Phase unbalance mitigation by three-phase damping voltage-based droop controllers in microgrids

• Presentation of a control strategy for phase unbalance mitigation in microgrids.
• In cooperation with voltage-based droop control to stabilise islanded microgrids.
• Analysis of the distortion damping resistance.
• Mathematical description of existing control coherently with the presented control.
• Unbalance mitigation and control equations of grid-forming and grid-following units.

Microgrids aggregate grid assets in geographically confined areas, allowing them to locally tackle grid issues and capture the value of their aggregated flexibility by bringing intelligence in the grid in a bottom-up approach. For the control of distributed generation units in single-phase islanded microgrids, the voltage-based droop (VBD) control strategy has already been developed. The VBD strategy can also be used in grid-connected microgrids, which makes a transition between both modes possible. Phase unbalance is a significant issue in three-phase microgrids and will become more pressing with increasing penetration of single-phase renewables. Therefore, the effect of several traditional control strategies for three-phase distributed generation units to phase unbalance is discussed. Subsequently, in this paper, a new control strategy is presented which extends the basic VBD control strategy so it can be used as a three-phase controller which at the same time mitigates unbalance in islanded and grid-connected microgrids, i.e., the three-phase damping VBD control. It is concluded that the presented three-phase damping VBD control method mitigates unbalance, allows the DG units to share the unbalanced currents, and is able to operate both in grid-connected and islanded systems.