High Voltage Testing and Engineering Commission

Current Publication

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

As early as in the 1960s, VLF tests (VLF, very low frequency, commonly meaning a
frequency of 0.1 Hz) were studied as an alternative to the DC tests previously used on
stators of large synchronous generators. VLF tests presented themselves as an
interesting alternative to DC tests, since the voltage distribution in the insulation at VLF
corresponds significantly better to the operating conditions. Due to the significantly
lower reactive power requirement for tests with VLF compared to tests with operating
frequency, VLF sources are potentially also smaller and thus more mobile.
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 stator insulation faults
are simulated numerically in order to investigate frequency-related differences in PD
behaviour. The implications of these simulations match well with PD measurements on a
complete stator as well as single test bars. It is shown that the VLF-PD measurement is of
substantially less sensitivity than the measurement at operating frequency for practically
significant insulation problems (slot or end corona protection damages). For the currently
used mica/synthetic resin stator insulation systems, PD measurements close to the
operating frequency will therefore remain the main diagnostic tool.

Complete publication as PDF