Charge Accumulation in Hybrid High Voltage Insulation

Doctoral thesis, 2003

This thesis deals with the novel idea of controlling the electric stress or field distribution in a hybrid air - solid-dielectric high voltage insulation by promoting accumulation of surface charges on the internal interfacial layers in the combined (hybrid) dielectric structure. Primarily, applications are expected to be found in high voltage dc systems and as a method to improve the resistance against insulation failure by overvoltages, e.g. lightning or switching surges. The unexploited concept introduced in this report is based on the fact that the insulation ability of an air-insulated high voltage system can be increased if the metal conductors are covered with dielectric coatings. The origin is a conditioning effect due to accumulation of charges on the coatings that significantly alters the internal electric stress distribution. A part of the electric stress across the air gap is transferred to and absorbed in the (thick) solid coatings. The outcome is an increase in the integrated breakdown strength of the hybrid system, which authorizes a further extended loading of it. The system conditioning is primarily accomplished by allowing a strictly limited discharge activity in the air gap. The charge formation and its effect on insulation ability of the hybrid insulation was experimentally investigated and also analysed in terms of electrostatic field models and (quasi) two- and three-dimensional numerical discharge computations. The analysed arrangement consisted of two flat dielectric-coated metal electrodes and was separated by an adjustable air gap. The received improvements in insulation ability during dc and impulse voltage applications were substantial and remarkable. Compared to the equivalent bare (uncoated) electrode system, the measured boost in dielectric performance was, for different air gaps, larger than between 40% and 130% for positive dc stress and larger than between 50% and 220% and positive impulse stress. Correspondingly, a significant transfer of the air gap electric stress was also recognized. Juxtaposed to the pure capacitive electric field distribution, electric stress transfers of up to at least 70% for positive dc (92 kV, air gap 6 mm) and 95% for positive impulse (52 kV, air gap 6 mm) load were established. Air gap stress off-loads of roughly the same magnitudes were obtained in the numerical discharge analyses. Thus, the predicted redistribution of the internal electric stress was effectively demonstrated by both experimental and computational means. Several dynamic features of the charge conditioning process were identified. It was e.g. recognized that the insulation concept presents an adaptive property of adjusting the insulation level to the encountered air gap electric stress.