Cascaded NPC/H-Bridge Inverter with Simplified Control Strategy and Superior Harmonic Suppression

Abstract

In recent decades the electric power systems has suffered significant power quality problems caused by the proliferation of non linear loads, such as arc furnace lighting loads adjustable ac drives etc., which causes a large amount of characteristic harmonics, low power factor and significantly deteriorates the power quality of the distribution system (Benslimane, 2007; Franquelo et. al., 2008; Gupta et al., 2008). The increasing restrictive regulations on power quality have significantly stimulated the development of power quality mitigation equipments. For high power grid connected systems, the classical two level or three level converters topology are insufficient due to the rating limitations imposed by the power semiconductors (Holmes & McGrath, 2001; Koura et al., 2007). Hence considerable attention has been focused on multilevel inverter topologies. This important multilevel technology has found widespread application in medium and high voltage electric drives, renewable energy – grid interface, power conditioning, and power quality application (Lai & Peng, 1996; Peng et al., 1996; Rodriguez et al., 2002; Sinha & Lipo, 1996; Tolbert et al., 1999). Multilevel converters offer several advantages compared to their conventional counterparts (Manjrekar & Lipo, 1988, 1998, 200; Corzine & Familiant, 2002; Lund et. al., 1999; Sneineh et. al., 2006; Park et. al., 2003; Zhang et al., 2002; Ding et. al., 2004; Duarte et al., 1997; Rojas &. Ohnishi, 1997). By synthesizing the AC output terminal voltage from several voltage levels, staircase waveforms can be produced, which in their turn approach the sinusoidal waveform with low harmonic distortion, thus reducing filters requirements. However the several sources on the DC side of the converter make multilevel technology difficult to control by the need to balance the several DC voltages. For the class of multilevel inverter called diode clamped, if a higher output voltage is required one of the viable methods is to increase the number of inverter voltage levels. For Neutral Point Clamped (NPC) inverter voltage can only be increased up to five level beyond which DC voltage balancing becomes impossible. For single Phase H Bridge inverter, an increase in the number levels leads to increase in the number of separate DC sources, thus the proposed hybrid model is developed by combining the NPC and Hbridge topologies (Wu et al., 1999).