JPH0886827A - Partial discharge detector - Google Patents

Abstract

PURPOSE: To accurately detect a partial discharge signal of a cable to be measured by setting internal impedance of a power source at a specified ratio of a characteristic impedance of the cable to be measured to connect an AC power source to the cable to be measured. CONSTITUTION: The rising end 2 of a cable 1 to be measured is connected to one end on the primary side of a balance transformer 3 and the rising end of a dummy cable 4 to the other end on the primary side of the balance transformer 3. A power source 10 is connected to a midpoint on the primary side of the balance transformer 3. The power source 10 has an AC power source 6 connected in series to an impedance Zg. The AC power source 6 is an ideal power source and the impedance Zg is made equal to an internal impedance of the power source 10. Then, the internal impedance of the power source 10 is set a half a characteristic impedance of the cable 1 to be measured while an equivalent impedance of a load on a secondary side load as viewed from the primary side of the balance transformer 3 is selected to double the characteristic impedance of the cable 1 to be measured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a partial discharge detecting device for detecting a partial discharge signal generated inside an insulator of a cable.

[0002]

2. Description of the Related Art As a method for observing or predicting the state of deterioration of a power cable, a high DC voltage is applied to the cable to be measured, and the partial discharge waveform generated inside the cable insulator is detected and analyzed. There is a method. However, when the power cable is in use, an AC voltage is applied to the cable. Therefore, in order to investigate the partial discharge waveform generated in the actual line, it is ideal to apply an AC test voltage to the power cable.

However, the power cable generally constitutes a large-capacity capacitor, and the charging current flowing through the detection portion of the partial discharge detection device becomes large. Therefore, the detection impedance is limited due to a problem such as heat generation, and many problems occur such that noise of the power source is easily mixed. Conventionally, in order to solve these problems, a measuring method as shown in FIG. 2 has been introduced.

FIG. 2 is a connection diagram showing an example of a conventional partial discharge detection device. The cable to be measured 1 is terminated by a rising termination 2. The detection of the partial discharge signal is performed by the signal detection unit 7 connected to the secondary side of the balance transformer 3. The rising end 2 of the cable 1 to be measured is connected to one end of the primary side of the balance transformer 3. Also,
The rising end 5 of the dummy cable 4 is connected to the other end on the primary side of the balance transformer 3. Dummy cable 4
Is a cable having a characteristic impedance substantially equivalent to that of the cable under test 1. An AC power source 6 is connected to the middle point of the primary side of the balance transformer 3.

The device shown in FIG. 2 operates as follows. First, when the AC power supply 6 is turned on, a charging current is supplied to each of the measured cable 1 and the dummy cable 4 via the coil on the primary side of the balance transformer 3. If the dummy cable 4 is a healthy power cable, a normal charging current flows in the right half of the primary side of the balance transformer 3.
On the other hand, when a partial discharge occurs in the cable to be measured 1, a partial discharge signal is superimposed on the normal charging current in the left half on the primary side of the balance transformer 3. As a result, a differential current between the current flowing from the AC power supply 6 toward the cable under test 1 and the current flowing toward the dummy cable 4 is induced on the secondary side of the balance transformer 3. This difference current is the AC power supply 6
From the current flowing into the cable 1 to be measured, only the partial discharge signal excluding the normal charging current is obtained. The signal detector 7 receives this and analyzes the partial discharge signal.

[0006]

The conventional partial discharge detection device as shown in FIG. 2 has the following problems. That is, the dummy cable 4 is not always a perfectly sound cable. Therefore, the partial discharge signal generated inside the dummy cable 4 is also detected by the signal detector 7. However, since this signal has the opposite polarity to the partial discharge signal of the cable under test 1, it is easy to identify the two.

However, in FIG. 2, the partial discharge signal generated in the cable to be measured 1 is propagated to the dummy cable 4 via the primary side of the balanced transformer 3. On the other hand, the partial discharge signal generated inside the dummy cable 4 is propagated to the cable 1 to be measured via the primary side of the balance transformer 3.
Then, during the measurement, the partial discharge signals generated in the cable under test 1 and the dummy cable 4 are reflected back and forth mutually, and a mixed waveform of these signals is detected by the signal detection unit 7 on the secondary side of the balanced transformer 3. Therefore, the signal detection unit 7 cannot determine which of these partial discharge signals is generated from the cable to be measured 1 and there is a problem that the analysis becomes difficult. The present invention has been made in view of the above points, and an object thereof is to provide a partial discharge detection device capable of accurately detecting a partial discharge signal by connecting an AC power supply to a cable to be measured. Is.

[0008]

A partial discharge detection device according to the present invention comprises a balanced transformer, a signal detector connected to the secondary side of the balanced transformer, and a midpoint of the primary side of the balanced transformer. An AC power source connected to the balanced transformer, a measured cable connected to one end of the primary side of the balanced transformer, and a dummy cable equivalent to the measured cable connected to the other end of the primary side of the balanced transformer. In the frequency band including the partial discharge signal frequency of the cable to be measured, the internal impedance of the power source is set to ½ of the characteristic impedance of the cable to be measured, and it is viewed from the primary side of the balanced transformer. The equivalent impedance of the secondary load is selected to be twice the characteristic impedance of the cable to be measured.

[0009]

[Function] Since the input impedance of the four-terminal network composed of the balanced transformer including the secondary load and the power supply becomes equal to the characteristic impedance of the cable under test, it is generated in the cable under test and reaches the signal detection section. The partial discharge signal detected as described above is not reflected back and forth on the primary side of the balanced transformer. Therefore, the signal detecting section can clearly distinguish the partial discharge signal of the cable under measurement from other noises.

