Repetitive Compensation of Fluctuating DC Link Voltage for Railway Traction Drives

AC-fed railway traction drives suffer from pulsating input power because of the single-phase pulse width modulation rectifier at the front end. An LC filter is often needed at the dc link to absorb the pulsating power. However, this greatly reduces the power density. An effective way to get rid of this clumsy filter is to compensate pulse width calculation of the traction inverter so that it does not produce harmful subharmonics (beat components) in the output voltage/current even with a fluctuating dc link. For limited switching frequency at high power level, it is difficult for conventional feed-forward compensation methods to achieve good results. A repetitive dc link voltage predictor is proposed in this paper to improve the compensation performance. It capitalizes on the repetitiveness of the dc link ripple and can predict the average value of the dc link voltage in the next switching period. An area-equalization-based algorithm is then used to calculate the required pulse width. Experiments show that compared with conventional methods, the proposed one can reduce the steady-state beat current by six to seven times. The dynamic beat current during load transients is also negligible when dynamic drop of the dc link voltage is kept below 10%.