Traction inverter, as a critical component in electrified transportation, has been the subject of many research projects in terms of topologies, modulation, and control schemes. Recently, some of the well-known electric vehicle manufacturers have utilized higher-voltage batteries to benefit from lower current, higher power density, and faster charging times. With the ongoing trend toward higher DC-link voltage in electric vehicles, some multilevel structures have been investigated as a feasible and efficient option for replacing the two-level inverters. Higher efficiency, higher power density, better waveform quality, and inherent fault-tolerance are the foremost advantages of multilevel inverters which make them an attractive solution for this application. This paper presents an investigation of the advantages and disadvantages of higher DC-link voltage in traction inverters, as well as a review of the recent research on multilevel inverter topologies for electrified transportation applications. A comparison of multilevel inverters with their two-level counterpart is conducted in terms of efficiency, cost, power density, power quality, reliability, and fault tolerance. Additionally, a comprehensive comparison of different topologies of multilevel inverters is conducted based on the most important criteria in transportation electrification. Future trends and possible research areas are also discussed.

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A Review of Multilevel Inverter Topologies in Electric Vehicles: Current Status and Future Trends

In recent years, multilevel inverters (MLIs) have emerged to be the most empowered power transformation technology for numerous operations such as renewable energy resources (RERs), flexible AC transmission systems (FACTS), electric motor drives, etc. MLI has gained popularity in medium- to high-power operations because of numerous merits such as minimum harmonic contents, less dissipation of power from power electronic switches, and less electromagnetic interference (EMI) at the receiving end. The MLI possesses many essential advantages in comparison to a conventional two-level inverter, such as voltage profile enhancement, increased efficiency of the overall system, the capability of high-quality output generation with the reduced switching frequency, decreased total harmonic distortions (THD) without reducing the power of the inverter and use of very low ratings of the device. Although classical MLIs find their use in various vital key areas, newer MLI configurations have an expanding concern to the limited count of power electronic devices, gate drivers, and isolated DC sources. In this review article, an attempt has been made to focus on various aspects of MLIs such as different configurations, modulation techniques, the concept of new reduced switch count MLI topologies, applications regarding interface with renewable energy, motor drives, and FACTS controller. Further, deep insights for future prospective towards hassle-free addition of MLI technology towards more enhanced application for various fields of the power system have also been discussed. This article is believed to be extremely helpful for academics, researchers, and industrialists working in the direction of MLI technology.

Multilevel Inverter: A Survey on Classical and Advanced Topologies, Control Schemes, Applications to Power System and Future Prospects

With the rapid development of modern energy applications such as renewable energy, PV systems, electric vehicles, and smart grids, DC-DC converters have become the key component to meet strict industrial demands. More advanced converters are effective in minimizing switching losses and providing an efficient energy conversion; nonetheless, the main challenge is to provide a single converter that has all the required features to deliver efficient energy for different types of modern energy systems and energy storage system integrations. This paper reviews multilevel, bidirectional, and resonant converters with respect to their constructions, classifications, merits, demerits, combined topologies, applications, and challenges; practical recommendations were also made to deliver clear ideas of the recent challenges and limited capabilities of these three converters to guide society on improving and providing a new, efficient, and economic converter that meets the strict demands of modern energy system integrations. The needs of other industrial applications, as well as the number of used elements for size and weight reduction, were also considered to achieve a power circuit that can effectively address the identified limitations. In brief, integrated bidirectional resonant DC-DC converters and multilevel inverters are expected to be well suited and highly demanded in various applications in the near future. Due to their highlighted merits, more studies are necessary for achieving a perfect level of reducing losses and components.

A Review on State-of-the-Art Power Converters: Bidirectional, Resonant, Multilevel Converters and Their Derivatives

Common-mode voltage (CMV) produces serious reliability and electromagnetic interference (EMI) issues in modern pulse-width modulated (PWM) electric drives. Such issues will become more prominent in the near future, as industry moves towards the introduction of wide-bandgap (WBG) semiconductor technologies operating at higher switching frequencies and also with greater dvdt. In this context, multiphase electric drive technologies can be of great interest, as their additional degrees of freedom can be exploited to reduce CMV. This work aims to study the potential of multiphase electric motor drive systems for CMV mitigation. To do so, a comprehensive review of the most recent scientific literature is conducted, mainly focusing on high impact works published recently. As a result, a clear and up-to-date picture of the most common multiphase technologies, i.e., (m+1)-leg, multiple three-phase, open-end and star-connected multiphase systems is provided along with their CMV reduction potential. Not only the topologies themselves but also modulation techniques are analysed and presented, mainly focusing on star-connected systems. As a conclusion, it can be stated that such multiphase systems are promising candidates to substitute conventional three-phase motor drives as, apart from their well-known advantages (efficiency, power density, power splitting, and fault tolerance), their CMV reduction potential is confirmed. The technical information provided in this work will help researchers and field engineers to design and develop high-performance multiphase electric drives. 

