JP3538475B2 - Field grounding relay - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field grounding relay,
In particular, the present invention relates to a field ground relay for detecting a ground fault in a field circuit of a generator including a generator motor and the like.

[0002]

2. Description of the Related Art A conventional field grounding relay supplies a predetermined DC voltage between the N side of a generator field circuit and the ground via a resistor, and generates a ground fault according to the magnitude of a current flowing to an output terminal. The presence or absence was determined.

According to the above-mentioned prior art, the ground fault is determined only by the magnitude of the current flowing to the output terminal. However, when the magnitude of the current is determined by detecting the peak value, the thyristor excitation type field is used. In the circuit, a harmonic voltage other than a DC voltage is generated as a field voltage, so that the harmonic voltage is induced by the stray capacitance to the output terminal of the field grounding relay, and the current flowing through the output terminal becomes an intermittent current, and a ground fault occurs. Is also intermittently detected.

[0004]

As a result, if there is no predetermined return delay timer, it can not be detected normally even if a ground fault occurs. There is a problem that the detection is erroneous due to the current flowing through the capacitor.

Further, since the ground fault is determined only based on the magnitude of the current, a highly reliable field ground relay cannot be provided, and the output of the field ground relay is not treated as a cutoff command, but an alarm command is issued. Was treated as

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide a high-performance field circuit which does not cause erroneous detection and erroneous non-detection even when applied to a thyristor excitation type field circuit. An object of the present invention is to provide a relay. Another object of the present invention is to provide a highly reliable field grounding relay that can handle its output as a direct cutoff command.

[0007]

In order to achieve the above object, a first aspect of the present invention is a field detecting device for detecting a ground fault in a field circuit in which a field winding of a generator and an excitation circuit are connected. In a ground fault relay, a DC voltage power supply connected between a negative electrode side of the field circuit and the ground via a predetermined resistor, a first filter circuit for extracting a DC voltage from a voltage across the resistor, and the field circuit A second filter circuit for extracting a DC voltage from a voltage between the negative electrode side and the ground, and determining whether a ground fault has occurred based on a ratio of an output voltage of the first filter circuit to an output voltage of the second filter circuit. A first judgment circuit, a second judgment circuit for judging the occurrence of a ground fault from the output voltage level of the first filter circuit, and respective judgment results of the first judgment circuit and the second judgment circuit are input. And logical AND circuit. To.

According to a second aspect of the present invention, there is provided the field grounding relay according to the first aspect, further comprising a logic that receives respective determination results of the first determination circuit and the second determination circuit in addition to the AND circuit. It is characterized by having a sum circuit.

According to a third aspect of the present invention, in the field earth fault relay according to the first aspect, the first determination circuit divides an output voltage of the second filter circuit by an output voltage of the first filter circuit. And a level determining circuit that determines that a ground fault has occurred when the output level of the dividing circuit has fallen below a predetermined value.

According to a fourth aspect of the present invention, in the field earth fault relay according to the first aspect, the first determination circuit divides an output voltage of the first filter circuit by an output voltage of the second filter circuit. And a level judging circuit for judging a ground fault when the output level of the dividing circuit exceeds a predetermined value.

According to a fifth aspect of the present invention, in the field grounding relay according to the first aspect, the first determination circuit amplifies an output voltage of the first filter circuit to a predetermined level; A level determining circuit that compares an output voltage of the circuit with an output level of the output voltage of the second filter circuit, and determines that a ground fault occurs when the output voltage of the amplifier circuit is equal to or higher than the output voltage of the second filter circuit; Characterized by the following.

According to a sixth aspect of the present invention, in the field grounding relay according to the first aspect, the first determination circuit amplifies an output voltage of the second filter circuit to a predetermined level; A level determining circuit that compares an output voltage of a circuit with an output level of an output voltage of the first filter circuit, and determines that a ground fault occurs when an output voltage of the amplifier circuit becomes equal to or less than an output voltage of the first filter circuit; Characterized by the following.

[0013]

[0014]

[0015]

[0016]

According to the first to sixth aspects of the present invention, even when the present invention is applied to a thyristor excitation type field circuit, erroneous detection and erroneous detection are not affected by the harmonic voltage contained in the excitation voltage. It is possible to provide a high-performance field-to-ground grounding relay without any problem.

[0017]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. FIG. 1 shows a first embodiment of the present invention (claims 1 and 2).
FIG. In the figure, A is a generator,
B is a generator field winding, C is an excitation circuit, and an excitation circuit C
A P-side field circuit D and an N-side field circuit E are connected between the motor and the generator field winding B. A field ground relay Ry1 for detecting a ground fault in the generator field circuit is connected between the field circuit E and the ground.

