A novel multi-string five-level PWM inverter for photovoltaic application

This paper presents a single-phase multi-string five-level PV inverter topology for grid-connected photovoltaic (PV) systems with a novel PWM control scheme. Three PV strings are cascaded together in parallel configuration and connected to a five-level inverter to produce output voltage in five levels: zero, +1/2Vdc; Vdc; −1/2Vdcand −Vdc. Two reference signals identical to each other with an offset equivalent to the amplitude of the triangular carrier signal were used to generate PWM signals for the switches. DSP TMS320F2812 is used to implement this PWM switching scheme together with a digital PI current control algorithm. The inverter offers much less THD and can operate at near unity power factor. The validity of the proposed inverter is verified through a prototype. The experimental results are compared with conventional single-phase multi-string three-level grid-connected PWM inverter.