For AC railway power supply systems with a different frequency than the public grid, high-voltage AC transmission lines are common, connected to the catenary by transformers. This study suggests an alternative design based on an high-voltage DC (HVDC)-feeder, which is connected to the catenary by converters. Such an HVDC line would also be appropriate for DC-fed railways and AC-fed railways working at a public-grid frequency. The converter stations between the public grid and the HVDCfeeder can be sparsely distributed, not denser than on 100 km distances, whereas the converters connecting the HVDC-feeder to the catenary are distributed denser. Their ratings can be lower than present-day substation transformers or converters, since the power flows can be fully controlled. Despite a relatively low-power rating, the proposed converters can be highly efficient because of the use of medium frequency technology. The proposed feeding system results in lower material usage, lower losses and higher controllability compared with the present solutions. Simulations of the proposed solution show clear advantages regarding transmission losses and voltages compared with conventional systems, especially for cases with weak feeding, and when there are substantial amounts of regeneration from the trains.

The railway power supply systems in many sparsely populated countries are relatively weak. Weak railway power supply systems causes problems with power quality, voltage drops, and high transmission losses.

For AC railway power supply systems with a different frequency than the public grid, high-voltage AC (HVAC) transmission lines are common, connected to the catenary by transformers.

In this paper an alternative design based on an HVDC feeder is suggested. The HVDC feeder is connected to the catenary by converters. Such an HVDC line would also be appropriate for DC-fed railways and AC-fed railways working at public frequency. The converter stations between the public grid and the HVDC feeder can be sparsely distributed, in the range of 100 km or more, whereas the converters connecting the HVDC feeder to the catenary are distributed with a much closer spacing. Their ratings can be lower than substation transformers or electro-mechanical converters, since the power flow can be fully controlled.

Despite a relatively low power rating, the proposed converters can be highly efficient due to the use of medium frequency technology. The HVDC-based feeding system results in lower material usage, lower losses and higher controllability compared to present solutions.

Simulations of the proposed solution show clear advantages regarding transmission losses and voltages compared to conventional systems, especially for cases with long distances between feeding points to the catenary, and when there are substantial amounts of regeneration from the trains.

Railways are the most energy-efficient land-based mode of transport, and electrification is the most energy-efficient way to power the trains. There are many existing solutions to supply the trains with electricity. Regardless of which particular technology is chosen, it is beneficial to interconnect the public power grids to grids supplying power to the railways. This paper shows that the most efficient, flexible, and gentle-for-the-public-grid way of doing that is through power electronic-based power converters. Converters offer great benefits regardless of whether the overhead contact lines are of DC-type or AC type, and regardless of the AC grid frequency. This paper presents neither new theory nor new experimental results. Based on already available information, this paper presents logical arguments leading to this conclusion from collected facts. Over time what used to be advanced and high-cost equipment earlier can nowadays be purchased at reasonable cost. It is obvious that for most electrically-fed railways, the use of modern power converters is attractive. Where the individual trains are high consumers of energy, the railway gradients are substantial, and the public grids feeding the railway are weak, the use of converters would be technically desirable, if not necessary for electrification.It is expected that more high-speed railways will be built, and more existing railways will be electrified in the foreseeable future. This paper could provide some insights to infrastructure owners and decision makers in railway administrations about value additions that converter-fed electric railways would provide.

This paper presents the main findings of a Master's Thesis project carried out in cooperation between Transrail and Royal Institute of Technology (KTH). The main objective was to create a plugin for checking the electric power system feasibility of a train traffic plan with an associated driving strategy created by TRAINS a Transrail software product. Secondary aims with the project was to study power system feasibilities during converter station outages, and to which extent optimal operation of the locomotive converters' reactive power assure power system feasibilities. In the developed optimal reactive power strategies, the main priority was to fulfill the desired traffic plans, whereas the secondary priority was to minimize railway power system power consumption. The case studies are applied on representative traffic scenarios and power system models representing the northern part of the Iron Ore line in Northern Sweden. The focus of the study is set on the IORE locomotives and the iron ore trains they haul. The optimized locomotive reactive power regards IORE, so also the investigated power system feasibilities of the traffic plans.

This study focuses on optimizing the operation of rotary railway-feeding converters. Since a large share of rotary converters can be expected to be in operation for decades to come in the railway power supply systems (RPSSs), it is important to make their operation as efficient as possible. The existing rotary converters may have unused capabilities particularly in load sharing, but also to some extent in reactive power compensation. Load-sharing improvement can be done in two steps; (1) coarsely by unit commitment within a converter station, (2) fine-tuned by controlling the terminal voltage of the converter station on the railway-side. The proposed optimization models minimize RPSS losses, including losses in the converters. The models are implemented and solved in the GAMS environment. The case studies are applied on Sweden-inspired RPSS designs and configurations, and the train load situations are varied. Ideas and experiences regarding improved computational efficiency for solving the problems are discussed.

