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This paper introduces the multi-neighboring reference vector discontinuous pulse-width modulation (MNRV DPWM) with offset voltage injection methodology tailored for single-phase diode-clamped four-level PWM inverters. By evenly utilizing multiple adjacent reference vectors with diverse charging/discharging characteristics of dc-link capacitors and employing duty compensators to adjust minor voltage discrepancies under practical conditions, precise tracking of commanded voltages is ensured even under transient conditions, while maintaining balanced voltages across the dc-link capacitors. This approach proposes various continuous/discontinuous modulation types using deliberately designed offset voltages to minimize dc-link voltage variation. Notably, setting the offset voltage to zero interestingly implements conventional virtual-vector PWM. The sum of the product of the duty ratios of the basis vectors and the phase currents was analyzed, and its relationship with the dc-link voltage fluctuation was mathematically rigorously examined. Through various simulations and prototype experiments, the steady-state operational characteristics of the proposed method have been validated, demonstrating lower average switching frequencies and superior total harmonic distortion performance compared to conventional method. The proposed method demonstrates overall superior performance in total harmonic distortion (THD) compared to the conventional carrier-overlapped PWM (COPWM) technique. At a power factor (pf) of 0.9, it provides comparable or slightly better performance, while at pf =0.3 and a modulation index (m) of 0.3, it exhibits significant improvements. Specifically, the proposed method reduces the output voltage THD to 86%-126% and the output current THD to 4.65%-5.45%, outperforming the COPWM, which recorded 121% and 7.35%, respectively. Additionally, proposed method reduces the average switching frequency by approximately 30% and the control operation time by about 22% compared to existing methods. The stability of the dc-link voltage equilibrium under transient conditions was also successfully verified.

In this paper, a three-phase diode-clamped four-level PWM inverter with offset voltage injection is proposed. The proposed inverter is based on multi-neighboring reference vector (discontinuous) pulse-width modulation (MNRV (D)PWM), where multiple adjacent reference vectors with different charging/discharging characteristics of the capacitors are selected depending on the location of the command voltage. These reference vectors are evenly utilized, and the duty ratio of the remaining reference vector as a degree of freedom is adjusted to satisfy the volt·time product. Additionally, the proposed MNRV (D)PWM can generate various (D)PWM forms by applying the offset voltage injection method. By extending the methodology applied to the four-level inverter, a general MNRV (D)PWM method applicable to general high-dimensional multilevel multiphase PWM inverters is also proposed. It is demonstrated that MNRV (D)PWM can be applied not only to dc/ac but also to dc/dc topologies. The ripple component of the dc-link voltage has been confirmed to be related to the three-phase summation of the product of the duty of the E voltage level vector and the phase current. Through various simulations and prototype experiments, the steady-state operating characteristics have been confirmed, demonstrating reduced average switching frequency and superior THD performance compared to conventional methods. Furthermore, proficient management of fluctuations in the dc-link voltage reference and sustained voltage equilibrium are exemplified.

In this paper, we propose a novel full-bridge (FB) diode-clamped four-level symmetric switching converter. Multi-neighboring reference vector discontinuous PWM (MNRV DPWM) for FB topology was implemented using a symmetric half-bridge (HB)-based control approach that incorporates an offset voltage addition method. A diode-clamped four-level symmetrical switching circuit with pulse-amplitude-modulation (PAM) was proposed and its working principle was explained in detail. Voltage gain, duty loss factor, and zero-voltage-switching (ZVS) condition were derived by analyzing the steady-state operating characteristics of the proposed converter. The required input and output capacitance values were also calculated explicitly. The voltages across the series-connected input DC-link capacitors were well balanced and the output voltage was controlled in a simple linear manner by PAM. The proposed method offers the advantage of reducing circulating current when compared to the existing phase-shift method, thanks to the inclusion of an additional power delivery period. The effectiveness of the proposed converter was verified through simulation and prototype experiments. The power conversion efficiency exceeded 91.44% at an output power ( $P_{O})$ of 250W or higher, with a peak efficiency of 95.23% achieved at $P_{O}=750\text{W}$ .

The Neutral Point Clamped (NPC) Multi-Level Inverters (MLI) have been ruling the power electronics industries for the past two decades. The Multi-Carrier Pulse Width Modulation (MCPWM) is common PWM techniques which are widely used in NPC-MLI applications. However, MCPWM is not having a good impact on the balancing of DC-link voltages, Common Mode Voltage (CMV) and limiting the Total Harmonics Distortion (THD). The Selective Harmonic Elimination (SHE) technique is introduced for reducing the THD, however all the switching angles should be maintained less than $\pi $ /2 to keep the eliminated harmonics at constant level which narrows down the modulation index range. Hence, in recent days Space Vector Modulation (SVM) technique is widely used in NPC-MLI, which gives better DC-link voltage balancing, self-neutral point balancing, near-zero CMV reduction, better-quality harmonics profile and switching loss minimization. Hence, it is a preferred solution for the majority of electrical conversion applications such as electric traction, high power industrial drives, renewable power generation, and grid-connected inverters, etc. The paper gives a comprehensive review of the SVM for NPC-MLI. First, this paper deliberates the state of art for two-level SVM and extends it to three-level (3L) SVM. Also compares the 3L SVM performance with other MCPWM techniques. Followed by the various modified MLI SVM techniques in terms of their implementations, DC-link capacitor balancing, and reduction of CMV. Further, the review of MLI SVM is widened to open-end winding Inverters and multiphase MLIs. The final part of this paper discussed the future trends and research directions on MLI SVM techniques and its applications.

Cascaded H-Bridge (CHB) converters have been widely employed in various applications, providing the advantages of modularity and multilevel voltage output. However, technically the CHB converters require isolated DC-link buses, which need to be regulated individually. In this context, a control scheme for the CHB converters features the voltage balancing algorithm. The voltage balancing control is typically performed by PI controller, where its control capability is limited by linear modulation region. In this work, the modulation based voltage balancing algorithm is proposed, which extends the balancing control capability and improves the dynamic performance. The proposed method is theoretically analyzed and validated through the simulation and experimental results.

This paper proposed a grid connected solar Photovoltaic (PV) Systems with a new voltage balancing converter suitable for Neutral-Point-Clamped (NPC) Multilevel Inverter (MLI). The switched-capacitors used in the proposed converter is able to balance the DC link capacitor voltage effectively by using proper switching states. The proposed balancing converter can be extended to any higher levels and it can boost the DC input voltage to a higher voltage levels without using any magnetic components. This feature allows the converter to operate with the boosting capability of the input voltage to the desired output voltage while ensuring the self-balancing. In this paper the proposed converter is used for a grid connected solar PV system with NPC multilevel inverter, which is controlled using vector control scheme. The proposed grid connected solar PV system with associated controllers and maximum power point tracking (MPPT) is implemented in Matlab/SimPowerSystem and experimentally validated using dSPACE system and designed converters. The simulation and experimental results show that the proposed topology can effectively balance the DC link voltage, extract maximum power from PV module and inject power to the grid under varying solar irradiances with very good steady state and dynamic performances.

In this paper a DC-link voltage balancing strategy for multilevel Cascaded H-Bridge (CHB) converter is proposed. Presented solution bases on optimal choice of active vector durations in Space-Vector Pulse Width Modulation (SV-PWM). It makes it possible to DC-link voltages control and to properly generate the output voltage vector in the case of DC-link voltage unbalance. Results of simulation and experimental researches on proposed control strategy are presented in the paper