The integration of significant volumes of intermittent renewable generation and distributed energy resources in electric power systems is forcing a debate on what electricity services are needed in a power system, how to price them and what agents are best suited for their provision. This paper defines and justifies the existence of a small set of primary electricity services that must exist in all power systems and regulatory contexts in order for the power system to operate satisfactorily. These services are determined by the placement of constraints on the planning and operation of power systems. As power system technologies evolve, new primary services may emerge, but will always be based on the placement of constraints on system operation and planning.
Keywords: electricity services; energy; operating reserves; black start; firm capacity; network connection; voltage control; power quality; constraint mitigation; loss reduction; regulation.