Start-Up and Transient Operation of a Bidirectional Chopper With an Auxiliary Converter

The emerging applications of large-capacity battery-powered motor drive systems, such as those used in dc electric railways in the recent years, have required a dramatic reduction in terms of size and weight of a bidirectional chopper that exists in these drive systems. However, a conventional medium-voltage large-capacity bidirectional chopper is equipped with a heavy and bulky inductor for current smoothing, which would pose many practical challenges when it is installed in a moving train car. The authors of this paper have proposed a bidirectional chopper with an auxiliary converter connected in series with the inductor intended for reducing its size and weight. However, the proposed converter requires the capacitors of the auxiliary converter to be initially charged in order to start its operation. This paper proposes a method for initial charging of the capacitors used in the auxiliary converter without utilization of external charging circuitry. In addition, the auxiliary converter can be operated as a solid-state dc circuit breaker to protect the chopper when a short-circuit fault in the switches of the main converter occurs. This paper also discusses the transient performance of the proposed chopper and its associated control system under current and voltage disturbances. A 150-V, 2-kW downscaled model is used for experimental verification.