JPH11183557A - Insulation degradation examining method for power cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To judge presence of harmful water tree in a power cable insulator and to assure remaining life time by a low frequency power source. SOLUTION: A CV cable 3 which is to be tested is applied with a VLF voltage which is 0.4-1 time of operation voltage through a harmonics removing filter 2 from a VLF power source 1 of 0.05-1 hz, and the current flowing the CV cable 3 and a reference capacitor 4, respectively, is inputted in a loss current measurement bridge 6 for obtaining phase deviation of] a third harmonics of the loss current relative to a basic wave, thus presence of water tree degradation is judged. Related to a power cable judged to contain the harmful water tree, a low frequency voltage of 3-5.5 times of operation voltage is applied for screening. The cable not broken by screening is assured life expectancy of 3-5 years.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for diagnosing insulation deterioration of a power CV cable.

[0002]

2. Description of the Related Art When a power cable is used for a long time, a deteriorated portion called a water tree is formed in an insulator due to the action of an electric field and moisture. This water tree evolves over time,
If left unattended to reduce the insulation performance of the power cable, it will eventually cause a dielectric breakdown accident during operation.
In order to prevent this dielectric breakdown accident, various techniques for diagnosing insulation deterioration of power cables have been developed. For example, a current (loss current) obtained by removing a current component whose phase leads the applied voltage by 90 ° from the current flowing through the insulator (loss current) is detected, and the state of distortion of the current waveform, that is, the magnitude of the harmonic component and the superimposed phase are determined. Japanese Patent Application Laid-Open Nos. Hei 5-80113 and Hei 7-151
A technique described in Japanese Patent Application Laid-Open No. 815 or the like is known.

[0003] Further, a method of guaranteeing a remaining life of several years for a cable that has passed a withstand voltage test based on a relationship between an AC breakdown voltage and a length of a water tree has been studied (for example, a special method). No. 8-58736).
Further, as a method of monitoring the insulation deterioration state of the power cable using a low-frequency power supply and detecting the presence of harmful water trees, Japanese Patent Application Laid-Open Nos.
A technique described in Japanese Patent Application Laid-Open No. 78061 is known.

[0004]

A method of determining a harmful level of a water tree by measuring a loss current by using a commercial frequency (hereinafter referred to as AC) power supply has a problem that the power supply is large and the portability is poor. The remaining life of the power cable could not be determined. A low-frequency (hereinafter, referred to as VLF) power supply is smaller and more portable than the AC power supply. However, since the voltage level at which the loss current is measured is not known, the loss current using the VLF power supply is low. And no harmful level of the water tree could be determined.
SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to grasp a harmful water tree of a power cable by applying a low frequency voltage and to operate the power cable having a harmful water tree. By performing a screening by destruction by applying a low-frequency voltage higher than the voltage, a low-frequency power source is used to determine harmful water trees in a power cable insulator and to guarantee the remaining life.

[0005]

When a sinusoidal voltage of a single frequency is applied to a cable with deteriorated water tree, a harmonic component is generated in the current, and the phase shift with respect to the fundamental wave component is caused by water tree deterioration. Indicates the degree of Therefore, it is possible to diagnose whether or not a harmful water tree exists by applying the VLF voltage to the cable and measuring the phase shift of the harmonic component.

[0006] The inventors of the present invention have conducted various experiments and studied. If the voltage stress on the insulator is 2 kV / mm or more, the presence or absence of deterioration of the cable is determined from the waveform of the loss current even when VLF is used. It became clear what we could do.
For example, since the voltage stress is 5 kV / mm depending on the operating voltage of the 66 kVC cable, it is possible to determine the harmful water tree in the CV cable insulator by applying VLF 0.4 times or more of the operating voltage. When determining whether the cable has deteriorated, it is desirable to perform the test at a voltage lower than the operating voltage of the cable. Therefore, when determining whether the cable has deteriorated from the waveform of the loss current using the VLF, It is considered that it is better to apply a voltage of 0.4 to 1 times the above. Furthermore, when the relationship between the water tree length and the AC and VLF breakdown voltage was examined, in the case of VLF, the cable in which the harmful water tree existed was broken at a voltage 3 to 5.5 times the AC operating voltage, and the cable was destroyed. It became clear that the missing cable could guarantee a remaining life of 3-5 years.

The present invention has been made in view of the above points, and solves the above-mentioned problems as follows. A low frequency voltage of 0.05 to 1 Hz, which is 0.4 to 1 times the operating voltage, is applied to the power cable, and the presence or absence of water tree deterioration is determined based on the third harmonic waveform distortion of the loss current generated at that time. .
Then, a screening is performed by applying a low-frequency voltage of 3 to 5.5 times the operating voltage to the power cable determined to have the harmful water tree to destroy the cable deteriorated with the water tree.

