Application of parallel three-level inverters to the speed control of induction motors in traction drives

This paper is concerned with the application of parallel operated neutral-point clamped three-level inverters to the speed control of induction motors in rapid transit system. Adopting three-level inverters can reduce total harmonic distortion and noise. It will also decrease the switching frequency of power transistors and increase efficiency. In this paper, two sets of inverters each in series with inductors are operated in parallel to drive two induction motors to propel electrical multiple unit. Voltage compensation is introduced to reduce difference current between inverters. Besides, parallel operation by inverters enhances availability, i.e., if one inverter has fault, the other inverter can provide partial power to induction motors. This paper presents analysis and design of the aforementioned system. Computer simulation shows that adopting the strategy of quadrature- and direct-axis currents as voltage compensation can not only reduce the difference current between inverters but also provide equilibrium current to induction motors. It will not be influenced by the switching frequency of inverters, switching sequence and series inductance. It is simple and can be extended to parallel operation of multi-sets of inverters.