Cross-linked polyethylene cables are used very frequently, which is widely recognized, but you may not know that cables must undergo withstand voltage tests before being put into operation. Common test methods include using an ultra-low frequency cable withstand voltage tester for AC withstand voltage tests, a series resonance test device for AC withstand voltage tests, and a DC high voltage generator for DC withstand voltage tests. The ultra-low frequency cable withstand voltage tester is mainly designed and manufactured for non-destructive insulation withstand voltage tests of cables and motors with voltages of 35kV and below.
Ultra-low frequency cable withstand voltage tester voltage waveforms mainly include sine wave and cosine wave.
First, in the ultra-low frequency system, the required power is very low. Theoretically, compared with the 50Hz system, the 0.1Hz system is 500 times smaller, so the equipment is small in size, light in weight, and the cost is close to that of the DC test system. The 0.1Hz ultra-low frequency test can effectively check the production quality and installation quality of rubber-plastic cables, generators, transformers and other equipment, assess the main insulation of generators and transformers, and the insulation strength of cable terminals and intermediate joints, and more sensitively detect obvious defects such as mechanical damage.
Second, when used for partial discharge measurement, it can suppress 50Hz AC interference.
Third, due to the principle and structure, the output voltage of the ultra-low frequency cable withstand voltage tester is currently relatively low, and it is generally only used for testing rubber-plastic cables of 35V and below and other capacitive electrical equipment. In the field of thermal breakdown, the breakdown voltage is inversely proportional to √ ef tan.6. In the high-frequency range, when the frequency changes, the changes in tan8 and the relative dielectric constant 8 of the dielectric are very small, so the breakdown voltage is inversely proportional to the square root of the applied voltage frequency f, and experimental results also confirm this.
For example, in engineering practice, when performing withstand voltage tests on cross-linked polyethylene power cables, if a 0.1Hz ultra-low frequency withstand voltage test is used, the test voltage cannot be the same as or similar to the rated operating voltage of the cable, otherwise even if the test is passed, the cable may break down during operation. According to the "Test method for ultra-low frequency (0.1Hz) withstand voltage of cross-linked polyethylene insulated power cables of 35kV and below", the test voltage is 3 times the rated voltage, but there are still cases on site where cables passed the test but exploded during operation.
Therefore, although the variable frequency series resonance device is expensive, large in size, and difficult to transport, relatively speaking, the series resonance test method has higher equivalence and is more stringent in assessing power cables.
