Modelling and control of three-phase PWM voltage source rectifiers- five-level NPC voltage source inverter-induction machine system

The three phase five-level Neutral Point Clamped (NPC) source voltage inverter (VSI) is a good topology for high voltage and high power applications where no semiconductor devices are available. However, it has an inherent problem of the midpoint (neutral point) balance. As consequence, the four capacitor voltages of the input DC voltage source of this converter are not equal, leading an output voltage which is asymmetric and has an average value different from zero. A large amount of research has looked at solving this problem. In this paper, we propose a new high voltage cascade constituted by two imbricate three phase PWM voltage source rectifiers and five-level NPC voltage source inverter. This cascade is applied to the great power induction machine drive. The midpoint balancing problem is analyzed and a simple solution that solves this balance is suggested. The proposed solution regulates the output voltage of each rectifier with line input currents and input voltages in phase. This algorithm is composed by one closed-loop for the output voltage control. The phase shift of the input currents is controlled without resorting to the control of input filter states. This assures that, in steady-state, the rectifier draws the desired reactive current. As an application, we study the speed performances drive of the three-phase great power induction machine fed by this system. This drive system can find applications in great power and high voltage fields as electrical traction and renewable energy.