Space vector pulse-width modulation for multilevel inverters and solution to modulation dependent problems

Abstract

The Space vector PWM (SVPWM) is a prominent modulation technique for multilevel inverters similar to two-level inverters. However, due to complex geometry of the space vector diagram and a large number of switching states, the implementation of SVPWM for multilevel inverters is considered complex. Furthermore, in linear range, maximum obtainable voltage is 90.7% of six-step. It can be increased further by properly utilizing the DC-link capacity through overmodulation. However, the implementation of SVPWM in overmodulation range is even more complex due to the nonlinearities of this region. To deal with these problems, a general SVPWM algorithm is proposed for multilevel inverters. The proposed algorithm is based on standard two-level SVPWM which greatly simplifies the modulation process. This algorithm is used to address following three problems, (1) Common mode voltage results in bearing currents that can lead to bearing failure. Schemes have been reported for multilevel inverters to reduce the common mode voltage. However, most of the schemes result in reduced modulation depth, high switching losses and high harmonic distortion. A scheme to reduce common mode voltage for cascaded inverters is proposed which can increase the voltage range of operation by about 17% and can produce lower THD than the previously proposed schemes. (2) The use of asynchronous PWM technique for the inverter produces subharmonics and interharmonics. These harmonics lead to several undesired effects on grid connected applications. This necessitates the need for synchronous PWM. The close loop control of synchronous PWM is complex especially during dynamics. The PWM for multilevel inverter is fairly complicated as compared to two-level inverter. Hence, aforementioned problem is more severe when multilevel inverter is used as a voltage source inverter. To deal with these problems a scheme is proposed for the close loop flux control of a grid connected cascaded multilevel inverter. (3) The 3-level NPC inverter is widely used topology. However, it is known to have neutral point fluctuation problem. At low modulation index, the fluctuations can be compensated using redundant switching states. But at higher modulation index and in overmodulation region, the neutral point fluctuation deteriorates the performance of the inverter. A simple SVPWM scheme is proposed for operating a 3-level NPC inverter at higher modulation indices including overmodulation range while maintaining the neutral point balance.