This paper presents an optimization model for simulations of railway power supply systems. It includes detailed power systems modeling, train movements in discretized time considering running resistance and other mechanical constraints, and the voltage-drop-induced reduction of possible train tractive forces. The model has a fixed number of stationary power system nodes, which alleviates optimized operation overtime. The proposed model uses SOS2 (Special Ordered Sets of type 2) variables to distribute the train loads to the two most adjacent power system nodes available. The impacts of the number of power system nodes along the contact line and the discretized time step length on model accuracy and computation times are investigated. The program is implemented in GAMS. Experiences from various solver choices are also discussed. The train traveling times are minimized in the example. Other studies could e.g. consider energy consumption minimization. The numerical example is representative for a Swedish decentralized, rotary-converter fed railway power supply system. The proposed concept is however generalizable and could be applied for all kinds of moving load power system studies.

In this paper a railway power system design based on an HVDC feeder is suggested. The converter stations between the public grid and the HVDC feeder can be sparsely distributed, in the range of 100 km or more, whereas the converters connecting the HVDC feeder to the catenary are distributed with a much closer spacing. The ratings of the catenary-connected ones can be lower than substation transformers or rotary converters, since the power conversion can be fully controlled. Simulations of the proposed solution show clear advantages regarding transmission losses and voltages compared to conventional systems, especially for cases with long catenary sections, and when there are substantial shares of regeneration from the trains.

In this paper an alternative railway power systems design based on an HVDC feeder is studied. The HVDC feeder is connected to the catenary by converters. Such an HVDC line is also appropriate for DC-fed railways and AC-fed railways working at public frequency.

A unit commitment optimal power flow model has been developed and is applied on a test system. In this paper, the model is presented in detail. The model, in the form of an MINLP program, uses unified AC-DC power flow to minimize the entire railway power system losses.

Simulations of the proposed solution show clear advantages regarding transmission losses and voltages compared to conventional systems, especially for cases with long distances between feeding points to the catenary, and when there are substantial amounts of regeneration from the trains.

In this paper an alternative railway power systems design based on an HVDC feeder is studied. The HVDC feeder is connected to the catenary by converters. Such an HVDC line is also appropriate for DC-fed railways and AC-fed railways working at public frequency. A unit commitment optimal power flow model has been developed and is applied on a test system. In this paper, the model is presented in detail. The model, in the form of an MINLP program, uses unified AC-DC power flow to minimize the entire railway power system losses. Simulations of the proposed solution show clear advantages regarding transmission losses and voltages compared to conventional systems, especially for cases with long distances between feeding points to the catenary, and when there are substantial amounts of regeneration from the trains.

Permanent magnet (PM) machines offer several advantages in traction applications such as high efficiencyand high torque per volume ratio. The iron losses in these machines are estimated mostly with empiricallaws taken from other types of machines or with finite element simulations (FEM). In the first part of thisthesis the objective is to define an accurate analytical model for the stator yoke, teeth and rotor of a PMmotor which should work well enough for all operating point (different loads and frequency).This analytical model is found using an iterative process. After building a loss matrix and flux matrix basedon FEM simulations, it is possible to curve fit each of the lines or the rows of the matrix in order to achievethe best fitting for every operating point. This is a very new approach; it was shown that it gives thepossibility, even with a very limited number of FEM simulations, to achieve an accurate estimation of thelosses.The second part of this report focuses on optimizing this analytical method, comparing it with otherpossibilities, analyzing limits and advantages. Special attention is also given to the effects of the losses onthe temperatures in different parts of the machine. In the last part of the thesis, the analytical model isused to test a new control strategy. Its goal is to reduce the total losses of the motor and optimize the ratiobetween torque and total losses for a given driving cycle.

Several research activities at KTH are carried out related to Isolated AC/DC converters in order to improve the design and efficiency. Concerning the improvement in the mentioned constraints, losses of the elements in the prototype converter are modeled in this thesis work. The obtained loss model is capable of calculating the losses under different circumstances. The individual contribution of losses for each element at different conditions can be obtained, which is further useful in improving the design and therefore, efficiency. The losses in different elements of the converter, including power semiconductor devices, RC-snubbers, transformer and filter inductor at different operating points can be computed by using the obtained model. The loss model is then validated by comparing the analytical results with the measurements. The results based on developed loss model show consistency with the measured losses. The comparison at different conditions shows that, the difference between measured and analytical results ranges between 10% to 20 %. The difference is due to those losses which are disregarded because of their negligible contribution. On the other hand, it is also observed that if the neglected losses are counted, the difference reduces up to 10%.

In order to transmit massive amounts of power generated by remotely located power plants, especially offshore wind farms, and to balance the intermittent nature of renewable energy sources, the need for a stronger high voltage transmission grid is anticipated. Due to limitations in ac power transmission the most likable choice for such a grid is a high-voltage dc (HVDC) grid. However, the concept of the HVDC grid is still under active development as different technical challenges exist, and it is not yet possible to construct such a dc grid. This paper deals with prospects and technical challenges for future HVDC SuperGrids. Different topologies for a SuperGrid and the possibility to use modular multilevel converters (M2Cs) are presented. A comprehensive overview of different submodule implementations of M2C is given as well as a discussion on the choice between cables or overhead lines, the protection system for the dc grid and dc-side resonance issues.

The modular multilevel converter (MMC) is the state-of-the-art voltage-source converter (VSC) topology used for various power-conversion applications. In the MMC, submodule failures can occur due to various reasons. Therefore, additional submodules called the redundant submodules are included in the arms of the MMC to fulfill the fault-safe operation requirement. The performance of the MMC with redundant submodules has not been widely covered in the published literature. This paper investigates the performance of the MMC with redundant submodules in the arms. Two different control strategies are used and compared for integrating redundant submodules. The response of the MMC to a submodule failure for the two strategies is also studied. Moreover, the operation of the MMC with redundant submodules is validated experimentally using the converter prototype in the laboratory.

