Commission d'étude des questions relatives à la haute tension

Publication actuelle

Potential of VLF PD measurements for diagnosis of stator insulation
of large hydro generators

As early as in the 1960s, the VLF test (VLF, very low frequency, commonly meaning a frequency of 0.1 Hz) was studied
as an alternative to the DC tests previously used on stators of large synchronous generators. It was clear from the
beginning that the voltage stress distribution in the generator insulation system at 0.1 Hz was much closer to the operating
conditions than with DC tests. The introduction mentions studies in which breakdown tests on 50/60-Hz and VLF
service-aged bars caused similar breakdowns across the main insulation, whereas significantly different breakdown
paths were observed at DC. The focus of the present study is on the applicability of the partial discharge (PD) diagnosis
of stators at VLF test frequency. For this purpose, typical isolation faults are simulated numerically in order to investigate
frequency-related differences in TE behaviour. The implications of these simulations match well with TE measurements
on a complete stator as well as single rods. For the TE measurements, a commercial TE measuring system
could be used without further adjustments. It is shown that the VLF-TE measurement is of substantially less sensitivity
than the measurement at operating frequency for practically significant insulation failures (e.g., slot or end corona protection
damages) for reasons of principle. For the currently used mica / synthetic resin insulation systems, the TE measurement
in the range of the operating frequency will therefore remain indispensable.

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