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  • 1.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    AC cables strengthening low frequency AC railway with purely active power loads2018In: 2018 IEEE Power & Energy Society General Meeting (PESGM), IEEE, 2018, , p. 5Conference paper (Refereed)
    Abstract [en]

    Converter-fed railway power systems traditionally use high voltage AC overhead transmission lines in the railway grid for increased loadability. An increased resistance to overhead high voltage AC transmission lines, may motivate cables as one alternative for the future. The focus of this paper is to compare a non-strengthened system with two cable solutions for comparing loadability, voltages, and reactive powers for different levels of load scaling. The studies confirmed that the obstacle of reactive power produced in lowly utilized cables is, even if less significant for low frequency AC, still present. A simplified load model is used representing trains with VSC-converters and three-phase motors as purely active loads regardless of motoring or regenerating. A previous study has been done on thyristor-based trains. The voltage levels while regenerating are higher than in the thyristor-train case study. Besides that the loadability for motoring and regenerating is higher with modern VSC-trains.

  • 2.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    AC cables strengthening railway low frequency AC power supply systems: a deepened study2017Conference paper (Refereed)
    Abstract [en]

    n railway power supply systems using ACfrequencies lower than the public grids of 50/60 Hz,high voltage AC overhead transmission lines in railwaygrid frequency are used as one measure of strengthen-ing the systems. An increased resistance to overheadhigh voltage AC transmission lines, may motivate ca-bles for future railway power systems.With the frequency of 50/60 Hz, reactive powerproduced in lowly utilized cables imposes an obstacle.For low frequency AC, this issue is less significant.Moreover, in converter-fed railways, no reactive powerwill leak into the feeding public grid.This paper studies AC cables in low-frequency ACrailway. Two reinforcement cable solutions are com-pared with no reinforcement. A simplified load model oftrains, with thyristor bridges and DC motors, is used

  • 3.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Administrativ bilaga till Slutrapport2017Report (Other academic)
  • 4.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Considering risks in power system operationand the consequences of different acceptedrisk levels2017Report (Other academic)
  • 5.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Optimal drift och dimensionering avbanmatningssystem: Slutrapport Elektraprojekt2016Report (Other academic)
  • 6.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Risker i drift av elkraftsystem och konsekvenser av olika accepterade risknivåer: Svensk kortversion2017Report (Other academic)
  • 7.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Slutrapport2017Report (Other academic)
  • 8.
    Abrahamsson, Lars
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Belea, Radu
    Atkins Sverige AB.
    Klerfors, Bertil
    Chalmers Technical University.
    Schütte, Thorsten
    STRI AB.
    Warner, Bruce
    ABB Switzerland Ltd.
    Modern methods for balancing of single phase loads when feeding a.c. Electrified railways2017In: Eb - Elektrische Bahnen, ISSN 0013-5437, Vol. 115, no 6-7, p. 378-384Article in journal (Refereed)
    Abstract [en]

    Conventional compensation circuits for phase balancing of single phase AC railways for 50 or 60 Hz have high cost for the associated transformers. A new version of the classical Steinmetz scheme, using three single phase transformers connected as a W with the phase angles -60°, 0° and +60°, can reuse the two standard single phase transformers of a V-connected feeding station with a third identical transformer added. Power factor compensation can easily be included. For neutral sections between different feeding systems, a neutral section converter is proposed

  • 9.
    Abrahamsson, Lars
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Laury, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Bollen, Math
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Evaluating a constant-current load model through comparativetransient stability case studies of a synchronous-synchronous rotary frequencyconverter fed railway2019Conference paper (Refereed)
    Abstract [en]

    This paper continues the pursuit of getting a deeperunderstanding regarding the transient stability of lowfrequencyAC railway power systems operated at 162⁄3 Hzsynchronously to the public grid. The focus is set on theimpact of different load models. A simple constant-currentload model is proposed and compared to a previously proposedand studied load model in which the train’s activepower is regulated.The study and comparison is made on exactly the samecases as and grid as with the already proposed and moreadvanced load model. The railway grid is equipped witha low-frequency AC high-voltage transmission line whichis subjected to a fault. The study is limited to railwaysbeing fed by different distributions of RFC (Rotary FrequencyConverter) types. Both AT (auto transformer) andBT (booster transformer) catenaries are considered.The RFC dynamic models are essentially Anderson-Fouad models of two synchronous machines coupled mechanicallyby their rotors being connected to the same shaft.The differences in load behaviour between the proposedconstant-current load model and the previously proposedand studied voltage-dependent active power load model areanalyzed and described in the paper.

