JP2019030073A - Zero-phase current differential relay - Google Patents

Abstract

To provide a zero-phase current differential relay capable of achieving both high-sensitivity fault detection and malfunction prevention at the same time.SOLUTION: A zero-phase current differential relay 40 protects a three-phase transformer 30 including a Y-connection winding 31. Each of phase currents Ia, Ib, Ic and a neutral point current IN of the Y-connection winding 31 are defined so that directions toward a neutral point 34 are the same polarity. The zero-phase current differential relay 40 includes: a relay calculation unit 50; a phase determination unit 60; and an operation determination unit 70. The relay operation unit 40 calculates a differential amount and a suppression amount on the basis of a zero-phase current and a neutral point current based on each phase current and determines whether the differential amount and the suppression amount are in an operation region. The phase determination unit 60 determines whether a phase of the neutral current with respect to the zero-phase current is in a first region including the same phase. On the basis of the determination result of the relay calculation unit 50 and the determination result of the phase determination unit 60, the operation determination unit 70 outputs a protection signal for protecting the three-phase transformer 30.SELECTED DRAWING: Figure 1

Description

  The present disclosure relates to a zero-phase current differential relay for protecting a transformer or a transmission line provided in a power system.

  The current differential relay is a relay that protects a section surrounded by a current transformer (CT) in a power device such as a transformer and a transmission line based on an input of a current signal from the CT. Among the current differential relays, one using a zero phase current in order to detect a ground fault in the protection section with high sensitivity is referred to as a zero phase current differential relay.

  Specifically, in the zero-phase current differential relay for protecting the transformer, the zero-phase current 3 × I0 (= Ia + Ib + Ic) calculated from the phase currents Ia, Ib, and Ic of the a-phase, b-phase, and c-phase, The differential amount ID is calculated from the difference current from the sex point current IN. That is, it is calculated as ID = | 3 × I0 + IN |. Here, | X | represents the amplitude value of X.

  On the other hand, with regard to the suppression amount IR, in the case of transformer protection, the maximum value suppression method, that is, the maximum value among | Ia |, | Ib |, | Ic |, | IN | An IR method is generally used (see, for example, Patent Document 1). Accordingly, it is possible to obtain a larger suppression amount IR than the scalar sum method, that is, a method using (| 3 × I0 | + | IN |) as the suppression amount.

  The zero-phase current differential relay is configured to operate when, for example, ID> K1 and ID> p × IR. Here, K1 is a set value of the minimum operating sensitivity, and p is a ratio set so as not to operate due to an error of the current transformer.

Japanese Patent Laid-Open No. 08-084433

  In the above maximum value suppression method, if a large load current flows during normal operation, even if a differential amount occurs due to a current transformer error, the suppression amount also increases in proportion to the load current, resulting in malfunction. There is nothing.

  However, the maximum value suppression method may be problematic in terms of detection sensitivity when a relatively small ground fault current flows through the transformer along with the load current as in the case of a resistive internal ground fault. In this case, the differential amount is the amplitude of the ground fault current (also referred to as a scalar value), but the suppression amount is the amplitude of the current synthesized from the load current and the ground fault current. Larger than the amount. Therefore, in order to detect an internal ground fault with high sensitivity, it is necessary to set the ratio (that is, the p value) as small as possible.

  On the other hand, the maximum value suppression method may malfunction in the case of an external short circuit failure. For example, if only one-phase CT is largely saturated due to an external short-circuit fault, the portion where the short-circuit fault current is reduced due to this CT saturation is the differential amount. When the degree of CT saturation is large, the differential amount approaches the amplitude of the short-circuit fault current. On the other hand, the suppression amount is the amplitude of the short-circuit fault current of the phase where CT saturation does not occur. Therefore, in order to prevent the zero-phase current differential relay from malfunctioning, it is necessary to set the above-described ratio (that is, p value) close to “1”.

  As described above, the maximum value suppression method has a problem that it is difficult to set a ratio that achieves both high sensitivity detection and malfunction prevention. This disclosure takes the above-mentioned problems into consideration, and an object thereof is to provide a zero-phase current differential relay capable of achieving both high-sensitivity failure detection and malfunction prevention.

  A zero-phase current differential relay according to one embodiment protects a three-phase transformer that includes a Y-connected winding. Each phase current and neutral point current of the Y-connection winding are defined so that the directions toward the neutral point have the same polarity. The zero-phase current differential relay includes a relay calculation unit, a phase determination unit, and an operation determination unit. The relay calculation unit calculates the differential amount and the suppression amount based on the zero-phase current and the neutral point current based on each phase current, and determines whether the differential amount and the suppression amount are in the operating range. The phase determination unit determines whether or not the phase of the neutral point current with respect to the zero-phase current is in a first region including the same phase. The operation determination unit outputs a protection signal for protecting the three-phase transformer based on the determination result of the relay calculation unit and the determination result of the phase determination unit.

  According to the above embodiment, since it is possible to determine whether or not an internal ground fault has occurred based on the determination result of the relay calculation unit and the determination result of the phase determination unit, both high-sensitivity failure detection and malfunction prevention are compatible. Thus, a zero-phase current differential relay capable of achieving the above can be provided.

It is a whole lineblock diagram including a zero phase current differential relay and a three phase transformer which is the protection object. It is a block diagram which shows an example of the hardware constitutions of the zero phase current differential relay 40 of FIG. It is an operating characteristic figure of a zero phase current differential relay. It is a figure explaining the settling method of the minimum sensitivity value K1 of FIG. It is a figure for demonstrating the flow of an electric current when an external short circuit fault arises between the b phase and c phase of the primary side winding 31 of FIG. FIG. 3 is a diagram for explaining a current flow when an internal short circuit failure occurs between the b phase and the c phase of the primary winding 31 of FIG. 1. It is a figure for demonstrating the flow of an electric current when an external ground fault occurs in the a phase of the primary side coil | winding 31 of FIG. It is a figure for demonstrating the flow of an electric current when an internal ground fault arises in the a phase of the primary side coil | winding 31 of FIG. It is a figure for demonstrating the change of the current waveform observed by a relay when a current transformer is saturated. It is a vector diagram which shows the relationship between the zero phase current observed by a relay, and a neutral point current. FIG. 6 is a diagram for describing a criterion for determining whether an internal failure or an external failure in the first embodiment. It is a vector diagram for demonstrating the production | generation method of the value of 90 degrees before the present time. It is a figure for demonstrating the method to determine whether the neutral point current IN exists in A1 area | region. FIG. 3 is a block sequence diagram of the zero-phase current differential relay according to the first embodiment. In the case of the zero phase current differential relay of Embodiment 2, it is a vector diagram which shows the relationship between a zero phase current and a neutral point current. FIG. 10 is a diagram for describing a criterion for determining whether an internal failure or an external failure in the case of the second embodiment. FIG. 5 is a block sequence diagram of a zero-phase current differential relay according to a second embodiment. FIG. 11 is an operation characteristic diagram of a relay calculation unit in the zero-phase current differential relay of the third embodiment. FIG. 6 is a block sequence diagram of a zero-phase current differential relay according to a third embodiment.

