Characterization of the breakdown voltage of vacuum interrupters by different procedures

This work compares two different procedures for the application of the Lightning Impulse Voltage Waveform (LIVW) with 1.2/50 μs front/half value times: Up & Down (UD) and Always Breakdown (AwBD) to obtain the cumulative breakdown voltage distribution for Vacuum Interrupters (VIs).The voltage pulses were applied to several VIs for medium voltage applications with fixed shield and for gap lengths ranging from 4mm to 15mm. The repeatability of the results for a given procedure, as well as discrepancies, were investigated and the results compared.The AwBD procedure, where the peak voltage of each pulse is set and kept at the same arbitrary high level to collect a breakdown for each pulse, was thoroughly investigated and some experimental evidences have been discussed. This procedure is effective to collect data on many breakdowns very quickly, due to both the speed of the voltage conditioning phase and because breakdown data are collected for each pulse.

I. Introduction

Vacuum Interrupters (VI) and Vacuum Circuit Breakers (VCB) started to penetrate the medium voltage market (e.g distribution networks) from the 70’s due to compact size, provided by the extraordinary breakdown strength of vacuum as insulating medium, reliability and long service [1][2]. R&D is ongoing to improve the performances of VI and VCB towards higher voltage levels, therefore the characterization of the withstand voltage as a function of the gap length, the shape of the electrodes and materials is matter of particular interest. VIs have standard requirements to withstand voltage pulses, much higher than the normal operating voltages (line-to-line voltage), and the Basic Insulation Level (BIL) is precisely codified (e.g. peak voltage and shape of the voltage waveform). In particular, VIs are designed to withstand (when the contacts are opened at the given nominal distance) surge voltages codified by the Lighting Impulse Voltage Waveform (LIVW), which is a voltage pulse having a front time and a time to half-value time of 1.2/50 μs respectively.