fault occurs on a power transmission line during a steady state is similar to the first operation shown in FIG. 3. In the second operation shown in FIG. 4, it is assumed that a change in current data occurs at time t3 due to some factor other than fault X. If the current data at the own terminal changes even though no fault has occurred on the transmission line PL, the current changes only at one terminal (terminal T-1 or terminal T-2) of the transmission line PL, which may cause unnecessary operation of the current differential relay 100 (current differential relay 100-1 or current differential relay 100-2). However, if there is an overcurrent relay 132 that uses current data at the opposite terminal, the current data at the opposite terminal is not affected by the corrupted current data at the own terminal, thereby preventing unnecessary operation of the current differential relay 100. Here, it is assumed that a change in current data occurs in the c-phase transmission line PL due to some factor, and only the current transformer CT-1c corresponding to the c-phase detects the change in current data.

[0042] In this case, in the current differential relaying device 100-1, the current differential relay 112 corresponding to the c-phase detects a change in the current data based on the current data I-1c and I-2c, even though no fault actually has occurred in the c-phase transmission line PL, and sets the detection signal DIFc to a "High" level. Then, the overcurrent relay 122 corresponding to the c-phase detects from the current data I-1c that an overcurrent is flowing in the c-phase transmission line PL and sets the detection signal OC-1c to a "High" level. The change current relay 124 corresponding to the c-phase also detects from the current data I-1c that the current flowing in the c-phase transmission line PL has changed by more than a predetermined current value and sets the detection signal ΔOC-1c to a "High" level. Meanwhile, the overcurrent relay 132 corresponding to the c-phase determines from the current data I-2c that no overcurrent is flowing in the c-phase transmission line PL and keeps the detection signal OC-2c to a "Low" level.

[0043] As a result, although the main relay unit 110 corresponding to phase c and the fail-safe relay unit 120 corresponding to phase c detect that the current data is changing even though no accident has actually occurred on the transmission line PL, the output unit 140 keeps the cutoff commands TC-1a to TC-1c at the “Low” level because the fail-safe relay unit 130 corresponding to phase c determines that the current flowing through the transmission line PL has not changed.

[0044] In this way, in the current differential relay device 100, if the main relay unit 110, the failsafe relay unit 120, and the failsafe relay unit 130 do not detect a change in the current flowing through the power transmission line PL, the current change is not determined to be due to the occurrence of accident X, and the interruption command TC-1 is not output to the circuit breaker CB-1.

[0045] This operation can be considered similarly to the case where a change in current due to some factor occurs in the c-phase transmission line PL and only the current transformer CT-2c corresponding to the c-phase detects the current change. More specifically, in the operation of the current differential relay 100-1 shown in Fig. 4, the main relay unit 110 corresponding to the c-phase and the fail-safe relay unit 120 corresponding to the c-phase detect a change in current data even though no fault has actually occurred in the transmission line PL, and the fail-safe relay unit 130 corresponding to the c-phase determines that the current flowing through the transmission line PL has not changed. In this case as well, the output unit 140 keeps the tripping commands TC-1a to TC-1c at the "Low" level.

[0046] As described above, the current differential relay 100 of the first embodiment includes a failsafe relay unit 120 that improves the reliability of the detection result of the fault X in the main relay unit 110 based on the current data I-1 on the system secondary side of the current transformer CT-1, and a failsafe relay unit 130 that improves the reliability of the detection result of the fault X in the main relay unit 110 based on the current data I-2 on the system secondary side of the current transformer CT-2. As a result, the current differential relay 100 of the first embodiment can improve the reliability of the detection result of the fault X in the main relay unit 110.

[0047] (Second embodiment) 5 is a diagram showing an example of the configuration of a current differential relay 100 according to the second embodiment. Two current differential relays 100a (current differential relays 100a-1 and 100a-2) according to the second embodiment each include a main relay unit 110 having a current differential relay 112, a fail-safe relay unit 120 having an overcurrent relay 122, a variation current relay 124, and an OR circuit 126, a fail-safe relay unit 130 (hereinafter referred to as the "fail-safe relay unit 130a") having a variation current relay 134, and an output unit 140 having an AND circuit 142 and an AND circuit 144. The current differential relay 100a has a configuration in which the fail-safe relay unit 130 included in the current differential relay 100 according to the first embodiment is replaced with the fail-safe relay unit 130a. The current differential relay 112 is an example of a "first means." The configuration of the overcurrent relay 122, the change current relay 124, and the OR circuit 126 is an example of a "second means." The change current relay 134 is an example of a "third means." The configuration of the AND circuit 142 and the AND circuit 144 is an example of a "fourth means."