  Hereinafter, each embodiment will be described in detail with reference to the drawings. The same or corresponding parts are denoted by the same reference numerals, and description thereof will not be repeated.

Embodiment 1 FIG.
[Configuration of zero-phase current differential relay and three-phase transformer]
FIG. 1 is an overall configuration diagram including a zero-phase current differential relay and a three-phase transformer that is a protection target thereof.

  Referring to FIG. 1, a zero-phase current differential relay 40 is a Y-Δ connection type three-phase transformer in which a primary winding 31 is a Y winding and a secondary winding 32 is a Δ winding. 30 is protected. Current transformers CTa, CTb, and CTc are provided for each phase on the primary side of the three-phase transformer 30. Further, a current transformer CTN is provided on the ground line 36 that connects between the neutral point 34 of the primary side winding of the three-phase transformer 30 and the ground pole 35. Current transformers CTa, CTb, CTc detect a-phase current Ia, b-phase current Ib, c-phase current Ic, respectively, and current transformer CTN detects neutral point current IN. A signal representing these phase currents Ia, Ib, Ic and a signal representing the neutral point current IN are taken into the zero-phase current differential relay 40.

  For the a-phase current Ia, b-phase current Ib, c-phase current Ic, and neutral point current IN, the currents in the direction toward the neutral point 34 of the three-phase transformer 30 have the same polarity as shown in FIG. (For example, positively).

  The zero-phase current differential relay 40 is configured by, for example, a digital protection relay configured based on a microcomputer. When the program is executed by the microcomputer, the zero-phase current differential relay 40 functions as a relay calculation unit 50, a phase determination unit 60, and an operation determination unit 70.

  The relay calculation unit 50 extends the function of a normal zero-phase current differential relay, and basically performs determination based on the differential amount ID and the suppression amount IR. Details of the operation of the relay calculation unit 50 will be described later with reference to FIGS. 3 and 4.

  The phase determination unit 60 is based on the phase relationship between the zero-phase current 3 × I0 (= Ia + Ib + Ic) calculated from the phase currents Ia, Ib, and Ic of the a-phase, b-phase, and c-phase, and the neutral point current IN. Determine whether it is an internal ground fault or an external ground fault. The internal fault means a fault that occurs in an internal protection section surrounded by the current transformers CTa, CTb, CTc, and CTN, and the external fault means a fault that occurs in a section outside these current transformers. Details of the operation of the phase determination unit 60 will be described later with reference to FIGS.

  The operation determination unit 70 determines whether an internal ground fault has occurred based on the determination result of the relay calculation unit 50 and the determination result of the phase determination unit 60. When it is determined that an internal ground fault has occurred, the operation determination unit 70 outputs a protection signal (also referred to as a relay output) for protecting the three-phase transformer 30. Details of the operation of the operation determination unit 70 will be described later with reference to FIG.

[Hardware configuration of zero-phase current differential relay]
FIG. 2 is a block diagram showing an example of a hardware configuration of the zero-phase current differential relay 40 of FIG. 2 has the same configuration as a so-called digital relay device. Referring specifically to FIG. 2, the zero-phase current differential relay 40 includes an input conversion unit 100, an A / D conversion unit 110, an arithmetic processing unit 120, an I / O (Input and Output) unit 130, and Is provided.

  The input conversion unit 100 includes auxiliary transformers 101_1, 101_2,... For each input channel. Input converter 100 represents signals representing phase currents Ia, Ib, and Ic output from current transformers CTa, CTb, and CTc of FIG. 1 and neutral point current IN output from current transformer CTN of FIG. An input unit to which a signal is input. Each auxiliary transformer 101 converts the current signals from the current transformers CTa, CTb, CTc and CTN into signals of a voltage level suitable for signal processing in the A / D converter 110 and the arithmetic processor 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 / D converter 114. The analog filter 111 and the sample hold circuit 112 are provided for each channel of the input signal.

  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 hold circuits 112_1, 112_2,. The A / D converter 114 converts the signal selected by the multiplexer 113 into a digital value.

  The arithmetic 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 225 for connecting them. The CPU 121 controls the overall operation of the zero-phase current differential relay 40. The RAM 122 and ROM 123 are used as the main memory of the CPU 121. The ROM 123 can store programs, setting values for signal processing, and the like by using a non-volatile memory such as a flash memory.

  The I / O unit 130 includes a digital input (D / I) circuit 132 and a digital output (D / O) circuit 133. The digital input circuit 132 and the digital output circuit 133 are interface circuits for performing communication between the CPU 121 and an external device.

[Operation of relay operation unit]
The relay calculation unit 50 firstly calculates the zero-phase current I0 of the symmetric coordinate method based on the information on the phase currents Ia, Ib, and Ic received from the current transformers CTa, CTb, and CTc, that is,
I0 = (Ia + Ib + Ic) / 3 (1)
Calculate In this specification, for convenience, three times the zero-phase current I0 (that is, 3 × I0) may be referred to as a zero-phase current.

Next, the relay calculation unit 50 calculates the differential amount ID and the suppression amount IR based on the calculated zero-phase current (3 × I0) value and information on the neutral point current IN received from the current transformer CTN. Calculate The differential amount ID is the amplitude of the vector sum of the zero-phase current (3 × I0) and the neutral point current IN, that is,
ID = | 3 × I0 + IN | (2)
Defined by Here, | ... | represents an amplitude value (also referred to as a scalar value). Further, the suppression amount IR is the sum of the amplitude of the zero-phase current (3 × I0) and the amplitude of the neutral point current IN (that is, the scalar sum).
IR = | 3 × I0 | + | IN | (3)
Defined by The reason why the scalar sum method is adopted as the suppression amount IR as described above is to remove the influence of the load current.

