What are the main methods of voltage withstand test for power cables

Cable withstand voltage test is a method of detecting faults in the presence of cables. During the testing process, it is susceptible to various factors, leading to problems such as faults and short circuits. Below are several main methods for cable withstand voltage testing.

1、 Super low frequency withstand voltage test

The Super low frequency (0.1Hz) withstand voltage test method first appeared in 1980. It is mainly a nondestructive testing method to observe whether there are insulation defects in cable operation. This method has been tested in a large number of laboratories and on-site tests. Low frequency voltage withstand test has good application effect in low-voltage power cable voltage withstand test. This method utilizes the principle of converting 50Hz AC power into DC voltage after rectification and filtering, and then converting the DC voltage into 1kHz AC voltage through an inverter circuit, followed by amplitude modulation processing at 0.1Hz.

Sinusoidal oscillator, amplitude modulated, 1 kHz. The AC voltage amplitude wave is converted into a beam wave with a 0.1Hz variation. It is mainly to use the high voltage formed between the high-voltage transformer and the voltage doubling circuit, with the sine wave as the main feature and the Varistor as the intermediary, so that the AC high-voltage load can output a 0.1Hz high-voltage sine wave waveform.

The main advantages of Super low frequency withstand voltage test are: 1. No damage; 2. High precision; 3. Small size, easy to carry. However, the output voltage level of this method is low, and it is mainly used for voltage withstand test of medium and low voltage cables.

2、 Modulation power frequency series resonance test

This method mainly utilizes the inductance of the reactor and the capacitance of the measured cable capacitance to generate resonance in a 50Hz power frequency environment, resulting in high voltage during the process.

The advantages of modulating the power frequency series resonance test are:

1. The output current waveform is basically a sine wave;

2. High specificity, only when the series resonant circuit meets the conditions for generating resonance can high voltage be formed, and a test is conducted on the cable to be tested.

If there is a problem, it will cause an abnormal circuit, equivalent to a short circuit in the cable. The high pressure will also experience a uniform decrease. Moreover, since the reactor can limit short-circuit current and the protection device is not affected, there is no need to install a resistance protection device. The disadvantages of this test method are complex operation, low system quality factor, low automation and high noise, which limit its application in practice.

3、 Frequency conversion series resonance test

The principle of frequency conversion series resonance test is similar to the above modulation power frequency series resonance test. The difference is that the frequency conversion series resonance test achieves the resonance of the test circuit by adjusting the output voltage frequency in the frequency conversion power supply; In frequency series resonance testing, the resonance of the test circuit is achieved by adjusting the inductance generated by the reactor at 50Hz power frequency.

Advantages of frequency conversion series resonance testing:

1. When the required power capacity for testing is lower than the power capacity of the tested cable, testing can be carried out at a power much lower than the required power, which can effectively improve the efficiency of on-site testing and overcome the shortcomings of modulation power frequency withstand voltage testing devices, making them more widely used in practical life.

2. The working frequency of the frequency conversion series resonance test is only 30~300Hz, which can improve the problem of low frequency, low loss and inconformity with the standard of the Super low frequency withstand voltage test.

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