JP3517737B2 - Ground fault directional relay for low voltage road - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low piezoelectric path for determining a ground-faulted low voltage side electric path among low voltage side electric paths including branch low voltage side electric paths of high and low voltage transformers in which low voltage side windings are commonly grounded. For ground fault direction relay.

[0002]

2. Description of the Related Art Conventionally, the problems to be solved by the invention
As a private electric facility such as a building or a factory, for example, a high-voltage side is 6.6 kv, a low-voltage side is a 3-phase three-wire high / low voltage transformer of 200 v, a high-voltage side is 6.6 kv, and a low-voltage side is 200 v
A v-100v single-phase three-wire high / low voltage transformer or the like is installed. In the case of the above-mentioned three-phase three-wire type high / low voltage transformer or single-phase three-wire type high / low voltage transformer, according to Article 24 of the interpretation of the technical standard for electrical equipment, the neutral point of the low voltage side winding or the low voltage side winding terminal One of them is required to perform class B grounding work (a common grounding pole work which is less than a predetermined grounding resistance value, and usually one customer is provided with only one pole). In FIG. 10, one of the low-voltage side winding terminals TE of the three-phase three-wire type high / low voltage transformer Tr1 and the neutral point N of the low-voltage side winding of the single-phase three-wire type high / low voltage transformer Tr2 are normally Common B type grounding electrode E, which is provided only for one pole for one customer
It is the electric circuit diagram which showed typically being connected to B.

Recently, the cables used for the low-voltage side electric line (including the branch low-voltage side electric line) of the three-phase three-wire type high-low voltage transformer Tr1 and the low-voltage side electric line of the single-phase three-wire type high-low voltage transformer Tr2 have recently been used. It has become long, and an electronic device incorporating a line filter or the like having a capacitor as a constituent element is often connected as a load. Therefore, in general, the apparent electrostatic capacitances (stray capacitances) C1 and C2 of the low-voltage side electric circuit are large.

In the low voltage side electric circuit as described above, FIG.
As shown in, the one-phase cable of the low-voltage side electric path of the three-phase three-wire high / low voltage transformer Tr1 is, for example, a D-type ground electrode ED (usually the conduit, control panel, casing of the operation panel, etc. are grounded). When a ground fault occurs in a conduit that is grounded at, the ground fault current from the ground fault point flows through paths 1 and 2 as indicated by broken lines. Among these, the current flowing in the path 1 is the ground fault resistance Rg, the D-type ground line EDL connected to the conduit, and the electrostatic capacitance (stray capacitance) C to ground.
1. Single-phase three-wire high-low voltage transformer Tr2 low-voltage side electric circuit, B-type branch ground wire EBL1 (branched from class B ground bus EBL0), three-phase three-wire high-low voltage transformer Tr1 low-voltage side electric line The current flowing in the closed circuit composed of
Ground fault resistance Rg, D type ground wire ED connected to conduit
L, electrostatic capacitance (floating capacitance) C2 to ground, flows into a closed circuit constituted by a branch low-voltage side electric circuit which is not grounded among the low-voltage side electric circuits of the three-phase three-wire high / low voltage transformer Tr1. Therefore, the single-phase three-wire high / low voltage transformer Tr that is not directly related to the ground fault
In some cases, the circuit breaker ELCB1 of the low voltage side electric circuit 2 and the circuit breaker ELCB2 of the branch low voltage side electric circuit of the three-phase three-wire high / low voltage transformer Tr1 are unnecessarily operated.

As described above, the cause of the unnecessary operation of the circuit breakers ELCB1 and ELCB2 of the low-voltage side electric circuit, which are not directly related to the ground fault, is the three-phase three-wire high / low voltage transformer Tr1 and the single-phase three-wire high / low voltage. The low-voltage side winding terminal TE of each transformer and the neutral point N of the low-voltage side winding are commonly connected to the class B grounding electrode EB that is the common grounding electrode of the transformer Tr2, and as described above, It is conceivable that the electrostatic capacitances (floating capacitances) C1 and C2 to the ground of the electric path are large.

Therefore, in the present invention, when any of the low-voltage side electric lines including the branch low-voltage side electric line of each high-low voltage transformer in which the low-voltage side winding is commonly grounded is ground-faulted, the ground-faulted low-voltage side electric line is surely made. It is an object to be solved to provide a ground fault direction relay for a low piezoelectric road which can be determined.