[0048] In the current differential relay 100a, the same reference numerals are used to designate components that are the same as those in the current differential relay 100. Therefore, detailed explanations of components in the current differential relay 100a that have the same configurations and operations as those in the current differential relay 100 will be omitted, and only the different configurations and operations will be explained.

[0049] Similar to the change current relay 124 included in the failsafe relay unit 120, the change current relay 134 is a relay that detects that the current flowing through the transmission line PL has changed by a predetermined current value or more based on the input current data I, and outputs a detection signal indicating this. However, the change current relay 134 detects that the current flowing through the transmission line PL has changed by a predetermined current value or more based on the current data I-2 of the current transformer CT-2 transmitted by the current differential relay 100-2, and outputs a detection signal (hereinafter referred to as "detection signal ΔOC-2") indicating that the current flowing through the transmission line PL has changed by a predetermined current value or more.

[0050] The failsafe relay unit 130a outputs the detection signal ΔOC-2 of the change current relay 134 to the AND circuit 142 provided in the output unit 140 as the detection signal in the failsafe relay unit 130a. The change current relay 134 is an example of a "second change current relay." The detection signal ΔOC-2 is an example of a "third detection signal."

[0051] With this configuration, the current differential relay 100a detects a fault X that has occurred in the power transmission line PL using the main relay unit 110, and improves the reliability of the detection result of the main relay unit 110 using the failsafe relay unit 120 and the failsafe relay unit 130a, similar to the current differential relay 100. Then, similar to the current differential relay 100, when a change in current in the power transmission line PL is caused by the occurrence of the fault X, the current differential relay 100a outputs a disconnection command TC-1 to the corresponding circuit breaker CB-1, instructing the disconnection of the power transmission line PL.

[0052] The operation of the current differential relay 100a can be considered similar to that of the current differential relay 100 shown in Figures 3 and 4, except that the detection signal OC-2 output by the overcurrent relay 132 is replaced with the detection signal ΔOC-2 output by the change current relay 134. Therefore, a detailed description of the operation of the current differential relay 100a will be omitted.

[0053] As described above, the current differential relay 100a of the second embodiment includes a failsafe relay unit 120 that improves the reliability of the detection result of the fault X in the main relay unit 110 based on the current data I-1 on the system secondary side of the current transformer CT-1, and a failsafe relay unit 130a that improves the reliability of the detection result of the fault X in the main relay unit 110 based on the current data I-2 on the system secondary side of the current transformer CT-2. As a result, the current differential relay 100a of the second embodiment can also improve the reliability of the detection result of the fault X in the main relay unit 110, similar to the current differential relay 100 of the first embodiment.

[0054] (Third embodiment) 6 is a diagram showing an example of the configuration of a current differential relay 100 according to the third embodiment. Two current differential relays 100b (current differential relays 100b-1 and 100b-2) according to the third embodiment include a main relay unit 110 having a current differential relay 112, a failsafe relay unit 120 having an overcurrent relay 122, a variation current relay 124, and an OR circuit 126, a failsafe relay unit 130 (hereinafter referred to as the "failsafe relay unit 130b") having an overcurrent relay 132, a variation current relay 134, and an OR circuit 136, and an output unit 140a having an AND circuit 146. The current differential relay 100b has a configuration in which the failsafe relay unit 130 included in the current differential relay 100 according to the first embodiment is replaced with the failsafe relay unit 130b, and the output unit 140 is replaced with the output unit 140a. The current differential relay 112 is an example of a "first means." The configuration of the overcurrent relay 122, the change current relay 124, and the OR circuit 126 is an example of a "second means." The configuration of the overcurrent relay 132, the change current relay 134, and the OR circuit 136 is an example of a "third means." The AND circuit 146 is an example of a "fourth means."

