**5. Conclusions**

Wind power will be the third largest renewable energy source around the world in 2023, and will provide approximately 30% of the total power demand during that year. However, this scenario gives rise to the problem of integrating the installed wind power capacity into the grid. Since the nature of wind is unpredictable, WPP installations are regarded as non-dispatchable, and hence TSOs and DSOs in different countries must carry out proper planning of network operation, and also forecast the participation of conventional power plants to compensate fluctuations in wind power generation. Moreover, voltage and frequency regulation problems may emerge in these situations.

In light of the above, transient stability analyses of WT and WPP dynamic models are required to forecast the electrical behavior of actual WTs before being connected to the grid. For this purpose, the International Electrotechnical Commission, through IEC 61400-27-1, defined four WT simulation models intended to represent actual WTs of different manufacturers. These generic WTs consist of a small

number of dynamic sub-models and parameters, and may be implemented and simulated in any software tool. Moreover, the Standard states that the responses obtained must be the same and independent of the software used. The present work demonstrates that, nevertheless, there may exist deviations between the two sets of results.

In order to analyze these differences, the generic Type 3 WT model, i.e., the DFIG WT, was implemented and simulated in MATLAB/Simulink and DIgSILENT-PowerFactory. Although MATLAB is widely used in the scientific literature, more specialized software tools, such as PowerFactory, are often used by TSOs and DSOs in different countries. Thus, the present work contributes to achieving a more widespread use of the recently developed generic WT models, also benefiting all stakeholders in the wind power industry.

The models were subjected to two different voltage dips, and their active and reactive power responses were compared with field measurements from an actual WT in operation. Validation errors following IEC 61400-27-1 guidelines were also estimated. In this way, it has been demonstrated that both simulation software tools are equally valid for the development of generic WT models. Beyond the specific applications of each tool, numerical differences between them are smaller than 2% for all the validation errors. Thus, it can be concluded that generic WT models can be implemented in the simulation software tool that fits better to the needs of the user.

With regard to the validation study, in general, the validation errors obtained between the field data series and the active and reactive power simulation responses of the models implemented in both software tools are low. The slight differences are due, firstly, to the limits of the model's accuracy, since, during the development of the Standard, some simplifications were introduced to design technically easy WT models (for instance, transient periods are not accurately represented by the generic WT models). Secondly, the errors may also be due to the limitations when the measurements were performed, such as the ones introduced by the tolerances of the measurement equipment, and also due to the conversion from instantaneous values to RMS values.

In addition, the errors between the two sets of simulated responses are near zero in the two Test Cases considered. Nevertheless, after carefully analyzing the differences in the implementation processes of both software tools and the behavior of each dynamic block, the errors between the active and reactive power responses may be attributed to two causes. The first cause could be the differences in the rate limiters used by the simulation tools. In these cases, PowerFactory applies a first-order filter to the input signal, the time constant of which affects its behavior and may dampen the effect of the filter. This affects the overall behavior of the WT, since IEC models use several of these rate limiters. The second cause concerns the time steps used. PowerFactory uses a variable step solver despite the time step being set as fixed. This may distort the dynamic responses of the WT model over time, since a fixed step solver is desirable for comparison purposes.

In summary, the outcomes of this work include the assessment of the advantages and disadvantages of the two power system simulation tools through the analysis of their implementation processes. Furthermore, the dynamic simulation of the generic Type 3 WT model is intended to provide a better understanding of IEC 61400-27-1, assessing its limitations and the accuracy of its originally-developed dynamic WT models, thus extending their field of application.

**Author Contributions:** Conceptualization, R.V.-R., A.L.-B. and E.G.-L.; Investigation, R.V.-R. and A.L.-B.; Methodology, R.V.-R., A.L.-B. and A.H.-E.; Project administration, E.G.-L.; Resources, F.J.-B.; Supervision, A.H.-E., F.J.-B. and E.G.-L.; Writing—Original draft, R.V.-R.; Writing—Review and editing, A.H.-E.

**Funding:** This research was funded by the Spanish Ministry of Economy and Competitiveness and European Union FEDER, gran<sup>t</sup> number ENE2016-78214-C2-1-R, as well as the Agreement signed between the UCLM and the Council of Albacete to foster Research in the Campus of Albacete.

**Acknowledgments:** The authors would also like to express their gratitude to the wind turbine manufacturer Siemens Gamesa Renewable Energy for the technical support received. Furthermore, the authors would like to thank the TC 88/WG 27 Committee "Wind turbines—Electrical simulation models for wind power generation", to his Convenor, Poul Sørensen, and to the 16 participating countries.

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