JPH0634698A - Partial discharge measurement method - Google Patents

Abstract

PURPOSE:To calibrate electric charge properly and measure partial discharge accurately by using a relatively high frequency where the impedance characteristics of a power cable converge. CONSTITUTION:A detection impedance Zd is connected to an insulation connection part Sa of a power cable S. Also, a pulse generator G is connected to the connection part Sa via a capacitor C. The cable S is pulse-wise divided into two parts by the impedance Zd and impedances at both sides of the connection part Sa are expressed by impedances Z1 and Z2, respectively. When calibrating electric charge of the cable S, a known amount of electric charge is supplied from the generator S. In this case, the impedance Zd becomes constant when the partial discharge is performed using a frequency which is 2kHz or higher where impedance characteristics are converged, thus performing calibration properly without being affected by the reflection within the cable S on calibration.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a partial discharge measuring method capable of accurately measuring the partial discharge amount of a power cable.

[0002]

2. Description of the Related Art Conventionally, partial discharge measurement has been studied as an initial defect detection of a CV cable line. In the power cable, the measurement sensitivity fluctuates due to the overlap of the reflection of the discharge pulse, and narrow band amplification was difficult to use for the detection of partial discharge.
High-frequency narrowband amplification is being actively studied because it is less affected by external noise.

[0003]

However, in the frequency band conventionally used, the impedance changes greatly, and the direct / indirect calibration ratio also changes, and the reliability of the measurement accuracy when measuring the partial discharge amount. But I can't get it.

An object of the present invention is to provide a partial discharge measuring method capable of accurately performing charge calibration by using a relatively high frequency with which the impedance characteristic converges and making an accurate measurement. .

[0005]

The partial discharge measuring method according to the present invention for achieving the above object uses a high frequency component to the extent that the impedance characteristic of the power cable converges when measuring the partial discharge. Then, the partial discharge amount in the power cable is measured.

[0006]

In the partial discharge measuring method having the above-mentioned structure, the amount of discharge in the electric power cable is measured by using the relatively high frequency component where the impedance characteristic converges.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail based on the illustrated embodiments. If the characteristic impedance of a cable such as a power cable is Zo, the propagation constant is γ, and the cable length is L, the impedance Zf at the near end when the far end of the cable is open,
And the impedance Zs at the near end when the far end is short-circuited is expressed by the following equation. Zf = Zo coth γL (1) Zs = Zo tanh γL (2) Here, the propagation constant γ is a complex number and is represented by the following equation. γ = α + jβ (3)

When there is no loss and α = 0, the impedances Zf and Zs are pure reactances, which are expressed by the following equation. Zf = −jZo cot βL (4) Zs = jZo tan βL (5) The characteristic impedance Zo is equal to the geometric mean of the impedances Zf and Zs and is expressed by the following equation. Zo ＝ （Zf ・ Zs） ^1/2・ ・ ・ (6)

Impedances Zf and Zs when a low frequency in which loss is negligible are input to the cable, assuming that the wavelength is λ, as can be seen from the equations (4) and (5), the cable length L is an integer of λ / 4. It takes a value of zero and infinity alternately as it doubles. In addition, the impedances Zf and Zs when a high frequency with large loss is input to the cable are given by equation (1) and
Since coth γL and tanh γL in the equation (2) converge to 1, both converge to the characteristic impedance Zo.

As a result, the characteristic of the absolute value of the impedance Zf with respect to frequency becomes Zo | c as the frequency becomes higher.
It can be seen that otβL | converges to Zo, and the characteristic of the absolute value of the impedance Zs converges from Zo | tanβL | to Zo.

FIG. 1 is a graph showing an outline of characteristics of absolute values of impedances Zf and Zs with respect to frequency. Here, the solid line represents the absolute value of the impedance Zf when the far end of the cable is open, and the dotted line represents the absolute value of the impedance Zs when the far end is short-circuited.

