Adaptive virtual impedance design for three-phase inverter under unbalanced grid connection

Author(s): You Hua, Lini Zhenga, Ruixuan Wanga, Zhen Lia*, Xi Chenb, Guoqing Hec and Xiangdong Liua
a School of Automation and Key Laboratory for Intelligent Control & Decision on Complex Systems, Beijing Institute of Technology, Beijing 100081, China
b GEIRI North America, CA, USA
c China Electric Power Research Institute, Beijing, China
International Journal of Smart Grid and Clean Energy, vol. 9, no. 1, January 2020: pp. 25-32
ISSN: 2315-4462 (Print)
ISSN: 2373-3594 (Online)
Digital Object Identifier: 10.12720/sgce.9.1.25-32

Abstract: As a critical part of energy conversion, the parallel inverters play an important role in the stability of distributed generation (DG) system. Although the conventional control, such as the virtual impedance-based droop control, can achieve the autonomous decoupling between active and reactive power irrespective of the condition of line impedance. However, under the unbalanced conditions, which has high occurrence due to the rural location of DG, the active and reactive power sharing are inevitably coupled so that the grid support capability is degraded, causing the stability issue to the whole system. In order to overcome this problem, this paper proposes an adaptive virtual impedance-based control strategy by purely decoupling the active and reactive power. The line impedance, i.e., the main origin of unbalances, is identified by establishing the unbalanced impedance modeling in full-dq transformation, which is further adopted as the adaptive virtual impedance generating the compensation of unbalanced voltages. Experimental results verify that the proposed method is effective to identify any unbalanced components and achieve the cancellation of unbalanced grid voltage so that the system stability is maintained.

Keywords: Parallel inverters, unbalanced conditions, virtual impedance, full-dq transformation, impedance identification
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