  This paper proposes a control method for allowing aggregates of
thermostatically controlled loads to provide synthetic inertia and primary
frequency regulation services to the grid. The proposed control framework is
fully distributed and basically consists in the modification of the thermostat
logic as a function of the grid frequency. Three strategies are considered: in
the first one, the load aggregate provides synthetic inertia by varying its
active power demand proportionally to the frequency rate of change; in the
second one, the load aggregate provides primary frequency regulation by varying
its power demand proportionally to frequency; in the third one, the two
services are combined. The performances of the proposed control solutions are
analyzed in the forecasted scenario of the electric power system of Sardinia in
2030, characterized by a huge installation of wind and photovoltaic generation
and no coil and combustible oil power plants. The considered load aggregate is
composed by domestic refrigerators and water heaters. Results prove the
effectiveness of the proposed approach and show that, in the particular case of
refrigerators and water heaters, the contribution to the frequency regulation
is more significant in the case of positive frequency variations. Finally, the
correlation between the regulation performances and the level of penetration of
the load aggregate with respect to the system total load is evaluated.