The feasibility of future multiterminal dc (MTDC) systems depends largely on the capability to withstand dc-side faults. Simulation models of MTDC systems play a very important role in investigating these faults. For such studies, the test system needs to be accurate and computationally efficient. This paper proposes a detailed equivalent model of the modular multilevel converter (MMC), which is used to develop the MTDC test system. The proposed model is capable of representing the blocked-mode operation of the MMC, and can be used to study the balancing control of the capacitor voltages. In addition, the operation of the MMC when redundant submodules are included in the arms can also be studied. A simplified model of a hybrid high-voltage dc breaker is also developed. Hence, the developed test system is capable of accurately describing the behavior of the MMC-based MTDC system employing hybrid HVDC breakers, during fault conditions. Using time-domain simulations, permanent dc-side faults are studied in the MTDC system. In addition, a scheme to control the fault current through the MMC using thyristors on the ac side of the converter is proposed.

Negative sequence currents are obtained during ac-side asymmetrical faults of converters in highvoltage direct current (HVDC) transmission systems. Consequently, second order harmonics in the dc-side voltage and current, unbalanced ac-side currents, and power oscillations can be observed. This paper presents a negative sequence current control (NSCC) scheme that eliminates second order harmonic ripples in the voltage and current of the dc-side during unbalanced grid conditions. Controllers for this purpose are investigated using a continuous model of the modular multilevel converter (M2C). The proposed scheme utilizes an open-loop controller for lower level control of the M2C. The continuous model used also has the capability to model blocking and deblocking events which may be used during protective actions. Simulation results reveal that the proposed NSCC scheme is effective in suppressing dc-side voltage and current ripples. Moreover, it keeps the ac-side phase currents balanced during asymmetrical fault conditions.

Simulation models of the modular multilevel converter (MMC) play a very important role for studying the dynamic performance. Detailed modeling of the MMC in electromagnetic transient simulation programs is cumbersome, as it requires high computational effort and simulation time. Several averaged or continuous models proposed in the literature lack the capability to describe the blocked state. This paper presents a continuous model, which is capable of accurately simulating the blocked state. This feature is very important for accurate simulation of faults. The model is generally applicable, although it is particularly useful in high-voltage dc applications.

High-voltage direct current (HVDC) grids using modular multilevel converters (M2Cs) have strongly been considered for the integration of distant renewable energy sources and also as a backbone to the existing ac-grids. The dynamic performance of the M2C is of particular interest in these grids. For electromagnetic transient (EMT) programs, modeling of HVDC-grids using detailed M2C models is unrealistic, as it requires extremely high computational effort and simulation time. In this paper an HVDC-grid test system is developed using a continuous simulation model of the M2C. The model is also capable of describing the blocking events of the M2C. Using time-domain simulations in PSCAD/EMTDC, the dynamic performance of the M2C in HVDC-grids under fault conditions is investigated. Simulation results reveal that the continuous M2C model can efficiently be used to study the dynamic performance of the M2C in HVDC-grids with high computational speed, under different fault conditions.

This paper presents the continuous model for the Modular Multilevel Converter (M2C). The model operates in two modes, either operating as a voltage source in deblocked mode or as a rectifying diode bridge in blocked mode. The model is validated by comparison with a detailed M2C model having 36 submodules per arm, using different control strategies. The comparison is based on time-domain simulations in PSCAD/EMTDC. The continuous model shows a very good agreement with the detailed model.

This diploma work is mainly focused on developing the control strategy for avariable speed drive as an alternative solution to a micro-hydro power plant. The detailed mathematical model for a micro-hydro system including a Kaplan turbine, mechanical shaft and electrical machines is presented and validated through simulations. A control strategy for an autonomous operation of a doubly-fed induction machine-based drive is developed for a wide range of speed. The drive can operate at a unity power factor.The possible applications of the analyzed system are also presented. As a positive side of the system, it is found that the direct interaction between the power electronic converters and the utility grid can be avoided by exploiting the proposed topology, which might lead to a better quality of the produced power in terms of harmonics. This could also lead to removal or reduction of the size of the harmonic filters that are being used in conventional doubly-fed induction generator installations. As regards to the drawbacks of the system, a comparison of converter and generator ratings between the analyzed solution and the conventional solution was performed. While the converters rating remain the same, there is one more electrical machine and the doubly-fed generator rating is slightly increased. Losses are also slightly larger due to the presence of the second machine.

This article describes a topology of three-phase transverse flux machine and the derivation of ananalytical model which does not exist in the literature. Three dimensional finite element analysis isalso used to get more accurate results. Steps carried out to obtain a better power factor are presented.