[0007]

The above-mentioned problems can be solved by the ground fault direction relay for a low piezoelectric path described in the claims. According to the ground fault direction relay for low-voltage road according to claim 1, when the effective value / phase calculating means calculates the effective value and the phase of the voltage between ground electrodes and the zero-phase current, the voltage between ground electrodes and the zero phase. The effective value and phase of the current are stored in the storage means at predetermined time intervals. The vector calculation means
A voltage vector based on the effective value and phase of the latest ground-to-ground voltage calculated by the effective value / phase calculation means and a voltage vector based on the effective value and phase of the ground-to-ground voltage stored in the storage means. A current vector based on the effective value and phase of the latest zero-phase current calculated by the effective value / phase calculation means as well as the difference, and a current vector based on the effective value and phase of the zero-phase current stored in the storage means. Calculate the current vector difference between and. Then, when the effective value of the current vector difference calculated by the vector calculating means exceeds the predetermined value, the ground fault determining means determines that the low voltage side electric circuit is grounded based on the phase difference of the current vector difference with respect to the phase of the voltage vector difference. It is determined whether there is a connection. Accordingly, when the low-voltage side electric path is ground-faulted, it is possible to reliably determine only the low-voltage side electric path that is grounded.

According to the ground fault direction relay for a low-voltage circuit of the present invention, when the ground fault judging means judges that the low voltage side circuit of the high / low voltage transformer is grounded, the circuit breaker of the low voltage side circuit is broken. Therefore, it is possible to prevent the circuit breaker of the low-voltage side electric circuit that is ground-faulted from performing a breaking operation and the circuit breaker of the low-voltage side electric circuit that is not ground-faulted to malfunction.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Next, embodiments of the present invention will be described. In Figure 1, the high voltage side is 6.6kv and the low voltage side is 2
One of the low-voltage side winding terminals TE of the 00v three-phase three-wire high / low voltage transformer Tr1 and the high-voltage side 6.6 kv and the low-voltage side 2
The neutral point N of the low-voltage side winding of the 00v-100v single-phase three-wire high / low voltage transformer Tr2 is set as a predetermined ground point, and the predetermined ground points TE, N and the B-type ground electrode (common ground electrode) EB are connected to each other. It is an electric circuit diagram showing that it is connected via each branch ground line EBL1, EBL2 and ground bus EBL0. It should be noted that a D-type ground electrode (for low-voltage device) for flowing a ground-fault current when the low-voltage side electric lines of the high- and low-voltage transformers Tr1 and Tr2, low-voltage line devices such as control panel, operation panel, etc. are ground-faulted. A grounding wire EDL is connected between the grounding electrode ED and the casing of the conduit, control panel, operation panel, or the like. The D-type ground electrode ED is
If the working voltage of the low piezoelectric path exceeds 300 V, a Class C ground electrode is used.

As shown in FIG. 1, a current detector ZCT1 for detecting the zero-phase current in the low-voltage side electric circuit of the three-phase three-wire high / low voltage transformer Tr1 is provided, and the current detector ZCT1 is provided.
When a ground fault occurs in the low-voltage side electric path of the three-phase three-wire high / low voltage transformer Tr1, the output terminal of discriminates the ground fault and
A ground fault direction relay 1 for a low-piezoelectric circuit that causes a circuit breaker MCCB1 connected to the low-voltage side electric circuit to perform an interruption operation as described below.
It is connected to the. Also, the ground fault direction relay for low-voltage road 1
Is a detection wiring V1L for detecting the voltage via the ground resistance between the above-mentioned type B grounding electrode EB and type D grounding electrode ED.
1, V1L2 are connected.

Similarly to the above, the single-phase three-wire high / low voltage transformer Tr
A current detector ZCT2 for detecting the zero-phase current of the low-voltage side electric circuit of No. 2 is provided, and the output terminal of the current detector ZCT2 is a low-voltage side electric path of the single-phase three-wire high / low voltage transformer Tr2. In the case of occurrence of, the ground fault is discriminated, and the ground fault direction relay 2 for low-voltage path is connected to operate the circuit breaker MCCB2 connected to the low voltage side electric circuit.
Further, to the ground fault direction relay 2 for the low piezoelectric path, detection wirings V2L1 and V2L2 for detecting the voltage via the ground resistance between the above-mentioned B-type ground electrode EB and D-type ground electrode ED are connected. The low-voltage ground fault direction relay 1 and the low-voltage road ground fault direction relay 2 have the same configuration, and the configuration thereof will be described in detail later.

