## 7 Experimental Investigation
### Discussion of Main Results and Comparison with Other Control Strategies  
The comparison between the simulation results and the experimental results by using the proposed online tuning mechanism are analyzed. Both results are compared in terms of the peak-to-peak capacitor voltage ripples, output current THD and rise time. For the peak-to-peak capacitor voltage ripples, the simulation result shows a value of 2 V while the experimental result indicates a value of 3 V. For the output current THD, the simulation result indicates 3.09% whereas the experimental result records 4.46%. The experimental result also reveals a higher rise time after a V\({}_{\text{in}}\) change from 60 V to 200 V, namely, 6 ms, as compared with that of the simulation result, which is only 0.1 ms. Overall, the experimental results are observed to have higher readings in the peak-to-peak capacitor voltage ripples, output current THD and rise time as opposed to those of the simulation results. Nonetheless, this difference is expected since the simulation condition normally assumes an ideal case while the experimental condition takes into account various factors in a real environment such as external disturbances, instrument limitations and hardware constraints.  
The experiment without the online tuning scheme shows that the capacitor voltage ripples are quite significant during the steady-state condition. Figure 15a,b show that the peak-to-peak capacitor voltage ripples reach up to 15 V. With the proposed online tuning method, the peak-to peak capacitor voltage ripples are able to be reduced to 3 V, as shown in Figure 13a,b. Figure 16 also displays that without the online tuning mechanism, V\({}_{\text{AB}}\) also takes the form of a nine-level waveform. I\({}_{\text{A}}\) and V\({}_{\text{AN}}\) also indicate satisfactory performances. The results show that the dynamic performance with the online tuning mechanism have been improved as compared with the results without tuning. The effectiveness of  
Figure 16: Experimental results of inverter’s waveforms (output current I\({}_{\text{A}}\), phase voltage V\({}_{\text{AN}}\), line-to-line voltage V\({}_{\text{AB}}\) and input voltage V\({}_{\text{in}}\)) without online tuning for a step response with V\({}_{\text{in}}\) changes between 60 V and 200 V. (**a**) Step-up response, (**b**) step-down response, (**c**) output current harmonics.  
the proposed PI controller with the proposed online tuning method has been validated through the improved performance achieved. At the same time, the inverter output shows satisfactory results, and the capacitor voltages are able to be balanced at the reference values with less ripples.  
As stated in Section 1, various control approaches have been proposed for the purpose of capacitor voltage balancing. These include those that apply a fuzzy controller, model predictive controller and proportional-resonant controller. Despite the fact that these control approaches can also offer desirable solutions, there is still a concern with regard to the real-time implementation issue. The fuzzy controller and model predictive controller may suffer from high computational cost. As for the proportional-resonant controller, the need for infinite gain in order to perform well would lead to a difficulty for practical implementation. On the other hand, the proposed controller is based on the PI configuration and it is well-known that owing to its simplicity, the PI controller has been widely used in industrial applications. Although there is an online tuning module in the proposed controller's structure, the fact that the tuning mechanism is simple will not result in high computation. Hence, the proposed control strategy that employs the proposed controller is indeed attractive and practical for possible real-time implementation in industry.