JP7499595B2 - Distance Relay - Google Patents

Description

This disclosure relates to distance relays.

JP 2007-20353 A (Patent Document 1) discloses a distance relay that protects a transmission line drawn from a busbar of a 1.5 breaker arrangement (also called a one and a half breaker arrangement).

In the 1.5 circuit breaker system, a first busbar and a second busbar are connected via a first circuit breaker, a second circuit breaker, and a third circuit breaker connected in series. A first end of the transmission line is connected between the first circuit breaker and the second circuit breaker. Furthermore, a first current transformer is connected between the first busbar and the first circuit breaker, a second current transformer is connected between the second circuit breaker and the third circuit breaker, and a first voltage transformer is connected in close proximity to the first end of the transmission line.

According to one embodiment of Patent Document 1, the power transmission line protection relay includes a distance relay element and an external fault detection means. The external fault detection means is provided to prevent the distance relay element from malfunctioning due to magnetic saturation of the current transformer (hereinafter also referred to as CT saturation).

Specifically, the distance relay element receives as input the combined fault current detected by the first current transformer and the second current transformer, and the power line voltage detected by the first voltage transformer. Based on these inputs, the distance relay element detects the impedance from the input position to the fault occurrence point, and whether the fault point is in the forward direction.

The external fault detection means includes a phase detection means and a sudden current change detection means. The phase detection means determines whether the phase of the first current detected by the first current transformer and the phase of the second current detected by the second current detection means are in phase (i.e., an internal fault) or out of phase (i.e., an external fault). The sudden current change detection means determines whether at least one of the first current or the second current has suddenly changed. These determinations are made within a short period of time until the current transformer is saturated. The external fault detection means locks the distance relay element so that it does not operate when at least one of the first current or the second current has suddenly changed and the phase of the first current and the phase of the second current are out of phase (i.e., an external fault).

JP 2007-20353 A

The above-mentioned Patent Document 1 does not take into consideration that the phase of the power supply voltage at the first end of the transmission line and the phase of the power supply voltage at the second end of the transmission line may differ depending on the load current. When the phase of the power supply voltage differs at both ends of the transmission line, the phase difference between the first current detected by the first current transformer and the second current detected by the second current transformer will not be 0° (in-phase) in the case of an internal fault, and will not be 180° (out-of-phase) in the case of an external fault. In this way, the phase difference between the first current and the second current deviates from the original phase difference, and the phase detection means may erroneously determine whether they are in-phase or out-of-phase.

For example, let us consider the case where a fault occurs on the second busbar. In this case, a through-current flows from the first busbar to the second busbar at the installation position of the first current transformer. In addition to this through-current, a transmission line current flows from the second end of the transmission line to the first end at the installation position of the second current transformer. Since the through-current is based on the power supply voltage on the first end side of the transmission line and the transmission line current is based on the power supply voltage on the second end side of the transmission line, the phases of the two may differ. When there is no CT saturation, this phase difference is usually within 30°, but when CT saturation occurs, the phase difference widens. As a result, the phase detection means may erroneously determine that the phases are the same when they should be determined to be opposite.

This disclosure has been made in consideration of the above problems. The purpose of this disclosure is to more reliably prevent malfunctions due to magnetic saturation of a current transformer in a distance relay for protecting a transmission line drawn from a busbar using a 1.5 circuit breaker system.

In one embodiment, a distance relay for protecting a power transmission system is provided. The power transmission system includes a power transmission line, a first busbar and a second busbar connected to a first end of the power transmission line in a 1.5 circuit breaker system, and a third busbar and a fourth busbar connected to a second end of the power transmission line, a first current transformer for detecting a first current flowing between the first end and the first busbar, a second current transformer for detecting a second current flowing between the first end and the second busbar, a first voltage transformer for detecting a first voltage at the first end, a third current transformer for detecting a third current flowing between the second end and the third busbar, and a fourth current transformer for detecting a fourth current flowing between the second end and the fourth busbar. The distance relay includes a distance determination unit, a direction determination unit, and an external fault determination unit. The distance determination unit calculates an impedance based on the value of a first composite current obtained by adding the detected values of the first current and the second current and the detected value of the first voltage, and determines whether the magnitude of the calculated impedance is within a set value. The direction determination unit determines whether the fault point is forward or backward based on the value of the composite current and the detected value of the first voltage. The external fault determination unit determines whether an external fault has occurred based on the sum of the detected values of the first current, the second current, the third current, and the fourth current detected at the same sampling time. The distance relay outputs a trip signal to the corresponding circuit breaker when the distance determination unit determines that the impedance is within the set value, the direction determination unit determines that the fault point is forward, and the external fault determination unit determines that there is no external fault.

According to the distance relay of the above embodiment, malfunctions due to magnetic saturation of the current transformer can be more reliably prevented by using the judgment result of the external fault judgment unit that judges whether or not there is an external fault based on the sum of the detected values of the first current, the second current, the third current, and the fourth current.

FIG. 1 is a circuit diagram showing an example of the configuration of a power transmission system. FIG. 2 is a block diagram showing an example of a hardware configuration of the power transmission line protection relay of FIG. 1 . FIG. 2 is a functional block diagram showing the functions of a distance relay element realized in the power transmission line protection relay of FIG. 1 . 4 is a conceptual timing diagram for illustrating an example of the operation of the distance relay element of FIG. 3. FIG. 11 is a functional block diagram showing the functions of a distance relay element realized in the power transmission line protection relay of embodiment 2. 6 is a conceptual timing diagram for explaining an example of the operation of the distance relay element of FIG. 5. FIG. 11 is a functional block diagram showing the functions of a distance relay element realized in the power transmission line protection relay of embodiment 3. FIG. 8 is a conceptual timing diagram for explaining an example of the operation of the distance relay element of FIG. 7 (when a rear fault occurs). FIG. 8 is a conceptual timing diagram for explaining an example of the operation of the distance relay element of FIG. 7 (when a forward fault occurs). FIG. 13 is a functional block diagram showing the functions of a distance relay element realized in the power transmission line protection relay of embodiment 4. 5 is a diagram for explaining a phase relationship between a first current and a second current. FIG. 11 is a flow chart illustrating the operation of the distance relay element of FIG. 10 . 11 is a conceptual timing diagram for explaining an example of the operation of the distance relay element of FIG. 10.

Each embodiment will be described in detail below with reference to the drawings. Note that the same or corresponding parts will be given the same reference symbols and their description will not be repeated.

Embodiment 1.
[Power transmission system configuration]
Fig. 1 is a circuit diagram showing an example of the configuration of a power transmission system. Note that the power transmission system 15 is configured as a three-phase AC system, but in Fig. 1, each bus bar and transmission line are represented by a single line for ease of illustration.

Referring to FIG. 1, the power transmission system 15 includes a power transmission line 20, a first busbar 21 and a second busbar 22 provided on the first end 23 side of the power transmission line 20, and a third busbar 24 and a fourth busbar 25 provided on the second end 26 side of the power transmission line 20. The first busbar 21 and the second busbar 22 are connected via a busbar tie line 16, and the third busbar 24 and the fourth busbar 25 are connected via a busbar tie line 17.

The power transmission system 15 further includes a first circuit breaker CB1, a second circuit breaker CB2, and a third circuit breaker CB3 connected in series on a busbar connection line 16 between a first busbar 21 and a second busbar 22, and a fourth circuit breaker CB4, a fifth circuit breaker CB5, and a sixth circuit breaker CB6 connected in series on a busbar connection line 17 between a third busbar 24 and a fourth busbar 25. A first end 23 of the power transmission line 20 is connected between the first circuit breaker CB1 and the second circuit breaker CB2, and a second end 26 of the power transmission line 20 is connected between the fourth circuit breaker CB4 and the fifth circuit breaker CB5.

Although not shown in FIG. 1, one end of a second transmission line is usually connected between the second circuit breaker CB2 and the third circuit breaker CB3. Furthermore, the other end of the second circuit breaker CB2 or one end of another third transmission line is connected between the fifth circuit breaker CB5 and the sixth circuit breaker CB6.

The above arrangement of busbars, circuit breakers, and transmission lines is called the 1.5 circuit breaker system. Circuit breakers CB1 to CB6 are normally controlled to the closed state, and when a fault occurs, the associated circuit breaker is controlled to the open state so that the faulty area is isolated from the power transmission system 15.

The power transmission system 15 further includes a first current transformer CT1, a second current transformer CT2, a first voltage transformer PT1, a third current transformer CT3, a fourth current transformer CT4, and a second voltage transformer PT2.

The first current transformer CT1 detects a first current I1 flowing through the busbar tie 16 between the first busbar 21 and the first circuit breaker CB1. The second current transformer CT2 detects a second current I2 flowing through the busbar tie 16 between the second circuit breaker CB2 and the third circuit breaker CB3. The first voltage transformer PT1 detects a first voltage V1 at a position close to the first end 23 of the transmission line 20. The third current transformer CT3 detects a third current I3 flowing through the busbar tie 17 between the third busbar 24 and the fourth circuit breaker CB4. The fourth current transformer CT4 detects a fourth current I4 flowing through the busbar tie 17 between the fifth circuit breaker CB5 and the sixth circuit breaker CB6. The second voltage transformer PT2 detects a second voltage V2 at a position close to the second end 26 of the transmission line 20.

As a transmission line protection relay for protecting the transmission line 20, a transmission line protection relay 30A is provided near the first end 23 of the transmission line 20, and a transmission line protection relay 30B is provided near the second end 26 of the transmission line 20.

The transmission line protection relay 30A receives a signal representing the first current I1 from the first current transformer CT1, receives a signal representing the second current I2 from the second current transformer CT2, and receives a signal representing the first voltage V1 from the first voltage transformer PT1. The transmission line protection relay 30B receives a signal representing the third current I3 from the third current transformer CT3, receives a signal representing the fourth current I4 from the fourth current transformer CT4, and receives a signal representing the second voltage V2 from the second voltage transformer PT2.

Furthermore, the power transmission line protection relay 30A and the power transmission line protection relay 30B are connected to each other via a communication path 27. The communication path 27 may be wired or wireless. The communication path 27 may also be a dedicated line or a network.

The power transmission line protection relay 30A transmits a signal representing a combined current IL of the first current I1 and the second current I2 to the power transmission line protection relay 30B via the communication path 27. The power transmission line protection relay 30B transmits a signal representing a combined current IR of the third current I3 and the fourth current I4 to the power transmission line protection relay 30A via the communication path 27. Note that the power transmission line protection relay 30A may transmit signals representing the first current I1 and the second current I2 to the power transmission line protection relay 30B instead of the combined current IL. Also, the power transmission line protection relay 30B may transmit signals representing the third current I3 and the fourth current I4 to the power transmission line protection relay 30A instead of the combined current IR.

