A single phase multi-string PV inverter with minimal bus capacitance

The multi-string photovoltaic (PV) inverter is of interest for building grid-connected PV systems because it offers a number of advantages compared to conventional centralized or single-stage inverters. Optimal operation of the system can be achieved since the maximum power point (MPP) of each string is controlled by a local dc-dc converter input stage. The input dc-dc converter also provides voltage decoupling, allowing the storage capacitance to be minimized by permitting increased voltage fluctuations on the intermediate dc bus. The electrolytic bus capacitor can therefore be replaced by a smaller, more-reliable film capacitor in order to increase the lifetime of the converter. This minimization of capacitance is not possible in a single-stage string inverter as a large input capacitor must directly decouple the PV string from the pulsating single phase output power. In this paper, the capacitance requirements of a two-stage string PV inverter are compared to those of a single-stage inverter and evaluated in light their impact on PV utilization efficiency. A complete simulation model, developed in PLECS and Simulink, is used to determine the interaction between the power conversion stages and the PV supply for the two systems and to demonstrate the relationship between capacitance, dc bus voltage fluctuations and overall system efficiency.