Research On Conyrol Strategy Between Modes Switching Of Micro-grid System

Faced with the increasingly severe energy crisis and intensified environmental problems,new energy power generation as a solution has the characteristics of green and clean,its proportion in power systems has increased year by year.Micro-grid consists of distributed generators(DG),battery energy storage and local load.It can reduce the costs caused by transportation line and obtain the electricity produced by DG as more as possible.So the micro-grid is the new trend of smart grid development.As the interface between micro-grid and the main grid,inverter plays an important role and its performance can directly affect the whole system's stability.So the micro-grid can transfer its operational mode by changing the operational mode of its inverter.Firstly,this paper analyzes theory of inverter with axial-regulators(ARI)by building its mathematical and physical model.Then the equal circuit consists of adjustable voltage source adjustable output impedance can be gotten.We have used the simulation software to prove its characteristics.The control parameters of the ARI are also discussed.The output characteristics of voltage amplitude and frequency under no-load condition are analyzed,and the whole output impedance can be inductive by adjusting the ARI virtual impedance.In the case of isolated island operation with load,the accurate compensation scheme is given based on the feedback of the power loss value on the virtual impedance to solve the problem of voltage amplitude and frequency dropping in the system.Based on ARI,this paper has proposed a method which can achieve synchronization quickly and smoothly.It contains synchronous detection unit and synchronous modulation units.The detection circuit does not only sense the difference between inverter output and the grid,but also feedback information to synchronous adjustment unit and send out grid-connecting command.According to the characteristics of the inverter,primary modulation and secondary modulation have been designed to adjust inverter output voltage.Using the linear relationships,rough adjustment of amplitude and frequency would be done refer to grid information firstly.Then the phase angle error would be reduced to zero by changing inverter frequency,here the frequency and phase angle synchronizations have been unified together.The secondary modulation completes the further optimization at the same time just to decrease transient shock.The simulation and experiment results are presented in the paper.Compared with the traditional control strategy,this proposed method has reduced the coefficients dependency issues and improved both the accuracy and adaptability effectively.In order to improve power reliability of emergency loads when the grid is fault,seamless transfer between grid-connected and stand-alone modes is the first issue to be concerned.This paper is based on the voltage-controlled inverter with axes.In the stand-alone mode,it can provide voltage and frequency supplies by V/f control.In the grid-connected mode,it regulates the output power by modulating the amplitude and phase of the equivalent voltage source.The essence while transferring between two modes doesn't change,both are voltage-controlled method.So it only needs to ensure the continuity of the reference signals by altering the axe-regulators,there will be less transient impact between grid-connected and stand-alone modes.The simulation results are presented to validate the effectiveness of the proposed control scheme.