Investigation of mica based insulation for high voltage machines subjected to repetitive pulsed voltage

The interest in the prediction and qualification of insulation life performance in high voltage machines fed by converters has steadily grown, since the use of these machines is gradually increasing. The voltage stress on the insulation system is different for converter fed machines compared to machines fed online with power frequency voltage. The common interest in this subject has lead to the release of a Technical Specification 60034-18-42, which gives recommendations on partial discharge resistant insulation system testing at voltage converter conditions. In this paper the set-up of test equipment for testing lifetime of mica based insulation exposed to converter generated repetitive pulsed voltage as well as to sinusoidal voltage is presented. The influence of repetition frequency as well as pulse rise time on the lifetime is compared to 50 Hz ac conditions. Partial discharges are believed to be the dominant ageing factor for voltage stresses above the partial discharge inception voltage. Increased repetition frequency shortens the lifetime as a function of the increase in frequency. However, indications are given that the pulse rise time may somewhat influence the ageing of mica based insulation in the case of pulsed voltage stress.

I. Introduction

Variable speed driven (VSD) machines, using frequency converters, are used in increasing numbers in industrial applications due to the benefits from flexible control and the price competitiveness of the system. Until the introduction of the VSD technique the most common source of fast rise time surges has been from circuit breaker operation, which could reach 100,000 operations over the lifetime of the machine. The switching used in the VSD technique, introduces thousands of transient voltage steps per second with short rise times.