Background and motivation

Grid-forming (GFM) control represents a promising solution for integrating high shares of converter-based resources into modern power systems. A GFM voltage-source converter (VSC) operates as an internal voltage source with defined ac-side output impedance, enabling the provision of critical services such as voltage support and frequency regulation. The design of its active and reactive power control loops governs the converter’s dynamic response to disturbances in the ac grid, thereby contributing to enhanced grid stability and resilience.

Under fault conditions, a GFM-VSC needs to fulfill two key operational objectives within a short timeframe. First, it needs to inject sufficient current—particularly reactive current—to support the terminal voltage magnitude. Second, it needs to track the active and reactive power references during both the occurrence and clearance of the fault, thereby ensuring the system returns to its intended operating point.

Achieving fast fault-current injection requires a slow internal voltage source dynamic, enabling the converter to behave as a stiff voltage source and deliver the necessary reactive current. In contrast, accurate and rapid power-reference tracking demands fast internal voltage source dynamics with high control bandwidth in both the active and reactive power control loops. These conflicting requirements for internal voltage source behavior present a fundamental challenge in the control design of GFM-VSCs.

Main work

(1) Problem formulation.

The trade-off between fault-current response and power-recovery speed is analyzed through an equivalent circuit model and phasor diagram analysis. A slower internal voltage source dynamic enables strong and rapid reactive current injection immediately after the voltage sag, while a faster dynamic improves power-tracking performance at the cost of reduced current response speed.

(2) Proposed control solution.

To address this trade-off, a coordinated control strategy is developed. First, reactive current setpoints are adaptively shaped based on the instantaneous terminal voltage, with values raised above nominal under deep sags to ensure ride-through compliance. Second, during power-reference transitions, high-bandwidth proportional paths from power error to frequency are activated in the active power loop, while additional proportional–integral branches are used in the reactive power loop. Moreover, feedforward paths from power references to current references are employed to accelerate the system’s dynamic response.

Results

The proposed control method allows the GFM-VSC to meet critical operational requirements under fault conditions.

(1) During fault occurrence, the converter injects reactive current rapidly and in accordance with grid-code specifications.

(2) During fault recovery, the adaptive control strategy enables quick restoration of both active and reactive power, ensuring the system rapidly returns to its intended operating state.