A Unified Grid-Forming and Grid-Following Primary Control Design With Optimized Enforcement of Grid Operational Constraints

Abstract

A novel, unified control design is proposed for software-defined inverters (SDIs) to operate in either grid-forming (GFM) or grid-following (GFL) mode in AC grids/microgrids and to switch between the two modes seamlessly. The proposed GFM and GFL controls are systematically developed and analytically synthesized based on the dynamic state space models of the inverter, its output filters, and its terminal voltage and current at the AC grid/microgrid. The proposed GFM and GFL controls have, a) a nonlinear nominal output tracking control as the seed design, and b) a constraint-enforcing control. The GFM nominal control tracks frequency, voltage-magnitude, and angle, while the GFL nominal control tracks frequency as well as real and reactive power references. Tracking of either GFM or GFL output vector is ensured under the recursive Lyapunov design paradigm, naturally resulting in consistent current and voltage control laws. This paradigm allows the implementation of constraint-enforcing control that ensures that the frequency, voltage, power, and current injections dynamically satisfy their operational constraints in the presence of load variations and intermittent renewable power sources. Stability and convergence of the proposed GFM and GFL controls, and constraint enforcement are concluded analytically, and their effectiveness is demonstrated through simulations.