**1. Introduction**

Recently, there has been a remarkable rise in the use of grid-supplied power. This can be attributed to an increased number of users and the expansion of high-power sectors. Traditional power production has led to a significant surge in global emissions, thereby causing detrimental effects on the environment. Significant progress has been made in integrating renewable energy sources such as solar and wind into the grid. Welcome to the world of photovoltaic (PV) systems, which have become the top choice for harnessing energy thanks to their incredible potential. In fact, worldwide, grid-connected solar PV capacity has soared to over 635 GW, satisfying approximately 2% of the global energy consumption [1].

Solar energy is a rapidly growing field, and one crucial aspect that has gained significant importance is power electronics. Researchers are actively engaged in the pursuit of developing highly efficient power electronic converters to enhance the overall performance of solar energy systems. In applications requiring medium and high power, MLIs are increasingly being employed. This is because MLIs provide several inherent advantages over two-level inverters, as mentioned in Table 1.

**Citation:** Nyamathulla, S.; Chittathuru, D. A Review of Multilevel Inverter Topologies for Grid-Connected Sustainable Solar Photovoltaic Systems. *Sustainability* **2023**, *15*, 13376. https://doi.org/ 10.3390/su151813376

Academic Editor: Mohamed A. Mohamed

Received: 26 July 2023 Revised: 22 August 2023 Accepted: 29 August 2023 Published: 6 September 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).


**Table 1.** Comparison between two-level and multilevel inverters (MLIs).

It has been predicted that renewable energy would contribute 29% of worldwide power output in 2020, up from 27% in 2019, that renewable energy generation would increase by more than 8% to 8300 TWh by 2021, and that solar PV and wind would account for two-thirds of the growth in renewable energy. The increase in renewable energy alone in China in 2021 was about half of what was predicted, followed by the United States, the European Union, and India, as shown in Figure 1a. China has continued to be the largest PV market, although there is growth in the United States due to continuous federal and state legislative support. In India, new solar PV capacity additions have recovered quickly from COVID-19-related delays in 2021. According to the IEA's 2021 Renewable Energy Market Update, by 2020, renewable energy was the only type of energy whose consumption increased despite the pandemic. To increase worldwide renewable power in 2021 and 2022, the renewable energy sector has looked at new additions. In addition, 270 GW went online in 2021 and 280 GW went online in 2022, continuing the remarkable level of renewable energy additions that are anticipated. This expansion has exceeded the yearly capacity rise record set in 2017–2019 by more than 50%, indicating that renewables have been responsible for 90% of the increase in global capacity in 2021 and 2022, as shown in Figure 1b.

Flexible alternating current transmission systems (FACTSs), customized power devices (CPDs), variable-speed drives (VSDs), active front-end converters (AFCs), and renewable energy sources for power generation are just a few of the many uses for MLIs [2–5]. MLIs can be classified as classical if they use the most common topologies, such as the diodeclamped multilevel inverter (DCMLI), flying capacitor multilevel inverter (FCMLI), and cascaded H-bridge (CHB) multilevel inverter, mentioned in Figure 1c. There has been a lot of interest in these topologies, but their practical implementation is highly impacted by the application, the system that is designed, and costs. The fundamental disadvantage of a DCMLI is its asymmetrical loss distribution. This, in turn, results in an irregular distribution of junction temperature, which, in turn, results in constraints on the inverter's power, current, and switching frequency at maximum junction temperature [6,7].

**Figure 1.** (**a**) Worldwide renewable power generation in 2020–2021; (**b**) net renewable capacity additions, by renewable energy market update 2021—IEA; (**c**) an outlook on the development of various MLI topologies.

Traditional MLIs, on the other hand, need a larger number of components to achieve the same number of output levels, have issues with capacitor voltage balance, and cannot increase their voltages [8]. Different reduced device count MLIs have been presented to address traditional MLIs' high component count [9–14]; however, these MLIs have lacked a voltage-boosting function. To eliminate capacitor voltage imbalance in typical MLIs, complicated control algorithms or multi-output boost converters have been implemented. These methods increase an inverter's weight, complexity, and expense. SC-MLIs minimize the drawbacks of traditional and reduced device count MLIs [15–22].

Researchers have continued to investigate and to develop additional topologies by implementing little or major modifications to conventional MLIs. As a result, MLIs with a lower device count have been developed, and this subset of MLIs has been dubbed RSC-MLIs, which have recently been the subject of many reviews. Newly designed RSC-MLIs for integrating renewable energy sources and driving applications are addressed in [23–25]. The incorporation of switched-capacitor (SC)-based circuits is one of the most widely used methods for designing better MLIs, and rapid progress has been made in the area of SC-MLI development since 2010. Pure SC-based switching circuits use a series–parallel switching conversion technique to take the available fixed DC-link voltage and produce a multilevel voltage using a reduced number of capacitors, power switches, and/or diodes. SC-MLIs are a valuable and interesting solution for many new applications due to the voltage step-up feature they offer, which includes self-voltage balancing for the involved capacitors, which is the result of a single-stage switching operation that eliminates the need for inductors and transformers [26–35]. The following is a list of the primary factors and propensities for SC-MLIs:


A wide range of new issues, design requirements, and real-world constraints of conventional MLIs have been highlighted in recent articles [36–45]. Different circuit designs are used to build MLIs using the SC concept. These include single, multiple, mid-pointclamped, and common-grounded (CG)-based DC-source structures [46–55]. Hybrid topological designs that integrate well-known integrated methods such as flying capacitor (FC) and switched boost (SB) technologies into the SC framework significantly raise their performance [56–82].

The significant contributions of this review include:


This review paper includes the following: Section 2 describes grid-connected solar PV systems and MLI background including MLI applications; different types of energy sources integrated with MLI-based systems, motivational factors for generating an RSC-MLI, and assessment parameters are discussed in great length. Section 3 presents the detailed MLI categorization and description of the structure and working principle of features for each reported RSC-MLI topology, and a variety of SC-MLIs with single or multiple DC-source, mid-point-clamped, and CGSC configurations are examined and then compared on several criteria, including the total number of power semiconductor switches, DC sources, passive elements, total standing voltage, efficiencies relative to the number of levels, and the structural motivations behind each concept. Section 4 provides a reliability assessment study to estimate the lifespan of an MLI. Section 5 provides the present challenges and recommendations. Finally, Section 6 concludes the article with some final thoughts.