The power transmission line protection relay 30A uses a distance relay system to determine whether or not a fault has occurred in the power transmission line 20 based on the detected values of the first current I1, the second current I2, the first voltage V1, and the composite current IR. The power transmission line protection relay 30B uses a distance relay system to determine whether or not a fault has occurred in the power transmission line 20 based on the detected values of the third current I3, the fourth current I4, the second voltage V2, and the composite current IL. Details of the operation of the power transmission line protection relays 30A and 30B will be described later with reference to Figures 3 and 4.

In the following description, the power line protection relays 30A, 30B may be referred to as distance relays 30A, 30B. Furthermore, the power line protection relays (distance relays) 30A, 30B are referred to collectively as power line protection relays (distance relays) 30 when referring to them collectively or when referring to an unspecified one. The first circuit breaker CB1, the second circuit breaker CB2, the third circuit breaker CB3, the fourth circuit breaker CB4, the fifth circuit breaker CB5, and the sixth circuit breaker CB6 are referred to collectively as circuit breakers CB. The first current transformer CT1, the second current transformer CT2, the third current transformer CT3, and the fourth current transformer CT4 are referred to collectively as current transformers CT.

[Hardware configuration of transmission line protection relay]
Fig. 2 is a block diagram showing an example of a hardware configuration of the power transmission line protection relay in Fig. 1. Since the power transmission line protection relay 30A and the power transmission line protection relay 30B in Fig. 1 have the same configuration, the power transmission line protection relay 30A will be representatively described below.

Referring to FIG. 2, the power line protection relay 30A has a configuration similar to that of a so-called digital relay device. Specifically, the power line protection relay 30A includes an input conversion unit 100, an A/D conversion unit 110, an arithmetic processing unit 120, and an I/O (Input and Output) unit 130.

The input conversion unit 100 includes auxiliary transformers 101_1, 101_2, ... for each input channel. The input conversion unit 100 takes in a signal representing the first current I1 output from the first current transformer CT1 in FIG. 1, a signal representing the second current I2 output from the second current transformer CT2, and a signal representing the first voltage V1 output from the first voltage transformer PT1 for each phase of the three-phase AC. Each auxiliary transformer 101 converts these input signals into signals of voltage levels suitable for signal processing in the A/D conversion unit 110 and the arithmetic processing unit 120.

The A/D conversion unit 110 includes analog filters (AF) 111_1, 111_2, ..., sample hold circuits (S/H) 112_1, 112_2, ..., a multiplexer (MPX) 113, and an A/D converter 114. The analog filter 111 and the sample hold circuit 112 are provided for each channel of the input conversion unit 100.

Each analog filter 111 is a low-pass filter provided to remove aliasing errors during A/D conversion. Each sample-and-hold circuit 112 samples and holds the signal that has passed through the corresponding analog filter 111 at a specified sampling frequency. The sampling frequency is, for example, 4800 Hz. The multiplexer 113 sequentially selects the voltage signals held in the sample-and-hold circuits 112_1, 112_2, .... The A/D converter 114 converts the signal selected by the multiplexer 113 into a digital value.

The calculation processing unit 120 includes a CPU (Central Processing Unit) 121, a RAM (Random Access Memory) 122, a ROM (Read Only Memory) 123, and a bus 124 connecting these. The CPU 121 controls the overall operation of the power line protection relay 30A by operating according to a program. The RAM 122 and the ROM 123 are used as the main memory of the CPU 121. The ROM 123 can store programs and setting values for signal processing by using a non-volatile memory such as a flash memory as a storage medium.

The I/O unit 130 includes a communication circuit 131, a digital input (D/I) circuit 134, and a digital output (D/O) circuit 135. The communication circuit 131 includes a receiver 132 that receives the combined current IR from the power line protection relay 30B, and a transmitter 133 that transmits the combined current IL to the power line protection relay 30B. The digital input circuit 134 and the digital output circuit 135 are interface circuits used when inputting and outputting digital signals between the CPU 121 and an external device. For example, the digital output circuit 135 outputs a trip signal TRS to a related circuit breaker CB in accordance with a command from the CPU 121.

The functions of the arithmetic processing unit 120 may be realized as an electronic circuit such as an ASIC (Application Specific Integrated Circuit) or an FPGA (Field Programmable Gate Array), or may be realized by combining two or more of a CPU, an ASIC, and an FPGA.

[Functional configuration of distance relay elements]
Fig. 3 is a functional block diagram showing the function of a distance relay element realized in the power transmission line protection relay of Fig. 1. Since the operation of the power transmission line protection relay 30A and the operation of the power transmission line protection relay 30B in Fig. 1 are similar, Fig. 3 representatively shows the function of a distance relay element 40A realized in the power transmission line protection relay 30A. Note that the function of the distance relay element 40A is realized, for example, by the CPU 121 in Fig. 2 operating according to a program.

As shown in FIG. 3, the distance relay element 40A includes an adder 35, filters (FIL) 36 and 37, a distance determination unit 41, a direction determination unit 42, a synchronization processing unit 43, an external fault determination unit 50, and logic operators 44 and 45.

The adder 35 calculates the value of the composite current IL by adding the detection value of the first current I1 and the detection value of the second current I2. The calculated composite current IL is input to the filter 37 and the synchronization processing unit 43, and is also output to the power line protection relay 30B via the transmitter 133.

The filter 36 extracts a sine wave component from the detection value of the first voltage V1. The extracted sine wave component of the first voltage V1 is input to the distance determination unit 41 and the direction determination unit 42. The filter 37 also extracts a sine wave component from the calculated composite current IL. The extracted sine wave component of the composite current IL is input to the distance determination unit 41 and the direction determination unit 42.

The distance determination unit 41 calculates the impedance from the sine wave component of the first voltage V1 and the sine wave component of the composite current IL. The distance determination unit 41 determines whether the calculated impedance is within a set value.

The direction determination unit 42 determines whether the phase of the sine wave component of the composite current IL is in-phase (i.e., forward) or out-of-phase (i.e., backward) with respect to the phase of the sine wave component of the first voltage V1.

The direction determination unit 42 is also provided with a memory function, and uses an added voltage obtained by adding the current detection value of the first voltage V1 and the detection value of the first voltage V1 n cycles before the current time (n is about 2 or 3) multiplied by a coefficient for the determination. This prevents the direction determination from becoming impossible due to a sudden decrease in the first voltage V1 immediately after the occurrence of a failure. Furthermore, the output of the direction determination unit 42 is locked at a point in time before the magnitude (for example, amplitude or effective value) of the added voltage also becomes small and the direction determination becomes impossible, and the determination result at that point in time is maintained. For example, the output of the direction determination unit 42 is fixed at a point in time when the magnitude of the first voltage V1 at both the current time and n cycles before the current time becomes smaller than a threshold value. Hereinafter, the function of latching the output of the direction determination unit 42 is referred to as the "output lock function" or "output latch function". In addition, the above-mentioned determination condition regarding the magnitude of the first voltage V1 for determining whether or not to cause the output lock function is referred to as the "latch condition".

The synchronization processing unit 43 receives the calculated value of the composite current IL as an input, and also receives the calculated value of the composite current IR from the power transmission line protection relay 30B. The synchronization processing unit 43 extracts the calculated value of the composite current IL at a sampling time that is earlier than the current time by the transmission delay of the communication path 27 so as to be synchronized with the sampling time of the received composite current IR on the second end side. The synchronization processing unit 43 outputs the values of the composite current IL and the composite current IR based on the detection values of the first current I1, the second current I2, the third current I3, and the fourth current I4 detected at the same sampling time to the external fault determination unit 50.

Note that various methods can be used to synchronize the timing of sampling the electrical quantity in the power transmission line protection relay 30A and the timing of sampling the electrical quantity in the power transmission line protection relay 30B, and there is no particular limitation. For example, a known method may be used in which the power transmission line protection relay 30A and the power transmission line protection relay 30B transmit a synchronization pulse to each other via the communication path 27. In this method, the transmission delay of the communication path 27 can be measured from the time difference between the transmission time and the reception time of the synchronization pulse in each of the power transmission line protection relays 30A and 30B. Then, the timing of sampling the electrical quantity at both ends of the power transmission line 20 can be synchronized based on the measured transmission delay.

The external fault determination unit 50 determines whether or not an external fault has occurred in the protection section of the transmission line 20 based on the combined current IL on the first end 23 side of the transmission line 20 and the combined current IR on the second end 26 side. This determination is performed until each current transformer CT is saturated. If an external fault has occurred, the external fault determination unit 50 outputs the logical value "true." Note that, for the sake of convenience, in the following description, the logical value "true" is associated with "1" and the logical value "false" is associated with "0."

Specifically, as shown in FIG. 3, the external fault judgment unit 50 includes a current differential judgment unit 51, a first fault judgment unit 52, a one-shot timer 53, a logical operator 54, an operation timer 55, and a recovery timer 56. The configuration and operation of the external fault judgment unit 50 will be described below.

The current differential determination unit 51 determines whether or not the absolute value of the sum of the instantaneous value of the combined current IL on the first end 23 side of the power transmission line 20 and the instantaneous value of the combined current IR on the second end 26 side is smaller than a threshold value K1. That is, at a sampling time t,
IL(t) = I1(t) + I2(t) ... (1)
IR(t) = I3(t) + I4(t) ... (2)
ID = | (IL (t) + IR (t)) | < K1 ... (3)
When the above formula (3) is satisfied, the current differential determination unit 51 outputs a logical value "true." According to Kirchhoff's current law, ID is zero in the case of an external fault, and has a value corresponding to the magnitude of the fault current in the case of an internal fault. Therefore, when the above formula (3) is satisfied, it indicates that a load current or an external fault current is flowing in the power transmission line 20.

The first fault determination unit 52 determines whether or not the combined current IL on the first terminal 23 side or the combined current IR on the second terminal 26 side has suddenly changed. Specifically, in the example of FIG. 3, T is a period (also referred to as a cycle) at the rated frequency,
ΣΔI＝｜IL(t)＋IL(t－T/2)｜＋｜IR(t)＋IR(t－T/2)｜ …(4)
When the value of the formula (4) is greater than the threshold value K2, that is, when the first failure determination unit 52 calculates
ΣΔI＞K2 … (5)
When the condition is satisfied, it outputs the logical value "true".