[0055] In the current differential relay 100b, the same components as those in the current differential relay 100 or the current differential relay 100a of the second embodiment are assigned the same reference numerals. Therefore, detailed explanations of the components in the current differential relay 100b that have the same configurations and operations as those in the current differential relay 100 or the current differential relay 100a will be omitted, and only the different configurations and operations will be explained.

[0056] Similar to the OR circuit 126 included in the failsafe relay unit 120, the OR circuit 136 performs a logical OR operation on the detection signal OC-2 of the overcurrent relay 132 and the detection signal ΔOC-2 of the change current relay 134. The OR circuit 136 outputs a detection signal (hereinafter referred to as "detection signal FS-2") that indicates the result of the logical OR operation. The detection signal FS-2 is an example of a "third detection signal."

[0057] The AND circuit 146 takes a logical AND of the detection signal DIF from the main relay unit 110, the detection signal FS-1 from the failsafe relay unit 120, and the detection signal FS-2 from the failsafe relay unit 130b. The AND circuit 146 outputs a detection signal representing the result of the logical AND as a shutdown command TC. More specifically, the AND circuit 146 outputs the shutdown command TC representing that the transmission line PL is to be shut off when the detection signal DIF represents that it has been determined that a fault has occurred in the power transmission line PL, the detection signal FS-1 represents that a fault has occurred in the power transmission line PL, and the detection signal FS-2 represents that a fault has occurred in the power transmission line PL.

[0058] The output unit 140a outputs the trip command TC of the AND circuit 146 as a trip command TC-1 to the corresponding circuit breaker CB-1. That is, similar to the output units 140 provided in the current differential relay 100 and the current differential relay 100a, the output unit 140a outputs the trip command TC-1 instructing to trip the power transmission line PL to the corresponding circuit breaker CB-1 when the main relay unit 110, the failsafe relay unit 120, and the failsafe relay unit 130b all detect that a fault has occurred in the power transmission line PL.

[0059] With this configuration, the current differential relay 100b detects a fault X that has occurred on the power transmission line PL using the main relay unit 110, and improves the reliability of the detection result of the main relay unit 110 using the fail-safe relay unit 120 and the fail-safe relay unit 130b, similar to the current differential relay 100 and the current differential relay 100a. Then, similar to the current differential relay 100 and the current differential relay 100a, the current differential relay 100b outputs a breaking command TC-1 to the corresponding circuit breaker CB-1 instructing the breaking of the power transmission line PL when a change in current in the power transmission line PL is caused by the occurrence of the fault X.

[0060] 3 and 4, the operation of the current differential relay 100b can be considered similar to that of the current differential relay 100, except that the detection signal OC-2 output by the overcurrent relay 132 is also the detection signal ΔOC-2 output by the change current relay 134. Therefore, a detailed description of the operation of the current differential relay 100b will be omitted.

[0061] As described above, the current differential relay 100b of the third embodiment includes a failsafe relay unit 120 that improves the reliability of the detection result of the fault X in the main relay unit 110 based on the current data I-1 on the system secondary side of the current transformer CT-1, and a failsafe relay unit 130b that improves the reliability of the detection result of the fault X in the main relay unit 110 based on the current data I-2 on the system secondary side of the current transformer CT-2. As a result, the current differential relay 100b of the third embodiment can also improve the reliability of the detection result of the fault X in the main relay unit 110, similar to the current differential relay 100 of the first embodiment and the current differential relay 100a of the second embodiment.

[0062] As described above, in the current differential relaying device of each embodiment, the main relay unit 110 detects a fault X that has occurred in the power transmission line PL based on the current flowing in the power transmission line PL at the terminal T-2, which is the opposite terminal of the terminal T-1 where the main relay unit 110 is located. In the current differential relaying device of each embodiment, the failsafe relay unit 120 improves the reliability of the detection result of the fault X by the main relay unit 110 based on the current flowing in the power transmission line PL at the terminal T-1 where the main relay unit 110 is located. Furthermore, in the current differential relaying device of each embodiment, the failsafe relay unit 130 improves the reliability of the detection result of the fault X by the main relay unit 110 based on the current flowing in the power transmission line PL at the terminal T-2, which is the opposite terminal of the terminal T-1 where the main relay unit 110 is located. That is, in the current differential relay of each embodiment, if the change in current in transmission line PL is caused by the occurrence of fault X, the current differential relay outputs shutdown commands TC-1 and TC-2 to circuit breakers CB-1 and CB-2 that shut off transmission line PL, thereby shutting off the connected transmission line PL, and if the change in current in transmission line PL is caused by some factor other than fault X, the current differential relay does not output shutdown commands TC-1 and TC-2, and keeps transmission line PL connected. In this way, in a power system to which the current differential relay of each embodiment is applied, unnecessary shutting off of transmission line PL can be avoided when the current changes due to factors other than fault X.