In FIG. 1, C1 and C2 are electrostatic capacities between the low-voltage side electric paths of the three-phase three-wire type high / low voltage transformer Tr1 and the single-phase three-wire type high / low voltage transformer Tr2 and the ground, and Rg is D This is the ground fault resistance when the low-voltage side electric path of the three-phase three-wire high / low voltage transformer Tr1 is grounded to the seed ground electrode ED. Also, R
E is a ground resistance between the B type ground electrode (common ground electrode) EB and the D type ground electrode ED.

In the electric circuit shown in FIG. 1, for example, 3
The operation principle of the low piezoelectric path ground fault direction relays 1 and 2 when the R phase of the low voltage side electric path of the phase 3-wire high / low voltage transformer Tr1 is grounded will be described. According to the Hoh-Thevenin theorem, Fig. 1
As shown in FIG. 2, when one phase of the low-voltage side electric circuit of the three-phase three-wire high / low voltage transformer Tr1, for example, R phase is ground-faulted at the ground fault point ES, the current distribution of each part of the electric circuit of FIG. , Superposition of the electric circuits of FIG. 3 (based on the generally known superposition theorem).

FIG. 2 shows a circuit in which a voltage source V-V0b is connected between the ground fault point ES and the ground fault resistance Rg in FIG.
The voltage source V-V0b represents the voltage generated between the ground fault point ES and the ground fault resistance Rg before the ground fault, and the direction of this voltage is the direction of the ground fault point ES and the ground fault before the ground fault. The direction of the voltage generated between the resistors Rg is the same. Therefore, in FIG. 2, the current does not flow from the ground fault point ES to the ground, and the current distribution of each part is equal to the current distribution before the ground fault.

FIG. 3 shows an electric circuit obtained by the Hoh-Thevenin theorem. As shown in FIG. 3, the voltage source V-V0b in the opposite direction to that in FIG. 2 is connected between the ground fault point ES and the ground fault resistance Rg, and the other voltage sources are short-circuited.

When the electric circuits of FIGS. 2 and 3 are superposed,
Since the directions of the voltage sources V-V0b are opposite to each other, they are canceled, and the ground fault point ES and the ground fault resistance Rg are short-circuited. This state represents a state in which the above-mentioned ground fault has occurred. That is, by overlapping the electric circuits of FIG. 2 and FIG.
It becomes the same as the electric circuit (Fig. 1) after the occurrence of the ground fault. This can be expressed as follows. Current distribution after occurrence of ground fault (Fig. 1) = Current distribution before occurrence of ground fault (Fig. 2) + Current distribution of Fig. 3 (1) Here, Fig. 4 is a rewrite of the electric circuit of Fig. 3 to make it easier to see.
It is the electric circuit shown in. In the electric circuit of FIG. 4, B
Voltage Va and current I0 between the seed ground electrode EB and the D seed ground electrode ED
The phase relationship between 1a and I02a is as shown in FIG. That is, the phases of the current I01a with respect to the voltage Va are the capacitances C1 and C2 between each low voltage side electric circuit and the ground, and the ground fault resistances Rg and B.
Depending on the value of the grounding resistance RE between the seed grounding electrode (common grounding electrode) EB and the D-type grounding electrode ED, it advances from 0 ° and becomes a range of 90 °,
The phase of the current I02a has a delay of 90 °. Therefore, by utilizing this phase difference, it is possible to distinguish between a case where a ground fault occurs in the low voltage side electric line and a case where a ground fault occurs in another low voltage side electric line.

