A Soft-switched Bi-directional Isolated Ac/dc Converter for Ac-fed Railway Propulsion Applications

A novel isolated ac/dc converter topology incorporates a voltage source converter (VSC) with capacitive snubbers and a cycloconverter, connected by a medium frequency transformer. The concept offers the possibility of bilateral power flow as well as three-level pulse width modulation on the ac side. It is shown that by alternately commutating the VSC and the cycloconverter it is possible to achieve either zerovoltage or zero-current switching conditions for all of the semiconductor elements. At low load the ac load current becomes insufficient for recharging the snubber capacitors of the VSC and the commutations may therefore become too slow. However, a quasi-resonant commutation mode for the voltage source converter is proposed that allows for sufficiently fast commutations down to zero load.