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The instability phenomena caused by converter–grid interactions can be prevented by designing controllers with adequate stability margins. Yet, the multiple-input–multiple-output (MIMO) dynamics of grid-connected voltage-source converters (VSCs) complicate the stability analysis for the controller design. To tackle this challenge, this article presents a loop-at-a-time stability analysis for grid-connected VSCs, which not only shows close correlations with the generalized Nyquist criterion for MIMO systems but also enables to quantify the stability margins of individual closed loops. Moreover, the interactions between the closed loops can be analyzed. Test cases with numerical sensitivity analysis, simulations, and field measurements of a converter validate the theory., QC 20221005

Modular multilevel converters (MMCs) can be configured to perform ac/ac conversion, which makes them suitable as railway power supplies. In this paper, a hierarchical control scheme for ac/ac MMCs for railway power supplies is devised and evaluated, considering the requirements and the operating conditions specific to this application. Furthermore, admittance models of the ac/ac MMC are developed, showing how the suggested hierarchical control scheme affects the three-phase and the single-phase side admittances of the converter. These models allow for analyzing the stability of the interconnected system using the impedance-based stability criterion and the passivity-based stability assessment. Finally, the findings presented in this paper are validated experimentally, using a down-scaled MMC., Not a duplicate with DiVA 1302284QC 20200217

Premise– Internationalisation has a number of positive effects on research and higher education.– Today, researchers at higher education institutions (HEIs) in Sweden collaborate to a greater extent with a larger number of countries than earlier.– Collaborations with researchers from countries whose scientific and political systems, culture and laws differ significantly from ours entail other and sometimes greater challenges.– There is an increased need for informed assessments of the contents and consequences of international collaborations before, during and after a project.– This document has been drawn up in collaboration between STINT, Karolinska Institutet, KTH Royal Institute of Technology and Lund University. The background to this work is the growing debate on values and ethical considerations in international collaborations in connection to HEIs in several countries.– The document is intended to serve as support for reflection and as the basis for discussion of strategic decisions on internationalisation. The purpose is to aid researchers, research directors, department heads, and university administration in assessing collaborations and structuring discussions on how the HEI, department or research group should approach international collaboration., QC 20240109

Energy storages have caught the attention of transportation community r the past several years. Rsecent developments in hybrid and plug-in ectric vehicles together with novel concepts in transportation such as ectric highways are the reasons for raising the role of energy orages in transportation to such a significant level. Performance mands for energy storage solutions vary significantly from one ansportation application to the other, making it difficult for the ientific community to converge to a single energy storage solution at caters all. This paper reviews the key performance demands of the jor transportation applications. It also investigates the aracteristics of emerging energy storage solutions and assess their itability for those reviewed transportation applications., QC 20190405

Modular multilevel converters (MMCs) can be configured to perform ac/ac conversion, which makes them suitable as railway power supplies. In this paper, a hierarchical control scheme for ac/ac MMCs for railway power supplies is devised and evaluated, considering the requirements and the operating conditions specific to this application. Furthermore, admittance models of the ac/ac MMC are developed, showing how the suggested hierarchical control scheme affects the three-phase and the single-phase side admittances of the converter. These models allow for analyzing the stability of the interconnected system using the impedance-based stability criterion and the passivity-based stability assessment. Finally, the findings presented in this paper are validated experimentally, using a down-scaled MMC., QC 20190404

Five folding box boards made on the same paperboard machine have been analyzed. The paperboards were from the same product series but had different grammage (235, 255, 270, 315, 340 g/m2) and different bending stiffness. The paperboards are normally used to make packages, and since the bending stiffness and grammage varies the packages performance will be different. Finite element simulations can be used to predict these differences. However, the stiffness and strength properties then need to be known. For efficient determination of the three-dimensional properties in MD, CD and ZD, it is proposed that the whole paperboard should be characterized with the following tests: in-plane tension, ZD tension, shear strength profiles and two-point bending. The stiffness and strength properties have with the proposed setups been determined at different relative humidity (20, 50, 70 and 90 % RH), and the mechanical properties have been evaluated as function of moisture ratio. The results showed a linear relation between mechanical properties and moisture ratio for each paperboard. The data was then normalized with data for the standard climate (50 % RH) and investigated as a function of moisture ratio. The results indicated that the normalized mechanical properties for all paperboards coincided along one single line and could therefore be expressed as a linear function of moisture ratio and two constants. Consequently, the study indicates that it is possible to obtain the mechanical properties of a paperboard, by knowing the structural properties for the preferred level of RH and the mechanical property for the standard climate (50 % RH and 23 °C)., QC 20210914Not duplicate with diva2:1378555 diva2:1426690 diva2:1593858

