A modular direct converter for transformerless rail interties

Transformerless rail interties are of high interest for the interconnection of the European 15kV/16.7Hz rail supply with the 50Hz industrial grid due to the direct connection to the rail supply without a 16.7Hz transformer. The MMLC is an emerging topology which offers high blocking voltages, high modularity and very high power quality. Two transformerless interties are presented and compared which are the “indirect MMLC” and the “direct MMLC”. The design for a 15MVA intertie and the calculation of the currents and voltages are the presented in detail. The cell capacitances are determined from the internal branch currents and voltages. Semiconductors loss calculations have been carried out in detail. The comparison of the indirect and the direct MML shows clear advantages of the direct MMLC.