Dissertations / Theses on the topic 'Medium Voltage DC'

DC fault protection is one challenge impeding the development of multi-terminal dc grids. The absence of manufacturing and operational standards has led to many point-to-point HVDC links built at different voltage levels, which creates another challenge. Therefore, the issues of voltage matching and dc fault isolation in high voltage dc systems are undergoing extensive research and are the focus of this thesis. The modular multilevel design of dual active bridge (DAB) converters is analysed in light of state-of-the-art research in the field. The multilevel DAB structure is meant to serve medium and high voltage applications. The modular design facilitates scalability in terms of manufacturing and installation, and permits the generation of an output voltage with controllable dv/dt. The modular design is realized by connecting an auxiliary soft voltage clamping circuit across each semiconductor switch (for instance insulated gate bipolar transistor – IGBT) of the series switch arrays in the conventional two-level DAB design. With auxiliary active circuits, series connected IGBTs effectively become series connection of half-bridge submodules (cells) in each arm, resembling the modular multilevel converter (MMC) structure. For each half-bridge cell, capacitance for quasi-square wave (quasi two- level) operation is significantly smaller than typical capacitance used in MMCs. Also, no bulky arm inductors are needed. Consequently, the footprint, volume, weight and cost of cells are lower. Four switching sequences are proposed and analysed in terms of switching losses and operation aspects. A design method to size converter components is proposed and validated. Soft-switching characteristics of the analysed DAB are found comparable to the case of a two-level DAB at the same ratings and conditions. A family of designs derived from the proposed DAB design are studied in depth. Depending on the individual structure, they may offer further advantages in term of installed semiconductor power, energy storage, conduction losses, or footprint. A non-isolated dc-dc converter topology which offers more compact and efficient station design with respect to isolated DAB – yet without galvanic isolation – is studied for quasi two-level (trapezoidal) operation and compared to the isolated versions. In all the proposed isolated designs, active control of the dc-dc converter facilitates dc voltage regulation and near instant isolation of pole-to-pole and pole-to-ground dc faults within its protection zone. The same can be achieved for the considered non-isolated dc-dc converter topology with additional installed semiconductors. Simulation and experimental results are presented to substantiate the proposed concepts.

This thesis investigates the design and analysis of modular medium-voltage dc/dc converter based systems. An emerging converter application is feeding offshore oil and gas production systems located in deep waters, on the sea bed, distant from the onshore terminal. The phase-controlled series-parallel resonant converter (SPRC) is selected as the dc/dc converter unit, for a 10kV dc transmission system. The converter has a high efficiency in addition to favourable soft switching characteristics offered by resonant converters which enable high frequency operation, hence designs with reduced footprints. The phase-controlled SPRC is studied in the steady-state and a new analysis is presented for the converter operational modes, voltage gain sensitivity, and analytically derived operational efficiency. The maximum efficiency criterion is used as the basis for selection of converter full load operational conditions. The detailed design of the output LC filter involves new mathem atical expressions for interleaved multi-module operation. A novel large signal dynamic model is proposed for the phase-controlled SPRC with state feedback linearization. The model preserves converter large signal characteristics while providing a tool for faster simulation and simplified closed loop design and stability analysis. Using this model, a Kalman filter based estimator is proposed and applied for sensorless multi-loop output voltage control. The objective is to enhance the single-loop PI control dynamic response and closed loop stability with no additional sensors required for the inner loop state variables. Dynamic performance and robustness of the converter to operational circuit parameter variations are achieved with three new robust controllers; namely, Lyapunov, sliding mode, and predictive controllers. Finally, converter multi-module operation is studied, catering for voltage and current sharing of the subsea load-side step-down converter. To achieve a step- down voltage, the phase-controlled SPRC modules are connected in an input-series connection to share the medium level transmission voltage. Output-series and output-parallel connections are used to reach higher power levels. A new sensorless load voltage estimator is developed for converters remotely controlled. Matlab/Simulink simulations and experimental prototype results are used to substantiate all the proposed analysis techniques and control algorithms.

