**6. Conclusions**

Dynamic behaviors and critical issues like the stability of DC-link capacitor voltage and grid injected active and reactive power in DFIG-based WTs under voltage sags and grid faults were investigated and robust and novel Adaptive Proportional Integral (API) and Proportional Resonant with Resonant Harmonic Compensator (PR+RHC) controllers were proposed. The proposed DC-voltage control method is implemented independent of rotor side control which mitigates voltage harmonics in DC-capacitors and stabilizes active and reactive power which results in enhanced reliability of DC-link capacitor, WT stability, and makes control systems adoptable for large scale DFIG converters.

The performance of the PI control scheme shows sensitivity, large oscillations, and slow convergence to normal and abnormal conditions as verified from our simulation results, Total Harmonic Distortion (THD) analysis, and performance indices tables (Integral Absolute Error (IAE), Integral Square Error (ISE) and Integral Time-weighted Absolute Error (ITAE). However, comparatively the proposed controllers, i.e., API and PR+RHC, provide a better dynamic response, less sensitivity, fast convergence, less oscillation, robust, ripple-free and fault tolerant performance under normal and abnormal conditions.

**Author Contributions:** I.K., K.Z. and W.U.D. propose the main idea of the paper. I.K. implements the mathematical derivations, simulation verifications and analyses. The paper is written by I.K., and is revised by K.Z., W.U.D., S.U.I., M.I., S.H. and H.-J.K. All the authors were involved in preparing the final version of this manuscript. Besides, this whole work was supervised by H.-J.K.

**Acknowledgments:** This Research was supported by BK21PLUS, Creative Human Resource Development Program for IT Convergence.

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