This thesis is an effort to investigate the operation and the performanceof modular multilevel converters (M2Cs). Proven to be the most promisingtopology in high-voltage high-power applications, it is necessary to put aneffort in understanding the physical laws that govern the internal dynamicsof such converters, in order to design appropriate control methods. AlthoughM2Cs belong to the well-studied family of voltage-source converters (VSCs),and claim a modular structure, their control is significantly more complicatedcompared to two- or three-level VSCs, due to the fact that a much highernumber of switches and capacitors are needed in such a topology. This thesishighlights the important parameters that should be considered when designingthe control for an M2C, through analyzing its internal dynamics, and alsosuggests ways to control such converters ensuring stable operation withoutcompromising the performance of the converter.Special focus is given on ac motor-drive applications as they are very demandingand challenging for the converter performance. Interactions betweenthe internal dynamics and the dynamics of the driven motor are experimentallyinvestigated. The problem of operating the converter when connectedto a motor standing still is visited, even under the condition that a greatamount of torque and current are requested, in order to provide an idea forthe converter requirements under such conditions. Finally, an optimization ofthe converter operation is suggested in order to avoid overrating the convertercomponents in certain operation areas that this is possible.All analytical investigations presented in this thesis are confirmed by experimentalresults on a laboratory prototype converter, which was developedfor the purposes of this project. Experimental verification proves the validityof the theoretical investigations, as well as the correct performance of thecontrol methods developed during this project on a real, physical converter,hoping that the results of this thesis will be useful for large-scale implementations,in the mega- or even giga-watt power range.

Inverter-based electrical-machine drives suffer from significantly higher losses compared to sinusoidal-supply-based alternatives, fed directly from the grid. Using multilevel inverters it becomes possible to partially mitigate the effects of the switched supply waveform, while keeping the advantages of variable-speed operation. This paper aims to evaluate the increase of the losses occurring in an induction motor (IM) fed by a modular multilevel converter (M2C), when compared to grid-connected operation, in order to evaluate the impact of the inverter-generated harmonics in the machine. It is confirmed that the losses created in the motor due to the harmonic content of the inverter-generated waveforms are very low, and almost equivalent to a purely sinusoidal supply. The investigation includes an analysis of the harmonic content from experimental waveforms obtained by an 11-kW IM laboratory setup, and it is further supported by measurements of the temperature rise in the IM-stator windings. It is concluded that the M2C could create the conditions even for high-power motors to be operated without any derating.

Modular multilevel converters require that the controller is designed so that the submodule capacitor voltages are equalized and stable, independent of the loading conditions. Assuming that the individual capacitor-voltage sharing is managed effectively, an open-loop strategy has been designed to ensure that the total amount of energy stored inside the converter always will be controlled. This strategy, using the steady-state solutions of the dynamic equations for controlling the total stored energy in each converter arm, has proven to be effective. The intention of this paper is to explain in a rigorous way the mechanism behind the suggested strategy and to prove that, when this open-loop strategy is used, the system becomes globally asymptotically stable. Experimental verification on a three-phase 10-kVA prototype is presented.

Modular multilevel converters (M2Cs) require that the controller is designed so that the submodule capacitor voltages are equalized and stable, independent of the loading conditions. Provided that the individual capacitor voltage sharing is managed effectively, an open-loop strategy can been designed to ensure that the total amount of energy stored inside the converter always will be controlled. This strategy, using the steady-state solutions of the dynamic equations for controlling the total stored energy in each converter arm, has proven to be effective. The intention of this paper is to explain in a rigorous way the mechanism behind the suggested strategy, and to prove that, when this open-loop strategy is used, the system becomes globally asymptotically stable.

Variable-speed drives have reduced voltage requirements when operating below the base speed. In a modular-multilevel-converter-based (M2C-based) motor drive it is then possible to operate with reduced voltage in the submodule capacitors, than at the base speed. In this sense, a greater capacitor-voltage ripple can be accommodated, without exceeding the maximum peak-capacitor voltage. This paper presents an analytical investigation for the optimal selection of the average capacitor voltage for M2Cs, when the motor is operating with rated torque, below the base speed. This method does not require any power exchange between the converter arms, so it keeps the conduction losses at the minimum level. Additionally, the method decreases the switching losses, due to the decreased capacitor-voltage level. The overall ratings of the converter remain the same as in the base-speed operation. It is shown that this method can be applied at a speed range between the base speed and down to approximately one third of it, i.e, an operating range that covers the requirements for typical pump- and fan-type applications. The results obtained from the analytical investigation are experimentally verified on a down-scaled laboratory prototype M2C.

Variable-speed drives have reduced voltage requirementswhen operating below the base speed. In a modularmultilevel-converter-based (M2C-based) motor drive it is thenpossible to operate with reduced voltage in the submodulecapacitors, than at the base speed. In this sense, a greatercapacitor-voltage ripple can be accommodated, without exceedingthe maximum peak-capacitor voltage. This paper presents ananalytical investigation for the optimal selection of the averagecapacitor voltage for M2Cs, when the motor is operating withrated torque, below the base speed. This method does not requireany power exchange between the converter arms, so it keepsthe conduction losses at the minimum level. Additionally, themethod decreases the switching losses, due to the decreasedcapacitor-voltage level. The overall ratings of the converterremain the same as in the base-speed operation. It is shownthat this method can be applied at a speed range betweenthe base speed and down to approximately one third of it,i.e, an operating range that covers the requirements for typicalpump- and fan-type applications. The results obtained from theanalytical investigation are experimentally verified on a downscaledlaboratory prototype M2C.