In the present invention, since the insulation deterioration diagnosis of the power cable is performed as described above, the third harmonic waveform of the loss current is distorted using a low-frequency power supply. Tree decisions can be made. Further, since the voltage to be applied for determining the harmful water tree may be equal to or lower than the operating voltage, more efficient diagnosis can be performed by applying a high VLF voltage from the beginning without causing unnecessary damage to the cable under test. be able to. In addition, since a low frequency voltage of 3 to 5.5 times the operating voltage is applied to the CV cable determined to have water tree deterioration, the cable having deteriorated water tree is destroyed.
The remaining life of the cable which has not been broken can be guaranteed for 3 to 5 years.

[0009]

Embodiments of the present invention will be described below. First, the relationship between the water tree length and the distortion of the third harmonic of the loss current was investigated in order to diagnose the insulation deterioration and determine the life of the CV cable using the VLF. FIG. 1 is a third harmonic waveform of a loss current when an AC alternating sine wave voltage is applied to an undegraded insulating sheet and a water tree-degraded insulating sheet, and FIG. b) shows the case of an insulating sheet deteriorated with water trees. As is clear from the figure, in the case of the water-deteriorated insulating sheet, the waveform is distorted at phases of 90 ° (π / 2) and 270 ° (3π / 2).

FIG. 2 shows a 22 k VCV cable (insulation thickness = 7).
mm) of the third harmonic waveform of the loss current in the deteriorated cable, which is consistent with the result of the insulating sheet shown in FIG.
It can be seen that even in the case of a CV cable, the presence or absence of deterioration can be determined from the waveform of the loss current. FIG. 3 is a diagram showing the water tree length and the phase shift of the third harmonic component with respect to the fundamental wave component when an AC voltage is applied to the insulator whose water tree has deteriorated. The horizontal axis indicates the water tree length. The percentage divided by the thickness and multiplied by 100 is shown, and the vertical axis shows the phase shift θ ₃ . As can be seen from FIG. 3, the water tree length increases as the phase shift θ ₃ approaches zero.
That is, it is understood that the deterioration is progressing.

FIG. 4 is a diagram showing the relationship between the AC breakdown voltage and the phase shift θ _3. The vertical axis shows the AC breakdown voltage and the horizontal axis shows the phase shift θ ₃ . As can be seen from the figure, AC breakdown voltage is lowered in accordance with the deviation theta ₃ phase approaches 0 °, and the result is out indicating that also degrade the electrical. In the figure, as an example, | θ ₃ | <20 °
Attention was required at this time (hazardous water tree was generated).
Based on the above, whether a harmful water tree exists by applying a sinusoidal voltage of a single frequency to the cable with deteriorated water tree and measuring the phase shift of the harmonic component with respect to the fundamental component Can be diagnosed.

The data shown in FIG. 1, FIG. 2, FIG. 3, and FIG. 4 are data when an AC voltage is applied. VLF is basically a sine wave, and the same result as AC is obtained. It is thought that it is possible. Therefore, the present inventor has proposed that
With respect to the VLF of Hz, an applied voltage capable of judging a water tree was examined. As a result, when the voltage stress was 2 kV / mm or more, it was confirmed that the determination of the harmful water tree was easily performed. Therefore, for example, the voltage stress at the operating voltage in the case of a 66 kVC cable (insulation thickness = 8 mm) is [60 × 15 / √ (3)] / 8 = 5 kV / mm. A hazardous water tree determination can be made.

Further, when determining whether or not a harmful water tree exists in the CV cable, it is necessary not to damage the CV cable. Therefore, it is desirable that the applied voltage when determining the presence or absence of the harmful water tree is lower than the normal operation voltage. From the above, in the present invention, the CV cable has an operating voltage of 0.4 to 1 times 0.0
A VLF of 5 to 1 Hz was applied, and the loss current was measured.
The presence or absence of the harmful water tree was determined from the phase shift of the harmonic, and the screening of the CV cable having the harmful water tree was performed.

Next, a VLF voltage of 0.05 to 1 Hz at which a cable with water tree deterioration can be broken was examined. FIG. 5 is a diagram showing the relationship between the water tree elongation obtained by the water needle model (22 kVC cable) experiment and the AC and VLF breakdown voltages. The vertical axis of FIG. 5 shows the breakdown voltage [kV], The axis indicates the remaining insulation thickness (the thickness of the normal part from the tip of the water tree).

