abstract: Modular multilevel converters (MMCs) have become an attractive technology for high power applications. One of the main challenges associated with control and operation of the MMC-based systems is to smoothly precharge submodule (SM) capacitors to the nominal voltage during the startup process. The existing closed-loop methods require additional effort to analyze the small-signal model of MMC and tune control parameters. The existing open-loop methods require auxiliary voltage sources to charge SM capacitors, which add to the system complexity and cost. A generalized precharging strategy is proposed in this thesis. For large-scale MMC-embedded power systems, it is required to investigate dynamic performance, fault characteristics, and stability. Modeling of the MMC is one of the challenges associated with the study of large-scale MMC-based power systems. The existing models of MMC did not consider the various configurations of SMs and different operating conditions. An improved equivalent circuit model is proposed in this thesis. The solid state transformer (SST) has been investigated for the distribution systems to reduce the volume and weight of power transformer. Recently, the MMC is employed into the SST due to its salient features. For design and control of the MMC-based SST, its operational principles are comprehensively analyzed. Based on the analysis, its mathematical model is developed for evaluating steady-state performances. For optimal design of the MMC-based SST, the mathematical model is modified by considering circuit parameters. One of the challenges of the MMC-based SST is the balancing of capacitor voltages. The performances of various voltage balancing algorithms and different modulation methods have not been comprehensively evaluated. In this thesis, the performances of different voltage-balancing algorithms and modulation methods are analyzed and evaluated. Based on the analysis, two improved voltage-balancing algorithms are proposed in this thesis. For design of the MMC-based SST, existing references only focus on optimal design of medium-frequency transformer (MFT). In this thesis, an optimal design procedure is developed for the MMC under medium-frequency operation based on the mathematical model of the MMC-based SST. The design performance of MMC is comprehensively evaluated based on free system parameters. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2020

This thesis aims to bring clarity to the dimensioning aspects and limiting factors of the modular multilevel converter (MMC). Special consideration is given to the dc capacitors in the submodules as they are a driving factor for the size and weight of the converter. It is found that if the capacitor voltages are allowed to increase by 10% the stored energy must be 21 kJ/MW in order to compensate the capacitor voltage ripple. The maximum possible output power can, however, be increased by injecting a second-order harmonic in the circulating current. A great advantage of cascaded converters is the possibility to achieve excellent harmonic performance at low switching frequencies. Therefore, this thesis also considers the relation between switching harmonics, capacitor voltage ripple, and arm quantities. It is shown that despite subharmonics in the capacitor voltages, it is still possible to achieve periodic arm quantities. The balancing of the capacitor voltages is also considered in further detail. It is found that it is possible to balance the capacitor voltages even at fundamental switching frequency although this will lead to a comparably large capacitor voltage ripple. Therefore, in order to limit the peak-to-peak voltage ripple, it is shown that a predictive algorithm can be used in which the resulting switching frequency is approximately 2–3 times the fundamental frequency. This thesis also presents two new submodule concepts. The first submodule simply improves the trade-off between the switching frequency and capacitor voltage balancing. The second submodule includes the possibility to insert negative voltages which allows higher modulation indices compared to half-bridge submodules. A brief comparison of cascaded converters for ac-ac applications is also presented. It is concluded that the MMC appears to be well suited for ac-ac applications where input and output frequencies are close or equal, such as in interconnection of ac grids. In low-frequency applications such as low-speed drives, however, the difficulties with handling the energy variations in the converter arms are much more severe in the MMC compared to the other considered topologies. / <p>QC 20141010</p>

Modular multilevel converter type voltage source converter (MMC-VSC) is a potential candidate for present and future HVdc projects. The d-q decoupled control system is widely used to control MMC-VSC systems. Selection of PI-controller parameters for MMC-VSC systems is a challenging task as control loops are not completely decoupled. Since there is no widely accepted method to tune these control loops, the industry practice is to use the trial and error approach that requires a great amount of time. Therefore, it is required to develop a systematic procedure to tune PI-controllers considering necessary system dynamics and also to propose guidelines for control system design. This thesis introduces a systematic procedure to determine PI-controller parameters for the d-q decoupled control system. A linearized state-space model of an MMC-VSC system is developed to calculate the frequency-domain attributes. The control tuning problem is formulated as an optimization problem which is general and any meta-heuristic method can be used to solve the problem. In this thesis, the simulated annealing is applied to solve the problem. The efficacy of the tuned parameters is tested on the electromagnetic transient model of the test system on the real-time digital simulators (RTDS). In addition, it is shown that the proposed method is suitable to tune PI-controller parameters for MMC-VSC systems connected to strong as well as weak ac networks. Further, this thesis investigates the effects of d-q decoupled controller parameters, phase-locked loop (PLL) gains, and measuring delays on the stability and performance of the MMC-VSC test system. It is shown that the converter controllers have greater influence on the system stability and the impact of PLL gains is negligible unless very high PLL gains are used. In addition, the negative impact of measuring delays in instantaneous currents and voltages is also analysed by performing eigenvalue and sensitivity analysis. Finally, a set of guidelines for control design of MMC-VSC systems is summarized. In general, the proposed control tuning procedure would be useful for the industry to tune PI-controllers of MMC-VSC systems. Furthermore, the proposed methodology is generic and can be adapted to tune of any dynamic device in power systems. / February 2017

