Research On Control Strategy For Seamless Switching Between Grid-connected And Island Operation For Micro-grid

The development and utilization technology of distributed generations have been gradually developed.Microgrid,as the intermediate transition of distributed power to the main grid,greatly reduces the impact and oscillation caused by the various distributed generations into the main grid.The inverter is the core part of the entire microgrid,which plays an important role in the field of electrical energy conversion and various renewable energy sources.It is very important for the safe and reliable operation of the microgrid to study the control strategy and control structure of the microgrid in connected-grid mode,off-grid mode,and the smooth switching between operating modes.In view of the defects,such as large oscillation and low robustness during load switching and mode switching,and poor anti-interference ability and poor dynamic and static characteristics in traditional proportional integral(PI),all these can no longer meet increasing control needs.In recent years,some new control methods about microgrid inverter have been introduced by domestic and foreign scholars,showing excellent adjustment ability and system performance,which further improved the control accuracy and reduced system sensitivity to disturbances.The main research content of this dissertation is based on the single-mode peer-to-peer microgrid grid-connected and off-grid switching technology,which takes two distributed generations(DG)running in parallel as an example.The thesis researches and designs separately the control strategies of the microgrid in the grid-connected and off-grid operation modes and the control strategies between modes switching.The main research content of this dissertation is divided into the following parts:(1)In grid-connected state,this thesis analyzes and designs the current inner loop controller based on harmonic compensation quasi proportional resonance(HCQPR).This method improves the control accuracy,and effectively eliminates the harmonic components in the inverter output current.In combination with the previous droop controller,the inverter's grid-connected operation control is completed.(2)In off-grid state,the thesis improvs the traditional PI controller.A voltage loop integral sliding mode control(ISMC)strategy is proposed.At the same time,in order to further improve the control accuracy and suppress sliding mode chatter,the thesis introduces the power function into the sliding mode approach rate,which enhances the robustness of the system and fulfills the control requirements of inverters for off-grid state.(3)This dissertation also studies and designs the control strategy of the microgrid between two states switching,proposes the pre-synchronization strategy before switching and adopts a method of transient adaptive nonlinear droop adjustment when switching.This method effectively reduces the influence of power fluctuation and distortion of voltage and current brought by the state transition process,and the method realizes seamless switching between connected-grid and off-grid operation modes.Finally,the proposed control strategies are simulated and verified.Taking advantage of Matlab/Simulink platform software to build a single-mode peer-to-peer microgrid simulation model.The simulation results show that the control strategy of the connected-grid and off-grid states and the switching strategy between the two states meet the control requirements,and it can achieve continuous and stable operation of the microgrid.