The first term in the above equation (4) represents the absolute value of the sum of the instantaneous value of the combined current IL(t) at sampling time t and the instantaneous value of the combined current IL(t-T/2) 180° electrical angle before the sampling time t. The second term in the above equation (4) represents the absolute value of the sum of the instantaneous value of the combined current IR(t) at sampling time t and the instantaneous value of the combined current IR(t-T/2) 180° electrical angle before the sampling time t. Therefore, since the combined currents IL and IR are sine waves, the value of ΣΔI will be 0 if there is no fault. On the other hand, regardless of whether the fault is internal or external, when a fault occurs, the current changes suddenly and the value of ΣΔI increases sharply.

Therefore, when the above formula (5) is satisfied, it indicates that an internal or external fault has occurred in the power transmission system 15. When both the above formulas (3) and (5) are satisfied, it indicates that an external fault has occurred in the power transmission system 15.

When the output of the first fault determination unit 52 changes from "0" to "1", the one-shot timer 53 continues to output "1" for the pulse interval T1. In general, the pulse interval T1 is set to about one cycle as the determination time until magnetic saturation of the current transformer CT occurs.

The operation timer 55 switches its output from "0" to "1" when the operation time T2 has elapsed since the output of the logical operator 54 changed from "0" to "1". The recovery timer 56 switches its output from "1" to "0" when the recovery time T3 has elapsed since the output of the operation timer 55 changed from "1" to "0". For example, the operation time T2 is set to about 1/8 to 1/6 cycle, and the recovery time T3 is set to be equal to or longer than the duration of magnetic saturation of the current transformer CT (for example, about 10 to 20 cycles).

Therefore, if the above formula (3) is satisfied continuously for at least T2 time (e.g., 1/8 to 1/6 cycle) within a time T1 (e.g., 1 cycle) after the above formula (5) is satisfied, the external fault judgment unit 50 outputs "1" for a time T3 (e.g., 10 to 20 cycles).

The logical operator 45 performs a logical AND operation between the negation of the logical value output from the external fault judgment unit 50 and the logical value output from the direction judgment unit 42, and outputs the result of the operation. The logical operator 44 performs a logical AND operation between the logical value output from the distance judgment unit 41 and the logical value output from the logical operator 45, and outputs the result of the operation.

Therefore, when the external fault determination unit 50 determines that there is no external fault (i.e., when the output of the external fault determination unit 50 is "0"), the output of the distance relay element 40A is determined by the results of the determinations of the distance determination unit 41 and the direction determination unit 42. That is, when the distance determination unit 41 determines that the impedance is within the set value and the direction determination unit 42 determines that there is a forward fault, the distance relay element 40A determines that an internal fault has occurred within the protection range of the power transmission line 20. As a result, the distance relay element 40A outputs a trip signal TRS to the corresponding first circuit breaker CB1 and second circuit breaker CB2. The trip signal TRS is continued for a time period predetermined by a recovery timer (not shown) or is latched by a latch circuit (not shown).

On the other hand, if the external fault determination unit 50 determines that an external fault has occurred (i.e., the output of the external fault determination unit 50 is "1"), the outputs of the logic operators 44 and 45 are fixed to "0" regardless of the determination result of the direction determination unit 42. This makes it possible to prevent malfunction of the direction determination unit 42 due to magnetic saturation of the current transformer CT.

In addition, if magnetic saturation occurs in the current transformer CT, the direction determination unit 42 will erroneously determine that the direction is forward, even though there is an external fault. Let us assume that when the direction determination unit 42 erroneously determines that the direction is forward, the determination result is latched by the memory function. At this point, the output of the external fault determination unit 50 is "1", so the output of the logical operator 45 becomes "0", and the distance relay element does not malfunction. However, when the recovery time T3 of the recovery timer 56 has elapsed, the output of the external fault determination unit 50 returns to "0". If the output of the direction determination unit 42 is latched in the forward direction (i.e., "1") at this point, the output of the logical operator 45 becomes "1", and the distance relay element malfunctions. In the distance relay element 40A of this embodiment, in order to prevent such malfunction, when the external fault determination unit 50 outputs "1", the direction determination unit 42 locks the determination result to backward (i.e., the output is "0"). As a result of the above, it is possible to prevent malfunctions caused by the memory function of the direction determination unit 42.

[Example of distance relay element operation]
Fig. 4 is a conceptual timing diagram for explaining an example of the operation of the distance relay element in Fig. 3. The timing diagram in Fig. 4 shows, from the top, the signal waveforms of the first current I1, the second current I2, the combined current IR, the output signal A0 of the distance determination unit 41, the output signals A1 and A2 of the direction determination unit 42, the output signal B of the current differential determination unit 51, the output signal C of the one-shot timer 53 based on the output of the first fault determination unit 52, the output signal D of the external fault determination unit 50, and the output signal E (trip signal TRS) of the logical operator 44, all of which are shown in Fig. 3. The output signal A2 of the direction determination unit 42 shows a waveform in this embodiment, and the output signal A1 shows a waveform in a comparative example.

In the following explanation, when the output of each judgment unit is "1" (logical value: true), it is considered to be a high level, and when the output of each judgment unit is "0" (logical value: false), it is considered to be a low level. Note that the signal waveforms shown in Figure 4 and the subsequent timing diagrams of Figures 6, 8, 9, and 13 have been exaggerated for the purpose of explanation, and are not shown as is in actual signal waveforms, nor are they shown to the correct scale.

The timing diagram in FIG. 4 shows a case where a ground fault occurs at fault point F of the second busbar 22 in FIG. 1. In this case, a through current flows from the first busbar 21 to the second busbar 22 through the busbar tie wire 16, and a combined current IR of the third current I3 and the fourth current I4 flows into the second busbar 22 through the transmission line 20. Therefore, the current detected by the second current transformer CT2 is a combined current of the through current (i.e., the first current I1) and the combined current IR (i.e., I3 + I4). Note that in FIG. 4, the current flowing from each busbar to the transmission line 20 is shown as positive in each current transformer CT.

Referring to FIG. 4, at time t11, no fault has occurred and the power transmission system 15 is in a normal state. In this case, the load current is detected by each current transformer CT. Since the magnitude of the load current is much smaller than the magnitude of the fault current, it is not shown in FIG. 4. When the load current is flowing, the aforementioned determination equation (3) is satisfied, and therefore the output signal B of the current differential determination unit 51 is at a high level.

At time t12, a ground fault occurs at fault point F in FIG. 1. It takes about one cycle from the occurrence of the fault for the distance determination unit 41 to calculate the impedance. Therefore, at time t20, the distance determination unit 41 determines that the calculated impedance is within the set value and that a fault has occurred in the power transmission system 15. As a result, the output signal A0 of the distance determination unit 41 switches to a high level.

It takes about 1/6 cycle from the occurrence of the fault for the direction determination unit 42 to detect the phase. Therefore, until that time, the determination result of the direction determination unit 42 includes the effects before the fault occurred, so the output of the direction determination unit 42 is indefinite. After that, until the effects of magnetic saturation of the current transformer CT appear, the output signal A2 of the direction determination unit 42 is at a low level (i.e., an external fault).

In the case of an external fault, the output signal B of the current differential determination unit 51 remains at a high level. In other words, the determination formula (3) described above is satisfied.

At time t13, 2 to 3 milliseconds (ms) after the occurrence of the fault (time t12), the first fault determination unit 52 detects a sudden change in current by determining that the above-mentioned formula (5) is satisfied. As a result, the one-shot timer 53 outputs a high-level output signal C for the pulse interval T1.

If the output signal B of the current differential determination unit 51 is maintained at a high level from time t13 to time t14 when the operation time T2 of the operation timer 55 has elapsed, the external fault determination unit 50 determines that an external fault has occurred. As a result, the output signal D of the external fault determination unit 50 is maintained at a high level only during the recovery time T3 of the recovery timer 56. Furthermore, at time t14, the external fault determination unit 50 latches the output signal A2 of the direction determination unit 42 to a low level (i.e., backward determination).

At the next time t15, the magnetic flux density generated by the second current I2 (i.e., the combined current of the first current I1, the third current I3, and the fourth current I4) exceeds the maximum magnetic flux density of the core of the second current transformer CT2, causing the core of the second current transformer CT2 to enter a magnetically saturated state. Note that at time t17, the direction of the current flowing through the current transformer CT is reversed, and the magnetic saturation of the second current transformer CT2 is eliminated. However, at time t18, magnetic saturation occurs again in the second current transformer CT2, and this magnetic saturation continues until time t19. Thereafter, the second current I2 detected by the second current transformer CT2 repeats the same phenomenon.

Due to magnetic saturation of the second current transformer CT2, the current differential determination unit 51 erroneously determines that there is an internal fault because |I1|>|I2|, and switches the output signal B to a low level. However, the output signal D of the external fault determination unit 50 is not affected by the magnetic saturation of the second current transformer CT2 because an external fault has been determined at time t14.

Furthermore, the output signal A2 of the direction determination unit 42 is fixed to a low level (i.e., backward determination) by the external fault determination unit 50 at time t14. Therefore, the output signal A2 of the direction determination unit 42 is not affected by the magnetic saturation of the second current transformer CT2.

When the output of the direction determining unit 42 is not locked backward by the external fault determining unit 50, the output signal waveform of the direction determining unit 42 is shown as A1 (A1a, A1b). As shown in FIG. 4, the output signal waveform A1 is affected by the magnetic saturation of the second current transformer CT2 at time t16 and switches to a high level (forward determination). If the output of the direction determining unit 42 is locked by the output lock function of the direction determining unit 42 itself in this state, the direction determining unit 42 will erroneously determine that the direction is forward, as shown by the solid output signal waveform A1a. On the other hand, if the voltage drop due to the fault is relatively small and the output lock function of the direction determining unit 42 itself does not work, the output signal waveform will show a low level (backward determination) during the period when magnetic saturation does not occur, and a high level (forward determination) during the period when magnetic saturation occurs, as shown by the dashed output signal waveform A1b.

The final output signal of the distance relay element 40A (output signal E of the logic operator 44) is the logical product of the logical value of the output signal A0 of the distance determination unit 41, the logical value of the output signal A2 of the direction determination unit 42, and the negation of the logical value of the output signal D of the external fault determination unit 50. Therefore, in the event of an external fault, the output signal E of the distance relay element 40A is at a low level, and no trip signal is output.

