Microgrid is widely accepted as an effective mean of integrating various distributed energy resources (DERs) through their interface converters to provide electric power of high quality and reliability. These distributed resources interface converters (DICs) are required to operate in an autonomous fashion without any communication for enhanced reliability. Conventionally, the real power-frequency droop control and the reactive power-voltage droop are adopted as the de-centralized control strategies in these DICs for the autonomous power sharing operations. However, the reactive power sharing of Q-V droop control often deteriorates if there are mismatched transmission line impedance characteristics. In this paper, a Q-V dot droop control method with V dot restoration mechanism is proposed to improve the sharing of reactive power. Its operation principle and control method are explained and analyzed. In addition, simulation and experimental results are presented to validate the effectiveness of the proposed method.
The microgrid is a very effective way of integrating various distributed generation resources into the ac power system. To provide electric power of high reliability, the converters in the microgrid often rely on autonomous droop control methods to reduce their dependence on communication and need to operate in both the islanded mode and the grid-connected mode. In this paper, a grid synchronization method for a multiconverter distributed generation system is proposed. The proposed grid synchronization method allows multiple droop-controlled converters to adjust the frequency, phase, and amplitude of their output voltages to prepare for grid connection. The entire synchronization process can be executed with very limited communication requirement. This paper provides detailed explanation on the proposed grid synchronization method. Experimental test results are presented to validate the effectiveness of the proposed method.
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