JP3236770B2 - Partial discharge measurement method for CV cable line - 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 of measuring a partial discharge of a CV cable line by indirect calibration, and more particularly to a method of selecting an amplification frequency of a partial discharge detection signal which is important when performing quantitative measurement in an indirect calibration method. Things.

[0002]

BACKGROUND OF THE INVENTION As one method of testing the insulation state of a CV cable line, a partial discharge measurement method has conventionally been adopted. In the partial discharge measurement, a so-called direct calibration method in which a charge is directly injected between the conductor of the cable and the shielding layer to detect a discharge based on minute defects in the insulator, and a direct contact with the charged part as described above. For example, there is an indirect calibration method in which a foil electrode is provided on an outer peripheral portion of an insulated connection portion of a cable line, a known charge is injected using electrostatic coupling, and a partial discharge is detected. This indirect calibration method has advantages in that the line does not need to be dismantled, and measurement can be performed in a live state.

[0006] Quantitative measurement of partial discharge by the indirect calibration method requires certain conditions. FIG. 2 shows a partial discharge measurement circuit, wherein a represents the capacitance of the sample (the capacitance of the cable line insulator to be tested), k represents the capacitance of the coupling capacitor, and Zd represents the detection impedance. .
In the case of direct calibration, the charge Q is directly injected into the sample, and in the case of indirect calibration, the charge q is injected into both ends of the detection impedance Zd.
It can be expressed by the relational expression of (1 + a / k) q. Here, when a = k, Q = 2q, that is, as long as such a condition is satisfied, quantitative measurement can be performed by setting the charge q of the indirect calibration to half the charge Q at the time of the direct calibration.

[0004] This condition is referred to as a condition of the ratio between direct calibration and indirect calibration (hereinafter referred to as the direct ratio) = 2. In the case of a CV cable line, if this condition, that is, the condition of direct ratio = 2, is known, the indirect calibration is performed. In this method, quantitative measurement of partial discharge can be performed. For example, when performing partial discharge measurement by indirect calibration at an insulated connection of a CV cable line, the direct ratio is determined by the cable impedance Z on one side of the insulated connection.
1 and the direct-current ratio = 2 when Z1 = Z2, the indirect calibration method can be applied if Z1 = Z2 is satisfied. However, in order to satisfy Z1 = Z2, it is necessary that the cable lengths on both sides of the insulated connecting portion are equal, but generally the cable lengths are not the same.

In order to substantially satisfy Z1 = Z2, attempts have been made to select the amplification frequency of the detected partial discharge signal based on the convergence characteristics of the cable impedance. That is, by setting the amplification frequency to a frequency at which the cable impedance converges to the surge impedance Z0, and by setting Z1 = Z2 = Z0, the direct ratio = 2 is achieved.

FIG. 3 shows a 6.6k length of 250m and 350m.
A simulated line is constructed by providing a CV cable insulation connection part of V, and the detected partial discharge signal is amplified in a narrow band resonance at a frequency from 2 MHz to 20 MHz, and is changed while confirming the maximum and minimum of the response. Shows the calculated direct ratio. As shown in the drawing, the direct ratio = 2 in the high frequency region of 13 MHz or higher, and the maximum of the arithmetic series is obtained in the lower frequency region. The state where the direct ratio of 13 MHz or more is 2 corresponds to the convergence operation of the cable impedance. In this case, if the detection signal is amplified at an appropriate frequency of 13 MHz or more, measurement by indirect calibration with quantitative characteristics can be performed. Can do it.

[0007]

However, when the amplification frequency is selected in the high frequency region as described above,
Since the propagation attenuation becomes more pronounced at higher frequencies, there is a problem that the measurement sensitivity in the longitudinal direction of the cable line is reduced. For example, in a case where a partial discharge measurement is performed at a certain insulated connection part in an actual line, even if the partial discharge measurement at the insulated connection part can be performed with high accuracy, it is several hundred meters
The partial discharge signal at the linear connection part which is separated by a large distance is greatly attenuated, and a substantial measurement cannot be performed. The problem of attenuation in the longitudinal direction of the line can be solved by selecting a frequency of about several MHz as the amplification frequency. However, such a low frequency region is not in a stable state with the direct ratio = 2, and the quantification of indirect calibration is performed. A problem arises in terms of gender.

