JP2000055974A - Measuring method for partial discharge at cable end connection part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure the partial discharge at end connection part neglecting connected electric apparatus. SOLUTION: Selecting frequency leading the impedance of an electric apparatus X to zero for measuring frequency and using this measuring frequency result in to be equal to shortening the end connection parts 12a and 12b and so the existence of electric apparatus X connected to the end connection parts 12a and 12b can be neglected. By impressing a testing voltage from the electric apparatus X side in this state, partial discharge generates near the end connection parts 12a and 12b and partial discharge signal obtained by extracting the signal of measuring frequency component selected by way of detecting foil electrodes 14a and 14b capable of regarding as capacitor and a signal detection circuit 15 can be detected with a measuring device 16. Even if external noises mix in both cables 11a and 11b, these noises are easily cancelled by differential connection of a balance circuit in the signal detection circuit 15 owing to the symmetry of the cables 11a and 11b so that only signal due to the discharge near the end connection parts 12a and 12b can be obtained with high sensitivity by the measuring device 16.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring a partial discharge at a cable terminal connecting portion to which an electric device is connected.

[0002]

2. Description of the Related Art For example, when a power cable is connected to an electric device such as a substation switchgear, it is necessary to grasp partial discharge characteristics of the entire system.
Electrical equipment and power cables have no problem because their discharge characteristics can be measured at the time of shipment from the factory, but the discharge characteristics at the terminal connection of the cable to the electrical equipment must be measured after construction.

[0003] Conventionally, for such partial discharge measurement of the terminal connection of a cable, a method of replacing the terminal connection of the cable with an insulated connection is known. In this case, if the impedance Z between the two phases is zero, the differential detection of the two phases is the same as that of the insulated connection, so that the terminals connected to the two-phase electric devices may be short-circuited.

[0004]

However, in reality, the work on site at the cable terminal connection is considerably large-scale, and an insulating layer or the like is formed, the electric equipment is separated therefrom, and the two-phase conductor is short-circuited. It is not easy to do. Therefore, it is desired to measure the discharge characteristics of the cable terminal connection part in a state where the electric equipment is connected.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a cable having a terminal connected to an electric device, a portion of the cable terminal connection capable of measuring a partial discharge characteristic of the terminal connection by ignoring the presence of the electric device using a specific frequency. An object of the present invention is to provide a discharge measurement method.

[0006]

According to the present invention, there is provided a method for measuring a partial discharge of a cable terminal connecting portion according to the present invention. A method for measuring a partial discharge, wherein a measurement frequency at which the impedance of the electric device connected between two-phase cables is close to zero or a minimum is obtained, and the measurement is performed on the anticorrosion layer near the terminal connection part of the two cables. The measuring foil circuit is attached to these detecting foil electrodes, the measuring circuit is connected to the detecting foil electrodes, and the calibrating foil electrodes are adhered to the detecting foil electrodes, respectively. Then, a test voltage is applied to the two cable termination connection portions, and a signal based on the measurement frequency component including the partial discharge of the two cables obtained by the detection foil electrode is forwarded by the measurement circuit. Extracting equilibrium from both cables, further calibrating this measurement signal using the calibration foil electrode and the calibration circuit, and measuring the magnitude of partial discharge at the terminal connection portion of both cables. I do.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the illustrated embodiment.

First, the impedance Z of the electric equipment X as viewed from the terminal to be subjected to the partial discharge measurement before the termination connection work is obtained, and the frequency characteristic (fZ characteristic) of the impedance Z is obtained.
Is evaluated to find a frequency f (usually several MHz) at which the impedance Z is almost zero, that is, it is considered that the terminals are short-circuited.

Note that the impedance Z being substantially zero means that the impedance Z is so small as to be negligible compared to the surge impedance of the power cable under test. For example, if the surge impedance of the cable is 20-
If it is about 30Ω, if it is smaller by one digit or more,
That is, if the impedance Z is about several Ω or less, it can be considered that the terminals are short-circuited.

In some cases, the impedance Z may not be considered to be zero as described above. In this case, a frequency at which the impedance Z becomes a minimum may be selected. Even with such a frequency, an approximate measurement is possible.

Frequency f at which impedance Z can be regarded as zero
Is used to perform partial discharge measurement ignoring the presence of the electric device X. In order to obtain this frequency f, before connecting a power cable to the electric device X, an oscillator 1 that outputs a variable frequency to the electric device X is connected to a resistor R and two lead wires 2 as shown in FIG. Connect via 3. Since the electric device X connected to the end of the power cable can be regarded as a multi-branch distributed constant system, the reflection characteristics vary depending on the frequency to be provided. That is, f in a multi-branch distributed constant
In the example of the -Z characteristic, the length of the specific section corresponds to the decrease in the impedance Z at the specific frequency. Therefore, the voltage V across the resistor R while changing the frequency from the oscillator 1
Then, the frequency f at which V ′ / V becomes sufficiently small, that is, the frequency f at which the impedance Z of the electric device X approaches zero is determined.