Modular multilevel converters are shown to have a great potential in the area of medium-voltage drives. Low-distortion output quantities combined with low average switching frequencies for the semiconductor devices create an ideal combination for very high-efficiency drives. However, the large number of devices and capacitors that have to conduct the fundamental-frequency current require more complex converter control techniques than its two-level counterpart. Special care needs to be taken for starting and operation at low speeds, where the low-frequency current may cause significant unbalance between the submodule capacitor voltages and disturb the output waveforms. In this paper, principles for converter operation with high torque in the whole speed range are investigated. Experimental results from a down-scaled 12-kVA prototype converter running a loaded motor at various speeds between standstill and the rated speed are also provided.

Modular multilevel converters (M2Cs) are shown to have a great potential in the area of medium-voltage drives. Low-distortion output quantities, combined with low average switching frequencies for the semiconductor devices create the ideal combination for very high-efficiency drives, both from an electric motor and an inverter point of view. With M2Cs the output voltage has such a low harmonic content that high-power motors can be operated without any derating. However, the large number of devices and the existence of capacitors that have to conduct the fundamental frequency current, requires more complex converter control techniques than its two-level counterpart. Special care needs to be taken under starting and operation with low frequency, where the low-frequency current may cause significant unbalance between the submodule capacitor voltages, disturb the output waveforms, and eventually cause the converter to trip. In this paper, principles for converter operation with high torque in the whole speed range, from standstill to rated speed will be investigated. The converter-control method utilizes estimation of the capacitor voltage variation, based on equations describing steady-state conditions. Experimental results from a down-scaled 12 kVA prototype converter running a loaded motor from zero up to the rated speed are provided in the paper.

The stacked polyphase bridges (SPB) converter topology is investigated in the presentthesis as a fault-tolerant choice for permanent-magnet synchronous reluctance (PMSyn-Rel) motor control. Integrated motor drive systems are studied as they offer great benefitsfor propulsion applications. Moreover, the importance of a modular topology, like theSPB, for an electric powertrain is discussed. The latter consists of a number of seriesconnected, 3-phase 2-level inverter submodules that supply separate sets of windings ina multi-star motor. The specifications of building a four-board SPB setup are examined,while the challenges of an active voltage balancing controller are analyzed. The designprocess is explained step-by-step and the final printed circuit boards (PCBs) are presented.Furthermore, the significance of low electromagnetic interference design for a converterthat requires high speed communication is highlighted. Finally, the prototype is testedthoroughly and the expected fault-tolerant capabilities are validated on a PMSynRel motor.

In this master thesis, a sensorless system has been developed for an induction machine in railway applications. The approach which was taken in this master thesis is a consequence of the expectations of the company: investigate thepossibilities of having the current generation of Alstom's drive system working without use of a speed sensor. Some tests have thus been performed in order to validate parts of this statement. However, because of the substantial time that would require an exhaustive investigation and a complete development, from simulation to tests on a real train, it has been decided to focus more on selecting a method relevant for Alstom's specications, and simulate it in order to identify issues. Thus, from an extensive literature search based on books and articles considered as references, several methods has been investigated and one have been selected for simulation. Two set-ups were used for simulations; one developed in Matlab and the other one using real-time simulators used by Alstom to test and validate its hardware and control software. From these tests, it was showed that for an accurate control, it is necessary to estimate, jointly with the speed, main motor parameters. But these motor parameters can hardly be all estimated at the same time and a strategy in order to estimate eciently these other parameters is proposed. Issues related to a practical implementation are also investigated and some conclusions are drawn for an implementation of a sensorless system in an Alstom train.

Measures to improve the modeling of steel-sheet laminations in electrical machine design tools are studied. Only the magnetic properties, namely, the permeability (BH curves) and the iron losses are addressed. The sensitivity of these properties upon dimensional, directional (anisotropy) and excitation variations, as well as upon the electrical machines manufacturing steps is evaluated. The studied electrical machines manufacturing step are: guillotine, laser-cutting, welding, pressing and punching. The properties of the delivered lamination coils and the various associated loss figures are also statistically benchmarked. The focus is on finding guidelines for incorporation of these sensitivities in design tools when needed. For this purpose, 5 Hz to 10 kHz tests are conducted on Epstein strips, on L-shaped segments and on standard stator laminations. Two different steel grades are studied. It is shown that the lateral dimension, anisotropy and welding influences are much more pronounced then those for punching, pressing and laser cutting and hence, need to be addressed in design tools. It is found that the commonly used Epstein test results of the coils are slightly inferior to the mean loss value of the online-testing (losses that are measured at every 1 m of the entire coil length). It is also noted that, the delivered laminations are almost always better than those ordered and the losses for each individual lamination coil are nearly constant.

Iron loss calculation in AC machines has aroused great interest over the years, in an attempt to make electrical machines more efficient. However, different iron loss models yield different results for the same machine, owing to their individual limitations. This element of uncertainty associated with obtained values is further enhanced in cases where knowledge of the flux density’s behavior is very elementary.  This thesis gives an overview of different iron loss models and applies them to a 15 MW induction motor equipped with fractional conductor winding. The aim is to analyze the results acquired using various models and determine the suitability of the investigated models. The two main categories of iron loss models implemented are the Steinmetz models and the loss separation model. Iron loss results confirmed that the basic Steinmetz equation is only suitable for sinusoidal flux density waveforms and its accuracy diminishes as the waveform becomes increasingly non-sinusoidal. Furthermore, parts of the stator where the magnetic field is most rotational were identified as the roots of stator teeth. However, rotational magnetic fields were found to have a small effect on stator yoke iron losses.

