Research On Extendable Switched-Capacitor Multilevel Inverter And Its Fault-Tolerant Strategy

The DC power generated by photovoltaic panels is supplied to the load or the grid through the inverters.Therefore,high-performance inverters play an important role in photovoltaic power generation system.The main advantages of multilevel inverters include realizing staircase voltages with a lower total harmonic distortion,mitigating electromagnetic interference,and allowing for switches with lower voltage stress.The development of traditional multilevel inverters is limited by a large number of devices,low voltage gain and single extension style.In order to solve these problems,this paper proposed two switched-capacitor multilevel inverters applying in photovoltaic power generation systems.In addition,a fault-tolerant strategy based on the proposed space-type switched-capacitor multilevel inverter was proposed to solve the low reliability problem caused by the large number of devices.The research contents of this paper mainly include:(1)In order to improve the boost capability and ouput power quality of inverters in the renewable energy power generation system,this paper proposed a switched capacitor inverter with low voltage stress and high-level characteristics.The proposed topology is composed of step-up unit,independent capacitor and switches,which reduces the harmonic content of inverter output voltage.Compared with other topologies,the performance of proposed inverter is better in terms of fewer switching devices and more output levels.Moreover,the inversion process could be realized without a back-end H-bridge to reduce the voltage stresses of switches effectively.In addition,the proposed inverter has the advantages of small voltage ripples and capacitors voltage self-balancing,which simplifies the complexity of the controller.(2)An extendable space-type switched-capacitor multilevel inverter was proposed to simplify the above topology.In the proposed inverter,charging paths of the capacitors are separated from their discharging paths,capacitors are charged by the source separately and connected by switches to increase the output level.The proposed inverter employing one source can be extended by adding capacitors and switches.Meanwhile,the advantages of low voltage stress and high-level characteristics are maintained.(3)A fault-tolerant strategy based on extendable space-type switched-capacitor multilevel inverter topology is proposed to enhance its reliability.The proposed strategy makes the inverter be capable of tolerating open-circuit faults.Under pre-fault and post-fault operations,it preserves capacitor voltage balancing,voltage boost capability,as well as the ability to supply inductive loads.Furthermore,the voltage stresses of the switches and the voltage ripples of the capacitors are decreased or remained under post-fault operations.Besides,this strategy can be used for short-circuit fault tolerance if quick fuses or other devices are used to isolate the short-circuit switches from the inverter.This paper includes the topologies,operation principles,modulation strategy and topology extensions of the proposed inverters.The calculations of capacitor ripples and the power loss are given.The operation strategy of the topology under open-circuit faults is explained.A comparative study with several switched-capacitors and fault-tolerant multilevel inverters is presented.Finally,simulation and experimental results are shown.