Rethinking the Active-Power–Frequency Loop in Grid-Forming Converters: A Constant-Frequency Voltage-Source Perspective

With the increasing penetration of renewable energy, power systems are transitioning from synchronous-machine-dominated to converter-dominated operation. Conventional VSG and droop control make the converter emulate synchronous-machine rotor dynamics by coupling active power with frequency. This design choice inevitably slows down the converter and may sacrifice its fast-regulation capability. The slow response of VSG-based GFMs is often treated as an inherent property of grid-forming operation, but it mainly arises from using the active-power–frequency loop to match synchronous-machine rotor dynamics. Rather than proposing another GFM control law, this paper revisits a basic assumption of VSG control: whether the active-power–frequency loop is still necessary when the converter is only required to provide fast support within its current margin, while steady-state support is assigned to the synchronous machine(SM). A voltage-oriented constant-frequency GFM strategy based on Q–Us droop control is adopted to decouple active-power regulation from frequency dynamics. Compared with conventional VSG control, the constant-frequency approach exhibits smaller voltage deviations and faster voltage recovery during disturbances. The results suggest that the relatively slow response associated with VSG control is primarily a consequence of its control design rather than an inherent requirement of grid-forming operation.