Fault Characteristic Analysis And Fault Ride Through Capability Research Of MMC-based Power Electronic Transformer

The power electronic transformer(PET),which integrates the functions of electrical isolation,voltage conversion,power flow control,reactive power compensation and power quality regulation,is expected to gradually replace the traditional power frequency transformer with a relatively simple function as the energy hub of distribution network.The faults of the PET have serious impacts on the power grid and users.It is of great significance to study fault characteristics and fault ride through capability of the PET.Therefore,the main work carried out in this paper is as follows:Firstly,an improved MMC based PET topology is proposed.The clamp double sub-module is introduced into the input stage of the PET,which has the DC side fault current self-cleaning capability.The isolation stage adopts the DAB structure and the output stage is a three-phase inverter.The basic control strategies of the input stage,isolation stage and output stage of the PET are briefly analyzed.The simulation model is built in PSCAD software,and the steady-state operation simulation of the PET is carried out.Secondly,this paper analyzes the characteristics of single-phase grounding fault in medium voltage AC side,single-pole grounding fault in medium voltage DC side,pole-to-pole short circuit fault in medium voltage DC side and power switch fault in input stage,which are closely related to the voltage of medium voltage DC bus in PET.Aiming at the single-phase grounding fault in medium voltage AC side and single-pole grounding fault in medium voltage DC side,the mathematical analysis method of instantaneous power of MMC bridge arm after fault is given.Aiming at the pole-to-pole short circuit fault in medium voltage DC side,the mathematical expressions of capacitor voltage and discharge current of the sub-module after fault are deduced,the influence factors of pole-to-pole short circuit fault characteristics and the overcurrent level after fault are given.Aiming at the power switch fault in input stage,the influence of typical short circuit and open circuit faults on power switch of PET is analyzed.For the above four types of faults,the correctness of the fault characteristics analysis is verified by PSCAD simulation.Finally,based on the analysis results of fault characteristics,this paper analyzes the fault ride through capability of PET under various faults.Aiming at the serious pole-to-pole short circuit fault in medium voltage DC side,this paper presents a soft start fault restart strategy and a method to select the maximum fault clearance time needed for successful restart,which have been verified in the engineering prototype of 10kV PET.Aiming at the sub-module fault caused by power switch,the design scheme of redundancy protection and control system is given,and an improved bypass circuit for clamp double sub-module redundancy protection is proposed.