JP3034651B2 - Diagnosis method for insulation of CV cable - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CV cable insulation diagnostic method for diagnosing the degree of insulation deterioration of a CV cable caused by water tree or the like under a live line.

[0002]

2. Description of the Related Art Generally, the insulation performance of an electric insulator is deteriorated due to aging of a power cable after installation. In particular, C
It is known that V-cables have dendritic cracks in the cross-linked polyethylene insulator, and so-called water trees that cause water to enter the cracks are the main causes of insulation deterioration. Such a decrease in insulation performance may progress if left unattended, and eventually lead to a large dielectric breakdown accident. Therefore, it is extremely important to grasp the change in the insulation resistance of the cable and to detect the deterioration at an early stage. For this reason, various insulation measurement methods have been conventionally known.In particular, in recent years, several methods for diagnosing in a live state without stopping power transmission at the time of measurement have been proposed. It is attracting attention because of its many advantages.

[0003] As a method of performing such constant monitoring,
Conventionally, a DC voltage is superimposed on a transmission AC voltage as described in, for example, Japanese Patent Publication No. 60-8465, and the insulation resistance of the cable is determined from the DC component of the cable leakage current detected as a result. The so-called DC superposition method or the so-called tan δ method of measuring a transmission voltage waveform and a current waveform and obtaining and evaluating a dielectric loss tangent as described in JP-A-60-185171 is generally used. . In particular, in the case of a CV cable, Japanese Unexamined Patent Publication No.
Assuming that the water tree has a current rectifying function in Japanese Patent Publication No. 202075, a method of measuring the direct current component of the cable leakage current during AC power transmission and estimating the distribution, length and volume of the water tree from the direction and the absolute value is known. It has been disclosed.

[0004] However, none of the above-mentioned known conventional methods can sufficiently satisfy the demand for early and accurate detection of insulation deterioration. That is, the DC superposition method described above is generally considered to have poor detection sensitivity with respect to the degree of deterioration, and there is a problem that detection is not performed unless the deterioration is extremely severe. Also several tens of volts during measurement
Therefore, a DC power supply needs to be separately prepared. On the other hand, although the tan δ method detects deterioration over the entire cable, it is known that the detection sensitivity to local deterioration like water trees is poor.

Further, in the case of JP-A-59-202075, which utilizes the rectifying action of a water tree, a so-called inner water conduction tree generated from the conductor side of the cable insulator and an outer water conduction tree generated from the sheath side are: Since the generated DC currents have opposite polarities, when both types of water trees are simultaneously generated, the detected DC currents cancel each other out, and there is a possibility that sufficient measurement cannot be performed.

The inventors of the present invention have studied the water tree phenomenon based on the above-described problems, and found that an AC voltage was applied to a power cable to be measured, and the amplitude of the applied AC voltage was gradually increased from zero. If the amplitude of the applied AC voltage reaches a certain value when a quasi-DC component of 10 Hz or less is detected in the grounding line current in the course of the process, it is based on the pulse noise superimposed on the AC voltage. It has been found that a pulsating current is detected, that is, the pulsating current is generated in a deteriorated portion of a cable insulator by being stimulated by a change in pulse noise of a line accompanying a live line, and its magnitude is It is related to the size of the water tree.

[0007]

Therefore, the present inventors have previously proposed a new insulation deterioration diagnosis method as Japanese Patent Application No. 1-167910, but there are the following problems in performing insulation deterioration diagnosis. It has been found. That is, the degree of deterioration of the insulator is determined based on the magnitude of the pulsating current detected from the ground line current. However, since the pulsating current signal is a minute signal, it is extremely difficult to evaluate. Further, if the pulsating current is simply amplified to facilitate the evaluation, the noise current component is also amplified at the same time, which also makes the evaluation difficult. It has also been found that the obtained pulsating current is affected by the magnitude of the noisy pulsating current which is originally superimposed on the AC power supply applied to the cable, and that it is necessary to correct it.

It is an object of the present invention to provide a CV cable insulation diagnosis method that can more reliably detect insulation deterioration by using the above-described pulsating current.