The field grounding relay Ry1 includes a resistor 2 connected in series with a predetermined DC voltage 1, a low-pass filter 3-1 for inputting a voltage between both ends of the resistor 2, a N-side field circuit E and a ground-to-ground connection. A low-pass filter 3-2 for inputting the output terminal voltage to
Level ratio determination circuit 4 for inputting the voltage output from each low-pass filter 3-1, 3-2, and level determination circuit 5 for inputting the voltage output from low-pass filter 3-1
And the determination result of the level ratio determination circuit 4 and the level determination circuit 5
And a logical product circuit 6 for inputting the result of the determination. The logical product circuit 6 outputs a ground fault detection result to the outside.

Next, the operation of this embodiment will be described.
If a ground fault occurs in the N-side field circuit E, a current obtained by dividing the DC voltage 1 by the sum of the resistance 2 and the ground fault resistance flows.
The voltage between the field circuit E on the side and the ground has a voltage level determined by the product of the current and the ground fault resistance, and the voltage across the resistor 2 has a voltage level determined by the product of the current and the resistance 2.

Here, assuming that the DC voltage 1 is V, the resistor 2 is R, the ground fault resistance is Rg, the voltage between the N-side field circuit E and the ground is V1, and the voltage across the resistor 2 is V2, V1 and V2 are It is represented by the following equation.

[0021]

(Equation 1)

The voltage across the resistor 2 is converted to a DC voltage by the low-pass filter 3-1, and the voltage between the N-side field circuit E and the ground is converted to a DC voltage by the low-pass filter 3-2. The circuit 4 determines the level ratio.

As a criterion for the level ratio determination circuit 4, the resistance 2 is a predetermined resistance value. When the resistance value of the ground fault to be detected becomes n times or less of the resistance 2, it is determined that a ground fault has occurred. In this case, the DC voltage between the N-side field circuit E input from the low-pass filter 3-2 and the ground is n times as large as the DC voltage across the resistor 2 input from the low-pass filter 3-1. It is determined that a ground fault has occurred if
The result of the determination is output as a logic signal.

For example, a logic "0" is output during a steady state where no ground fault has occurred, and a logic "1" is output when it is determined that a ground fault has occurred. Further, the level of the DC voltage output from the low-pass filter 3-1 to which the voltage across the resistor 2 is input is determined by the level determination circuit 5. The level determination circuit 5 determines that a ground fault has occurred when a voltage equal to or higher than a predetermined voltage level is input, and outputs a determination result as a logic signal, similarly to the level ratio determination circuit 4.

As the input voltage of the level determination circuit 5, since the voltage value of the DC voltage 1 and the resistance value of the resistor 2 are apparent values in advance, a DC voltage proportional to the current determined by the ground fault resistance is input. As a criterion, when the current flowing through the ground fault resistance is equal to or more than a predetermined value, it is determined that a ground fault has occurred.

However, the voltage across the resistor 2 and the voltage between the N-side field circuit E and the ground are basically DC, but when the excitation circuit C is of the thyristor excitation type, the DC voltage is used as the field voltage. In addition, harmonic voltage is generated and N
An AC voltage is induced in the voltage between the field circuit E on the side and the ground. Due to the influence of the induced AC voltage, the current flowing through the resistor 2 and the ground fault resistor includes an AC current in addition to the DC current, and the voltage across the resistor 2 and the voltage between the N-side field circuit E and the ground are: It does not become a complete DC voltage. The low-pass filters 3-1 and 3-2 are used to determine the level ratio and the level using this voltage as a DC voltage.

The AND circuit 6 includes a level ratio determination circuit 4
And the logic output of the level determination circuit 5 is both "1", the ground fault occurrence state is output to the outside as a ground fault occurrence state.

Further, when a ground fault occurs in the P-side field circuit D and the generator field winding B, the sum of the excitation voltage at the ground fault point and the DC voltage 1 is applied to the resistor 2 and the ground fault resistor. And the voltage across the resistor 2 becomes a voltage determined by the product of the current and the resistor 2. The voltage between the N-side field circuit E and the ground is the product of this current and the ground fault resistance. Is obtained by subtracting the excitation voltage at the ground fault point from the voltage determined by.

