JPH0378588B2 - - Google Patents

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 cables, when the cables are live.

It is well known that when rubber-plastic insulated power cables are used under water, their insulation performance gradually deteriorates. This phenomenon of insulation performance deterioration is due to water trees that occur starting from the cable conductor or internal semiconducting layer (so-called "internal water tree"), or water trees that originate from the outer semiconducting layer (so-called "outer conductive water tree"). This is due to "water tree deterioration." On the other hand, a method to diagnose 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 recent years, in live lines (charging lines), grounding transformers on distribution busbars have been A method is sometimes adopted in which a single-polar DC voltage is applied from a neutral point, the DC current flowing between the metal shielding layer of the cable to be measured and the ground is measured, and the insulation resistance is measured from that value.However, The present inventors made the following discovery in the process of investigating the water tree phenomenon. That is, a negative polarity DC voltage was applied from the conductor side to the deteriorated cable where the external water tree occurred, as in a normal DC voltage application test. When voltage is applied, the insulation resistance value calculated from the DC leakage current is not much different from a normal cable with no water trees, which means that when measuring leakage current using normal DC voltage,
This indicates that it is not possible to accurately determine cable deterioration.

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 power cable.

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

In the above, the reason why measuring the commercial frequency component in the zero-sequence current is effective for determining cable insulation deterioration will be explained.

The equivalent circuit of a cable insulator for a three-phase AC line can be expressed as shown in Figure 1. In the figure, V〓a, V〓b, V〓c are the voltages of each phase of the line, I〓a, I〓b, I〓c are the charging currents of each phase of the cable, Z〓a, Z〓b, Z〓 c is the impedance of each phase of the cable, and I〓o is the zero-sequence current. In the case of the target three-phase AC, the charging currents I〓a, I〓b, and I〓c are as follows (1) and (2), respectively.
，
It is shown by equation (3).

I〓a＝I〓 ₁ +I〓 ₂ +I〓 ₀ (1) I〓b＝aI〓 ₁ +a ² I〓 ₂ +I〓 ₀ (2) I〓c＝a ² I〓 ₁ +aI〓 ₂ +I〓 ₀ ( 3) However, I〓 ₁ is the positive sequence current, I〓 ₂ is the negative sequence current, and I〓 ₀ is the zero sequence current. Further, 1+a+a ² =0.

Here a is (-1/2+j√3/2), a ² is (-1
/2 −j√3/2).

When the three-phase AC voltage is balanced and the cable insulation is normal (i.e., Z〓a=Z〓b=Z〓c),
I〓a+I〓b+I〓c=0, and I ₀ =0.

However, due to water tree deterioration, the impedance of the cable insulation becomes unbalanced (for example, Z〓a≠
When Z〓b≠Zc or Z〓a＝Z〓b≠Z〓c occurs, I〓a＋I
〓b＋
I〓c≠0, and I ₀ ≠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 applies when the three-phase AC voltage is balanced, but when it is unbalanced (｜V〓a｜≠｜V〓b｜≠｜V〓c
|) Even in the case of a normal cable (Z〓a=Z〓b=Z〓〓c), I〓a + I〓b + I〓c≠0, and I〓 ₀ ≠0, that is, a zero-sequence current flows. Further, even when the three-phase AC voltage includes harmonic components, a zero-sequence current corresponding to the frequency of the harmonic components flows.

Therefore, in order to determine the cable insulation deterioration state based on the zero-sequence current, it is necessary to measure and calculate the commercial frequency components and each phase voltage in the zero-sequence current, 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, an embodiment of the present invention will be described with reference to FIGS. 2 and 3. In addition, Fig. 2 shows a three-core cross-linked polyethylene insulated power cable (CV) in which three cross-linked polyethylene insulated wire cores are collectively covered with a common sheath; As an example of a case where the target is a cable made by twisting polyethylene insulated cables (CVT), 1
is a power transformer, 2 is a high voltage bus, 3 is a grounding transformer, 4 and 4' are cables to be measured, 5 is a grounding wire from the cable metal shielding layer, 6 is a zero-sequence current detection section, 7
8 is a phase voltage detection unit, and 8 is an insulation deterioration determination device.
The insulation deterioration determination device 8 has a wave circuit, an amplification circuit, a data digital display section, and a digital and analog output section. The digital output from the insulation deterioration determination device is connected to a large-scale computer for data processing and data file processing.

Examples of the zero-sequence current detection section 6 are shown in FIGS. 4 and 5. Figure 4 shows the detection impedance, which uses a resistor, and is set to a value of 10Ω or less, which satisfies type 2 grounding. Figure 5 uses inductance as the detection impedance, and the reactance should be 20mH or less, a value that satisfies type 2 grounding.

As is clear from the above explanation, according to the present invention, it is possible to accurately know the insulation deterioration of cables under live line conditions, thereby preventing cable breakage accidents and, by extension, power outage accidents associated with this. It can be prevented.

[Brief explanation of drawings]

Fig. 1 is an equivalent circuit diagram of a cable insulator for a three-phase AC line, Figs. 2 and 3 are explanatory diagrams of embodiments of the present invention, and Fig. 4 is a zero-sequence current detection diagram of Figs. 2 and 3. FIG. 5 is a diagram showing an example in which a resistor is used as the detection impedance of the section, and FIG. 5 is a diagram showing an example in which an inductance is used as the detection impedance. 1...Power transformer, 2...High voltage bus, 3...
Grounding transformer, 4, 4'...Cable under test, 5
... Grounding wire, 6 ... Zero-phase current detection section, 7 ... Phase voltage detection section, 8 ... Insulation deterioration determination device.

Claims (1)

[Claims] 1. A cable characterized by detecting the deteriorated state of cable insulation by measuring the commercial frequency component in the zero-sequence current appearing in the grounding wire of a three-phase AC line constructed using a rubber/plastic insulated power cable. live wire insulation diagnosis method.