Space Charge Behavior Characteristics And Mechanism Of Polyimide At The Edges Of Pulse Voltage

With the development of UHVDC transmission and grid connection of new energy generation,power electronic technology and equipment play an increasingly important role in power system.Different from traditional AC and DC equipment,power electronic equipment operates under the pulse voltage with high frequency and steep rise time,which leads to insulation degradation and failure early.Space charge is an important factor resulting in insulation degradation.Thus,in this paper,polyimide,a common insulating material used in power electronics equipment,is taken as the research object to explore the dynamic characteristics and behavior mechanism of space charge at the edge of pulse voltage.Firstly,a measurement method for dynamic behavior of space charge at pulse edge was proposed based on piezoelectric sensing method and a joint platform used for PEA test and charge dynamic characteristics at pulse edge was established.Then,the function and principle of each module of the test system are introduced,and the parameters of circuit components are calculated and selected.Secondly,a series of experiments were carried out to investigate the dynamic behavior of space charge at the edge of pulse voltage.The results show that there are differences in the internal space charge distribution characteristics before and after the rising and falling edge of pulse voltage,and it is found that there exists charge vibration behavior at the pulse edge.The vibration amplitude depends on the rise and fall times:the vibration amplitude increases dramatically with the shortening of pulse edge time.Besides,as the time of applying voltage increases,a large amount of heteropolar space charges accumulate in the material,leading to polarity of charge vibration waveform reversal.Then,the above experimental phenomena are described from the perspective of forces acting on the space charge.The forces acting on space charge are divided into electric field force and material stress.It is believed that the suddenly increased resultant force at the pulse edge is caused by abrupt change of electric field force leading to charge vibration,and the direction of space charge vibration and migration at the edge of pulse voltage depends on the direction of resultant force acting on the space charge.The mathematical model of space charge vibration under pulse voltage is established,and the relationship between the rise time of pulse voltage and the maximum value of charge vibration is further analyzed by mathematical calculation.In addition,the mechanism of charge vibration is deeply discussed from the perspective of the spectrum characteristics of pulse voltage and charge vibration waveforms.Finally,the damage mechanism of charge vibration on insulating materials under pulsed voltage is analyzed,and the space charge accumulation characteristics under nanosecond and microsecond pulse voltage are compared.The reasons why the insulation is more likely to deteriorate under the nanosecond pulsed electric field are expounded from the aspect of the degree of electric field distortion.