Large - Scale Optimal Control of Interconnected 1 Natural Gas and Electrical Transmission Systems ∗

4 We present an optimal control model that captures spatiotemporal interactions between gas 5 and electric transmission networks. We use the model to study flexibility and economic oppor6 tunities provided by coordination. A case study in the Illinois system reveals that coordination 7 enables the delivery of significantly larger amounts of natural gas to the power grid. In particular, 8 under a coordinated setting, gas-fired generators act as distributed demand response resources 9 that can be controlled by the gas pipeline operator. This enables more efficient control of pres10 sures and flows in space and time and overcomes delivery bottlenecks. We demonstrate that the 11 additional flexibility not only benefits the gas operator but also leads to more efficient power grid 12 operations and results in increased revenue for gas-fired power plants. We also use the optimal 13 control model to analyze computational issues arising in these complex models. We demonstrate 14 that the interconnected Illinois system gives rise to a highly nonlinear optimal control problem 15 with 4,400 differential and algebraic equations and 1,040 controls that can be solved with a state16 of-the-art sparse optimization solver. 17