Research On Photovoltaic Grid-connected Control Strategy Based On Modular Cascade Solid-State Transformer

In recent years, The contradiction between the development of world economy and fossil energy supply is increasingly serious, and the problem of environmental pollution becomes worse, so the utilization of renewable energy, such as solar energy, tidal energy and wind power, becomes the significant issue to relieve the pressure of energy supply and environmental problems. How to build DC microgrid, which is based on the distributed renewable energy resources (DRER) and distributed energy storage device (DESD), has become a popular topic in the renewable energy research. Solid-state transformer (SST) is now regarded as an emerging technology in distributed power system. With its special three-port structure, it can perfectly integrate DC microgrid, AC microgrid and the traditional power system. This technology is appreciated by renewable energy researchers in recent years. This paper aims to research on the smart SST interfaced DC microgrid, then a control strategy for PV is proposed.First of all, the paper theoretically analyze the SST, which is the platform of this research, and systematically review the future distributed power system based on the SST. The technology of SST is introduced and studied. The topology of SST, which is presented by the FREEDM center of North Carolina University, uses the advanced power electronic devices and optimal power electronic control method to replace the traditional transformer, the advantages are analyzed. And the topology of Modular Cascade Solid-state Transformer is introduced, in order to solve the problem of power and voltage balance, some strategies are analyzed. Then a systematic review of SST interfaced DC microgrid is introduced, some power management strategies, which use SST to seamlessly switch DC microgrid from islanding mode to SST-enabled mode, are proposed.Secondly, this paper analyzes the topology of modular cascade SST in detail, and studies the working principle. The main circuit is analyzed in modeling. In order to solve the problem of power and voltage balance, cascade rectifier uses a single-phase d-q vector control strategy with common duty ratio, and DAB uses an output voltage following input voltage control strategy. Based on the control strategy, a set of 6kw modular cascade SST hardware parameters and control parameters are designed in detail.Thirdly, this paper researches the Photovoltaic Grid-connected Generation System based on SST. Solar energy emerges in the research of distributed power because of its advantages:unlimited, universal and convenient. Anew type of distributed photovoltaic system based on SST is proposed in this paper, a control strategy, which can output maximum power of photovoltaic energy, is presented. Then the battery as a typical case of DESD is studied. And a feasible power management strategy for DC microgrid, which consists PV and battery, is proposed.Finally, a simulation model and an experimental prototype are built, the Photovoltaic Grid-connected Generation System based on SST is verfied, emulational and experimental waveforms demonstrate the feasibility of the proposed control strategy.