Research And Design Of Bidirectional SST Topology Structure And Control Strategy

In recent years,the distributed power generation equipment becomes higher and higher in power system.So the control of distributed equipment and power generation absorption at local area become the research difficulties.At the same time,the power electronic equipment has already applied in the generation,transmission,distribution and consumption of electricity in power system.Power-electronics-enabled power system will be the new tendency in the future.A novel structure of bi-directional solid-state transformer(SST)is designed for the access problem of various distributed equipment.Based on the novel structure,the topology,mathematic model,control strategy and power optimization are designed and researched,to realized the plug-and-play of various distributed equipment and power exchange between that and distribution network in local area.The main contributions of this paper are as follows:1.A structure of novel hybrid ac/dc type SST is proposed.Compared with the conventional ac type SST,the dc bus is accessible for storage,PV and dc load in the proposed structure of SST.SST is a high frequency and full-controllable power electronic circuit.Compared with the electronic transformer,the volume and quality of SST reduced immensely and the power density increased.In addition,the power quality is controllable greatly.Corresponding topology circuits for different distributed equipment are selected according to the difference of character of those.Besides,the improved control strategies are designed to realize the stable operation of multiple modules in system.2.To solve the problems of frequent power fluctuation and dc bus voltage stability caused by access of multiple distributed equipment,an unified droop control strategy for SST and storage based on dc bus signal and an adaptive droop dc bus voltage control strategy for a grid-connected inverter are proposed,respectively.Characteristic of“resource" or "load”for SST and storage can be switched adaptively,and dynamic power regulation is achieved by droop control.Droop control for "resource" and "load" is similar with the characteristic for"generator-load" in conventional power system,whose purpose is to restrain the power fluctuation and to realize the stable control of dc bus voltage.3.Dynamic response speed of system is slow owing to three stage cascaded structure of SST.Respectively,the feed-forward control strategy based on high voltage dc bus signal for rectifier and the feed-forward control strategy based on low voltage dc bus signal for dualactive bridge(DAB)circuit are proposed in this paper.The response speed of dc bus voltage increases effectively and the dc voltage to load sensibility reduces greatly.