B. Commutation

The inverter should always be in the state of safety switch during the commutation process, so the problems of shoot-through and high voltage stress during state cutover should be avoided. To distinguish positive current flow from negative current flow, the circuit can be separated into two parts to comprehend the state commutation.


Moreover, as shown in Figure 12, under the conduction of the state machine, the switching frequency of switch S5 to S8 is the same as modulation frequency, while switching frequency of switch S1 to S4 is the same as carrier frequency. Moreover, the voltage stress of power devices can be limited to Vdc/4. To sum up, shoot-through problems and overvoltage issues will not be caused by the commutation of current and state cutover. Accordingly, the modulation scheme which is proposed in this section is appropriate for the ANPC-5L inverter.

**Figure 13.** State cutover and current commutation. (**a**) V8 to V6. (**b**) V8 to V7-1. (**c**) V6 to V5-1. (**d**) V5-1 to V7-1. (**e**) V5-1 to V2-1.

### **5. Proposed Control Method for Soft Start-Up**

The overvoltage issue of the ANPC-5L inverter includes two kinds of problems causing high voltage stress. The first kind is caused by the dead zone of the output voltage switching stage of the half-bridge circuit in series, and the second kind is caused by the soft start process. In the traditional inverter's soft start-up scheme, the dynamic change in voltage may lead to the overvoltage of low-voltage devices.

Start-up is an indispensable process in the control of the photovoltaic inverters, especially among power converters with flying capacitors. There will be very large current stresses on DC-bus capacitors, flying capacitors, and voltage stresses on power switches during the buildup of capacitor voltage if the procedure is not well controlled. Connecting current-limiting resistance in series with voltage sources can limit these stresses in conventional ways. In addition, when bus capacitors are pre-charged but flying capacitors are not fully charged, the voltage stress of several switches will increase by using normal working states, as shown in Figure 14.

Motor windings are used as part of a boost circuit to build up the voltage of flying capacitors with a constant pre-charging current for ANPC-5L converters, and the voltage stress of several switches will double. A pre-charge method applied to flying capacitor multilevel inverters is proposed in [26]; however, it requires plenty of AC contactors and even low-voltage DC power supply, which is not suitable for photovoltaic application.

It is clear that further efforts need to be made to reduce the voltage stress of power devices in the process of soft start-up, as well as to produce flying capacitor pre-charging means with less additional auxiliary circuits. The following part presents an analysis and proposes methods to settle these challenges.

**Figure 14.** High voltage stress without optimal control.

As shown in Figure 15, there are twelve devices in each phase and switches S3, S4, S5 and S8 are used to connect CF with CUP and CDOWN in parallel. Then, these capacitors can be charged by the DC-link voltage source in the meantime. Assuming the DC-link voltage source is constant, by controlling the main switches S3, S4, S5 and S8, the voltage of the flying capacitor CF takes priority over the voltage of CUP and CDOWN, reaching its reference voltage to ensure that the voltage stress of the switches is not higher than VDC/4.

**Figure 15.** Proposed soft start-up process. (**a**) State 1. (**b**) State 2. (**c**) State 3. (**d**) Sequence diagram.

In a summary, the proposed soft start-up method can be divided into three states, as follows:

• State 1: When the upper bus capacitance CUP and the lower bus capacitance CDOWN are zero, and the flying capacitor Cf is not charged and the contactors K1 and K2 are disconnected, the ANPC five-level single-phase converter is in the initial condition with no energy in the capacitors. As shown in Figure 15a, since the main switches S3, S4, S5 and S8 are turned on and the contactor K2 is connected, the DC-link voltage

source charges the upper bus capacitor CUP, the lower bus capacitor CDOWN, and the flying capacitor Cf simultaneously through the current limiting resistance R1. The voltage-divider resistances, R2 and R3, are placed in parallel with each bus capacitor to avoid the influence of the unbalanced characteristics of the upper and lower bus capacitors. In Figure 15d, the voltage of the flying capacitor Cf and bus capacitors increases gradually from t1 to t2;


The proposed method is applicable to the single-phase of the ANPC-5L inverter and pre-charges through the original PV DC voltage source in the photovoltaic application. The power resistance R1 has an impact on the charge current of the whole startup process, but will not influence the final capacitor voltage. While the converter is in H-bridge topology or in three phase topology, it is still sufficiently practical for the tolerance of the voltage stresses. This method uses the DC side power supply to charge the capacitor in the converter, and has the advantages of simple structure, convenient control, fewer additional auxiliary branches, and reliable soft start-up.
