**1. Introduction**

Continuous development and improvements of Photovoltaic (PV) system designs along with related technologies, such as Wide Bandgap (WBG) GaN/SiC devices, Digital Signal Processor (DSP)- and Field-Programmable Gate Array (FPGA)-based control units have gradually decreased their costs. This allows new solutions featuring high e fficiency and easy implementation which make them commercially attractive. At the same time, power rates and voltage operation ranges determine the availability of certain PV applications, especially in small-scale installations. In addition to efficiency and power density, the reliability of PV inverters is the key factor influencing the feasibility of single-phase industrial implementations [1,2], where Full-Bridge (FB) Voltage-Source Inverters (VSIs) are mostly used. Many DC–AC solutions for connecting PV modules to a single-phase grid are discussed in Reference [3]. The relative costs assessed based on the calculated ratings, component surveys at di fferent vendors, and linear regression analysis were also taken into account in the evaluation.

The Z-Source Inverter (ZSI) [4] is an alternative to VSIs and Current-Source Inverters (CSIs) due to its ability to provide buck–boost operation within the single stage and its improved reliability based on its natural immunity against short-circuit. Its benefits have made it a promising solution for PV

systems and have urged investigations in this area, which has resulted in many DC–DC and DC–AC topologies for single-phase and three-phase applications [5–15].

The Quasi-Z-Source Inverter (QZSI) was derived from the ZSI and has become a desirable topology for PV applications [5] due to its inheritance of all the advantages of ZSI enhanced by lower component ratings and continuous input current. The application of multilevel inverters has advantages in higher power designs, where the high voltage stress on the inverter's switches can be avoided [16–19]. The combination of the QZSI with the Three-Level (3L) Neutral-Point-Clamped (NPC) inverter has created a new promising topology, described in detail in Reference [6]. It features certain advantages such as low voltage stress on the power switches, single-stage buck–boost operation, continuous input current, short-circuit immunity, and low total harmonic distortion of the output voltage and current.

A detailed comparative study of basic and derived impedance-source networks for buck–boost inverter applications is provided in Reference [7], mostly for three-phase applications. The investigation of loss distribution was addressed recently in References [8,9] for QZSI-based topologies along with methods for their reduction and e fficiency improvement.

Many publications devoted to the ZSI- and QZSI-derived solutions for PV, wind, and Microgrids applications have appeared recently [10–14]. They address certain issues, such as current harmonics reduction, voltage gain improvement, leakage current reduction, etc. The authors of Reference [11] emphasize the use of the coupled-inductor and SiC devices to optimize power density. A good comparison of impedance-source networks suitable for DC and AC applications by means of the passive components' number and size, semiconductor devices stress, and range of the input voltage variation is provided in Reference [15]. The increased voltage stress across semiconductors was reported as the main drawback of ZSI/QZSI. High-voltage gain solutions with additional magnetics may mitigate this.

An extreme high e fficiency of 99.4% was reported for a three-phase 50 kW full-SiC PV string inverter in Reference [20]. Another full-SiC solution for a 25 kW three-phase PV string inverter demonstrated 97.7% peak e fficiency [21]. These are examples of extra high e fficiency, which, however, can be achieved much easier in high-power systems. The latter includes a detailed step-by-step explanation and design guidelines for all the components of the system.

Some low-power low-voltage designs are presented in References [22–26]. An example of an e fficient converter based on the zeta inverter topology using 300 V Si + 1200 V SiC Metal–Oxide–Semiconductor Field-E ffect Transistors (MOSFETs) is provided in Reference [22], with efficiency up to 95%; however, the nominal power was 220 W and the maximal was 440 W. A CSI-based single-phase solution for leakage current reduction is shown in Reference [23], where Insulated-Gate Bipolar Transistors (IGBTs) were used.

Several 350–400 W designs based on a quasi-switched-boost inverter with an e fficiency of 91.3–94% are reported in References [24,25]. A good analysis of power losses, e fficiency, and temperature is provided in Reference [26] for a CSI-based solution with SiC MOSFETs; additionally, power losses for all-SiC and hybrid approaches were analyzed, but the experimental results are not shown in the paper.

