Voltage-Support Control of Inverter-Based Resources During Unbalanced Faults Addressing Grid Impedance Asymmetry

Voltage support is an essential requirement for inverter-based resources to maintain the stability and reliability of modern power grids during faults. Short-circuit events are among the most severe disturbances, as they can lead to equipment failure and power outages. Therefore, various fault ride-through (FRT) strategies have been developed to support the grid voltage during such events. As most faults are asymmetric, they cause asymmetry in the grid impedance and unbalanced voltages. However, existing control strategies only consider voltage unbalance, whereas they largely overlook the asymmetry in the grid impedance. Consequently, even during asymmetric short-circuit events, the equivalent grid impedance at the point of common coupling is still assumed to be symmetric. This work proposes a generalized optimal voltage-support FRT control that accounts for grid asymmetry. First, an analytical derivation of the grid impedance matrix is presented. Then, a generalized voltage-support strategy is formulated that explicitly accounts for impedance asymmetry. Numerical simulations on a modified version of the CIGRÉ low-voltage benchmark microgrid and experimental results from a laboratory system validate the proposed strategy.