*6.2. Fault Mode*

The last experiment is conducted to evaluate the performance of SHMS and HMSCZS in the fault mode where the PV module fails. Initially, the CHB inverter and PV modules are operating in normal mode as indicated in the first experiment. Then the second PV module is removed. As can be seen from Figure 19a,b, both the HMSCZS and SHMS can keep the CHB inverter stable and operating at a unity power factor without interruption. As is presented in the Figure 19a, by using the HMSCZS, the fault H-bridge unit can operate only in "0" or "+PWM" mode when VG is positive and "0" or " −PWM" mode when VG is negative. In such condition, the fault H-bridge unit is always in discharge status and unable to realize the equalization of the charge-discharge. Consequently, as presented in Figure 19c, the DC-side voltages of all H-bridge units diverge from the references in the steady state, which may result in low generated power of the CHB inverter. However, as presented in the Figure 19b, by utilizing the SHMS, the fault H-bridge unit can operate in "+1", "+PWM", "−1" or " −PWM" mode whether the grid voltage (VG) is positive or negative. Therefore, as depicted in the Figure 19d, the SHMS is capable of realizing the equalization of the charge-discharge of the fault H-bridge unit and maintaining the DC-side voltage balance of the other H-bridge units. In order to compare the output power of the two methods under fault mode, the total output power of the CHB inverter is recorded by the upper computer respectively. As can be seen from Figure 20, with HMSCZS, the total output power of the CHB inverter ranges from 919.2–986.4 W and the average is about 948.8 W. When SHMS is utilized, the total output power of the CHB inverter ranges from 972.3–988.6 W and the average is about 978.2 W. Compared with the HMSCZS, the efficiency of the CHB inverter can be improved about 2.89% by adopting the SHMS. Furthermore, the efficiency of the CHB inverter with both methods is presented in Figure 21. As can be seen from Figure 21, the SHMS is capable of improving the efficiency of the CHB inverter compared with the HMSCZS. Therefore, under the fault mode, the SHMS is able to make the DC-side voltages reach the references, thus maintaining a higher energy yield.

**Figure 19.** Experimental results under fault mode: grid voltage (VG), grid current (IS), and the second H-bridge output voltage (VH2) with: (**a**) HMSCZS and (**b**) SHMS; DC-side voltages of all H-bridge units with: (**c**) HMSCZS and (**d**) SHMS.

**Figure 20.** Experimental results of the total output power of the CHB inverter under fault mode with: (**a**) HMSCZS and (**b**) SHMS.

**Figure 21.** Experimental results of the efficiency of the CHB inverter under the fault mode with different methods.
