**4. Experimental Study**

#### *4.1. Experimental Setup and Tested Prototypes Description*

The general approach to the experimental verification is shown by the structure of the experimental setup in Figure 4, which was intended to facilitate a comparison of the proposed solutions by means of an efficiency study and verification of theoretical statements. The experimental setup includes the following equipment: programmable DC power supply (PV array simulator) Chroma 62150H-1000S [43]; high-performance power analyzer YOKOGAVA WT1800 [44]; oscilloscope Tektronix MSO 4034B [45]; 2L QZSI or 3l NPC QZSI as a PV inverter incorporating output LCL filter (Figure 5); resistive load for up to 3 kW output power; both converters were controlled by FPGA Cyclone IV EP4CE22E22C8 [46]; the temperatures of the inverter switches (Tsw) and heatsinks (Tswh), QZS-stage diodes (TqzsD) and heatsinks (TqzsDh), clamping diodes (TclD) and heatsinks (TclDh) in 3l NPC QZSI, were measured using an infrared thermal camera Fluke Ti10 [47]; passive element values were measured using HM8118 LCR Bridge/Meter [48].

**Figure 4.** The structure of the experimental verification setup.

**Figure 5.** Experimental prototypes of the 2L QZSI (**a**) and the 3L NPC QZSI (**b**).

#### *4.2. Operation Waveforms and Characteristics of the Prototypes*

The experimental waveforms are shown below in Figures 6 and 7 for both the 2L QZSI and the 3L NPC QZSI. Figure 6 shows the experimental results in the open loop mode of the 2L QZSI. Figure 6a,b correspond to the nominal power point 1800 W with the input voltage 360 V. The operation in the boost mode (power point of 1 kW) is presented in Figure 6c,d. To boost input voltage of 200 V up to 360 V at DC-link, the duty cycle DS = 0.225 was applied. The impact of the ST states on the ripples can be observed.

**Figure 6.** Operation waveforms of 2L QZSI at a nominal power level of 1800 W (**<sup>a</sup>**,**b**) and in boost voltage mode at a reduced power level of 1000 W (**<sup>c</sup>**,**d**).

**Figure 7.** Operation waveforms of 3L NPC QZSI at a nominal power level of 1800 W (**<sup>a</sup>**,**b**) and in boost voltage mode at a reduced power level of 1000 W (**<sup>c</sup>**,**d**).

The appropriate waveforms for 3l NPC QZSI are depicted in Figure 7.

The diagrams presented are similar. The LF ripple of the input current was about 30% for both solutions that slightly exceeded the analytically calculated level of 25%. However, the input currents for both converters were in the CCM and the HF ripples for the 2L QZSI and the 3L NPC QZSI corresponded to the calculated value of 8%. The voltage ripples at C1 are 1.5–1.6% and ripples at C2 were 1.2–1.4%, which completely satisfied or nearly the calculated values of 2% and 1%, correspondingly.

In addition, Figure 8 shows the experimental diagrams of the PV-inverter operation in the closed-loop grid-connected mode for the nominal and reduced input power. It can be observed that the input current ripples had an LF ripple that corresponded to the double-frequency ripple. In the second case (Figure 8b), the current ripple had an HF component. It should be noted that a closed-loop system stabilizes the behavior of the converter. The observed current ripples completely corresponded to the theoretical expectation.

**Figure 8.** Grid-connected operation of 3L NPC QZSI at a nominal power level of 1800 W (**a**) and in boost voltage mode at a reduced power level of 900 W (**b**).
