**5. Conclusions**

An alternative approach to improve dynamic performance of inter-connected power systems with relevant penetration of wind resource is discussed and evaluated. A new frequency control strategy is proposed by introducing demand-side contribution to frequency control, including primary and secondary response under frequency excursions. This demand-side contribution is carried out through the integration of frequency-sensitive load controllers into thermostatically controlled residential loads. Additionally, a supplementary control loop synthesizing virtual inertia for wind power plants is included as well, considering generation rate constraints and a governor dead-band. A combined frequency control strategy is thus proposed and evaluated.

According to the results, frequency deviations were significantly reduced in comparison to classical scenarios by including a combined solution with demand-response and wind power plant participation (peak frequency excursions were reduced by 23%). Similarly, the necessities of both primary and secondary regulation reserves from the supply-side could decrease significantly under power imbalance conditions. From the demand-side, minor effects on the controlled loads were allowed to satisfy minimum comfort levels required by the customers, with set-point temperature variations lower than 2 ◦C. Consequently, the results show a relevant reduction in supply-side reserve requirements (26.8%) and thus a suitable solution to integrate both controllable loads and wind power plants into the grid frequency stability.

**Author Contributions:** Data curation, I.M.-B.; Formal analysis, Á.M.-G. and E.G.-L.; Funding acquisition, E.G.-L.; Investigation, A.D.H.; Resources, T.G.-S.; Supervision, Á.M.-G.; Visualization, T.G.-S.; Writing—original draft, A.F.-G.; and Writing—review and editing, Á.M.-G.

**Funding:** This work was partially supported by by the Regional Seneca Foundation of Spain through the research projects 08747/PI/08 and 19379/PI/14. This work was also supported by project AIM, Ref. TEC2016-76465-C2-1-R (AEI/FEDER, UE).

**Acknowledgments:** This work was financially supported by the Regional Seneca Foundation of Spain through the research projects 08747/PI/08 and 19379/PI/14.

**Conflicts of Interest:** The authors declare no conflict of interest.
