Cascaded adaptive control for grid-forming inverters and model-free microgrid synchronization

• Adaptive cascaded control applied to grid-forming inverter. • Voltage direct-tracking using a model reference adaptive controller. • Controller validated through extensive tests with wide parameter variations. • Comparison between adaptive and fixed-gain control techniques. • Model-free adaptive control applied to microgrid synchronization. This paper presents a cascaded adaptive control strategy for the inner loop of a grid-forming inverter. The control structure combines an inner proportional regulator for current limiting with an outer Model Reference Adaptive Controller (MRAC) for voltage regulation, tracking references from the primary control layer. The primary control utilizes a droop mechanism with virtual impedance to enhance active and reactive power decoupling. For grid-reconnection scenarios, the secondary control implements an adaptive proportional-integral-derivative (PID) controller in the synchronization loop, effectively compensating for non-deterministic communication delays while maintaining a simple design framework. The control system is validated through hardware-in-the-loop (HIL) simulations, evaluating five critical scenarios: steady-state operation (islanded and grid-connected), reconnection transients, unplanned islanding, and a microgrid fault. A comparative analysis with a fixed-gain controller demonstrates the superior performance of the proposed adaptive approach in managing uncertainties and dynamic grid conditions, with consistent improvements across all scenarios and error reductions of up to 90% in key performance indices.