The current distribution of the electric circuit of FIG. 3 can be obtained from the equation (1) by the following equation. Current distribution in Fig. 3 = current distribution after occurrence of ground fault (Fig. 1) -current distribution before occurrence of ground fault (Fig. 2) Expression (2) Further, between the B type ground electrode EB and the D type ground electrode ED in Fig. 4. The voltage Va and the currents I01a and I02a of are calculated by the following equations. Va = RE * Ia = RE * (I0-I0b) = V0-V0b Formula (3) I01a = I01-I01b Formula (4) I02a = I02-I02b Formula (5) In addition, each said formula is the type of voltage ( 100V, 200V, 4
00V), circuit method (single-phase two-wire system, single-phase three-wire system, three-phase three
Line type, three-phase four-wire type), and a circuit to which a voltage other than the grounded line is applied, regardless of the number of transformer banks and the number of branch circuits (branch low-voltage side circuit shown in FIG. 10). Is always satisfied when is grounded.

As is apparent from the above description, the voltage Va and the currents I01a and I02a are the same as the voltage V0 after the ground fault,
Current I01, I02 to voltage V0b before ground fault, current I0
It is obtained by subtracting 1b and I02b.
The above-mentioned low-voltage ground fault direction relay 1 has a case where the current I01a obtained by the above calculation exceeds a preset setting value, that is, the low-voltage side electric line of the high-low voltage transformer Tr1 is in a ground fault state. Compares the phase of the current I01a with the phase of the voltage Va as a reference to determine whether or not a ground fault has occurred in the low-voltage side electric circuit. Similarly, the ground fault direction relay 2 for a low piezoelectric path is in a state where the current I02a obtained by the above calculation exceeds a preset set value, that is, the low piezoelectric path of the high / low voltage transformer Tr2 is grounded. In this case, the current I is based on the phase of the voltage Va.
By comparing the phases of 02a, it is determined whether or not a ground fault has occurred in the low voltage side electric circuit.

FIG. 6 is a circuit block diagram showing the configuration of the ground fault direction relay 1 for a low piezoelectric road. The block diagram shown in FIG. 6 shows a circuit block for determining whether or not the low-voltage side electric path of the high / low voltage transformer Tr1 described above has a ground fault. Random relay 2
Is basically configured similarly to FIG.

As shown in FIG. 6, the aforementioned current detector ZC is used.
T1 is connected between the terminals Z1 and Z2, and is connected to the class B ground resistance and D
Detection wiring V1 for detecting the voltage between the seed ground resistors
L1 and V1L2 are connected to terminals V1 and V2, respectively.

The over-input protection / filter circuit 2a connected to the terminals Z1 and Z2 and the over-input protection filter circuit 2b connected to the terminals V1 and V2 prevent the input of an excessive voltage and prevent the internal circuit from operating. It is a circuit for electrically protecting and removing high frequency noise components.

The over-input protection / filter circuit 2a, the effective-value operation circuit 3a connected to the over-input protection / filter circuit 2b, and the effective-value operation circuit 3b are the effective value of the current detected by the current detector ZCT1, and , Detection wiring V1L
1 is a circuit for calculating the effective value of the voltage between the B-type ground resistance and the D-type ground resistance detected by V1L2.

Further, the phase calculation circuit 4a and the phase calculation circuit 4
Reference numeral b is a circuit that calculates the phases of the current detected by the current detector ZCT1 and the voltages detected by the detection wirings V1L1 and V1L2. Each phase is obtained by calculating the difference from the reference phase. In the present invention, since the voltage / current phase before and after the ground fault with different detection times is obtained as shown in the above formula (2), the reference phase must be unchanged before and after the ground fault.
Therefore, for example, the phase of the power supply voltage (100 VAC commercial power supply) of the ground fault direction relay 1 for the low piezoelectric road is used as a reference for the calculation.

The timer circuit 5 includes an effective value arithmetic circuit 3b and an effective value and phase of the current detected by the current detector ZCT1 output from the effective value arithmetic circuit 3a and the phase arithmetic circuit 4a.
And the detection wiring V1L output from the phase calculation circuit 4b
1, the effective value and the phase of the detected voltage by V1L2 are stored in the storage circuits 6a and 6b at intervals of t1 seconds of the set time. The data stored in the storage circuits 6a and 6b (the detected current, the effective value and the phase of the detected voltage) are rewritten every t1 seconds, and only the latest data is stored.

The vector operation circuit 7a is a current detector ZC.
The effective value and phase of the current detected this time by T1 and the current detector ZCT1 stored in the storage circuit 6a.
Based on the effective value and the phase of the detected current according to the above, the above-mentioned current I01a is obtained by vector calculation.

