JPS59202076A - Detection of insulation deterioration of cable - Google Patents

Abstract

PURPOSE:To attain to enhance the detection accuracy of cable insulation deterioration, by measuring positive and negative polarity DC leak currents by adding a stray current negating circuit. CONSTITUTION:DC voltage is applied through impedance 32 from a DC voltage generating apparatus 31 with a polarity change-over device and DC leak currents are measured by DC leak current measuring circuits 41, 42. At this time, various earthing potentials are generated at earthing joints 21-25. Stray current negating circuits 51, 52 consisting of minute DC voltage generating sources of 0-5 volt and voltage dividing variable resistors are added to joints 25, 27 or 26, 28 of the measuring circuits 41, 42 in parallel to said measuring circuits and the variable resistors are controlled prior to applying positive and negative polarity DC voltages to bring the currents flowing the measuring circuits 41 or 42 to zero and, thereafter, DC voltages are applied to measure leak currents.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for detecting insulation deterioration of an electric cable. As a conventional method for detecting insulation deterioration of an electric cable, a method is generally known in which a DC voltage is applied to the cable insulator and a DC leakage current at that time is measured to detect the deterioration state of the cable insulator. Furthermore, recently, a method of more accurately detecting the deterioration state based on the characteristic difference between positive and negative polarity DC leakage currents when positive and negative polarity DC voltages are applied has been studied. However, when this method is applied to an actual installed cable, stray current may be included in the DC leakage current measurement due to the influence of the ground potential difference along the cable, resulting in incorrect judgment, especially when looking at differences due to polarity. may occur.

An object of the present invention is to eliminate the drawbacks of the prior art described above, to more accurately measure positive and negative polarity DC leakage currents, and to improve the accuracy of cable insulation deterioration detection.

In other words, the gist of the present invention is to eliminate direct current leakage currents of positive and negative polarity by adding a stray current canceling circuit that can be adjusted to minimize the influence of stray currents before, during, or after measuring leakage direct current. There is something to be measured. By the way, cables are generally used in a fairly long length.

On the other hand, long cables are grounded at multiple points to ensure safety and prevent induction. Phenomena may generate electric potential. It has already been confirmed that by applying positive and negative DC voltages to the insulation of the cable to be examined and knowing the difference in the characteristics of the DC leakage current, the determination of the deterioration status of the insulation can be dramatically improved. . On the other hand, in this measurement, a closed circuit including the above-mentioned ground potential difference is often used in the measurement circuit. Also,
Stray currents based on the ground potential difference are generated over a relatively long period of time (
The longitude required for leakage current measurement) often have approximately the same polarity and a constant value. Therefore, in this situation, the positive
When measuring negative polarity leakage current, positive and negative polarity leakage currents are measured including stray currents of polarity that increase one polarity and decrease the other. In some cases, the difference in leakage current characteristics may be significant.
Conversely, it is also possible that the characteristic difference between positive and negative polarity leakage currents is reduced, leading to erroneous judgments. Therefore, in order to reduce such errors as much as possible and measure positive and negative polarity leakage currents more accurately, it is necessary to detect stray currents and correct the leakage currents.

However, if a stray current canceling circuit is added, the measured current becomes -1 the leakage current of the cable insulator, making it easier to examine the polarity difference.

