ASSESSING NEW SF6 REPLACEMENT GASES THROUGH PULSED TOWNSEND EXPERIMENTS

Abstract: The topic of SF6 replacement is still lively discussed in the high voltage community. Especially in the last years, research was intensified to find an alternative gas or gas mixture with similar or better insulation properties but lower global warming potential than SF6. Due to the almost unlimited number of possible gas mixtures, the high voltage laboratory at ETH re-initiated the search by using very efficient methods. One of the key elements is a highly automated Pulsed Townsend experiment with which the swarm parameters of gases and gas mixtures are measured. Traditionally, in high voltage engineering the voltage withstand strength of gases is determined by breakdown experiments. With our Pulsed Townsend experiment we have chosen a different way to determine the insulation properties of a gas or gas mixture. In this contribution it is shown how the electrical properties of alternative gases can be assessed by analyzing displacement currents measured under swarm conditions for different electric field strengths, pressures, and mixing ratios. Based on approximately four years of experience in operating a Pulsed Townsend experiment we designed an experiment that is improved with respect to the accuracy of E/N-determination as well as ease of handling, maintenance and photocathode replacement. This contribution focuses on the design aspects of improved E/N determination, mainly the electric field distribution created by the used electrodes. It was found that a misalignment of the electrodes and a protruding photocathode have the largest influence on the electric field in the gap. We conclude that with our new experiment setup, a total accuracy of 0.17 % in E/N is achieved. Example waveforms are measured in pure CO2 to demonstrate the achieved accuracy