Many years of operation experience at home and abroad shows that DC voltage withstand test is not suitable for rubber plastic power cable in handover or preventive test, and AC voltage withstand test (frequency range 30-300hz) should be used to ensure the safe movement of cable. This is an epoch-making change in the development history of preventive test of power equipment. However, DC voltage withstand test is still effective for oil immersed paper insulated power cables, and should be continued to be used. It is not suitable to use large capacity power frequency test transformer on site because of the large capacity of the cable. The frequency modulation series resonance high voltage test equipment has the advantages of light weight, small power supply capacity (only 1 / 30 ~ 1 / 80 of the power supply capacity of the test transformer), good voltage waveform, easy to handle and suitable for field use.
This test equipment is specially designed and manufactured for field AC withstand voltage test of rubber and plastic power cable, and is applicable to handover test and preventive test. The main components of the whole set of equipment include: frequency conversion control box, resonance reactor, excitation transformer, capacitor voltage divider, etc.
Fig. 1 wiring diagram of tuned series resonance test principle
The working principle wiring of frequency modulation series resonance high voltage test equipment is shown in Figure 1. The 380V / 50Hz AC power supply outputs a 30-300hz frequency adjustable voltage through the frequency converter, which is sent to the excitation transformer to boost to 0-2000v, and forms a high-voltage main harmonic circuit through the resonance reactor L (the resonance reactor can be used in series or in parallel depending on the tested cable and voltage level and length) and the tested cable Cx. The capacitor voltage divider is pure capacitance type, which is used to measure the test voltage.
Firstly, the frequency converter sends a lower voltage ue to the main resonance circuit through the excitation transformer T, and adjusts the output frequency of the frequency converter. When the frequency reaches the resonance frequency, the output frequency of the frequency converter is adjusted
The voltage UCX on the tested object CX reaches the maximum value. At this time, the inductive reactance
ZL= 
of the resonance reactor is equal to the capacitive reactance of the tested
object, which forms a state of mutual compensation (offset). In the main resonance circuit, there is only equivalent resistance component (effective resistance of winding and equivalent core loss, etc.), which can generate a large current under a small UE voltage, so as to make the tested cable CX harmonious High voltage is generated at both ends of the reactor group. After finding the resonance frequency, keep the frequency f unchanged, and then slowly increase the UE voltage, so that the expected test voltage value UX can be achieved on the tested cable Cx.
During resonance, the relationship between voltage and current on the tested object is
The ratio of output voltage UCX to excitation voltage UE is the comprehensive quality factor Q of the test circuit
It can be seen from the above working principle:
1. The higher the quality factor, the smaller the power capacity required.
2. The resonance reactor L and the tested cable CX are in resonance state, which forms a good filter circuit, so the output voltage UCX sine wave shape is good.
3. When the tested cable breaks down, the resonance condition is lost, and the current of high-voltage circuit and low-voltage power circuit is reduced instead, so the arc at the insulation breakdown will not expand the fault point, which is convenient for maintenance.
