Improved PWM Approach for Cascaded Five-Level NPC H-Bridge Configurations in Multilevel Inverter

Multilevel inverter (MLI) design for high voltage and high power applications faces challenges such as unequal power losses and junction temperatures. To address the issues of decreased efficiency and reliability, this paper introduces two innovative pulse width modulation (PWM) approaches, such as Enhanced Level Shifted PWM (ELS-PWM) and Enhanced Phase Shifted PWM (EPS-PWM). They are designed for five-level neutral point clamped (NPC) H-Bridge Configurations (FLNPCHC), which are further cascaded to form MLIs. These methods leverage modified modulating waves and restructured triangular waves to improve power quality, balance power distribution, and ensure uniform power allocation across the inverter legs. The result is identical switch utilization rates, reduced switching times, lower junction temperatures, and enhanced overall efficiency of the FLNPCHC-MLI. These approaches are validated through MATLAB simulations and tested using a 1 kVA laboratory prototype. The outcomes demonstrate significant improvements in total harmonic distortion (reduced to approximately 2.3%) and efficiency (increased to approximately 97.8%), offering a promising solution for high-voltage applications.

development of advanced power electronic systems.Multilevel inverters (MLI) have gained prominence due to their highquality output waveforms, reduced harmonic distortion, and improved efficiency [1], [2].By employing cascaded H-Bridge configurations and connecting multiple H-Bridge units in series, MLIs can tackle high voltage and power conversion applications while overcoming the challenges of power losses, junction temperature disparities, and efficiency limitations associated with traditional pulse width modulation (PWM) techniques [3], [4], [5], [6], [7], [8].Implementing a five-level neutral point clamped (NPC) H-Bridge configuration [9] can address these issues by offering improved power quality, balanced power distribution, and improved overall efficiency, making it a more robust solution for high voltage direct current applications [10].When applied to five-level NPC inverters, traditional modulation techniques such as level-shifted pulse width modulation (LS-PWM) [11] and phase-shifted pulse width modulation (PS-PWM) [8] have the limitations discussed above, leading to reduced efficiency and reliability.In light of these challenges, there is a growing need to develop enhanced modulation techniques that can address the issues associated with traditional PWM techniques while maintaining high power quality and efficiency in cascaded five-level NPC H-Bridge Configurations (FLNPCHC) based MLI [12].
In the field of LS-PWM for five-level inverters, carrier-based continuous PWM (CPWM) and discontinuous PWM (DPWM) strategies have been introduced for T-Type three-level (3 L) converters, reducing common-mode voltage and switching loss while controlling neutral-point voltage [13].These strategies have been successfully adapted to control a five-level NPC H-bridge inverter, optimizing voltage level management [14], [15].Analytical DPWM for 3 L inverters with unbalanced DC-link voltages, offering insights into voltage balancing for FLNPCHC, has been presented [16].A carrier-based DPWM for a five-level flying capacitor rectifier with unbalanced DC-link voltages, managing voltage fluctuations in FLNPCHC, is also discussed [17].The application of LS-PWM in FLNPCHC is crucial for voltage level management, reduction of switching losses, and minimization of harmonic distortion.This research emphasizes the integration of complex topologies and control algorithms into LS-PWM strategies for FLNPCHC, focusing on voltage balance and neutral-point voltage fluctuations to enhance power quality and performance.
The PS-PWM research for five-level inverters includes a hybrid DPWM strategy for 3 L inverters under two-phase load conditions [18].This strategy is adaptable to FLNPCHC, with the aim of enhancing performance and efficiency.Another study explores a carrier-based DPWM for single and parallel 3 L T-Type converters, focusing on neutral point potential balancing, which is extendable to FLNPCHC [19].A novel low-frequency virtual space vector PWM (SVPWM) for a single-phase NPC 3 L inverter, which improves output average voltage and bus voltage utilization [20].Adapting PS-PWM techniques to FLNPCHC can significantly improve voltage regulation, performance, and efficiency.Minimizing switching losses and reducing harmonics are key benefits of these adaptations.The development of advanced control algorithms and voltage balancing techniques within PS-PWM, combined with existing methods, promises optimal control strategies for FLNPCHC, advancing power electronics systems.
Significant advancements in cascaded H-Bridge (CHB) multilevel converters are evident.Space vector PWM (SVPWM) strategies have been developed for CHB inverters to improve the voltage balancing of the DC-link [21].A model-based dead-time compensation strategy for CHB converters was introduced, enhancing output waveform quality, addressing phase current polarity issues [22].Fault-tolerant strategies using auxiliary H-bridge cells have been developed for CHB converters to effectively restore faults [23].Online diagnosis and ridethrough operations in CHB converter-based STATCOMs, enabling quick resolution of single IGBT open-circuit faults, have been a focus [24].State-of-Charge (SoC) balancing methods have been developed for battery energy storage systems using CHB converters [25].Phase-shifted PWM-based fault-tolerant approaches have been proposed for battery storage systems, ensuring seamless operation during faults [26].Demonstrations of Maximum Power per Ampere modulation in CHB converters, which minimize