Inertia Enhancement Control Strategy Based on Frequency-Locked Loop for Grid Connected Converter

[Objective] With the increasing penetration of renewable energy sources， traditional rotating machines have been replaced with power electronic converters， causing a considerable decline in the overall inertia of power systems. This reduction in inertia poses serious challenges to the frequency stability and system disturbance rejection. To overcome these challenges， grid-connected converters that can actively provide synthetic inertial support are required. [Methods] First， based on a power-sharing control strategy， this study analyzes the pole-zero distribution characteristics of conventional phase-locked loop （PLL）-based control structures and highlights the associated stability degradation under weak grid conditions. Next， an inertia enhancement control scheme based on an improved frequency-locked loop （FLL） structure is proposed to address these problems. By proportionally integrating the frequency-derivative signal with the reference active power， the proposed method effectively mitigates power oscillations and enhances the inertial response of the converter. Furthermore， a modified model predictive control strategy is employed to replace the conventional inner current loop， significantly improving the transient performance of the system. Further， the effectiveness of the proposed control strategy is validated through hardware-in-the-loop simulations.Comparative studies of frequency step responses demonstrated the superiority of the proposed FLL-based structure in maintaining frequency stability under weak grid conditions. The active power control strategy under the improved FLL framework is detailed， along with the principle of inertia emulation and tuning of the associated parameters. [Results] The frequency response of the system is substantially improved and synthetic inertia enhancement is achieved. [Conclusions] The proposed method prevents high-frequency noise issues caused by the direct differentiation of frequency signals and eliminates stability problems typically introduced by PLL.