PLL and DFT feed-forward control for railway single-phase line-converter synchronisation

Power-electronic converters connected to a railway supply grid have to meet specific interference limitations. In the majority of cases all frequency components except the odd multiples of the fundamental oscillation of the grid voltage are strictly limited. Hence all advanced control schemes are operating grid-synchronised. Because these control schemes additionally need accurate information of the actual grid voltage often a combination of Phase-Locked Loop (PLL) and Discrete Fourier Transform (DFT) is used. In railway-grid applications there are special requirements to the transient response time of the synchronisation. In this paper the transient response time is notably reduced by using a sophisticated feed-forward control. Based on a brief introduction of the DFT this feed-forward control is derived in detail. Simulation results verify the excellent operation of the PLL and DFT combination even if the line voltage is disturbed by stationary and dynamic grid intrusions or pantograph arcing.