New soft switching system for three-phase three-level voltage source inverters

The paper deals with a new soft switching system in three-phase three-level voltage source inverters. In the majority of soft switching systems applied until now, additional capacitors are connected in parallel to main transistors and very often special inductors are connected in series to auxiliary transistors. In cases when disturbances in the control system of the voltage source inverters would occur, the main or auxiliary transistors can be damaged because capacitors can be abrupt discharged through the main conductive transistors or sudden interruption of the inductor current can cause overvoltages which may be dangerous for semiconductor elements. Above mentioned disadvantages do not occur in the proposed soft switching system and the control method of the transistor switching is simpler in comparison to the existing soft switching systems. This paper presents switching principles of all transistor of the three-phase three-level voltage source inverter, and guidelines for the selection of elements of this system are briefly discussed. The working correctness of the new soft switching system was validated during laboratory tests, and attention was also paid to certain limitations of the control algorithm. The proposed system is recommended for medium and high power inverters.