JPH0692995B2 - Insulation monitoring method in low piezoelectric path - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for monitoring insulation of electric lines such as distribution lines, and more particularly to a method for monitoring the insulation of electric lines by utilizing the characteristics of harmonics of the electric lines themselves.

[0002]

2. Description of the Related Art In a power distribution line, for some reason, the insulation of a power line such as a power distribution line is deteriorated, which may lead to a ground fault and a ground fault relay may operate. When this ground fault relay operates, it takes a long time to investigate and restore the accident point. Therefore, there is a demand for measures to prevent the occurrence of the accident. For this purpose, various insulation monitoring devices that constantly monitor the insulation of electric circuits have been proposed.

Leakage current due to insulation deterioration (deterioration) can be detected by a zero-phase current transformer, but especially Δ and V
A charging current due to the capacitance to ground always flows in a one-wire grounding circuit such as a wire connection, and the current detected by the zero-phase current transformer always includes this charging current. Therefore, an error occurs in the current due to the actual deterioration of insulation, and it becomes particularly remarkable in the case of a slight decrease in insulation, which makes it impossible to perform accurate detection. It may be possible to obtain this by vector calculation from the normal zero-phase current and interphase current or line voltage, but since the phase of each phase is different, it is not possible to separately obtain the resistance component and the capacitance. .

In order to solve this, the conventional structure shown in FIG.
The method shown in (B) is adopted. That is, FIG. 3 (A)
Is applied to the second type ground wire 42 of the three-phase transformer 41 by the signal applying device 4
3, a low frequency signal of about 20 Hz is applied from the signal applying device 43, and the return of the signal is detected by the detecting device 44. In the figure, c is the capacitance of the electric path, and r is the earth-to-ground insulation resistance of the electric path.

Further, as shown in FIG. 3B, a signal applying line 45 is provided between the electric line 40 and the ground, the signal applying device 43 is provided on the signal applying line 45, and the detecting device 44 for detecting the return of the signal. There is also a method of providing a return signal to monitor the insulation state of the electric circuit.

[0006]

However, as shown in FIG. 3 (A), when a signal is applied to the second type grounding wire, an electric wire that meets the requirements of the electrical technical standards must be used for the grounding wire. However, this is not preferable because the entire insulation monitoring device becomes large and the ground resistance is affected.

In the method of FIG. 3B, there is a problem that the detection device cannot detect it because there is a return via the second type ground wire 42 on the transformer side.

In view of the above problems, it is an object of the present invention to provide an insulation monitoring system of this kind that does not require a signal application device and does not affect the definition of electrical technical standards.

[0009]

Means for Solving the Problems In the present invention, means for solving the above-mentioned problems are as follows: from a harmonic component existing in a low-voltage path grounded by a single wire, to a third nth harmonic current (where n is an integer). And harmonic voltage is detected, these detected current,
The current of the resistance of the electric circuit is obtained from the phase relation of the voltage, and the insulation state of the electric circuit is monitored by this current.

[0010]

When the insulation of the electric path is lowered, the current flowing through the ground wire increases. The third nth harmonic current component is extracted from this current, and the harmonic voltage component in phase with this is also extracted from the voltage of the electric path, and the current and voltage are A / D converted and vector-calculated by the arithmetic processing unit. , The resistance state is obtained and the current is constantly monitored to monitor the insulation state of the electric circuit.

Voltages and currents in electric lines such as distribution lines always contain harmonic components, and the third nth order (n
Since the harmonics have the same phase in each phase even in the three-phase circuit, the resistance component and the capacitance component caused by the harmonics have the same phase in each phase. Therefore, the same phase as the harmonic voltage can be taken out as a resistance.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on an embodiment shown in the drawings.

In a circuit such as a distribution line, a harmonic component is always present in the circuit itself, and the third nth (n is an integer) harmonic of this harmonic component is in-phase with each phase in the three-phase circuit. Become. The present invention has been made paying attention to this point.
FIG. 1 is a circuit diagram of an embodiment of the present invention, which will be described in detail below with reference to FIG.

In FIG. 1, reference numeral 1 is a transformer for supplying electric power to a three-phase electric line 10 of a low voltage distribution line. Reference numeral 2 is a second type grounding wire, 3 is a current detecting section, the current detecting section 3 is a current transformer CT provided in the grounding wire 2, and a band for detecting a harmonic current component from the detected current of the current transformer CT. It consists of a pass filter F _B1 and an amplifier A _P. The harmonics may be the 3nth order (n is an integer), that is, any of the 3rd, 6th, and 9th order harmonics, but the lower order harmonics are easier to detect, especially the 3rd order harmonics. The third harmonic component is used in the present embodiment because the most components are contained and are easy to detect. Therefore, the bandpass filter F _B1 is a filter that passes only the third harmonic current.
Reference numerals 4 and 5 respectively denote band-pass filters F _B2 and F _B3 for detecting the third harmonic voltage component from the R-phase and S-phase of the electric path 10 and an amplifier A _P for amplifying the output of the band-pass filter. . These detectors 3, 4,
The signals detected by the band pass filters F _B1 , F _B2 , and F _B3 of _No. 5 are appropriately amplified by the amplifier A _P , and the A / D conversion device 7
Is converted into a digital signal by. 8 is an arithmetic processing unit, which is A
Each digital signal from the D / D converter 7 is input, the vector operation described later is performed to calculate the resistance current, and the insulation resistance of the electric circuit is calculated. When the insulation resistance value is below a predetermined value, an alarm and display are given. Alarms and displays are provided via the section 9.