  Next, the relay calculation unit 50 performs an operation determination according to the following equations (4) and (5) based on the differential amount ID and the suppression amount IR.

ID> K1 (4)
ID> p × IR (5)
Here, K1 is a set value indicating the minimum operating sensitivity (hereinafter referred to as the minimum sensitivity value K1), and p is a ratio set so as not to operate due to a CT error or the like. When the suppression amount IR is equal to or less than K1 / p, the operation is determined according to the equation (4), and when the suppression amount IR is greater than K1 / p, the operation is determined according to the equation (5).

  FIG. 3 is an operation characteristic diagram of the zero-phase current differential relay. FIG. 4 is a diagram for explaining a method for setting the minimum sensitivity value K1 in FIG.

  Referring to FIGS. 3 and 4, relay operation unit 50 is configured such that the case where both of the above equations (4) and (5) are satisfied is in the operating range. Here, as shown in FIG. 4, the minimum sensitivity value K1 is set so as to increase in proportion to the maximum amplitude of the phase currents Ia, Ib, Ic and the neutral point current IN.

Specifically, when the maximum value (max) of | Ia |, | Ib |, | Ic |, | IN | is defined as Imax, the minimum sensitivity value K1 is within the range of 0 <Imax ≦ K0 / q. ,
K1 = K0 (6)
Is defined as a constant value. In the range of Imax> K0 / q,
K1 = q × Imax (7)
It is defined to be proportional to Imax. Here, K0 and q are settling values. In the present specification, the multiplication symbol is indicated by “x” or “*”.

[Operation of phase detector]
Next, the operation of the phase determination unit 60 in FIG. 1 will be described. In the following, first, in what direction the current flows in the primary and secondary sides of each current transformer according to the type of failure (ie, external short circuit, internal short circuit, external ground fault, internal ground fault) explain. Next, how the secondary current of each current transformer changes when the current transformer is saturated will be described. Next, based on the above considerations, the phase relationship between the zero-phase current (3 × I0) and the neutral point current IN, depending on the difference between the internal ground fault and the external ground fault and whether the current transformer is saturated or not. Will be described. Finally, a method for determining whether there is an internal ground fault or an external ground fault will be described.

(1. About the primary current of the current transformer according to the type of failure)
FIG. 5 is a diagram for explaining the flow of current when an external short circuit fault occurs between the b-phase and the c-phase of the primary winding 31 of FIG. In FIG. 5, when the power source is connected to both the primary side and the secondary side of the three-phase transformer 30 (that is, in the case of both-ends power source), the c-phase line 37 c is changed to the b-phase line 37 b. A current generated in each line when the external short-circuit current 41 flows is schematically shown in FIG. In the case of an external short-circuit fault, fault currents Ib and Ic from the power supply on the secondary side of the three-phase transformer 30 flow through the b-phase current transformer CTb and the c-phase current transformer CTc.

  FIG. 6 is a diagram for explaining a current flow when an internal short-circuit failure occurs between the b-phase and the c-phase of the primary winding 31 of FIG. In FIG. 6, when the internal short circuit current 42 flows from the b-phase line 37 b to the c-phase line 37 c in the case where the power source is connected to both the primary side and the secondary side of the three-phase transformer 30. The current generated in each line is schematically shown. In the case of an internal short-circuit fault, fault currents Ib and Ic from the power supply on the primary side of the three-phase transformer 30 flow through the b-phase current transformer CTb and the c-phase current transformer CTc.

As shown in FIG. 5 and FIG. 6, the current provided in the b-phase current transformer CTb, the C-phase current transformer CTc, and the ground line 36 regardless of whether the fault is an internal short-circuit fault or an external short-circuit fault. For the currents Ib, Ic, IN that respectively flow through the primary side of the transformer CTN,
Ib = −Ic (8)
IN = 0 (9)
Is established. When the current transformers CTb and CTc are not saturated, the above equation (8) is also established for the secondary side of the current transformers CTb and CTc. On the other hand, when the short circuit current is excessive and at least one of the current transformers CTb and CTc is saturated, the above equation (8) is not established for the secondary side of the current transformers CTb and CTc. It should be noted that the above equation (9) always holds regardless of whether the current transformers CTb and CTc are saturated.

  FIG. 7 is a diagram for explaining the flow of current when an external ground fault occurs in the a phase of the primary winding 31 of FIG. Also in the case of FIG. 7, a case where a power source is connected to both the primary and secondary electric lines of the three-phase transformer 30 is shown.

  Referring to FIG. 7, the fault current from the secondary power source of the three-phase transformer 30 is generated in each phase of the primary winding 31 that is Y-connected through the neutral point 34 of the primary winding 31. Flowing into. The fault current of each phase of the primary side winding 31 flows to current transformers CTa, CTb, CTc provided on the primary side lines of the respective phases of the three-phase transformer 30.

  The external ground fault current 43 flows from the failure point 48 of the a-phase line 37a on the primary side of the three-phase transformer 30 to the ground. A part of the external ground fault current 43 is provided in each phase line through the ground pole 35, the ground line 36, the neutral point 34, and the primary side three-phase windings 33a, 33b, 33c, respectively, and is a current transformer. It flows to CTa, CTb, CTc.

Therefore, for the currents Ia, Ib, Ic, IN flowing through the primary sides of the current transformers CTa, CTb, CTc, CTN, respectively,
Ia + Ib + Ic = −IN (10)
Is established.

  When none of the current transformers CTa, CTb, CTc, CTN is saturated on the secondary side of the current transformers CTa, CTb, CTc, CTN, the relationship of the above equation (10) is established. Usually, since the fault phase current Ia is large among the currents Ia, Ib, and Ic of each phase, the relationship of Ia = −IN is generally established. Accordingly, the current phase on the secondary side of the a-phase current transformer CTa is substantially opposite to the phase of the neutral point current IN. Further, the secondary-side current phase of the a-phase current transformer CTa is substantially equal to the phase of the zero-phase current 3 × I0. Therefore, the phase of the zero-phase current 3 × I0 is substantially opposite to the phase of the neutral point current IN.

  On the other hand, when at least one of the current transformers CTa, CTb, CTc, and CTN is saturated, the relationship of the above equation (10) is not established. The phase relationship between the a-phase current Ia and the neutral point current IN in this case will be described later with reference to FIGS.

