Modular Battery Systems for Electric Vehicles based on Multilevel Inverter Topologies - Opportunities and Challenges

Modular battery systems based on multilevel inverter (MLI) topologies can possibly overcome some shortcomings of two-level inverters when used for vehicle propulsion. The results presented in this thesis aim to point out the advantages and disadvantages, as well as the technical challenges, of modular vehicle battery systems based on MLIs in comparison to a conventional, two-level IGBT inverter drivetrain. The considered key aspects for this comparative investigation are the drive cycle efficiency, the inverter cost, the fault tolerance capability of the drivetrain and the conducted electromagnetic emissions. Extensive experiments have been performed to support the results and conclusions.In this work, it is shown that the simulated drive cycle efficiency of different low-voltage-MOSFET-based, cascaded seven-level inverter types is improved in comparison to a similarly rated, two-level IGBT inverter drivetrain. For example, the simulated WLTP drive cycle efficiency of a cascaded double-H-bridge (CDHB) inverter drivetrain in comparison to a two-level IGBT inverter, when used in a small passenger car, is increased from 94.24% to 95.04%, considering the inverter and the ohmic battery losses. In contrast, the obtained efficiency of a similar rated seven-level cascaded H-bridge (CHB) drivetrain is almost equal to that of the two-level inverter drivetrain, but with the help of a hybrid modulation technique, utilizing fundamental selective harmonic elimination at lower speeds, it could be improved to 94.85%. In addition, the CDHB and CHB inverters’ cost, in comparison to the two-level inverter, is reduced from 342€ to 202€ and 121€, respectively. Furthermore, based on a simple three-level inverter with a dual battery pack, it is shown that MLIs inherently allow for a fault tolerant operation. It is explained how the drivetrain of a neutral point clamped (NPC) inverter can be operated under a fault condition, so that the vehicle can drive with a limited maximum power to the next service station, referred to as limp home mode. Especially, the detection and localization of open circuit faults has been investigated and verified through simulations and experiments.Moreover, it is explained how to measure the conducted emissions of an NPC inverter with a dual battery pack according to the governing standard, CISPR 25, because the additional neutral point connection forms a peculiar three-wire DC source. To separate the measured noise spectra into CM, line-DM and phase-DMquantities, two hardware separators based on HF transformers are developed and utilized. It is shown that the CM noise is dominant. Furthermore, the CM noise is reduced by 3dB to 6dB when operating the inverter with three-level instead of two-level modulation

Modular Multilevel Converters: Recent Achievements and Challenges

The modular multilevel converter (MMC) is currently one of the power converter topologies which has attracted more research and development worldwide. Its features, such as high quality of voltages and currents, high modularity and high voltage rating, have made the MMC a very good option for several applications including high-voltage dc (HVdc) transmission, static compensators (STATCOMs), and motor drives. However, its unique features such as the large number of submodules, floating capacitor voltages, and circulating currents require a dedicated control system able to manage the terminal variables, as well as the internal variables with high dynamical performance. In this paper, a review of the research and development achieved during the last years on MMCs is shown, focusing on the challenges and proposed solutions for this power converter still faces in terms of modeling, control, reliability, power topologies, and new applications

Modular multilevel converter with embedded batteries as a motor controller.

This thesis details the design of the control system and hardware for a prototype of the new inverter topology the modular multilevel converter with embedded batteries for electric vehicle applications. Within this topology, the battery cells incorporated within the battery pack are directly integrated into the motor controller/ power converter by replacing the individual module capacitors with batteries. Since the batteries are directly connected to the module switching circuit, the batteries can be individually balanced using the same technique as an active battery management system, without the need for external energy-shunting hardware. A control algorithm for balancing the embedded batteries without affecting the motor control scheme with significantly unbalanced battery cells is presented and discussed. A multilevel space vector modulation scheme using the abc-reference frame for the selection of space vectors is developed. Initial testing of both the simulation model and prototype was carried out using a static RL load to test the PWM scheme and battery SOC balancing scheme. A Field-oriented control scheme was then designed and implemented for controlling a salient pole surface-mounted PMSM. The performance of the converter as a motor controller was assessed in terms of ability to balance the SOC of the embedded module batteries and total harmonic distortion over the course of the operating torque-speed range. Simulation of the control system on simulated hardware has been carried out in MATLAB; these simulation results verify the theoretical analysis. Then further verified and analysed using the developed laboratory-scale embedded battery MMC prototype

Modular multilevel converter with thyristor DC-link switch for full-torque variable-speed drives

The modular multilevel converter (MMC) is a promising topology for medium-voltage drive applications due to its high-quality output waveforms, low device switching frequency and voltage rating. However, the large cell capacitor voltage ripple is a severe challenge faced by MMC at low motor speeds. Recently, a hybrid MMC (HMMC) topology is proven to be a competitive solution because of its lower cell capacitor voltage ripple and no common-mode voltage (CMV) problem compared with other methods. However, the dc-link switch with IGBT limits HMMC to be applied for high-voltage applications. This paper uses the thyristor instead of IGBT as the dc-link switch. To ensure the thyristor can be softly turned on and safely turned off, a new control scheme is proposed. When using this proposed scheme, HMMC can also tolerate the failure of thyristor's turning-off without shutting down the system, improving the reliability effectively. The cell capacitor voltage ripple analysis is presented considering the effects of the thyristor switching process. In addition, a decoupled energy balancing control is utilized to suppress the fluctuation of dc current. Experimental results obtained from a 380 V/7.5 kW downscaled prototype validate the effectiveness of starting up a motor from the standby mode to rated speed with full-torque