[Effects of the First Embodiment]
A case where external fault determination unit 50 in Fig. 3 is not provided is taken as Comparative Example 1, and a case where phase detection means disclosed in the above-mentioned Japanese Patent Laid-Open Publication No. 2007-20353 (Patent Document 1) is provided instead of current differential determination unit 51 in Fig. 3 is taken as Comparative Example 2. Below, the effects of embodiment 1 will be described while comparing distance relay element 40A described above with Comparative Examples 1 and 2.

First, in the case of Comparative Example 1, in which the external fault determination unit 50 is not provided, the output signal E of the distance relay element 40A is determined by the logical product of the logical value of the output signal A0 of the distance determination unit 41 and the logical value of the output signal A2 of the direction determination unit 42. However, for example, if an external fault occurs at fault point F in FIG. 1, causing magnetic saturation in the second current transformer CT2, the direction determination unit 42 will erroneously determine that there is a forward fault. As a result, the distance relay element 40A will malfunction.

In contrast, in the case of the distance relay element 40A of this embodiment, the external fault determination unit 50 determines whether or not there is an external fault before magnetic saturation of the current transformer CT occurs. Then, if the external fault determination unit 50 determines that there is an external fault, the output signal of the logic operator 45 is fixed to a low level, and the output signal A2 of the direction determination unit 42 is also fixed to a low level (rearward determination). Therefore, it is possible to prevent malfunction of the distance relay element 40A caused by magnetic saturation of the current transformer CT.

On the other hand, in the case of Comparative Example 2, the phase detection means determines whether the first current I1 detected by the first current transformer CT1 and the second current I2 detected by the second current transformer CT2 are in phase (i.e., an internal fault) or out of phase (i.e., an external fault). This determination is performed before magnetic saturation of the current transformer CT occurs. If the phase detection means determines that the phases of the first current I1 and the second current I2 are out of phase (i.e., an external fault), the distance relay element is locked so that it does not operate.

However, while the phase calculations performed by the phase detection means can be performed correctly for sine waves at the rated frequency, calculation errors occur for transient distorted wave waveforms that occur when a fault occurs.

Furthermore, for example, when an external fault occurs at fault point F in FIG. 1, the first current I1 detected by the first current transformer CT1 is a through current in the direction from the first busbar 21 to the second busbar 22. The second current I2 detected by the second current transformer CT2 is a composite current of this through current and a current flowing in from the second end 26 side of the transmission line 20. In other words, the second current I2 is equal to a composite current of the first current I1, the third current I3, and the fourth current I4. The first current I1 is based on the power supply voltage on the first end 23 side of the transmission line 20, while the third current I3 and the fourth current I4 are based on the power supply voltage on the second end 26 side of the transmission line 20, so there is a possibility that the phases of the two currents are shifted by up to about 30°. The combination of the transient distorted waveform that occurs when a fault occurs and the effects of a phase shift in the power supply voltages at both ends of the transmission line can cause the phase detection means to erroneously determine that the two are in phase when they should actually be out of phase.

In contrast, in the case of the distance relay element 40A of this embodiment, the current differential determination unit 51 determines whether the fault is internal or external based on the sum of the instantaneous values of the first current I1 and second current I2 on the first end 23 side and the instantaneous values of the third current I3 and fourth current I4 on the second end 26 side at the same sampling time. Based on Kirchhoff's current law, the sum is non-zero in the case of an internal fault and zero in the case of an external fault. By comparing the absolute value of this sum with a threshold value, it is possible to reliably determine whether or not there is an external fault even if the power supply phase on the first end 23 side and the power supply phase on the second end 26 side are significantly different.

In this embodiment, if the external fault determination unit 50 determines that an external fault has occurred, the determination result of the direction determination unit 42 is locked to the rearward determination. As a result, erroneous determinations due to the memory function of the direction determination unit 42 can be avoided.

In this embodiment, each transmission line protection relay 30 uses the detection current at the other end of the transmission line 20. For this reason, each transmission line protection relay 30 includes a transmitter 133 that transmits a composite current of the detection currents at its own end to the transmission line protection relay 30 at the other end, a receiver 132 that receives the composite current of the detection currents at the other end from the transmission line protection relay 30 at the other end, and a synchronization processing unit 43 for synchronizing the detection current at its own end with the detection current at the other end. Furthermore, a communication path 27 is provided that connects the transmission line protection relays 30A and 30B at both ends of the transmission line 20.

When the communication path 27 in FIG. 1 is configured with optical fiber, the signal transmission delay time is about 2 to 3 milliseconds, so the external fault determination unit 50 can determine whether or not there is an external fault within a short time (e.g., 3 to 5 milliseconds) before CT saturation occurs. However, for example, when the communication path 27 is configured with a metal cable, the transmission delay time can reach a maximum of 10-odd milliseconds. In this case, the determination by the external fault determination unit 50 will be delayed compared to the direction determination by the direction determination unit 42. A solution to this problem will be explained in embodiment 2.

In addition, if each of the power transmission line protection relays 30A, 30B includes a current differential relay element in addition to a distance relay element, the ID in formula (3) may be calculated using the differential current calculated by the current differential relay element.

Embodiment 2.
In the second embodiment, a case will be described in which the transmission delay time of the communication path 27 is large, and therefore the judgment by the external fault judgment unit 50 is delayed compared to the direction judgment by the direction judgment unit 42. As will be described in detail below, an operation timer 63 is provided in the path of the output signal of the direction judgment unit 42 to delay the change in the output from the direction judgment unit 42. An operation time T4 of the operation timer 63 is set to be approximately equal to the transmission delay time of the communication path 27.

[Functional configuration of distance relay elements]
Fig. 5 is a functional block diagram showing the function of a distance relay element realized in the power transmission line protection relay of embodiment 2. Since the operation of the power transmission line protection relay 30A and the operation of the power transmission line protection relay 30B in Fig. 1 are similar, Fig. 5 representatively shows the function of the distance relay element 40B realized in the power transmission line protection relay 30A. Note that the function of the distance relay element 40B is realized, for example, by the CPU 121 in Fig. 2 operating according to a program.

Specifically, the distance relay element 40B in FIG. 5 differs from the distance relay element 40A in FIG. 3 in that it further includes a second fault determination unit 60, a recovery timer 61, a logical operator 62, and an operation timer 63.

The second fault judgment unit 60 receives the detection value of the first current I1 and the detection value of the second current I2. The second fault judgment unit 60 judges whether the detected current on the own terminal side, i.e., the first current I1 or the second current I2, has suddenly changed. That is, the second fault judgment unit 60 calculates the following, where T is the period at the rated frequency:
ΔI1(t) = |I1(t) + I1(t - T / 2) | ... (6)
ΔI2(t) = |I2(t) + I2(t - T / 2) | ... (7)
The right side of the above formula (6) represents the absolute value of the sum of the instantaneous value of the first current I1(t) at sampling time t and the instantaneous value of the first current I1(t-T/2) 180° electrical angle before the sampling time t. The right side of the above formula (7) represents the absolute value of the sum of the instantaneous value of the second current I2(t) at sampling time t and the instantaneous value of the second current I2(t-T/2) 180° electrical angle before the sampling time t.

Furthermore, the failure determination unit 60 sets Ist as a set value,
ΔI1(t)+ΔI2(t)＞Ist …(8)
is satisfied, the logical value "true" is output. When there is no fault, the values of ΔI1(t) and ΔI2(t) are approximately 0. Regardless of whether the fault is internal or external, when a fault occurs, the first current I1 and the second current I2 change suddenly, so the sum of ΔI1(t) and ΔI2(t) becomes greater than the threshold value. Since the second fault determination unit 60 determines the presence or absence of a fault using only the current detection value on its own end, it can determine the presence or absence of a fault more quickly without being affected by the transmission delay of the communication path 27, compared to the first fault determination unit 52, which uses the current detection values on both its own end and the other end.

The recovery timer 61 switches its output from "1" to "0" when a recovery time T5 has elapsed since the output of the second fault determination unit 60 changed from "1" to "0". The logical operator 62 calculates and outputs the logical product of the logical value of the output signal of the direction determination unit 42 and the logical value of the output signal of the recovery timer 61.

The operation timer 63 changes its own output from "0" to "1" after the lapse of operation time T4 from when the output of the logical operator 62 changes from "0" to "1". This makes it possible to delay the output signal from changing to "1" by the operation time T4 when the direction determination unit 42 erroneously determines that an internal fault exists despite the fact that it is an external fault due to magnetic saturation of the current transformer CT. The operation time T4 is set to be approximately equal to the transmission delay time of the communication path 27.

The logical operator 45 calculates the logical AND of the logical value of the output signal of the operation timer 63 and the negation of the logical value of the output signal of the external fault determination unit 50. Therefore, even if the output of the external fault determination unit 50 is delayed by the transmission delay time of the communication path 27 when an external fault occurs, the effect of the change in the output of the direction determination unit 42 can be delayed by the operation time T4 of the operation timer 63. As a result, it is possible to prevent malfunction of the distance relay element 40B caused by magnetic saturation of the current transformer CT.

Other aspects of Figure 5 are similar to those of Figure 3, so the same or corresponding parts are given the same reference symbols and will not be described repeatedly.

[Example of distance relay element operation]
Fig. 6 is a conceptual timing diagram for explaining an example of the operation of the distance relay element in Fig. 5. The timing diagram in Fig. 6 shows, from the top, the signal waveforms of the first current I1, the second current I2, the combined current IR, the output signal A0 of the distance determination unit 41, the output signals A1 and A2 of the direction determination unit 42, the output signal B of the recovery timer 61 based on the output of the second fault determination unit 60, the output signal C of the operation timer 63 based on the output of the direction determination unit 42, the output signal D of the current differential determination unit 51, the output signal E of the one-shot timer 53 based on the output of the first fault determination unit 52, the output signal F of the external fault determination unit 50, and the output signal G (trip signal TRS) of the logical operator 44 shown in Fig. 5.

The first current I1, the second current I2, the composite current IR, the output signal A0 of the distance determination unit 41, and the output signals A1 and A2 of the direction determination unit 42 have the same signal waveforms as those in FIG. 4 of the first embodiment. Specifically, at time t31, an external fault occurs at fault point F of the second busbar 22 in FIG. 1. Between time t33 and time t35 and between time t36 and time t40, magnetic saturation occurs in the second current transformer CT2. As a result, the second current I2 detected by the second current transformer CT2 becomes approximately zero.