  FIG. 8 is a diagram for explaining a current flow when an internal ground fault occurs in the a phase of the primary winding 31 of FIG. Also in the case of FIG. 8, a case where a power source is connected to both the primary side and the secondary side of the three-phase transformer 30 is shown.

  Referring to FIG. 8, the failure current from the secondary power source of the three-phase transformer 30 is the primary phase winding 33 a Y-connected through the neutral point 34 of the primary winding 31. , 33b, 33c. Further, the fault current flowing through the b-phase and c-phase windings 33 b and 33 c flows to the current transformers CTb and CTc on the primary side of the three-phase transformer 30. The fault current flowing through the a-phase winding 33a flows from the fault point 49 to the ground as an internal ground fault current 44.

  A part of the internal ground fault current 44 that flows from the failure point 49 of the a-phase line 37a to the ground passes through the grounding electrode 35, the grounding wire 36, and the neutral point 34, and each of the primary windings 31 that are Y-connected. It flows in the phase windings 33a, 33b, 33c. Further, the fault current flowing through the b-phase and c-phase windings 33 b and 33 c flows to the current transformers CTb and CTc on the primary side of the three-phase transformer 30. The fault current flowing through the a-phase winding 33a flows from the fault point 49 to the ground as an internal ground fault current 44.

  On the other hand, the current Ia from the power source connected to the primary side of the three-phase transformer 30 flows through the primary side of the current transformer CTa connected to the a-phase winding 33a of the three-phase transformer 30. The phase of the a-phase current Ia is substantially in phase with the neutral point current IN. After passing through the current transformer CTa, the current Ia flows from the failure point 49 to the ground as the internal ground fault current 44.

  When the current transformers CTa, CTb, CTc, and CTN are not saturated, the primary side of the current transformers CTa, CTb, CTc, and CTN is also applied to the secondary currents of the current transformers CTa, CTb, CTc, and CTN. The same relationship is established with the side. Usually, since the fault phase current Ia is large among the currents Ia, Ib, and Ic of each phase, the phase of the secondary current Ia of the a phase current transformer CTa is almost the same as the phase of the zero phase current 3 × I0. . Therefore, the phase of the zero-phase current 3 × I0 is substantially the same as the phase of the neutral point current IN.

  When at least one of the current transformers CTa, CTb, CTc, CTN is saturated, the above phase relationship changes. The phase relationship between the a-phase current Ia and the neutral point current IN in this case will be described next with reference to FIGS.

(2. Change in secondary current of current transformer when current transformer is saturated)
FIG. 9 is a diagram for explaining a change in the current waveform observed by the relay when the current transformer is saturated.

  Referring to FIG. 9, the current waveform on the secondary side of the current transformer when the current transformer is non-saturated is indicated by a solid line (CT non-saturated). When the current transformer is saturated, the output on the secondary side of the current transformer suddenly attenuates due to magnetic saturation, as shown by the broken line (CT saturation) in FIG. When this waveform is input to the zero-phase current differential relay 40, the high-frequency component is removed by the filter circuit (for example, 111 in FIG. 2) inside the relay, so that the one-dot chain line (CT saturation + filter) in FIG. The waveform shown is obtained. The signal having the waveform of the one-dot chain line is used for the relay calculation. Thus, when the current transformer is saturated, the waveform observed by the relay is in a leading phase compared to the original current waveform.

(3. Phase relationship between zero-phase current and neutral point current)
FIG. 10 is a vector diagram showing the relationship between the zero-phase current and the neutral point current observed by the relay. 10A to 10D, vector diagrams are divided into four patterns depending on the difference between the a-phase ground fault external fault and the a-phase ground fault internal fault and the presence or absence of saturation of the current transformer. ing.

  A case where the current transformer CTa is saturated due to an a-phase ground fault external failure will be described with reference to FIG. First, when the current transformer CTa is not saturated, the phase of the zero-phase current (3 × I0) is substantially opposite to the neutral point current IN as described with reference to FIG. Here, when the current transformer CTa is saturated, the a-phase current Ia advances and changes to the phase side, so that the vector of the zero-phase current (3 × I0) is in the region indicated by hatching in FIG. Moving.

  Note that the degree of the leading phase differs depending on the degree of saturation of the current transformer. Therefore, there is variation in the moving range of the vector of the zero-phase current (3 × I0). This is the same in the case of FIGS.

  A case where the current transformer CTN is saturated due to an a-phase ground fault external failure will be described with reference to FIG. First, when the current transformer CTN is not saturated, the phase of the zero-phase current (3 × I0) is substantially opposite to the neutral point current IN as described with reference to FIG. Here, when the current transformer CTN is saturated, the neutral point current IN advances and changes to the phase side, so that the vector of the neutral point current IN moves to the area indicated by hatching in FIG. .

  With reference to FIG. 10C, the case where the current transformer CTa is saturated due to an a-phase ground fault internal failure will be described. First, when the current transformer CTa is not saturated, the phase of the zero-phase current (3 × I0) is substantially the same as that of the neutral point current IN, as described with reference to FIG. Here, when the current transformer CTa is saturated, the a-phase current Ia advances and changes to the phase side, so that the vector of the zero-phase current (3 × I0) is in the area indicated by hatching in FIG. Moving.

  With reference to FIG. 10D, the case where the current transformer CTN is saturated due to an a-phase ground fault internal failure will be described. First, when the current transformer CTN is not saturated, the phase of the zero-phase current (3 × I0) is substantially the same as that of the neutral point current IN, as described with reference to FIG. Here, when the current transformer CTN is saturated, the neutral point current IN advances and changes to the phase side, so that the vector of the neutral point current IN moves to the area indicated by hatching in FIG. .

  From the above, although there is a partial overlap between the external fault and the internal fault, in the case of the external ground fault, the neutral point current IN vector and the zero phase regardless of whether the current transformers CTa and CTN are saturated or not. The angle formed with the vector of the current (3 × I0) is generally greater than 90 ° (on the opposite phase side). On the other hand, in the case of an internal ground fault, the angle formed by the vector of the neutral point current IN and the vector of the zero-phase current (3 × I0) is approximately 90 regardless of whether or not the current transformers CTa and CTN are saturated. Less than ° (in-phase with each other).

(4. Determination method of internal ground fault or external ground fault)
A method for determining whether there is an internal ground fault or an external ground fault based on the phase relationship between the zero-phase current (3 × I0) and the neutral point current IN will be described.

