JPH0511009A - Diagnosing method for insulation of cv cable - Google Patents

Abstract

PURPOSE:To accurately diagnose the insulation of a power cable by applying an AC voltage across the power cable and detecting a pulsating current from a grounding conductor. CONSTITUTION:An AC power source S is connected to the conductor 1a of a power cable 1 to be diagnosed and a transformer 2 which detects a grounding conductor current (i) is coupled between the shielding layer 1b of the cable 1 and earth, with the output of the transformer 2 being connected to a display 6 through the first low-pass filter 3 which removes an AC component and takes out a pulsating current component only from the output of the transformer 2, the first resonator 4 which only extracts signals having a specific frequency, and variable gain amplifier 5. The power source S is connected with a transformer 7, the second low-pass filter 8, and second resonator 9 and the output of the resonator 9 is connected to the gain controlling input terminal of the amplifier 5 so as to adjust the gain of the amplifier 5 in accordance with the output of the resonator 9.

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 diagnosis method for diagnosing a degree of insulation deterioration of a CV cable due to a water tree or the like under a live line.

[0002]

2. Description of the Related Art Generally, a power cable is deteriorated in insulation performance of an electric insulator due to aging after installation. In particular, C
It is known that a V-cable causes a tree-like crack in a cross-linked polyethylene insulator, and a so-called water tree in which moisture enters the crack is a main cause of insulation deterioration. Such deterioration 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 detect the deterioration early. For this reason, various insulation measurement methods have been conventionally known, but in recent years, in particular, several methods have been proposed for diagnosing in a live state without stopping power transmission during measurement, and constant state monitoring is also possible. It is attracting attention because of its many advantages.

As a method of performing such constant monitoring,
Conventionally, for example, as described in Japanese Patent Publication No. 60-8465, a DC voltage is superimposed on a transmission AC voltage, and the insulation resistance of the cable is obtained and evaluated from the DC component of the cable leakage current detected as a result. A so-called DC superposition method or a so-called tan δ method, which measures a transmission voltage waveform and a current waveform and obtains and evaluates a dielectric loss tangent, is generally used, as described in JP-A-60-185171. .. Further, especially in the case of a CV cable, Japanese Patent Laid-Open No. 59-59
In Japanese Patent Laid-Open No. 202075, it is assumed that the water tree has a current rectifying action, and a method of measuring the DC 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 absolute value thereof. It is disclosed.

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 in the first section is generally considered to have poor detection sensitivity to the degree of deterioration, and there is a problem in that it cannot be detected unless it is considerably deteriorated. Also, several tens of volts at the time of measurement
A DC superimposed voltage of a certain degree is required, and therefore a DC power supply must be prepared separately. On the other hand, it is known that the tan δ method detects deterioration over the entire cable, but the detection sensitivity for local deterioration such as water tree is poor.

Further, in the case of Japanese Patent Laid-Open No. 59-202075, which utilizes the rectifying action of a water tree, a so-called inner water guiding tree generated from the conductor side and an outer water guiding tree generated from the sheath side of the cable insulator are Since the generated DC currents have mutually opposite polarities, when both types of water trees are generated at the same time, the detected DC currents may cancel each other out, and sufficient measurement may not be possible.

On the basis of the above problems, the present inventors have studied the water tree phenomenon. As a result, an AC voltage is applied to the power cable to be measured, and the amplitude of the applied AC voltage is gradually increased from zero. In the process, when a quasi-DC component of less than 10 Hz is detected in the ground line current, when the amplitude of the applied AC voltage reaches a certain value, it is based on the pulse noise superimposed on the AC voltage. We found that a pulsating current is detected, that is, this pulsating current is generated in the deteriorated part of the cable insulator by being stimulated by the change in the pulse noise of the line accompanying the live line, and its magnitude is It is related to the size of the water tree.

[0007]

Therefore, the inventors of the present invention have previously proposed a new insulation deterioration diagnosing method as Japanese Patent Application No. 1-167910, but there are the following problems in diagnosing insulation deterioration. 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, but the pulsating current signal is a very small signal, which makes evaluation extremely difficult. Further, if the pulsating current is simply amplified to facilitate the evaluation, the noise current component is also amplified at the same time, which makes the evaluation difficult. It was also found that the obtained pulsating current is also affected by the magnitude of the noisy pulsating current that is originally superimposed on the AC power source applied to the cable, and it is necessary to correct it.