It takes about one cycle from the occurrence of the fault for the distance determination unit 41 to calculate the impedance. Therefore, at time t41, the distance determination unit 41 determines that the calculated impedance is within the set value and that a fault has occurred in the power transmission system 15. As a result, the output signal A0 of the distance determination unit 41 switches from "0" to "1".

In addition, due to the occurrence of an external fault at time t31, the direction determination unit 42 initially determines that the fault is in the rear direction, and the output signals A1 and A2 become "0". When magnetic saturation occurs in the second current transformer CT2 at time t33, the direction determination unit 42 erroneously determines that the fault is in the front direction due to the influence of magnetic saturation. As a result, the output signal of the direction determination unit 42 switches from "0" to "1". After that, the output signal of the direction determination unit 42 basically shows the change in the dashed waveform A1b in FIG. 6. However, depending on the value at the time when the magnitude of the first voltage V1 (however, the voltage value two or three cycles ago multiplied by a coefficient due to the memory function is added) becomes smaller than the threshold value, the output signal of the direction determination unit 42 is fixed to a high level (A1a) or a low level (A2). Furthermore, as will be described later, when the external fault determination unit 50 determines that the fault is in the external direction, the output signal of the direction determination unit 42 is fixed to a low level (A2).

At time t32 after the occurrence of an external fault at time t31, the second fault judgment unit 60 detects a sudden change in the first current I1 and the second current I2. After that, the output signal B of the second fault judgment unit 60 is "1" for 0.5 cycles. Furthermore, the recovery timer 61 maintains the output signal B at "1" for the recovery time T5, and then switches the output signal B to "0" at time t43.

After an external fault occurs at time t31, the first fault determination unit 52 detects a sudden change in the combined currents IR and IL. This causes the one-shot timer 53 to output a one-shot pulse as the output signal E for the pulse interval T1. However, the rising edge of this one-shot pulse is delayed until time t37 due to the effect of the transmission delay time of the communication path 27.

At the next time t33, the second current transformer CT2 becomes magnetically saturated, so that the second current I2 becomes nearly zero. As a result, at time t34, the direction determination unit 42 erroneously determines that there is a forward fault, and its output signal A1 switches from "0" to "1." As a result, the output signal C of the operation timer 63 gradually increases so as to switch to "1" after the operation time T4 has elapsed.

In addition, due to the effect of magnetic saturation of the second current transformer CT2 at time t33, the current differential determination unit 51 erroneously determines that there is an internal fault, and the output signal D switches from "1" to "0." However, this signal change is delayed until time t39 due to the effect of the transmission delay time of the communication path 27.

If the output signal D of the current differential determination unit 51 is maintained at a high level from time t37 when the output signal of the one-shot timer 53 switches to "1" until time t38 when the operation time T2 of the operation timer 55 has elapsed, the external fault determination unit 50 determines that an external fault has occurred. As a result, the output signal F of the external fault determination unit 50 is maintained at a high level only during the recovery time T3 of the recovery timer 56. Furthermore, the external fault determination unit 50 latches the output signal A2 of the direction determination unit 42 to a low level (i.e., backward determination) at time t38. As a result, the output signal C of the operation timer 63 is fixed at a low level.

The final logical value of the output signal of the distance relay element 40B (output signal G of the logical operator 44) is the logical product of the logical value of the output signal A0 of the distance determination unit 41, the logical value of the output signal C of the operation timer 63 based on the output of the direction determination unit 42, and the negation of the logical value of the output signal F of the external fault determination unit 50. Therefore, in the event of an external fault, the output signal G of the distance relay element 40B is at a low level, and no trip signal is output.

[Effects of the second embodiment]
Since the external fault determination unit 50 uses both the self-end current and the other-end current, the determination of the external fault determination unit 50 is delayed by the transmission delay time of the communication path 27. Therefore, when the transmission delay time of the communication path 27 is large, the determination result of the external fault determination unit 50 cannot be obtained before the current transformer CT becomes magnetically saturated, which is an inconvenience. Therefore, in the distance relay element 40B of the second embodiment, the operation timer 63 is used to delay the change in the output signal of the direction determination unit 42 due to the magnetic saturation of the current transformer CT by the operation time T4. The operation time T4 is set to be approximately equal to the transmission delay time of the communication path 27. As a result, when an external fault occurs, the calculation timing of the direction determination and the calculation timing of the external fault determination can be matched, so that the malfunction of the distance relay element 40B due to the magnetic saturation of the current transformer CT can be prevented.

Embodiment 3.
In the second embodiment, the operation timer 63 is used to delay the output change of the direction determination unit 42 by the operation time T4, thereby preventing an erroneous output of the direction determination unit 42 due to magnetic saturation of the current transformer CT when an external fault occurs. However, the operation timer 63 similarly delays the forward output of the direction determination unit 42 when an internal fault occurs, and therefore unnecessarily delays the operation of the distance relay element even when magnetic saturation of the current transformer CT does not occur. In the third embodiment, the output of the forward determination result of the direction determination unit 42 is delayed only when a rear fault occurs. This makes it possible to prevent unnecessary delays in the operation of the distance relay element. A specific description will be given below with reference to the drawings.

[Functional configuration of distance relay elements]
Fig. 7 is a functional block diagram showing the function of a distance relay element realized in the power transmission line protection relay of embodiment 3. Since the operation of the power transmission line protection relay 30A and the operation of the power transmission line protection relay 30B in Fig. 1 are similar, Fig. 7 representatively shows the function of the distance relay element 40C realized in the power transmission line protection relay 30A. Note that the function of the distance relay element 40C is realized, for example, by the CPU 121 in Fig. 2 operating according to a program.

Specifically, the distance relay element 40C in FIG. 7 differs from the distance relay element 40A in FIG. 3 in that it includes a fault direction determination unit 70 based on the own terminal current and an operation timer 63. The fault direction determination unit 70 includes a second fault determination unit 60, a recovery timer 61, a phase determination unit 71, and a logical operator 72.

The operation of the second fault determination unit 60 and the recovery timer 61 in FIG. 7 is the same as the operation of the second fault determination unit 60 and the recovery timer 61 in FIG. 5. Therefore, they are given the same reference numerals and the description will not be repeated.

The phase determination in the phase determination unit 71 utilizes the fact that in the case of an internal fault, the waveforms of the first current I1 and the second current I2 are substantially the same, and in the case of an external fault, the waveforms of the first current I1 and the second current I2 are substantially reversed. In other words, if the difference between the instantaneous values of the first current I1 at two sampling times is Δ ^# I1(t) and the difference between the instantaneous values of the second current I2 at the same two sampling times is Δ ^# I2(t), then
In case of forward failure, Δ ^# I1(t) × Δ ^# I2(t) > 0 … (9)
In case of rear failure, Δ ^# I1(t) × Δ ^# I2(t) < 0 … (10)
The above formula (9) holds true when the phase difference of the second current I2 with respect to the first current I1 is in a range of greater than -90° and smaller than +90°. The phase determination unit 71 outputs "1" when it determines that there is a forward fault, and outputs "0" when it determines that there is no forward fault.

For example, the sampling period for relay calculation is 30° electrical angle, and the first current I1 that is n·30° electrical angle before the current time t is expressed as I1(t-n), and the first current I1 that is n·30° electrical angle after the current time t is expressed as I1(t+n), where n is a positive integer. The second current I2 is expressed in a similar manner. In the above equations (9) and (10), when #=1, there is a time difference of 30° electrical angle between the two sampling times. When #=2, there is a time difference of 60° electrical angle between the two sampling times. When #=3, there is a time difference of 90° electrical angle between the two sampling times.

First, the phase determination unit 71 determines at the current sampling time t:
Δ ¹ I1(t) = I1(t) - I1(t - 1) ... (11A)
Δ ¹ I2(t) = I2(t) - I2(t - 1) ... (11A)
and judge whether the above formula (9) is satisfied.

When the above formula (9) is satisfied at the current sampling time t, the phase determination unit 71 determines at the next sampling time t+1 as follows:
Δ ² I1(t+1) = I1(t+1) - I1(t-1) ... (12A)
Δ ² I2(t+1) = I2(t+1) - I2(t-1) ... (12A)
and judge whether the above formula (9) is satisfied.

If the above formula (9) is satisfied at the next sampling time t+1, the phase determination unit 71 determines at the next sampling time t+2 as follows:
Δ ³ I1(t+2) = I1(t+2) - I1(t-1) ... (13A)
Δ ³ I2(t+2) = I2(t+2) - I2(t-1) ... (13A)
In this manner, when the above formula (9) is satisfied at three consecutive sampling times, the phase determining unit 71 determines that a forward fault has occurred.

If magnetic saturation of the current transformer CT occurs shortly after the occurrence of an external fault, the phase determination at the three sampling times as described above takes too long, and the distance relay element 40C may malfunction. In this case, the phase determination unit 71 may determine that a forward fault has occurred if the above formula (9) is satisfied at two sampling times, the current sampling time t and the next sampling time t+1.

When there is a phase difference between the power supply voltage at the own end and the power supply voltage at the other end, if an external fault occurs at fault point F of the second busbar 22 in Fig. 1, a phase difference may occur between the first current I1 detected by the first current transformer CT1 and the second current I2 detected by the second current transformer CT2. In this case, it is necessary to narrow the phase range for forward determination. For example, in order to ensure that the phase difference between the first current I1 and the second current I2 is within a range greater than -60° and less than +60°, the phase determination unit 71
Δ ^# I1(t) × Δ ^# I2(t) > 0 … (14A)
Δ ^# I1(t) × Δ ^# I2(t-1) > 0 … (14B)
Δ ^# I1(t-1) × Δ ^# I2(t) > 0 … (14C)
Determine whether all of the conditions are satisfied.

As described above, formula (14A) is satisfied when the phase difference of the second current I2 relative to the first current I1 is greater than -90° and less than +90°. Formula (14B) is satisfied when the phase difference of I2 ∠-30° relative to I1 is in the range of -90° to +90°, i.e., in the range of -120° to +60°. Formula (14C) is satisfied when the phase difference of I2 relative to I1 ∠-30° is in the range of -90° to +90°, i.e., in the range of -60° to +120°. Therefore, when all of the above formulas (14A) to (14C) are satisfied, the phase difference of the second current I2 relative to the first current I1 is in the range of greater than -60° and less than +60°. As a result, even if the phase of the first current I1 and the phase of the second current I2 are shifted by 30°, the phase determination unit 71 can correctly determine that there is a forward fault. Note that the determination of whether each of the equations (14A) to (14C) is true is performed at the second or third sampling time, as in the case of equation (9) above.