[0010]

The present invention will be described in detail below with reference to the embodiments shown in the drawings. FIG. 1 is a schematic view showing an embodiment of the partial discharge detection device of the present invention. The apparatus shown in the figure has substantially the same configuration as the conventional partial discharge detection apparatus shown in FIG. 2 in appearance. That is, the rising end 2 of the cable 1 to be measured is connected to one end of the primary side of the balance transformer 3, and the rising end 5 of the dummy cable 4 is connected to the other end of the primary side of the balance transformer 3. . In addition, at the midpoint of the primary side of the balance transformer 3, the power source 1
0 is connected. The power source 10 is configured such that an AC power source 6 and an impedance Zg are connected in series. This AC power supply 6 is an ideal power supply, and impedance Z
It is assumed that g is equal to the internal impedance of the power source 10. The present invention is characterized in that the power source impedance Zg is adjusted to a predetermined value. For this reason, the impedance of the adjustment resistor (not shown) is actually added to the impedance of the power source itself to obtain this Zg.

On the other hand, a signal detector 7 is connected to the secondary side of the balance transformer 3. In the present invention, when the equivalent impedance of the signal detecting section 7 viewed from the primary side of the balance transformer 3, that is, the secondary side load is ZL, this ZL is selected as a value described below. The characteristic impedance of the cable to be measured and the characteristic impedance of the dummy cable 4 are both selected as ZO.

First, the equivalent circuit of the balance transformer 3 is obtained using FIG. In (a) of the figure, and are terminals of the balance transformer 3. The impedance of the load seen from this terminal-is ZL as described above. Also,
The load impedance of the balanced transformer 3 seen from the terminal-is ZL / 4 according to the calculation in the figure. "1/2" takes into consideration the turn ratio of the terminal portion which is the intermediate tap of the balance transformer 3. The impedance seen from the terminal-is also ZL / 4. That is, the equivalent circuit of the balance transformer 3 which becomes ZL when viewed from-and ZL / 4 when viewed from-and-is as shown in FIG.

FIG. 4 shows an equivalent circuit of the circuit shown in FIG. 1 in which the equivalent circuit of the balance transformer 3 is added. In the figure, the terminal connecting the cable to be measured 1 is-, and the terminal connecting the dummy cable 4 is-. At this time,
The impedance Z viewed from the terminal- of this four-terminal network is as shown in the equation (1) in the figure. Here, in the present invention, the internal impedance Zg of the power source and the impedance ZL of the signal detecting portion are selected so that Zg = (ZL / 4) = ZO / 2.

By doing so, the driving point impedance of Z becomes ZO and the transfer impedance becomes "0", as shown in the equation (2) of FIG. Therefore, as shown in FIG. 5, the impedance ZO of the cable under test 1 becomes equal to the input impedance of this four-terminal network, and the partial discharge signal generated in the cable under test 1 connected to the terminal-is once detected by the signal detector. When it reaches 7 and is detected, it is not reflected in the original direction thereafter, and unnecessary round-trip reflection can be prevented. The same applies to the dummy cable 4 side.

With the above configuration, noise that is difficult to separate is not input to the signal detecting section 7. The present invention is not limited to the above embodiments. In the above-described embodiment, the power supply 10 is configured by using the AC power supply. However, even when the DC power supply is used as the power supply 10, the charging current having the AC component flows, and thus the similar partial discharge signal measurement can be performed. is there. The impedance values in the above description may all be in an appropriate frequency band including the partial discharge signal frequency of the cable. This is because it is sufficient to obtain impedance mapping in this band.

[0016]

The partial discharge detection apparatus of the present invention described above shows the measured cable, the dummy cable, the internal impedance of the power source, and the equivalent impedance of the secondary load as seen from the primary side of the balance transformer. , Since each value was selected to be an appropriate value in the frequency band including the partial discharge signal frequency of the cable to be measured, it is possible to prevent the partial discharge signal emitted from the cable to be measured from being reflected back and forth through the balanced transformer, and to make a true measurement. It becomes possible to clearly distinguish the partial discharge signal to be input, input it to the signal detection unit, and analyze it.

[Brief description of drawings]

FIG. 1 is a schematic view showing an embodiment of a partial discharge detection device of the present invention.

FIG. 2 is a schematic view showing an example of a conventional partial discharge detection device.

FIG. 3 is an equivalent circuit explanatory diagram of a balance transformer.

FIG. 4 is an equivalent circuit of the embodiment shown in FIG.

FIG. 5 is an explanatory diagram of impedance matching.

[Explanation of symbols]

 1 cable to be measured 2,5 rising end 3 balance transformer 4 dummy cable 6 AC power supply 7 signal detector 10 power supply

Claims (1)

[Claims] 1. A balance transformer, a signal detector connected to the secondary side of the balance transformer, an AC power source connected to a midpoint of the primary side of the balance transformer, and a balance transformer. A cable to be measured connected to one end on the secondary side and a dummy cable equivalent to the cable to be measured connected to the other end on the primary side of the balanced heisei unit are provided, and include a partial discharge signal frequency of the cable to be measured. In the frequency band, the internal impedance of the power source is set to ½ of the characteristic impedance of the cable to be measured, and the equivalent impedance of the secondary load viewed from the primary side of the balanced transformer is measured. A partial discharge detection device characterized by being selected to have twice the characteristic impedance of the cable.