Master of Science
Department of Electrical and Computer Engineering
Sanjoy Das
Noel Schulz
Electrical power, although convenient form of energy to distribute and use, cannot easily be stored in large quantities economically. Most electrical power generated by utility plants is consumed simultaneously in real time. However, in some cases, energy storage systems become crucial when power generated from sources does not fulfill peak power load demand in a power system or energy storage systems are needed as backup. Due to these reasons, various technologies such as batteries, ultracapacitors (UC), superconducting magnetic energy storage (SEMS) and flywheels are beneficial options for energy storage systems. Shipboard power systems must use one or more energy storage systems in order to backup the existing power system if locally generated power is unavailable. This will lessen the effect of voltage sags on power quality, and improve system reliability. This report mainly focuses on the design of a Boost DC-DC converter and the integration of that converter with a previously designed battery storage model, as well as the effect of varying loads at the end of the converter.

Modern data center power architecture developing trend is analyzed, efficiency improvement method is also discussed. Literature survey of high frequency isolated power conversion system which is also called solid state transformer is given including application, topology, device and magnetic transformer. Then developing trend of this research area is clearly shown following by research target. State of art wide band gap device including silicon carbide (SiC) and gallium nitride (GaN) devices are characterized and compared, final selection is made based on comparison result. Mostly used high frequency high power DC/DC converter topology dual active bridge (DAB) is introduced and compared with novel CLLC resonant converter in terms of switching loss and conduction loss point of view. CLLC holds ZVS capability over all load range and smaller turn off current value. This is beneficial for high frequency operation and taken as our candidate. Device loss breakdown of CLLC converter is also given in the end. Medium voltage high frequency transformer is the key element in terms of insulation safety, power density and efficiency. Firstly, two mostly used transformer structures are compared. Then transformer insulation requirement is referred for 4160 V application according to IEEE standard. Solid insulation material are also compared and selected. Material thickness and insulation distance are also determined. Insulation capability is preliminary verified in FEA electric field simulation. Thirdly two transformer magnetic loss model are introduced including core loss model and litz wire winding loss model. Transformer turn number is determined based on core loss and winding loss trade-off. Different core loss density and working frequency impact is carefully analyzed. Different materials show their best performance among different frequency range. Transformer prototype is developed following designed parameter. We test the developed 15 kW 500 kHz transformer under 4160 V dry type transformer IEEE Std. C57.12.01 standard, including basic lightning test, applied voltage test, partial discharge test. 500 kHz 15 kW CLLC converter gate drive is our design challenge in terms of symmetry propagation delay, cross talk phenomenon elimination and shoot through protection. Gate drive IC is carefully selected to achieve symmetrical propagation delay and high common mode dv/dt immunity. Zero turn off resistor is achieved with minimized gate loop inductance to prevent cross talk phenomenon. Desaturation protection is also employed to provide shoot through protection. Finally 15 kW 500 kHz CLLC resonant converter is developed based on 4160V 500 kHz transformer and tested up to full power level with 98% peak efficiency.
Master of Science