[0063] The current differential relay of each of the above-described embodiments has been described as being configured to correspond to a power system that transmits power between two terminals T, that is, as being configured to include a failsafe relay unit 120 and a failsafe relay unit 130 that correspond to the two terminals T. However, in the current differential relay of each of the embodiments, the number of terminals T that each failsafe relay unit corresponds to is not limited to two, and may be three or more terminals T. The configuration and operation of the current differential relay in this case should be equivalent to the configuration and operation of the current differential relay 100 of the first to third embodiments.

[0064] According to at least one of the embodiments described above, a current differential relay (e.g., 100-1) that detects a fault (X) in an electric power system (1) and outputs a trip command (TC-1) to a circuit breaker (e.g., CB-1) when the fault is detected includes first means (110) that periodically samples the current at its own terminal (e.g., T-1) and the currents at all other terminals (e.g., T-2) of the electric power system at the same time, calculates a difference current from the sampled data, and detects a system fault based on the magnitude of this difference current, second means (120) that detects an overcurrent or a current change in the current at its own terminal, third means (130) that detects an overcurrent at another other terminal, and fourth means (140) that generates a trip command output from all operating conditions of the first means, second means, and third means, thereby making it possible to improve the reliability of detecting a system fault (X) in an electric power system.

[0065] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents. [Explanation of symbols]

[0066] 1···Power system, 100, 100-1, 100-2, 100a, 100a-1, 100a-2, 100b, 100b-1, 100b-2···Current differential relay device, 110···Main relay section, 112···Current differential relay, 120···Fail-safe relay section, 122···Overcurrent relay, 124···Variable current relay, 126···OR circuit, 130, 130a, 13 0b···Fail-safe relay section, 132···Overcurrent relay, 134···Variable current relay, 136···OR circuit, 140, 140a···Output section, 142, 144, 146···AND circuit, AC, AC-1, AC-2···Generator, T, T-1, T-2···Terminal, PL···Transmission line, CT, CT-1, CT-2···Current transformer, CB, CB-1, CB-2···Circuit breaker

Claims

1. A current differential relay that detects a fault in a power system and outputs a trip command to a circuit breaker when a fault is detected. a first means for periodically sampling a terminal current of the power system and all terminal currents other than the terminal current of the power system at the same time, calculating a difference current from the sampled data, and detecting a system fault based on the magnitude of the difference current; a second means for detecting an overcurrent or a current change in the current at the terminal; a third means for detecting an overcurrent at another terminal; a fourth means for generating a trip command output from all operating conditions of the first means, the second means, and the third means; A current differential relay device comprising:

2. A current differential relay that detects a fault in a power system and outputs a trip command to a circuit breaker when a fault is detected. a first means for periodically sampling a terminal current of the power system and all terminal currents other than the terminal current of the power system at the same time, calculating a difference current from the sampled data, and detecting a system fault based on the magnitude of the difference current; a second means for detecting an overcurrent or a current change in the current at the terminal; a third means for detecting a change in current at another terminal; a fourth means for generating a trip command output from all operating conditions of the first means, the second means, and the third means; A current differential relay device comprising:

3. A current differential relay that detects a fault in a power system and outputs a trip command to a circuit breaker when a fault is detected. a first means for periodically sampling a terminal current of the power system and all terminal currents other than the terminal current of the power system at the same time, calculating a difference current from the sampled data, and detecting a system fault based on the magnitude of the difference current; a second means for detecting an overcurrent or a current change in the current at the terminal; a third means for detecting an overcurrent or a current change at another terminal; a fourth means for generating a trip command output from all operating conditions of the first means, the second means, and the third means; A current differential relay device comprising:

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