power cable Comparison of two test methods for
Because the capacitance of the cable line is very large, if the power frequency voltage test is adopted, there must be a large capacity power frequency test transformer, and the field test is difficult to achieve; Therefore, the traditional withstand voltage test method is DC withstand voltage test. Because the DC insulation resistance of the cable is very large (generally above 10G), the charging current is very small during the DC withstand voltage test, and the test equipment has the advantages of small capacity, light weight, good mobility, etc; However, the DC withstand voltage test method has many shortcomings for XLPE AC cables both in theory and in practice. Mainly reflected in: 1. A general principle of high voltage test technology, the test voltage applied on the test object must simulate the operating conditions of the high voltage apparatus. The conclusion of passing or not passing the high voltage test shall represent whether the weak points in the high voltage apparatus are harmful to the future operation. This means that the failure mechanism in the test should have the same physical process as that in the operation of electrical appliances. 2. Under DC voltage, the electric field distribution of cable insulation depends on the volume resistivity of the material, while under AC voltage, the electric field distribution depends on the dielectric constant of each medium. In particular, the distribution of DC electric field strength and AC electric field strength in cable accessories such as cable terminals and connector boxes are completely different, Moreover, the mechanism of insulation aging under DC voltage is different from that under AC voltage. Therefore, the DC withstand voltage test cannot simulate the operation condition of XLPE cable. 3. XLPE cable will produce "memory" effect under DC voltage, storing and accumulating unipolar residual charges. With the "memory" caused by the DC withstand voltage test, it takes a long time to release the DC bias. If the cable is put into operation before the DC residual charge is fully released, the DC bias voltage will be superimposed on the peak power frequency voltage, making the voltage value on the cable far exceed its rated voltage, which may lead to cable insulation breakdown. 4. During the DC withstand voltage test, electrons will be injected into the polymer medium to form space charges, which will reduce the electric field strength there and make it difficult to breakdown. Space charges are easily generated at semiconductor protrusions and dirt spots of XLPE cables. However, if the surface flashover of the cable terminal or breakdown of the cable accessories occurs during the test, it will cause wave oscillation on the cable core. At the place where space charges have been accumulated, the polarity of the oscillation voltage rapidly changes to a different polarity, resulting in a significant increase in the electric field strength there, which may damage the insulation and cause multi click breakdown. 5. One of the fatal weaknesses of XLPE cable is that it is easy to generate water tree in the insulation. Once water tree is generated, it will quickly change into electrical tree under DC voltage and form discharge, accelerating the insulation degradation, so that breakdown will occur under the power frequency voltage after operation. The pure water tree can maintain a fairly durable pressure position under AC working voltage, and can maintain for a period of time.