A valuable and interesting experimental comparison presented in Reference [27] is devoted to three topologies of a three-phase Two-Level (2L) inverter: a QZSI, a VSI with a boost converter, and a VSI with an interleaved boost converter. A detailed description of the methodology for comparison could be a very good reference for such an analysis. However, since the investigated input voltage range was 400–600 V, the operation of the QZSI was not assessed completely by means of the boost mode and the California Energy Commission (CEC) e fficiency was not reported.

The most relevant solutions reported for single-phase and three-phase PV applications and supported by experimental verification are listed in Table 1. It should be mentioned that di fferent solutions have been used to reach certain installed goals and satisfy some specific requirements.



In some cases, different semiconductor technologies were tested. Thus, in Reference [1], different WBG and Si devices were investigated and evaluated (650 V GaN switches by Transphorm, RFMD and GaN Systems, 650 V SiC switches by RoHM, 900 V SiC by Wolfspeed and F5 series IGBT switches by Infineon). The final choice was to use 900 V SiC devices due to the voltage margin of 200% over the maximum DC bus voltage. The power levels of different PV applications could vary significantly. Particularly, the topologies discussed in Table 1, have been verified by experimental prototypes in the range from 220 W to 50 kW.

A 1800 W single-stage distributed PV plant was taken as a case study in Reference [28]. The experimental results of the developed 1 kW two-string prototype with different PV strings at various PV conditions are shown in Reference [29]. The industrial PV-string inverter SMA Sunny Boy 1600TL with a maximum input power of 1700 W was investigated in Reference [30].

The main requirements for off-grid and grid-connected PV systems include efficiency, reliability, and high-power density. These features could be available by providing low-input current ripple as well as low DC-link voltage ripple. This results in high-output current quality with the minimal possible requirements to the output filter. The importance of the power decoupling between the

modules and the grid is discussed in Reference [3]. Some theoretical and simulation results for the 2L QZSI and the 3L NPC QZSI are reported in References [31–33].

To improve the reliability of the system and achieve higher power density by the reduction of redundant passive components, the approach of interleaving is often used in VSI. It enables significant reduction of the current ripple in QZS-stage inductors and the voltage ripple at the DC-link [27,31,33–35]. A topology of the Interleaved QZSI (IQZSI) under the Simple Boost Control (SBC) was proposed in Reference [34] for PV applications. Its certain benefits, including the reduced output THD and QZS-stage passive elements, potentially lead to higher power density of the system. To improve utilization of the DC-link and achieve higher gain, the Maximum Boost Control (MBC) [36] with appropriate modification was required. It smoothes out variation in the Shoot-Through (ST) duty cycle. The operation of MBC in IQZSI revealed the importance and proved the necessity of power decoupling in such PV systems [35]. Additionally, some control approaches for 3l NPC QZSI are proposed in [37–39].

Although many solutions were claimed as suitable for PV applications, in most of the listed studies, the case study tasks for PV applications are not positioned in detail. Moreover, there are numerous works that present SiC-based solutions, including those built on QZS network, however, the discussions on the feasibility and experimental investigations of the alternative approaches for 2L and multi-level approaches based on Si, SiC, and Si+SiC designs are absent. The peak and especially the CEC efficiency [40] of the proposed PV solutions are often not analyzed in the papers. The calculation for the passive components is usually significantly simplified and in practical experience, some capacitors or inductors can be smaller or with an increased ripple [15]. Thus, our study aimed to discuss the most urgen<sup>t</sup> peculiarities in the implementation of the 2L full-SiC and the 3L Si–SiC inverters based on the QZS network and to share our experiences to advance the application of these solutions in PV systems.

The paper is organized as follows. Section 2 outlines the main specifications of the case study system, provides the system parameters, and explains both of the converters with the control approach. Section 3 presents the design guidelines for element selection. Section 4 describes the experimental prototypes built based on the 2L QZSI and the 3L NPC QZSI topologies, explains the structure of the experimental setup along with the equipment used, and demonstrates the obtained results, including operation waveforms, measured efficiency, and temperature dependencies. Section 5 presents a comparative evaluation of both topologies followed by the conclusions provided in Section 6.
