An LQR-based Robust Voltage Controller for Grid Forming Inverters during Blackstart

The wide adoption of renewable energy resources has led to the increasing penetration of power electronic inverters on the grid. Grid following (GFL) inverters are replacing conventional synchronous machines that provide rotational inertia to the power system, which can lead to frequency stability issues during grid contingencies. Recently, grid forming (GFM) inverters have emerged as a way to reliably incorporate renewable sources by actively regulating voltage and frequency of the power system. For many grid forming inverters, the internal voltage control loop needs to be robust against grid impedance changes. In this paper, we introduce a robust voltage controller based on linear-quadratic-regulator (LQR) during system blackstart. We propose a tuning procedure that ensures stability over a wide range of grid impedances. The effectiveness of this control structure is then demonstrated with a realistic blackstart process in PSCAD®, which includes energization of transformers, line equipment, induction motors and passive loads of different sizes.