Next, an embodiment of the present invention will be described with reference to FIGS. 1 and 2. Electrical equipment 11. ．． 12 are connected by a cable 13 and further grounded at points 21-24. In order to detect insulation deterioration, a DC voltage is applied to such a stem from the DC voltage generator 31 with plug-type switching device through the impedance 32, and the DC leakage current is determined by the DC leakage current measuring circuit 41 or 42. do. At this time, the ground connection points 21-2
5, various ground potentials occur. In Figure 1, this is isometrically represented as a battery symbol! It was shown in DC leakage current measurement circuit 41
Alternatively, a stray current flows between the 420 connection points 25, 27 or 26, 28 because the current measuring circuit is in a state equivalent to being short-circuited due to the low impedance. It is also possible to measure these currents before applying the DC leakage current and calculate and correct the DC leakage current before applying the DC voltage, but in order to know the change in leakage current over time, it is necessary to always perform this calculation. It's an inconvenience that you have to do. Konodame connection point 25.2
7 or 26.28 in parallel with the DC leakage current measurement circuit.
A stray current canceling circuit 51 or 52 consisting of a minute DC voltage source of ~5\・' and a voltage dividing variable resistor is added, and a positive or (
d. Adjust the variable resistor before applying the negative polarity DC voltage to set the current flowing in the current measurement circuit 41 or 42 to O,
Apply DC voltage and measure leakage ■: Current. As a result, since the change in stray current within the measurement period (usually within one hour) is small, the measured current leaks out:
This can be regarded as a flow, and the subsequent deterioration detection/determination operation becomes extremely simple.

In addition, if there is a possibility that the change in stray current is large, please refer to 11.
, Ichimata - If the above adjustment is made before applying the negative polarity DC voltage, the influence of the change will be reduced. In addition, the DC leakage current after the DC voltage impression is read and the correction is made to make the influence of the stray current even smaller than in the case of 11.

In FIG. 1, points 61 to 65 and other points (r o i
It is also possible to configure the circuit so that a switch is placed at point 79++ and the DC leakage current of the cable under test can be measured by opening and closing the switch. Additionally, the present invention provides cable shielding layer 14.
It is possible to configure the circuit even when there is no. In this embodiment, a DC voltage is applied from the cable conductor side, but if the cable shielding layer side is insulated from the earth, a DC voltage is applied from the shielding layer side and flows through the corrosion protection layer to the earth. 1
It is also possible to configure a circuit that cancels the non-current component '1 and detects only the leakage current flowing to the conductor side. In addition, the resistors 8] and 82 in the DC leakage current measuring circuit 41 in Fig. 2 are placed to adjust and protect the measurement range of the measuring circuit, and a gear, knob, or arrester is connected in parallel for protection. It may also be inserted into.

In the stray current cancellation circuit 51, the polarity of one set of batteries is changed, and the variable constant voltage DC%' using an AC power source is used.
It is also possible to cancel any stray current by using a resistor, adjusting a series resistor, etc. Furthermore, DC Shimotsuki - power source for application:
31. By selecting the impedance 32, the cable 13 is supplied with AC voltage and current from the electrical device 11 or 1-12.
□Flow can also be measured.

As described above, the present invention can be applied to the detection of insulation deterioration of cables at installation sites where stray current due to earth electricity (i7) is expected, and as a result, it is possible to detect sudden power outages. It is possible to prevent sudden accidents due to insulation deterioration, which have a major impact on society, such as signal outages, and the economic and social effects can be said to be extremely large.

[Brief explanation of the drawing]

Figure 1 shows an example of the method for detecting insulation deterioration of cables according to the present invention M1.
FIG. 2 is a diagram showing an example of the structure of the first stray current canceling circuit used in the embodiment of FIG. 1. (11,]2: Electric device (electrical device or signal source)) (]3
．． Cable) (14, Cable shielding layer) (]5. Earth) (21 to 26: Ground connection points) (27, 28: Measurement circuit connection points) (31° DC voltage generator) (32: Connection impedance) ( 41, 42: DC leakage current measurement circuit) (51, 52:
Stray current cancellation circuit) (61-65: circuit configuration, closing point) (81-83
:Adjustment or protection resistance)

Claims (1)

[Claims] 1. A method of applying positive and negative DC voltages between the conductor of the cable to be measured and the ground or shielding layer, and detecting and determining the insulation deterioration of the cable from the positive and negative DC leakage currents flowing through the cable at this time. A method for detecting insulation deterioration of a cable, characterized in that a stray current canceling circuit is added to reduce the influence of ground stray current along the cable.