This paper applies a control method based on current control and sum-capacitor-voltage estimation to the direct ac/ac modular multilevel converter. As capacitor voltages are estimated, their measurements are not needed in the high-level control, which simplifies the communication between the main controller and the submodules of the converter. The stability of the internal dynamics of the converter, using the aforementioned control method, is studied using Lyapunov stability theory, proving that the system is globally asymptotically stable. The behavior of the converter is simulated focusing on three-phase 50 Hz to single-phase 16 (2)/(3) Hz conversion, which is typical for railway power supply systems of some European countries. Simulation results are in agreement with the expected behavior of the converter, both in steady-state and dynamic situations., QC 20161209

The current from an electrical arc furnace (EAF) in a steel plant is highly unsymmetrical, i.e. it comprises a large amount of negative sequence. This causes problems with the voltage stability in the capacitor submodules for a static synchronous compensator (STATCOM), when implemented using the modular multilevel converter (M2C) concept. In order to counteract the uncontrollability of the capacitor voltage, a common practice is to circulate a zero-sequence fundamental current or a direct current. The method brings the benefit of re-balancing the active power among the individual phases while it requires over-rating of the valve current or dc voltage of the STATCOM. This would in practice give a more expensive design. This paper presents a benchmark of various M2C-based STATCOM designs considering the valve overrating. The STATCOM topologies will be benchmarked with the EAF modeled for all possible combinations of positive-and negative-sequence current. The rating comparison considering the dc capacitor ripple and zero-sequence harmonic injection are discussed., QC 20161209

The paper describes the design of a single-sided linear induction motor (SLIM) intended for an Articulated Funiculator (AF). A procedure is proposed where first an analytical design is carried out to obtain a preliminary geometry. The analytical design is based on approximate equivalent circuits under constrained conditions. In the second step a two-dimensional finite element method (2D-FEM) analysis is performed to validate the accuracy of the analytical models. The modification and optimization are conducted to acquire the best design., QC 20161213

A continuum material model is developed for simulating the mechanical response of high-density cellulose-based materials subjected to stationary and transient loading. The model is formulated in an infinitesimal strain framework, where the total strain is decomposed into elastic and plastic parts. The model adopts a standard linear viscoelastic solid model expressed in terms of Boltzmann hereditary integral form, which is coupled to a rate-dependent viscoplastic formulation to describe the irreversible plastic part of the overall strain. An anisotropic hardening law with a kinematic effect is particularly adopted in order to capture the complex stress-strain hysteresis typically observed in polymeric materials. In addition, the present model accounts for the effects of material densification associated with through-thickness compression, which are captured using an exponential law typically applied in the continuum description of elasticity in porous media. Material parameters used in the present model are calibrated to the experimental data for high-density (press)boards. The experimental characterization procedures as well as the calibration of the parameters are highlighted. The results of the model simulations are systematically analyzed and validated against the corresponding experimental data. The comparisons show that the predictions of the present model are in very good agreement with the experimental observations for both stationary and transient load cases., QC 20151027

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., QC 20150930

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., QC 20150930

With increasing railway traffic, the demand for electrical power increases. However, railway power systems are often weak causing high transmission losses and large voltage drops. One possible method for strengthening the railway power supply system is to implement a High Voltage Direct Current (HVDC) feeder in parallel to the Overhead Contact Line (OCL). The HVDC feeder is connected to the OCL by converters. This paper describes different properties and characteristics of such an HVDC feeder solution. An AC/DC unified Optimal Power Flow (OPF) model and unit commitment is used to obtain proper control of the converters. The non-linear load flow and converter loss equations, and the binary variables for the unit commitment, lead to an optimization problem of Mixed Integer Non- Linear Programming (MINLP) type. The optimization problem is formulated in the software GAMS, and is solved by the solver BONMIN. In each case, the objective has been to minimize the total active power losses., QC 20150608

