JPS59202071A - Insulation diagnosis of cable during hot-line work - Google Patents

Abstract

PURPOSE:To make it possible to accurately measure the deterioration in the insulation of a power cable during hot-line work, by performing judgement by measuring a zero phase current with commercial frequency of the earthing wire of a three-phase AC line. CONSTITUTION:The zero phase current with commercial frequency of the earthing wire 5 from the metal shield layer of a cable 4 to be measured forming a three-phase AC line connected to a high voltage bus 2 is detected by a zero phase current detector 6. An insulating deterioration judge apparatus 8 operates and separates the zero phase current based on the impedance imbalance of a cable insulator by said detected value and the each phase current from a phase voltage detecting part 7 to determine the zero phase current at a balance time when the insulator comes to zero in normal. Therefore, the insulating deterioration of a power cable during hot-line work can be accurately measured.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for diagnosing the insulation of power cables, particularly rubber-plastic insulated power couples, when the wires are live.

When Com Plastic Insulating Capable is used under water, its insulation performance gradually deteriorates (R61).
,well known. This 7-edge performance deterioration phenomenon is caused by water 1 that occurs starting from the cable conductor or internal semiconducting layer.
-1) Water tree deterioration 1. is caused by - (so-called "internal water 1-!J-1)" or water tree originating from the external semiconducting layer (so-called "outer water conduction 1-!J-1"). On the other hand, a method for diagnosing the insulation performance of this cable is to periodically measure the dielectric loss tangent, DC leakage current, partial discharge, etc. of the cable after stopping the charging line. However, this method is not practical because the charging line must be stopped each time.In view of this, in recent years, in live lines (charging lines), it is necessary to A method is sometimes adopted in which a DC voltage of polarity is applied, a DC current flowing between the metal shielding layer of the cable to be measured and the top and bottom is measured, and the insulation resistance is measured from the value.However, the inventor of the present invention made the following discovery in the process of investigating the water 1-RI phenomenon.In other words, a deteriorated cable in which external water-conducting water 1-cou occurred was connected to the negative pole from the conductor side in the same manner as in the normal DC voltage application test. When a DC voltage of
A normal cable with no leakage and a dog plug, which indicates that it is not possible to accurately determine cable deterioration by measuring leakage current using normal DC voltage.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art described above and to provide a method that can accurately determine the insulation deterioration of a live special power cable.

That is, the gist of the present invention is to detect the deterioration state of cable insulation by measuring the zero-sequence current of the commercial frequency appearing in the grounding wire of a three-phase AC line constructed using cables on rubber/plastic insulation. It is in.

In this section, we will explain why commercial frequency zero-sequence current measurement is effective for determining cable insulation deterioration.

The equivalent circuit of a cable insulator for a three-phase AC line can be expressed as shown in Figure 1. In the figure, vertical aXΩb,, V, and C are the voltages of each phase of the line, Ta, Ib, and Ic are the charging currents of each phase of the cable, Zrl, Z+), ZC is the impedance of each phase of the cable, and fo is zero. It is a phase current. In the case of target three-phase AC, charging currents 1a, fb, and fc are expressed by the following equations (1), (2), and (3), respectively.

1, b = a L -t-a' 12 +J, o
(2JIc'= a2L + a I2
+IO(3) current. It is still 1+a+a2-0.

Two, ru.

If the three-phase AC voltage is balanced and the cable insulation is normal, @ (i.e. Za = Zb − Zc) is
a+I+)+IC=0, and To20.

However, if water tree deterioration causes an unbalance in the impedance of the cable insulation (for example, ZaN9\ne or Za=Zb')Zc), Ia-1-Ib+ic
\0, Io';:0, that is, a zero-sequence current flows.

That is, by measuring the zero-sequence current, it is possible to know the deterioration of the cable insulator.

The above is a case where the three-phase AC voltage is balanced, but
In the case of unbalanced (1 standing a1' = IVI) l # IVc
I) has a normal cable (;ja=zb=zc)
flows even in the case of Furthermore, even when the three-phase AC voltage contains harmonic components, a zero-sequence current corresponding to the frequency of the harmonic components flows.

Therefore, in order to determine the state of cable insulation deterioration based on the zero-sequence current, it is necessary to measure the zero-sequence current at the commercial frequency, measure and calculate each phase voltage, and calculate the zero-sequence current based on the impedance imbalance of the cable insulation. By separating the zero-sequence current, it becomes possible to determine the insulation deterioration of the cable from this zero-sequence current.

Next, the present invention will be explained in detail with reference to FIGS. 2 and 3. In addition, Fig. 2 shows a cross-linked polyethylene insulating cable (CV) consisting of three cross-linked polyethylene insulated wire cores collectively coated with a common base. As an example of a case where the target is a cable made by twisting insulated cables (CV'll'), 1 is a power transformer, 2 is a high voltage bus, 3 is a grounding transformer, and 4.4' is an illumination transformer. 5 is a grounding wire from the cable metal shielding layer, 6 is a zero-sequence current detection section, 7 is a phase voltage detection section, and 8 is an insulation deterioration determination device. The insulation deterioration determination device 8 has a Toba circuit, an amplification circuit, a data digital display section, and a digital and analog output section. The tit/tal output from the insulation deterioration determination device is connected to a large-scale electronic computer, where data processing and cheat file are performed.

Examples of the zero-sequence current detection section 6 are shown in FIGS. 4 and 5. FIG. 4 shows a detection impedance using a resistor, which is set to a value of 10Ω or less, which satisfies the second type grounding. Figure 5 shows the detection impedance using 7 inductors, and the reactance is 20, a value that satisfies type 2 grounding.
1ηI-I or less.

As is clear from the following explanation, according to the present invention, it is possible to accurately know the insulation deterioration of cables under live line conditions, and to prevent cable breakdown accidents and, by extension, the resulting power outages. Accidents can be prevented.

[Brief explanation of drawings]

Figure 1 is an equivalent circuit diagram of a cable insulator for a three-phase AC line.
FIG. 2 and FIG. 3 are detailed explanatory diagrams of the present invention, respectively;
FIG. 4 is a diagram showing an example in which a resistor is used as the detection impedance of the zero-sequence current detection section in FIGS. 2 and 3, and FIG. 5 is a diagram showing an example in which an inductance is used in the detection impedance. (1 Power transformer) (2, High voltage disconnection wire) (3 Grounding transformer) (4, 4': Cable under test) (5 Grounding wire) (6: Zero phase current detection section) (7 Phase voltage detection Part) (8° Insulation Deterioration Judgment Device) Procedural Manual (Voluntary) 1 Case Display Showa I] 1958 Special a'l Immediate No. 7
6884 No. 2 Name of the invention Cable live insulation diagnosis method 4, Agent 100 Address 2-1-2 Marunouchi, Chiyoda-ku, Tokyo Telephone Tokyo (216) 1G+, 1. (Main representative) 5.
Figures 2 and 3 of the drawings to be corrected. 6. Details of the amendments are as shown in the attached sheet. 7. Drawing showing Figures 2 and 3 after the revised list of attached documents. 1 or more

Claims (1)

[Claims] Cable activity characterized by detecting the insulation deterioration state of the cable by measuring the zero-sequence current of the commercial frequency appearing in the grounding wire of the three-phase AC line constructed using the 1com plastic insulated cable cable. Line insulation diagnosis method.