Two-Stage Reactive Power and Voltage Optimization Control Strategy for Hybrid Wind Farms Considering System Strength
: A two-stage reactive power and voltage optimization control strategy is proposed for hybrid wind farms comprising grid-following (GFL) and grid-forming (GFM) wind turbines. The proposed method incorporates reactive power balancing to enable coordinated regulation among different devices, enhance voltage control capability under weak-grid conditions, and mitigate reactive power imbalance caused by heterogeneous control characteristics. First, a linearized optimization model is developed for the coordinated reactive power and voltage regulation of GFL and GFM wind turbines. In the day-ahead stage, equivalent short-circuit ratio (ESCR) constraints are incorporated to coordinate reactive compensation devices while ensuring operational economy, voltage security, and adequate system strength. In the intraday stage, an adaptive optimization strategy driven by wind power fluctuation levels is introduced, in which both control objectives and optimization time scales are dynamically adjusted. During periods of significant wind power fluctuations, a reactive power balancing optimization mechanism is activated to enhance coordination among wind turbines, while reactive power balancing zones are further established to determine reference reactive power values for GFL wind turbines. Finally, the effectiveness of the proposed method is validated through a case study of a hybrid wind farm consisting of 34 doubly-fed induction generators (DFIGs).