The vector operation circuit 7b has a detection wiring V1.
The effective value and phase of the voltage between the class B ground resistance and the class D ground resistance detected this time by L1 and V1L2, and the storage circuit 6
Based on the effective value and the phase of the detection voltage by the detection wirings V1L1 and V1L2 stored in b, the above-mentioned voltage V
a is obtained by vector calculation.

The level determination circuit 8 is a vector operation circuit 7.
When the current I01a obtained by a exceeds a preset level set value, a logic H signal is output.

The phase discrimination circuit 9 is a vector operation circuit 7b.
When the phase of the current I01a calculated by the vector calculation circuit 7a is delayed by approximately 90 ° with respect to the phase of the voltage Va calculated in step 1, the low voltage of the single-phase three-wire high / low voltage transformer Tr2 described above. When it is determined that a ground fault has occurred in the side electric line and a logical L signal is output, the phase of the current I01a advances from 0 ° with respect to the phase of the voltage Va and is within 90 °, the high level is detected. It is determined that the low-voltage side electric path of the low-voltage transformer Tr1 has a ground fault, and a logic H signal is output.

The AND circuit 10 outputs the logic H signal only when the signals output from the level judgment circuit 8 and the phase judgment circuit 9 are both logic H signals. That is, the current I0
1a exceeds the preset level setting value,
When the phase of the current I01a advances from 0 ° with respect to the phase of the voltage Va and is within a range of 90 °, the high / low voltage transformer T
A logic H signal indicating that the low-voltage side electric path of r1 is grounded is output.

The time settling circuit 11 has a t that can be set arbitrarily.
The logic H signal is output when the logic H signal is continuously output from the AND circuit 10 for 2 hours, and erroneous determination due to noise or the like is prevented.

When the logic H signal is output from the time settling circuit 11, the output circuit 12 activates the output relay X1 and turns on the operation display lamp 13 so that the low voltage side electric circuit of the high / low voltage transformer Tr1 is grounded. Display what you have done. The a-contact (make contact) of the output relay X1 is connected to a trip circuit (not shown) so as to activate the trip coil of the circuit breaker MCCB1 to cause the circuit breaker MCCB1 to perform a breaking operation. Also, output relay X1
B contact (break contact) is the high and low voltage transformer Tr1.
Is connected to an alarm circuit (not shown) for alarming that the low-voltage side electric circuit has a ground fault.

Further, the power supply circuit 14 receives the 100 volt voltage from the commercial power supply input from the terminals P1 and P2 via the over-input protection filter circuit 15, and each circuit of the ground fault direction relay 1 for the low piezoelectric path. Outputs the DC voltage required by. In addition, the ground fault direction relay 1 for the low piezoelectric road described above
The configuration shown in the block diagram of is an example, and the present invention is not limited to this.

In the above description of the embodiment, two high- and low-voltage transformers, namely, a three-phase three-wire high-low voltage transformer Tr1 and a single-phase three-wire high-low voltage transformer Tr2, are used as the high-low voltage transformers subject to the ground fault judgment. However, the voltage type (100V, 200V, 400
V), circuit system (single-phase two-wire system, single-phase three-wire system, three-phase three-wire system, three-phase four-wire system), number of banks of transformers and electric circuits (Fig. 1).
(Including the branch low-voltage side electric circuit as shown in 0) is not limited.

7, 8 and 9 are three-phase three-wire type 400V
2 shows an example of connection of the low-voltage ground fault direction relay 1 in the case of a circuit, a three-phase four-wire system 400V, and a single-phase two-wire system 100V circuit. In any case, the voltage between the class B ground resistance and the class D ground resistance (in the case where the voltage of the low-voltage side electric circuit is a 400 V circuit, it becomes the class C ground resistance, and its ground electrode is shown as EC) and the low-voltage side electric circuit. The zero-phase current of is detected. still,
In the three-phase three-wire 400V circuit of FIG. 7, C0 is a small-capacity capacitor used to measure the potential of the neutral point of the low-voltage side winding of the high-voltage transformer Tr1a, that is, the potential of the class B ground resistance. Therefore, as shown in FIG. 7, a small-capacity capacitor C0 is connected in a star shape, and the voltage between the neutral point and the C-class ground resistance is detected as the voltage between the B-class ground resistance and the C-class ground resistance. is doing. The reason is that in the case of the high / low voltage transformer Tr1a, the low voltage side electric circuit is a three-phase three-wire 400V circuit, and the neutral wire is not drawn out.

