A Distributed-Controlled Harmonic Virtual Impedance Loop for AC Microgrids

Proportional harmonic current sharing among inverters is desired in ac microgrid with nonlinear loads. Usually, virtual impedance is emulated to achieve good harmonic current sharing at the cost of increased harmonic voltage distortion in the system. Existing schemes overcome this issue by communicating voltage and/or current values among inverters, thereby requiring significant data exchange. Further, some of them suffer from single point of failure because of their centralized architecture. This article proposes a distributed controller, which emulates harmonic voltage sources in addition to the virtual impedance, thereby reducing voltage distortion in the system. Furthermore, the proposed scheme reduces congestion in the communication network, as compared to the existing communication-based methods, as only two bits per harmonic frequency is communicated among inverters. The effect of virtual impedance on the stability of the system is investigated. Additionally, the impact of communication delay on the current sharing error is studied in this article. Finally, the proposed technique is validated through simulation and experimentation on a scaled-down laboratory prototype.