Equivalent networks for medium-term hydro-thermal scheduling with variable head cascaded reservoirs and transmission constraints

In this paper we investigate the use of equivalent networks to reduce variable and constraint spaces of medium-term hydro-thermal scheduling problems with variable head cascaded reservoirs and transmission constraints. We take advantage of the fact that in DC power flow formulations, which are suitable for these problems, the Ward equivalent yields exact equivalent networks as far as topology is concerned. Numerical tests based on modified versions of the IEEE 14-bus and IEEE RTS-96 networks with hydro plants from the Brazilian power system illustrate how the consideration of such constraints significantly influences optimal cascaded reservoir operation over the planning horizon. Additional analysis shows how medium-term hydro-thermal scheduling benefits from equivalent networks in providing optimal transmission-constrained dispatches with lower computational requirements.