  • 10.
    Abrahamsson, Lars
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Laury, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Bollen, Math
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Further studies on the transient stability of synchronous-synchronous rotary frequency converter fed railways with low-frequency AC high-voltage transmission2018In: International Journal of Energy Production and Management, ISSN 2056-3272, E-ISSN 2056-3280, Vol. 3, no 4, p. 266-276Article in journal (Refereed)
    Abstract [en]

    This paper continues the pursuit of getting a deeper understanding regarding the transient stability of low-frequency AC railway power systems operated at 16 2/3 Hz that are synchronously connected to the public grid. Here, the focus is set on such grids with a low-frequency AC high-voltage transmission line subject to a fault. The study here is limited to railways being fed by different distributions of Rotary Frequency Converter (RFC) types. Both auto transformer (AT) and booster transformer (BT) catenaries are considered. No mixed model configurations in the converter stations (CSs) are considered in this study. Therefore, only interactions between RFCs in different CSs and between RFCs, the fault, and the load can take place in this study. The RFC dynamic models are essentially two Anderson-Fouad models of synchronous machines coupled mechanically by their rotors being connected to the same mechani- cal shaft. Besides the new cases studied, also a new voltage-dependent active power load model is presented and used in this study.

  • 11.
    Abrahamsson, Lars
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Serrano Jimenez, Daniel
    University Carlos III.
    Laury, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Bollen, Math
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    AC cables strengthening railway low frequency AC power supplysystems2017In: ASME/IEEE 2017 Joint Rail Conference, ASME Press, 2017Conference paper (Refereed)
    Abstract [en]

    In present-day railway power supply systems using an AC frequency lower than the one in the public power system of 50/60 Hz, high voltage overhead transmission lines are used as one measure of strengthening the railway power supply system grids. This option may be economically beneficial, compared to strengthening the grid purely by increasing the density of converter stations or increasing the cross section areas of the overhead catenary wires. High voltage AC transmission lines in the railway power supply system allow larger distances between converter stations than would otherwise be possible for a given amount of train traffic. Moreover, the introduction of AC transmission lines implies reduced line losses and reduced voltage level fluctuations at the catenary for a given amount of train traffic. However, due to the increased public and government resistance for additional overhead high voltage AC transmission lines in general, different alternatives will be needed for the future improvements and strengthening of railway power systems. For a more sustainable transport sector, the share and amount of railway traffic needs to increase, in which case such a strengthening becomes inevitable. Earlier, usage of VSC-HVDC transmission cables has been proposed as one alternative to overhead AC transmission lines. One of the main benefits with VSC-HVDC transmission is that control of power flows in the railway power systems is easier and that less converter capacity may be needed. Technically, VSC-HVDC transmission for railway power systems is a competitive solution as it offers a large variety of control options. However, there might be other more economical alternatives reducing the overall impedance in the railway power system. In public power systems with the frequency of 50/60 Hz, an excess of reactive power production in lowly utilized cables imposes an obstacle in replacing overhead transmission lines with cables. In low frequency AC railway power system, the capacitive properties are less significant allowing longer cables compared to 50/60 Hz power systems. Moreover, in converter-fed railways, some kind of reactive compensation will automatically be applied during low-load. At each converter station, voltage control is already present following the railway operation tradition. Therefore, in this paper, we propose AC cables as a measure of strengthening low-frequency AC railway power systems. The paper compares the electrical performances of two alternative reinforcement cable solutions with the base case of no reinforcement. The options of disconnecting or toggling the cables at low load as well as the automatic reactive compensation by converter voltage control are considered. Losses and voltage levels are compared for the different solutions. Investment costs and other relevant issues are discussed.

