Fast Frequency Support from Grid-Forming Converters under dc- and ac-Side Current Limits

A decentralized nonlinear feedback controller is proposed for droop-based grid-forming converters (GFCs) with both dc-side current limitation and ac-side current limit to provide fast frequency support from renewable resources after a large generation trip or load increase. The controller is designed to guarantee asymptotic stability for a generic grid with multiple synchronous generators and GFCs represented by a reduced-order model. The controller is based on the integral sliding mode approach and requires frequency information of the neighboring buses if GFCs are connected to them. The proposed approach also leads to a quantifiable frequency support mechanism that aims to maximize the improvement in frequency nadir and minimize the steady-state frequency deviation. In addition, it allows a GFC to refrain from participating in frequency support if it chooses to do so. Simulations on the electromagnetic transient (EMT) model of a 4-machine 2-area system with 1 GFC and a detailed phasor model of the modified IEEE 16-machine system with 4 GFCs equipped with inner control loops demonstrate the effectiveness of the suggested control.