The Medium Voltage Direct Current (MVDC) distribution represents a promising technology for future shipboard power systems. In such a topic, during the last years, universities and reserch centers have proposed technical solutions to achieve the important targets of MVDC technology, for instance fuel saving, reducing power system weight/space, reconfigurability in case of fault and enhanced power quality. Conversely, the main challenge to face regards voltage control, which has to be capable for guaranteeing the paramount requirement of stability. In regards to this aspect, a possible instability may arise due to the presence of high-bandwidth controlled load converters, modeled as Constant Power Loads (CPLs). Such non-linear loads are seen from the system as negative incremental resistances which are the cause of voltage instability in presence of a perturbation (e.g. load connection, generating system disconnection). The thesis has been realized in the Laboratory of Grid Connected and Marine Electric Power Generation and Control (EPGC Lab.), at the University of Trieste. The aim is to develop voltage control strategies to solve the CPL issue in a realistic multi-converter MVDC Integrated Power System, which is conveniently designed considering a real cruise line MVAC distribution. In such a system, voltage instability may be engage by different approaches, exploiting plant solutions (addition of dedicated filters, addition of energy storage devices) or control solutions. The latter is followed in this thesis: in this case voltage actuators (DC/DC power converters) are used to compensate for the voltage instability: therefore, on one hand (load side) power converters are responsible for the non-linear loads’ issue but, on the other (generators side), they may be utilized to contribute in its solution, thus ensuring a stable behavior. The stabilizing approach foresees the employment of different control techniques, whose theory is focused in the thesis. Starting from the simplier State Feedback (SF), two techniques are mostly studied in the multi-converter arrangement, i.e the Active Damping (AD) and the Linearization via State Feedback (LSF). The AD is a control method to transiently increase the filter resistances in order to damp the voltage oscillations: one of the main pros is the simple implementation on digital controllers, whereas the drawback regards its limited stabilizing action. Therefore, strategies based on Active Damping are to be used to stabilize non-critical systems. Conversely, LSF is a well-performing technique to obtain a notable cancellation of the non-linearities related to CPLs, by exploiting the DC/DC converters to apply a proper non-linear control function. Against the notable capability in stabilizing critical systems, great attention is to be paid in control function’s estimation: inaccurate system parameters or errors in controller’ feedbacks may invalidate the LSF approach, determining a partial loop-cancellation, therefore a non-linear resulting power system. Final simulations are aimed in testing AD and LSF, implemented in global and local control strategies: the former strategy has the purpose to solve the instability directly on CPLs, whereas the second one ensures the bus stability.
La distribuzione in media tensione continua (Medium Voltage Direct Current, MVDC) rappresenta una tecnologia promettente per i sistemi elettrici navali del futuro. A tal riguardo, negli ultimi anni, università e centri di ricerca hanno proposto soluzioni tecniche tali da raggiungere gli obiettivi propri della tecnologia MVDC: fra gli altri, risparmio di carburante, riduzione del peso/ingombro dell’impianto elettrico, riconfigurabilità a fronte di guasti e miglioramento della power quality. D’altra parte, la più grande sfida da affrontare riguarda la regolazione della tensione che deve risultare in grado di garantire il requisito fondamentale della stabilità. Relativamente a questo aspetto, una possibile instabilità si manifesta in presenza di convertitori di carico a banda elevata, modellizzabili come carichi a potenza costante (Constant Power Loads, CPLs). Tali carichi non-lineari vengono visti dal sistema come resistenze incrementali negative, le quali rappresentano la causa dell’instabilità della tensione a fronte di un disturbo (per esempio connessione di carico, disconnessione di un sistema di genenerazione). La tesi è stata realizzata presso il Laboratorio Grid Connected and Marine Electric Power Generation and Control (EPGC Lab.), presso l’Università degli Studi di Trieste. Lo scopo è quello di sviluppare strategie per il controllo della tensione in grado di risolvere la questione CPL, considerando un possibile impianto elettrico integrato (multi-convertitore) in MVDC, convenientemente progettato a partire dalla distribuzione reale MVAC di una nave da crociera. Nel sistema visto, l’instabilità di tensione può essere affrontata secondo diversi approcci, sfruttando soluzioni impiantistiche (aggiunta di filtraggio dedicato, aggiunta di energy storage) oppure soluzioni controllistiche. Il secondo approccio è quello seguito nella presente tesi: gli attuatori di tensione (convertitori DC/DC) vengono usati in questo caso per compensare l’instabilità di tensione. Quindi, da una parte (lato carico) i convertitori sono responsabili del problema dei carichi non-lineari, dall’altro (lato generatori) possono essere utilizzati per contribuire alla sua soluzione, garantendo un comportamento stabile. L’approccio stabilizzante previsto prevede l’utilizzo di diverse tecniche di controllo, analizzate nella tesi dal punto di vista teorico. A partire dalla tecnica semplice State Feedback (SF), altre due tecniche sono state studiate per il caso di sistema multi-converter, ovvero l’Active Damping (AD) e il Linearization via State Feedback (LSF). L’AD è un metodo di controllo per incrementare transitorialmente la resistenza dei filtri, in modo tale da smorzare le oscillazioni di tensione: uno dei principali vantaggi è quello relativo alla semplice ingegnerizzazione su controllori digitali, mentre lo svantaggio riguarda la limitata azione stabilizzante. Pertanto, strategie basate sull’AD devono considerarsi valide per stabilizzare sistemi non critici. D’altra parte, LSF è una tecnica molto valida per ottenere una buona cancellazione delle non-linearità dei CPL, per mezzo dell’azione di convertitori DC/DC in grado di applicare un’opportuna funzione di controllo non-lineare. A fronte di una notevole capacità nello stabilizzare sistemi critici, grande attenzione va posta nella stima della funzione di controllo: conoscenza inaccurata dei parametri o errori nei feedback ai controllori possono invalidare l’approccio LSF, causando una parziale cancellazione, quindi un sistema risultante non-lineare. Le simulazioni finali hanno lo scopo di testare le tecniche AD e LSF, implementate in strategie di controllo locale e globale: la prima strategia ha lo scopo di risolvere l’instabilità direttamente sui CPL, mentre la seconda assicura la stabilità del bus.

