Research On Power Cooperative Control Of Multi-port Cascaded Solid State Transformer

With the profound changes in the domestic and foreign economic situation, a new round of energy reform from traditional fossil energy to clean new energy has been quietly launched. In order to deal with global climate change, promote economic development, adjust energy structure, Europe and other developed countries had put forward the development of smart grid one after another. Chinese government had clearly proposed to strengthen the construction of smart grid in the government work report.As an important part of the future smart grid, solid state transformer (SST) is a new kind of intelligent equipment achieving voltage conversion and power transmission with power electronic devices and transform techniques. Compared with the traditional transformer, SST has the following advantages, small volume, light weight, environmental friendliness, fault isolation, regulation and control of the voltage and current in grid side. SST also provides the AC and DC ports to interface with the distributed storage of new energy resources, AC and DC load, to achieve grid power distribution cooperatively. The characteristics mentioned above help to improve the efficiency and reliability of power grid. Limited by the level of the current power electronic devices, the capacity and voltage level of SST could be promoted by cascade module. The cascaded topology is easy to be expanded and encapsulated, but the topology has voltage and power balance problems among the modules.This paper takes the cascaded solid state transformer applied in distribution network into research, and a series of control strategies based on the cooperation method are proposed. Firstly, aiming at the voltage and power balance control problems of cascaded SST in distribution network, we get the internal power transmission model of SST and the equivalent conditions of voltage and power balance through the analysis of input stage and the middle stage switch equivalent circuits. Thus, a common duty ratio control strategy based on single-phase dq coordinate transformation is proposed for the rectifier stage to balance inner power and input/output voltage superposition control is presented for DAB to achieve the voltage balance. Then, focus on the power management of distributed micro sources, users and main grid integrated in the distribution network, in order to achieve power interaction between distrituted generation(DG), user and the grid, a cooperative control strategy based on the low-voltage DC port voltage of SST is proposed.Finally, a corresponding simulation model is built with PSCAD/EMTDC software platform and a small power test prototype is built in the laboratory. Both simulation and experimental results verify the validity and reliability of proposed cooperative control strategies.