[0009]

According to the present invention, there is provided a method for diagnosing insulation of a CV cable, comprising the steps of: applying an AC voltage having a basic AC frequency from a power supply to a power cable to be measured; In a CV cable insulation diagnosis method for detecting a ground wire current flowing through a cable insulator from a ground wire of a cable and detecting a degree of deterioration of the cable insulator, an AC component of the basic AC frequency is removed from the ground wire current. A first step of extracting only a signal value of a specific low frequency via an amplifier, and removing an AC component of the basic AC frequency from the AC voltage and extracting a magnitude of a signal value of the specific low frequency component. Adjusting the amplification of the amplifier in the first step according to the second step and the magnitude of the signal value obtained in the second step; Correcting the voltage variation, and having a third step of detecting the degree of deterioration of the cable insulation based on a signal value obtained by the first step.

[0010]

According to the CV cable insulation diagnostic method having the above-described structure, an AC voltage is applied to a power cable, an AC component is removed from a ground wire current obtained from a ground wire, a specific low frequency component is detected, and a power supply is detected. And removes the AC component to obtain a similar specific low-frequency component and correct it.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method according to the present invention will be described in detail with reference to the illustrated embodiment. FIG. 1 is a block diagram for implementing the deterioration diagnosis method of the present invention. In the figure, reference numeral 1 denotes a test power cable such as a CV cable to be measured, which has a capacitance Ck between the conductor 1a and the shielding layer 1b. S is an AC power supply, one end of which is grounded and the other end of which is a test power cable 1
To the side of the conductor 1a. A current transformer 2 is coupled to a ground line between the shield layer side 1b of the test power cable 1 and the ground, and the output of the current transformer 2 is a first low-pass filter 3, a first resonator 4, a variable It is connected to a display 6 via a gain amplifier 5. Further, a transformer 7, a second low-pass filter 8, and a second resonator 9 are connected to the AC power supply S, and an output of the second resonator 9 is connected to a gain control input terminal of the variable gain amplifier 5. Note that, instead of the current transformer 2, a resistor may be inserted into the ground line to perform detection using the resistor.

When a voltage is applied to the test power cable 1 from the AC power supply S, a charging current flows through the capacitance Ck of the cable, and a grounding current i flows from the shielding layer 1b side to the ground through a grounding line. Will flow. The current transformer 2 detects a ground line current i from the ground line, and outputs a first low-pass filter 3.
50 Hz or 60 Hz applied from the ground line current i.
An AC component consisting of a basic AC frequency of Hz is removed, and only a low-frequency pulsating component is extracted.

As the first low-pass filter 3, for example, a constant K-type filter is used. The basic AC component is removed from the ground line current i generated by the application of the AC power supply S to the cable conductor 1a, and only the pulsating current component is removed. And outputs this output to the narrow-band type first resonator 4. The first resonator 4 extracts only a specific low frequency component from the pulsating flow component.

As the first resonator 4, various narrow-band type resonators can be used. For example, a parallel T-type CR circuit often used in a low frequency band can be used. This parallel T-type CR feedback amplifier circuit (Twin-T circuit) has a resistance value of 2
It consists of a parallel circuit of a T-type circuit consisting of two resistors Ra, Ra and a capacitor Cb, and a T-type circuit consisting of two capacitors Ca, Ca of the same capacity and a resistor Rb, wherein the capacitor Cb and the resistor Rb each have Cb = 2.・ Ca, Rb = Ra / 2.

The parallel T-type CR circuit configured as described above has a frequency characteristic that allows a frequency signal of f = 1 / 2π · Ca · Ra to pass as a maximum signal. Only one signal of a certain specific frequency can be extracted from the flow components. Further, this signal is amplified by a variable gain amplifier 5 and transmitted to a display 6 such as a pen graph or an oscilloscope.

At the same time, the AC power source S is connected to the second low-pass filter 8 via the transformer 7 to remove the applied AC component, and the second resonator 9 extracts the same specified signal extracted by the first resonator 4. The frequency is extracted, and the gain of the variable gain amplifier 5 is adjusted with a signal proportional to the magnitude. That is, the second
If the output of the resonator 9 is large, the gain of the variable gain amplifier 5 is adjusted to be small, and if it is small, the gain is adjusted to be large.