Soft DC Links are power electronic converters enabling the control of power flow between distribution feeders or networks. This thesis considers the use of Soft DC Links in Medium Voltage (MV) distribution networks to improve network operation while facilitating the integration of distributed generators (DGs). Soft DC Links include Soft Open Points (SOPs) and Medium Voltage Direct Current (MVDC) links. An SOP can be installed to replace mechanical switchgear in a network, providing controllable active power exchange between connected feeders, as well as reactive power compensation at each interface terminal. The deployment of an MVDC link enables power and voltage controls over a wider area, and facilitates the effective use of available capacity between adjacent networks. The benefits of using SOP and MVDC link in MV distribution networks were investigated. A multi-objective optimisation framework was proposed to quantify the operational benefits of a distribution network with an SOP. An optimisation method integrating both global and local search techniques was developed to determine the set-points of an SOP. It was found that an SOP can improve network operation along multiple criteria and facilitate the integration capacity of DGs. A Grid Transformer-based control method of an MVDC link was proposed, which requires only measurements at the grid transformers to determine the operation of an MVDC link. Control strategies considering different objectives were developed. The proposed control method is used in the ANGLE-DC project, which aims to trial the first MVDC link in Europe by converting an existing AC circuit to DC operation. It was found that an MVDC link can significantly increase the network hosting capacity for DG connections while reducing network losses compared to an AC line. An impact quantification of Soft DC Links was carried out on statistically-similar distribution networks, which refer to a set of networks with similar but different topological and electrical properties. A model was developed to determine the optimal allocation of Soft DC Links. It was found that a Soft DC Link can reduce the network annual cost under a wide range of DG penetration conditions. The statistical analysis provides distribution network planners with more robust decisions on the implementation of Soft DC Links.

A novel multilevel converter that is especially suited for high speed multi-megawatt switched reluctance motor drives operating at the medium voltage level is presented. The drive is capable of variable speed, four-quadrant operation. Each phase leg of the converter contains an arbitrary number of cascaded cells connected in series with the phase winding. Each cell contains a half-bridge chopper connected to a capacitor. The converter is named the cascaded chopper cell converter. The modular nature of the converter with the ability to add redundant cells makes it very reliable, which is a key requirement for medium voltage drive applications. A comprehensive control algorithm that overcomes the challenges of balancing and controlling cell capacitor voltages is also proposed. A suitable startup algorithm to limit startup current and switching losses, as well as ensure that cell capacitor voltages remain controlled at startup, is suggested. Details of the drive design such as component sizing and control parameter selection are also discussed. A detailed simulation model is developed and explained, and simulation results are provided for primary validation. Operation with standard current and speed control is first simulated. Then a scheme that gives way to a controller that operates the drive in single-pulse mode is developed and presented. This single-pulse control scheme controls the turn-on and turn-off angles, as well as the energization voltage level, in order to obtain high efficiency. Practical considerations related to the drive such as reliability, efficiency, and cost considerations are also discussed. Finally, a detailed comparison of the proposed converter to another competing converter is performed. Besides its scalability to high voltages and powers, the reliability and efficiency of the proposed converter makes it also a candidate for sub-megawatt applications requiring minimum downtime, or any application where high efficiency or improved performance is required. A small part of this work is also dedicated to brushless dc machines. Control methods for a new converter for brushless dc machines are proposed and verified via simulation. The main advantage of this converter with the proposed control is that it allows exact control of torque or speed up to twice the rated speed, without resorting to current phase advancing or other flux-weakening techniques.
Ph. D.

