Abstract
Abstract:
Leakage current monitoring is still relied upon when
on-site condition assessment or stability analysis of metal oxide
varistors is to be conducted. The non-availability of adequate
testing equipment, capable of directly measuring resistive component
of varistor current, has paved the way to indirect diagnostic
techniques which makes use of third harmonic component of
varistor-arrester leakage current. The major shortcoming related
to this method consists of high probability of measurement errors
and subsequent misinterpretation of actual condition of varistor
units should third harmonic-frequencies not be compensated
in the supply voltage, prior to measurement. In this study, 90
identical low voltage varistor samples acquired from three leading
manufacturers are subjected to distorted and non-distorted
ac supply voltage. The magnitude of third harmonic currents
observed in both measurement conditions, is deduced from the
decomposition of time-domain leakage current waveforms into
the frequency-domain using the flattop window algorithm of
the Fast Fourier Transform. The large-sample statistical test
of Hypothesis methodology is invoked to analyse the difference
between the mean third harmonic currents of the varistor
samples before and after harmonic injection in the supply. The
average percentage of third harmonic injected in the supply
voltage is 5%. However, the results obtained suggest over 95 %
confidence that the measurement deviation in the third harmonic
current-based assessment of varistor-arresters upon 5% third
harmonic distortion in the supply voltage lies between 32%and
67%.