JPS5848861B2 - How to detect water trees in cables - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a detection method for detecting the presence or absence of water trees, which often occur in polyethylene cables.

Water treeing is caused by moisture penetrating the insulation in cables and abnormal electric field areas such as voids in the insulation and protrusions in the semiconducting layer, and it causes a significant deterioration of the insulation properties of the cable. It is.

Recently, damage to polyethylene insulated cables has been occurring frequently due to this water tree, and this has become a major problem.

Therefore, as a method of predicting the occurrence of such water trees, a method using a combination of tan δ and DC leakage current has been recommended.

However, the method using tan δ is a numerical value that indicates the average deterioration of the cable, and is hardly suitable for discovering phenomena such as water trees where deterioration is progressing locally.

In other words, if the cable has deteriorated to an extreme degree and water treeing occurs along the entire length of the cable, it can be detected, but
As many cases of water tree occurrence show, near joints,
This is because there are many cases where the problem occurs locally, such as at the rising edge of the cable.

In short, it is impossible to detect such a state of deterioration using the tan δ method.

Next, the method using direct current leakage current has the advantage of being able to detect local deterioration, but in the case of polyethylene, the specific resistance value is high, so there are at least 100 or more water trees in the radial direction of the cable insulation coating layer. If it is not stretched, it cannot be detected.

Furthermore, since the measured value is small, it is difficult to understand the characteristics unless a high voltage is applied.

Due to this, there is a red that introduces a new danger of degrading the insulation performance of the cable during measurement.

As described above, conventional methods are still insufficient, and no effective concrete measures have yet been found.

In addition, the problem of water pollution is not limited to manufacturers.
There has been an urgent need to develop an effective detection method for users such as electric power companies, from the standpoint of preventing accidents and predicting the remaining life of existing cables.

Therefore, as a method to solve the above problem, the present inventors have already proposed a method of detecting the presence or absence of water trees from the discharge characteristics of the cable (Japanese Patent Application No. 11534/1986).
No. 1) (see Japanese Patent Publication No. 57-32343).

Specifically, the cable to be measured is charged with a DC power source, then discharged, the amount of charge Q stored in the cable is integrated from zero to 30 seconds, and deterioration is determined based on this value. It is something.

However, with this method, if the cable to be measured is a long cable actually installed, the cable's own capacity C
increases, the amount of charge Q during charging and discharging increases, and the aftereffect current (current with a time delay) in the water-tripping generation area is masked and cannot be detected.

This drawback cannot be improved even if the voltage at the time of measurement is increased, as it only increases the instantaneous discharge current.On the contrary, if the voltage is increased, the current at the time of measurement becomes unstable,
This also brings about a new drawback in that it poses a danger to the human body.

In addition, increasing the measurement voltage increases the space charge stored in polyethylene, which causes the problem that the measurement results become unstable.

Therefore, in the present invention, the current fraction during discharge is I = Io+
It is expressed as Id (I... total current, Io... instantaneous discharge current, Id... aftereffect current), and the instantaneous discharge current is about 1 to 2 seconds ( In reality, it seems that the discharge lasts for a short time, but due to the delay in the operation of the picoammeter and the recorder, it becomes approximately zero (this is the extent of the discharge). By bypassing the instantaneous discharge current using the method, and then measuring the discharge characteristics of the aftereffect current stored in the water-trimming section, the pico amplifier can be used to run long cables without danger and with a stable current. The aim is to provide a method that can be adequately detected with a meter.

The figure shows an example of a detection circuit that implements the detection method of the present invention, where 1 indicates a cable to be measured, and 1a
is its conductor, 1b is its polyethylene insulation layer, and 1c is its shielding layer.

S1 is a changeover switch, for example a vacuum switch; 2 is a DC power source; 3 is a current measuring device, for example a picoammeter;

A charging circuit is formed between the two.

S2 is a bypass switch of a bypass circuit connected in parallel to the current measuring device 3, and the changeover switch S
It is linked to 1.

5 is a recorder for recording current.

First, the movable contact piece a of the changeover switch S is put on the fixed contact b side, and the cable insulation layer 1b is turned on by the DC power supply 2.
to charge.

A charging time of about 5 minutes is enough for the aftereffect current to be approximately saturated.

Strictly speaking, it takes a very long time, but in the case of this measurement, this level of current sensitivity is sufficient for measurement.

Once the battery is sufficiently charged in this manner, the movable contact a of the changeover switch S1 is placed on the fixed contact C side to form a discharge circuit and discharge.

At this time, the movable contact piece d of the bypass switch S2, which is interlocked via the timer 4, is on the side of the fixed contact e.

Therefore, the initial instantaneous discharge current is discharged to the ground side.

When the discharge is performed for 2 seconds in this manner, the timer 4 is activated and the movable contact piece d of the switch S2 is separated from the fixed contact e side and opened.

Therefore, the aftereffect current with a time delay is measured by the current measuring device 3 and recorded on the recorder 5.

This naturally includes a small amount of instantaneous discharge current, but this is not a problem since it does not adversely affect the detection of the water litter generation area.

Since the instantaneous discharge current is bypassed as described above, the aftereffect current value shows a current value on the order of mttA even at a voltage of 100 cm, and a picoammeter is sufficient as a current measuring device.

Note that a voltage of 100 to 500 V is suitable for the measurement.

As is clear from the above, according to the present invention, during discharge,
The initial instantaneous discharge current is bypassed, and the discharge characteristics of the aftereffect current are measured after a predetermined period of time has elapsed.
Even in the case of a long cable that is actually installed, there is an effect that it is possible to easily detect the part where water litter occurs.

Further, in this case, there is no need to make the measurement voltage so high, so there is no danger and there is no possibility that the current during measurement will become unstable.

Furthermore, a picoammeter on the order of mμA can be used as a current measuring device, which is economical.

[Brief explanation of the drawing]

The figure is a circuit diagram showing an example of a detection circuit for implementing the detection method of the present invention. 1...Cable, 1a...Conductor, 1b
...Polyethylene insulation layer, 1c ... Shielding layer, 2 ... DC power supply, 3 ... Current measuring device, 4 ... Timer, 5... Recorder, S1... Changeover switch, S2...
Bypass switch.

Claims (1)

[Claims] 1. In a detection method in which the cable to be measured is charged, then discharged, and the presence or absence of water trees is detected from the discharge characteristics, the instantaneous discharge current at the initial stage of discharge is bypassed, and then after a predetermined delay, the aftereffect current is detected. A method for detecting water trees in a cable, characterized by measuring discharge characteristics.