JP3049657B2 - Partial discharge measurement method - Google Patents

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 a 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 power cables, measurement sensitivity fluctuates due to the overlap of discharge pulse reflections, making it difficult to use narrow-band amplification to detect partial discharge.
Studies on high-frequency narrow-band amplification are being actively conducted because of the small influence of external noise.

[0003]

However, in the frequency band conventionally used, the impedance greatly changes, and the direct / indirect calibration ratio also fluctuates, so that the reliability of the measurement accuracy when measuring the partial discharge amount is high. Is not easily obtained.

[0004] It is an object of the present invention to provide a partial discharge measurement method that can perform charge calibration correctly and perform accurate measurement by using a relatively high frequency at which the impedance characteristics converge. .

[0005]

A partial discharge measuring method according to the present invention for achieving the above object uses a high frequency component such that the impedance characteristics of a power cable converge when measuring a partial discharge. Then, the amount of partial discharge in the power cable is measured.

[0006]

The partial discharge measuring method having the above-described configuration measures the amount of discharge in the power cable using a relatively high frequency component whose impedance characteristics converge.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to 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 opened,
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 α = 0 without loss, impedances Zf and Zs are pure reactances, respectively, and are expressed by the following equations. Zf = −jZo cot βL (4) Zs = jZotan 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)

Assuming that the wavelength is λ, the impedances Zf and Zs when a low frequency with negligible loss is input to the cable are, as can be seen from equations (4) and (5), the cable length L is an integer of λ / 4. The value alternates between zero and infinity each time the number doubles. In addition, when a high frequency with a large loss is input to the cable, the impedances Zf and Zs are expressed by Equation (1) and
Since cothγL and tanhγL 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 the frequency becomes Zo | c as the frequency increases.
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 the outline of the characteristics of the absolute values of the impedances Zf and Zs with respect to the frequency. Here, the solid line represents the absolute value of impedance Zf when the far end of the cable is opened, and the dotted line represents the absolute value of impedance Zs when the far end is short-circuited.

FIG. 2 is a schematic diagram of a 700 m
FIG. 7 is a graph chart showing characteristics of an absolute value of impedance with respect to a frequency of a test cable having a length of FIG. In the resistance voltage division method, a test cable is connected via a resistor, and the impedance of the test cable is determined from a shared voltage of the test cable. The impedance is 0.053 kΩ with respect to 1 V on the vertical axis in FIG. The solid line represents the case where the far end of the test cable is opened, and the dotted line represents the case where the far end is short-circuited. FIG. 3 is a graph showing the characteristics of the absolute value of the impedance of a test cable having a length of 100 m. FIG. 4 is a graph showing the characteristics of the absolute value of the impedance of a test cable having a length of 10 m. FIG.

As can be seen by comparing FIGS. 2 to 4, the characteristics of the absolute value of the impedance of the cable greatly change with the length and frequency of the cable. Also, as shown in FIG. 2, if the cable is sufficiently long and 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 a C of 1200 m using a resistance voltage dividing method.
FIG. 7 is a graph 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 indicates the case where the far end of the CV cable is opened, and the dotted line indicates 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 greatly changes, so that it is difficult to determine the ratio between direct and indirect calibration. However, if a frequency of 2 MHz or more at which the impedance converges is used, the impedance will be constant and calibration will be performed correctly. This frequency is determined by the attenuation characteristics and length of the cable, and eliminates the problems associated with reflection, as with infinite length lines and matching.

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

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

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

[0019]

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

[Brief description of the drawings]

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

FIG. 2 is a graph illustrating a characteristic of an absolute value of impedance with respect to a frequency of a 700 m test cable.

FIG. 3 is a graph illustrating characteristics of the absolute value of impedance with respect to the frequency of a 100 m test cable.

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

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

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

[Explanation of symbols]

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

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-167483 (JP, A) JP-A-2-167484 (JP, A) JP-A-3-138574 (JP, A) JP-A-4-167 84778 (JP, A) Yoshinori Tsunoda: "Impedance Characteristics and Partial Discharge Measurement of Power Cables" (Proceedings of the IEEJ Power and Energy Division Conference No. 342, published July 15, 1992) ( 58) Field surveyed (Int.Cl. ⁷ , DB name) G01R 31/12

Claims (1)

(57) [Claims] 1. A method for measuring partial discharge, comprising: measuring a partial discharge amount in a power cable using a high frequency component such that impedance characteristics of the power cable converge when measuring partial discharge. .