According to the power characteristics of PET(Power Electronic Transformer),an ap-proach to improving power system dynamic characteristics with PET is proposed. The secondary side of PET is connected with the transmission line and its primary side with an infinite-bus system,and it can be equivalent to a controllable voltage source. A mathematical model of the power system with generator excitation and PET is established,in which the power angle,angular velocity and terminal voltage are state variables and the excitation voltage of generator,amplitude and phase-angle of the controllable voltage source are control variables. An optimal coordinated control strategy for the generator excitation and PET is deduced. Simulation shows that,the rapid power exchange and bi-directional power flow between PET and system occur increases system damping and suppresses system oscillation under disturbances. This project is supported by Natural Science Foundation of Hubei Province(2001ABB026).

A three-phase four-wire distribution system electronic power transformer(EPT) implemented with the input stage,isolation stage and output stage was presented and its load characteristics were analyzed.A simple and appropriate control was developed to obtain good characteristics based on analyzing the demand of the primary system and the loads.In this control scheme the dual-loop decouple control in synchronous rotating d-q frame was adopted in the input stage and the dual-loop control based on voltage instantaneous value was introduced in the output stage.The load characteristics of the electronic power transformer were investigated in detail by simulations under Matlab/Simulink.The results show that the EPT has good performances with the proposed control scheme under balance and imbalance loads,which makes the input current sinusoidal and keeps the load voltage in normal value.

This paper presents the design method of high power IGBT snubbers for electronic power transformer in distribution system,which includes choosing the type of snubbers and parameters calculating.Simulation is carried out for each kind of topology and parameter,the characteristics of different circuits are analyzed and compared,and then optimal scheme is summarized.Simulation results show that the snubbers can effectively restrict the du/dt and the over-voltage across the IGBT when turned off.