**7. Conclusions**

In this paper, transient characteristics of DFIG-WT short-circuit current under converter control are analyzed and a detailed analytical calculation model of short-circuit total current of DFIG-WT, including GSC current and stator current, is deduced. Based on the established model, the characteristics of steady-state fault current of DFIG-WT are further studied. The accuracy of the theoretical analysis and mathematical deduction is verified by comparing simulation test and LVRT field test data. The main work of this paper can be summarized and conclusions can be drawn as follows:


The results and conclusions in this paper could provide theoretical references for short-circuit current calculation of power systems with DFIG-WTs connected to the grid, as well as optimizing settings and redesigns for relevant protection, such as transformer protection with second harmonic restraint. Moreover, from the aspect of control strategies of convertors, if the fluctuation of DC bus voltage could be suppressed, the second harmonic content contained in the short-circuit current should also be decreased.

**Author Contributions:** Conceptualization, J.L. and T.Z.; methodology, J.L.; software, J.L.; validation, J.L. and T.Z.; formal analysis, J.L.; investigation, J.L.; resources, Z.W.; data curation, J.L.; writing—original draft preparation, J.L.; writing—review and editing, J.L., T.Z. and Z.W.; visualization, J.L.; supervision, T.Z. and Z.W.; project administration, T.Z. and Z.W.; funding acquisition, Z.W.

**Funding:** This work was funded by the National Key Research and Development Plan of China (2016YFB0900640) and Research Project of State Grid Corporation of China (5211TZ16000F).

**Acknowledgments:** We acknowledge the North China Electric Power Research Institute (NCEPRI), which provided technical support and LVRT field test data for this research.

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