A current source inverter with series AC capacitors for transformerless grid-tied photovoltaic applications

The Current Source Inverter (CSI) is one of the simplest power converter topologies that can convert DC to AC and feed power generated from photovoltaic (PV) cells into the AC grid with a single power conversion stage over the whole PV voltage range. The CSI also provides smooth DC current which is one of the requirements of the PV cells as well as preventing reverse current using unidirectional switches. However, the CSI operates with low efficiency at lower PV voltages, which is where the PV cells produce maximum output power. This low efficiency is caused by large differences in voltage levels between the PV side and the grid side across the converter. This thesis presents an alternative topology to the three-phase CSI by connecting an AC capacitor in series with each AC phase line of the CSI circuit. The presence of the series AC capacitors in the CSI topology allows the AC voltage levels to be adjusted to match the voltage levels of the PV cells. Therefore, the CSI with series AC capacitors is able to operate with optimal DC-AC voltage levels. Performance of the proposed topology is evaluated in comparison to the standard CSI and five other converter topologies based on transformerless circuit concepts selected from those already available in the market and suitable converters discussed in the literature. All converter topologies were modeled and simulated with the SABER simulation software package. The CSI with series AC capacitors prototype was constructed in order to validate the feasibility of the proposed topology and the performance of the proposed topology in comparison to the standard CSI. Simulation results show that the CSI with series AC capacitors provides improved efficiency and better input/output power quality in comparison to the standard CSI. The proposed topology also achieves the lowest output line current distortion, lowest voltage stress across the circuit components and lowest estimated cost of power semiconductors when compared to all considered topologies. Experimental results are also presented to validate the simulation results