JPS6330584B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a simple method for detecting water trees that may occur in the insulation of rubber-plastic insulated power cables. Generally, in rubber-plastic insulated cables, one of the causes of insulation deterioration is water trees caused by the expansion of water voids in the insulator. Predicting this water tree in advance is an important technical issue in order to prevent cable destruction accidents. Conventionally, as water tree detection means, a method of measuring direct current leakage current, a method of measuring tan δ, or a method of using both of these methods are known. However, these detection methods have the following drawbacks. That is, in the method of measuring DC leakage current, the current does not increase until the cable is about to break down, so water trees cannot be detected in advance. Furthermore, in the method of measuring tan δ, the measured value indicates average deterioration over the entire length of the cable, so it cannot detect partial deterioration that often occurs when water trees occur in actually installed cables. The object of the present invention is to provide a simple water tree detection method for rubber-plastic insulated power cables that overcomes these drawbacks. In the present invention, by comparing the DC leakage current of the cable before and after applying a DC cutting wave between the conductor of the test cable, which is a rubber-plastic insulated cable, and the cable shielding metal tape, This object is achieved by a method for detecting water trees in rubber/plastic insulated power cables, which is characterized by detecting water trees in the insulation of the cable. The invention will now be explained by way of example with reference to the drawings. As shown in FIG. 1, in the DC cutting wave generation circuit used in the method of the present invention, a ball gap 2 is inserted in parallel between a cable conductor 3 and a cable shielding metal tape 4 in a cable 1 to be tested. A DC high voltage generator 5 is connected to this. A constant DC voltage V _p is applied to the conductor cable 1 by the DC high voltage generator 5, and then the ball gap 2 is short-circuited. At this time, a high voltage is repeatedly applied between the conductor 3 of the cable 1 to be tested and the cable shielding metal tape 4, which is a rectangular damped oscillatory wave having a waveform as shown in FIG. 2 and having a period T. If a water tree is generated in the cable insulation, this short-circuit damped oscillation wave causes electric field concentration in the water tree portion, and an electric tree is generated from the water tree portion. Next, the degree of deterioration can be determined by comparing the measured value of the DC leakage current of the conductor cable 1 with the measured value of the DC leakage current before application of the DC cutting wave. That is, if the DC leakage current value before and after the application of the DC cutting wave remains unchanged, water trees do not occur and the cable under test is normal. In addition, if the DC leakage current value after applying the DC cutting wave is larger than the DC leakage current value before applying the DC cutting wave, water trees have occurred in the insulation of the cable under test, causing deterioration. It can be determined that Note that the DC voltage V _p applied to the cable under test does not need to be a very high voltage, and it is preferable to apply a voltage that does not deteriorate the normal part of the cable. Next, the present invention will be explained with reference to Examples and Comparative Examples. Example 1 Three-phase 6KV cross-linked polyethylene insulated cable (3×
200mm ² , length 850m) was used. As shown in FIG. 1, this cable was connected so that a ball gap was inserted in parallel between the cable conductor and the cable shielding metal tape, and this was connected to a DC high voltage generator. First, a DC high voltage generator
By applying a constant DC voltage of 5 KV to the cable and then shorting the ball gap, a rectangular damped wave vibration of period T having a waveform as shown in Fig. 2 is generated between the cable conductor and the cable shielding metal tape. It was generated in The DC leakage current (1-minute value) was measured for each of the three-phase conductors of the cable before and after the DC cutting wave was applied, and the presence or absence of water trees was detected by comparing these measured values. This result is the first
Shown in the table. Table 1 also shows the AC breakdown voltages measured for each of the three phase conductors of the cable. For the AC breakdown voltage, five cables with an effective length of 10 m for each phase were cut from the test cable before DC voltage was applied, and the average value of these five cables was shown. Example 2 A three-phase 6KV cross-linked polyethylene insulated cable (3×
150 mm ² , length 230 m) was used in the same manner as in Example 1, and the DC leakage current (1 minute value) and AC breakdown voltage were measured before and after applying the DC cutting wave for each of the three-phase conductors of the cable. was measured. The results are shown in Table 1. Comparative example 1 New three-phase 6KV cross-linked polyethylene insulated cable (3 x 200mm ² , long 400m) as the test cable
In the same manner as in Example 1, except that A. was used, the DC leakage current (1 minute value) and A.
C. Breakdown voltage was measured. The results are shown in Table 1.

【table】

[Table] (Note) Red, white, blue: Colors of green bodies in each phase.
_I1 : DC leakage current value of the cable before applying the DC cutting wave.
I ₂ : 〃 Later cable
〃.
As is clear from Table 1, the DC leakage current value of the cable under test after applying a DC cutting wave between the conductor of the rubber-plastic insulated power cable and the cable shielding metal tape is the same as that of the cable before application. Cables with a DC leakage current value greater than
Trees were present in the insulator, and its AC breakdown voltage was also found to be very low compared to a new product without water trees, shown as a comparative example. As is clear from the above explanation, the water tree detection method for rubber/plastic insulated power cables of the present invention has speed and accuracy that are impossible with conventional methods, and has a great practical effect in predicting insulation deterioration. There is.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing an example of a DC cutting wave generation circuit used in the method of the present invention, and Fig. 2 is a schematic diagram showing an example of a DC cutting wave generation circuit used in the method of the present invention. FIG. 2 is a diagram showing an example of a voltage waveform. DESCRIPTION OF SYMBOLS 1...Test cable, 2...Ball gap, 3...Conductor, 4...Cable shielding metal tape, 5...DC high voltage generator, _Vp ...DC voltage, T...period.

Claims (1)

[Claims] 1. In detecting the presence of water trees in a rubber-plastic insulated power cable, the DC leakage current of the cable is measured before and after applying a DC cutting wave between the conductor of the cable under test and the cable shielding metal tape. A method for detecting water trees in a rubber/plastic insulated power cable, characterized in that the occurrence of water trees in the insulator of the cable is detected by comparison.