  FIG. 11 is a diagram for explaining a criterion for determining whether an internal failure or an external failure in the case of the first embodiment. FIG. 11 shows the phase relationship of the neutral point current IN with respect to the zero-phase current (3 × I0). Specifically, the phase of the neutral point current IN with respect to the zero-phase current (3 × I0) is divided into three regions.

  The A1 region is a phase region having a predetermined phase in the range of −90 ° to 0 ° as a lower limit and a predetermined phase in the range of 0 ° to 90 ° as an upper limit. When the neutral point current IN is in this A1 region when the zero-phase current (3 × I0) is used as a reference, the phase determination unit 60 determines that the three-phase transformer 30 has an internal ground fault.

  The A2 region is a phase region having a predetermined phase in the range of 90 ° to 180 ° as the lower limit and an upper limit of the predetermined phase in the range of 180 ° to 270 °. Alternatively, the A2 region is a phase region in which a predetermined phase in a range of −270 ° to −180 ° is a lower limit and a predetermined phase in a range of −180 ° to −90 ° is an upper limit. When the neutral point current IN is in the A2 region when the zero-phase current (3 × I0) is used as a reference, the phase determination unit 60 determines that the three-phase transformer 30 has an external ground fault.

  The A3 area is an area that is neither the A1 area nor the A2 area. When the neutral point current IN is in the A2 region when the zero-phase current (3 × I0) is used as a reference, it is difficult for the phase determination unit 60 to determine whether it is an internal failure or an external failure. However, since the magnetic saturation is transient, the phase of the neutral point current IN shifts from the A1 region to the A2 region after waiting for about two to three cycles. If the phase of the neutral point current IN does not change from the A3 region even after waiting for about two to three cycles, it is determined that the neutral point current is saturated due to a very large internal fault current, and an internal fault is determined. This is because, in the case of an external fault, the fault current passing through the fault phase current transformer always passes through the winding impedance of the transformer, so the magnitude of the fault current of the external fault is the fault current of the internal fault. This is because it is considered to be smaller than the case.

  As an example, as shown in FIG. 11, the phase range may be divided so that the positive phase and the negative phase are symmetrical with respect to the zero-phase current (3 × I0). In the example of FIG. 11, it is assumed that each of φ1, φ2, and φ3 has a predetermined phase angle in the range of 0 to 90 °, and that a relationship of φ1 + φ2 + φ3 = 180 ° holds. In this case, the A1 region is a phase region from −φ1 to + φ1. The A2 region is a phase region from −180 ° to − (φ1 + φ2) and from (φ1 + φ2) to 180 °. The A3 region is a phase region from − (φ1 + φ2) to −φ1 and from φ1 to (φ1 + φ2).

  Furthermore, as an example, φ1 = φ2 = φ3 = 60 ° may be set. Since the state of magnetic saturation is determined by the magnetic characteristics of the iron core used in the current transformer, the characteristics of the three-phase transformer and the power system, the actual values of φ1, φ2, and φ3 are determined through experiments or simulations. It is necessary to set the optimal value.

(5. Specific determination method of phase region)
As a method for determining the phase relationship between the zero-phase current (3 × I0) and the neutral point current IN (ie, in which region of A1 to A3 the phase of the neutral point current IN is). Any known method can be used. For example, it can be determined using the following formula for calculating the phase angle.

In general, the sampling intervals of the current IA and the current IB are set every 30 °, the current values of the currents IA and IB are respectively IA [m] and IB [m], and the values 90 ° before the current time are IA [m−3 ] And IB [m-3]. Then, the cosine of the phase difference θ of IB [m] with respect to IA [m] is obtained by using the amplitude | IA | of the current IA and the amplitude | IB |
| IA | × | IB | × cosθ = IA [m] × IB [m] + IA [m-3] × IB [m-3] (11)
It is expressed.

  Here, when IA [m] × IB [m] + IA [m−3] × IB [m−3] ≧ 0, cos θ ≧ 0. Therefore, when the right side of the above formula (11) is 0 or more, it indicates that IB [m] is within a phase difference range of −90 ° to + 90 ° with respect to IA [m].

  As shown in FIG. 12, the values of IA [m-3] and IB [m-3] are the currents IA [m] and IB [m] at the current time point and IA [m] that is 30 ° before the current time point. -1] and IB [m-1].

  FIG. 12 is a vector diagram for explaining a method of generating a value 90 degrees before the current time. As shown in FIG. 12, by subtracting the value obtained by multiplying I [m], which is the current value, by √3 from the value obtained by doubling I [m−1], which is 30 ° before the current time, It is possible to generate I [m−3], which is a value 90 ° before the current time.

  If the above method is expanded, a value at an arbitrary sampling time can be generated by linearly combining I [m], which is the current value, and I [m−1], which is 30 ° before the current value. it can.

  FIG. 13 is a diagram for explaining a method of determining whether or not the neutral point current IN is in the A1 region. In FIG. 11, it is assumed that φ1 = 60 °.

  Referring to FIG. 13A, first, the neutral point current IN [m] is 3 × I0 [m], and the hatched region (−30 ° vector of FIG. , −90 ° to + 90 °). Specifically, on the right side of the formula (11), 3 × I0 [m−1] is substituted for IA [m], IN [m] is substituted for IB [m], and 3 is set for IA [m−3]. × I0 [m-4] is substituted, and IN [m-3] is substituted for IB [m-3]. Then, depending on whether the calculation result on the right side of Expression (11) is positive or negative, that is, whether cos θ is positive or negative, the phase of the neutral point current IN [m] is in the hatched region of FIG. It can be determined whether or not.

  Referring to FIG. 13B, in the same manner as described above, the phase of the neutral point current IN [m] is 3 × I0 [m] with respect to the hatched region (+30 in FIG. 13B). It can be determined whether or not it is in the range of −90 ° to + 90 ° with respect to the vector of °. In this case, on the right side of Expression (11), 3 × I0 [m + 1] is substituted for IA [m], IN [m] is substituted for IB [m], and 3 × I0 [ m-2] is substituted, and IN [m-3] is substituted for IB [m-3]. However, since [m + 1] is data after 30 ° from the present time, in practice, all data are delayed by 30 °, and 3 × I0 [m] is substituted for IA [m], and IB [m] IN [m−1] is substituted into IA [m−3], 3 × I0 [m−3] is substituted into IA [m−3], and IN [m−2] is substituted into IB [m−3]. As a result of these determinations, if it can be determined that IN [m] is in the hatching area in any case, it can be determined that IN [m] is in the A1 area.

