## 4 Discussion
### Experimental Results  
To verify the proposed control strategy, a low-power experimental prototype of a three-phase MMC was developed, as demonstrated in Figure 7. The experimental parameters are listed in Table 1. The MMC is controlled using the \"\(\mu\)-_tech_\" research and development control platform. The experimental results of the PQ, VSG, CVB-PQ, and CVB-VSG control strategies applied to the MMC are shown in Figures 8 and 9, with a change in the active power injected into the grid from 80 W to 120 W at time \(t=60\) s.  
Figure 6: Total harmonic distortion of grid current. Figure 7: Experimental platform of the three-phase MMC. Figure 8: Experimental results of applying VSG and PQ control to the MMC. (**a**) Three-phase voltages of the MMC. (**b**) Three-phase voltages and currents of the grid. (**c**) Voltages of SM capacitors in phase B. (**d**) Phase A circulating current. (**e**) DC current. (**f**) Active and reactive power. Figure 9: Experimental results of applying CVB\(-\)VSG and CVB\(-\)PQ control to the MMC. (**a**) Three-phase voltages of the MMC. (**b**) Three-phase voltages and currents of the grid. (**c**) Voltages of SM capacitors in phase B. (**d**) Phase A circulating current. (**e**) DC current. (**f**) Active and reactive power. Figures 8a and 9a depict the steady-state waveforms of the MMC output voltage. It is evident that the waveform exhibits improved characteristics when utilizing the CVB-PQ and CVB-VSG control strategies (Figure 9a), where five balanced and stable levels are observed. In contrast, the PQ and VSG strategies (Figure 8a) result in unstable and unbalanced voltage levels, leading to the disappearance of internal levels. Figures 8b and 9b illustrate the steady-state currents and voltages of the grid. The voltage waveform appears sinusoidal, while the current ranges from 1.5 A to 1.8 A, corresponding to variations in the power reference. Comparing the CVB-PQ and CVB-VSG control strategies (Figure 9b) with the PQ and VSG strategies (Figure 8b), it is evident that the former exhibit improved current waveforms with fewer fluctuations. The voltages of the SM capacitors in phase B are presented in Figures 8c and 9c for the PQ and VSG control strategies, as well as the CVB-PQ and CVB-VSG control strategies, respectively. The voltages of the SM capacitors are paired and stable (1 and 3, 2 and 4), but they differ from the reference voltage Vdc/N, which is equal to 50 V. When there is a change in injected active power, the voltages of the SM capacitors become unbalanced and deviate towards either \(V_{dc}\) = 100 V or 0 V for the PQ and VSG control (see Figure 8c). Without the implementation of the CVB method, the voltages across the SM capacitors are subject to substantial deviations, potentially compromising the stability and efficiency of the system. The capacitors within the SMs are engineered to handle defined active power thresholds, which are established based on the broader system architecture and the rated capacity of the MMC. Nevertheless, the observed voltage deviations stem not from inadequate capacitor sizing but from the suboptimal selection of which SMs are actively engaged or bypassed during operation. For the same capacitor, more capacitance would indeed reduce voltage deviations by lowering ripple (\(\Delta\)V = I \(\Delta\)t/C), but this comes at the cost of size, expense, slower dynamics, and potential inefficiency. The system may require more time to charge and discharge the capacitors during rapid power demand variations. However, for the CVB-PQ and CVB-VSG control (see Figure 9c), the voltages of the SM capacitors remain stable, balanced, and equal to the reference voltage. The fluctuation range of the SM voltages is between 48 V and 51 V for the CVB-VSG control and between 44 V and 51 V for the CVB-PQ control. The circulating currents of the PQ, VSG, CVB-PQ, and CVB-VSG controls are illustrated in Figures 8d and 9d, respectively.