This paper put emphasis on change agents within the universities and how local initiatives can be systematically approached and ramped up. Rooted in the challenges and constraints that have been addressed in past educational program initiatives, the case consists of specific focus areas to leverage impact. Universities continuously strives to provide the best conditions for an inspiring and prosperous learning environment, and to provide educational programs with teaching of excellent educational quality. KTH is no exception and therefore the university management has initiated a pedagogical program starting in 2014. One of the first thing initiated within the framework of this pedagogical program is the creation of a group of 24 pedagogical developers.

The focus for the pedagogical developers is to facilitate the opportunities for KTHs faculty to work together and create consensus on educational development in different teaching teams. This paper presents the University's pedagogical developers' initiative as a whole and how this has been outlined in detail to reach specific redesign targets. The School of Industrial Engineering and Management pedagogical group consists of five practicing teachers that besides this new role also engage heavily in various courses of the School's departments. Since the pedagogical initiative is aligned with several important CDIO aspects, e.g. the learning environment, formats of formative feedback, assessment and examination there is also importance to reassure this in the existing Master level programs.

At KTH the five-year comprehensive Master of Science in Engineering programs concern distinct vocational educations in which the CDIO aspects are very important. At the same time the programs has been divided in a basic level (B.Sc. in Engineering) of three years and a advanced level (M.Sc.) of two years. This has for instance made it harder to align the progression between first cycle level and second cycle level regarding for instance the CDIO efforts (e.g. oral and written communication, teamwork). This paper will therefore discuss and enhance how the pedagogical programme, we as pedagogical developers, can support and strengthen the initiation and implementation of the CDIO aspects in the education.

The tapped-inductor buck (TI-buck) converter has shown to be a suitable solution for auxiliary power supply for modular multilevel converter submodules. Such application features a large step-down voltage conversion, made at relatively low output power. This converter operates in discontinuous conduction mode with zero voltage switching of the high-voltage valve. This paper treats the dynamic behaviour of the aforementioned converter. First, an average output current model of the converter is developed and a small signal model is obtained. Then, a closed-loop output voltage control, which uses the switching frequency as control variable, is designed and implemented using a microcontroller. Measurements on a down-scaled prototype shows that the control system provides a well-controlled average output voltage, which is stable under significant load variation. Finally, a solution for implementing the start-up of the converter is presented and tested.

In a voltage source converter (VSC) based HVDC system, the modulation scheme used is an important factor in achieving a desired harmonic performance with allowable semiconductor losses. In this paper, the use of selective Harmonic Elimination Pulse Width Modulation (HEPWM) for a three-level Neutral Point Clamped (NPC) converter in VSC based HVDC application will be discussed. Steady state performance of theconverter system in terms of harmonics and losses will be evaluated using MATLAB and PSCAD/EMTDC simulation. Simulation results show a 37% improvement in total semiconductor loss with better harmonic performance by using the three-level solution compared to the two-level solution with the stated modulation scheme. 

In this paper a full soft-switching high step-up DC-DC converter is introduced as an alternative approach to module integrated converters for photovoltaic applications. The presented operation principle and key equations can be used as design guidelines for component and parameter estimation in practical applications. The proposed DC-DC converter was verified by help of simulations and experiments. Power loss analysis based on the semiconductor datasheet values showed that the converter tends to achieve an efficiency of 92.8% at the maximum power point.

In this master thesis, the magnetic properties of SiFe laminations after cutting and welding

are studied. The permeability and the iron loss density are investigated since they are

critical characteristics for the performance of electrical machines. The magnetic measurements

are conducted on an Epstein frame for sinusoidal variations of the magnetic ux

density at frequencies of 50, 100 and 200 Hz, according to IEC 404-2. Mechanical cutting

with guillotine and cutting by means of ber and CO2 laser are performed. The inuence

of the ber laser settings is also investigated. Especially the assisting gas pressure and

the power, speed and frequency of the laser beam are considered.

In order to increase the cutting e ect, the specimens include Epstein strips with 1,

2 and 3 additional cutting edges along their length. It is found that mechanical cutting

degrades the magnetic properties of the material less than laser cutting. For 1.8% Si

laminations, mechanical cutting causes up to 35% higher iron loss density and 63% lower

permeability, compared to standard Epstein strips (30 mm wide). The corresponding

degradation for laser cut laminations is 65% iron loss density increase and 65% permeability

drop. Material of lower thickness but with the same Si-content shows lower

magnetic deterioration. Additionally, laser cutting with high-power/high-speed characteristics

leads to the best magnetic characteristics among 15 laser settings. High speed

settings have positive impact on productivity, since the cutting time decreases.

The inuence of welding is investigated by means of Epstein measurements. The test

specimens include strips with 1, 3, 5 and 10 welding points. Experiments show an iron

loss increase up to 50% with a corresponding 62% reduction in the permeability.

A model that incorporates the cutting e ect is developed and implemented in a FEMbased

motor design software. Simulations are made for a reference induction motor.