Accordingly, the present invention solves the above-mentioned problems, and in the partial discharge measurement of a CV cable line by the indirect calibration method, the partial discharge over a long distance of the line without being affected by propagation attenuation without impairing quantitative performance. It is an object to provide a method capable of performing a measurement.

[0009]

According to the method for measuring partial discharge of a CV cable line according to the present invention, a known charge is injected into a cable line from a detection end of the CV cable line by using electrostatic coupling. In the method of measuring a partial discharge by amplifying a signal detected by impedance, the detection signal is amplified with a resolution that is not affected by the overlap of reflected pulses in the line, and this amplified signal is a low-frequency signal in which the cable impedance does not converge. Analyzing the frequency spectrum in the region, selecting a specific frequency from the spectrum avoiding the resonance frequency depending on the cable length of the line, and performing partial discharge measurement using the frequency as the amplification frequency of the detection signal. It is a feature.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention has great significance in that, first, a detection signal is amplified with a resolution which is not affected by the overlap of reflected pulses in a line. According to our research,
When a measurement pulse is injected into a cable line, reflection of the pulse occurs at the connection part, etc., but the cable span between the connection parts of a general cable line is about several hundred meters, and the cable in the cable corresponding to this distance It has been found that the direct ratio greatly fluctuates at a period of about several hundred kHz due to the overlap of the reflected pulses. In addition, it has been found that the variation has a variation lower limit near the direct ratio = 2.

Conventionally, in this kind of partial discharge measurement method, the detection signal is amplified with a resolution of about 1 MHz for the convenience of measurement, but repetition of about several hundred kHz cannot be read at such a resolution. Therefore, in spite of the advantage that the propagation attenuation is small in the low frequency region where the cable impedance is not converged, the quantitativeness cannot be secured because the frequency at which the direct ratio is approximately 2 cannot be determined. It was excluded from the amplification frequency.

In view of the above findings, the present invention amplifies the detection signal with a resolution that is not affected by the overlap of reflected pulses in the line. The resolution is determined based on the span of the line or the like. However, in the case of a line having a cable span between the above-mentioned connecting parts of about several hundred meters, if the signal is amplified with a resolution of 100 kHz or less, preferably about 10 to 50 kHz, the line is directly converted. This is preferable because a period of about several hundred kHz of the interval ratio can be sufficiently read.

Next, the frequency spectrum of the signal thus amplified is analyzed in a low frequency region where the cable impedance does not converge. Thus, the fluctuation state of the direct ratio can be accurately grasped. The low-frequency region where the cable impedance is not converged indicates a frequency of about 10 MHz or less, as is clear from FIG. 3, but the present invention does not cover a high-frequency region where propagation attenuation occurs. Performing the following spectrum analysis in the low frequency region is substantially sufficient.

Then, an appropriate frequency is selected as an amplification frequency of the detection signal from the spectrum, but this is nothing but selecting a frequency where the condition of the direct ratio = 2 is substantially satisfied. As described above, in the low frequency region, the direct ratio fluctuates at a period of about several hundred kHz with a fluctuation lower limit around 2. The maximum point is the resonance frequency depending on the cable length of the line. In the present invention, the partial discharge measurement is performed by avoiding this resonance frequency and using a frequency at which the direct ratio is approximately 2 as the amplification frequency of the detection signal. In addition, 10M
There are a plurality of frequencies at which the state of direct ratio = 2 is established even in a low frequency region of less than or equal to Hz, but the higher the frequency, the more the above-mentioned problem of propagation attenuation occurs. Therefore, it is desirable to select a frequency as low as possible. 5-7MHz, especially 1
What is necessary is just to select in the range of 5 MHz. In a frequency region that is too low, there is usually no point where the direct ratio = 2, and it is not possible to select an amplification frequency in this region.

[0015]

An embodiment of the present invention will be described below in detail. FIG. 1 is a circuit diagram showing a case where the partial discharge measurement method according to the present invention is applied to the insulated connection portions IJ of the CV cables 11 and 12. The insulated connecting portion IJ is configured such that a reinforcing insulator is put on the connecting portion of the cable conductor, a shielding layer, an anticorrosion layer, and the like are provided thereon, and an insulating cylinder 3 of the shielding layer is further provided. Reference numerals 21 and 22 and 61 and 62 denote foil electrodes attached to the anticorrosion layer of the insulated connecting portion IJ.

