Outer loop control design of grid-forming converters for enhanced transient stability and current limiting capability

Current limiting is necessary to ensure the hardware integrity of grid-forming converters (GFMs), but with implications for the grid-forming ability itself. Implementing a threshold virtual impedance (TVI) is a promising current limiting technology, as it maintains the voltage source characteristic behind an impedance. In this paper, a new P̃/f droop control is proposed to enhance the transient stability of a droop control-based GFM under TVI current limiting control for large fault events, which is achieved by replacing the real-time active power, P , with a modified value, P̃ , whereby the voltage drop caused by the TVI control is not included. Existing work has demonstrated that the virtual impedance ratio, σVIX/R , has a conflicting effect on system dynamics and transient stability, i.e. a small σVIX/R results in a well-damped current response, but a very limited transient stability margin, while a large σVIX/R results in a poorly-damped current response, but an acceptable transient stability margin. Under the proposed P̃/f droop control, this dilemma is avoided, i.e. a small σVIX/R not only results in a well-damped current response, but also further enhances the transient stability. Finally, since the standard dispatchable virtual oscillator control (dVOC)-based GFM doesn't incorporate current limiting control, here, the same TVI current limiting control (again, a small σVIX/R is recommended for fault events) is added to the outer control loop of the original dVOC-based GFMs, such that the above P̃/f droop characteristic is provided for a dVOC-based GFM.