Research On Fault Detection And Fault-Tolerance Technology Of Cascaded Solid State Transformer

Recently, with the rise of new energy and distributed power generation, and with the proposition of the concept of smart grid, power electronic transformer (PET) will play an increasingly important role in power system. At present, the power electronic devices are widely used in the field of aerospace and industrial equipments. The reliability of power electronic devices has become a key technology to guarantee the safe running of spacecrafts and industrial equipment. As a power electronic device, fault detection technology and fault-tolerrence control of PET are highly significant both in theory and practice.Firstly, introduction is made about the development and principle of PET. Summary of fault detection and fault-to lerrence control technology for power electronic devices is given afterwards. This article issues a structure of cascaded PET and divides it into three stages namely the input stage, the intermediate stage, and the output stage. Mathematical models of the three stages are made. Furthermore, control strategies are studied for each stage, with verification via simulation. Secondly, for the three stages of the cascade PET corresponding fault detection method and fault-tolerant control methods are investigated. In the aspects of fault detection, two methods are studied for the input stage. One method is based on analysis of s witching frequency harmonics and the other is based on pulse voltage comparasion. Both methods take the H-bridge unit as the detection unit. A method based on Park transformation and neural network is applied to the output stage, by way of analyzing three-phase output current. All methods for fault detection are verified by simulation. In the aspects of fault-tolerant control, with a fault-tolerant unit containing an H-bridge unit of input stage and a DAB unit of intermediate stage, this article proposes a fault-tolerant control strategy based on double zero-sequence voltage injection and DC voltage optimization. Zero-sequence voltage injection leads to power redistribution of each phase. The output stage applies a three-phase four-switch fault tolerant reconfigurable architecture to achieve fault-tolerant operation through the seven-segment SVPWM modulation.MATLAB/Simulink platform of a cascaded PET is established. Simulation of the control strategies presented in this paper verifies the performance of the cascaded PET. Fault detection methods and fault-to lerrence control methods presented in the paper are also verified by way of simulation. The software fault detection methods of input stage and output stage can be used for online fault detection. Fault-tolerance control strategies of all stages have proved the correctness and feasibility.