An object of the present invention is to provide a method for diagnosing insulation of a CV cable which can more reliably detect insulation deterioration by utilizing the above-mentioned pulsating current.

[0009]

In order to achieve the above object, a method for diagnosing insulation of a CV cable according to the present invention applies an AC voltage from a power source to a power cable to be measured, and then a ground wire current from the ground wire. A first step of detecting an AC component from the ground line current and extracting only a signal of a specific low frequency and a step of removing an AC component from the AC current and extracting only the specific low frequency component. And a third step of detecting the degree of deterioration of the cable insulator by correcting the value obtained in the first step by the value obtained in the second step. Characterize.

[0010]

According to the method for diagnosing insulation of a CV cable having the above-mentioned structure, an AC voltage is applied to the power cable to remove an AC component from the installation line current obtained by the ground line to detect a specific low frequency component, and a power supply. The AC component of is removed and a similar specific low frequency component is obtained and corrected.

[0011]

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 carrying out the deterioration diagnosis method of the present invention. In the figure, reference numeral 1 denotes a power cable under test such as a CV cable to be measured, which has a capacitance Ck between the conductor 1a and the shield layer 1b. S is an AC power supply, one end of which is grounded and the other end of which is the power cable for test 1
Connected to the conductor 1a side. In addition, the current transformer 2 is coupled to the ground wire between the shield layer side 1b of the test power cable 1 and the ground, and the output of the current transformer 2 is the first low-pass filter 3, the first resonator 4, and the variable It is connected to the display 6 via the gain amplifier 5. Further, the AC power source S is connected to the transformer 7, the second low-pass filter 8, and the second resonator 9, and the output of the second resonator 9 is connected to the gain control input terminal of the variable gain amplifier 5. In addition, instead of the current transformer 2, a resistance may be inserted in the ground line to perform detection by the resistance.

Here, when a voltage is applied from the AC power source S to the power cable under test 1, a charging current flows in the electrostatic capacitance Ck of the cable, and the ground wire current i is passed from the shield layer 1b side to the ground through the ground wire. Will flow. The current transformer 2 detects the ground line current i from the ground line, and the first low pass filter 3
50Hz or 60 applied from the ground wire current i
The AC component of Hz is removed and only the low-frequency pulsating flow component is extracted. As the first low-pass filter 3, for example, a constant K type filter is used, and the basic AC component is removed from the ground line current i generated by the application of the AC power source S to the cable conductor 1a to output only the pulsating component. This output is input to the narrow band type first resonator 4. The first resonator 4 extracts only a specific low frequency component from the pulsating flow component. This first
As the resonator 4, various narrow band types can be used, but for example, a parallel T-type CR circuit which is often used in a low frequency band can be used. This parallel T-type CR feedback amplifier circuit (Twin-T circuit) consists of a T-type circuit consisting of two resistors Ra and Ra of the same resistance value and a capacitor Cb, and two capacitors Ca and Ca and a resistor Rb of the same capacity. It is composed of a parallel circuit of T-type circuit, and the capacitor Cb and the resistor Rb are Cb = 2 · Ca and Rb
= Ra / 2. Parallel T-type C configured in this way
The R circuit has a frequency characteristic of allowing a frequency signal of f = 1 / 2π · Ca · Ra to pass with the maximum signal, and selects a signal of frequency f, that is, a certain specific frequency from among pulsating flow components. Only the signal can be extracted. Further, this signal is amplified by the variable gain amplifier 5 and transmitted to the display 6 such as a pen graph or an oscilloscope.