  • 12.
    Laury, John
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Bollen, Math
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    A rotary frequency converter model for electromechanical transient studies of 16 (2/3) Hz railway systems2019In: International Journal of Electrical Power & Energy Systems, ISSN 0142-0615, E-ISSN 1879-3517, Vol. 106, p. 467-476Article in journal (Refereed)
    Abstract [en]

    Railway power systems operating at a nominal frequency below the frequency of the public grid (50 or 60 Hz) are special in many senses. One is that they exist in a just few countries around the world. However, for these countries such low frequency railways are a critical part of their infrastructure.

    The number of published dynamic models as well as stability studies regarding low frequency railways is small, compared to corresponding publications regarding 50 Hz/60 Hz public grids. Since there are two main type of low frequency railways; synchronous and asynchronous, it makes the number of available useful publications even smaller. One important reason for this is the small share of such grids on a global scale, resulting in less research and development man hours spent on low frequency grids.

    This work presents an open model of a (synchronous-synchronous) rotary frequency converter for electromechanical stability studies in the phasor domain, based on established synchronous machine models. The proposed model is designed such that it can be used with the available data for a rotary frequency converter.

    The behaviour of the model is shown through numerical electromechanical transient stability simulations of two example cases, where a fault is cleared, and the subsequent oscillations are shown. The first example is a single-fed catenary section and the second is doubly-fed catenary section.

  • 13. Laury, John
    et al.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Bollen, Math
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Challenges with increased share of power electronic generation in railway power supply systems.In: The Electricity Journal - ElsevierArticle in journal (Refereed)
  • 14. Laury, John
    et al.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Bollen, Math
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Impact of reduced share of rotary frequency converter in a low-frequency synchronous railway grid: A transient stability study.2019Conference paper (Refereed)
  • 15.
    Laury, John
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Bollen, Math
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Modified voltage control law for low frequency railway power systems2017In: Proceedings of the 2017 IEEE/ASME Joint Rail Conference, ASME Press, 2017Conference paper (Refereed)
    Abstract [en]

    In today's Swedish and Norwegian low frequency railway power system the voltage at a converter is controlled such that its voltage will drop with increased reactive power output. However, for low frequency railways the influence of active power on voltage is larger compared to public power systems and alternative methods are interesting to investigate. This paper presents a modified voltage control law for increased load sharing between converter stations and reduce the risk for converter overload in low frequency railways power systems. The modified voltage control law is derived mathematically and tested with different droops for two case studies. The results confirms the increased load sharing between the converter stations. The results are analysed and discussed; ideas are presented to counteract some of the negative impacts of the modified voltage control law

  • 16.
    Laury, John
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Bollen, Math
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Transient Stability of a Rotary Frequency Converter fed railway, interconnected with a parallel low frequency high voltage transmission system2018In: WIT Transactions on the Built Environment, ISSN 1746-4498, E-ISSN 1743-3509, Vol. 181, p. 15-24Article in journal (Refereed)
    Abstract [en]

    Using low frequency High Voltage Transmission systems (HV-T) in parallel with the catenary systemstrengthens the railway system by reducing the total impedance of the railway grid. A consequence ofthe reduced impedance is that converter stations are electrically closer to each other.Inside a converter station, different types of Rotary Frequency Converters (RFCs) are used. It is not wellexplored how different RFCs behaves and interacts with each other during and after a large disturbance,like a short circuit.The dynamics of an RFC are modelled by using the Andersson-Fouad model of a synchronous machine.The study presented in this paper investigates interactions inside and between converter stations, withdifferent types of RFC, for an HV-T system in parallel with a Booster Transformer catenary system.The numerical simulation results show, for instance, that the main power oscillation take place inside aconverter station with mixed configuration of RFC type after fault clearance.

  • 17.
    Laury, John
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Bollen, Math
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Transient stability of rotary frequency converter fed low frequency railway grids: The Impact of Different Grid Impedances and Different Converter Station Configurations2018Conference paper (Refereed)
    Abstract [en]

    One method of strengthening low frequency AC railway grids is to upgrade Booster Transformer (BT) catenary systems, to Auto Transformer (AT) catenary systems. An AT catenary system has lower equivalent impedance compared to a BT system. Thus, an upgrade makes the existing converter stations electrically closer.

    Converter stations may have different types of Rotary Frequency Converters (RFCs) installed in them, and it is not well explored how different RFCs behaves and interact during and after a large disturbance.