FIG. 2 shows an actual measurement of 700 m using the resistance voltage dividing method.
It is a graph figure when the characteristic of the absolute value of impedance with respect to the frequency of the sample cable of length is measured. The resistance voltage dividing method is a method in which a test cable is connected via a resistor and the impedance of the test cable is obtained from the voltage shared by the test cable. The impedance is 0.053 kΩ with respect to the vertical axis of 1 V in FIG. The solid line represents the case where the far end of the sample cable is open, and the dotted line represents the case where the far end is short-circuited. Further, FIG. 3 is a graph diagram when measuring the characteristic of the absolute value of the impedance of the test cable having a length of 100 m, and FIG. 4 is the graph showing the characteristic of the absolute value of the impedance of the test cable having a length of 10 m. It is a graph figure at the time of doing.

As can be seen by comparing FIGS. 2 to 4, the characteristic of the absolute value of the impedance of the cable changes greatly with the length and frequency of the cable. Further, when the cable is sufficiently long as shown in FIG. 2, if a sufficiently high frequency is input to the cable, the impedance value converges to a certain value Zo as shown in FIG.

FIG. 5 shows C of 1200 m using the resistance voltage dividing method.
It is a graph chart when the characteristic of the absolute value of the impedance with respect to the frequency of the V cable is measured, and the impedance is 0.053 kΩ with respect to 1 V on the vertical axis. The solid line represents the case where the far end of the CV cable is open, and the dotted line represents the case where the far end is short-circuited. In this case, it can be seen that the impedance value converges at a frequency of about 2 MHz.

When charge calibration is performed using a relatively low frequency at which the impedance does not converge, the impedance changes greatly, so it is difficult to determine the ratio of direct / indirect calibration. However, if a frequency of 2 MHz or more where the impedance converges is used, the impedance becomes constant and the calibration is performed correctly. This frequency is determined by the attenuation characteristics and length of the cable, and the problems associated with reflection are eliminated, as is the case with infinite lines and matching conditions.

That is, if the partial discharge is measured by using the frequency component of 2 MHz or more, the impedance becomes constant, so that the calibration can be performed correctly without being affected by the reflection in the cable.

FIG. 6 is a circuit configuration diagram for calibrating the power cable S. The detection impedance Zd is connected to the insulated connection Sa of the power cable S. In addition, a pulse generator G is connected to the insulating connection Sa through a capacitor C.
Connect. The power cable S is pulse-divided into two by the detection impedance Zd, and the impedances on both sides of the insulating connection portion Sa are expressed as impedances Z1 and Z2, respectively. When performing charge calibration of the power cable S, a known charge amount is injected from the pulse generator G and calibration is performed.

At this time, if the impedances Z1 and Z2 are equal, the response of the pulse connected in parallel with the detection impedance Zd for indirect calibration is the impedance for direct calibration.
It doubles the response when injected into either Z1 or Z2.
The actual discharge occurs at the direct calibration position Z1 or
It is the part of Z2, and the detection impedance Zd
The charge of half the size injected in parallel with can be substituted. This is the principle of indirect calibration, and it is necessary to make impedances Z1 and Z2 equal so that the response of indirect calibration is twice the response of direct calibration. Therefore, the frequency at which the impedance is constant is selected as the measurement frequency. To do.

[0019]

As described above, the partial discharge measuring method according to the present invention can perform charge calibration properly by using a relatively high frequency component in which the impedance characteristic of the power cable converges, and further the high frequency narrowing can be performed. Since the frequency component of the band is used, the influence of external noise is small.

[Brief description of drawings]

FIG. 1 is a graph showing characteristics of impedance absolute value with respect to frequency.

FIG. 2 is a graph diagram when characteristics of absolute value of impedance with respect to frequency of a test cable of 700 m are measured.

FIG. 3 is a graph diagram when characteristics of absolute value of impedance with respect to frequency of a 100 m test cable are measured.

FIG. 4 is a graph when the characteristics of the absolute value of impedance with respect to the frequency of the 10 m test cable are specified.

FIG. 5 is a graph when the characteristics of the absolute value of impedance with respect to the frequency of a 1200 m CV cable are measured.

FIG. 6 is a circuit configuration diagram when performing calibration.

[Explanation of symbols]

 S Power cable Sa Insulation connection C Capacitor G Pulse generator Z1, Z2 Impedance Zd Detection impedance

Claims (1)

[Claims] 1. A method for measuring partial discharge, which comprises measuring a partial discharge in the power cable by using a frequency component having such a high frequency that an impedance characteristic of the power cable converges when measuring the partial discharge. .