Recent research into industrial applications of electric power conversion shows an increase in the use of renewable energy sources and an increase in the need for electric power by the loads. The Medium-Voltage DC (MVDC) concept can be an optimal solution. On the other hand, the Modular Multilevel Converter (MMC) is an attractive converter topology choice, as it has advantages such as excellent harmonic performance, distributed energy storage, and near ideal current and voltage scalability. The fault response, on the other hand, is a big challenge for the MVDC distribution systems and the traditional MMCs with the Half-Bridge submodule configuration, especially when a DC short circuit fault happens. In this study, the fault current behavior is analyzed. An alternative submodule topology and a fault operation control are explored to achieve the fault current limiting capability of the converter. A three-phase SiC-based MMC prototype with the Full-Bridge configuration is designed and built. The SiC devices can be readily adopted to take advantage of the wide-bandgap devices in MVDC applications. The Full-Bridge configuration provides additional control and energy storage capabilities. The full in-depth design, controls, and testing of the MMC prototype are presented, including among others: component selection, control algorithms, control hardware implementation, pre-charge and discharge circuits, and protection scheme. Systematical tests are conducted to verify the function of the converter. The fault current behavior and the performance of the proposed control are verified by both simulation and experiment. Fast fault current clearing and fault ride-through capability are achieved.
Master of Science

L'intégration croissante des ressources énergétiques natives en courant continu à l'échelle de réseaux électriques, telles que les systèmes photovoltaïques, les véhicules électriques, les systèmes de stockage et les centres de données, remet en question le choix conventionnel des systèmes de distribution en courant alternatif (AC) au niveau de la moyenne tension (MV). Cette recherche vise à évaluer la viabilité technique et économique des réseaux de distribution MV AC/DC en développant des outils de planification pour les réseaux hybrides.L'étude propose des modèles de planification optimale des systèmes de distribution pour traiter les connexions MVDC point à point et les topologies hybrides, qui incorporent des ressources DC connectées au système par le biais de convertisseurs DC/DC, en utilisant des formulations telles que la programmation linéaire en nombres entiers mixtes, quadratique et conique de second ordre. Parmi les principales contributions de cette recherche, une analyse documentaire approfondie des hypothèses de planification a été réalisée, la proposition d'un modèle linéaire de pertes pour les stations de conversion AC/DC et DC/DC tenant compte de l'efficacité variable en fonction de la charge, l'incorporation de contraintes topologiques pour tenir compte des topologies radiales par morceaux, et l'évaluation des avantages économiques pour un large ensemble de paramètres dans le cadre des marchés de l'électricité dérégulés.Les perspectives futures comprennent l'étude de l'scalabilité des modèles proposés à des réseaux de distribution à plus grande échelle, l'exploration des tendances émergentes dans la résolution de l'optimisation et l'inclusion de choix liés à la conception dans les outils de planification
The increasing integration of utility-scale DC-native energy resources, such as photovoltaic systems, electric vehicles, storage systems, and data centers, challenges the conventional choice of AC distribution systems at the Medium Voltage (MV) level. This research aims to evaluate the technical and economic viability of AC/DC MV distribution networks by developing planning tools for hybrid networks.The study proposes optimal distribution system planning models to address point-to-point MVDC connections and hybrid topologies, which incorporate DC resources connected to the system through DC/DC converters, using formulations such as Mixed Integer Linear, Quadratic and Second Order Conic Programming. Some of the key contributions of this research include an extensive literature review of planning hypotheses, the proposal of a linear model of losses for AC/DC and DC/DC conversion stations accounting for part-load efficiency, the incorporation of topological constraints to accommodate piece-wise radial topologies, and the assessment of economic benefits across a wide range of parameters within the framework of unbundled electricity markets.Future perspectives include investigating the scalability of the proposed models to larger-scale distribution networks, exploring emerging trends in the optimization solving and including design-related choices in the planning tools