[0035]

According to the present invention, when any of the low-voltage side electric lines including the branch low-voltage side electric line of each high-low voltage transformer in which the low-voltage side winding is commonly grounded, is ground-faulted, the ground-faulted low-voltage side electric line Can be reliably determined.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram showing an embodiment of the present invention.

FIG. 2 is an electric circuit diagram in which the electric circuit of FIG. 1 is drawn based on the Hoh-Thevenin theorem.

FIG. 3 is an electric circuit diagram obtained by the Hoh-Thevenin theorem.

FIG. 4 is an electric circuit diagram in which the electric circuit of FIG. 3 is simplified.

FIG. 5 is a vector diagram for determining a ground fault of a low voltage side electric circuit.

FIG. 6 is a circuit block diagram of a ground fault direction relay for a low piezoelectric path.

FIG. 7 is an electric circuit diagram showing a connection example of a ground fault direction relay for a low piezoelectric road.

FIG. 8 is an electric circuit diagram showing a connection example of a ground fault direction relay for a low piezoelectric path.

FIG. 9 is an electric circuit diagram showing a connection example of a ground fault direction relay for a low-voltage path.

FIG. 10 is an electric circuit diagram for explaining a conventional problem.

[Explanation of symbols]

1,2 Low-voltage ground fault relay 2a, 2b Over-input protection / filter circuit 3a, 3b RMS value calculation circuit 4a, 4b Phase calculation circuit 5 timer circuit 6a, 6b memory circuit 7a, 7b vector operation circuit 8 level judgment circuit 9 Phase discrimination circuit 10 AND circuit 11 hours settling circuit 12 Output circuit 13 Operation indicator lamp X1 output relay Tr1 3-phase 3-wire high / low voltage transformer Tr2 Single-phase 3-wire high / low voltage transformer N neutral point TE Low voltage side winding terminal ZCT1 current detector ZCT2 current detector VL1, VL2 detection wiring EB Class B grounding electrode ED D type ground electrode

Front Page Continuation (72) Hideki Fujita Inventor Hideki Fujita 1 20-20 Kitakousan, Otaka-cho, Midori-ku, Nagoya, Aichi Chubu Electric Power Co., Inc. Power Technology Research Center (56) References JP 2001-352663 (JP, A) ) JP 2001-339847 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H02H 3/32-3/52

Claims (2)

(57) [Claims] 1. A common grounding electrode for grounding the low voltage side windings of a plurality of high and low voltage transformers to a common ground and a low voltage side electric circuit connected to the low voltage side windings of the respective high and low voltage transformers are ground-faulted. In the case of voltage detection means for detecting the voltage between the ground electrodes generated between the ground electrode for low piezoelectric device for flowing the ground fault current,
Current detection means for detecting the zero-phase current of the low-voltage side electric circuit connected to the low-voltage side winding of each of the high-low voltage transformer,
Effective value / phase calculating means for calculating the effective value and phase of the ground-to-ground voltage and the zero-phase current, and the effective value of the ground-to-ground voltage and the zero-phase current calculated by the effective value / phase calculating means. Storage means for storing the value and the phase, and the effective value
A voltage vector based on the latest effective value and phase of the voltage between ground electrodes calculated by the phase calculation means, and a voltage vector based on the effective value and phase of the voltage between ground electrodes stored in the storage means. A current vector based on the difference and the latest effective value and phase of the zero-phase current calculated by the effective value / phase calculating means, and a current vector based on the effective value and phase of the zero-phase current stored in the storage means. Vector calculation means for vector-calculating the current vector difference between the current vector difference and the current vector difference relative to the phase of the voltage vector difference when the effective value of the current vector difference calculated by the vector calculation means exceeds a predetermined value. A ground fault direction relay for a low piezoelectric road, comprising: a ground fault determination means for determining whether or not the low-voltage side electric line is grounded based on a phase difference. 2. The ground fault direction relay for a low piezoelectric road according to claim 1, wherein the ground fault judging means operates a circuit breaker of the low voltage side electric line judged to have a ground fault. Folding direction relay.