Modified Carriers Pulse Width Modulation for Cascaded H-bridge Inverters

Cascaded H-bridges has been a topic of interest in drives and grid inverter applications, as the topology has an inherent ability to produce output power with low distortion. The multilevel output voltage waveform produced by the topology can increase the apparent switching frequency in direct proportion to the number of cascaded cells. When identical H-bridge cells are used, the inverter is assumed to load each H-bridge symmetrically. The conventionally used pulse width modulation (PWM) methods, such as phase-shifted PWM and level-shifted PWM, suffer from either inferior line-to-line voltage or asymmetric loading of the H-bridges. This paper presents a carrier based PWM method for cascaded H-bridges, that provides symmetric load sharing for the H-bridge cells, H-bridge specific DC link voltage compensation, and high-quality line-to-line voltage waveform. The applicability of the modulation method is demonstrated with simulation of a photovoltaic cascaded H-bridge inverter.