The lead wires 2 are connected to the foil electrodes 21 and 22.
The other ends are connected to the detection impedance Zd. PG denotes a pulse generator, which is configured so that a known charge can be injected from other foil electrodes 61 and 62 to the CV cables 11 and 12 through lead wires 63 and 64 by utilizing electrostatic coupling. Have been.
In the present embodiment, the case where the detection foil electrodes 21 and 22 and the pulse injection foil electrodes 61 and 62 are separately provided in the insulating connection portion IJ is illustrated, but a pair of foil electrodes has both functions. It can also be done.

The detection impedance Zd is connected to an amplifier 4 such as a narrow band resonance amplifier for amplifying a voltage (detection signal) generated at both ends thereof with a resolution of 100 kHz. Sweeps the signal amplified in the range of 10 MHz, analyzes the frequency spectrum, selects an appropriate frequency that makes the direct ratio = 2 from this frequency range by calculation, and displays the detected signal waveform based on the amplified frequency. The vessel 5 is connected.

When actually performing the partial discharge measurement, first, a known charge is applied to the foil electrodes 61 and 62 by the pulse generator PG.
From the CV cable line using electrostatic coupling.
The injected pulse is detected by the other foil electrodes 21 and 22. Here, if the insulator is sound, a signal substantially equal to the injection pulse waveform will be detected at the detection impedance Zd. When a defect such as a protrusion exists, a minute discharge occurs in the insulator (partial discharge occurs), and this signal is detected by the detection impedance Zd in a form superimposed on the injection pulse waveform. Will be.

The signal containing the detected partial discharge pulse is amplified by the amplifier 4 at a resolution (100 kHz) which is not affected by the overlap of the reflected pulses in the line as described above. The range is swept, and the detection signal amplified at an appropriate frequency at which the direct ratio = 2 is displayed. According to this method, since the measurement is performed in the state of the direct ratio = 2, the same result as when directly calibrating with the calibration charge twice as much as the injected charge without exposing the cable conductor and the shielding layer is obtained. Obtainable.

Since a low frequency region of 10 MHz or less is used, propagation attenuation in the longitudinal direction of the cable line hardly occurs. Therefore, even in the case where there is a straight normal connection portion or a termination portion at a point several hundred meters away from the insulated connection portion IJ shown in FIG. 1, a partial discharge signal generated at these points can be measured with high sensitivity.

[0021]

According to the method for measuring partial discharge of a CV cable line of the present invention as described above, in the indirect calibration method, the detection signal is detected in a low frequency region which is not affected by propagation attenuation in the longitudinal direction of the cable line. Can be accurately selected so as not to impair the quantitativeness of the measurement. Therefore, there is an excellent effect that the partial discharge measurement over a long distance of the CV cable line can be performed with high sensitivity.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an example of a circuit configuration for implementing a method of the present invention.

FIG. 2 is a circuit diagram showing a general partial discharge measurement circuit.

FIG. 3 is a graph showing a measurement result of a direct ratio.

[Explanation of symbols]

 11,12 CV cable 21,22 Foil electrode for detection 23,24 Lead wire 4 Amplifier 5 Measuring instrument 61,62 Foil electrode for pulse injection IJ Insulated connection PG Pulse generator Zd Detection impedance

Claims (3)

(57) [Claims] 1. A method for measuring a partial discharge by injecting a known charge from a detection end of a CV cable line into a cable line using electrostatic coupling and amplifying a signal detected by a detection impedance at this time. In, while amplifying the detection signal with a resolution that is not affected by the overlap of reflected pulses in the line, and analyzing the frequency spectrum in a low frequency region where the cable impedance is not converged,
Selecting a specific frequency from the spectrum, avoiding a resonance frequency depending on the cable length of the line, and performing partial discharge measurement using the frequency as an amplification frequency of the detection signal, wherein the partial discharge is measured. Measuring method. 2. The method according to claim 1, wherein the detection signal is amplified with a resolution of 100 kHz or less and a frequency spectrum is analyzed. 3. The frequency spectrum analysis range is 1
The method for measuring partial discharge of a CV cable line according to claim 1, wherein the partial discharge is in a low frequency region of 0 MHz or less.