[Zero-phase current differential relay operation including operation determination unit]
Next, the operation of the entire zero-phase current differential relay 40 including the operation determination unit 70 of FIG. 1 will be described.

FIG. 14 is a block sequence diagram of the zero-phase current differential relay according to the first embodiment.
Referring to FIG. 14, relay calculation unit 50 determines whether or not equation (4) described above is satisfied, and whether or not equation (5) is satisfied. A determination unit 52 and an AND gate 53 are included. The output of the AND gate 53 is asserted when the determination results of the determination unit 51 and the determination unit 52 are both satisfied.

  The phase determination unit 60 determines whether or not the neutral point current IN is in the A1 region of FIG. 11, and determines whether or not the neutral point current IN is in the A3 region of FIG. And a determination unit 63 that determines whether the amplitude of the zero-phase current (3 × I0) and the amplitude of the neutral point current IN are both greater than the threshold value k3. If at least one of the amplitude of the zero-phase current (3 × I0) and the amplitude of the neutral point current IN is extremely small, the accuracy of the phase calculation cannot be guaranteed, and thus the determination unit 63 is provided.

  The operation determination unit 70 includes AND gates 73 and 74, an on-delay timer 71 with an operation time t 1, an on-delay timer 72 with an operation time t 2, and an OR gate 75. The operation time t1 is several milliseconds, for example, and the operation time t2 is several cycles, for example.

  The output of the AND gate 73 is asserted when the determination results of the determination unit 61 and the determination unit 63 are satisfied and the output of the AND gate 53 is asserted. The output of the AND gate 73 is output to the outside as a relay output via the on-delay timer 71 and the OR gate 75.

  Therefore, when the determination unit 61 determines that the neutral point current IN is in the A1 region (that is, an internal failure) and the relay calculation unit 50 determines that the operation is the zero-phase current differential relay. The zero-phase current differential relay 40 outputs a signal representing an internal failure to the outside after the state continues for several milliseconds.

  On the other hand, the output of the AND gate 74 is asserted when the determination results of the determination unit 62 and the determination unit 63 are satisfied and the output of the AND gate 53 is asserted. The output of the AND gate 74 is output to the outside as a relay output via the on-delay timer 72 and the OR gate 75.

  Therefore, it is determined by the determination unit 62 that the neutral point current IN is in the A3 region (that is, it is difficult to determine whether an internal failure or an external failure), and the relay calculation unit 50 operates in the operating range of the zero-phase current differential relay. If it is determined that there is, the zero-phase current differential relay 40 outputs a signal indicating an internal failure to the outside only when the state continues for several cycles.

[effect]
The zero phase current differential relay of the first embodiment described above has the following effects.

  (i) As described in the above equation (3), according to the present embodiment, the amount of suppression includes the zero-phase current (3 × I0) due to Ia, Ib, and Ic on the line side and the neutral point current IN. Scalar sum is adopted. In this case, since the load current is balanced in three phases, the suppression amount IR is not affected by the load current and is determined only by the ground fault current. Therefore, at the time of an internal ground fault in which the zero-phase current (3 × I0) and the neutral point current IN are in the same phase, the differential amount becomes substantially equal to the suppression amount. For this reason, it is possible to detect a ground fault with high sensitivity without extremely reducing the ratio settling value.

  (ii) As described with reference to FIG. 4 and equations (6) and (7), the minimum sensitivity value K1 of the differential amount ID of the zero-phase current differential relay 40 can be set in proportion to the amplitude of the load current. I did it. Although the error of the current transformer increases in proportion to the increase in load current, the minimum sensitivity value K1 is increased in proportion to the load current, so that erroneous output can be prevented.

  In setting the minimum sensitivity K1, only the error of the current transformer is considered, and it is not necessary to consider the saturation of the current transformer. Therefore, the merit of the high sensitivity detection described in the above (i) is not impaired.

  (iii) As a measure against saturation of the current transformer, it is possible to determine whether an internal ground fault or an external ground fault has occurred based on the phase angle between the zero-phase current (3 × I0) and the neutral point current IN. did. In this case, if it is difficult to determine whether there is an internal ground fault or an external ground fault due to the saturation of the current transformer, the zero-phase signal is output after the operating time has elapsed using an on-delay timer having an operating time of several cycles. The current differential relay 40 was made to operate. Therefore, even when the current transformer is saturated due to an external ground fault, even if the neutral point current IN transiently enters the above-mentioned region where internal / external failure is difficult to determine with respect to the zero-phase current, During the time when the neutral point current IN exists in the transient region, the determination result is not output by the on-delay timer. Therefore, the zero-phase current differential relay 40 can be prevented from malfunctioning.

Embodiment 2. FIG.
In the normal state, a load current flows through the current transformers CTa, CTb, and CTc, while no current flows through the current transformer CTN for the neutral point current IN. Therefore, an inexpensive current transformer having a lower saturation voltage than the current transformers CTa, CTb, and CTc tends to be easily selected as the current transformer CTN for the neutral point current IN. That is, the current transformer CTN is more likely to be magnetically saturated than the current transformers CTa, CTb, and CTc. For this reason, when an external ground fault occurs, magnetic saturation often occurs in the current transformer CTN for the neutral point current IN and does not occur in the line current transformers CTa, CTb, and CTc. Alternatively, even when magnetic saturation occurs in both of these, the current transformer CTN has a higher degree of saturation.

  Therefore, in the second embodiment, a case where only the saturation of the current transformer CTN is considered and the saturation of the current transformers CTa, CTb, and CTc is not considered will be described.

  FIG. 15 is a vector diagram showing the relationship between the zero-phase current and the neutral point current in the case of the zero-phase current differential relay of the second embodiment. FIG. 15A corresponds to FIG. 10B and shows a case where the current transformer CTN is saturated due to an a-phase ground fault external failure. FIG. 15B corresponds to FIG. 10D and shows a case where the current transformer CTN is saturated due to an a-phase ground fault internal failure.

  As shown in FIGS. 15A and 15B, when only saturation of the current transformer CTN is considered, there is no overlap of neutral point current IN vectors between the external ground fault and the internal ground fault. . Therefore, it is not necessary to set the A3 area described in FIG.

