A Distributed-Controlled Harmonic Virtual Impedance Loop for AC Microgrids

Proportional harmonic current sharing among inverters is desired in AC microgrid with non-linear loads. R/R-L based virtual impedance is emulated to achieve good harmonic current sharing at the cost of increased harmonic voltage distortion. To reduce voltage distortion, negative resistance emulation is suggested, which suffer from reduced stability margin. Some of other methods require system knowledge to determine value of virtual impedance and are not adaptive to change in system. To address this, communication based methods are suggested in literature, which require exchange of harmonic voltage / current values, thereby requiring significant data exchange. Some of existing methods are based on centralized control architecture suffering from single point of failure. To address these issues, a distributed controller is proposed, which emulates harmonic voltage sources in addition to virtual impedance, thereby cancelling the effect of virtual impedance and reducing voltage distortion. Furthermore, the proposed scheme reduces congestion in communication network, as compared to existing communication-based methods, as only two bits per harmonic frequency is communicated among inverters. The effect of virtual impedance on system stability is investigated. Impact of communication delay on current sharing error is studied. The proposed technique is validated through simulation and experimentation on a scaled-down laboratory prototype.