A Carrier-Based Implementation of a Nearest Three Space-Vector PWM Strategy of Three-Level Inverter

PWM techniques of neutral-point-clamped three-level inverters (NPC-TLI) aim at equally distributing the DC bus voltage among two split capacitors along with the generation of the average three-phase voltage. The virtual space vector class of PWM (VPWM) can make the ripple in the capacitor voltage zero at every switching cycle by ensuring that the carrier cycle average of the neutral point current is zero. The nearest three space-vector PWM (NPWM) synthesizes the average output voltage with better quality and at a lower switching frequency when compared with VPWM. With NPWM, it is not possible to achieve zero average neutral point current over a carrier cycle at all operating conditions. But by adjusting the distribution of dwell time among two redundant states through a feedback controller, it is possible to minimize the ripple in the capacitor voltage. This also ensures dynamic balancing of average voltage. Hence, this NPWM with the voltage balancing scheme is a popular choice and termed as general NPWM (GNPWM) in this work. The space vector implementation of GNPWM involves complex computation. This paper presents a carrier-based algorithm for implementing GNPWM that does not require sector identification etc. It obtains the duty ratios of six independent switches of TLI directly from the three reference output voltage signals through minimal computation. The proposed algorithm is validated through MATLAB simulation and experiment on a 2.5 kW hardware prototype.