the rms current value in dc sources, thus improving efficiency, have been made [27].A grid supportbased module power balancing strategy for PV systems using CHB inverters effectively manages power imbalances [28].Low computational burden MPC strategies for single-phase CHB converters have been introduced, reducing execution time and grid current distortion [29].Achievements in fast dynamics and fixed switching frequency in CHB converters using dual-layer modulated MPC schemes have been reported [30], [31].The focus has been placed on online inductance identification and adaptive control methods for CHB converters, ensuring stability across varying grid impedances, especially in on-board traction applications [32].The combination of non-linear PSMC and linear MPC in Model Predictive Sliding Control for CHB converters, enhancing dynamic current reference tracking, has been developed.Addressing cell failure in CHB inverters has been achieved with a CMV injection method, minimizing load and impact on inverter functionality [33].Addressing unbalanced conditions with multi-objective model-free predictive control for CHB inverters has improved accuracy in various applications [34].Introducing a hybrid symmetric multilevel CHB converter topology, which significantly improves output levels, has been a notable advancement [35].The integration of flexible power control in PV-Battery Hybrid CHB converters, managing overmodulation risks in PV modules, has been a focus [36].Improvements in capacitor voltage balance control during unbalanced grid conditions in CHB-based STAT-COMs have been made using optimal decoupled discontinuous modulation strategies [37], [38].Enhancements in smart grid interoperability with Power Line Communication methods in CHB converters have been demonstrated, facilitating power and data transmission [39].A hierarchical multifactorial error correction method has been based on FCS-MPC prediction error analysis in CHB inverters, which improves predictive control accuracy [40].Rapid and robust fault identification has been enabled in CHB converters using multiple voltage-based fault diagnosis approaches without additional sensors, improving diagnostic robustness [41].
In [42], [43], an optimal reliability-oriented DPWM strategy is proposed for single-phase five-level T-type inverters in PV systems, demonstrating the potential of novel PWM techniques to improve FLNPCHC performance and efficiency in solar applications.Combining different modulation techniques, such as hybrid pulse frequency modulation (PFM) and the PS-PWM control scheme for the single leg-integrated discontinuous conduction mode (DCM) boost inverter in [44], can achieve optimal performance in FLNPCHC.Custom-tailored PWM strategies, such as the carrier-based DPWM scheme with optimal PWM sequences for a five-level flying capacitor rectifier in [45], showcase their potential to manage voltage fluctuations and improve power quality in FLNPCHC.Research such as [46] focuses on active vector modification for the suppression of circulating currents in parallel inverters, offering information on advanced PWM techniques that address specific issues faced by FLNPCHC, including circulating currents and voltage balancing [47].By synthesizing existing knowledge and building upon proposed PWM techniques, new control strategies can be developed that meet the unique requirements of FLNPCHC, leading to improved performance, efficiency, and reliability in power electronics systems.This paper seeks to address a critical issue in power electronics by developing and proposing two enhanced PWM techniques for cascaded FLNPCHC-MLI, surpassing the limitations of traditional modulation methods [1].These advancements contribute significantly to the progress of power electronics applications in renewable energy systems and various other high-performance voltage regulation and control applications with the objective of achieving the following.
r Create and implement two unique PWM techniques spe- cific to the needs of five-level NPC cascaded H-bridge MLI, considering their operational principles, component characteristics, and potential constraints, to overcome existing method limitations and optimize the inverter topology.
r Extensively assess the proposed PWM techniques through simulations and real-world experiments to validate their practical usability, robustness in various conditions, and effectiveness in resolving the identified challenge.
r Perform an in-depth performance comparison between the proposed PWM methods and conventional modulation techniques, focusing on power quality, efficiency, and reliability, to underscore the benefits of the novel methods in elevating the overall performance of five-level NPC cascaded H-bridge inverters.The significant contributions of this research based on the addressing of the identified issues are as follows.
r An extended version of [1] is presented in this paper, where a more detailed analysis of the proposed EPS-PWM and ELS-PWM techniques is provided.
r The scope of the previous work is extended by applying these techniques to cascaded three-phase FLNPCHC-MLI.r In the paper, a switching analysis is presented, in which improvements in switch utilization, reductions in switching times, lower junction temperatures, and enhanced overall efficiency are reported for the FLNPCHC-MLI system.The organization of this paper is structured to ensure a complete understanding of the research topic.In Section I, an introduction and literature review are provided, while Section II offers a detailed presentation of the proposed PWM techniques, their implementation, and benefits.Finally, Section III details the comparison of simulation and experimental results, summarizing the key findings and potential future research directions in Section IV.