The results indicate a 30% increase in the iron losses compared to a model that does

not consider the cutting e ect. In case of laser cut core laminations, this increase reaches

50%. The degradation prole considers also the deteriorated magnetizing properties. This

leads to increased nominal current up to 1.7% for mechanically cut laminations and 3.4%

for laser cut la

Accurate knowledge of winding temperature is critical for the control, protection, and real-time monitoring of high-performance electric machines. Lumped parameter and finite element analyses can be used to model thermal stress, but both have drawbacks in applications where fast estimates of local temperature distributions are necessary. To overcome this, a closed-form solution for the steady-state stator temperature distribution over one slot pitch in a radial air gap electric machine is presented. Machine symmetry and material thermal properties are used to create a representative layer model in which a solution to Laplace's equation is developed. In addition to lumped parameter and three-dimensional (3D) finite element models, the method is verified through experimental results. Analytical model winding temperature predictions are within about 2.5% of finite element model predictions. Estimates of stator slot, tooth, and end-winding temperatures are within 7% of experimental measurements. The results are shown to have value for parametric machine design and protection.

A closed-form analytical solution for temperature distribution in a rectangular slot structure is presented. The methodology models windings, lamination and slot insulation as three distinct regions and formulates a boundary value problem for each region. The solutions follow Poisson's and Laplace's equations. The closed-form nature of the solution allows it to be used for electric machine design and protection. The model is compared with a lumped parameter model, and verified using three-dimensional finite element analysis (FEA) and experimental measurements on a linear induction machine. Slot temperature predictions using the proposed model are within 3.2% of FEA. Model temperature estimates in the slot, lamination, and end winding are all within 6.7% of experimental measurements. This temperature estimation accuracy suggests that the proposed approach is suitable for applications where fast and flexible thermal modeling is necessary.

The research for electric vehicles has been growing during last years and the development of electric drive trains can be considered a main challenge. This thesis presents the electric drive train of the research concept vehicle (RCV) 2013, with particular focus on electric machines, motor controllers, and the communication system. In the first part of this thesis, the electric drive train configuration and components are described. In-wheel motors are proposed which is a permanent magnet synchronous machine (PMSM). This technology allows the use of autonomous corner modules (ACM) increasing the quality and safety of the system. Each of the four in-wheel motors has a controller enabling the use of torque or speed control mode. Furthermore, a dSPACE unit provides the total control of the system by CAN bus. Additionally, the dSPACE ControlDesk interface used to control the drive system is presented. In the second part, the heat sink of the AC Drive is investigated by measurements and analytical calculations. Furthermore, the motor temperature at different loads is also presented and discussed. Finally, the efficiency of an in-wheel motor (PRA 230) is studied. Also the efficiency of the motor controller is estimated and discussed.

Recently, with growing application of wind power, the system based on the doubly fedinduction generator (DFIG) has become the one of the most popular concepts. Theproblem of connecting to the grid is also gradually revealed. As an effective solution toconnect offshore wind farm, VSC-HVDC line is the most suitable choice for stabilityreasons. However, there are possibilities that the converter of a VSC-HVDC link canadversely interact with the wind turbine and generate poorly damped sub-synchronousoscillations. Therefore, this master thesis will derive the linear model of a single DFIG aswell as the linear model of several DFIGs connecting to a VSC-HVDC link. For thelinearization method, the Jacobian transfer matrix modeling method will be explainedand adopted. The frequency response and time-domain response comparison betweenthe linear model and the identical system in PSCAD will be presented for validation.

In the first year of the studies in the five-yearMaster of Science programme in Electrical Engineering at KTH,there is a course named “Electro project”. The students that takethis compulsory course choose between several projects and thispaper describes a newly invented project called Free Fall, whichpractically and theoretically demonstrates in a simple andinteresting way important electrical engineering equations forthe students. The students study, calculate and build a gondolathat first accelerates when it falls freely along a tower from itstop and then decelerate with eddy current brakes. The studentslearn apart from modelling of physical phenomena, mechanicalconstruction, validation, measurements and collaboration withinthe group as well as with the laboratory staff.

The growing need for simpler and cheaper manufacturing process has led to the research into

fractional-slot concentrated-wound (FSCW) motors. This concept has been widely investigated

for surface-mounted permanent magnet (SMPM) machines. This thesis studies the same

concept applied for synchronous reluctance machines (SynRM).

In this thesis, a FSCW, 15 kW, 4-pole, Permanent-Magnet-Assisted Synchronous Reluctance

Machine (PMaSynRM) is designed and optimized using finite element method (FEM) based

simulations for a set of given technical specifications. Initially, the existing synchronous machine

topologies are investigated and later two novel motor designs are introduced and optimized,

namely, a FSCW-SynRM and a FSCW-PMaSynRM with ferrite magnets. Moreover, the

influence of replacing ferrite material with Neodymium Iron Boron (NdFeB) in the FSCWPMaSynRM

is analyzed. Detailed investigations are performed in order to compare the impact

of material at different temperatures. Variation of the torque-speed capabilities with temperature

and a safe operating temperature range where the magnets are not demagnetized are

identified. The variation of overload capability with temperature is also studied. Finally, a

comparison between the new proposed designs and other existing standard design topologies

is performed.