Design and Control of Power Converters 2020

In this book, nine papers focusing on different fields of power electronics are gathered, all of which are in line with the present trends in research and industry. Given the generality of the Special Issue, the covered topics range from electrothermal models and losses models in semiconductors and magnetics to converters used in high-power applications. In this last case, the papers address specific problems such as the distortion due to zero-current detection or fault investigation using the fast Fourier transform, all being focused on analyzing the topologies of high-power high-density applications, such as the dual active bridge or the H-bridge multilevel inverter. All the papers provide enough insight in the analyzed issues to be used as the starting point of any research. Experimental or simulation results are presented to validate and help with the understanding of the proposed ideas. To summarize, this book will help the reader to solve specific problems in industrial equipment or to increase their knowledge in specific fields

Hybrid modular multilevel converter (MMC) applications under over-modulation

The Modular Multilevel Converter (MMC) has become a prominent converter topology due to its several advantages: high modularity, high scalability, low harmonic distortion, and high efficiency. In particular, due to its modular structure, it is possible to increase its voltage rating by stacking extra cells. The MMC has been successfully applied to high voltage direct current (HVDC) transmission systems and drive applications. Most MMC-based projects use the half-bridge sub-module (HBSM) as a building block to reduce semiconductor losses. Despite the MMC advantages, there are still open challenges regarding its control and operation. For instance, in the HBSM-based MMC, the minimum dc-port voltage cannot be lower than two times the AC output voltage. The over-modulation operation of the MMC, i.e. operation with reduced dc-link voltage, has shown some benefits. For instance, the MMC can operate with a reduced dc-port voltage in HVDC applications to avoid flashovers under extreme atmospheric conditions. In addition, for back-to-back MMC-based drive applications, it is possible to reduce the energy arm oscillations by controlling the dc-port voltage. The operation with a reduced dc-port voltage can be accomplished by using full-bridge sub-modules (FBSM) instead of the HBSMs. However, this solution has higher semiconductor losses. A possible alternative is to use a hybrid MMC. In this case, each arm is composed of HBSMs and FBSMs. However, in the hybrid MMC, the capacitor voltages of the HBSMs and FBSMs may drift apart if the converter operates with reduced dc-port voltage because the arm current becomes unipolar, i.e. the arm currents do not have zero-crossing angles. This thesis presents two control strategies to ensure the cell capacitor voltage balance of the hybrid MMC operating in over-modulation. A decoupled control is developed and shown to regulate the inner and outer converter variables independently. An optimisation problem is proposed to ensure the local balance between HBSMs and FBSMs. In addition, a closed-loop controller is considered to correct any mismatch between the control and actual system parameters. The proposed controller is validated through simulation and experimental results. In particular, a 5 kW hybrid MMC of 18 cells has been built to validate the proposed strategies. Finally, this thesis presents the control systems and experimental evaluation of a hybrid back-to-back (BTB) modular multilevel converter (MMC) for drive applications. The grid-side converter is a hybrid MMC composed of half-bridge sub-modules (HBSMs) and full-bridge sub-modules (FBSMs), while the drive-side converter is an HBSM-based MMC. The proposed topology can operate with a variable dc-port voltage. By controlling the dc-port voltage as a function of the machine operational point, it is possible to reduce the high sub-module capacitor voltage oscillations in the machine-side MMC during low machine speed. An experimental rig composed of 36 cells was built and tested to validate the proposed control

Partitioning And Interface Requirements Between System And Application Control For Power Electronic Converter Systems

Applications of power electronics in power systems are growing very rapidly and changing the power system infrastructure in terms of operation speed and control. Even though applications of power electronics are wide spread, the cost and reliability of power electronics are the issues that could hinder their penetration in the utility and industrial systems. The demand for efficient and reliable converter controllers gave rise to modularized converter and controller design. The objective of this dissertation is to determine the appropriate partitioning and interface requirements between the system and application control layers for power electronic converters so that the minimum set of system layer to application layer control interfaces is compatible across all power electronic controllers. Previous work, using the Open System Architecture (OSA) concept has shown that there is a set of common functions shared by different converters at the low-level control layers. It has also shown that, depending on the application, there is a variation in control functions in application/middle control layers. This functional variation makes it difficult to define system functionality of power converters at upper control layers and further complicates the investigation into the partition requirements of system to application control layer. However, by analyzing the current or voltage affected by a converter in terms of orthogonal components, where each component or group of components is associated with a power-converter application, and the amount of required DC bus energy storage, a common functionality can be observed at the application control layer. Therefore, by establishing common functionality in terms of affected current or voltage components, a flexibility of operation can be realized at upper control layers that will be a major contribution towards standardizing the open system architecture. In order to a construct functional flexible power converter control architecture, the interface requirements to the system control layer and the partitioning between the system control layer and application control layer need to be explored. This will provide flexibility of system design methodology by reducing the number of constraints and enabling system designers to explore possible system architectures much more effectively

A dual modular multilevel converter with shared capacitor sub-module for MV open-end stator winding machine drives

This paper proposes a new dual modular multilevel converter (MMC) topology as a medium-voltage drive for adjustablespeed applications incorporating open-end stator winding machines. A novel concept of sharing one capacitor between each two adjacent-arm sub-modules (SMs) of MMC phaselegs, operating with out-of-phase modulation, is realized through new SM arrangement. This concept allows the MMC to utilize half the number of the SM capacitors, compared to a traditional MMC topology. Additionally, the sizing requirement of the shared capacitor is diminished, which significantly reduces the volume of the drive system and its stored energy. The switching scheme of the shared capacitor between two oppositely modulated SMs eliminates the problem of capacitor wide voltage fluctuations, independent of the operating frequency. Further, the proposed MMC can efficiently operate at near zero frequency, therefore a machine speed-range from zero speed to the rated speed is possible under rated torque operating condition. The proposed MMC topology is elucidated in detail, and its effective performance is verified through simulation