Partial Discharge Characteristics Of Epoxy Resin Of Saturable Reactor Under Exponential Decay Pulse Voltage

As the key equipment for protecting the thyristor in the UHV DC converter valve,the saturable reactor plays a significant role in ensuring the normal operation of the converter valve.However,the insulation of a saturable reactor has long been subjected to a pulse voltage with a large amplitude and a steep rising edge,Partial discharge(PD)due to the pulse voltage is the main cause of insulation failure of the saturable reactor.Nowadays,saturated reactor insulation can only be designed according to the partial discharge standard under sinusoidal voltage,and there is no insulation design criterion under the exponential decay pulse voltage.Therefore,it is of great significance to study the PD mechanism of epoxy resin of the saturable reactor under the exponential decay pulse voltage.Firstly,this paper designed an exponential decay pulse voltage source for simulating the electrical stress of saturable reactors based on the new topology of bipolar Marx circuit and pulse transformer.The circuit of the pulse voltage source was designed for waveform requirements.In order to verify the feasibility of the proposed circuit,the Pspice simulation software was used to build the circuit model,and the simulation results were obtained which has verified the feasibility of the circuit.On this basis,the test system of the pulse generator was built,and the performance of the pulse generator was tested.The pulse voltage source can output bipolar exponential decay microsecond pulse.The amplitude of the positive pulse voltage is adjustable from 0 to 12.12 kV,the pulse width is 11.40 ?s,and the rising edge is 1.18 ?s.The amplitude of the negative pulse voltage is adjustable from 0~12.12 kV,the pulse width is 68.20 ?s,the rising edge is 21.38 ?s.All parameters are basically the same as the designed waveform parameters.Secondly,considering that the saturable reactor may introduce defects during the actual casting process,epoxy air-gap defects and needle-plate defects were fabricated and the construction of the experimental platform was completed.On this basis,the PD experiments of epoxy resin samples under exponential decay pulse voltage were carried out.The partial discharge inception voltage(PDIV)peak,the PD number,the average PD amplitude and the phase resolved partial discharge(PRPD)patterns were obtained.At the same time,PD experiments at 25 °C and 110 °C were carried out and the PD characteristics were compared.Compared with the PD under sinusoidal voltage,fewer PD and larger PD are obtained under pulse voltage.The PDIV peak of the air-gap defects is higher,while the PDIV peak of the needle-plate defect is lower.Finally,in order to illustrate the PD mechanism of two epoxy resin samples under exponential decay pulse voltage,this paper explained the partial discharge mechanism under exponential decay pulse voltage from the perspective of equivalent circuit model and space charge dissipation/accumulation of epoxy resin air gap defects and needle plate defects.Combining the electrical conductivity of epoxy resin and the change of space charge with temperature,this paper explained the difference of partial discharge behavior at 25 °C and 110 °C.Temperature increases the electrical conductivity of the epoxy resin and increases the decay rate of the space charge,which in turn affects the PD characteristics.This paper also compares and analyzes the PD characteristics under the exponential decay pulse voltage and sinusoidal voltage,which can provide a basis for the insulation design of the saturable reactor under the exponential decay pulse voltage.