A. FLNPCHC-MLI System Description
The three-phase cascaded five-level MLI topology based on NPC H-Bridge Configurations (FLNPCHC) is illustrated in Fig. 1.Each phase of MLI consists of n cells and each cell consists of FLNPCHC.Each FLNPCHC cell has 2 legs (leg-1 and leg-2) and each leg has four switches with two antiparallel diodes.Insulated gate bipolar transistor (IGBT) Here, i ∈ {1, 2, . . .n} denotes i th FLNPCHC cell and x ∈ {a, b, c} represents the phases of the three-phase MLI.Each FLNPCHC cell is supplied by an isolated DC power source with voltage v i dc and is further cascaded in series for each phase, as shown in Fig. 1.DC-link voltages across each cell are considered constant, i.e., v i dc ≈ V dc ∀i and the differential voltage across each leg of i th FLNPCHC cell is v i x12 .

B. Proposed Modulation Strategies for FLNPCHC-MLI
In the domain of high-power applications, modulation strategies such as LS-PWM and PS-PWM stand out as exceptional choices.Owing to their superior harmonic attributes and ease of execution, these PWM techniques have found widespread utilization within the domain of power electronics.Illustrations of the modulation methods for LS-PWM and PS-PWM applied to phase-a are presented in Figs. 2 and 3. Here, each FLNPCHC cell employs dual modulating waves (v + ma and v − ma ) of equal magnitude but opposite phase, and a pair of triangular waveforms, depicted in Fig. 2. Modulating waves (v + ma and v − ma ) can be explicitly defined as Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.where V m and ω denote the per-unit (pu) amplitude and angular frequencies of the phase-a voltage; n is the number of cascaded cells in the phase.In the LS-PWM method, two triangular waves (v tr1 and v tr2 ) are level shifted with respect to each other, shown in Fig. ) of leg-1 for the i th FLNPCHC cell in phase-a can be represented as Similarly, the gating signals {S i x21 , S i x22 , S i x23 , S i x24 } and pole voltage (v i a2o ) of leg-2 for i th FLNPCHC cell in phase-a can be evaluated as The differential voltage across each leg can be deduced as ), as shown in Fig. 2. In the PS-PWM method, the triangular waves (v tr3 and v tr4 ) are phase shifted by angle θ [48].Triangular waves v tr3 and v tr4 have amplitude varies in the range of [−1, 1] V to accommodate the pu amplitude of modulating waves v + ma and v − ma .With l c as the number of levels in the output voltage, the phase shift angle (θ) can be defined by the factor 360 • /(l c − 1).Like LS-PWM, PS-PWM generates four gating signals by comparing the modulating signal v + ma (v − ma ) with v tr3 and v tr4 .Using (3) and ( 5), the pole voltages v i a1o and v i a2o can be generated, respectively, which further synthesize the output voltage v i a12 as shown in Fig. 3.Under this LS-PWM, the operating frequency of each switch is consistent, but the power distribution between the inverter legs is not balanced due to an unequal utilization rate of the switches, as shown in Fig. 2. With PS-PWM, the power allocation among the inverter legs shows a relative balance; however, the elevated operating frequency of each switch amplifies switching losses, thereby diminishing the inverter's efficiency.Within the LS-PWM and PS-PWM techniques, the amplitude and frequency parameters of the triangular waves are identical.Consequently, every power switch operates at an identical switching frequency, as illustrated in Fig. 3.
To address these challenges associated with the LS-PWM and PS-PWM methods, this paper introduces two refined PWM approaches, i.e., ELS-PWM and EPS-PWM.Achievement of power balance is feasible with ELS-PWM and EPS-PWM through uniform utilization of switches.The minimization of switching losses can be accomplished by employing reduced switching intervals, which increases the efficiency of the inverter.These proposed PWM techniques are presented in Fig. 4, with Fig. 4(a) representing ELS-PWM, and Fig. 4(b) delineating EPS-PWM.These proposed PWM strategies include both restructured triangular waves and improved modulating waves, characterized by the modulation index m a .Specialized modulating wave configurations are implemented to realize both power equilibrium and a contraction in switching durations, as displayed in Fig. 4 where v x represents the overall DC offset and equal to the amplitude V m .Modulating waves (v + * ma and v − * ma ) fed to both Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

