Control Strategy Research For Converter Under Microgrid’s Abnormal Status

Technology on microgrid is one of the most promising technology for the electric network. In the system, the converters based on electrical and electronics are responsible for the system link voltage supply in island mode; in grid connecting mode, it can output and absorb current to achieve energy optimization; it can also adjust the voltage error between grid voltage and microgrid voltage. But the unbalance of grid voltage and load in microgrid makes the control of converter more difficult. To improve the output power quality for converter, this thesis mainly researches the converter’s control strategy in abnormal state of microgrid. It is mainly about these aspects:(1) Stated research’s current status of the microgrid and its abnormal conditions, compared different research achievements and concluded their own advantages and disadvantages. Based on the abnormal working conditions’impacts to the grid and microgrid, proposed the research purpose of this thesis.(2) Established the mathematical modeling for normal state of microgrid, researched the output characteristic of converter’s droop control and analyzed the control coefficients’ affection of the3-loop control for the system, used the virtual impedance to improve the droop control characteristic. Finally designed the droop control.(3) Established the mathematical modeling for abnormal state of microgrid connecting to the grid. Based on this, obtained the unbalance measurement, which is mainly used to separate the positive sequence and negative sequence of voltage. To improve the output power quality of the converters, improved droop control according to different constraint conditions. Established the mathematical modeling for this control and improved the droop control based on this. Analyzed the control and proved its correction theoretically. According to this, designed the control structure. Finally, designed the simulation coefficients and proved the efficiency of the control.(4) Established the mathematical modeling for abnormal status of microgrid in island mode. The research is mainly on the negative-sequence current’s rational division between multi-parallel converters. Used the improved droop control based on negative-sequence virtual impedance and designed the coefficients of the control for simulation and experiment. Finally proved the efficiency of the control.(5) Established the mathematical modeling for abnormal and normal state of microgrid in transition from island to synchronization. Analyzed the conventional control for transition. Analyzed the transfer function and simplified it. Then got the limitation of this method on abnormal status. Furthermore, adding the negative-sequence error adjustment to the conventional control and got the improved transition control. The control was proved on simulation and experiments with designing coefficients.