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

FIG. 2 is a graph showing the output voltage of the variable gain amplifier 5 with respect to the applied voltage. The horizontal axis shows the applied voltage and the vertical axis shows the current I. When an AC voltage is applied to the power cable 1 under test, an electric charge corresponding to the applied AC voltage is induced in the shield layer 1b by electrostatic coupling with the conductor 1a, and due to this time change, the AC applied to the ground is changed. The ground wire current i that fluctuates at a cycle similar to the frequency of the voltage will flow, but in addition to this, when there is water tree deterioration in the insulator 1c, the above-mentioned pulsating current is superimposed. Become. As described above, the pulsating current is a current other than the current that depends on the applied AC voltage, is generated in the deteriorated portion due to the pulse noise superimposed on the AC power supply S, and is equal to or lower than the frequency of the applied AC voltage. Frequency current. Normally, the frequency of the applied AC voltage is 50 Hz or 60 Hz, so for example 50 H
First to allow frequency signals below z or 60 Hz to pass
If the circuit of the low-pass filter 3 is designed, only the pulsating current 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 it is in terms of ground capacity.

Since the magnitude of this 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 the sound test power cable 1 in which the water tree does not exist in the cable insulator 1c, the magnitude of the current obtained by the indicator 6 is almost constant as indicated by A, regardless of the voltage change. On the other hand, when a water tree exists, the current value increases as the voltage increases, as shown by B.

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

Therefore, the measured value B is obtained by adjusting the gain of the variable gain amplifier 5 by the magnitude of the pulsating current directly obtained from the AC power source S, that is, the signal passed through the second low pass filter 8. The measured value B is nothing more than the calibrated measured value C according to the magnitude of the pulse noise included in the AC power source S, and the degree of deterioration of the insulator 1c can be diagnosed more accurately.

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

[0019]

As described above, according to the CV cable insulation diagnosis method of the present invention, a pulsating current component in the cable ground wire current is detected, and a single frequency signal is detected from the pulsating current 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 source, so the degree of water tree deterioration can be determined regardless of the original pulse noise magnitude. It can be accurately detected, and more accurate deterioration diagnosis can be performed.

[Brief description of drawings]

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

FIG. 2 is a graph of measurement results.

[Explanation of symbols]

 1 Power cable under test 1a Conductor 1b Shielding layer 1c Insulator 2 Current transformer 3, 8 Low-pass filter 4, 9 Resonator 5 Variable gain amplifier 6 Indicator 7 Transformer S AC current i Ground wire current

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[Procedure amendment]

[Submission date] November 8, 1991

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 1

[Correction method] Change

[Correction content]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 009

[Correction method] Change

[Correction content]

[0109]

In order to achieve the above object, a method for diagnosing insulation of a CV cable according to the present invention applies an AC voltage from a power source to a power cable to be measured, and then a ground wire current from the ground wire. A first step of detecting an AC component from the ground line current and extracting only a signal of a specific low frequency; and a step of removing an AC component from the AC voltage and extracting only the specific low frequency component. And a third step of detecting the degree of deterioration of the cable insulator by correcting the value obtained in the first step by the value obtained in the second step. Characterize.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0010

[Correction method] Change

[Correction content]

[0010]

According to the insulation diagnosis method for a CV cable having the above-mentioned structure, an AC voltage is applied to the power cable to remove the AC component from the ground line current obtained by the ground line to detect a specific low frequency component, and at the same time, to supply the power. The AC component of is removed and a similar specific low frequency component is obtained and corrected.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0013

[Correction method] Change

[Correction content]

At the same time, the AC power source S is removed via the transformer 7 by the second low-pass filter 8 to remove the applied AC component.
The same specific frequency extracted by the first resonator 4 is extracted by the second resonator 9, and the gain of the variable gain amplifier 5 is adjusted by a signal proportional to this magnitude. That is, if the output of the second resonator 9 is greater adjusted to reduce the gain of the variable gain amplifier 5, the gain is smaller in the opposite large
It is adjusted to hear.

Claims (1)

Claim: What is claimed is: 1. An AC voltage is applied from a power source to a power cable to be measured, a ground line current is detected from the ground line, an AC component is removed from the ground line current, and a specific low frequency is detected. The first step of extracting only the signal of the above, the second step of removing the alternating current component from the alternating current and extracting only the specific low frequency component, and the value obtained in the first step as the first step. And a third step of detecting the degree of deterioration of the cable insulator by correcting the value obtained in the second step.