Modular Multilevel Converters (MMC) are being widely considered for medium to high voltage DC generation systems. Integrated system design optimization of the generator-MMC system through multi-objective optimization is of interest, because such an approach allows the trade-off between competing objectives (for example, mass and loss) to be explicitly and quantitatively identified. In this work, such an optimization based design paradigm for MMC based generator rectifier systems is developed. To formulate the design problem as a multi-objective optimization problem, it is required that the system waveforms can be obtained to facilitate the imposition of constraints and the estimation of power losses. Similarly, it is also desired to include detailed electric machine magnetic and electrical analysis in design optimization, as well as aspects such as the inductor and heat sink design. Such development typically requires detailed component design and simulation models for the electric machine and converter which are computationally expensive. As an alternative, the proposed work utilizes an electric machine metamodel, heat sink metamodel, and high-speed steady-state simulation model for the MMC to facilitate multi-objective optimization minimizing system metrics of interest while satisfying system constraints. Using the developed component simulation and design models, a multi-objective optimization based design of an MMC based generator-rectifier system is conducted.

This paper studies the possibility of building a modular multilevel converter (M2C) using silicon carbide (SiC) switches. The main focus is on a theoretical investigation of the conduction losses of such a converter and a comparison to a corresponding converter with silicon-insulated gate bipolar transistors. Both SiC BJTs and JFETs are considered and compared in order to choose the most suitable technology. One of the submodules of a down-scaled 3 kVA prototype M2C is replaced with a submodule with SiC JFETs without antiparallel diodes. It is shown that the diode-less operation is possible with the JFETs conducting in the negative direction, leaving the possibility to use the body diode during the switching transients. Experimental waveforms for the SiC submodule verify the feasibility during normal steady-state operation. The loss estimation shows that a 300 MW M2C for high-voltage direct current transmission would potentially have an efficiency of approximately 99.8% if equipped with future 3.3 kV 1.2 kA SiC JFETs. / © 2011 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.QC 20111220

Nowadays high power density has become an emerging need for the medium-voltage (MV) high-power converters in applications of power distribution systems in urban areas and transportation carriers like ship, airplane, and so forth. The limited footprint or space resource cost such immensely high price that introducing expensive advanced equipment to save space becomes a cost-effective option. To this end, replacing conventional Si IGBT with the superior SiC MOSFET to elevate the power density of MV modular converters has been defined as the concentration of this research work. As the modular multilevel converter (MMC) is the most typical modular converter for high power applications, the research topic is narrowed down to study the SiC MOSFET-based MMC. Fundamentals of the MMC is firstly investigated by introducing a proposed state-space switching model, followed by unveiling all possible operation scenarios of the MMC. The lower-frequency energy fluctuation on passive components of the MMC is interpreted and prior-art approaches to overcome it are presented. By scrutinizing the converter's switching states, a new switching-cycle control (SCC) approach is proposed to balance the capacitor energy within one switching cycle is explored. An open-loop model-predictive method is leveraged to study the behavior of the SCC, and then a hybrid-current-mode (HCM) approach to realize the closed-loop SCC on hardware is proposed and verified in simulation. In order to achieve the hybrid-current-mode SCC (HCM-SCC), a high-performance Rogowski switch-current sensor (RSCS) is proposed and developed. As sensing the switching current is a critical necessity for HCM-SCC, the RSCS is designed to meet all the requirement for the control purposes. A PCB-embedded shielding design is proposed to improve the sensor accuracy under high dv/dt noises caused by the rapid switching transients of SiC MOSFET. The overall system and control validations have been conducted on a high-power MMC prototype. The basic unit of the MMC prototype is a SiC Power Electronics Building Block (PEBB) rated at 1 kV DC bus voltage. Owing to the proposed SCC, the PEBB development has achieved high power density with considerable reduction of passive component size. Finally, experimental results exhibit the excellent performance of the RSCS and the HCM-SCC. / Ph. D.

This thesis examines alternatives for power supply for a heavy truck application based on five different modular multilevel converter configurations that ultimately feed a 3-phase motor. Advantages and disadvantages of the different configurations are being discussed as well as other important factors that play a role in what configuration that is beneficial for the intended application. How half- or full-bridge submodules and battery cells relate to each other to achieve a desired voltage are being explained and calculated. Power losses of the converter submodules are being calculated as well as how a specific battery capacity, with increasing average power consumption, performs uphill according to set requirements. It turns out to be the double-armed modular multilevel converter configurations that has the best performance when it comes to utility, energy storage and the lowest power losses.

Batteries are becoming popular in the trend of electrification. Performance andlifespan of a battery pack are closely related to how it has been utilized. Withproper balancing control to slow down aging process, variances of capacity andresistance between battery cells can be maintained at a better level. Among balancing methods, dissipative balancing is still the most common method for itssimplicity in control, low cost and high speed. Non-dissipative balancing methods like converter-based and capacitor-based are of researchers’ interest becauseof less heat generated and superior efficiency. In this thesis, the converter-based balancing method is investigated. A modular multilevel converter (MMC) with Pulse-Width Modulation (PWM) pattern iscompared with another MMC with Nearest-Level Modulation (NLM). The speedto balance six battery sub-modules, output power and battery current harmonicsare examined.

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