JP2006030008A - Method of detecting defect in power cable and connecting part - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To safely and surely specify a defective position in the inside, without having to stop power transmission, or without generating electrical breakdowns in a power cable or a connecting part. <P>SOLUTION: Radiation is emitted to the power cable and the connecting part, while conformed with a phase of a voltage waveform not generated with partial discharge, in a discharge waveform, in a diagram obtained by measuring the partial discharge of the power cable and the connecting part. A radiographic photograph of the inside is photographed, and defects of the power cable and the connecting part inside are detected based on an photographed image. More specifically, the radiation is emitted during a time of any selected from 0 (sec)-0.178 T(sec), 0.322T (sec)-0.678 T(sec) and 0.822 T(sec)-1 T(sec), where T is one period of a voltage phase (T=0.02 sec in 50 Hz). The radiation is emitted, in particular, to the markedly deteriorated cable, while being limited to the first quadrant and the fourth quadrant out of the voltage phase. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a power cable used for power transportation and a defect detection method for a connection part, and more particularly to a power cable and a defect detection method for a connection part that detect internal defects by radiation.

Conventionally, a method for detecting defects in an insulator by partial discharge measurement has been used as a method for detecting defects in power cables and connecting portions. In addition, a technique has also been proposed in which X-rays, γ-rays, and the like are irradiated onto a diagnostic target line, an internal observation photograph is taken with the radiation, and an internal defect is detected by decoding the photograph (for example, Patent Document 1). reference).
In addition, maintenance of oil-immersed insulated cables such as OF cables and POF cables is conducted by insulating oil analysis, radiation internal diagnosis, and monitoring of oil amount and hydraulic pressure.
Of these, the analysis of insulating oil is widely performed as a method for diagnosing abnormalities inside cables. If there is an abnormality inside, the oil is decomposed by heat and discharge, and the generated gas and characteristics are deteriorated.
Radiation internal diagnosis is performed when cables are installed on sloping ground or in places where there is a lot of traffic and a lot of vibration. This is because, due to the relative movement between the cable core and the outer aluminum cover, a discharge occurs due to the displacement and damage of the insulating paper and the disturbance of the shielding layer, which eventually causes dielectric breakdown. However, analysis of insulating oil is often used instead of internal radiation diagnosis.

Monitoring of the oil amount / hydraulic pressure is performed as a means for early occurrence of oil leakage abnormality due to damage to the OF cable or a minute crack in the metal sheath due to metal fatigue. However, information about internal abnormalities cannot be obtained.
Of these maintenances, the number of lines that employ a live-line oil-collecting connector is increasing for oil collection for insulating oil analysis. Therefore, it has become possible to perform insulating oil analysis even in a live line. Moreover, since monitoring of oil quantity and oil_pressure | hydraulic is performed visually, it is performed also in a live line.
However, the radiation internal diagnosis is usually performed in a state where the track is stopped, and therefore, the frequency of implementation is low.
JP 2003-65975 A

Among the methods for detecting defects in the power cable and the connection part, the partial discharge measurement can detect the presence or absence of any abnormality in the power cable or the connection part based on the obtained result. It is impossible to determine which part is located.
In addition, although radiographic photographs are effective as a method for identifying and identifying internal abnormal locations, it is usually necessary to stop power transmission before photographing.
This is because radiation is an energy source that promotes the discharge in the case where a partial discharge occurs at an abnormal location, so that it is possible to avoid the possibility of causing a dielectric breakdown during imaging.
However, since stopping power transmission sometimes gives a great burden to power cable customers, radiation imaging after stopping power transmission is often difficult to implement. Therefore, if radiography can be performed without stopping power transmission, it is expected that these problems can be solved. However, the harmfulness of defects occurring inside the power cable or connection section varies. Therefore, it is virtually impossible to set radiation irradiation conditions without knowing the degree and to perform radiography during power transmission safely and without dielectric breakdown.

On the other hand, insulation oil analysis is very effective as an internal diagnosis method for oil-immersion insulated cables of OF and POF in oil-immersed insulated cables, but because the amount of insulation oil is very large, detection of abnormalities in insulation oil analysis is delayed. There are cases.
In addition, in a special installation place such as an inclined land or a cable with a large core displacement such as a POF cable, it is faster to detect an abnormality and to obtain more information by directly checking the inside with radiation rather than an insulating oil analysis. However, when using radiation, since the inspection cable line is stopped as described above, the frequency of use is very low.
The reason why the line is stopped when the inspection is performed in this way is that, as described above, the insulation characteristic of the power cable in the live line is deteriorated by the irradiation of radiation.
That is, when radiation is irradiated to a defective portion of a bubble or a gap generated by displacement or damage of insulating paper, the air in the defective portion is ionized to generate electrons. If a partial discharge is generated in the cable, the number of partial discharges is increased by supplying electrons, which promotes deterioration and involves a risk of dielectric breakdown.
The present invention was made in order to avoid the above-described problems, and without failing to stop power transmission and without causing dielectric breakdown of the power cable or connection portion, it is possible to safely and reliably internal defect positions. It aims to make it possible to specify.

