**6. Experimental Verifications**

Since SM faults make the MMCs present unequal equivalent capacitances per arm and per phase-leg bases, this section emulates the steady-state impacts of SM faults through the deliberate use of unequal cell capacitances and arm inductances on experimental test rig of a single-phase grid-connected MMC with three cells per arm, see Figure 15. The MMC test rig is equipped with the following controllers: 1) active and reactive power controller, which sets P = 2kW and Q = 0, 2) inner current controller, circulating current suppression controller, 3) horizontal/common-mode capacitor voltage sum, 4) level-shifted pulse width modulation with 1 kHz carrier frequency and sorting-based capacitor voltage balancing.

**Figure 15.** Experimental test of a 3-cell HB-MMC.

Figures 16 and 17 display experimental results when control schemes A and B are employed. Figure 16a,b and Figure 17a,b present upper and lower arm capacitor voltage sums and upper and lower arm currents for the control schemes A and B, respectively. Although both methods appear to work well, it is worth noticing that the Scheme-B exhibits slightly lower ripples in the common-mode current. In line with simulations, the FFT shows that the Scheme-B exhibits slightly lower contamination of the common-mode current by the fundamental frequency component compared to that of the control method A. Despite the few SM numbers of the experimental test rig, the presented experimental results point toward the same conclusions as simulation cases discussed earlier.

**Figure 16.** Experimental waveforms of the 3-cell MMC that employs internal Scheme-A: (**a**) upper and lower capacitor voltage sums, (**b**) upper and lower arm currents, and (**c**) common-mode current with the measured fundamental frequency content of 6.6%.

**Figure 17.** Experimental waveforms of the 3-cell MMC that employs internal Scheme-B: (**a**) upper and lower capacitor voltage sums, (**b**) upper and lower arm currents, and (**c**) common-mode current with the measured fundamental frequency content of 5.4%.

#### **7. Conclusions**

This paper has investigated the impacts of MMC internal asymmetries due to SM faults or severe deviations of the SM capacitance or arm inductance values from their nominal values, particularly, on the following aspects: control range, power quality in ac and dc sides, and distribution of voltage stresses across the SM capacitors and semiconductor switches. Moreover, the paper has presented a direct method for fundamental frequency ripple suppression from the dc link, and its performance is compared to a well-performing conventional controller for managing MMC internal dynamics. The investigation has found that the control for actively balancing arm capacitor voltage sums (Scheme-A) is better than the direct fundamental ripple suppression method (Scheme-B) from the ac side viewpoint, but it pollutes dc current with a fundamental frequency ripple. In contrast, the Scheme-B is better than its counterpart A, from the dc side viewpoint; nevertheless, it increases the risk of dc injection into the ac grid and reduction of ac control range.

**Author Contributions:** Conceptualization: G.P.A. and S.W.; Methodology: S.W. and F.S.A.; Software: S.W. and F.S.A.; Validation: G.P.A. and S.W.; Formal Analysis: F.S.A. and S.W.; Investigation: S.W. and G.P.A.; Resources: F.S.A.; Data Curation: S.W.; Writing—Original Draft Preparation: S.W. and F.S.A.; Writing—Review and Editing: G.P.A.; Visualization: S.W. and F.S.A.; Supervision: G.P.A.; Project Administration: G.P.A and F.S.A.; Funding Acquisition: F.S.A. and G.P.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was funded by King Abdulaziz University, Jeddah, Saudi Arabia, and King Abdulah City for Atomic and Renewable Energy, Riyadh, Saudi Arabia, under grant no. (KCR-KFL-14-20). Therefore, the authors gratefully acknowledge their technical and financial support.

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