I. Introduction
As a compact and high-efficiency solution to power conversion, the grid-connected three-phase pulsewidth modulation (PWM) rectifiers play a central role in energy storage systems, flexible ac transmission, and utility interfaces with renewable energy resources. Faults that occur in the power rectifiers endanger the operation of the overall system and, under certain circumstances, lead to the shutdown of the entire system. The reliability of the power rectifiers has gained increasing attention in the past decades [1]– [14]. Over the past several years, substantial research efforts have been focused on fault-tolerant power rectifiers considering the failures of power semiconductor devices [15]– [34]. The fault-tolerant operation of the power rectifier is composed of two procedures: fault detection and fault tolerance. Fault detection is a prerequisite of fault-tolerant operation [15]. For fault detection, considerable investigations have been conducted, and many control strategies have been proposed [16]– [19]. The purpose of fault detection is to locate the fault source within the power rectifier. With knowledge of the fault locations, the corresponding control strategy can be applied to maintain fault-tolerant operation. Thus, the focus of this paper is the fault-tolerant technique. Among semiconductor switch failures, the open-circuit and short-circuit faults of switching devices (insulated-gate bipolar transistors (IGBTs) and metal–oxide–semiconductor field-effect transistors) are the major faults [20], [21]. A three-phase four-switch (TPFS) topology is presented in [22]– [30], where the dc-bus midpoint is connected to the faulty phase of the rectifier. In the post-fault operation, the power rectifier is equivalent to the TPFS. With the advantage of the reduced number of power switches and a minimized hardware reconfiguration, the topology presented in [22]– [30] is a promising option for a fault-tolerant system. The three-phase grid-connected PWM power rectifier is widely implemented in solar energy, wind energy, and active power filters. For the safety considerations of the interconnection, the power rectifiers are mandated to meet the technical specifications, such as the total harmonic distortion (THD) and the reactive power fluctuation. In post-fault operation, the poor performance of the power rectifier, such as the unbalanced input ac currents and the high THD of the ac currents, deteriorates the stability of the voltage at the point of common coupling, which causes a secondary fault. This may, in turn, lead to the shutdown of the overall system of renewable energy resources. Therefore, the comprehensive design and optimization, including the optimized modulation approach, detailed modeling, and reliable control algorithm, is essential to prevent the occurrence of a secondary fault.