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Stability of Low-Frequency AC Railways: Models and Transient Stability
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Low-frequency AC railway grids are unique in the sense that a only few countries around the world uses them, still however, they are an important parts of their countries infrastructures. Due to the usage of a dierent frequency than the public grid of the country, conversion of frequency is needed for the interconnection. The frequency conversion is done by machine based rotary frequency converters or power electronic based static frequency converters.

When reinforcing with new power conversion capacity, mostly static frequency converters are installed since rotary frequency converters for railways have not been manufactured for some time. As more static frequency converterare introduced, the share of rotary frequency converters is reduced. It is not well explored how the stability of low-frequency AC railways is affected with a large share of static frequency converters.

In this thesis, the main goal has been to obtain knowledge of the stability of low-frequency AC railway grids, with focus on synchronous ones. The electromechanical stability of a synchronous low-frequency AC railway is explored through numerical simulations, where the transient stability is the main focus.

The main contributions of this thesis is proposing a model of a rotary frequency converter, proposing a model of a static frequency converter, and transient stability simulations. The model of the rotary frequency converter uses established machine models, whereas the static frequency converter model has been developed with help of measurements. It can be concluded that the proposed static frequency converter model captures the main behaviour of the measurements of a static frequency converter.

The transient stability of synchronous AC railway grids is studied, through numerical simulations. The studied cases are for instance dierent railway grid congurations with dierent types rotary frequency converters and railway grids with mixes of static frequency converters and static frequency converter.

The main conclusion is that the rotary frequency converter fed synchronous railway grids studied are transiently stable, and the studied railway grids where rotary frequency converters are gradually replaced with static frequency converter are also transiently stable. However, it was found that the studied railway grids obtain a heavier oscillatory behaviour when there is a mix of rotary frequency converters and static frequency converters.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2019.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electric Power Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-71795ISBN: 978-91-7790-292-8 (print)ISBN: 978-91-7790-293-5 (electronic)OAI: oai:DiVA.org:ltu-71795DiVA, id: diva2:1270596
Public defence
2019-02-28, Skellefteå, 10:00 (English)
Opponent
Supervisors
Available from: 2018-12-27 Created: 2018-12-13 Last updated: 2019-02-14Bibliographically approved
List of papers
1. Transient stability of rotary frequency converter fed low frequency railway grids: The Impact of Different Grid Impedances and Different Converter Station Configurations
Open this publication in new window or tab >>Transient stability of rotary frequency converter fed low frequency railway grids: The Impact of Different Grid Impedances and Different Converter Station Configurations
2018 (English)Conference paper, Published 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.

Place, publisher, year, edition, pages
ASME Press, 2018
Series
Proceedings of the ASME Joint Rail Conference, ISSN 1559-9531, E-ISSN 2160-1380
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electric Power Engineering
Identifiers
urn:nbn:se:ltu:diva-70235 (URN)10.1115/JRC2018-6247 (DOI)000438662100091 ()2-s2.0-85050882038 (Scopus ID)978-0-7918-5097-8 (ISBN)
Conference
ASME Joint Rail Conference (JRC 2018), Pittsburgh, PA, APR 18-20, 2018
Available from: 2018-08-07 Created: 2018-08-07 Last updated: 2018-12-13Bibliographically approved
2. Transient Stability of a Rotary Frequency Converter fed railway, interconnected with a parallel low frequency high voltage transmission system
Open this publication in new window or tab >>Transient Stability of a Rotary Frequency Converter fed railway, interconnected with a parallel low frequency high voltage transmission system
2018 (English)In: WIT Transactions on the Built Environment, ISSN 1746-4498, E-ISSN 1743-3509, Vol. 181, p. 15-24Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
WIT Press, 2018
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electric Power Engineering
Identifiers
urn:nbn:se:ltu:diva-70287 (URN)10.2495/CR180021 (DOI)2-s2.0-85057204459 (Scopus ID)978-1-78466-285-1 (ISBN)
Conference
Comprail 2018, 16th International Conference on Railway Engineering Design & Operation, 2-4 July 2018, Lisbon, Portugal
Funder
Swedish Transport Administration
Note