That is, the ratio between the voltage between both ends of the resistor 2 and the voltage between the N-side field circuit E and the ground is not equal to the ratio between the resistor 2 and the ground fault resistance. Is lower by the excitation voltage at the ground fault point, the determination level of the level ratio determination circuit 4 is higher in sensitivity than the ground fault of the N-side field circuit E.

FIG. 2 is a circuit diagram of a second embodiment (corresponding to claims 3 and 4) of the present invention. As shown in the drawing, this embodiment differs from the embodiment of FIG. 1 in that a field grounding relay Ry2 is used in place of the field grounding relay Ry1 of FIG. Since they are the same, the same portions are denoted by the same reference numerals, and redundant description will be omitted.

The field grounding relay Ry2 of the present embodiment connects a predetermined DC voltage 1 and a resistor 2 in series with the DC voltage 1, and connects the N-side field circuit E to the ground via the resistor 2. To the low-pass filter 3-1 and the output terminal voltage between the N-side field circuit E and the ground is input to the low-pass filter 3-2. The voltages output from the filters 3-1 and 3-2 are input to a division circuit 4-1 and the output of the division circuit 4-1 is output to a level determination circuit 4.
-2, the voltage output from the low-pass filter 3-1 is input to the level determination circuit 5, and the determination result of the level determination circuit 4-2 and the determination result of the level determination circuit 5 are input to the AND circuit 6. The AND circuit 6 is configured to output a ground fault detection result to the outside.

Next, the operation of the present embodiment will be described.
If a ground fault occurs in the N-side field circuit E, a current obtained by dividing the DC voltage 1 by the sum of the resistance 2 and the ground fault resistance flows.
The voltage between the field circuit E on the side and the ground has a voltage level determined by the product of the current and the ground fault resistance, and the voltage across the resistor 2 has a voltage level determined by the product of the current and the resistance 2.

Here, the DC voltage 1 is V, the resistance 2 is R, the ground fault resistance is Rg, the voltage between the N-side field circuit E and the ground is V1,
Assuming that the voltage between both ends of the resistor 2 is V2, V1 and V2 are represented by the following equations.

[0034]

(Equation 2)

The voltage between both ends of the resistor 2 is converted to a DC voltage by the low-pass filter 3-1, and the voltage between the N-side field circuit E and the ground is converted to a DC voltage by the low-pass filter 3-2. Input to 4-1.

The dividing circuit 4-1 uses a low-pass filter 3-2.
The input DC voltage between the N-side field circuit E and the ground is divided by the DC voltage across the resistor 2 input by the low-pass filter 3-1.
Judge level by -2.

As the level judgment reference of the level judgment circuit 4-2, the resistance 2 has an apparent resistance value in advance. If the determination is made, when the division value V1 / V2, which is the output voltage of the division circuit 4-1, falls below the voltage level of n (V1 / V2 ≦ n), it is determined that a ground fault has occurred, and the determination result is obtained. Is output as a logic signal.

For example, a logic "0" is output during a steady state where no ground fault has occurred, and a logic "1" is output when it is determined that a ground fault has occurred. Further, the level of the DC voltage output from the low-pass filter 3-1 to which the voltage across the resistor 2 is input is determined by the level determination circuit 5.

The level determination circuit 5 determines that a ground fault has occurred when a voltage equal to or higher than a predetermined voltage level is input, and outputs the determination result as a logic signal as in the level determination circuit 4-2.

Normally, the voltage value of the DC voltage 1 and the resistance value of the resistor 2 are obvious values in advance.
Is input as a DC voltage proportional to the current determined by the ground fault resistance, and it is determined that a ground fault has occurred when the current flowing through the ground fault resistance is equal to or greater than a predetermined value.

Here, the voltage across the resistor 2 and the voltage between the N-side field circuit E and the ground are basically DC,
When the excitation circuit C is of a thyristor excitation type, a harmonic voltage other than a DC voltage is generated as a field voltage, and an AC voltage is induced by the stray capacitance to a voltage between the N-side field circuit E and the ground.

Due to the influence of the induced AC voltage, the current flowing through the resistor 2 and the ground fault resistor includes an AC current in addition to the DC current, and the voltage across the resistor 2 and the N-side field circuit E
The voltage between the ground and the ground is not a complete DC voltage. The low-pass filters 3-1 and 3-2 are used to determine the level ratio and the level using this voltage as a DC voltage.

Here, FIG. 3 shows a circuit example of the low-pass filters 3-1 and 3-2. As shown in FIG. 3, the low-pass filter is a third-order low-pass filter combining a resistor 3a, a capacitor 3b, and an operational amplifier 3c.
Hz or less, 100 included in the input voltage Vin
The AC component above Hz is attenuated below -60 dB,
The DC component is output as the output voltage Vout without being attenuated.

