Analysis and Evaluation Methods for UHV AC Retrofitting Schemes and Application Effects under Renewable Energy Transmission Scenarios

Aiming at the challenge of large-scale centralized integration of high-proportion renewable energy, and addressing current bottlenecks in ultra-high voltage (UHV) AC power grids—such as low utilization rates of transmission corridors and poor active power flow controllability—this paper proposes various upgrading and transformation schemes for UHV AC ring networks. These schemes aim to simultaneously enhance the renewable energy accommodation and operational regulation capabilities of the AC grid. First, targeting typical UHV double-circuit transmission corridors, three topological schemes are designed: direct AC integration of renewable energy, single-circuit voltage source converter-based high voltage direct current (VSC-HVDC) transformation, and double-circuit full VSC-HVDC transformation. Second, a unified DC power flow mathematical model is established to theoretically derive and reveal the power flow control characteristics under traditional AC integration, single-circuit VSC-HVDC transformation, and full VSC-HVDC transformation. Finally, a multi-dimensional evaluation system is constructed, encompassing power flow control flexibility, static and transient voltage stability, and short-circuit current levels. Simulations and analyses are then conducted using the Power System Dispatcher-Based Performance Analysis (PSD-BPA) program based on the 2040 planning data for the East China Power Grid. The research results indicate that the double-circuit full VSC-HVDC transformation scheme can overcome the physical constraints of the AC grid’s natural impedance, achieving active regulation of transmission power. Simultaneously, it effectively limits short-circuit currents by leveraging its nonlinear high-impedance characteristics. This scheme serves as an effective technical pathway for resolving the challenges of high-proportion renewable energy transmission and ensuring the safe and stable operation of receiving-end power grids.