The Coordinated Control Strategy Research Among Multiple Distributed Sources In Microgrids

Electrical grids tend to be more distributed, intelligent, and flexible in recent years. Microgrid which acts as a small system that can integrate distributed sources, energy storage devices and dispersed loads has got lots of attention. New energy sources such as photovoltaic cells, wind turbines often use power electronic converter as interface to microgrid, like voltage source inverter(VSI), while conventional sources like diesel engine often interface with microgrid via synchronous generator. All distributed sources are connected to the ac bus of microgrid through line impedance. In order to share the loads among distributed sources properly, droop control is generally used. This paper proposes an improved droop control method and designs two coordinated control strategy for microgrid system based on droop control.In traditional P-V droop control, taking the effects of line impedance and local load into account, this paper proposes a new strategy that adjusts each VSI’s droop coefficient automatically according to each VSI’s output active power: every VSI works in traditional P-V droop control in the beginning, all VSI’s output active power is gathered by central controller to calculate given power which is sent back to each VSI’s local controller, then adjust each VSI’s P-V droop coefficient through an PI controller to realize power sharing among all VSIs. This strategy can eliminate the effects of line impedance and local load, can apply to two conditions of load sharing and proportional load sharing and don’t need the information of line impedance.This paper builds a microgrid simulation system which contains a photovoltaic unit, a synchronous generator and two battery-powered VSIs and designs two coordinated control strategy for this system based on droop control. In the condition where the photovoltaic unit follows maximum power point tracking and the synchronous generator adopts P-f droop control, this paper analyzes the operation characteristic of microgrid system with the battery-powered VSIs adopting either P-f droop control or P-V droop control. In islanded mode, the battery-powered VSIs adopt improved P-f or improved P-V droop control to achieve power sharing between two battery powered VSIs; in grid-connected mode, battery powered VSI adopts traditional P-f droop control and traditional P-V droop control to research the power flow condition between microgrid and main grid for comparative study.Finally, in an experimental platform of two VSIs in parallel, this paper conducts islanded experiment with automatically adjusting droop coefficient control strategy compared to traditional P-V droop control. The experiment results under the condition of load sharing and proportional load sharing validate the feasibility of the improved strategy.