Abstract
Stator winding insulation is the part of the hydrogenerator experiencing the highest number and the most damaging failures. Partial discharge (PD) measurement, both offline and online, are commonly used for condition assessment and monitoring of electrical machines. The main concern of using very low frequency (VLF) methods is the changed electrical field distributions compared to that at power frequency. Hence, PD measurements performed at VLF should be carefully assessed and compared to PD measurements at power frequencies. In this work, offline PD measurement at VLF are presented and compared to power frequency online measurements of a 50-year-old hydrogenerator in Norway, using statistical analysis of phase resolved PD recordings. It is found that both offline VLF and online assessment can identify unnormal PD activity in a specific phase, although the phase resolved PD patterns are not similar for VLF offline and online assessment.
Highlights
The total installed capacity of larger hydrogenerators in Norway is more than 32 000 MVA
The main purpose of this paper is to compare online and offline very low frequency (VLF) Partial discharge (PD) measurements on a hydrogenerator stator winding in Norway being more than 50 years in service
The VLF source was connected to the LV terminal at the neutral point side, whereas PD data was recorded using a 5 nF coupling capacitor and PD monitoring system connected to the HV terminal
Summary
The total installed capacity of larger hydrogenerators in Norway is more than 32 000 MVA. Most of the hydrogenerators was installed from 1955 to 1995. The technical lifetime of such generators is expected to be more than 50 years dependent on type of loading and operation. A significant percentage of the population has likely reached its expected lifetime. It is determined that stator insulation damages are the significantly most frequent failures and those that produce the greatest extent of damage [1]. There is a need for reliable diagnostic methods to detect degradation of the insulation system of the stator bars. The development and use of reliable diagnostic techniques will likely reduce assessment uncertainty and make the condition estimate more accurate
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