Abstract
Droop-controlled inverters, which are the main interfaces between distributed generators (DGs) and grid AC bus, are widely adopted in today’s microgrids. Because the DGs are usually dispersed along with the microgrids, which may spread out in a wide area, the impedances of transmission lines that connect different DGs and the point of common coupling (PCC) may be different. Large circulating currents among paralleled inverters would be induced eventually, which not only results in decreased system efficiency, but also makes some DGs impossible to operate at their rated power. In this paper, the relationship between circulating current and line impedances among paralleled inverters with conventional droop control is analyzed in detail. To mitigate the circulating current, an adaptive virtual output inductance control method for the DG inverter is proposed. With the help of the added adaptive virtual inductances, huge reduction on the circulating currents can be obtained regardless of the differences in line impedances. The implementation details of the proposed method and the optimum design rules for system parameters are elaborated, which are swiftly followed by the operational principle and stability analyses. Because neither communications nor global signals are needed in the proposed method, it is a total discrete approach, which evidently indicates high flexibility and scalability. The validity of the proposed method is finally verified by simulations and experiments.
Highlights
Microgrid, which is a promising structure to integrate renewable and distributed energy sources in a more efficient and reliable way, has greatly attracted people’s attention recently [1,2,3,4]
The micro-sources such as photovoltaic cells, small wind turbines, energy storage systems are integrated to the grid in the form of distributed generation, which are further defined as distributed generators (DGs) [8]
The intended contribution of this paper is to propose an adaptive virtual output inductance control method to solve the difficulty of suppressing circulating currents among paralleled inverters in the microgrid
Summary
Microgrid, which is a promising structure to integrate renewable and distributed energy sources in a more efficient and reliable way, has greatly attracted people’s attention recently [1,2,3,4]. In [14], a robust droop control method, which adopts an integration element in the reactive power control loop, is proposed This method can fully avoid the influence of line impedances, it features deteriorated flexibility and scalability due to the need of detecting voltage signal at PCC and adding complex communication system to share it with DG inverters. They all suffer from the same problem related to low flexibility and scalability To address this issue, some other researchers proposed novel droop control methods, which avoid the use of common signals or communications among DGs, to mitigate the circulating currents. The intended contribution of this paper is to propose an adaptive virtual output inductance control method to solve the difficulty of suppressing circulating currents among paralleled inverters in the microgrid.
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