Ce travail concerne l’étude et la réalisation d’une architecture multicellulaire de conversion d’énergie électrique haute tension avec étage intermédiaire alternatif moyenne fréquence destinée à la traction électrique ferroviaire. L’objectif de ce travail est de diminuer la masse et le volume de l’étage de conversion alternatif-continu que l’on retrouve dans les engins de traction conçus pour circuler sur les réseaux alternatifs 25kV-50Hz ou 15kV-16Hz2/3. La recherche de gains sur l’étage de conversion alternatif-continu s’applique aussi bien sur les automotrices où l’on cherche à gagner de la place disponible pour y placer des passagers que sur les locomotives ou encore sur les motrices de TGV où l’on recherche un gain de masse étant donné que ces engins sont en limite de charge à l’essieu. Le contexte de la haute tension implique l’utilisation d’interrupteurs de forts calibres en tension pour limiter au maximum le nombre de cellules de conversions utilisées. D’un autre côté, la recherche de gains sur le transformateur nécessite une fréquence de découpage élevée, génératrice de pertes en commutation dans les interrupteurs. L’architecture de conversion retenue permet par l’association de structures duales d’obtenir des conditions de commutation douce, ce qui est favorable à une montée en fréquence avec des interrupteurs de forts calibres en tension. Le convertisseur élémentaire associe un onduleur de tension commandé au blocage et un commutateur de courant commandé à l’amorçage. Afin d’évaluer le rendement de l’architecture considérée, un prototype d’un bloc de conversion élémentaire, d’une puissance de 280 kVA, a été réalisé au laboratoire PEARL. Les interrupteurs sont réalisés sur la base de modules IGBT 6,5kV/200A. Les essais en commutation douce ont permis d’évaluer, dans des conditions de fonctionnement réelles, les pertes dans les modules IGBT. Compte tenu de ces résultats, il est possible de déterminer les limites de fonctionnement de la structure de conversion et d’effectuer un dimensionnement en considérant le compromis rendement-poids-volume pour un engin de traction donné
This thesis concerns the study and the rating of a high voltage multicellular converter with an intermediate medium frequency stage dedicated to railway traction. The objective is to reduce the weight and the volume of the AC-DC conversion stage which is implemented in railway engines running on 25kV-50Hz or 15kV-16Hz2/3 railways. Reduction on weight and size of the AC-DC converter may be applied on multiple unit trains where the transformer causes room loss for passengers and on locomotives and high speed trains where the axle load is limited. On one hand high voltage switches are required in order to minimize the number of cells used to build the converter. On the other hand, reducing the size and the weight of the transformer requires a high switching frequency, causing high commutation losses. To achieve soft switching conditions with high voltage semiconductors, the proposed topology is based on an association of dual structures. Each elementary converter combines a controlled turn-off voltage source inverter and a controlled turn-on current source inverter. In order to estimate the efficiency of the new topology, a prototype of one elementary cell working at 280 kVA, was built at the Power Electronics Associated Research Laboratory (PEARL). The switches are standard 6.5 kV/200A IGBTs modules. Soft-switching tests, in real operating conditions, allow evaluating IGBTs and diodes switching losses. Thanks to these results, it is possible to find the structure operating limits and to size the transformer considering the trade-off between the system efficiency and the transformer weight

This Diploma thesis deals with assessment of the influence of two fundamental types of resistance welders operation on chosen power quality parameters at the point of common coupling (PCC) of the power network. The assessed parameters of power quality are the total harmonic distortion of the supply voltage, asymmetry of the supply voltage and the level of short term flicker at the point of common coupling. The assessment is based on the comparison of the results of computer simulations done in PSCad 4.2.0 with the requirements of technical standards. The outcome of this thesis is the determination of allowable limits for physical and operation parameters for the general arrangement of a power distribution network and a resistance welder that should guarantee the power quality compliance.

碩士
國立成功大學
電機工程學系
106
As the progress of wide-bandgap (WBG) semiconductor devices and the rise of distributed energy resources (DER), medium-voltage solid-state transformer (SST) becomes more and more popular in recent years. Medium-voltage SST features high-performance and fantastic functionality. The SST proposed in this thesis is applied in 11.4 kVAC distribution system, and the DC stage of the SST is composed of 13 modules. Operating principle, steady-state analysis and components design of the converter as well as the parameter design, insulation consideration and loss optimization of the transformer, are described in detail in this thesis. Meanwhile, the simulation software SIMPLIS and COMSOL Multiphysics® are used to ensure the validity. Finally, a prototype converter with input voltage 1.52 kVDC, output voltage 380 VDC and output power 10 kW is designed and realized to verify the feasibility of the module applied in SST.

