Optimised harmonic elimination modulation extended to four-leg neutral-point-clamped inverter.

Uninterruptible power supply (UPS) is the promising candidate to ensure high quality of delivering power from renewable energy resources to the AC loads especially sensitive and critical ones. In this study, optimised harmonic elimination modulation technique is extended to four-leg neutral-point-clamped (NPC) with neutral point voltage balancing and it has been experimentally validated. It is demonstrated that by controlling the fourth leg to eliminate more harmonics while operating at low switching frequency makes the four-wire NPC an ideal candidate for an UPS application in energy conversion systems. Non-triplen, 5th to 23rd harmonics orders from the output voltage waveforms are eliminated by optimally computing the switching angles for phase legs. Moreover, switching angles computed for the fourth leg are also considered to help removing triplen harmonics containing 3rd, 9th, 15th, and 21st orders from phase voltages. It is mathematically proved that the designed selective harmonic elimination technique ensures the neutral point voltage balancing. The obtained experimental results confirm the proper performance of the proposed technique in supplying symmetrical/asymmetrical loads from renewable energy resources while keeping the inverter neutral point voltage balanced. [ABSTRACT FROM AUTHOR]