Electrical Charges on Polymeric Insulator Surfaces and their Impact on Withstand Performance

Licentiatavhandling, 2009

Use of polymeric materials for high voltage outdoor insulation systems is increasing continuously. Insulating polymers gradually replace traditional materials like porcelain and glass and become dominant at ultra high voltages, e.g., in 1000 kV ac and 800 kV dc overhead power transmissions, which are presently being developed. Polymeric materials in such systems are exposed to extremely high electric stresses that can become strong enough to initiate discharge activities in the surrounding air, e.g., in form of corona. Such discharges lead to generation of free electrical charges, which, while being deposited and accumulated at gas-solid interfaces, may alter the electric field distribution in the insulation system and result in unexpected performance. The presented work focuses on developing computational tools for analyzing three constitutive processes of the phenomenon involved; namely, corona discharge in air and charging of polymeric materials surfaces, relaxation of deposited surface charges at gas-solid interfaces and evaluation of flashover characteristics of polymeric insulators in presence of surface charges. Physical backgrounds, mathematical formulations and computer implementations of the three developed models are presented in the thesis. Validation of the approaches used is carried out by comparing results of computer simulations with those obtained from measurements and available in the literature. The verified models are further employed to analyze several study cases. Thus, positive impulse corona modes in air in a needle-plane electrode arrangement, charging of thick polymeric materials samples by burst and glow positive coronas in air and effect of humidity on corona current and charge generation during corona activity are investigated using the developed model of corona discharge. Further, relaxations of charges deposited on surfaces of silicone and ethylene-propylene-diene-monomer based rubber samples are considered. The corresponding computer model accounts for several mechanisms of charge decay, e.g., neutralization by atmospheric ions, charge injection and transport in the solid material. Finally, the effect of pre-deposited surface charges on flashover characteristics of a polymeric post insulator is examined. Effects of the magnitude of the pre-deposited surface charge, its location on insulator surface and properties of the dielectric material of the insulator are considered. The obtained regularities of the flashover characteristics are analyzed in terms of electric field distributions in the vicinity of the insulator surface.