Varying Negative Sequence Virtual Impedance Adaptively for Enhanced Unbalanced Power Sharing Among DGs in Islanded AC Microgrids

Inverter based distributed generators (DGs) are increasingly being employed for interfacing renewable energy sources into the grid. Droop control is a very popular technique for ensuring parallel operation of these sources based on local measurements alone, in which the inverters are made to emulate the behaviour of synchronous generators of the power system. However, several drawbacks of the droop control technique are being addressed actively, over the years by various researchers. One of the prominent issues is the inaccurate power sharing among the distributed generators (DGs) due to the feeder/line impedances. The performance further deteriorates in the presence of unbalanced loads. This work presents an adaptive technique for adjusting the negative sequence (NS) virtual impedance of the DG, based on its output current, without the need of communication or extra sensors, to achieve enhanced unbalanced power sharing. The proposed control is shown to have the capability of seamless integration with the existing conventional and inverse droop techniques. The methodology is validated through simulations for different DG ratings, controls and on a modified 13 bus system. Small signal eigenvalue analysis about the operating point is used to determine the stable range of control parameters and the strategy is validated on an experimental setup in the laboratory.