# A Secondary Reconfigurable Inverter and Its Control Strategy
## 1 Introduction  
With the development of power electronics technology, voltage source inverters have been widely used [1; 2; 3; 4]. The power semiconductor devices of the inverter and its control circuit constitute the weakest link, which is vulnerable to being faulty. Its reliability problems have not been solved effectively [5; 6; 7]. The usual faults in the inverter are short circuits and open circuits of power semiconductor devices. The wrong triggering signals, the auxiliary power failure, the device over-voltage breakdown, the avalanche breakdown, and the thermal breakdown can lead to a short circuit fault on the power semiconductor device. Since the existence of a short circuit fault time is too short, it is hard to diagnose. Hence, the diagnosis and protection of short circuit faults are mostly based on the design of thehardware circuit. This has been summarized in the literature [8]. The fast fuse can also be put into the inverter circuit to turn the short circuit fault into the open circuit, which uses the open circuit diagnosis method to handle the faults. It is therefore convenient to implement the topology reconfiguration and the fault-tolerant control strategy [9; 10]. The main reasons causing the open circuit of the power switches are the device rupture, the bonding line breaking or welding, the loss of driving signals or the circuit failure, etc. A power semiconductor device fault will make the inverter operate in an abnormal state and affect the system performance, and the voltage and current stresses of the other devices will be increased. The system will crash, so it must be diagnosed in time. To improve the dependability of inverters, fault-tolerant strategies have been investigated by a large number of studies. These can be divided into redundant and nonredundant fault-tolerant strategies. Redundant fault-tolerant strategies include switch-redundant topologies [9], neutral leg topologies [10], additional phase leg topologies [11], cascaded inverter topology [12], etc. In the modular multilevel inverter, many standardized redundant modules are integrated into the branch to allow the system redundancy [13]. However, these solving methods are often complex. The nonredundant fault-tolerant inverter structure reconstructs the faulty inverter. Three-phase four-switch (TPFS) inverters have been intensively researched as the fault-tolerant topology structure when a three-phase six-switch (TPSS) inverter is faulty, where one phase terminal is connected to the midpoint of the DC side capacitors. The TPFS inverter can improve the system reliability and has the potential for quick recovery of partial system performance [14; 15]. The development of the four-switch inverter technology in the field of motor drive is relatively mature [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30], including DC Motors, IM Drives etc. However, it was not fault-tolerant. The literature [28; 29] has adopted the traditional fault-tolerant TPFS inverter in the microgrid. However, the control strategy was complicated. There are many fault-tolerant control strategies when an open circuit fault occurs. Fault-tolerant control strategies have been proposed for diodes of NPC (neutral point clamped) inverters [31], flying-span capacitor inverters [32], cascade multilevel inverters [33; 34], T-type three-level inverters [35; 36], etc.. Additional TRIACS (TRIode alternating current semiconductor) are added for fault-tolerant operations [31; 32; 33; 34; 35; 36; 37; 38]. However, if the traditional fault-tolerant inverter undergoes a power semiconductor device fault on any bridge, the bridge arm needs to be removed, and the control algorithm in the normal running state needs to be converted into the control algorithm in the faulty state, which is complicated. In response to these problems, a topology of a secondary reconfigurable inverter and the corresponding control strategy are proposed in this article. The reconfigurable fault-tolerant space is much larger than that of the traditional fault-tolerant inverter.