A Non-linear Adaptive Virtual Impedance (NLAVI) based Approach for Sharing Asymmetrical Currents

Abstract

Increased single-phase penetration of converter based distributed generators (DGs) such as rooftop solar photo-voltaic (PV) units exasperates the asymmetry in the distribution system. The unbalanced single phase loads may give rise to issues such as increased currents, heating, neutral point voltage shifts etc. From the considerations of efficiency and safety, it is desirable to have a close to perfectly balanced three-phase system. In view of this, several control modifications for the sharing of unbalanced/ asymmetrical currents amongst DGs is being reported recently in the literature. This work proposes a modified non-linear adaptive virtual impedance (NLAVI) shaping technique to achieve closer to proportional sharing of unbalanced currents in an islanded active distribution network. In addition, a seperate negative sequence control loop improves the unbalance at the point of common coupling (PCC) voltage. Simulation results for a system with two converters: three-leg and four-leg inverter based DGs and some preliminary controller hardware-in-loop (CHIL) validations on the proposal have been presented. Further, CHIL results for a hybrid system with one three-leg and one four-leg inverter based DG are also discussed.