Railway mass transit systems like subways play a fundamental role in the concept of sustainable cities. In these systems, the amount of passengers strongly fluctuates along the day. Hence, in order to provide a proper service without incurring disproportionate energy consumption, operation at different traffic densities is required. The majority of underground systems are DC-electrified. Standard DC voltages in railway systems are low for historical and safety reasons. In the rush hours, the large number of trains demanding power of the system may lead to overloaded substations and voltage dips. This problem is partially mitigated by means of substation-transformer tap regulation, which allows operators to increase the no-load voltage. High no-load voltage has a beneficial effect at all trafficdensity scenarios in terms of transmission losses. However, at the same time it effectively reduces the system's capacity to absorb regenerated energy, which may lead to inefficient energy consumption figures during off-peak hours. In this paper, the sensitivity of system energy consumption to no-load voltage has been analyzed. Several traffic-density scenarios in a case-study system are explored. As a result, a scheduled no-load voltage scheme is proposed for the operation of the system. This operation strategy improves energy efficiency without incurring a high investment cost. The only costs related to this proposed method are the costs of wear-andtear in tap-changers. In case there are devices such as energy storage systems installed in the system, there would be additional operation costs related to a simultaneous update of the voltage limits for their operation., QC 20150610

In recent years, interest has risen in using electrified rail operations as an environmentally friendly and efficient way of producing passenger and freight services. New high-speed lines are being designed and built throughout Europe and China, and there is also an increasing interest in high speed rail services in southeast Asia and the United States. To meet this demand, the electrification of railways has rendered increased interest both for new lines and for upgrading existing systems to higher power capacity. Historically, the first rail electrifications were designed as low-voltage dc systems, where the dc voltage was typically generated by dc generators. The limitations in the voltage capability of dc machines and related equipment limited the voltage considerably. More than 100 years later, we still have dc traction systems generated by rectifiers connected to the three-phase grid with voltages in the range of 650 or 750 V up to 3 kV. Because of large voltage drops and excessive losses, it is required that the connecting points to the threephase system are located with a relatively short mutual distance., QC 20170309

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., QC 20130610. Updated from submitted to published.

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., QC 20120926

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., QC 20130813

QC 20131016

QC 20120926

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., QC 20130218

In this paper, we present a novel maintenance concept based on condition monitoring and dynamic maintenance packaging, by showing how to connect the information flow from low-level sensors to high-level operations and planning under uncertainty. Today, condition-based maintenance systems are focused on data collection and custom-made rule based systems for data analysis. In many cases, the focus is on measuring "everything" without considering how to use the measurements. In addition, the measurements are often noisy and the future is unpredictable which adds a lot of uncertainty. As a consequence, maintenance is often planned in advance and not replanned when new condition data is available. This often reduces the benefits of condition monitoring. The concept is based on the combination of robust, dynamically adapted maintenance optimization and statistical data analysis where the uncertainty is considered. This approach ties together low-level data acquisition and high-level planning and optimization. The concept has been illustrated in a context of rail vehicle maintenance, where measurements of brake pad and pantograph contact strip wear is used to predict the near future condition, and plan the maintenance activities., QC 20110201

A central part of a medium frequency conversion system is the transformer. A medium frequency transformer for railway propulsion may be stressed beyond 30kV with voltage slopes around 50kV/mu s depending on topology while the geometric dimensions are small. A 170kVA transformer prototype operated at 4 kHz has been developed and tested. A transient thermal model of the transformer is developed and verified experimentally. The transient thermal behavior is important in propulsion applications where the conversion system is often heavily overloaded. Simulations indicate that the overloading capability of the transformer is comparable to a conventional traction transformer. Experimental results are in good agreement with the model., QC 20110304

QC 20120216

Experiences from the Swedish hybrid locomotive (T43H) are presented. The locomotive has a series hybrid propulsion system with a comparatively small diesel engine and a large battery pack. Original layout was a traditional diesel-electric power train. Indications from switch operations show fuel savings of 37 to 50 % with the hybrid locomotive. The Swedish experiences are thus in parity with North American experiences showing savings between 30 and 80 %. The evaluation showed that hybrid locomotives in switch duty have significant advantages compared to conventional diesel locomotives in terms of reducing fuel consumption and improving environmental performance., QC 20131011

Being a modern electrical engineer does not only require state of the art skills in areas such as transfer and processing of information, electronics, systems engineering, and biomedical electrical engineering; it also requires generic engineering skills such as oral and written communication, team building, interpersonal skills, and the ability to take the responsibility of solving assigned tasks. In the development of a new electrical engineering curriculum, it was considered a challenge to introduce these generic engineering skills in a more applicable way. The approach described in this paper has been very successful in implementing and maintaining a challenging first year project based engineering course with a high level of training of generic engineering skills., QC 20150624

This paper describes a gate control method where an IGBT is controlled in its linear region by means of closed loop control in order to regulate the voltage slope during turn-on and to clamp the voltage of an anti-parallel diode in a source commutated converter. Controlling the voltage slope may be necessary in a high voltage converter to avoid emission of EMI or to avoid triggering oscillations which may cause insulation failure. Controlling the switching trajectory without influence from the device characteristics is important where series-connection is necessary to increase the overall blocking voltage. The control method has been verified by means of a prototype., QC 20150624