TABLE I MODIFIED MODULATING AND TRIANGULAR WAVES EXPRESSIONS
the ELS-PWM and EPS-PWM methods are considered similar and can be represented as ( 7)- (8).
The alteration in modulating waves for phase-a in the inverter's design leads to a changed switching state and even distribution of power across the switches in both inverter legs.The proposed ELS-PWM approach incorporates two level-shifted triangular waves (v * tr1 and v * tr2 ), while the EPS-PWM method utilizes two triangular waves (v * tr3 and v * tr4 ) shifted by a specific phase angle Θ.The modified expressions for these modulating waves and triangular waves are listed in Table I  ).As the underlying working principles of both proposed methods are identical, this paper discusses EPS-PWM as an illustrative example.Fig. 4(b) reveals how the modulating wave is divided into four parts, corresponding to positive and negative half-cycles of the output voltage.The portions 1 and 2 relate to the positive half-cycle, while the portions 3 and 4 correspond to the negative half.During the portion 1 , S i a11 , S i a12 are activated and leg-2 is in action; in the portion 2 , leg-1 acts by activating S i a23 , S i a24 .Similarly, in the portion 3 , S i a13 , S i a14 are ON and leg-2 acts; in the portion 4 , the leg-1 acts and S i a21 , S i a22 are activated.This sequential switching ensures a uniform loss distribution between both legs.Fig. 5 illustrates the functionality of the five-level inverter in correspondence with the four working modes, and Table II details all switching states for these modes, along with the status of the input capacitors C a1 and C a2 .The proposed ELS-PWM and EPS-PWM schemes use the Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.same modified reference waveforms to reduce power losses, and the one modified waveform is used for cells in the same phase.Therefore, the 3-phase cascaded H bridge based on NPCs requires three reference waveforms v + mx ∈ {v + ma , v + mb , v + mc } and its 180 Taking v * x ∈ {v * a , v * b , v * c } as reference voltage waveforms for phases a, b, and c, respectively, and the output voltage of each phase can be expressed as Unlike the cells in the same phase which use one reference waveform, the triangular waves of the cells in the same phase have a phase shift from one cell to another.The phase change between cells can be expressed as Θ = 360 • /(2n).This phase change is decided by the number of cascaded cells in the phase (n) or the output voltage levels (l = nl c + 1).Fig. 3 shows the reference waveforms and the phase-shifted triangle waveforms of a single cell corresponding to one phase of the traditional PS-PWM technique.It can be observed from Fig. 3 that all triangular waves have the same amplitude and frequency.Therefore, all the power devices in the cascaded cells operate at the same switching frequency.