It was found that FSCW-SynRM present lower efficiency, power factor and higher torque ripple

than DW-SynRM. However when ferrite magnets are inserted in FSCW-PMaSynRM the

efficiency, power factor and the flux-weakening capability exceed the values of the DW-SynRM.

Moreover, by using NdFeB instead of ferrite in FSCW-PMaSynRM, the torque ripple, the fluxweakening

capability and the overload capability improve and a wider safe temperature range

for no demagnetization is achievable. Finally, it is found that DW-PMaSynRM with ferrite

presents the same efficiency level as FSCW-PMaSynRM with ferrite, but higher power factor

and lower torque ripple. However FSCW-PMaSynRM with ferrite have other advantages, such

as shorter end-winding length, good fault-tolerant capability and simpler and cheaper

manufacturing process.

Cyber Physical Energy Systems (CPES) development requires the combination of distributed intelligence to fulfill the future complex tasks and reach the increase the energy demands. Electrical Industrial Systems (EIS) are in continuous evolving integrating new technologies allowing to a better performance and increase the efficiency. This paper applies the consensus protocol for Multi-Agent Systems (MAS) to control the speed of multiple induction motors. In this paper, the behaviour of the system under different disturbances and scenarios has been simulated, thus, confirming the suitability and simplicity of this method for coordinating the control actions.

This paper presents a steady state analysis of the operation modes of the quasi-Z-source (qZS) derived push-pull DC/DC converter topology. It was derived by the combination of the qZS network and coupled inductors. The output stage of the converter consists of a diode bridge rectifier and an LC-filter. This topology provides a wide regulation range of the input voltage and galvanic isolation. These features fit the requirements for the integration systems of renewable energy sources, such as PV panels, variable speed wind turbines, and fuel cells. A converter can operate in continuous (CCM) and discontinuous conduction mode (DCM). Switching period is divided into four and six intervals for CCM and DCM, respectively. Equivalent circuits and analytical expressions for each interval are presented. The DC gain factor for each mode is derived. To simplify our analysis, coupled inductors were substituted with a model that consists of an ideal transformer and magnetizing inductance. Leakage inductances are neglected because the coupling coefficient in this topology should be close to unity. In DCM the converter operation depends on the active duty cycle and the duty cycle of the zero current condition. Two solutions are possible for the DC gain factor in DCM. It is theoretically impossible to achieve the unity DC gain factor in DCM if the turns ratio of coupled inductors is equal to or more than one. The proposed topology was simulated with PSIM software in two operating points. Experimental verification proves our theoretical and simulation results.

Nowadays, it is problematic to connect only one power semiconductor switch directly to the grid due to the high voltage range. In order to solve this difficulty, a new type of power converter has been introduced as a solution in high power applications. Multilevel Converters use high speed switching components, avoiding the problem of linking them directly to the grid by connecting single devices among multiple DC levels. Differents Multilevel topologies have been developed in the last few years. Multilevel Converters are more complex to modulate than the two level traditional converters because of the number of switching alternatives that are available. The latest and most promising such topology for high power applications is the Modular Multilevel Converter (M2C). Several control and modulation methods have been suggested for this topology. The aim of this master thesis project is to deeply investigate and evaluate one of them, based on a carrier phase-shifted Pulse Width Modulation (PWM) techniques. Four different control topologies using phase shift PWM techniques on M2C are studied and explored in this work. These topologies include the following loops of control: Averaging Control based on the currents inside the converter, Individual Balancing Control based on the output current and capacitors voltages, and Arm Balancing Control based on the voltage difference between the arms of the converter. The operation principle of an M2C is presented. This project proposes a switching frequency that meets the two required criteria: low enough to maintain cost feasibility, and high enough to reach a harmonic performance target. Additionally, this work proposes an analytic expression for the output voltage spectrum of the converter, which enables prediction of harmonic performance. Three distinct simulations were performed each one using different control topologies and switching frequencies. The first controller simulated took into account the Averaging Control topology, based on the circulating current. Within this topology both individual and arm balancing techniques are also explored. A second controller is also simulated using Averaging Control, based on the arms currents, as well as the other control loops. For the last case an Averaging Control, based on the arm currents, without the Arm Balancing is simulated. The results of each simulation are discussed and compared. Finally, these topologies are implemented and verified experimentally on a 10-KVA M2C prototype. The experiments are performed using only one phase and 11-level modulation methods. The controller efficiency is studied and verified through step response analysis.

Parallel connection of silicon carbide power modules is a possible solution in order to reach higher current ratings. Nevertheless, it must be done appropriately to ensure a feasible operation of the parallel-connected power modules. High switching speeds are desired in order to achieve high efficiencies in medium and high-power applications but parasitic elements may give rise to a non-uniform current sharing during turn-on and turn-off, leading to non-uniformly distributed switching losses. This paper presents the switching performance of parallel-connected power modules populated with several silicon carbide metal-oxide-semiconductor field-effect-transistors chips. It is experimentally shown that turn-on and turn-off switching times of approximately 50 ns and 100 ns, respectively, can be reached, while an acceptably uniform transient current sharing is obtained. Moreover, based on the obtained results, an efficiency of approximately 99.35% for a three-phase converter rated at 312 kVA with a switching frequency of 20 kHz can be estimated.