If the line constant of the electric device X is known, the frequency f can be calculated by a method called EMTP based on the known line constant, and this frequency f may be used.

FIG. 2 is a block diagram of an embodiment for implementing the method according to the present invention. The anticorrosion layer 13 in the vicinity of the terminal connection portions 12a and 12b of the two cables 11a and 11b connected to the electric device X which is a multi-branch distributed constant system.
a, 13b, the detection foil electrodes 14a, 1
4b, and these detection foil electrodes 14a, 14b
Is connected to a measuring device 16 via a signal detection circuit 15 for differentially detecting the output of the measuring device 16. In addition, the detection electrodes 14a, 14b
The calibration foil electrodes 17a and 17b are adhered to the vicinity of, and the calibration circuit 18 is connected to these calibration foil electrodes 17a and 17b.

By arranging the ring-shaped magnetic bodies 19a and 19b on the grounding wires of the insulating layers of the cables 11a and 11b, the impedance becomes high at a high frequency used as a measurement frequency in the present invention, and it is as if the insulating connection portions were provided. Is reproduced, while the ground state is ensured at the commercial frequency.

When the previously determined frequency f is selected as the measurement frequency, the terminal connection 1 is selected at the selected frequency f.
This is equivalent to a short circuit between the terminals 2a and 12b, so that the presence of the electric device X connected to the terminal connection portions 12a and 12b can be ignored.

Therefore, a test voltage of a commercial frequency can be applied from the electric device X side. By applying this test voltage,
If partial discharge occurs near the terminal connection portions 12a and 12b, the detection foil electrodes 14a and 14b can be regarded as capacitors.
, A discharge signal is obtained. Although various frequencies are mixed in the partial discharge, by extracting a signal of the selected measurement frequency component in the signal detection circuit 15, a portion generated in the vicinity of the terminal connection portions 12a and 12b ignoring the presence of the electric device X is extracted. Only the discharge signal is obtained and can be measured by the measuring device 16.

In a three-phase power cable, two-phase combinations as described above can be sequentially performed to repeatedly measure.

The cables 11a and 11b are externally supplied.
Noise is mixed into the cable 11a,
11b is easily canceled by the differential connection of the balance circuit in the signal detection circuit 15 due to the symmetry of the signal detection circuit 11b.
In the case of 6, only signals due to the discharge near the terminal connection portions 12a and 12b can be obtained with high sensitivity.

In addition, by injecting a known charge from the power supply of the calibration circuit 18 through the calibration foil electrodes 17a and 17b, indirect calibration is performed, that is, a discharge generated inside the cable is injected into an external detection unit. Perform calibration. By this calibration, the magnitude of the discharge signal obtained by the measuring device 16 can be properly evaluated.

The commercial frequency voltage can be applied from the electric equipment itself or from the far ends of a plurality of connected cables as in the embodiment. This voltage can be equivalent to that during operation.

[0021]

As described above, the method for measuring partial discharge at the cable terminal connection according to the present invention uses a measurement frequency that makes the impedance of the connected electric equipment zero.
Since the noise is removed by the differential detection operation and the indirect calibration is performed, the detection sensitivity of the partial discharge is improved, and the quantitative measurement can be performed.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of impedance measurement of an electric device.

FIG. 2 is a configuration diagram of an embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Oscillator 11a, 11b Cable 12a, 12b Termination connection part 13a, 13b Corrosion protection layer 14a, 14b Detection foil electrode 15 Signal detection circuit 16 Measuring instrument 17a, 17b Calibration foil electrode 18 Calibration circuit X Electric equipment

Claims (1)

[Claims] 1. A method for measuring a partial discharge of a cable terminal connection when an electric device is connected to the cable terminal connection, wherein the impedance of the electric device connected between two-phase cables is close to zero or Obtain the minimum measurement frequency, affix a detection foil electrode to the anticorrosion layer in the vicinity of the terminal connection portion of both cables, connect a measurement circuit to these detection foil electrodes, Affixing the calibration foil electrodes in the vicinity, connecting these calibration foil electrodes to the calibration circuit, applying a test voltage to the two cable termination connection portions, and obtaining the two cables obtained by the detection foil electrodes A signal based on the measurement frequency component including the partial discharge is extracted equilibrium from the two cables by the measurement circuit, and the measurement signal is further calibrated using the calibration foil electrode and the calibration circuit. And measuring a partial discharge magnitude at a terminal connection portion of the two cables.