Unified Viewpoint of Soft Startup and Rate Limiter for SOGI-FLL

This letter demonstrates that the second-order generalized integrator-based frequency-locked loop (SOGI-FLL) with a soft startup/rate limiter essentially incorporates an adaptive feedforward relying on
a priori
information of transient input. When a fair parameter is tuned, the soft startup and the limiter give a similar performance. The shortcoming is that they have a deceleration blind zone, depending on the frequency constraints and SOGIs gain. If the allowed rate of change of frequency (RoCoF) is fixed, the fast response FLL can weaken this shortcoming. Specifically, if the RoCoF of low-inertia grids is considered, they could produce a severely biased estimation under the harmonics and dc offsets. The experimental results are given to validate the claims.