Online monitoring of a power slip-ring on the shaft of a wind power generator

Abstract

The experimental results obtained from an online monitoring study of out-of-round defects in the slip-rings of wind turbine generators are presented. Five defects were involved, in which the slip-ring diameter varied from the normal value of 224 mm over large ranges of polar angle. The angular dependence of the radius changes was different for each defect and the amplitudes of the changes varied from 0.035 mm to 0.110 mm over the five defects. The defect detection sensor was a shock accelerometer embedded in the brush contacting the slip-ring and connecting the rotating and static parts of the generator. The time dependences of the brush acceleration produced by each defect, and also by a perfect slip-ring, were measured over continuous time intervals of 10 s for 11 slip-ring rotation speeds varying from 370 to 3827 rlmin. This covers the practical range of wind speed conditions. Fast Fourier transforms (FFTs) of this data were performed in order to present the acceleration data in the frequency domain, especially because such presentations promise rapid identification of sharp changes in the defect profiles with angle. Plots of the Fourier coefficients (acceleration amplitudes) as a function of the ring rotation speed are presented for the first four harmonics. These curves contain one or two maxima or show evidence of an approach to a maximum at higher revolution speeds. A qualitative explanation of these maxima is proposed and an outline is also presented of a quantitative model of the measured acceleration data, which is to be the subject of further work. In the first analysis of data, a strong and systematic dependence of these maximum acceleration amplitudes on the defect amplitudes was found. This result holds promise that the embedded accelerometer can form a suitable basis for an online alarm system in which an automatic warning is issued when the acceleration amplitude for any harmonic exceeds a level previously decided upon and set by the turbine operators. Further analysis of the data combined with modelling is in progress in order to investigate the possibility of obtaining the detailed profile of out-of-round defects in slip-rings from online time-dependent brush acceleration measurements.