What are the main methods of power cable withstand voltage test?
Cable withstand voltage test is a way to find faults in the presence of cables. It is easily affected by various factors during the test, resulting in problems such as failure and short circuit. The main methods of several cable withstand voltage tests are described below.
First, ultra low frequency withstand voltage test
The ultra-low frequency (0.1Hz) withstand voltage test method first appeared in 1980, and it is mainly a non-destructive test method for observing whether there is insulation defect in cable operation, and the method has been verified by a large number of laboratory tests and field tests. The low-frequency withstand voltage test has a good application effect in the low-voltage power cable withstand voltage test. The method utilizes the principle of converting 50 Hz alternating current into direct current voltage through rectification and filtering, and then converting the direct current voltage into an alternating current voltage of 1 kHz through an inverter circuit, and then performing amplitude modulation processing by a 0.1 Hz sinusoidal oscillator, and after amplitude modulation, 1 kHz. The amplitude wave such as the AC voltage is converted into a 0.1 Hz varying beam wave. It is mainly based on the high voltage formed between the high voltage transformer and the voltage multiplying circuit, which is mainly characterized by a sine wave and is mediated by a varistor, so that the high AC voltage load output is a 0.1 Hz high voltage sinusoidal waveform. The advantages of the ultra-low frequency withstand voltage test are mainly: 1 no damage; 2 high accuracy; 3 small size, easy to carry. However, the output voltage level of this method is low, and it is mainly used in the medium and low voltage cable withstand voltage test.
Second, the modulated power frequency series resonance test
The method mainly utilizes the inductive reactance of the reactor and the capacitive reactance of the measured cable capacitance to generate a resonance in a 50 Hz power frequency environment, and a high voltage is generated in the process. The advantages of the modulated power frequency series resonance test are: 1 the output current waveform is basically a sine wave; 2 is high in specificity, and only after the series resonant circuit satisfies the condition for generating resonance, a high voltage is formed, and the cable to be tested is once tested. If there is a problem, it will cause a loop abnormality, which is equivalent to a short circuit of the cable. The high voltage will also appear to be uniformly dropped. Moreover, since the reactor can limit the short-circuit current, the protection device is not affected, and thus it is not necessary to install a resistor protection device. The disadvantage of this test method is that the operation is complicated, the system quality factor is not high, the automation level is low, and the noise is large, which limits its application in practice.
Third, the frequency conversion series resonance test
The principle of frequency conversion series resonance test is similar to the above-mentioned principle of modulated power frequency series resonance test. The difference is that the frequency conversion series resonance test realizes the resonance of the test circuit by adjusting the output voltage frequency in the variable frequency power supply; In the frequency series resonance test, the resonance of the test circuit is realized by the inductance generated by the regulating reactor at the 50 Hz power frequency. The advantage of the variable frequency series resonance test is that when the required power supply capacity of the test is lower than the power supply capacity of the cable under test, the test can be performed at a much lower power than the required power supply, which can effectively improve the efficiency of the field test and can overcome The shortcomings of the modulated power frequency withstand voltage test device make it more widely used in real life. In addition, the operating frequency of the variable frequency series resonance test is only 30 to 300 Hz, which can improve the problem that the frequency of the ultra-low frequency withstand voltage test is too low and the loss is small, which is inconsistent with the actual loss. Therefore, the results obtained by the variable frequency series resonance withstand voltage test are precise, comprehensive and reliable.