In the present invention, the above problem is solved as follows.
(1) In the discharge waveform obtained by the partial discharge measurement of the power cable and the connection portion, radiation is applied to the power cable and / or the connection portion in accordance with the phase of the voltage waveform where the partial discharge does not occur. A photograph is taken, and a defect in the power cable and / or connection portion is detected from the taken image.
(2) When one period of the voltage phase is T (T = 0.02 sec for 50 Hz), 0 (sec) to 0.178 T (sec), 0.322 T to 0.678 T (sec), or 0. Radiation is irradiated between any one of 822T to 1T (sec), an internal radiograph is taken, and a defect inside the power cable and the connecting portion is detected from the taken image.
(3) In the above (2), for a power cable that is particularly deteriorated, the voltage phase is limited to the second quadrant and the fourth quadrant, and is 0.322T to 0.5T (sec), or , 0.822 T to 1 T (sec) is irradiated with radiation.

  According to the present invention, it is possible to detect a location of a defect in a power cable or a connection portion where a defect exists by applying radiation without stopping power transmission in a voltage application state, that is, in an actual cable line. Thus, stable diagnosis / maintenance of equipment can be performed without risk of dielectric breakdown.

The present inventors experimented how the occurrence of partial discharge of an oil-immersed insulated cable changes due to radiation irradiation. As a result, the following was confirmed.
-The occurrence of partial discharge is limited to the first and third quadrants of the applied voltage phase (commercial frequency), and the occurrence location is limited to the first and third quadrants even if the intensity of radiation energy and the irradiation time are changed. Is done.
・ When partial discharge is occurring, the frequency of partial discharge increases due to irradiation of radiation.
-The starting voltage of partial discharge is reduced by 10% at the time of irradiation as compared to the case without irradiation.

From the above, it was found that the following may be performed when irradiating radiation in a live wire.
(A) As shown in FIG. 1, it is considered that a new discharge will not be produced if radiation is applied during a phase where a partial discharge does not occur. Almost all oil-immersed cables have a partial discharge temporarily only when an abnormal voltage such as lightning or switching surge is applied, and no partial discharge is always generated.
Therefore, partial discharge measurement is performed on the power cable or connection part that is the object of investigation for the presence or absence of defects to detect the presence of defects.
Then, radiation irradiation in radiography is performed only at a phase angle at which partial discharge does not occur in the measured partial discharge waveform.
By repeating the radiation imaging for a certain period of time, it is possible to avoid insufficient exposure and to perform photography with good resolution.

(B) Since the partial discharge start voltage decreases by 10% due to radiation irradiation, the start of partial discharge can be suppressed unless radiation is applied to a region exceeding 90% of the voltage waveform.
In FIG. 2, if one period of the sine wave is T (0.02 sec for 50 Hz), the region (I) (0.178T to 0.322T) and the region (II) (0.678 to. 822T) corresponds to 0.9 or more of a sine wave, and if this portion is not irradiated with radiation, no new partial discharge is generated by irradiation.
In other words, if a region of 0 to 0.178T, 0.322T to 0.678T, and 0.922 to 1T is irradiated with radiation for one period of the applied potential phase, no new partial discharge occurs.

(C) On the other hand, in the case of an oil-immersed insulated cable that is significantly deteriorated, it is considered that partial discharge always occurs. In this case, the partial discharge is widely distributed in the first quadrant and the third quadrant. In FIG. 3, the region (III) (0 to 0.322T) and the region (IV) (0.5T to 0. 0). If it is not irradiated to (822T), a new partial discharge does not occur.
That is, the region of 0.322T to 0.5T and 0.822T to 1T may be irradiated for one period of the applied potential phase. Here, the OF cable that is significantly deteriorated is, for example, a cable that is planned to be repaired by detecting 10 ppm or more of acetylene in an insulating oil analysis.
Although the oil-insulated insulated cable has been described above, the radiation irradiation method for suppressing the occurrence of partial discharge described above can be similarly applied to a cable using polyethylene or rubber as an insulator.

Hereinafter, the present invention will be described based on examples.
(1) Example 1
In the present invention, an experiment was first conducted using an oil-immersed paper sheet, a short model cable of oil-impregnated paper, a defective OF connection part, and a defective POF connection part. Kraft paper was used as oil-impregnated paper, and mineral oil, synthetic oil (hard), and bolybden oil were used as oil. A punch hole of φ5 mm was made in each sample defect, and a defect thickness of ½ to １ ／ of the insulation thickness was provided.
The sheet test, the model cable test, the OF connection part, and the POF connection part all showed the same tendency, and the results shown in Table 1 were obtained when the partial discharge start voltage when radiation was not irradiated was V.

