Due to the absence of widely available and economically feasible storage, each electricity system has to maintain a constant balance between supply and demand by adapting the generating power. Load also needs to be adapted to forecasted and unforecasted changes in supply and demand as well as to unexpected faults in the system, such as the unscheduled shutdown of a generating unit or transmission line. Such load following allows the alignment of the overall system supply with daily, weekly and seasonal demand variations.