FIG. 2 is a graph showing the output voltage of the variable gain amplifier 5 with respect to the applied voltage. The horizontal axis indicates the applied voltage, and the vertical axis indicates the current I. When an AC voltage is applied to the test power cable 1, an electric charge corresponding to the applied AC voltage is induced in the shielding layer 1b by electrostatic coupling with the conductor 1a. The ground line current i that fluctuates at a cycle substantially equal to the frequency of the voltage flows. In addition, when the water tree deterioration is present in the insulator 1c, the above-described pulsating current is superimposed. Become. The pulsating current is a current other than the current depending on the applied AC voltage as described above, and is considered to be generated in the deteriorated portion due to the pulsed noise superimposed on the AC power supply S, and is lower than the frequency of the applied AC voltage. Frequency current. Usually, the frequency of the applied AC voltage is 50 Hz or 60 Hz.
If the circuit of the first low-pass filter 3 is designed to pass a frequency signal of not more than 60 Hz or 60 Hz, only the pulsating component can be detected from the ground line current i. The frequency of the pulsating flow component to be detected is arbitrary, but the higher the frequency, the more disadvantageous in terms of ground capacity. Therefore, the frequency is preferably about 5 Hz or less, and for example, 1 Hz is suitable.

Since the magnitude of the pulsating flow component increases as the degree of water tree deterioration increases, the degree of water tree deterioration can be estimated by obtaining this magnitude.
For example, in a sound test power cable 1 in which a water tree does not exist in the cable insulator 1c, the magnitude of the current obtained by the display 6 is substantially constant as indicated by A regardless of the voltage change. On the other hand, when the water tree is present, the current value increases as shown in B as the voltage increases.

In this case, when the gain of the variable gain amplifier 5 is fixed, the value obtained by the display 6 is changed according to the influence of the magnitude of the pulse noise superimposed on the AC power supply S in FIG. Is obtained. This measured value C
Does not always uniquely represent the size of the water tree, and is affected by the pulse noise in the AC power supply S, so that it is difficult to accurately determine the degree of the water tree.

Then, when the magnitude of the pulsating current directly obtained from the AC power supply S, that is, the gain of the variable gain amplifier 5 is adjusted by the signal passed through the second low-pass filter 8, a measured value B is obtained. The measured value B is nothing but a result of calibrating the measured value C according to the magnitude of the pulse noise included in the AC power supply S, and the degree of deterioration of the insulator 1c can be diagnosed more accurately.

In the above-described embodiment, the first and second low-pass filters 3 and 8 and the second resonators 4 and 9 are used.
Are provided separately, but they can be used in common by switching them with a switch. Further, in the present embodiment, a case where the present invention is applied to a single-core power cable having a small charging current due to the cable capacitance Ck is used as an example, but the present invention is applicable to a three-core batch power cable or a three-core batch measurement of a single-core three-wire power cable line. Is similarly applicable.

[0022]

As described above, according to the method for diagnosing insulation of a CV cable according to the present invention, a pulsating component in a cable ground wire current is detected, and a single frequency signal is further detected from the pulsating component. The extracted and amplified output signal is similarly corrected by the magnitude of the pulse noise of the same frequency signal obtained from the AC power supply, so that the degree of water tree deterioration is regardless of the magnitude of the original pulse noise. Accurate detection can be performed, and more accurate deterioration diagnosis can be performed.

[Brief description of the drawings]

FIG. 1 is a block circuit configuration diagram for implementing the present invention.

FIG. 2 is a graph showing measurement results.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Test power cable 1a Conductor 1b Shielding layer 1c Insulator 2 Current transformer 3, 8 Low-pass filter 4, 9 Resonator 5 Variable gain amplifier 6 Display 7 Transformer S AC current i Ground line current

Claims (1)

(57) [Claims] 1. Basically, from a power supply to a power cable to be measured.
An AC voltage having an AC frequency is applied to the power cable.
Detecting a ground line current Le of the ground line through the cable insulation CV Ke for detecting the degree of deterioration of the cable insulation
In insulation diagnosis method Buru, before from the ground line current
A first step of removing an AC component of the basic AC frequency and extracting only a signal value of a specific low frequency via an amplifier; and removing an AC component of the basic AC frequency from the AC voltage and removing the specific low frequency. a second step and, the signal obtained in the second step of extracting magnitude <br/> of the signal values of the component
The amplifier in the first step depending on the magnitude of the value
Voltage fluctuation of the power supply by adjusting the amplification degree of the power supply.
A third step of detecting the degree of deterioration of the cable insulator based on the signal value obtained in the first step. Method.