Comparison of Offline VLF PD Measurements and Online PD Measurements on a 50-Year-Old Hydrogenerator Stator in Norway

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-yearold 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.

Analysis of partial discharge sources in stator insulation system using variable excitation frequency

“…Therefore, the shielding effect is less at low frequencies leading to high discharge magnitude and a higher number of discharge pulses. Further, there will be an increase in electric field concentration at the tip of stress grading tape at low-frequency excitation [18]. This increase in the electric field at low frequency can further intensify discharges at 0.1 Hz as compared to the power frequency.…”

“…The results indicate that PRPD patterns of slot discharge were asymmetric at all excitation frequencies in the range of 50-0.1 Hz. In [18], the authors suggest that discharges in end-windings, which are not present during online PD measurement, can occur during very low-frequency PD measurement.…”

Effects of Frequency and Temperature on Partial Discharge Characterization of Stator Windings

“…Gulski et al [9] reported that charge magnitudes and inception voltages of four different large motors were comparable at 50 Hz and 230 Hz damped AC. In [10], a comparative study between VLF off-line and on-line PD monitoring measurements on a 95-MVA-hydro generator was carried out. In a follow-up study [11], inception voltage and charge magnitude were reported to be comparable at 0.1 Hz and 50 Hz.…”

Use of Data-Driven Approaches for Defect Classification in Stator Winding Insulation

“…The shapes of H n (ϕ) and H qn (ϕ) have characteristic features for different PD sources [9]. Skewness and kurtosis have widely been used for the quantitative classification of PDs in artificial defects [9,10] and hydro generators [11]. In relation to a normal distribution, skewness refers to the degree of asymmetry of a distribution: Sk is positive (negative) for a leftward (rightward) shift of a normal distribution.…”