The logical operator 72 calculates the logical product of the logical value of the output signal of the recovery timer 61 and the logical value of the output signal of the phase determination unit 71. Therefore, the fault direction determination unit 70 outputs "1" to the operation timer 63 if it determines that there is a forward fault, and outputs "0" if there is not.

The operation timer 63 delays the rising edge of the output signal of the direction determination unit 42 by the operation time T4. Here, the operation timer 63 sets the operation time T4 to 0 milliseconds when the failure direction determination unit 70 determines that there is a forward failure, and sets the operation time T4 to a value approximately equal to the transmission delay time of the communication path 27 when the failure direction determination unit 70 determines that there is a rearward failure.

Other aspects of Figure 7 are similar to those described in Figures 3 and 5, so the same or corresponding parts are given the same reference symbols and will not be described repeatedly.

[Example of distance relay element operation]
Figures 8 and 9 are conceptual timing diagrams for explaining an example of the operation of the distance relay element of Figure 7. The timing diagram of Figure 8 shows the case where a rear fault (external fault at fault point F in Figure 1) occurs, and the timing diagram of Figure 9 shows the case where a forward fault occurs.

The timing diagrams of Figures 8 and 9 show, from top to bottom, the signal waveforms of the output signal A0 of the distance determination unit 41 shown in Figure 7, the output signals A1 and A2 of the operation timer 63 based on the output of the direction determination unit 42, the output signal B of the recovery timer 61 based on the output of the second fault determination unit 60, the output signal C of the phase determination unit 71, the output signal D of the fault direction determination unit 70 (logic operator 72), the output signal E of the current differential determination unit 51, the output signal F of the one-shot timer 53 based on the output of the first fault determination unit 52, the output signal G of the external fault determination unit 50, and the output signal H (trip signal TRS) of the logic operator 44. Note that the output signal A2 of the operation timer 63 based on the output of the direction determination unit 42 is the waveform in this embodiment, and the output signal A1 is the waveform in the comparative example.

Referring to FIG. 8, at time t51, an external fault occurs at fault point F of the second busbar 22 in FIG. 1. It takes about one cycle from the occurrence of the fault for the distance determination unit 41 to calculate the impedance. Therefore, at time t55, the distance determination unit 41 determines that the calculated impedance is within the set value and that a fault has occurred in the power transmission system 15. As a result, the output signal A0 of the distance determination unit 41 switches from "0" to "1".

After an external fault occurs at time t51, the second fault determination unit 60 detects a sudden change in the first current I1 and the second current I2 at time t52. Thereafter, for 0.5 cycles until time t53, the output signal B of the second fault determination unit 60 is "1". Furthermore, the recovery timer 61 maintains the output signal B at "1" for the recovery time T5, and then switches the output signal B to "0" at time t61. The phase determination unit 71 determines that there is no forward fault at time t52, and maintains the output signal C at "0". Therefore, the output signal D of the fault direction determination unit 70 indicates "0". As a result, the operation time T4 of the operation timer 63 is set to a value approximately equal to the transmission delay time of the communication path 27.

After the external fault occurs at time t51, the direction determination unit 42 initially determines that it is a rear fault, and its output signal becomes "0". When magnetic saturation occurs in the second current transformer CT2, the direction determination unit 42 erroneously determines that it is a forward fault, and the output signal of the direction determination unit 42 switches from "0" to "1" at time t54 (see signal waveform A1). As a result, the output signal A2 of the operation timer 63 gradually increases, switching to "1" at time t60 after the operation time T4 has elapsed.

When an external fault occurs at time t51, the first fault determination unit 52 detects a sudden change in the combined currents IR and IL. This causes the one-shot timer 53 to output a one-shot pulse as the output signal F for the pulse interval T1. However, the rising edge of this one-shot pulse is delayed until time t57 due to the effect of the transmission delay time of the communication path 27.

The output signal E of the current differential determination unit 51 indicates "1" when an external fault first occurs. However, when the second current transformer CT2 becomes magnetically saturated, this influence causes the current differential determination unit 51 to erroneously determine that an internal fault has occurred. As a result, the output signal E of the current differential determination unit 51 switches from "1" to "0". However, this change in the output signal E is delayed until time t56 due to the influence of the transmission delay time of the communication path 27. Thereafter, when the polarity of the AC fault current changes and the magnetic saturation of the second current transformer CT2 is eliminated, the output signal E of the current differential determination unit 51 switches from "0" to "1" at time t58.

At time t58, both the output signal E of the current differential determination unit 51 and the output signal F of the one-shot timer 53 become high level. Then, when both signals are maintained at high level until time t59 when the operation time T2 of the operation timer 55 has elapsed, the external fault determination unit 50 determines that an external fault has occurred. As a result, the output signal G of the external fault determination unit 50 is maintained at high level only during the recovery time T3 of the recovery timer 56. Furthermore, at time t59, the external fault determination unit 50 latches the output signal of the direction determination unit 42 to low level (i.e., backward determination). As a result, the output signal A2 of the operation timer 63 is fixed at low level.

The final logical value of the output signal of the distance relay element 40C (output signal H of the logical operator 44) is the logical product of the logical value of the output signal A0 of the distance determination unit 41, the logical value of the output signal A2 of the operation timer 63 based on the output of the direction determination unit 42, and the negation of the logical value of the output signal G of the external fault determination unit 50. Therefore, in the event of an external fault, the output signal H of the distance relay element 40C is at a low level, and no trip signal is output.

Next, referring to FIG. 9, at time t51, an internal fault occurs in the power transmission line 20 in FIG. 1. Since it takes about one cycle from the occurrence of the fault for the distance determination unit 41 to calculate the impedance, at time t55, the output signal A0 of the distance determination unit 41 switches from "0" to "1."

After an internal fault occurs at time t51, the second fault determination unit 60 detects a sudden change in the first current I1 and the second current I2 at time t52. Thereafter, the output signal B of the second fault determination unit 60 is "1" for 0.5 cycles until time t53. Furthermore, the recovery timer 61 maintains the output signal B at "1" for the recovery time T5, and then switches the output signal B to "0" at time t61. The phase determination unit 71 determines that a forward fault has occurred at time t52, and switches the output signal C from "0" to "1". Therefore, the output signal D of the fault direction determination unit 70 switches from "0" to "1" at time t52. As a result, the operation time T4 of the operation timer 63 is set to 0 milliseconds.

After an internal fault occurs at time t51, the direction determination unit 42 determines that it is a forward fault. As a result, the output signal of the direction determination unit 42 switches from "0" to "1" at time t54. Note that since the operation time T4 of the operation timer 63 is set to 0 milliseconds, the output signal A2 of the operation timer 63 also switches from "0" to "1" at time t54. Note that in the case of an internal fault, magnetic saturation of the current transformer CT does not occur, so the output signal A2 of the operation timer 63 remains at a high level.

When an internal fault occurs at time t51, the current differential determination unit 51 detects the internal fault. As a result, the output signal E of the current differential determination unit 51 switches from "1" to "0". However, this change in the output signal E is delayed until time t56 due to the effect of the transmission delay time of the communication path 27.

When an internal fault occurs at time t51, the first fault judgment unit 52 detects a sudden change in the composite currents IR and IL. This causes the one-shot timer 53 to output a one-shot pulse as the output signal F during the pulse interval T1. However, the rising edge of this one-shot pulse is delayed until time t57 due to the influence of the transmission delay time of the communication path 27. While the output signal F of the one-shot timer 53 indicates "1", the output signal E of the current differential judgment unit 51 indicates "0", so the output signal G of the external fault judgment unit 50 remains at a low level.

The final logical value of the output signal of the distance relay element 40C (output signal H of the logical operator 44) is the logical product of the logical value of the output signal A0 of the distance determination unit 41, the logical value of the output signal A2 of the operation timer 63 based on the output of the direction determination unit 42, and the negation of the logical value of the output signal G of the external fault determination unit 50. Therefore, in the event of an external fault, the output signal H of the distance relay element 40C changes from low level to high level at time t55, and a trip signal is output.

[Effects of the Third Embodiment]
According to the third embodiment, the fault direction determination unit 70 of the distance relay element 40C determines that a forward fault has occurred when the changes in the first current I1 and the second current I2 detected by the current transformers CT1 and CT2 on the local side are of the same polarity. In the case of a forward fault, the operation time of the operation timer 63 for delaying the output signal of the direction determination unit 42 is set to 0. This makes it possible to prevent a delay in the determination time for the forward fault determination. In addition, when the fault direction determination unit 70 determines that a forward fault has not occurred, it sets the operation time of the operation timer 63 to a value based on the transmission delay time of the communication path 27. This makes it possible to prevent a malfunction of the distance relay element 40C due to magnetic saturation of the current transformer CT.

The phase determination by the fault direction determination unit 70 uses the changes in the first current I1 and the second current I2. This makes it possible to remove the DC component contained in the fault current, enabling more accurate determination.

In addition, when the phase of the power supply voltage on the first end 23 side of the transmission line 20 differs from the phase of the power supply voltage on the second end 26 side of the transmission line 20, the range of the phase of the AC component of the second current I2 relative to the phase of the AC component of the first current I1 is set narrower. For example, when the phases of the power supply voltages on both ends of the transmission line 20 differ by a maximum of α degrees, the fault direction determination unit 70 determines that there is a forward fault when the phase difference of the AC component of the second current I2 relative to the AC component of the first current I1 is greater than -(90-α) degrees and less than +(90-α) degrees. This allows for correct determination even when there is a phase difference in the power supply voltages on both ends of the transmission line 20.

Embodiment 4.
When there is a transmission failure in the communication path 27 or a synchronization failure in the synchronization processing unit 43, the external fault determination unit 50 does not operate normally. Therefore, even if the operation time T4 of the operation timer 63 is set to the transmission delay time of the communication path 27 when an external fault occurs, the distance relay element may malfunction. The fourth embodiment presents a solution to this problem.

[Functional configuration of distance relay elements]
Fig. 10 is a functional block diagram showing the function of a distance relay element realized in the power transmission line protection relay of embodiment 4. Since the operation of the power transmission line protection relay 30A and the operation of the power transmission line protection relay 30B in Fig. 1 are similar, Fig. 10 representatively shows the function of the distance relay element 40D realized in the power transmission line protection relay 30A. Note that the function of the distance relay element 40D is realized, for example, by the CPU 121 in Fig. 2 operating in accordance with a program.