碩士
國立高雄海洋科技大學
輪機工程研究所
99
Medium voltage DC (MVDC) electrical distribution system that is a novel technique for sea and undersea vehicles has been extensively studied by ship building corporations and end-users in recent years. In this type of distribution systems, the steady-state stability issue is important because of the presence of different types of power converters and both of AC and DC power supply in the system. Power flow analyses can be carried to address this issue. This thesis aims to build a set of power flow analysis model for ship MVDC distribution systems with different types of power converters. A rigid power flow model of the power converters in steady-state is first. Derived to consider medium and low voltage characteristics of electric power system on this type of ship. A Newton-Raphson algorithm based unified AC/DC power flow solution model then is developed to incorporate AC-system and DC-system models and the AC/DC interface buses for system planning purpose. With the developed analysis model, the steady-state operating characteristics of the power-generation/distribution system and power converters for a given set of busbar loads and power generations in this type of distribution system can be accurately determined for providing a better system planning result. The analysis results can provide engineers useful information for planning and designing similar systems.

Grounding the frame of photovoltaic (PV) panel is a necessity for the safety of humans. This leads to the formation of large capacitance between PV cells and ground. Hence, to reduce leakage current due to parasitic capacitance between PV cells and ground, the DC output voltage of a photovoltaic panel is normally kept below 1 kV. Conventionally, for medium voltage (3.3 kV-66 kV) AC grid integration of PV panel, the DC output of PV is rst converted to 400V AC and is connected to a 400V collection grid through a line frequency transformer (LFT). This LFT provides isolation and limits circulating current among the PV modules. Another step-up LFT is used to connect 400V AC grid to the medium voltage (MV) transmission grid. These line frequency transformers are bulky and expensive. The line side lters are placed on the low voltage side of the rst LFT and hence, experience high currents leading to higher copper losses. To avoid the limitations of LFTs, power converters with high frequency transformer (HFT) are becoming popular. The HFT is fed from a DC side inverter (DSI) and the output of HFT (which is high frequency AC) is converted to line frequency AC using power electronic converters. This type of converter is known as high frequency link (HFL) DC to AC converter. State-of-the-art HFL DC to AC converters mostly employ a multi-stage power conversion technique where an isolated DC to DC converter is cascaded with an inverter. The stages are controlled independently. The inter-stage voltage sti DC-link is maintained with large electrolytic capacitor. But such an approach requires higher amount of ltering and use of electrolytic capacitor a ects long-term reliability. Moreover, the capacitor voltage needs to be tightly regulated to protect the devices. The grid interfaced inverter is high frequency hard-switched resulting in reduced e ciency. These drawbacks are overcome in a single-stage power conversion approach where the inter-stage lter capacitor is removed and all the power devices are either soft or line frequency switched resulting in reduction in switching loss and improvement in e ciency. In literature, to replace the step-up LFT and to directly integrate the converter to the medium voltage grid, a popular solution is the usage of cascaded multilevel power conversion. Generally, the above discussed multi-stage converter is employed as modules in a cascaded multilevel con guration to produce medium voltage. Moreover, some existing topologies use single-stage converters in a cascaded multilevel con guration to produce medium voltage, but the grid side converters are high frequency switched, leading to higher loss. In this thesis, a new topology is proposed to overcome the drawbacks of existing cascaded multilevel power conversion topologies. In the thesis, a new single-stage high frequency link cascaded multilevel converter topology is proposed for MV grid integration of solar power. A single-stage high frequency link DC to AC converter is used as a module. The DC side of each module is connected to a PV source. The AC sides of multiple such modules are connected in series in a cascaded fashion to interface with the MV AC grid. Proposed modulation of the DC to AC module results in zero voltage switching (ZVS) of the DC side converter and line frequency switching of the AC side converter. ZVS happens for most part of the line cycle. Over a switching cycle, the operation of this module is similar to a phase-shifted full bridge (PSFB) DC to DC converter. In the PSFB converter, during switching transition, the parasitic capacitance of AC side diode bridge along with leakage inductance of HFT forms a resonating circuit. This resonating circuit leads to high voltage stress on the secondary side devices. An active snubber is designed to restrict the voltage overshoot. The operation of PSFB converter, considering all parasitics, is not explored in literature. In this thesis, a detailed analysis of the operation of the PSFB and step-by-step design methodology is given. The hardware is designed and tested with DC input voltage of 400 V, DC output voltage of 1240 V, output power of 1.5 kW and switching frequency of 20 kHz. Experimental results validate the analysis. A method is proposed to observe medium voltage waveforms with the standard low-voltage probe. A method to remotely control the medium voltage converter is developed to ensure safety.