JPS61234372A - Insulation deterioration test for power cable - Google Patents

Abstract

PURPOSE:To obtain highly reliable test results to match actual use, by using an AC voltage as such to be applied to the core. CONSTITUTION:An AC voltage is used as such as to be applied to a sample cable 1. When a deterioration test is carried out by the measuring circuit of this invension, a switch S1 is connected to the terminal (a) position to apply the AC voltage between the terminal and a shield layer 1b grounded to the core 1a of the cable 1. Subsequently, the switch S1 is connected to the terminal (b) position while the core 1a is grounded initially through a resistance R and then, after the switch S2 is closed, it is grounded through the resistance R. After a specified time, the switch S1 is connected to the terminal (c) position to release the grounding of the core 1a while the residual voltage of the cable 1 is measured with a voltometer 2. The deterioration in the cable 1 is decided from the residual voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an insulation deterioration testing method for determining the insulation deterioration of a power cable based on its residual voltage.

[Technical background of the invention] After applying DC voltage to the core wire of the power cable.

When the core wire is grounded for a short time and then opened, a residual voltage (recovery voltage) appears in the core wire.

This residual voltage occurs because the charge accumulated in the cable insulation due to the application of the DC voltage cannot be discharged within a short period of time due to the dielectric relaxation phenomenon. In comparison, the residual voltage becomes larger. Therefore, a method of determining the insulation deterioration of a cable from this residual voltage has been conventionally implemented.

By the way, when an AC voltage is applied to a power cable and when a DC voltage is applied, the potential distribution in the insulator is different, and the difference becomes particularly noticeable when electrode irregularities (defects) are present. And the power cable
An alternating current voltage is applied during actual use.

For the above reasons, the conventional test method described above in which a DC voltage is applied to the cable cannot obtain test results suitable for actual use, and therefore there is a risk of erroneously determining insulation deterioration.

[Object of the Invention] In view of these conventional problems, the present invention seeks to provide a method for testing insulation deterioration of power cables that can obtain highly reliable test results suitable for actual use.

[Summary of the Invention] In the present invention, the above object is achieved by applying an alternating current voltage as the voltage applied to the core wire of the cable.

[Example] The present invention will be described in detail below with reference to one drawing.

First, the principle of generating residual voltage will be briefly explained. Now, the core wire and shielding layer (sheath) of the test cable are
The xwfl side electrode in the figure is considered to be the x=0 side (ground side), and after a voltage is applied between these electrodes, a space charge as shown in the figure is generated in the insulator between the electrodes. think.

In the figure, when the electrode on the x=Q side is open, the potential distribution Vf(x
) is formed, and a voltage Vf(Q ) is observed at the xzQ side electrode.

On the other hand, when this x=fl side electrode is grounded, the potential distribution inside the insulator becomes Vs(x) shown in the figure.

This Vs(x) is a combination of Vf(X) and the potential distribution -Vb(x) necessary to make the voltage of the x=0 side electrode zero.

Vs(x)=Vf(x) Vb(x)...
(1) Vs(fi)=Vf((+) Vb(fl)=
0[Vf(fi)=Vb(fl)] ・−・・(
2).

Now, consider the case where the electrode on the x=Q side is once grounded and then opened in a state where the potential distribution shown in FIG. 2 is formed.

If -Vb(x) is maintained during the open period and the space charge distribution does not change at all, the condition of equation (2) is maintained as is, so the voltage at x=-Q side electrode is always zero.

However, in reality, the space charge moves and disappears over time during the open period due to the internal electric field it forms and thermal diffusion.

The value of Vf(Q) decreases from Vf(Q)=Vb(Q) to Vf(fl)=0. At this time, if the value of -vbB) is maintained as it was at the time of grounding, then (1
) As is clear from the equation, the potential of the electrode on the X=Ω side changes asymptotically from the zero value immediately after the ground is opened to the value -vb(Q) over time, and this is observed as the residual voltage. be done. Figure 4 shows x=Q at time t=0.
This shows an example of the residual voltage vs. time characteristic when the side electrode is disconnected from ground, and the residual voltage is larger for a deteriorated cable than for a healthy cable.

The residual voltage mentioned above is not only caused by the dielectric relaxation phenomenon (polarization) in the cable insulator, but also by positive or negative charges injected from the defective part of the cable and trapped in the insulator. It also occurs when the shielding layer (shielding layer) is cursed, and the generation of residual voltage due to the latter becomes particularly noticeable when urine-tree deterioration occurs in the insulator. The residual voltage caused by the latter cannot be detected sufficiently by the conventional method of applying a DC voltage to the cable.

FIG. 1 shows an example of a measuring circuit for carrying out the test method according to the present invention, and as shown in the figure, in the present invention, an alternating current voltage is used as the voltage applied to the test cable 1.

When performing a deterioration test using this measurement circuit, connect the switch S to the terminal a side and apply an alternating current voltage between the core wire 1a of the cable l and the grounded shielding layer lb for a predetermined time,
After that, connect the switch S1 to the terminal side and connect the core wire 1a.
is initially grounded via a resistor R, and then grounded via a resistor R that closes F2 and is grounded. Then, after a predetermined period of time, the switch Sl is connected to the terminal C side to release the grounding of the core wire 1a, and the residual voltage of the cable 1 is measured by the voltmeter 2.

The deterioration of the cable 1 is determined from the residual voltage, and the table below shows an example of the measurement conditions and criteria.

Margins below [Effects of the Invention] In the present invention, since an alternating current voltage is applied as the voltage applied to the cable during testing, deterioration can be determined in accordance with the actual usage state of the cable. That is, compared to conventional testing methods, insulation deterioration such as water tree deterioration can be detected more accurately and a highly reliable determination effect can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an example of a residual voltage measuring circuit applied to the test method according to the present invention, and Fig. 2 is a schematic diagram showing the electric charge distribution in the test cable and its insulator. Conceptual diagram, Figure 3 is a graph illustrating the potential distribution in the insulator when the core wire of the cable is open and grounded, and Figure 4 is a graph illustrating the generation of residual voltage. It is. 1 ...... Test cable 1a ... Core wire 1b ...... Shielding layer (sheath) 2 ...
··voltmeter

Claims (1)

[Claims] In the test method for determining the insulation deterioration of the power cable by applying a voltage to the core wire of the power cable, then grounding the core wire for a predetermined period of time, and then measuring the residual voltage of the core wire after the core wire is released from the grounded state, the above-mentioned A power cable insulation deterioration testing method characterized by using an alternating current voltage as the voltage applied to the core wire.