Breakdown and partial discharge in dry air under non-uniform electric field for MVDC applications

Abstract

Medium and high voltage gas insulated switchgear (GIS) are key installations in the electrical power transmission and distribution network. The equipment in these substations are insulated in high pressure gas in order to guarantee their proper functioning. Insulation using SF6 gas is widely used in GIS because of its high dielectric and thermal performance, and its non-toxicity. This solution has made it possible to develop efficient and more compact GIS. However, SF6 is also responsible for significant environmental impact due to its high global warming potential. Replacing SF6 with another insulation system would require changing the design of the switchgear or the operating conditions. For example, insulation with air requires 3 times more inter-electrode distances than with SF6 if the same pressure value is maintained in the switchgear. Increasing the air pressure improves insulation performance while maintaining the same compactness. In this context, this work aims to investigate the performance of dry air (80% N2 + 20% O2) under non-uniform electric field conditions for medium voltage switchgear applications.
Experimental/Modeling methods
In this work, both experimental and simulation investigations have been carried out for breakdown and partial discharges measurements in dry air. The breakdown voltage (BDV) and partial discharge inception voltage (PDIV) as a function of the electric field non-uniformity factor have been studied. The investigated non-uniformity factors have been chosen with consideration of their values in medium voltage switchgears, they are ranging between 0.05 and 0.5. The dielectric strengths of dry air under MVDC are investigated at different pressures. The experimental work has been compared with numerical study of PDIV and BDV based on streamer criterion using Comsol Multiphysics numerical software.
Results/discussion/conclusions
The results show that the maximum breakdown field decreases as a function of the field non uniformity factors for strongly non-uniform electric field (non-uniformity factor less than 0.3), then it stables for weakly non-uniform electric field (non-uniformity factor higher than 0.3). The breakdown maximum field is not dependent on the polarity of the DC voltage for non-uniformity factors higher than 0.2. Otherwise, the maximum field seems to be lower under positive DC voltage for field factors under 0.2. Simulation results of the maximum field obtained using the streamer criterion are in good agreement with the experimental PDIV results.

Ayyoub Zouaghi, Caterina Toigo, Rako Arel Phenix, Paco, Frank Jacquier, Alain Girodet .

Presented at ICD 2024