Control Strategy Research For Parallel-connected Distributed Generation In A Low-voltage Microgrid

A microgrid mainly contains the renarable resources, energy storage devices andpower electronic devices. The inverter that is considered as the power electronic interfacecan make microgrid operate on the grid-connected mode. In addition, it plays an importantrole in the control and operation of microgrid. When microgrid operates on islandingmode, it often adopts droop control. However, this control will produce circulation due tothe inconsistent of local loads and line impedances, decreasing system efficiency. In orderto reduce or eliminate the circulation and realize the output power sharing, this paperpresents a novel droop control strategy based on adjusting droop coefficient adaptively.Considering that the line impedance is resistive in the low-voltage microgrid, thus weadopt the P-V droop control, and choose one inverter that operates in traditional P-V droopcontrol as a reference. Then transmiting the information of its output active power to otherinverters to regulate the P-V droop coefficients of the slave inverters adaptively by theactive power difference. Finally the system circulation can be emilited. Compared with theexisting power sharing control strategies, this adaptive adjustment of droop coefficientcontrol method shows many advantages, such as simple and easy to implement, highprecision of output voltage and accurate power sharing when considering local loads.However, the information of line impedance values are needed in the implementationof this method, at the same time, the equivalent grid impedance (including gridimpedance and the line impedance from PCC to grid) has an influence on the dectionaccuracy of the line impedances, and thus this paper puts forward a strategy to detectfundamental wave line impedances accurately in grid-connected operation model, whichnot only considers the impedances between DGs and PCC, but also concerns the lineimpedance from PCC to grid. This strategy adopts droop control to grid-connectedinverters and derives voltage and current variations by disturbing the reference voltages.Finally, based on the model of virtual grid-connected inverter, the values of the lineimpedances and grid equivalent impedance can be obtained. This method is simple andfeaible. In addition, we can obtain the line impedances and grid equivalent impedance values through only one disturbance.Simulation and experiment based on TMS320F2812have been preformed to verifythe theoretical analysis, the simulation and experimental results verify the effectivenessand feasibility of the proposed strategies.