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With the increasing concerns for the traditional energy shortage and environment issues, distributed generation (DG) systems based on renewable energy sources (RES) have experienced a fast development in recent years. With more DG units being integrated into the power system, a more recent concept, called microgrid, is developed by grouping a cluster of loads and parallel DG units in a local area. This paper addresses the issue of real and reactive power control for DG units in a low voltage (LV) microgrid during the autonomous islanding operation. The traditional method for power control in parallel DG systems is the frequency and voltage magnitude droop method, which is based on the assumption of a mainly inductive line impedance, and is subject to power control couplings when implemented in a LV microgrid, where the line resistance to reactance ratio (R/X) is high. It is also revealed in this paper that the traditional droop control can lead to stability concerns in a LV microgrid. To achieve accurate and decoupled real and reactive power control and at the same time, to improve the system stability, a virtual frequency-voltage frame control is proposed, where the original voltage and frequency frame is transformed to a virtual frame to realize a completely decoupled relationship between real and reactive power. Details of frame transformation control and small signal stability analysis are presented. Both simulation and experimental results are provided in this paper.
Date of Conference: 17-20 May 2009