An internal model approach to (optimal) frequency regulation in power   grids with time-varying voltages
  This paper studies the problem of frequency regulation in power grids under unknown and possible time-varying load changes, while minimizing the generation costs. We formulate this problem as an output agreement problem for distribution networks and address it using incremental passivity and distributed internal-model-based controllers. Incremental passivity enables a systematic approach to study convergence to the steady state with zero frequency deviation and to design the controller in the presence of time-varying voltages, whereas the internal-model principle is applied to tackle the uncertain nature of the loads. 
Sebastian Trip, Mathias B\"urger, Claudio De Persis
cs.SY math.OC