  FIG. 16 is a diagram for explaining a criterion for determining whether an internal failure or an external failure in the case of the second embodiment. FIG. 16 shows the phase relationship of the neutral point current IN with respect to the zero-phase current (3 × I0). Specifically, the phase of the neutral point current IN with respect to the zero-phase current (3 × I0) is divided into two regions.

  The A1 region is a phase region in which a predetermined phase in the range of −90 ° to 0 ° is the lower limit and a predetermined phase in the range of 90 ° to 180 ° is the upper limit. When the phase of the neutral point current IN is in this A1 region when the zero-phase current (3 × I0) is used as a reference, the phase determination unit 60B determines that the three-phase transformer 30 has an internal ground fault.

  The A2 region is a phase region other than the A1 region. When the phase of the neutral point current IN is in this A2 region when the zero-phase current (3 × I0) is used as a reference, the phase determination unit 60B determines that the three-phase transformer 30 has an external ground fault.

  As shown in FIG. 16, the above relationship can be expressed using phase angles φ4, φ5, and φ6. Here, φ4 + φ5 + φ6 = 360 °, 0 ° <φ4 <90 °, and 90 ° <φ5 <180 °. In this case, the A1 region is a phase range from −φ4 to + φ5. The A2 region is a phase range from + φ5 to (φ5 + φ6). For example, φ4 = 60 °, φ5 = 120 °, and φ6 = 180 °.

  FIG. 17 is a block sequence diagram of the zero-phase current differential relay according to the second embodiment. The block sequence diagram of FIG. 17 corresponds to FIG. 14 of the first embodiment.

  Referring to FIG. 17, zero-phase current differential relay 40B according to the second embodiment includes a relay calculation unit 50, a phase determination unit 60B, and an operation determination unit 70B. The configuration of relay operation unit 50 in FIG. 17 is the same as that in FIG. 14, and therefore description thereof will not be repeated.

  The phase determination unit 60B determines whether or not the neutral point current IN is in the region A1 in FIG. 16, and the amplitude of the zero-phase current (3 × I0) and the amplitude of the neutral point current IN And a determination unit 63 for determining whether or not the threshold value is larger than the threshold value k3.

  The operation determination unit 70B includes an AND gate 73 and an on-delay timer 71 having an operation time t1. The operation time t1 is several milliseconds, for example. The output of the AND gate 73 is asserted when the determination results of the determination unit 61 and the determination unit 63 are satisfied and the output of the AND gate 53 is asserted. The output of the AND gate 73 is output to the outside through the on-delay timer 71 as a relay output.

  Therefore, when the determination unit 61 determines that the neutral point current IN is in the A1 region (that is, an internal failure) and the relay calculation unit 50 determines that the operation is the zero-phase current differential relay. The zero-phase current differential relay 40B outputs a signal indicating an internal ground fault to the outside after the state continues for several milliseconds.

[effect]
As described above, when magnetic saturation occurs only in the current transformer CTN for the neutral point current IN, the device configuration can be simplified. Since other effects are the same as those of the first embodiment, description thereof will not be repeated.

Embodiment 3 FIG.
In the third embodiment, a case will be described in which the setting of the operation area of relay computing unit 50C can be changed. In this case, according to the determination result of the phase determination unit 60, the operation range of the relay calculation unit 50C and the operation time of the on-delay timer are changed. Since the definitions of the differential amount ID and the suppression amount IR are the same as those described in the expressions (2) and (3) of the first embodiment, the description will not be repeated.

  FIG. 18 is an operational characteristic diagram of the relay computing unit in the zero-phase current differential relay of the third embodiment. The motion determination area in FIG. 18 is applied to the case of the A3 area in FIG.

  Referring to FIG. 18, relay calculation unit 50 </ b> C performs an operation determination according to the following equations (12) and (13) in the case of area A <b> 1 in FIG. The operation is determined according to (12) to (14).

ID> K1 (12)
ID> p × IR (13)
ID> r × IR-K4 (14)
In the above formula (14), r and K4 are settling values (where r> p). The operation region (applied in the region A3) satisfying the above equations (12), (13) and (14) is a region hatched in FIG. As shown in FIG. 18, when the suppression amount IR is equal to or less than K1 / p, the operation determination is performed according to the equation (12). When the suppression amount IR is greater than K1 / p and less than or equal to K4 / (rp), the operation is determined according to the equation (13). When the suppression amount IR is larger than K4 / (rp), the operation is determined according to the equation (14).

  The operation region (applied in the region A1) satisfying only the above equations (12) and (13) is a region hatched in FIG. Compared with the operation region of FIG. 3, the operation region of FIG. 18 is narrower in a range where the suppression amount IR exceeds the threshold value of K4 / (rp).

  The condition of Expression (14) may be transiently entered into the operating range of Expression (13) when CT is saturated at the time of an external failure, and is set to avoid that possibility as much as possible.

FIG. 19 is a block sequence diagram of the zero-phase current differential relay according to the third embodiment.
Referring to FIG. 19, zero-phase current differential relay 40 </ b> C includes a relay calculation unit 50 </ b> C, a phase determination unit 60, and an operation determination unit 70.

  The relay calculation unit 50C includes a determination unit 51 that determines whether or not the above equation (12) is satisfied, a determination unit 52 that determines whether or not the above equation (13) is satisfied, and the above-described equation (12). A determination unit 54 that determines whether or not Expression (14) is satisfied, and AND gates 53 and 55 are included. The output of the AND gate 53 is asserted when the determination results of the determination unit 51 and the determination unit 52 are both satisfied. The output of the AND gate 55 is asserted when the determination results of the determination unit 51, the determination unit 52, and the determination unit 54 are all satisfied.

  Since the configuration of phase determination unit 60 is the same as that of FIG. 14 of the first embodiment, description thereof will not be repeated. The configuration of the operation determination unit 70 is the same as in the case of FIG. 14, but the determination conditions input to the AND gates 73 and 74 are different from the case of FIG.

  Specifically, the output of the AND gate 73 is asserted when the determination results of the determination unit 61 and the determination unit 63 are satisfied and the output of the AND gate 53 is asserted. The output of the AND gate 73 is output to the outside as a relay output via the on-delay timer 71 and the OR gate 75.

  Therefore, it is determined by the determination unit 61 that the phase of the neutral point current IN is in the A1 region (that is, an internal failure), and the differential amount ID and the suppression amount IR are shown in FIG. If it is determined that (the above equations (12) and (13)), the zero-phase current differential relay 40 outputs a signal indicating an internal failure to the outside after the state continues for several milliseconds. .