According to Table 1, the method of irradiating radiation in the region other than (I) and (II) in FIG. 2 or the region other than (III) and (IV) in FIG. It can be seen that the state can be almost equivalent to the case.
Therefore, by this method, the radiographing at the time of the hot line does not promote the deterioration of the cable, and the hot-line picture can be taken.

(2) Example 2
Next, a plurality of 66 kV 1 × 325 mm2 OF cables (15 m in length) in which a piece of broken copper wire with a diameter of 1 mm and a length of 8 mm is artificially wound between layers of laminated oil-immersed insulating paper are prepared and used as samples. It was.
When a commercial frequency voltage of 38 kV, which is an operating voltage, was applied to this cable, it became clear that partial discharge occurred.
FIG. 4 shows a partial discharge measuring circuit used in this example.
As shown in the figure, a commercial frequency voltage is applied from the transformer 1 to the OF cable 2 to connect the detection impedance 3 between the sheath and the ground of the OF cable 2, and the coupling capacitor 4 is connected between the terminal and the ground of the transformer 1. A partial circuit was measured by connecting a partial discharge measuring device 6 to the detection impedances 3 and 5. This partial discharge occurrence state is shown in FIG.

First, X-ray irradiation was performed according to a normal work procedure in a state where a commercial frequency voltage of 38 kV was applied to the cable. As a result, in the sample, partial discharge rapidly progressed during X-ray irradiation, causing dielectric breakdown. This can be considered as evidence showing the danger of performing X-ray irradiation without stopping power transmission.
Next, using a second cable having the same defect, an experiment was performed in which X-ray irradiation was performed with a limited phase angle.
FIG. 5 shows the waveform of the partial discharge occurrence state described above, but since FIG. 5 is an overwriting of the partial discharge waveform for about 10 minutes, the phase in which the discharge is occurring and the occurrence are shown. There is enough phase distinction.

In view of this, X-ray irradiation was performed on the defective portion only in the range of the phase angles of A and B where no partial discharge was generated as shown in FIG. FIG. 6 is a diagram showing the state of X-ray irradiation. The X-ray source 10 and the X-ray film 11 are arranged as shown in the figure, the X-ray source 10 irradiates the OF cable 2 with X-rays, Line photographs were acquired.
The X-ray irradiation control is performed by opening and closing the shutter 13 of the X-ray source 10 by the shutter control computer 12 based on the result of measuring the partial discharge by the partial discharge measuring device 6, and discharge is generated. Or, the shutter is controlled to close at a phase angle that may occur. The applied voltage is 38 kV as described above. X-ray irradiation was performed for 40 minutes including the time for closing the shutter. During this time, the X-ray film 11 was fixed as shown in FIG.

As a result, dielectric breakdown did not occur during this 40 minutes, and the measured partial discharge did not show any signs of defect deterioration such as an increase in charge amount or an increase in the number of pulses.
The film was developed after the completion, and it was confirmed by the developed photograph that the metal foreign matter was sandwiched between the insulating paper layers, and it was shown that the defect portion can be specified.
In the above description, the case where X-rays are used as the radiation source has been described. However, other radiation sources such as γ-rays can be used depending on the purpose.

It is a figure which shows the generation | occurrence | production situation of partial discharge. It is a figure explaining the case where radiation is irradiated in area | regions other than (I) and (II). It is a figure explaining the case where a radiation is irradiated in area | regions other than (III) and (IV). It is a figure which shows the example of a partial discharge measurement circuit. It is the wave form diagram which showed the partial discharge generation | occurrence | production situation in the OF cable with a defect. It is the figure which showed the experimental equipment condition when radiation imaging was performed by limiting an irradiation phase angle while performing partial discharge measurement.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Transformer 2 OF cable 3,5 Detection impedance 4 Coupling capacitor 6 Partial discharge measuring device 10 X-ray source 11 X-ray film 12 Shutter control computer 13 Shutter

Claims (3)

A method for detecting defects in a power cable and a connection part in a live line state by radiography,
In the discharge waveform obtained by the partial discharge measurement of the power cable and connection part, radiation is applied to the power cable and connection part in accordance with the phase of the voltage waveform where partial discharge does not occur, and an internal radiograph is taken. ,
A method for detecting a defect in a power cable and a connection part, wherein a defect in the power cable and the connection part is detected from the photographed image. A method for detecting defects in a power cable and a connection part in a live line state by radiography,
When one period of the voltage phase is T (T = 0.02 sec for 50 Hz),
0 (sec) to 0.178 T (sec)
0.322T to 0.678T (sec)
0.822T to 1T (sec)
Irradiate between any of these to take an internal radiograph,
A power cable and a defect detection method for a connection part, wherein a defect inside the power cable and the connection part is detected from the photographed image. Especially for power cables that are significantly degraded, of the above voltage phases, limited to the second and fourth quadrants,
0.322T to 0.5T (sec)
0.822T to 1T (sec)
The method for detecting a defect in a power cable and a connection part according to claim 2, wherein radiation is irradiated during any of the above.

Images