Optimal Photovoltaic Multi-String Inverter Topology Selection Based on Reliability and Cost Analysis

Inverters are the most vulnerable parts of the photovoltaic (PV) power plants. Therefore, choosing an appropriate inverter topology to maximize the reliability and availability of the PV power plants is very important, especially in the large scale power plants. This paper proposes a novel index named Total Financial Losses (TFL) to compare different inverter topologies from reliability and energy losses points of view. Initial cost, repair cost, financial losses due to the downtime and element losses, and environmental conditions are considered in evaluating the TFL index. Using this universal index, the optimal inverter topology and its switching frequency are determined when the TFL is minimized. Considering the fact that the reliability of power electronic devices is strongly dependent on their power losses and ambient conditions, the exact failure rates of vulnerable elements such as capacitors, switches, diodes, and cooling system are calculated. The system availability and frequencies of encountering the failure states are determined using the Markov method. Finally, the optimal photovoltaic inverter of the 150 kW power range is selected out of commonly used two-level and three-level topologies.