  On the other hand, the output of the AND gate 74 is asserted when the determination results of the determination unit 62 and the determination unit 63 are satisfied and the output of the AND gate 55 is asserted. The output of the AND gate 74 is output to the outside as a relay output via the on-delay timer 72 and the OR gate 75.

  Therefore, it is determined by the determination unit 62 that the phase of the neutral point current IN is in the A3 region (that is, it is difficult to determine whether an internal failure or an external failure), and the differential calculation unit 50 and the suppression amount IR Is determined to be within the operating range shown in FIG. 18 (that is, the above equations (12), (13), and (14)), the zero-phase current differential relay 40 is in a state where the state continues for several cycles. A signal indicating an internal failure is output to the outside only.

[effect]
According to the above configuration, even if an external failure occurs due to saturation of the current transformer, the differential amount ID is close to the suppression amount IR and occupies the operating range (determination of the gate 53 in FIG. 19). Even if the condition is satisfied, if the phase determination unit 60 determines that the area is the A3 area, the operation range of the ratio differential relay is changed. That is, the operating range of the ratio differential relay is changed from the operating range shown in FIG. 3 (that is, determination of gate 53 in FIG. 19) to the operating range shown in FIG. 18 (that is, determination of gate 55 in FIG. 19). The operation region shown in FIG. 18 is a narrower region in which the differential amount ID and the suppression amount IR may exist when CT is saturated at the time of an external failure than the operation region shown in FIG. Furthermore, when the phase determination unit 60 determines that the region is the A3 region, the on-delay timer 72 acts, so that the reliability can be increased in terms of preventing malfunctions with respect to CT saturation. Since other effects of the third embodiment are the same as those of the first embodiment, description thereof will not be repeated.

Modified example.
In the first to third embodiments, the vector difference current (that is, | 3 × I0−IN |) may be used as the suppression amount, the amplitude of the zero-phase current (3 × I0), and the neutral point current. The larger one of the amplitudes of IN (hereinafter referred to as the zero-phase maximum value) may be used.

  When a vector difference current is used as the suppression amount IR, the suppression amount due to an external failure (Ia and IN are opposite in phase when an a-phase ground fault external failure) increases, while an internal failure (when an a-phase ground fault external failure occurs) Since the amount of suppression at Ia and IN is substantially the same phase), more stable operating characteristics can be obtained.

  In addition, when the zero-phase maximum value is used as the suppression amount IR, the suppression amount is halved due to an external failure compared to the scalar sum, so the ratio setting needs to be considered accordingly. The same effect is obtained. Note that both the vector difference current and the zero-phase maximum value have the advantage that they are not affected by the load current because they are the suppression amounts using the zero-phase current.

  The embodiment disclosed this time must be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  30 three-phase transformer, 33 windings, 34 neutral point, 35 grounding pole, 36 grounding wire, 37a, 37b, 37c line, 40, 40B, 40C zero-phase current differential relay, 50, 50C relay operation unit, 60 , 60B Phase determination unit, 70, 70B Operation determination unit, 71, 72 On-delay timer, CTN, CTa, CTb, CTc Current transformer.

Claims (8)

A zero-phase current differential relay for protecting a three-phase transformer including a Y-connection winding,
Each phase current and neutral point current of the Y-connection winding is defined so that the directions toward the neutral point have the same polarity.
Relay calculation that calculates the differential amount and the suppression amount based on the zero-phase current based on each phase current and the neutral point current, and determines whether the differential amount and the suppression amount are in the operating range And
A phase determination unit that determines whether or not the phase of the neutral point current with respect to the zero-phase current is in a first region including the same phase;
A zero-phase current differential relay comprising: an operation determination unit that outputs a protection signal for protecting the three-phase transformer based on a determination result of the relay calculation unit and a determination result of the phase determination unit. The phase determination unit determines whether the phase of the neutral point current with respect to the zero-phase current is in the first region, the second region, or the third region;
The first region is a phase region having a predetermined phase in a range of −90 ° to 0 ° as a lower limit and a predetermined phase in a range of 0 ° to 90 ° as an upper limit,
The second region is a phase region having a predetermined phase in a range from 0 ° to 180 ° as a lower limit and a predetermined phase in a range from 180 ° to 270 ° as an upper limit,
2. The zero-phase current differential relay according to claim 1, wherein the third area is a phase area that is neither the first area nor the second area. The operation determination unit is in a state where it is determined that the phase of the neutral point current with respect to the zero-phase current is in the first region, and that the differential amount and the suppression amount are in the operation region. Output the protection signal when maintained for a period of 1,
The operation determination unit is in a state where the phase of the neutral point current with respect to the zero-phase current is in the third region, and the state where the differential amount and the suppression amount are determined to be in the operation region is The zero-phase current differential relay according to claim 2, wherein the protection signal is output when the second period longer than the first period is maintained. The relay calculation unit determines whether the differential amount and the suppression amount are included in a first operation range and whether they are included in a second operation region that is narrower than the first operation region. ,
The operation determination unit determines that the phase of the neutral point current with respect to the zero-phase current is in the first region, and that the differential amount and the suppression amount are in the first operation region. Outputting the protection signal when the state is maintained for a first period;
The operation determination unit determines that the phase of the neutral point current with respect to the zero-phase current is in the third region, and that the differential amount and the suppression amount are in the second operation region. The zero-phase current differential relay according to claim 2, wherein the protection signal is output when a state is maintained for a second period longer than the first period. The phase determination unit determines whether the phase of the neutral point current with respect to the zero-phase current is in the first region or the second region;
The first region is a phase region having a predetermined phase in a range of −90 ° to 0 ° as a lower limit and an upper limit of a predetermined phase in a range of 90 ° to 180 °,
The zero-phase current differential relay according to claim 1, wherein the second region is a phase region that is not the first region.   The operation determination unit, when it is determined that the phase of the neutral point current with respect to the zero-phase current is in the first region, and that the differential amount and the suppression amount are in the operation region, The zero-phase current differential relay according to claim 5, which outputs the protection signal.   The minimum sensitivity value in the operating range is set to increase as the maximum value of the amplitude of each phase current and the amplitude of the neutral point current increases. Zero-phase current differential relay described in 1. The zero-phase current differential relay according to any one of claims 1 to 7, wherein the suppression amount is a sum of three times the amplitude of the zero-phase current and the amplitude of the neutral-point current.

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