Next, an example of input / output waveforms of the low-pass filters 3-1 and 3-2 is shown in FIG. As shown in FIG. 4, the input voltage Vin includes an AC voltage and a DC voltage whose fundamental cycle is a sixth harmonic component of the commercial frequency generated by thyristor excitation, and the output voltage Vout cuts the AC component. It is output as a DC voltage.

Referring again to FIG. 2, the logical product circuit 6 comprises the logic output of the level determination circuit 4-2 and the level determination circuit 5
Only when both of the logic outputs are "1", the ground fault occurrence state is output to the outside as a ground fault occurrence state.

When a ground fault occurs in the P-side field circuit D and the generator field winding B, the sum of the excitation voltage at the ground fault point and the DC voltage 1 is applied to the resistor 2 and the ground fault. The current divided by the added value of the resistance flows, the voltage between both ends of the resistance 2 becomes a voltage determined by the product of this current and the resistance 2, and the voltage between the N-side field circuit E and the ground is the current and the ground fault resistance. The voltage is obtained by subtracting the excitation voltage at the ground fault from the voltage determined by the product.

That is, the ratio between the voltage between both ends of the resistor 2 and the voltage between the N-side field circuit E and the ground is not equal to the ratio between the resistor 2 and the ground fault resistance. Is lower by the excitation voltage at the ground fault point, the determination level of the level ratio determination circuit 4-1 is more sensitive than the ground fault of the N-side field circuit E. Further, the current flowing through the resistor 2 also increases due to the effect of the excitation voltage at the ground fault point, so that the determination level of the level determination circuit 5 is higher in sensitivity than the ground fault of the N-side field circuit E.

As described above, according to this embodiment, even when the present invention is applied to a thyristor excitation type field circuit, it is not affected by the harmonic voltage included in the excitation voltage and has a high error-free and error-free detection. A high-performance field grounding relay can be realized.

Further, a relative value judgment result for judging the occurrence of a ground fault by a ratio of a voltage between both ends of a predetermined resistor to a voltage between both ends of the ground fault resistor, and an absolute value judgment of a ground fault current level based on a voltage level between both ends of the predetermined resistor. By satisfying the AND condition with the result, it is possible to realize a highly reliable field grounding relay with extremely few erroneous detections, and it is possible to directly handle the output of the field grounding relay as a cutoff command.

The dividing circuit 4 of the second embodiment shown in FIG.
Instead of dividing by -1, the voltage V2 across the resistor 2 input from the low-pass filter 3-1 is divided by the DC voltage V1 between the N-side field circuit E input from the low-pass filter 3-2 and the ground. When the voltage level output from the dividing circuit 4-1 exceeds the predetermined level (V2
/ V1 ≧ 1 / n) to determine that a ground fault has occurred.
By outputting a logic signal to the second embodiment, the same effect as in the second embodiment can be obtained.

FIG. 5 is a circuit diagram of a third embodiment (corresponding to claim 5) of the present invention. As shown in the figure, this embodiment is different from the embodiment of FIG. 1 in that the field grounding relay R of FIG.
in that a field grounding relay Ry3 is used in place of y1.
Since the other points are the same, the same portions are denoted by the same reference numerals, and redundant description will be omitted.

The field grounding relay Ry3 of this embodiment is shown in FIG.
In this embodiment, an amplifier circuit 4-3 and a level comparison circuit 4-4 are used instead of the level determination circuit 4 of the field grounding relay Ry1, and the other configurations are the same.

Next, the operation of this embodiment will be described.
The DC voltage V2 across the resistor 2 output from the low-pass filter 3-1 is amplified to a predetermined level by an amplifier circuit 4-3, and the DC voltage n · V2 output from the amplifier circuit 4-3 is amplified.
And a DC voltage V1 between the N-side field circuit E output from the low-pass filter 3-2 and the ground, and a level comparison circuit 4-4.
To compare the levels. When a DC voltage between the N-side field circuit E input from the low-pass filter 3-2 and the ground becomes equal to or higher than a voltage obtained by amplifying the DC voltage at both ends of the resistor 2 input from the amplifier circuit 4-3 by a predetermined factor. (N · V2 ≧ V1)
By determining that a ground fault has occurred and outputting a logic signal to the AND circuit 6, the same effect as in the embodiment of FIG. 1 can be obtained. Here, if the amplification degree of the amplifier circuit 4-3 is determined to be n times or less of the resistance 2, the amplification degree may be n times as large.

