  Frequency control reserves are an essential ancillary service in any electric
power system, guaranteeing that generation and demand of active power are
balanced at all times. Traditionally, conventional power plants are used for
frequency reserves. There are economical and technical benefits of instead
using energy constrained units such as storage systems and demand response, but
so far they have not been widely adopted as their energy constraints prevent
them from following traditional regulation signals, which sometimes are biased
over long time-spans. This paper proposes a frequency control framework that
splits the control signals according to the frequency spectrum. This guarantees
that all control signals are zero-mean over well-defined time-periods, which is
a crucial requirement for the usage of energy-constraint units such as
batteries. A case-study presents a possible implementation, and shows how
different technologies with widely varying characteristics can all participate
in frequency control reserve provision, while guaranteeing that their
respective energy constraints are always fulfilled.
