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The necessity of conducting AC withstand voltage tests on high-voltage electrical equipment and the application of resonance boosting methods in power systems. In order to accurately determine whether electrical equipment meets operating conditions, prevent equipment damage, and ensure safe system operation, one effective measure is to conduct insulation withstand voltage tests on equipment. The insulation withstand voltage test includes two testing methods: DC withstand voltage and AC withstand voltage. The DC withstand voltage test is widely used due to its device's small capacity, small size, light weight, and easy portability. But not all electrical equipment can be explained through DC withstand voltage tests, such as rotor withstand voltage tests for generators, cross-linked power cables GIS、 Voltage withstand test of transformers, etc; Practice has shown that only AC withstand voltage testing can achieve the actual effect of withstand voltage testing for certain electrical equipment. It can effectively detect more dangerous concentrated defects and has decisive significance in determining whether electrical equipment can be safely put into operation. It is an important means to avoid insulation accidents in electrical equipment and the most direct and effective method to identify the insulation strength of electrical equipment.

The resonant boost method is currently the most suitable method for large capacitive samples in power systems, which can be divided into power frequency resonant boost method and frequency conversion resonant boost method. The power frequency AC high voltage test has a wide range of applications, but the tuning is relatively complicated, and the induction method is usually used; The variable frequency AC high voltage test is limited by the test standards, but it is easy to tune and operate, usually using frequency modulation method. After the power frequency is defined as 45H z-55H z (even defined as 45H z-65H z), especially when frequency and inductance methods are used simultaneously, the system can easily meet the frequency requirements of regulations and the requirements of equipment for wide range capacitance samples.

The principle of frequency conversion series resonance withstand voltage test

Thevariable frequency series resonant withstand voltage test deviceconsists of a variable frequency power supply, excitation transformer, high-voltage reactor, and capacitive voltage divider. By changing the inductance and test frequency of the test system, the circuit is in a resonant state. At this time, the circuit impedance Z (ω o)=R is pure resistance, and the voltage and current are in phase. In this way, most of the capacitive current on the test sample in the test circuit is offset by the inductive current on the reactor, and the power supply only provides the energy of the active power consumed in the circuit. The basic principle and equivalent circuit are shown in Figure 1:

Figure 1

Advantages of series resonant AC withstand voltage test device in power system applications:

(1) Series resonant power supply generates high voltage and high current through resonance between resonant reactor and tested capacitor. The power supply only needs to provide the active power consumption in the system. Therefore, the required power supply for the test is only 1/Q of the test capacity, eliminating the need for bulky high-power voltage regulating devices and high-capacity power frequency test transformers. The weight and volume of the equipment are greatly reduced. The weight of the resonant excitation power supply system is generally 1/3-1/5 of that of a regular testing device.

(2) The high-voltage reactor L and the tested equipment Cx are in a resonant state, forming a good filtering circuit. Therefore, the output voltage Uc is a good sine waveform, which can effectively prevent harmonic peaks from causing false breakdown of the test sample during the withstand voltage test.

(3) Series resonant AC withstand voltage test, if the insulation weak point of the test sample is broken down, the resonance condition is destroyed, the circuit immediately loses resonance, the voltage on the test sample drops instantly, the energy is quickly released, and no overvoltage is generated on the test sample. The entire circuit current quickly drops to 1/Q of the normal test current. Therefore, the series resonant withstand voltage test can effectively find the insulation weak point of the test sample, and the short-circuit current at the insulation breakdown point does not pose a risk of expanding the fault point.