Mode Switching Strategy Research Of Grid/Islanded In Microgrid

With the development of the technology of distributed generation,the impact of the distributed generator on the grid becomes more serious.One of the solutions is the microgrids made up of the distributed generators and the local load.So the chief value of the microgrids is enhancing the reliability of power supply for consumers,while the requirements of the consumers are more than this.It is inevitable that the microgrids provide more service for the additional requirements of consumers.On the other side.The distributed generators are implemented via the interface of power electronic devices.It enhances the flexibility of the control for the distributed generators,but also increases the difficulty of maintaining the stability of the microgrids.The control of microgrids takes full advantage of the flexibility of distributed and meets the requirements of consumers.So the control schemes in this paper are used to ensure the voltage and frequency performance when facing the outside interference and during the microgrid mode switching.Firstly,on the basis of the power distribution mechanism of multi-inverters in paralleled micro grids,it is proved that condition for ideal sharing of loads is their equivalent output impedances of parallel-operated inverters should be in inverse proportion to their power rating.But this condition is difficult to meet in practice.For a power droop control structure with a voltage current loop,an improved power droop robust control and the equivalent output impedance is proposed which is robust against the parameter drifts.To ensure the voltage stability in the microgrid under the condition of outside interference,a mixed control method combing with the Multi-Agent technology is proposed.There are two levels in the Multi-Agent system.In the lower level,the decentralized controller is proposed.In the upper level,the coordinated controller uses the adaptive sliding mode control.And build the risk indices of voltage stability to decide whether to start the upper controller.In the small disturbance,the decentralized controller can effectively compensate the voltage variation.If the large disturbance occurs,the coordinated controller starts.The two controllers restrain the interference to maintain the voltage stability.Secondly,regarding the inrush current when a inverter connects to the micro-grid and the DC voltage fluctuation when a inverter is cut off from the grid,this paper proposes a dual-mode control and seamless transfer control method of three-phase inverter.The control scheme combines steady-state control with transfer control.Wherein,the transfer control consists of soft-start virtual impedance and single loop current control.To avoid reverse energy flow,phase lead regulation of droop control and its frequency regulation scheme are presented in the processes of grid-connecting and grid steady controls.The control method can inhibit both the impacts of the converter output current and the grid-injection current when a inverter connects to the grid,and can effectively reduce the grid-injection current to avoid the imbalance of instantaneous current when a inverter is cut off from the grid.Finally,the simulations are used to show that the two control strategies are the effectiveness and rationality.