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Discussion about the value of insulation resistance of power cable

Information Source:Henan Huadong Cable Co., Ltd  Release date:2014-08-16

1 Specification for insulation resistance of power cable

In the DL/T596-1996 "preventive test electrical equipment regulations", the insulation resistance of the cable value using a "self regulation". This is because when the power cable test project leakage current and DC voltage test for DC, such as insulation resistance is poor, generally can be found in DC leakage current test, the value of insulation resistance, only as a comparison before and after the pressure test for reference. Q/CSG10007-2004 "preventive test procedures" in electric power equipment preventive test, according to the experience, the insulation resistance of provisions of paper insulated power cables and plastic insulated power cable is greater than 1000M Ω, the purpose is to avoid leakage damage power cable current and DC voltage in the dc

2 The relationship between power cable resistance and temperature

Cable factory in the test report is given a 20oC cable insulation resistance value per km, if the cable 500 meters, insulation resistance value when converted to km with the values in Table 1 shows that the cable insulation resistance value is qualified. All kinds of insulation resistance of a cable is converted to a length of 1KM, see the 20oC temperature values are shown in Table 1
Insulation resistance of cable factory is given in the test report is the value of the insulation resistance of the cable sample values, it is not using megger or high resistance meter measured, but by comparison method, are compared with the standard resistor. Due to different cable length of the insulation resistance of different values, in order to unify the rules to the 1KM scale, conversion, the conversion formula for the insulation resistance of Rl--- Rl=Rs L in every kilometer length value (M Ω· KM); Rs--- sample resistance value of insulation resistance (M Ω); L--- sample cable length measurement. (KM)
For example: the sample cable length of 10 meters of insulation resistance value is 32400 M Ω, the insulation resistance of the cable per kilometer is Rl=Rs · L
Rl=32400 * 0.01
=324M
The insulation resistance of the cable by temperature influence is bigger, generally with increasing temperature reduction rule is reduced, so the measurement of insulation resistance to the recording environment temperature, (to record the soil temperature in situ measurements). Cable manufacturing plant given insulation resistance value is converted to 20oC, insulation resistance, the conversion formula is R20---- R20=K · Rl for 20oC; K--- insulation temperature correction coefficient
For example:
When the insulation resistance is measured by the environment temperature is 30 ℃ for 312M Ω conversion to the insulation resistance at 20 ℃ for R20=K R
=1.41 * 312
=440 M

3 The relationship between resistance and soil temperature of power cable insulation

The measurement of power cable insulation resistance and DC leakage current can not be used to record temperature as the temperature conversion basis. Because the power cables buried in the soil, temperature and cable around the different seasons of the year, basically the total temperature, usually in the 120cm below the moist soil temperature 15 ~ 18 ℃, plus cable before each test has power more than two hours, the cable core temperature of cables would drop to the soil temperature., if the temperature conversion, also can be used as the basis for soil temperature. Table 3 gives a long 200KM, 10KV power cable insulation resistance and leakage current measurement data, visible five times test insulation resistance and DC leakage current corresponding values were very close, is the temperature difference, the insulation resistance of no abnormal change; if the conversion according to the temperature record, is beautiful. Relatively large, may be the insulated cable misjudge good as problems.

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