A. Prototype Description
A laboratory prototype of a 3-phase FLNPCHC inverter of 1 kVA has been successfully developed to demonstrate the effectiveness of novel PWM techniques in grid applications.The inverter operates with an input voltage and a power rating of 100 V and 1000 W, respectively.It features a complex configuration of Insulated Gate Bipolar Transistors (IGBTs) with a total count of 4 × 2 × 5 × 3, and uses the IGBT G4PH50UD based on the International Rectifier for efficient performance.The inverter uses HCPL 3120 gate driver ICs, which are arranged in a 4 × 2 × 5 × 3 array, similar to IGBTs.The total number of 4 × 5 × 3 STTH10LCD06 power diodes is also used to make the NPC structure.TMS320F280335 microcontroller is used as an essential component to execute the control algorithms.The device operates at a sampling frequency f sa = 100 kHz and a switching frequency f sw = 20 kHz, optimizing performance and efficiency.Auxiliary capacitors are utilized with values C a1 and C a2 both at 500 μF to stabilize the DC-link voltage.The parasitic capacitances C p and C n are 150 nF, considered in the design to minimize unwanted effects.The grid to which the inverter is connected has a voltage of 115 V and features an inductance L g = 2 mH.The architecture comprises IGBTs, diodes, and DC-link capacitors to produce a multi-level output voltage waveform.This waveform exhibits a reduced harmonic content, thus improving the quality of the power supplied to the grid.A TMS320F28335 digital signal processor serves as the heart of the operation, handling the PWM algorithms and switching of the IGBTs.The prototype is also equipped with safety features such as overcurrent, overvoltage, and short circuit protection, ensuring robust and reliable operation.Monitoring capabilities are also built into the system to continuously evaluate inverter performance and identify potential problems, making it a compact, efficient, and reliable power conversion solution.

B. Simulation Study of Proposed PWM Techniques
MATLAB simulations are used to verify the traditional LS-PWM, PS-PWM, and the two advanced PWM methods, considering parameters like an input DC source of 100 V, filter inductance of 2 mH, and a switching frequency of 3 kHz.
1) Five-Level Inverter Simulation Waveforms and THD Studies: Initially, the output voltage and current THDs of the LS-PWM and PS-PWM methods are described.Fig. 6 illustrates that without a filter, the THDs for both techniques are 19.4% and 33.93%, with most harmonics focused at 6 and 12 kHz.Subsequently, Fig. 7 depicts the output voltage and current waveforms for the improved LS-PWM and PS-PWM (Fig. 7(a)-(d) represent the enhanced versions).The phase-a output voltage 2) Comparison of Switching Times Between Conventional and Proposed PWM Methods: Fig. 8 shows the switching times for one full cycle for both conventional and proposed PWM methods.The figure illustrates the unequal switching times for conventional LS-PWM in Fig. 8(a), and the continuous highfrequency operation of PS-PWM in Fig. 8(b), which, although it may provide good output quality, leads to increased loss and lower efficiency.On the other hand, the two proposed methods in Fig. 8(c) and (d) maintain equal switching durations for both inverter legs, resulting in equal power distribution and uniform losses between the legs, thus improving efficiency.The subsequent power loss computation verifies this equal distribution and efficiency enhancement due to reduced switching times in the proposed PWM methods.
Power losses with the ELS-PWM: Power losses with the EPS-PWM: Here, P Sai , P Sbi , P Daj , and P Dbj (where i = 1, 2, . . . 4 and j = 1, 2) represent power device losses.Under the proposed methods, the power losses of the two inverter legs are nearly symmetric, indicating a uniform power sharing.However, in conventional LS-PWM and PS-PWM, the power losses are greater, with non-uniform distribution among the switches as specified below.
Power losses with the conventional LS-PWM: Power losses with the conventional PS-PWM: 3) Performance for Modular FLNPCHC Based MLI: In the analyzed three-phase system, a total of 15 FLNPCHC units are employed, with each phase incorporating 5 FLNPCHC units as demonstrated in Fig. 1.This particular configuration serves the purpose of testing the system's performance under varying conditions.For this experiment, a three-phase load is connected to the system.The phase impedance of this load is set at 10 + j3.5 Ω, a complex value that accounts for both resistance and reactance in the circuit.Each FLNPCHC unit operates with a DC-link voltage of 48 V , which is aligned with a standard battery voltage level.This standardization simplifies both design and integration of the units into existing systems.Furthermore, each FLNPCHC unit incorporates two capacitors with a capacitance value of 470 μF each.These capacitors play a pivotal role in filtering and smoothing the input DClink voltage.Both proposed PWM strategies are evaluated in Fig. 9: EPS-PWM in Fig. 9(a) and ELS-PWM in Fig. 9(b).The ELS-PWM strategy showcases a better THD for line voltage, measuring around 8.25%.In contrast, the EPS-PWM strategy exhibits a THD of 9.67%.However, when we focus on the phase voltage THD, ELS-PWM registers nearly 8.98%, while EPS-PWM demonstrates an astonishingly low 11.25%.For both strategies, the phase current THD remains relatively constant at 1.89 ± 0.1%, indicating comparable performance in minimizing current ripples.Key Observations: 1) ELS-PWM outperforms EPS-PWM in terms of reducing line voltage THD; 2) Conversely, EPS-PWM, despite its higher line voltage THD, performs remarkably well in minimizing phase voltage THD; 3) EPS-PWM delivers fewer voltage levels, which may have an impact on its THD performance; 4) The specific choice of 470 μF for capacitance and 48 V for the DC-link voltage are likely tailored to achieve optimal harmonic performance; and 5) Performance metrics can vary depending upon the load impedance, suggesting the need for further evaluation under diverse load conditions.Fig. 10 shows a comparison between DC-link voltage variations under different conditions for a system employing the proposed ELS-PWM.Fig. 10(a      Fig. 12 depicts % current THD versus output power in watts for four different PWM schemes.The four PWM schemes are represented as LS-PWM, ELS-PWM, PS-PWM, and EPS-PWM, each plotted with a distinct color and symbol.The graph shows that as the output power increases from 0 to 1000 W, the current THD decreases for all schemes.It is evident from the trend lines that ELS-PWM and EPS-PWM have a lower THD across the output power range compared to those of LS-PWM and PS-PWM.The purpose of the graph is to analyze and compare the current THD performance of conventional and proposed PWM techniques at various power levels.

