Carbon Emissions Resulting from Different Power Flow Models for Dispatch

Optimal power flow (OPF) seeks to minimize the cost of electric power generation subject to physical constraints. The objective function in the OPF problem is typically defined in terms of dollars, and not in terms of environmental objectives such as plant emissions. However, the mix of generators that result in the lowest system cost does not always fully correlate with the mix of generators that result in the lowest system emissions. This can be further exacerbated under a DC OPF framework, which utilizes slack bus generators (often fast-ramping gas plants) to ensure AC feasibility. This paper analyzes the difference in emissions under different power flow models to quantity how cost-based objectives in OPF have impacts on the resulting system emissions. The IEEE 118-bus system is considered as our initial test case, using demand data from CAISO. Our results indicate that the choice of slack bus(es) can heavily influence the difference in emissions. We also propose an alternative DC OPF framework which reduces utilization of the slack bus (thus reducing emissions) and avoids possible constraint violations.