Specifically, the distance relay element 40D in FIG. 10 has a different configuration of the fault direction determination unit 70 from the fault direction determination unit 70 in FIG. 7. As shown in FIG. 10, the fault direction determination unit 70 includes a second fault determination unit 60, a recovery timer 61, a first phase determination unit 71A, a second phase determination unit 71B, and logical operators 72, 73, 75, and 76. The first phase determination unit 71A and the second phase determination unit 71B constitute the phase determination unit 71.

The operation of the second fault determination unit 60 and the recovery timer 61 in FIG. 10 is the same as the operation of the second fault determination unit 60 and the recovery timer 61 in FIG. 5 and FIG. 7. Therefore, they are given the same reference numerals and the description will not be repeated.

The first phase determination unit 71A determines whether or not a forward fault has occurred, taking into consideration the phase shift of the power supply voltage at both ends of the power transmission line 20. For example, the first phase determination unit 71A determines whether or not the phase difference of the second current I2 relative to the first current I1 is in a range greater than -60° and less than +60°, based on the above-mentioned equations (14A) to (14C).

The second phase determination unit 71B determines whether or not a rear fault has occurred, taking into consideration the phase shift of the power supply voltage at both ends of the power transmission line 20. For example, the second phase determination unit 71B
Δ ^# I1(t) × Δ ^# I2(t) < 0 … (15A)
Δ ^# I1(t) × Δ ^# I2(t-1) < 0 … (15B)
Δ ^# I1(t-1) × Δ ^# I2(t) < 0 …(15C)
It is determined whether or not all of the above formulas (15A) to (15C) are satisfied. When all of the above formulas (15A) to (15C) are satisfied, the phase difference of the second current I2 with respect to the first current I1 is in a range greater than +120° and smaller than +240°. The determination of whether each of the formulas (15A) to (15C) is satisfied is performed at two or three sampling times, as described in the formulas (11A), (11B) to (13A), and (13C).

The logic operator 72 calculates the logical product of the logical value of the output signal of the recovery timer 61 and the logical value of the output signal of the first phase determination unit 71A. The calculation result of the logic operator 72 is output to the operation timer 63.

The logical operator 73 calculates the logical product of the logical value of the output signal of the recovery timer 61 and the logical value of the output signal of the second phase determination unit 71B. The calculation result of the logical operator 72 is output to the operation timer 63 and the direction determination unit 42.

The logical operator 75 calculates the logical AND of the negation of the logical value of the output signal of the logical operator 72 and the logical value of the output signal of the logical operator 73. Therefore, while the second fault determination unit 60 determines that a fault has occurred and the output of the recovery timer 61 is "1", if the second phase determination unit 71B determines that the direction is backward, the output of the logical operator 75 becomes "1". In other cases, the output of the logical operator 75 becomes "0". The output signal of the logical operator 75 is input to the direction determination unit 42.

The logical operator 76 calculates the logical AND of the negation of the logical value of the output signal of the logical operator 72, the negation of the logical value of the output signal of the logical operator 73, and the logical value of the output signal of the recovery timer 61. Therefore, while the second fault determination unit 60 determines that a fault has occurred and the output of the recovery timer 61 is "1", if the first phase determination unit 71A does not determine that the direction is forward and the second phase determination unit 71B does not determine that the direction is backward (hereinafter, this state will be referred to as "indeterminate"), the output of the logical operator 76 will be "1". In other cases, the output of the logical operator 76 will be "0". The output signal of the logical operator 76 is input to the direction determination unit 42.

The distance relay element 40D further includes a logical operator 74 that performs a logical sum operation. In the power line protection relay 30A of the fourth embodiment, the synchronization processing unit 43 outputs a signal indicating whether or not there is a synchronization failure to the logical operator 74. The receiver 132 also outputs a signal indicating whether or not there is a transmission failure to the logical operator 74. The logical operator 74 outputs "1" when there is a synchronization failure or a transmission failure, and outputs "0" when there is neither a synchronization failure nor a transmission failure. The result of the logical sum operation of the logical operator 74 is input to the first phase determination unit 71A, the second phase determination unit 71B, and the operation timer 63. Therefore, the first phase determination unit 71A, the second phase determination unit 71B, and the operation timer 63 can detect whether or not there is a transmission failure or a synchronization failure.

The operation of the direction determination unit 42 and the operation timer 63 based on the output of the fault direction determination unit 70 and the logical operator 74 will be described later with reference to Figures 12 and 13. Other points in Figure 10 are similar to those described in Figures 3, 5, and 7, so the same or corresponding parts are given the same reference symbols and will not be described repeatedly.

[Regarding the Phase Relationship Between the First Current I1 and the Second Current I2]
Fig. 11 is a diagram for explaining the phase relationship between the first current and the second current. Fig. 11 shows the phase relationship of the second current I2 with respect to the phase of the first current I1. The phase determination unit 71 determines whether the phase of the second current I2 with respect to the phase of the first current I1 is on the in-phase side (forward region 91), on the reversed phase side (rear region 92), or in an intermediate region (indeterminate region 93) between the in-phase side and the reversed phase side.

Specifically, referring to FIG. 11, when the above-mentioned formulas (14A) to (14C) are satisfied, it can be determined that the phase difference of the second current I2 relative to the first current I1 is in the forward region 91. Therefore, when the second fault determination unit 60 determines that a fault has occurred, it can be concluded that a forward fault has occurred.

On the other hand, if the above-mentioned formulas (15A) to (15C) are satisfied, it can be determined that the phase difference of the second current I2 relative to the first current I1 is in the rear region 92. Therefore, if the second fault determination unit 60 determines that a fault has occurred, it can be concluded that a rear fault has occurred.

The case that is neither the forward region 91 nor the backward region 92 is called the indeterminate region 93. When there is a phase difference in the power supply voltage at both ends of the transmission line 20, and the phase difference between the first current I1 and the second current I2 is in the indeterminate region 93, it is not possible to reliably determine whether there is a forward fault or a backward fault.

Distance Relay Element Operation
Fig. 12 is a flowchart showing the operation of the distance relay element in Fig. 10. Hereinafter, the operation of the distance relay element 40D, in particular the setting of the operation time T4 of the operation timer 63 and the control of the direction determination unit 42 will be described with reference to Figs. 10 and 12.

In step S100 of FIG. 12, if the fault direction determination unit 70 determines that the fault is in the forward direction (YES in step S100), the fault direction determination unit 70 sets the operation time T4 of the operation timer 63 to 0 milliseconds (step S150). This allows the distance relay element 40D to immediately output a trip signal based on the output signal of the distance determination unit 41 and the output signal of the direction determination unit 42 in the case of a forward fault that is not affected by magnetic saturation of the current transformer CT. If NO in step S100, the process proceeds to step S110.

In the next step S110, the case where the fault direction determination unit 70 determines that the fault is a rear fault (YES in step S110) will be described. In this case, if the latch condition of the direction determination unit 42 is satisfied (YES in step S160), the fault direction determination unit 70 latches the determination result of the direction determination unit 42 as a rear determination (step S170). For example, as the latch condition, a condition is adopted in which the magnitude of the composite voltage of the first voltage V1 at the current time and the coefficient multiplied first voltage V1 n cycles before the current time (n is set to an integer value of about 2 or 3) is equal to or less than a threshold value. On the other hand, if the latch condition is not satisfied (YES in step S160), the fault direction determination unit 70 sets the output of the direction determination unit 42 to a rear determination (does not lock) (step S180). If NO in step S110, that is, if the phase difference of the second current I2 relative to the first current I1 is in the indefinite region 93, the process proceeds to step S120.

In the next step S120, if there is neither a transmission failure nor a synchronization failure (NO in step S120), the failure direction determination unit 70 sets the operation time T4 of the operation timer 63 to a value approximately equal to the transmission delay time of the communication path 27 (step S190). On the other hand, if there is a transmission failure or a synchronization failure (YES in step S120), the failure direction determination unit 70 advances the process to step S130.

In the next step S130, if the phase difference between the second current I2 and the first current I1 is in the indefinite region 93 and the latch condition is met (YES in step S130), the fault direction determination unit 70 latches the determination result of the direction determination unit 42 as a backward determination (step S170).

On the other hand, if the phase difference between the first current I1 and the second current I2 is in the indefinite region 93 and the latch condition is not satisfied (NO in step S130), the fault direction determination unit 70 sets the operating time T4 of the operation timer 63 to the saturation time of the current transformer CT (step S140).

Figure 13 is a conceptual timing diagram for explaining an example of the operation of the distance relay element of Figure 10. The timing diagram of Figure 13 shows a case where a backward fault (external fault at fault point F in Figure 1) occurs and a transmission failure or synchronization failure occurs.

The timing diagram of FIG. 13 shows, from the top to bottom, the signal waveforms of the output signal A0 of the distance determination unit 41 shown in FIG. 10, the output signals A1 and A2 of the operation timer 63 based on the output of the direction determination unit 42, the output signal B of the recovery timer 61 based on the output of the second fault determination unit 60, the output signal C1 of the first phase determination unit 71A, the output signal C2 of the second phase determination unit 71B, the output signal D of the logical operator 74, the output signal E1 of the logical operator 72, the output signal E2 of the logical operator 73, the output signal F of the current differential determination unit 51, the output signal G of the one-shot timer 53 based on the output of the first fault determination unit 52, the output signal H of the external fault determination unit 50, and the output signal I (trip signal TRS) of the logical operator 44. Note that the output signal A2 of the operation timer 63 based on the output of the direction determination unit 42 is the waveform in this embodiment, and the output signal A1 is the waveform in the comparative example.

Referring to FIG. 13, a transmission failure occurs at time t71. During the transmission failure, the output signal F of the current differential determination unit 51 and the output signal G of the first failure determination unit 52 both indicate "0". Therefore, the output signal H of the external failure determination unit 50 also indicates "0".

At the next time t72, an external fault occurs at fault point F of the second busbar 22 in FIG. 1. As a result, the distance determination unit 41 determines that a fault has occurred in the power transmission system 15 because the calculated impedance is within the set value. As a result, at time t77, the output signal A0 of the distance determination unit 41 switches from "0" to "1".

When an external fault occurs at time t72, the second fault judgment unit 60 detects a sudden change in the first current I1 and the second current I2 at time t73. After that, the output signal B of the second fault judgment unit 60 is "1" for 0.5 cycles until time t75. Furthermore, the recovery timer 61 maintains the output signal B at "1" for the recovery time T5, and then switches the output signal B to "0" at time t61.

