Charge Transport And Conductivity Modification Of Silicone Rubber Composites For HVDC Cable Acessories Under Electric-Thermal Compound Field

High voltage direct current(HVDC)cable accessory is the key equipment of the HVDC transmission system.HVDC cable accessory insulation suffers from charge accumulation caused by the non-uniform electric field distribution.This intensifies the electric field distortion and the aging of insulation.Temperature and overvoltage also intensify charge injection and accumulation.This dissertation prepares silicone rubber(SR)composites with nonlinear conductivity and studies the charge transport and conductivity under thermal-electric field.This dissertation aims to provide the theory and experiment fundament for the development of nonlinear-conductivity-composite insulation for HVDC cable accessories.The main research work and achievements are as follows:(1)The SiC/SR composites are prepared and the nonlinear conductivity of Si C/SR composites is obtained at 30 ～ 90 ℃.It is found that as the Si C content increases,the conductivity-temperature coefficient change from positive to negative.A low conductivity-temperature coefficient composite is obtained,providing the experimental fundament for suppressing the electric field distortion in radial direction caused by temperature gradient.(2)The surface charge and the space charge properties of SiC/SR composites under DC field are obtained.It is found that charge accumulation increases the field in sample and exceeds the threshold field,which leads to the fast transport of carrier through percolation network formed by Si C particles.The surface charge and space charge dissipation is accelerated.High temperature increases the energy of carrier thermal motion,which rises the carrier de-trap propability and accelerates charge dissipation.It is proven that Si C/SR composites can modify charge transport under temperature and DC compound field.(3)The surface charge properties of SiC/SR composites under impulse superimposed DC electric field are obtained.Under heteropolar impulse superimposed DC voltage,a heteropolar charge region is formed in sample.These heteropolar charges attract charges generated by DC voltage to move to the central region of sample,which intensify the charge accumulation.Under homopolar impulse superimposed DC voltage,the surface charge potential is increased by the impulse voltage,which rises the electric filed in sample.The increase in temperature also intensify the carrier thermal motion.These reasons lead to the acceleration of surface charge dissipation.It is proven that the Si C/SR composites can suppress the surface charge properties under temperature and homopolar impulse superimposed DC compound field.(4)The surface charge properties of SiC/SR composites under polarity reversal electric field are obtained.It is found that the inside of the sample central reion has more heteropolar charges than the surrounding regions,which intensifies the induced electric field,leading to high decay rate of surface potential.As a result,the surface potential at the central point is lower than outside positions.The induced electric field between the surface charges and the heteropolar charges in sample increases the conductivity after polarity reversal.This accelerates the surface charge transport to ground and the neutralization between heteropolar charges.