Coordinated Control of Parallel Three-Phase Four-Wire Converters in Autonomous AC Microgrids

Abstract

The coordinated control of parallel three-phase four-wire converters in autonomous AC microgrids is investigated in this paper. First, based on droop control, virtual impedance is inserted in positive-, negative-and zero-sequences to enhance system damping and imbalance power sharing. Then, to facilitate virtual impedance design, small signal models of the three-sequence equivalent circuits are established respectively. Corresponding indexes are proposed to comprehensively evaluate the impact of sequence virtual impedance on current sharing accuracy, voltage quality at the point of common coupling (PCC) and system stability. In addition, constraint of DC-link voltage is also considered to avoid over modulation when subjected to unbalanced loads. Furthermore, to address the PCC voltage degradation resulting from virtual impedance, a voltage imbalance compensation method, based on low-bandwidth communication, is proposed. Finally, simulation and experimental results are provided to verify the correctness of the theory model, indicating that the proposed method can achieve PCC voltage restoration while guaranteeing the current sharing accuracy with desirable dynamics.

Keywords

• AC microgrids
• sequence virtual impedance
• three-phase four-wire (3P4W) converter
• voltage imbalance compensation