Common-mode voltage mitigation in multiphase electric motor drive systems

This paper presents an efficiency evaluation of three silicon carbide (SiC) inverter topologies: two-level (2L) voltage source inverter (VSI), 3L neutral-point clamped (NPC), and 3L T-type inverter. Their efficiency is evaluated for powertrains rated at 400 V and 800 V, and using SiC MOSFET devices rated at 650 V and 1200 V operating at a switching frequency of 30 kHz. Firstly, the efficiency is evaluated at different operating load currents, on a per-unit scale. Secondly, the efficiency curves are mapped into torque-speed 2D maps of 120 kW interior permanent magnet (IPM) motors. Thirdly, the resulting efficiency maps are employed in an electric vehicle (EV) model, in order to study the performance of the three inverters on standard drive cycles. At the vehicle level, the energy consumption of the vehicle using the studied inverters is analyzed. It is found that 3L SiC-based inverters are most competitive in the 800 V powertrain. When compared to VSI, NPC and T-type offer 0.6% and 1.2% energy consumption savings. In 400 V, only T-type enjoys 0.9% energy savings over VSI.

Efficiency Evaluation of 2L and 3L SiC-Based Traction Inverters for 400V and 800V Electric Vehicle Powertrains

Traction inverter has been the subject of many studies due to its essential role in the proper performance of the drive system. With the recent trend in increasing the input voltage in battery-powered electric vehicles, multilevel inverters have been proposed in the literature as a promising substitute for conventional two-level traction inverters. A critical aspect of utilizing multilevel structures is employing proper control and modulation techniques. The control system structure must be capable of handling a number of key issues, like capacitor voltage balancing and equal power loss sharing, which arise in multilevel topologies. This paper presents a review of the present-day traction drive systems in the industry, control and modulation techniques for multilevel structures in the inverters, as well as the principal challenges that need to be addressed in the control stage of the multilevel traction inverter. A comparison has been made between different methods based on the most important criteria and requirements of the traction drive system. Finally, future trends in this application are presented and some suggestions have been made for the next generation of traction drives.

A Review of Modulation and Control Techniques for Multilevel Inverters in Traction Applications

This paper presents a comprehensive analysis and comparison between the 2-level voltage source inverter, 3-level neutral point clamped, and 3-level active neutral point clamped inverters in traction inverters. In this comparison Silicon Carbide MOSFET, Gallium Nitride MOSFET and Silicon IGBT are employed. An analytical method for calculating the inverter power loss is also presented in this paper. Simulation results are conducted using MATLAB/Simulink and PLECS at different operating conditions. A permanent magnet synchronous motor is used as the load. The analysis and comparison have been conducted at different operating points regarding the speed and torque. The performance of each inverter topology is also investigated at different switching frequencies for these operating conditions. Moreover, two drive cycle analyses are also studied and included in this paper.

Comparative Analysis of 2-Level and 3-Level Voltage Source Inverters in Traction Applications

This paper presents a closed-form formulation to model the switching loss in single-phase dual active bridge converters (DAB). The proposed model is general and covers all modulation techniques with one, two, or three control parameters. Moreover, in this paper, the switching loss is minimized in a DAB converter controlled by the extended phase shift modulation (EPS) utilizing the model. In other words, the proposed loss model is used in a standard nonlinear optimization approach targeting the switching loss. By this optimization, the efficiency of the DAB converter is improved notably. Particularly, in IGBT-based converters with high switching loss, the optimization approach reveals significant improvement. In the designed 10kW DAB converter controlled by EPS modulation with optimal parameters, the switching loss is reduced by 22% compared to the traditional single-phase shift modulation (SPS). The total efficiency is increased by 0.6%, employing the proposed model and the optimization process.

Modeling and Minimization of Switching Loss in Dual Active Bridge Converters

This paper presents a detailed PCB design for a 70kW 3-level Active Neutral Point Clamped traction inverter. The proposed design is focused on stray inductance reduction and thermal performance optimization. The voltage overshoot and maximum allowable stray inductance of the busbar are investigated in detail. Current density and its effect on the PCB temperature are analyzed using 3D finite element analysis.

Amirreza Poorfakhraei

Papers

A Review of Multilevel Inverter Topologies in Electric Vehicles: Current Status and Future Trends

A Review of Modulation and Control Techniques for Multilevel Inverters in Traction Applications

Comparative Analysis of 2-Level and 3-Level Voltage Source Inverters in Traction Applications

Modeling and Minimization of Switching Loss in Dual Active Bridge Converters

A 70kW 3-Level Active Neutral Point Clamped Traction Inverter PCB Design for Stray Inductance and Thermal Performance Optimization