Measurement of the magnitude of internal discharges in power capacitors

Abstract

An experimental investigation of the effects of internal inductance on the measurement of internal discharges in power capacitors has been carried out. It has been revealed that, for tests on large capacitor units, substantial errors of measurement, loss in sensitivity and deterioration in response characteristics are possible. The error in measurement can be corrected if the self-resonant frequency of the capacitor under test is known. All the effects, can largely be avoided in high-voltage-capacitor units by constructions that avoid low self-resonant frequencies. Such constructions appear to be easier to realise in capacitors rated at 7–10kV than in those rated at 1–1.5kV. In medium-voltage units, the effects are substantial for ratings above 5kV Ar. The feasibility of making discharge tests on capacitor banks by sectionalising the bank, and by in situ tests on pairs of capacitors installed in the bank, has been studied. It is concluded that testing by sectionalising will be impracticable on most sites, but that in situ tests will be possible but laborious.