Expansion planning of hybrid HVAC/HVDC transmission systems under wind power integration and generation flexibility constraints

The inherent variability of wind farm output intensifies power system flexibility requirements. Thermal generation units provide critical flexibility through their ramp rate capabilities, which directly influence both generation economics and transmission network expansion planning (TNEP). However, thermal plant operators often restrict ramp rates to avoid equipment stress, creating a key planning challenge: determining optimal transmission expansion under these operational constraints. This paper proposes a novel bi-level optimization framework for TNEP that integrates short-term wind power and load demand fluctuations in electricity markets. The proposed methodology employs a 24-hour operational horizon to capture renewable generation variability and demand patterns while evaluating 2 critical flexibility factors: (1) thermal unit ramp rate impacts and (2) high-voltage direct current (HVDC) line integration. The upper-level problem optimizes transmission investments, while the lower-level solves market-clearing problems to maximize social welfare. Through duality theory, the Mathematical Program with Equilibrium Constraints is reformulated as a tractable Mixed-Integer Linear Programming problem. Case study results demonstrate that enhanced thermal flexibility (via higher ramp rates) yields multifaceted benefits: a reduction in investment costs, increased social welfare, and complete wind curtailment elimination alongside lower thermal generation and emissions. Also, the hybrid AC/DC approach outperforms AC-only systems, reducing required transmission corridors while reducing costs by up to 20%. Optimal ramp rates (15–35%) prove most effective for curtailment reduction, with HVDC lines further enhancing grid flexibility.