    Using the Anderson-Fouad model of synchronous machines to describe the dynamics of RFCs, several case studies have been performed through numerical simulations. The studies investigate the interactions within and between converter stations constituted with different RFC types, for BT as well AT catenary systems.

    The numerical studies reveal that replacing BT with AT catenary systems, results in a more oscillatory system behaviour. This is seen for example in the power oscillations between and inside converter stations, after fault clearance.

  • 18.
    Laury, John
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Bollen, Math
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Transient stability analysis of low frequency railway grids2016In: WIT Transactions on the Built Environment, ISSN 1746-4498, E-ISSN 1743-3509, Vol. 162, p. 213-223Article in journal (Refereed)
    Abstract [en]

    This paper investigates the replacement of Rotary Frequency Converters(RFCs) with Static Frequency Converters (SFCs) in the Swedish synchronous-synchronous Railway Power Supply System (RPSS) operating at1623Hz. Thereis a need to investigate how such a system behaves when RFCs are partly replacedwith SFCs, as the SFCs do not have any physical inertia.Most of the transient stability studies published for synchronous-synchronousRPSS address the behaviour of a single RFC or SFC against an infinite bus.However, there are very few studies investigating a system with several RFCs,SFCs or a mix of both in the same converter station. This paper presents the resultsof a transient stability study with both RFCs and SFCs. The investigated casesconsist of faults at different locations, and present the results when an RFC isreplaced with an SFC in a converter station.

  • 19.
    Mahmood, Yasser Ahmed
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science. Kungliga tekniska högskolan, KTH.
    Ahmadi, Alireza
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Verma, Ajint K.
    Stord/Haugesund University College.
    Reliability Evaluation of Traction Power Capacity in Converter-fed Railway Systems2016In: IET Generation, Transmission & Distribution, ISSN 1751-8687, E-ISSN 1751-8695Article in journal (Refereed)
    Abstract [en]

    A Traction Power Supply System (TPSS) supplies electricity for train propulsion on electrified railways. In many countries, electrified railways have single-phase low-frequency AC power systems fed by frequency converters (FCs). Outages of power will significantly weaken the TPSS, cause operational problems and ultimately lead to traffic disruption in the form of speed reduction, train delays, or cancellations. The aim of this study is to propose an approach to analyse the reliability of power conversion capacity to discover the reasons for ‘loss of load”. The paper assumes train delays are caused by converter outages, shortages of reserve power capacity for unexpected train loads, or a combination of the two. It considers the availability performance of the converter station and evaluates the reliability of the conversion power capacity. It proposes a power-traffic factor (PTF) to aggregate and quantify train power demands. The PTF can be related to the availability of traction power capacity and will identify areas with risks of power shortages. The study finds the shortage in power leading to train delays in some regions of the system is due to a shortage of reserve power capacity

  • 20.
    Serrano-Jiménez, Daniel
    et al.
    Carlos III University of Madrid, Av. de la Universidad 30, 28911 Leganés, Spain.
    Abrahamsson, Lars
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Castaño-Solís, Sandra
    ETSIDI – Technical University de Madrid, Ronda de Valencia 3, 28013 Madrid, Spain.
    Sanz-Feito, Javier
    Carlos III University of Madrid, Av. de la Universidad 30, 28911 Leganés, Spain.
    Electrical railway power supply systems: Current situation and future trends2017In: International Journal of Electrical Power & Energy Systems, ISSN 0142-0615, E-ISSN 1879-3517, Vol. 92, p. 181-192Article in journal (Refereed)
    Abstract [en]

    Railway electrification is experiencing a very important transformation process today. The need of increasing its capacity has evidenced the drawbacks of conventional systems of dealing with the higher power required, whilst maintaining reasonable costs. Furthermore, the current trend undertaken by electrical systems towards smarter grids involves to rethink carefully the direction of evolution of these systems. However, the different technical progress at the time of railway electrification, and the particular historical and economic characteristics of the countries, have led to a great variety of configurations that require different solutions. The first main objective of this article is to classify and describe the principal electrical railway power supply systems existing and the most important proposals for their improvement found in the literature. The second main objective is to make a comparison of all presented systems based on economic and technical criteria capable of assessing their suitability and future projection. The right choice of the feeding system is decisive for the development of more competitive, efficient and reliable railway systems.

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