Performance of insulators under HVDC stress.

Abstract

Ongoing industrialization and expansion in developing economies around the world calls for an upgrade and reinforcement of the current electrical power systems with the use of high voltage direct current (HVDC) transmission and distribution systems. A major factor for these systems is the choice of suitable insulators required by the transmission lines. Insulators under HVDC are expected to function irrespective of the various stresses associated with their use, varying and extreme environmental conditions. The electric field distribution around insulators under HVDC stress is different to that of insulators under an AC stress due to electrical characteristics (resistivity and permittivity) and the space charge accumulation in the air-solid interface of the insulation system. This contributes to the mechanism of breakdown for insulators under DC stress. This study contributes to the knowledge and understanding of insulator breakdown voltage under HVDC stress. Experiments were carried out with a 22 kV silicone rubber insulator according to IEC 60060-1 standards, to understand the breakdown voltage under impulse, AC and HVDC in both dry and wet conditions. The results indicated that the impulse breakdown occurred at a higher voltage, compared to DC and AC, as expected due to the short duration of the applied voltage. It was noted that the breakdown voltage for negative DC was higher than the positive DC, an indication that the space charge generation and distribution may be the cause. Breakdown tests were carried out on 22 kV silicone rubber and glass cap-and-pin insulators under AC, negative and positive polarity HVDC stress where the effect of the surface charge was investigated. The course of these surface charges was modified through a corona source and quantified by measuring the leakage current along the insulator surface. The results were analyzed and showed that the choice of insulator material plays a crucial role in DC insulation system: silicone rubber being an organic material exhibited more current than the non-organic glass insulators. To further aid the understanding of the differences in the electric field, the test arrangements were analysed using finite element models. The simulation results showed that under the application of DC voltage, the presence of space charges on the insulator surface distorts the electric field distribution along the insulators and the field becomes resistive as against capacitive under AC voltage when there is an increase in surface conductivity.