Study of Current and Electromotive Force Waveforms in Order to Improve the Performance of Large Pm Synchronous Wind Generator

Investigation on Multi-star Structures for Large Power Direct-drive Wind Generator

“…For the 3-phase system, it was shown that the DC current presents high amplitude oscillations with a frequency six times the frequency of the phase currents [2]. The envelope of the DC current waveform results from the positive part of each envelope of phase currents, excepting the commutation periods, when the DC current decrease from its maximum value to zero.…”

“…Unfortunately, the low-order harmonics in the DC current can not be removed and the best way to reduce them with a multi-star system is to have a shift between stars equal to θ = π/m [2].…”

“…The DC bus currents of each converter have the same waveform, but the oscillations are not synchronous [Figures 9(a) In the case of 3-phase system, the DC bus current contains a rich spectrum of 6k harmonics: H 6 = 6%, H 12 = 14%, H 18 = 15%, H 24 = 14%, H 30 = 14%, H 36 = 10% [2]. For the 9-phase system with 20 • shift, it is easy to observe that the 6 and 12 order harmonics have vanished.…”

“…A previous work [2] has highlighted that the DC bus voltage has to be equal to twice the peak-to-peak value of the EMF in order to remove electromagnetic torque oscillations. Unfortunately, the low-order harmonics in the DC current can not be removed and the best way to reduce them with a multi-star system is to have a shift between stars equal to θ = π/m [2].…”

“…The influence of the number of stars, respectively, their phase shift and the waveform of the electromotive force (EMF) are investigated. The quality criteria of multi-star systems are compared to those obtained with a 3-phase system detailed in [2].…”