Research On Fault Ride Through Mechanism And Control Strategy Of Power Electronic Transformer Under Power Grid Fault

With fossil energy is exhausted,more and more distributed energy is connected to the grid in order to make better use of renewable energy,but it also puts forward higher requirements for power quality,stability and flexibility of power system.Therefore,a power electronic transformer(Power Electronic Transformer,PET)composed of power electronic devices and high frequency transformers came into being.Power electronic transformer through modern power electronic transformation technology in addition to the realization of the traditional power transformer voltage,electrical isolation and power transmission functions,but also has the advantages of high controllability,fault crossing and small occupation,as one of the most important devices in the power gri d,it has a huge development prospects.This paper first describes the research background of PET and the current situation of development at home and abroad,and then focuses on the operation characteristics of cascaded PET in the power grid,aiming at th e asymmetric fault of the power grid and the transient fault of the power grid,the main work content and the innovative results obtained are as follows:1)study the cascade type PET.First,the input cascade H bridge,isolation stage bidirectional active full bridge converter and output stage three-phase four-bridge inverter are modeled.Then the input stage adopts the total voltage control strategy,and adopts the modulation method of carrier phase shift;the isolation stage adopts the uniform voltage control;and the output stage adopts the voltage and current double closed loop control.Finally,all levels and the overall control strategy are simulated and verified.2)operation strategy of cascade type PET under asymmetric fault of power grid is studied.cascaded PET are modeled under asymmetric faults,pointing out the importance of fast and accurate extraction of positive and negative sequence electrical components to control,and three extraction methods are studied.then,zero sequence voltage,negative sequence current injection method is proposed to solve the problem of dc side voltage imbalance between input stages,and simulation verification is carried out.3)proposed a cascaded PET fault crossing control strategy under the instantaneous fault of power grid.Firstly,by analyzing the operation characteristics of the PET under reclosing mechanism,it is pointed out that if some control strategy is not adopted,the PET may appear overcurrent when facing reclosing.and then,the mechanism PET impulse current generation during reclosing is analyzed.Finally,according to different initial working conditions and the maximum allowable current of input stage,the ra nge of power adjustment of distributed power supply is obtained for power compensation,and then the impulse current suppression strategy combined with distributed power supply access is proposed to improve the PET’s operation ability.Finally,the RT＿LAB system is used to construct the semi-physical experiment platform,and the cascade PET fault crossing control strategy is designed and verified digitally.Experimental results show that the proposed cascade PET impulse current suppression strategy is feasible and effective.