1. Measuring Insulation Resistance
It should be carried out on each phase separately, with the other two phase conductors, the metal shield or metal sheath and armor layer at both ends of the cable connected. For this test, it is only necessary to note that the cable is a capacitive device, and the situations that should be noted when performing insulation resistance and absorption ratio tests on capacitive devices. For example: sufficient discharge before and after the test, connect after starting the fire, disconnect before stopping the megger, etc.
Insulation resistance changes slightly with temperature; for ambient temperature, soil temperature needs to be recorded for buried cables. The temperature correction coefficient for viscous impregnated paper insulated cables is shown.
Cable
2. DC Withstand Voltage and Leakage Current Test
Oil-paper insulated cables only undergo DC withstand voltage, not AC withstand voltage. Because ACIgincrease may lead to thermal breakdown;In the hot state, the electric field distribution is uneven, which can easily damage the cable. It should be noted: the cable core wire is connected to negative polarity; when the cable is damp, moisture carries positive charge. If the core is connected to negative polarity, moisture will concentrate towards the core wire, increasing moisture in the insulation and leakage current, making defects easier to detect. If the core wire is positive polarity, moisture will permeate towards the lead sheath, reducing moisture in the insulation and decreasing leakage current, making defects harder to detect.
3. Rubber and Plastic Cable Test
Rubber and plastic cables refer to PVC, cross-linked polyethylene, and EPR insulated cables. They are characterized by large capacity, high voltage levels, light structure, and easy bending, and have gradually replaced oil-paper insulated cables.
The difference between cross-linked polyethylene cables and the familiar oil-impregnated paper belted cables, besides the phase-to-phase main insulation being cross-linked polyethylene plastic, is the presence of two layers of semiconductor adhesive coating. On the outer surface of the core wire, a layer of semiconductor adhesive is applied to overcome corona and free discharge, providing a good transition between the core wire and the insulation layer. On the outer surface of the phase-to-phase insulation, within the copper tape shield layer, a second layer of semiconductor adhesive is applied. The copper tape shield layer is only a0.1mmthick thin copper tape, forming the phase-to-phase shield layer.
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