With the continuous acceleration of social development, the demand for electricity is increasing day by day. With the advantage of high power supply reliability, power cables are more and more used in the transmission and distribution domains. At present, cross-linked polyethylene (“XLPE“) insulated power cables are safe and convenient, and easy to operate and maintain. They have replaced oil-paper insulated power and gradually replaced PVC insulated power cables and oil-filled power cables. However, the operation and research in recent years have proved that the insulating material of the cross-linked cable is prone to branch discharge during operation, causing insulation aging, damage, and even affecting its use. because Therefore, a full understanding of the insulation characteristics of power cables, and effective discovery and prevention of defects in the insulation are of great significance for ensuring the safe operation of equipment and systems.

The test of power cable before delivery is completed by the power cable manufacturing department, including routine test, sampling test, form test and pre-qualification test.

1.1 Routine test
Routine test is also called factory test, the main purpose is to inspect
Whether each product has defects caused by accidental factors.

1.1.1 Conductor DC resistance
This test is to check whether the conductor cross-section meets the specified size. If the cross-section is too small or impure conductor materials are used, the DC resistance of the conductor will increase; conversely, if the cross-section is too large and the cable outer diameter is constant, the insulation thickness will become thinner. This situation does not meet the requirements. The DC resistance value of the conductor at 20°C should comply with the relevant ground conductor resistance standards.

1.1 Routine test
Routine tests are also called factory tests, and their main purpose is to check whether each product has defects caused by accidental factors.
1.1.1 Conductor DC resistance
This test is to check whether the conductor cross-section meets the specified size. If the cross-section is too small or impure conductor materials are used, the DC resistance of the conductor will increase; conversely, if the cross-section is too large and the cable outer diameter is constant, the insulation thickness will become thinner. This situation does not meet the requirements. The DC resistance value of the conductor at 20°C should comply with the relevant ground conductor resistance standards.

1.1.2 AC voltage test
The test is carried out on the entire cable. The test time for cables of 110kV and below is 15min. The main insulation of the cable should not be broken under the specified test voltage. The test voltage value U is shown in Table 1: 220kV cable should gradually increase the test voltage To 318kv, keep for 30min, the insulation should not be broken down.

1.2 Sampling test
Sampling test is mainly to verify whether the key performance of the product in the production process meets the design requirements. Generally, 10% sampling is done regularly and quantitatively, and sampling tests are mostly destructive.

1.2.1 Structural dimension inspection
Check the size of the cable structure. The average insulation thickness should not be less than the fixed standard value δ, and the weakest point at any point should not be less than 096~0.1mm. The average thickness of the sheath should not be less than 0.858~0.1mm (in the sheath) The surface is cylindrical) or 0.858~0.2mm (the sheath surface is irregular cylindrical).

1.2.2 4h AC withstand voltage test The test is carried out on a short cable sample.
The test voltage is shown in Table 2. For cables of 110kV and above, the 4h high voltage test is not carried out in the sampling test, but the capacitance value of the cable should be measured, and the capacitance value should not exceed 8% of the nominal value.

1.2.3 Thermal extension test
The purpose of the hot extension test is to check the degree of cross-linking of the cross-linked polyethylene insulation. If the insulation is not well vulcanized and the degree of cross-linking is very low, the elongation under the specified conditions will be very large, and the permanent elongation after cooling is also Big. If the cable cross-linking degree is not enough, the heat resistance and mechanical properties of the cable are very poor, so this is an important index for evaluating the insulation performance of the cable.

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