Inner-Loop Controllers for Grid-Forming Converters

This paper presents a detailed discrete-time implementation of an inner-loop voltage controller with a current limiter for grid-forming converters with an LC filter connected to the grid. The proposed approach utilizes a state feedback control law that depends on the states of both the converter and the internal model controller. Additionally, a resonant controller is designed to accurately track the fundamental frequency of the grid voltage. The stability of the proposed controller is analyzed for both virtual impedance and virtual admittance implementations. To validate the controller's effectiveness, simulation results are provided using a Hardware-in-the-Loop (HIL) setup, alongside a comparison of different virtual impedance implementations.