Research On Modeling And Control Strategy Of Power Electronic Transformer Interfaced DFIG System

With the continuous increase of wind power installed capacity,the impacts of wind power integration on the power grid have become more serious.Wind energy conversion systems with the interface of fundamental frequency transformers have some disadvantages.Power electronic transformer(PET)is a kind of power transmission device.It is made up of high-frequency transformer and power electronic converters,which can reduce the amount of magnetic material of fundamental frequency transformer,improve power quality,limit fault current and so on.It is of great significance to study how to replace the traditional fundamental frequency transformer as the interface between the power grid and the wind energy conversion system.This paper mainly studies the modeling and control strategy of power electronic transformer interfaced doubly fed induction generation(DFIG)system.Firstly,a comprehensive analysis of the research status of PET interfaced wind energy conversion system is carried out.On this basis,the topology adopted in this paper is selected.An optimized control scheme is proposed for PET isolation level by employing an evaluation coefficient to investigate the influence of the disturbances of inner and outer phase shift rario on the transmission power and backflow power,and obtain the combination of the inner and outer phase shift rario when the backflow power is minimum,which achieves the purpose of reducing the converter loss.Secondly,the mathematical model of each part of the PET interfaced DFIG wind energy conversion system is established,and the general mathematical model of the system is derived.The simulation model of DFIG wind energy conversion system based on PET interface is built.The startup control process of PET is designed to reduce the impact of surge current on the startup of transformers and switching devices.Based on PSCAD simulation,the operating conditions of the system under normal grid voltage are analyzed.Theoretical analysis and simulation results demonstrate the feasibility of PET interfaced DFIG wind energy conversion system and the effectiveness of system control strategies.Finally,two kinds of power quality disturbances are studied in this paper,such as voltage sag and voltage swell,which are frequent in DFIG wind energy conversion system.The transient processes of DFIG wind energy conversion system during grid voltage sag and voltage swell are analyzed.An improved control strategy of PET high voltage level is proposed to achieve fault ride through during power grid voltage sag,which adopts the chopper protection on the DC side of cascade H bridge submodule.The correctness and effectiveness of the proposed control strategy are verified through the simulation analysis of PET interfaced DFIG wind energy conversion system with different sag amplitudes and different sag durations.Simulation results indicate that the DFIG wind energy conversion system based on PET interface can ride through low voltage and meet existing low voltage ride through standards.Similarly,the simulation results of PET interfaced DFIG wind energy conversion system during the grid voltage swell show that the system also can ride through high voltage and meet the existing high voltage ride through enterprise standards.