## 7 Challenges and Future Perspectives
(3) By implementing novel voltage balancing techniques or external circuits, the size of auxiliary capacitors and voltage ripples in single-_DC_-source topologies can be reduced. (4) Designing resonant converters based on single-_DC_-source _MLIs_. (5) To decrease the complexity of the controllers, new switching techniques with integrated voltage balancing algorithms are being designed. The selection of the appropriate multilevel inverter (_MLI_) topology has always been a challenge because it needs fewer switches and isolated _DC_ sources to generate a greater number of voltage levels. In comparison to multiple-_DC_-source _MLI_ topologies, single-_DC_-source _MLC_ topologies are now considered more suitable for many power system applications, such as renewable energy (_RE_) conversion systems and electrified transportation. Furthermore, the use of a single-_DC_-source _MLI_ to increase the power rating and minimize the switching frequency while maintaining acceptable power quality is an important necessity and a persistent challenge for the industry [182]. * In combining multilevel converters with bidirectional converters, the selection of a proper _MLI_ topology and balancing the state-of-charges (_SOCs_) of batteries are the two key challenges in _MLI_-based battery storage systems (_BSSs_). _MLIs_ are not only used in large-scale renewable energy-integrated grid applications, but also electric vehicle (_EV_) charging, vehicle-to-grid (_V2G_), and _EV_ operation. A challenging aspect of this field is the selection of an efficient and cost-effective _MLI_ topology. The increasing use of _MLIs_ in _BSSs_ over the last ten years has advanced battery technologies, topologies, and control techniques. Voltage and _SOC_ imbalances between battery packs could be a challenge in _MLC_-based BSSs due to electrochemical variations in battery packs and the use of second-life batteries [183]. The study by [184] presented an up-to-date study of the existing state-of-the-art _MLC_-based _BSSs_, taking into account the most recent contributions in the field of battery technology. Because of their long lifespan and high power density, Li-ion batteries are increasingly being used in _MLI_-based _BSSs_. However, research on metal-air battery technology indicates that it is a viable candidate for _EV_ applications due to its high energy density and low cost. _MLIs_ are already known to be superior to standard two- or three-level converter schemes due to their low harmonic distortion, small scale, and less reliance on magnetic circuits. This accounts for the increasing interest in _MLIs_ over the last decade [184]. In summary, promising research directions for future work can include, but are not limited to: (1) The creation of new types of _MLIs_ with fewer components. (2) Improving the _PWM_ system and/or _SOC_ balancing control to reduce the negative effects of voltage and _SOC_ imbalances between battery packs. (3) The incorporation of renewable energy sources, especially wind, into _BSSs_ through _MLIs_ to mitigate the negative effects of _RESs'_ intermittent nature. * A combination of both converters as previously mentioned in Section 6 can yield better performance, but the challenges remain. The study by [185] focused on the state of the art, challenges, and requirements of multilevel converters in industrial applications. To reduce energy waste and improve power quality, new highly efficient power electronic technologies and control strategies are required. There is enormous potential for energy efficiency improvement in electric motor-driven systems. Medium-voltage (_MV_) drives are widely used in several industries, such as the oil and gas industry, manufacturing plants, and process industries. The topologies and control of line- and motor-side converters, as well as power semiconductor switching devices, are all challenges in the design of powered _MV_ drives. Three topologies have been successfully introduced as standard products for MV industrial drives by [185]: these include 3 Level-_NPC_, 4 Level-_FLC_, and 5 Level-_CHB_.