At time t73, the first phase determination unit 71A determines that the fault is not forward, and the second phase determination unit 71B determines that the fault is backward. As a result, the output signal C1 of the first phase determination unit 71A and the output signal E1 of the logical operator 72 remain at "0", but the output signal C2 of the second phase determination unit 71B and the output signal E2 of the logical operator 73 switch from "0" to "1". However, since the latch condition of the direction determination unit 42 is not satisfied at this point, the fault direction determination unit 70 sets the output signal A2 of the direction determination unit 42 to "0" (backward determination) but does not lock it (step S180 in FIG. 12).

At the next time t74, magnetic saturation occurs in the second current transformer CT2, and the phase difference between the first current I1 and the second current I2 advances to the indefinite region 93. As a result, the output signal C2 of the second phase determination unit 71B and the output signal E2 of the logical operator 73 switch from "1" to "0". Furthermore, the fault direction determination unit 70 sets the operation time T4 of the operation timer 63 to the decay time of the magnetic saturation of the current transformer CT (i.e., the CT saturation time) (step S140 in FIG. 12).

When an external fault occurs at time t72, the direction determination unit 42 initially determines that it is a rear fault, and its output signal becomes "0". When magnetic saturation occurs in the second current transformer CT2, the direction determination unit 42 erroneously determines that it is a forward fault, and the output signal of the direction determination unit 42 switches from "0" to "1" at time t76 (see signal waveform A1). As a result, the output signal A2 of the operation timer 63 gradually increases, switching to "1" at time t79 after the operation time T4 has elapsed. Here, the operation time T4 is set to the CT saturation time.

At time t78, the polarity of the AC fault current switches, and the magnetic saturation of the second current transformer CT2 is temporarily eliminated. As a result, the output signal A2 of the operation timer 63, which is based on the output of the direction determination unit 42, returns to "0". In this way, by setting the operation time T4 of the operation timer 63 to the CT saturation time, it is possible to prevent malfunction of the distance relay element 40D due to CT saturation.

At time t80, magnetic saturation of the second current transformer CT2 occurs, causing the output signal of the direction determination unit 42 to switch from "0" to "1" (see signal waveform A1). As a result, the output signal A2 of the operation timer 63 gradually increases so as to switch to "1" after the operation time T4 has elapsed.

At time t81, the latch condition is satisfied in the direction determination unit 42. At this time t81, the determination result of the phase determination unit 71 is in an indefinite state (i.e., both output signals E1 and E2 are "0" and the output signal B of the recovery timer 61 is "1"), so the output signal of the direction determination unit 42 is locked at "0" (rearward determination) (step S170 in FIG. 12).

The final logical value of the output signal of the distance relay element 40D (output signal I of the logical operator 44) is the logical product of the logical value of the output signal A0 of the distance determination unit 41, the logical value of the output signal A2 of the operation timer 63 based on the output of the direction determination unit 42, and the negation of the logical value of the output signal H of the external fault determination unit 50. Because a transmission failure has occurred, the output signal H of the external fault determination unit 50 is fixed to "0". As described above, the fault direction determination unit 70 also controls the direction determination unit 42 so that it does not malfunction due to magnetic saturation of the second current transformer CT2. Therefore, the output signal H of the distance relay element 40D is maintained at a low level, and no trip signal is output.

[Effects of the Fourth Embodiment]
According to the above-mentioned fourth embodiment, when it is impossible to determine whether the current is forward or backward based on the current at the own end alone due to the phase difference between the power supply voltages at both ends of the power transmission line 20, it is determined to be indefinite. When there is no transmission failure in the communication path 27 and no synchronization failure in the synchronization processing unit 43, and the phase relationship between the first current I1 and the second current I2 is indefinite, the operation time T4 of the operation timer 63 is set to the transmission delay time. This makes it possible to prevent unnecessary operation of the distance relay element 40D.

On the other hand, if there is a transmission failure in the communication path 27 or a synchronization failure in the synchronization processing unit 43, and the phase relationship between the first current I1 and the second current I2 is uncertain, the external fault determination unit 50 will not operate normally, and there is a possibility of malfunction even if the operation time T4 of the operation timer 63 is set to the transmission delay time of the communication path 27. Therefore, in this case, if the latch condition is not satisfied, the operation time T4 of the operation timer 63 is set to the CT saturation time. As a result, while the current transformer CT is magnetically saturated, the output of the direction determination unit 42 will not switch from "0" to "1", thereby preventing malfunction of the distance relay element 40D.

The embodiments disclosed herein should be considered in all respects as illustrative and not restrictive. The scope of the present invention is indicated by the claims, not the above description, and is intended to include all modifications within the meaning and scope of the claims.

15 Power transmission system, 16, 17 Busbar connection line, 20 Power transmission line, 21 First busbar, 22 Second busbar, 23 First end, 24 Third busbar, 25 Fourth busbar, 26 Second end, 27 Communication path, 30A, 30B Power transmission line protection relay (distance relay), 40A to 40D Distance relay element, 41 Distance determination unit, 42 Direction determination unit, 43 Synchronization processing unit, 44, 45, 54, 62, 72, 73, 74 Logic operation unit, 50 External fault determination unit, 51 Current differential determination unit, 52 First fault determination unit, 53 One-shot timer, 55, 63 Operation timer, 56, 61 Recovery timer, 60 Second fault determination unit, 70 Fault direction determination unit, 71 Phase determination unit, 91 Forward region, 92 Backward region, 93 Undefined region, 132 Receiver, 133 Transmitter, 134 circuit, CB1-CB6 circuit breaker, CT1-CT4 current transformer, F fault point, I1 first current, I2 second current, I3 third current, I4 fourth current, IL, IR combined current, PT1-PT2 voltage transformer, T1 pulse interval, T2, T4 operating time, T3, T5 release time, TRS trip signal, V1 first voltage, V2 second voltage.

Claims (11)

A distance relay for protecting a power transmission system, comprising:
The power transmission system includes:
Power lines and
a first busbar and a second busbar connected to a first end of the transmission line in a 1.5 circuit breaker manner, and a third busbar and a fourth busbar connected to a second end of the transmission line;
a first current transformer that detects a first current flowing between the first end and the first bus;
a second current transformer configured to detect a second current flowing between the first end and the second bus;
a first voltage transformer for detecting a first voltage at the first end;
a third current transformer that detects a third current flowing between the second end and the third bus;
a fourth current transformer that detects a fourth current flowing between the second end and the fourth bus;
The distance relay includes:
a distance determination unit that calculates an impedance based on a value of a first combined current obtained by adding together a detected value of the first current and a detected value of the second current and that calculates an impedance based on a detected value of the first voltage and determines whether or not a magnitude of the calculated impedance is within a set value;
a direction determination unit that determines whether a fault point is in the forward direction or the rearward direction based on a value of the first combined current and a detected value of the first voltage;
an operation timer that delays an output signal of the direction determination unit by an operation time;
a receiver that receives the detection values of the third current and the fourth current transmitted from the second end side of the power transmission line via a communication path;
an external fault determination unit that determines whether or not an external fault has occurred based on an addition result of each of the detection values of the first current, the second current, the third current, and the fourth current detected at a same sampling time,
The distance relay outputs a trip signal to a corresponding circuit breaker when the distance determination unit determines that the impedance is within a set value, the direction determination unit determines that the fault point is forward, and the external fault determination unit determines that there is no external fault. The external failure determination unit
a current differential determination unit that determines whether or not an absolute value of an addition result of each of the detected values of the first current, the second current, the third current, and the fourth current that are detected at the same sampling time is within a threshold value;
2. The distance relay according to claim 1, further comprising: a first fault determination unit that detects the occurrence of a fault in the power transmission system based on a change in the first composite current and a change in a second composite current obtained by adding together a detection value of the third current and a detection value of the fourth current. A fault direction determination unit that determines whether or not a fault occurs in a forward direction based on the first current and the second current,
The distance relay according to claim 1 , wherein the fault direction determination unit sets the operation time to 0 when it determines that the fault is a forward fault. The distance relay according to claim 3 , wherein the failure direction determination unit sets the operation time based on a transmission delay time of the communication path when it is determined that the failure is not a forward failure. The failure direction determination unit is
a second fault determination unit that detects a fault occurrence in the power transmission system based on a change in the first current and a change in the second current;
a phase determination unit that determines whether or not a phase of the second current with respect to a phase of the first current is on the same phase side;
5. The distance relay according to claim 3 , wherein the fault direction determination unit determines that a fault has occurred in the power transmission system when the second fault determination unit detects that a fault has occurred in the power transmission system and the phase determination unit determines that the fault is on the in -phase side. The failure direction determination unit is
a second fault determination unit that detects a fault occurrence in the power transmission system based on a change in the first current and a change in the second current;
a phase determination unit that determines whether a phase of the second current relative to a phase of the first current is on the in-phase side, on the reversed phase side, or in an intermediate region between the in-phase side and the reversed phase side,
The fault direction determination unit determines that a fault has occurred in the power transmission system when the second fault determination unit detects that a fault has occurred in the power transmission system and the phase determination unit determines that the fault has occurred on the in-phase side,
The distance relay according to claim 3 , wherein the fault direction determination unit determines that a fault has occurred in a rear direction when the second fault determination unit detects that a fault has occurred in the power transmission system and the phase determination unit determines that the fault is on the reverse phase side. The distance relay according to claim 6, wherein the fault direction determination unit sets the operating time based on a transmission delay time of the communication path when neither the forward fault nor the backward fault occurs, a transmission failure of the communication path is not detected, and a synchronization failure of sample timing is not detected between the first end side and the second end side of the power transmission line. The distance relay according to claim 6 or 7, wherein the fault direction determination unit sets the operating time based on a decay time of magnetic saturation of a current transformer or locks the determination result of the direction determination unit to a backward determination when a transmission failure of the communication path or a synchronization failure of sample timing between the first end side and the second end side of the transmission line is detected instead of the forward fault or the backward fault. The distance relay according to claim 6 , wherein the failure direction determination unit, when determining that the failure is a rearward failure, sets the determination result of the direction determination unit to rearward or locks it to the rearward determination. The phase determination unit
10. The distance relay according to claim 5 , wherein whether or not the relay is on the in-phase side is determined based on a sign of a product of a difference between detection values of the first current at two sampling times and a difference between detection values of the second current at the two sampling times. The distance relay according to claim 1 , wherein the external fault determination unit, when determining that an external fault has occurred, locks the determination result of the direction determination unit to a backward determination.

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