Transmission-State-Dependent Carbon-Trading Settlement in UPFC-Assisted Preventive Optimal Power Flow

Low-carbon OPF studies usually include carbon cost or emission limits in generation-side dispatch, while carbon-accounting studies often evaluate emissions after dispatch. The link between transmission-side flow control and carbon settlement has received less attention. In particular, it remains unclear how UPFC-driven changes in branch flows and losses should affect allowance balance, trading volume, and settlement cost in preventive OPF. This paper therefore proposes a transmission-state-dependent carbon-trading settlement method for UPFC-assisted preventive OPF. Different from post-dispatch carbon accounting, the proposed method calculates carbon-trading quantities from the same network-feasible dispatch solution that determines UPFC-regulated flows, network losses, thermal generation, and contingency feasibility. The model is written in equivalent current injection form. A social-learning artificial bee colony algorithm with feasibility repair is used to solve the constrained problem, with infeasible trial solutions repaired according to power-balance, operating-limit, and carbon-trading constraints. The Taipower 345 kV transmission system is used for testing. Under the annual average daily load condition, line losses are reduced from 27,126.55 MWh to 21,281.92 MWh, while carbon emissions decrease from 300,988.72 tCO2 to 279,527.70 tCO2, corresponding to reductions of 21.55% and 7.13%. The generator-side profit remains positive at TWD 11,000,763. The combined UPFC-PCOPF case also satisfies the screened post-contingency line-loading and voltage constraints. The full-year cases show a similar reduction pattern. These results suggest that carbon-trading settlement is not a fixed post-dispatch accounting item. In UPFC-assisted dispatch, it is linked to transmission losses and thermal generation.