C. Experimental Validation
The assessment of the PWM techniques, namely conventional LS-PWM, PS-PWM, proposed ELS-PWM, and EPS-PWM, has been conducted through experimental verification.The experimental parameters are consistent with those used in the simulation, and the results pertaining to the five-level output voltage of the inverter, the load current, and output voltage THD for the conventional and proposed methods are detailed in this section.
In the initial stage, Fig. 13(a)-(d) depict the waveforms corresponding to traditional LS-PWM and proposed ELS-PWM methods.It is noticeable that the THD in output voltages is found to be 20.7% and 16.3% for these techniques, with the majority of harmonics collected at frequencies of 6 kHz, respectively.Moving to PS-PWM methodologies, the voltage and current output waveforms of the traditional PS-PWM method are illustrated in Fig. 14(a) and (b).Within this context, the THD for the inverter's output voltage amounts to 34.7%, with higher-order harmonics primarily centered at a 12 kHz frequency.Similarly, Fig. 14(b) and (d) show the voltage and current output waveforms of the proposed EPS-PWM technique, recording a THD in the output voltage of 15% and a harmonic concentration at 12 kHz.Furthermore, the transient response testing of the five-level inverter using the proposed PWM methods (as presented in Fig. 15) substantiates the robustness of the transient response, while a comparative study of efficiencies in Fig. 16 clearly shows the efficiency gains achieved through reduced switching times and ON durations.Fig. 17 illustrates the % voltage THD analysis across various power levels for different Pulse Width Modulation (PWM) schemes.It compares the conventional LS-PWM and PS-PWM with the proposed ELS-PWM and EPS-PWM schemes.As the output power increases from 0 to 1000 W, the ELS-PWM and EPS-PWM exhibit lower voltage THD compared to the conventional PWM schemes, indicating cleaner and more efficient power output.The EPS-PWM scheme shows the most significant reduction in voltage THD, maintaining a nearly constant low THD level even as output power increases.This suggests that the EPS-PWM scheme is particularly effective in maintaining power quality under varying power outputs.In contrast, the conventional PWM voltage THD remains consistently higher, underscoring the improved performance of the proposed PWM techniques in reducing harmonic distortion.
An in-depth evaluation of the prolonged lifetime efficacy of the proposed ELS-PWM and EPS-PWM methods was carried out through the temperature measurements of the IGBTs.The internal temperature analysis of the inverter's IGBTs reveals that conventional LS-PWM and PS-PWM methods register the highest mean temperatures.However, advanced LS-PWM and PS-PWM modulation strategies enable an even temperature distribution among the inverter switches, resulting from equivalent utilization durations of the IGBTs, as depicted in Fig. 18.Although the conventional LS-PWM method exhibits elevated IGBT temperatures in leg-1 due to prolonged operation (as shown in Fig. 18), the traditional PS-PWM method maintains uniform temperatures between IGBTs but with increased power losses.According to theoretical insights and simulation results, the application of inventive enhanced LS-PWM and PS-PWM strategies decreases inverter power losses through balanced usage and minimized IGBT switching times, as demonstrated in Fig. 18.Consequently, enhanced modulation approaches lead to a reduction in average temperatures of 3.5 • C and 4.5 • C, respectively, compared to conventional LS-PWM and PS-PWM techniques.