FIG. 6 is a circuit diagram of a fourth embodiment (corresponding to claim 6) of the present invention. As shown in the figure, this embodiment is different from the embodiment of FIG. 1 in that the field grounding relay R of FIG.
in that a field grounding relay Ry4 is used in place of y1.
Since the other points are the same, the same portions are denoted by the same reference numerals, and redundant description will be omitted.

The field grounding relay Ry4 of this embodiment is shown in FIG.
In this embodiment, an amplifier circuit 4-3 and a level comparison circuit 4-4 are used instead of the level determination circuit 4 of the field grounding relay Ry1, and the other configurations are the same.

Next, the operation of the present embodiment will be described.
N-side field circuit E output from low-pass filter 3-2
Amplifying circuit 4-3 amplifies (attenuates) the DC voltage V1 between the power supply and the ground to a predetermined level, and outputs the DC voltage V1 / n output from the amplifying circuit 4-3 and the resistor 2 output from the low-pass filter 3-1. DC voltage V2 at both ends of the level comparison circuit 4-4
To compare the levels. N input from the amplifier circuit 4-3
When the voltage obtained by amplifying (attenuating) the DC voltage between the field circuit E on the side and the ground by a predetermined factor is equal to or less than the DC voltage across the resistor 2 input from the low-pass filter 3-1 (V1 / n ≦ V
In 2), it is determined that a ground fault has occurred, and a logic signal is output to the AND circuit 6, whereby the same effect as in the embodiment of FIG. 1 can be obtained. Here, the amplification degree of the amplifier circuit 4-3 may be 1 / n times the amplification degree (attenuation), assuming that the ground fault is determined by n times or less of the resistance 2.

FIG. 7 is a circuit diagram of a fifth embodiment of the present invention. As shown in the figure, the present embodiment is different from the embodiment of FIG. 1 in that a field grounding relay Ry5 is used instead of the field grounding relay Ry1 of FIG. Since they are the same, the same portions are denoted by the same reference numerals, and redundant description will be omitted.

The field grounding relay Ry5 of this embodiment is shown in FIG.
A logical AND circuit 7 is added to the circuit configuration of the field grounding relay Ry1 of the first embodiment, and the logical output signal of the level ratio determining circuit 4 and the logical output signal of the level determining circuit 5 are transmitted to the logical OR circuit 7 in addition to the logical AND circuit 6. If any one of the logic signals is "1" (ground fault determination state), the logical sum circuit 7 can output a ground fault alarm command to the outside, and the respective logic signals are output. If both are "1" (ground fault determination state), the logical product circuit 6 outputs a cutoff command to the outside due to the occurrence of ground fault. This embodiment can also obtain the same effects as the embodiment of FIG.

FIG. 8 is a circuit diagram of a sixth embodiment of the present invention. As shown in the figure, the present embodiment differs from the embodiment of FIG. 1 in that a field grounding relay Ry6 is used instead of the field grounding relay Ry1 of FIG. Since they are the same, the same portions are denoted by the same reference numerals, and redundant description will be omitted.

The field grounding relay Ry6 of this embodiment is similar to the one shown in FIG.
The low-pass filter 3-1 and the low-pass filter 3-2 are omitted from the circuit configuration of the field grounding relay Ry1 of FIG.
And the voltage V1 between the N-side field circuit E and the ground.
Is directly input to the level ratio determination circuit 4 and the voltage V2 across the resistor 2 is directly input to the level determination circuit 5, and the voltage across the resistor 2 and the N-side field circuit E and the ground The level ratio between the voltages between both ends of the resistor 2 is determined by the level determination circuit 5.

In particular, when the level ratio determination of the level ratio determination circuit 4 and the level determination of the level
By providing a return delay time, even when an input voltage of the level ratio determination circuit 4 and the level determination circuit 5 includes an AC component, the output of the level ratio determination circuit 4 and the level determination circuit 5 does not become an intermittent output. By operating as described above, the same effect as the embodiment of FIG. 1 can be obtained. The present invention can be applied by appropriately combining the above-described embodiments and their application examples, and the same effects as those of the embodiment of FIG. 1 can be obtained.

[0062]

As described above, according to the first aspect of the present invention,
According to claim 6 , even when applied to a thyristor excitation type field circuit, a high-performance field ground fault free from erroneous detection and erroneous detection without being affected by the harmonic voltage included in the excitation voltage. A relay can be realized. Further, the output of the field grounding relay can be handled as a direct cutoff command.

