Comparison of Droop-Based Single-Loop Grid-Forming Wind Turbines: High-Frequency Open-Loop Unstable Behavior and Damping

TL;DR

This paper compares droop-based grid-forming controls in wind turbines, revealing that droop-I control introduces high-frequency open-loop instability that cannot be mitigated by traditional damping methods, impacting overall system stability.

Contribution

It provides a detailed analytical and case study comparison showing that droop-I control causes inherent high-frequency instability in wind turbine systems, unlike droop control.

Findings

Droop-I control shifts high-frequency poles causing instability.

Open-loop instability cannot be mitigated by tuning control parameters.

Active damping may fail under droop-I control due to unstable poles.

Abstract

The integration of inverter-interfaced generators introduces new instability phenomena into modern power systems. This paper conducts a comparative analysis of two widely used droop-based grid-forming controls, namely droop control and droop-I control, in wind turbines. Although both approaches provide steady-state reactive power-voltage droop characteristics, their impacts on high-frequency (HF) stability differ significantly. Firstly, on open-loop (OL) comparison reveals that droop-I control alters HF pole locations. The application of Routh's Stability Criterion further analytically demonstrates that such pole shifts inevitably lead to OL instability. This HF OL instability is identified as a structural phenomenon in purely inductive grids and cannot be mitigated through control parameter tuning. As a result, droop-I control significantly degrades HF stability, making conventional gain and phase margins insufficient for evaluating robustness against parameter variations. Then, the performance of established active damping (AD) is assessed for both control schemes. The finding indicates that AD designs effective for droop control may fail to suppress HF resonance under droop-I control due to the presence of unstable OL poles. Case studies performed on the IEEE 14-Bus Test System validate the analysis and emphasize the critical role of HF OL instability in determining the overall power system stability.