Online Construction of Small-Signal Stability Region for Wind Farms With Reduced-Order Impedance Model and Two-Layer Optimization

Incidents of wideband oscillations occurred in wind farms, urgently necessitating small-signal stability assessment. The wind speed and the output power are time-variant, resulting in varying impedance characteristics and stability performance of the wind turbine generators (WTGs). To achieve accurate and efficient stability assessment, this paper proposes an online construction method of the stability region based on a piecewise affine impedance model. Firstly, the piecewise affine impedance of the WTG is offline constructed as a function of the operating points. During online applications, the impedance is rapidly identified with knowledge of the readily measured operating points. As such, the efficiency of the impedance calculation is significantly improved, and the impedance of the WTGs with unknown parameters can be constructed. To address the grid side with data-driven methods, a dynamic mode decomposition-based piecewise affine approach is introduced to identify the grid impedance. Secondly, a two-layer optimization is proposed to determine the stability region boundary of the WTGs. An enhanced generalized Nyquist criterion and a composite shape algorithm for a rapid solution are developed. Finally, numerical simulations and hardware experiments are performed for validation. The experiments include a physical platform for a single-PMSG stability test and a hardware-in-loop platform for multi-WTG stability region assessment.