As can be seen from FIG. 5, a cable having a harmful water tree can be destroyed by applying a VLF voltage that is about three times the AC voltage to a cable that has been deteriorated by the water tree. Further, the relationship between the test voltage and the remaining life was examined for a 22 to 33 kVCV cable (insulation thickness t = 6 mm).
(60 kV / 6 mm = 10 kV / m)
m), the estimated remaining life of the cable that did not break is 3
When a voltage of 70 kV is applied to a cable having an insulation thickness of 6 mm (70 kV / 6 mm = 12 kV / mm),
The estimated remaining life of the unbroken cable was found to be 5 years. In the case of 60 kV, the operating voltage (22 to 33 k
V / √ (3) = 13 kV to 20 kV), which is 3 to 4.5 times. In the case of 70 kV, the operating voltage is 3.5 to 5.5 times, so that the operating voltage is 3 to 5.5 times. By applying the VLF, it is possible to perform screening of a cable having a harmful water tree.

Although the above data is data for a 22-33 kVCV cable, it is considered that the data can be traced to a 66 kVCV cable. In the case of a 66 kVCV cable, the operating voltage is 60 × 1.15 / {(3)}.
Since it is 40 kV, the VLF voltage for screening the cable having the harmful water tree (remaining insulation thickness t = 3 mm) is 40 kV × 3 to 5.5 = 120 to 220 kV.

FIG. 6 shows an example of the measuring circuit used in the present invention.
FIG. 3 shows a CV cable using the measurement circuit shown in FIG.
Loss current by applying VLF to the
Re θ _ThreeWas measured. In FIG. 6, 1 is 0.05 to 1H
z removes harmonics contained in the VLF power supply.
Filter, 3 is the cable under test, 4 is the standard condenser
5 is a voltage divider, CV cable 3 under test, standard
VLF power supply 1 for capacitor 4 and voltage divider 5
CV cable 3 to be tested via the harmonic elimination filter 2
Was applied with a VLF voltage 0.4 to 1 times the operating voltage. So
Then, the measured CV cable 3 and the standard capacitor 4
Input the current flowing to each to the loss current measurement bridge 6
The power supply voltage level is calculated based on the current flowing through the CV cable 3 to be measured.
The loss obtained by removing the component that is 90 ° ahead of the phase
The current signal is input to the waveform analyzer 7 and the loss with respect to the fundamental wave
Phase shift θ of the third harmonic of current_ThreeI asked.

The phase shift θ _{3 was} measured as described above, and a CV cable having a harmful water tree was determined based on the phase shift θ ₃ . As described above, screening was performed by applying a voltage of 3 to 5.5 times the operating voltage to these CV cables, for example, for 10 minutes. As a result of the above test, the method of the present invention makes it possible to determine a cable having a harmful water tree, and for a cable that has not been destroyed by an insulation deterioration diagnosis and a pressure resistance test for 3 to 5 years. It has been confirmed that the remaining life of can be guaranteed.

[0019]

As described above, in the present invention,
The following effects can be obtained. (1) Since insulation deterioration diagnosis of a power cable can be performed using a small-sized low-frequency power supply, the test apparatus can be configured at low cost and can be made portable. (2) When judging a harmful water tree, the operating voltage is set to 0.
Since the test is performed by applying a low frequency voltage of 4 to 1 times, no extra damage is given to the cable under test. In addition, a cable having a hazardous water tree can be screened by applying a low-frequency voltage higher than the operation voltage. For this reason, it is possible to guarantee not only the harmful water tree but also the remaining life of the cable that has not been destroyed as a result of the screening.

[Brief description of the drawings]

FIG. 1 is a diagram showing loss current waveforms of an undegraded sheet and a deteriorated sheet.

FIG. 2 is a diagram showing a loss current waveform of a deteriorated cable.

FIG. 3 is a diagram illustrating a relationship between a water tree length and a phase shift θ ₃ of a third harmonic.

FIG. 4 is a diagram illustrating a relationship between AC destruction performance and a phase shift θ ₃ of a third harmonic.

FIG. 5 is a diagram showing a relationship between a residual insulating thickness and an AC / VLF breakdown voltage.

FIG. 6 is a diagram illustrating an example of a measurement circuit used in the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Low frequency power supply 2 Harmonic elimination filter 3 Cable under test 4 Standard capacitor 5 Voltage divider 6 Loss current measurement bridge 7 Waveform analyzer

Claims (1)

[Claims] 1. A method of diagnosing insulation deterioration of a power cable by applying a low frequency voltage of 0.05 to 1 Hz, wherein a low frequency voltage of 0.4 to 1 times the operating voltage is applied to the power cable. Then, the presence or absence of water tree deterioration is determined based on the waveform distortion of the third harmonic of the loss current generated at that time. For the power cable determined to have water tree deterioration, the operating voltage is 3 to 5.5 times the operating voltage. Apply low frequency voltage,
A method for diagnosing deterioration of power cable insulation, characterized in that screening is performed by destroying a cable with water tree deterioration.