This paper describes an ac/dc converter system consisting of a voltage-source converter (VSC) with purely capacitive snubbers and a two-phase by three-phase cycloconverter, connected via a medium-frequency (MF) transformer. By alternately commutating the two converters, it is possible to achieve beneficial switching conditions for all semiconductor devices. A commutation and modulation algorithm is described, which allows for pulsewidth-modulation control of the output voltage while maintaining soft switching. Low-load operation of the converter is a potential difficulty because the load current may be insufficient for recharging the snubber capacitors of the VSC. However, if the cycloconverter is used to momentarily short circuit the transformer, a quasi-resonant commutation mode of the VSC can be achieved, making a fast and soft commutation of the VSC down to zero load possible, without an auxiliary circuit. Furthermore, the design and operation of a 40-kVA prototype converter system are described. The experimental results from the prototype clearly show the practical feasibility of the studied concept., 39th Annual Meeting of the IEEE-Industry-Applications-Society Location: Seattle, WA Date: OCT 03-07, 2004

A conceptual study for a Swedish dual-mode locomotive is presented. The primary energy sources are AC electricity from an overhead catenary system or a diesel-electric generator system. The study focuses on analysing how freight operations and locomotive utilisation can be improved and how it can affect environmental aspects of freight operations. Based on project experiences and operative demands, a system proposal has been presented. The rated power output is considerably higher in electric mode compared to diesel mode. The reason for this being higher power demand on electrified main lines compared to branch lines and yards lacking overhead catenary where the power need is lower. The starting tractive effort will however be the same regardless of electric mode or diesel mode, making it possible to haul the same train weight in both propulsion modes. The study has shown that dual-mode locomotives for Swedish freight operations are most interesting in order to improve operations, degree of utilisation and environmental performance., QC 20110127

A common issue in electric railway operation is the wearofthepantographcontact strip. This is especially pronounced with ice or rime frost on the overhead line that causes arcing between the contact strip and the contact wire. To monitor the contact strip it is possible to utilise the fact that arcing between the contact strip and the overhead line generates a DC component in the AC locomotive current. The paper presents a method forconditionbased maintenanceofthe contact strip. The wearofthe contact strip is predicted bymonitoringthe running distanceofthepantographas well as the DC componentofthe locomotive current. To evaluate the method, measurements have been carried out on a Swedish Re locomotive during winterconditions., QC 20150701, TIMM

QC 20150209

QC 20120117

The migration to line side power conversion topologies comprising a medium frequency transformer in AC-fed propulsion systems may reduce size and weight of the conversion system. The switching frequency is a crucial factor affecting the viability of such a conversion system. An increased switching frequency of a mutually commutated conversion system comprising a medium frequency transformer employing soft switching is verified by single pulse measurements. A test bench to characterize standard IGBT modules under soft switching has been build and loss measurements on a 3.3 kV IGBT module are presented for hard and soft switching. A loss model of the investigated topology is developed and the maximum switching frequency for a converter equipped with the investigated module is estimated for hard and soft switching., QC 20100723

QC 20130619

Design considerations are presented for a medium frequency transformer in a line side power conversion system for electric railway traction. The system employs soft commutation in order to reduce switching losses and allow for high operating frequency of the transformer. Two different designs are evaluated; The first one is dry insulated and the second one is liquid-immersed. Calculations indicate that a 1 MVA transformer operating at 4 kHz could be realized with an active weight of below 150 kg for both dry and liquid-immersed transformer concepts. The transformers are modeled magnetically, electrically and thermally, and a geometric optimization procedure based on a cost function is applied to achieve an optimum design., QC 20100723

An ac/dc converter topology comprises a voltage source converter (VSC) with capacitive snubbers and a 2-phase by 3-phase cycloconverter, connected by an MF transformer. By alternately commutating the two converters it is possible to achieve soft switching conditions for all of the semiconductor elements. Furthermore, it is shown that resonant commutation of the voltage source converter is possible by utilizing the cycloconverter in order to momentarily short-circuit the transformer terminals. This allows for soft commutation of the VSC down to zero load. Furthermore, the design and operation of a 40 kVA prototype converter system based on the studied concept is described., QC 20101014. 39th Annual Meeting of the IEEE-Industry-Applications-Society. Seattle, WA. OCT 03-07, 2004

QC 20131127