A discontinuous PWM scheme for capacitor voltage balancing in three level NPC traction inverter drive

A carrier based discontinuous pulse width modulation (DPWM) strategy for capacitor voltage balancing in a three level neutral point clamped (NPC) traction inverter drive for Electric Vehicles (EVs) is developed and presented in this paper. The voltage unbalance is compensated by injecting an offset signal with the three phase reference voltages. The offset signal is derived from the three phase stator currents and capacitor voltage differential. The scheme requires only single carrier and two auxiliary signals per phase for implementation. The proposed modulation scheme exhibits good control over two dc link capacitor voltages for a wide range of torque-speed variations. The effectiveness of the proposed scheme are verified through simulation by developing a MATLAB/SIMULINK model for induction motor (IM) based traction inverter drive while employing indirect field oriented control (IFOC) strategy for closed loop control. Finally, the modulation scheme is validated through experimentation in a small scale prototype NPC inverter.