A Fault-Tolerant Method for Cascaded H-Bridge-Based Photovoltaic Inverters With Improved Active and Reactive Power Injection Capability in Postfault Condition

Abstract

This article proposes a fault-tolerant strategy for the cascaded H-bridge-based photovoltaic (PV) inverters with a single failure in power switches. In the introduced method, the faulty leg is bypassed after a switch failure, and the faulty cell (FC) continues to operate with the remaining healthy leg. After bypassing the faulty leg, one voltage level is missed at the ac side of the FC, and if other cells operate normally, a dc offset will appear at the ac terminal voltage, leading to dc current injection to the grid. To tackle this problem, in one of the healthy cells, which is called halved H-bridge cell (HHC), the leg complementary to the faulty leg in the FC is bypassed. In this faulty condition, FC and HHC cannot operate in maximum power point. Therefore, the control system is modified to get the maximum possible active power from the existing PV arrays. Using the proposed configuration and the modified control system, all PVs are kept in use, and consequently, the drawn active power increases compared to the conventional bypassing methods and existing partially bypassing methods. Moreover, in the postfault condition, the ability of the system to inject reactive power to the grid is improved since the suggested method synthesizes higher voltage than the alternative methods at the ac side of the inverter. Mathematical equations describing the system behavior in the postfault condition are derived, and the improvement in active and reactive power injection capability is analytically proved. The effectiveness of the proposed method is verified through simulations and experiments.