JPH077028B2 - Method for measuring DC component of power cable - Google Patents

Description

Description: TECHNICAL FIELD The present invention diagnoses the insulation performance of a power cable under a hot line (in the state of applying an AC voltage). Therefore, the DC component of the current flowing between the conductor and the shielding layer of the power cable is detected. It concerns a method of measuring.

[Prior art]

It is known that the magnitude of the DC component of the current flowing between the conductor and the shielding layer of the power cable under the live line, that is, the current flowing through the insulating layer is closely related to the deterioration of the insulating layer. Therefore, by measuring this DC component, it is possible to detect the water tree or the like generated in the insulating layer of the CV cable.

Conventionally, this DC component is measured as shown in FIG. The power cable 1 has a structure in which a shield layer 3 is provided on the outer periphery of a conductor 2 via an insulating layer (not shown), and the conductor 2 is connected to a high voltage bus bar 4 and is in a live state. Further, for example, a GPT (grounding type instrument transformer) 5 is connected to the high voltage bus bar 4, and its neutral point is grounded. Both ends of the shield layer 3 are grounded in a normal operating state, but when measuring a DC component, a parallel circuit of an AC grounding capacitor 6 and a DC current measuring device 7 is provided between one end of the shield layer 3 and the ground. And ground the other end. As a result, the shield layer 3 is in a state of being insulated from the ground in terms of direct current and being low impedance grounded in terms of alternating current.

By doing this, the DC component flows from the high voltage charging section through the route of the conductor 2 of the power cable → the same insulating layer → the same shielding layer 3 → DC current measuring device 7 → ground → GPT5 → high voltage charging section as indicated by the one-dot chain line arrow. It will be. Therefore, if the value of the DC current measuring device 7 is read, the DC component of the current flowing through the insulating layer of the power cable can be measured.

When the high-voltage bus 4 is connected to a grounding transformer or a neutral point grounding type transformer, the neutral point grounding part of the transformer may be used instead of the GPT5.

〔problem〕

However, when another power cable 11 that is not a measurement target is connected to the same phase of the GPT5 as shown in the figure, the DC component of the current flowing between the conductor 12 and the shield layer 13 of the other power cable 11 is a dotted arrow. And the measured value of the DC current measuring device 7 is affected.

When multiple power cables are connected to the same phase of the transformer with the neutral point grounded in this way, an error occurs in the measured value of the DC component of the cable to be measured, and diagnosis of insulation performance is performed. You may make a mistake.

[Means for solving problems and their effects]

In view of the above problems of the prior art, an object of the present invention is to combine three transformers (GPT, grounding transformer, neutral point grounding type transformer, single phase transformer) whose neutral point is grounded. When multiple power cables are connected to the same phase (such as a neutral point grounding type transformer), the DC component of the current flowing between the conductor and the shield layer can be measured accurately for one of the power cables. To provide a way to do well.

In order to achieve this object, the present invention connects a parallel circuit of an AC grounding capacitor and a DC current measuring device between the neutral point of the transformer and the ground in which a plurality of power cables are connected to the same phase. At the same time, the shield layer of the cable to be measured is connected to the ground side of the parallel circuit, and the shield layer of the cable to be non-measured is connected to the neutral point of the transformer, and then flows to the DC current measuring device in that state. It is characterized by measuring a direct current.

In this way, the DC component of the non-measurement target cable does not flow into the DC current measuring device, and it becomes possible to accurately measure the DC component of only the measurement target cable.

〔Example〕

An embodiment of the present invention will be described in detail below with reference to FIG.

In FIG. 1-, the same parts as those in FIG. This measurement method is different from the conventional method in that the parallel circuit of the AC grounding capacitor 6 and the DC current measuring device 7 is connected to the GPT5.
Is connected between the neutral point and the ground, the shield layer 3 of the measurement target cable 1 is connected to the ground side of the parallel circuit by the electric wire 8, and the shield layer 13 of the non-measurement target cable 11 is neutral to the GPT5. The measurement is performed with the electric wire 9 connected to the point.

In this way, the current flowing between the conductor 2 of the measurement target cable 1 and the shielding layer 3 flows as shown by the alternate long and short dash line, and its DC component flows to the DC current measuring device 7, but the current flowing between the conductor 12 of the non-measurement target cable 11 and The current flowing between the shield layers 13 flows as indicated by the dotted arrow and ceases to flow into the DC current measuring device 7.
Therefore, the DC current measuring device 7 can accurately measure only the DC component of the current flowing between the conductor 2 and the shield layer 3 of the cable 1 to be measured.

In the above-mentioned embodiment, the shield layer of the non-measurement target cable and the neutral point of the GPT are directly connected by an electric wire, but the same may be achieved by connecting them through a DC cut filter similar to the AC grounding capacitor 6. The effect is obtained.

Although only one non-measurement target cable is shown in the above-mentioned embodiment, the same is true when a plurality of non-measurement target cables are connected to the same GPT.

In the drawings of the above-described embodiment, the power cable for one phase is shown for convenience, but it goes without saying that the power cable is similarly connected to other phases. Also, the power cables are three single-core power cables and three-phase three-core power cable 1
There are cases of books.

〔The invention's effect〕

As described above, according to the present invention, a plurality of power cables are connected to the same phase of a transformer with a neutral point grounded, and a current flowing between a conductor and a shield layer for one of the power cables. When measuring the DC component of, it is possible to measure only the DC component of the current flowing between the conductor of the measurement target cable and the shield layer without being affected by the current flowing between the conductor of the non-measurement target cable and the shield layer. Therefore, the measurement accuracy can be improved and the reliability of insulation performance diagnosis can be improved.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a DC component measuring method of a power cable according to an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional measuring method. 1: cable to be measured, 2: conductor, 3: shield layer, 4: high voltage busbar,
5: GPT, 6: AC grounding capacitor, 7: DC current measuring device, 8/9: Electric wire, 11: Non-measurement target cable, 12: Conductor, 1
3: Shield layer.

Claims (1)

[Claims] 1. A direct current component of a current flowing between a conductor and a shield layer for one power cable among a plurality of power cables connected to the same phase of a transformer whose neutral point is grounded. In the method of measuring, between the neutral point of the transformer and the ground, a parallel circuit of an AC grounding capacitor and a DC current measuring device is connected, and the shielding layer of the cable to be measured is connected to the ground side of the parallel circuit. And connecting the shielding layer of the non-measurement target cable to the neutral point of the transformer, and measuring the DC current flowing through the DC current measuring device in that state, measuring the DC component of the power cable. Method.