[0063] Also, according to claim 1 of the present invention, according to claim 3 wherein
According to item 6 , a relative value judgment result for judging the occurrence of a ground fault by a ratio between a voltage between both ends of a predetermined resistor and a voltage between both ends of the ground fault resistor, and an absolute value judgment of a ground fault current level based on a voltage level between both ends of the predetermined resistor A highly reliable field grounding relay with extremely few erroneous detections can be realized by the resulting AND condition.

Further, according to the second aspect of the present invention, a relative value determination result determined by a ratio between a voltage between both ends of a predetermined resistor and a voltage between both ends of a ground fault resistor, and a ground fault current level based on a voltage level between both ends of the predetermined resistor. The ground fault occurrence warning command can be output based on the OR condition of the absolute value determination result of, and even if any of the ground fault determination units becomes abnormal, the ground fault occurrence warning command can be output, and It is possible to realize a highly reliable field grounding relay with extremely few non-detections.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram of a first embodiment of the present invention.

FIG. 2 is a circuit diagram of a second embodiment of the present invention.

FIG. 3 is a circuit diagram of a low-pass filter to which the present invention is applied.

FIG. 4 is an input / output waveform diagram of a low-pass filter circuit to which the present invention is applied.

FIG. 5 is a circuit diagram of a third embodiment of the present invention.

FIG. 6 is a circuit diagram of a fourth embodiment of the present invention.

FIG. 7 is a circuit diagram of a fifth embodiment of the present invention.

FIG. 8 is a circuit diagram of a sixth embodiment of the present invention.

[Explanation of symbols]

A: generator, B: generator field winding, C: excitation circuit, D:
P-side field circuit, E ... N-side field circuit, Ry1, Ry2
, Ry3, Ry4, Ry5, Ry6 ... field earthing relay, 1 ... DC voltage, 2 ... resistor, 3-1, 3-2 ... low-pass filter, 3a ... resistor, 3b ... capacitor, 3c ... operational amplifier, 4 ... Level ratio judgment circuit, 4-1 ... division circuit, 4-2 ...
Level determining circuit, 4-3: amplifying circuit, 4-4: level comparing circuit, 5: level determining circuit, 6: logical product circuit, 7: logical sum circuit.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02H 7/06 H02P 9/14, 9/30

Claims (6)

(57) [Claims] A field grounding relay for detecting a ground fault in a field circuit to which a field winding of a generator and an excitation circuit are connected,
A DC voltage power supply connected between the negative electrode side of the field circuit and the ground via a predetermined resistor; a first filter circuit for extracting a DC voltage from a voltage across the resistor; A second filter circuit that extracts a DC voltage from a voltage between the first filter circuit and a first determination circuit that determines whether a ground fault has occurred based on a ratio between an output voltage of the first filter circuit and an output voltage of the second filter circuit. A second determination circuit that determines the presence or absence of a ground fault from the output voltage level of the first filter circuit, and an AND circuit that inputs respective determination results of the first determination circuit and the second determination circuit. Field earth fault relay characterized by being constituted. 2. The field grounding relay according to claim 1, further comprising: an OR circuit that receives respective judgment results of the first judgment circuit and the second judgment circuit in addition to the AND circuit. Characteristic field grounding relay. 3. The field grounding relay according to claim 1, wherein the first determination circuit divides an output voltage of the second filter circuit by an output voltage of the first filter circuit, A field earth fault relay comprising a level judgment circuit for judging a ground fault when an output level of the division circuit becomes equal to or lower than a predetermined value. 4. The field grounding relay according to claim 1, wherein the first determination circuit divides an output voltage of the first filter circuit by an output voltage of the second filter circuit, And a level determining circuit for determining a ground fault when an output level of the dividing circuit is equal to or higher than a predetermined value. 5. The field grounding relay according to claim 1, wherein the first determination circuit amplifies an output voltage of the first filter circuit to a predetermined level; A level determination circuit that compares an output level of an output voltage of the second filter circuit and determines that a ground fault occurs when an output voltage of the amplifier circuit is equal to or higher than an output voltage of the second filter circuit. Characteristic field grounding relay. 6. The field grounding relay according to claim 1, wherein the first determination circuit amplifies an output voltage of the second filter circuit to a predetermined level; A level determination circuit that compares an output level of an output voltage of the first filter circuit and determines that a ground fault occurs when an output voltage of the amplifier circuit becomes equal to or less than an output voltage of the first filter circuit. Characteristic field grounding relay.