FAQ about DC Dielectric and Leakage Test for Electric Cables

Question 1：The paper insulated power cable only adopts the DC withstand voltage test.

(1) The capacity of the cable is large. AC withstand voltage test needs test transformer with large capacity，but there is no such test conditions on-site.

(2) AC voltage withstand test may cause free discharge in the gap of paper insulated cable, which damages the cable. When the voltage is the same, the AC voltage is more harmful to the cable insulation than the DC voltage.

(3) The leakage current can be measured at the same time of  AC voltage withstand test. According to the value of the leakage current and the relationship between the leakage current and the test voltage, the insulation condition of the cable can be determined.

(4) If there are some local voids in the paper insulation, the DC voltage is mostly distributed on the parts that are related to the defects. Therefore, it is easier to expose the local defects of the cable.

Question 2：Polyethylene cable should not be tested by DC high voltage

The distribution of electric field of XLPE cable under AC and DC voltage is different. XLPE cable insulation layer is made of polyethylene by chemical cross-linking. It is an integral type insulation structure. Its dielectric constant is 2.1~2.3, and it is generally not affected by temperature change. Under AC voltage, the electric field distribution in the insulation layer of XLPE cable is determined by the dielectric constant, that is, the electric field strength is inversely proportional to the dielectric constant, and this distribution is relatively stable.

Under the influence of DC voltage, the electric field intensity in the insulation layer is directly allocated according to the insulation resistance factor, and the distribution of the insulation resistance coefficient is uneven. This is because the inevitable by-product into a certain amount in XLPE process, such as methane, acetophenone, polyvinyl alcohol, they have a relatively small coefficient of insulation resistance. The distribution of radial insulation layer is uneven. So that under DC voltage, the electric field in the XLPE cable insulation layer’s distribution is different from the ideal cylinder insulation structure, and related to the material inhomogeneity.

Question 3：The main insulation of oil filled cable doesn't do DC withstand voltage test when it is put into operation

(1) Surveillance by other tests. In operation, the external force may have damaging effects on the self-contained oil filled cable, which can be measured by the outer sheath insulation resistance and to monitor the oil pressure. Insulation aging can be carried out through the oil monitoring, performance changes so unnecessary for DC voltage test.

(2) High voltage and difficult test. The voltage level of self-contained oil filled cable is high, so the test voltage is high, but also in the terminal head around and many other electrical equipment, is generally difficult for high voltage test.

Based on the above reasons, the main insulation of the capacitive oil filled cable will not be subjected to DC voltage withstand test except in special circumstances.

Question 4：When measuring the DC leakage current of the cable, the pointer of the micro meter has a swing

If there is no impact on cable terminal and dirt test power instability and other factors, in the measurement, DC microammeter cyclical swing, possibly because of porosity defects in local cable test in. Porosity defect breakdown occurs at a certain voltage, leakage current increases, the cable capacitance is gap after discharge breakdown. When the electric cable exit charging voltage was gradually increased, the gap is again breakdown; then, the insulating gap again to get recovery. Thus, the periodic oscillation of the micro meter in the measurement is repeated.

Question 5：Precautions for measuring leakage current of cables over 10kV

Measurement of 10kV and power cable leakage current combined with DC voltage. The test voltage is divided into 4~5 grades and rises to 3-6 times the rated voltage. Because of the higher voltage, corona discharge may occur at the end of the lead and the cable when the voltage increases. After the DC test voltage exceeds 30kV, the leakage current of electric cable with good insulation increases, so the leakage current with test voltage rise and rapid growth of the phenomenon, does not necessarily mean that the power cable is defective. At this time, we must use the interelectrode insulating layer, barrier or cover, and bold lead, increasing the lead to the distance and other measures to reduce stray corona discharge leakage current, and then judge real power cable insulation level according to the measurement results.