A new simple control approach of M2LC for AC railway applications

The 15kV, 16.7Hz or 25kV, 50Hz AC electrification system is commonly used in railways worldwide. This voltage is transformed to a lower voltage level which is applicable for traction converters of train applying a large and heavy transformer that takes up a lot of space in locomotive. To overcome this problem as for train space limitation, different topologies have been suggested (e.g. M₂LC, and series ac/dc/ac/dc converters) that these topologies are reviewed in this paper. A proper approach to decrease the volume and weight of this transformer, is using of a Modular Multi-Level Convertor (M²LC) in structure of train that is connected to overhead networks directly and increases the operation frequency to several hundred Hz in a constant voltage level in transformer input. So a compact and light medium frequency (MF) transformer will be used in train instead of ordinary transformers. However, the control of M²LC is very complex because of it uses the high number of switches in its structure. This paper presents an easy, simple and new approach to control of M₂LC that will be seen proper for AC traction applications. It will be presented in both patterns to generate voltage based on a simple PWM method as well as with simulation results.