**5. Conclusions**

Multi-areas interconnected power systems are analyzed under power imbalance conditions and with high wind energy integration. From the supply-side, conventional and renewable resources are considered, including thermal, hydro-power, wind and PV power plants. Wind power integration accounts for between 25% and 40%, corresponding to current percentages in some European countries. Tie-line power is limited to a maximum value of 10%, in line with recent EU directives. Different cases are compared and analyzed, depending on frequency control strategies applied by wind power plants. According to the results, frequency responses are improved by including wind power plants in frequency control, in comparison with simulations where this task is only performed by conventional generation units. Of the different cases, the nadir reductions are maximized when only wind power plants within the area submitted to imbalances are considered. In this case, the nadir is reduced between 40% and 50% in the area submitted to imbalanced areas in comparison to conventional generational unit scenarios. Moreover, these nadir values are also reduced in the other areas between 20% and 30%. When wind power responses of all areas are considered, higher frequency oscillations and lower nadir reductions can be reached in comparison with only conventional generation unit

#### *Energies* **2018**, *11*, 2775

scenarios. Stabilization time remains almost constant under different situations, and very similar to simulations where only conventional units respond under frequency excursions. Subsequently, the authors sugges<sup>t</sup> including only wind power plant frequency response within the area submitted to imbalances, avoiding additional frequency oscillations coming from wind power plants located in the other areas.

**Author Contributions:** Data curation, A.F.-G.; formal analysis, A.F.-G.; methodology, A.F.-G., A.V.-R. and A.M.-G.; software, A.F.-G.; supervision, A.V.-R. and E.G.-L.; visualization, A.M.-G.; writing—original draft, A.M.-G.; writing—review & editing, E.G.-L.

**Funding:** This work was supported by 'Ministerio de Educación, Cultura y Deporte' of Spain (ref. FPU16/04282). The authors are grateful for the financial support from the Spanish Ministry of the Economy and Competitiveness and the European Union —ENE2016-78214-C2-2-R.

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