FIG. 2 is a vector diagram of the above detected currents and voltages. _V'R3 and _V'S3 are the third harmonic voltage detected from the R-phase and S-phase voltages V _R3 and V _{S3 of} FIG. Is. Since the third harmonic are in phase with each phase of the three-phase, V _'R3,
V'S3 is in phase. I ′ _03C is the third harmonic current of the current I _03C flowing due to the electrostatic capacity of the electric path, which is a 90 ° advance current with respect to the third harmonic voltage. _I'03
Is a third harmonic current of the current I ₀₃ flowing through the ground line 2 and increases when the insulation degree of the electric path decreases. I _03r is a resistance component current flowing due to the third harmonic, which has the same phase as the harmonic voltage. The detection current I ′ ₀₃ and the detection voltage V ′ _R3 (or V ′ _S3 )
And the phase angle θ of the detected current I ′ ₀₃ is calculated by vector calculation.

The third harmonic in the three-phase electric circuit is generated in one cycle at an electrical angle of 60 degrees of the fundamental wave in each phase, so that all the third harmonics in each phase are in phase. Therefore, since the resistance component and the capacitance component that flow due to the third harmonic have the same phase in each phase, the same phase component as the third harmonic voltage can be extracted as the resistance component.

In addition, the reason why the two voltage detecting sections are provided is that the voltage value of the third harmonic may be different depending on each phase even if the phases are in phase, so that when the resistance value is displayed, that value is displayed. Is calculated by (V ′ _R3 + V ′ _S3 ) / I _03r , and an error will occur if calculation is made with only one phase voltage, and this error is eliminated. Therefore, if the voltage value is the same for each phase, or only the current value is displayed, or if only the phase relationship needs to be known, only one voltage detection unit is required.

Further, since the voltage detection of the voltage detection units 4 and 5 is divided by the resistors R ₁ and R ₂ and detected, the current of this resistance is included in the detection current I ₀₃ , so the resistance value is In the case of displaying, the resistance values of R ₁ and R ₂ are known in advance, and therefore the calculation processing unit may subtract the resistance values.

In the above embodiments, the case where the third harmonic contained in a large amount in the electric path is used has been described. However, since the third nth harmonic is all in phase, it is not limited to the third harmonic and is easy to extract. The 3nth harmonic may be appropriately selected and used.

In the case of the neutral grounding of the Y connection, the capacitance to ground can be balanced, and the current for the capacitance is canceled out. Therefore, it is not necessary to apply the present invention. It is applicable when it occurs. However, in this case,
When the resistance value is to be displayed, the voltage detector of FIG. S. T Must be provided for each phase.

[0021]

As described above, according to the present invention, a harmonic component is always present in the electric line.
Taking advantage of the fact that the second harmonic is in phase with each phase, and that the resistance and capacitance of the current flowing between each phase and the ground are in phase respectively and the resistance and capacitance can be separated, Since the insulation status is monitored, the conventional signal application device is not required, and since no signal is applied to the ground wire, the ground wire can be used as it is in accordance with the regulations of the electrical engineering standards. Therefore, the insulation monitoring device can be manufactured at low cost and the construction cost can be reduced. Further, since it does not affect the ground resistance, ideal insulation monitoring can be realized.

[Brief description of drawings]

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

FIG. 2 is a vector diagram for explaining the present invention.

FIG. 3 is an explanatory diagram of a conventional insulation monitoring method.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Transformer 2 ... Ground wire 3, 13 ... Current detection part 4, 5, 14 ... Voltage detection part 7 ... A / D conversion device 8 ... Arithmetic processing part 9 ... Alarm and display part 10 ... Electric circuit F _{B1 to} F _B8 … Bandpass filter

Claims (1)

[Claims] 1. A method for monitoring insulation of a one-wire grounded circuit,
From the current flowing in the ground wire and the voltage of the electric line, the 3rd nth harmonic component (where n is an integer) that is inherent in the electric line is detected, and the current of the resistance component is detected from the phase relationship between the detected harmonic current and harmonic voltage Insulation monitoring method in a low piezoelectric path, characterized in that the insulation state of the electric circuit is monitored by this current.