D. Applications and Future Scope
LS-PWM and PS-PWM methods are crucial in power electronic systems for a wide range of real-world applications.They are used in motor drives to control speed and torque, ensuring the desired performance and efficiency.In renewable energy systems, these methods help achieve high efficiency and improved power quality by converting DC power from solar panels or wind turbines into AC power.They also play a role in electric vehicle charging stations, affecting the overall efficiency and power quality of the charging process.In addition, LS-PWM and PS-PWM methods are utilized in uninterruptible power supplies to maintain stable power during outages or fluctuations.They contribute to mitigating harmonics and improving power quality in distribution networks through active power filters.In high-voltage direct current transmission systems, they enable efficient and flexible power transfer between AC grids.Finally, variable-frequency drives use these methods to control the speed of industrial equipment, enabling energy-efficient operation and precise control.In general, LS-PWM and PS-PWM methods are essential for efficient, reliable, and high-performance power electronic systems.
Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.Future research on ELS-PWM and EPS-PWM methods can explore several directions to enhance their performance and applicability across various domains.An area of focus is developing advanced modulation techniques to minimize switching losses, reduce harmonics, and optimize the performance of power electronic systems.Researchers could also investigate adaptive control algorithms that adjust modulation parameters based on operating conditions, ensuring optimal system efficiency and reliability.Another potential direction is to design novel multilevel converter topologies to reduce component count and complexity, leading to more cost-effective solutions.The integration of advanced materials and emerging semiconductor devices could be explored to further increase the efficiency and power density of converters employing ELS-PWM and EPS-PWM techniques.Research on fault-tolerant modulation strategies could improve the robustness and reliability of the system, enabling power electronic systems to withstand faults and maintain stable operation.Developing tools for the accurate modeling and simulation of LS-PWM and PS-PWM-based systems will facilitate the analysis and optimization of system performance.Lastly, applying these methods to emerging fields, such as smart grid systems, energy storage solutions, and

IV. CONCLUSION
The implementation of enhanced LS-PWM (ELS-PWM) and enhanced PS-PWM (EPS-PWM) techniques in cascaded FLNPCHC-MLI has successfully addressed challenges faced in high voltage (HV) applications.These novel PWM methods, utilizing modified modulating waves and rearranged triangular waves, have significantly improved power quality, power balance between inverter legs, and uniform power allocation.The results of the study include the attainment of identical switch utilization rates and reduced switching times, resulting in lower junction temperatures and increased efficiency for FLNPCHC-MLI.The effectiveness of these proposed modulation techniques was demonstrated through MATLAB simulations and further validated experimentally using a 1 kVA laboratory prototype of FLNPCHC-MLI.By employing ELS-PWM and EPS-PWM techniques, the current total harmonic distortion was significantly improved to approximately 2.3%, and the inverter achieved an impressive efficiency of around 97.8%.This proves that the proposed solution is a promising approach for HV applications.
The potential for further exploration and optimization of PWM techniques in multi-level inverters for HV applications presents opportunities for improved performance and efficiency.Future research could focus on refining algorithms, expanding applicability to a wider range of power electronics, and integrating the enhanced PWM methods into advanced control strategies, such as model predictive control, to increase both the performance and reliability of HV systems.
(a) and (b).These alterations in modulating waves are made by employing existing modulating waves (namely v + ma and v − ma ) and offset voltages (specifically v x1 and v x2 ).The offset voltages v x1 and v x2 shown in Fig. 4(a) and (b) can be represented as

Fig. 7 .
Fig. 7. Enhanced LS-PWM and PS-PWM methods output voltage and current waveforms along with the THD.
) displays the voltage response, v ca1 and v ca2 , to a load change where the load doubles, indicating how the voltage variation increases with load variation.Fig. 10(b) illustrates the DC-link voltage response to changes in Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

Fig. 8 .
Fig. 8. Switching times comparison of the conventional and proposed LS-PWM, PS-PWM methods.

Fig. 12 .
Fig.12.Current THD analysis at various power levels for conventional and proposed PWM schemes.

Fig. 17 .Fig. 18 .
Fig.17.Voltage THD analysis at various power levels for conventional and proposed PWM schemes.