The complete set of frequency conversion series resonance test equipment consists of a frequency conversion power supply, excitation transformer, reactor, and capacitive voltage divider; The capacitor and reactor of the test sample form a series resonant connection, and the voltage divider is connected in parallel to the test sample to measure the resonant voltage on the test sample and provide an overvoltage protection signal. The frequency modulation power output is coupled to the series resonant circuit through an excitation transformer, providing excitation power for series resonance.
Working principle of the complete set of frequency conversion series resonance test equipment: It is known that when the circuit frequency f=1/2 π√ LC, resonance occurs in the circuit. At this time, the voltage on the test sample is Q times the output voltage of the excitation transformer high voltage end. Q is the system quality factor, that is, the voltage resonance multiple, usually ranging from tens to hundreds. First, the output frequency of the frequency conversion power supply is adjusted to cause series resonance in the circuit, and then the output voltage of the frequency conversion power supply is adjusted under the condition of circuit resonance to achieve the test value of the test sample voltage; Due to the resonance of the circuit, a smaller output voltage of the variable frequency power supply can generate a higher test voltage on the test sample CX. The working principle of the variable frequency series resonance test set is shown in the following figure:
Due to the fact that the cable is a capacitive load, the power frequency voltage output by the step-up transformer to the cable will have a capacitance rise effect. The size of the capacitance rise is related to the size of the cable capacitance and the resonance between the step-up transformer and the cable capacitance. Usually, the capacitance rise may exceed 20% to 30%. Therefore, a voltage divider needs to be connected in parallel at the output end of the step-up transformer to accurately measure the test voltage on the cable and prevent the insulation of the cable from being damaged due to high voltage on the cable. The advantages of this test system are that the circuit is simple, the operation is convenient, and it can pressurize and burn cables with insulation defects to discover the fault point; The disadvantage is that the system has a large volume, the output power is the same as the input power, and the power consumption is high. When the pin is broken down, the high voltage output of the boosting transformer directly discharges to the ground, which can easily cause an increase in ground potential, equipment damage, and threaten personal safety. Moreover, due to the difficulty in controlling the degree of cable combustion, several layers of cables are often burned out, causing unnecessary losses to the cable factory.
Due to the needs of power development, cables produced by cable factories have increasingly higher voltage levels, larger cross-sectional areas, and longer lengths. Therefore, the capacity of factory voltage withstand testing equipment is also increasing; Due to its own defects, the typical test system for step-up transformers can no longer meet the requirements of cable factory withstand voltage testing, especially with the deepening of the two network transformation, the demand for overhead insulated wires and high-voltage cross-linked cables is increasing. The complete set of variable frequency series resonance test equipment is suitable for the handover and preventive testing of high-capacity, high-voltage capacitive test samples such as power cable series resonance, power transformer series resonance, generator sets (hydro generators or thermal power generators), motor series resonance, switchgear series resonance, GIS switches, etc.