Comparative assessment of single-loop droop controlled grid-forming converter and its damping enhancement

As the penetration of power electronic interfaced power sources increases rapidly, grid-forming controlled voltage source converters (VSCs) are deemed as an effective alternative to synchronous generators. As a typical grid-forming control scheme, droop control is commonly utilized due to its simplicity of construction and stability of operation. VSCs driven by droop controls, such as multi-loop droop control (MLDC) and single-loop droop control (SLDC) are capable of actively establishing voltage and frequency of grid-forming converters. However, the transient performance of SLDC at different faults is less explored to date. In this work, an in-depth comparative assessment of large-signal stability against the established MLDC is first presented by means of phase portrait. It is revealed that, both control strategies have similar transient behaviors with their basic scheme. However, with the introducing of the low-pass filters which can emulate the inertia, SLDC generally exhibits a larger power angle overshoot during the fault and therefore has poorer transient stability as compared that of MLDC. To address this issue, this work improves the damping of SLDC by incorporating differential compensation link into it. Furthermore, a parameter design guideline that balances damping and inertia, hence the transient stability and frequency stability, is proposed. The analysis is supported by real-time hardware-in-the-loop experimental results.