Konferensartikel i tidskrift

Available from: 2018-08-09 Created: 2018-08-09 Last updated: 2018-12-13Bibliographically approved
3. A rotary frequency converter model for electromechanical transient studies of 16 (2/3) Hz railway systems
Open this publication in new window or tab >>A rotary frequency converter model for electromechanical transient studies of 16 (2/3) Hz railway systems
2019 (English)In: International Journal of Electrical Power & Energy Systems, ISSN 0142-0615, E-ISSN 1879-3517, Vol. 106, p. 467-476Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Low frequency railways, 16 (2/3)  Hz, Modelling, Simulations, Transient stability, Rotary frequency converter, Motor generator set, Multi machine system
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electric Power Engineering
Identifiers
urn:nbn:se:ltu:diva-71471 (URN)10.1016/j.ijepes.2018.10.017 (DOI)000454377000043 ()2-s2.0-85055732778 (Scopus ID)
Funder
Swedish Transport Administration
Note

Validerad;2018;Nivå 2;2018-11-07 (inah)

Available from: 2018-11-06 Created: 2018-11-06 Last updated: 2019-09-13Bibliographically approved
4. Challenges with increased share of power electronic generation in railway power supply systems.
Open this publication in new window or tab >>Challenges with increased share of power electronic generation in railway power supply systems.
(Swedish)In: The Electricity Journal - ElsevierArticle in journal (Refereed) Submitted
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:ltu:diva-71799 (URN)
Available from: 2018-11-28 Created: 2018-11-28 Last updated: 2018-12-13
5. Impact of reduced share of rotary frequency converter in a low-frequency synchronous railway grid: A transient stability study.
Open this publication in new window or tab >>Impact of reduced share of rotary frequency converter in a low-frequency synchronous railway grid: A transient stability study.
2019 (English)Conference paper, Published paper (Refereed)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electric Power Engineering
Identifiers
urn:nbn:se:ltu:diva-71798 (URN)
Conference
Proceedings of the 2019 IEEE/ASME Joint Rail Conference
Note

Accepted.

Available from: 2018-11-28 Created: 2018-11-28 Last updated: 2019-01-08
6. A simplified static frequency converter model for electromechanical transient stability studies of 16 2/3 Hz railways
Open this publication in new window or tab >>A simplified static frequency converter model for electromechanical transient stability studies of 16 2/3 Hz railways
(English)Manuscript (preprint) (Other academic)
Abstract [en]

With increased share of Static Frequency Converters (SFCs) in 16 2/3 Hz railway grids concerns about stability have increased. Stability studies for such low-frequency railway grids are few, and models that describe SFC dynamics are especially few. This paper presents an open SFC model for electromechanical stability studies in the phasor domain, suited for 16 2/3 Hz synchronous railway grids. The developed and proposed SFC model is implemented in MatLab Simulink, together with grid and loads. Numerical studies are made, in which the proposed SFC model is validated against both measured RMS-phasor amplitude of voltage and current at the railway grid side of an SFC. The SFC model developed is able to reproduce the measured RMS voltage and current with an acceptable accuracy.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electric Power Engineering
Identifiers
urn:nbn:se:ltu:diva-71796 (URN)
Available from: 2018-11-28 Created: 2018-11-28 Last updated: 2019-01-30
7. Evaluating a constant-current load model through comparativetransient stability case studies of a synchronous-synchronous rotary frequencyconverter fed railway
Open this publication in new window or tab >>Evaluating a constant-current load model through comparativetransient stability case studies of a synchronous-synchronous rotary frequencyconverter fed railway
2019 (English)Conference paper, Published 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.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:ltu:diva-71801 (URN)
Conference
Proceedings of the 2019 IEEE/ASME Joint Rail Conference
Note

Submitted to JRC 2019. Not yet accepted. /20181205

Available from: 2018-11-28 Created: 2018-11-28 Last updated: 2018-12-13
8. Further studies on the transient stability of synchronous-synchronous rotary frequency converter fed railways with low-frequency AC high-voltage transmission
Open this publication in new window or tab >>Further studies on the transient stability of synchronous-synchronous rotary frequency converter fed railways with low-frequency AC high-voltage transmission
2018 (English)In: International Journal of Energy Production and Management, ISSN 2056-3272, E-ISSN 2056-3280, Vol. 3, no 4, p. 266-276Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
WIT Press, 2018
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electric Power Engineering
Identifiers
urn:nbn:se:ltu:diva-71797 (URN)10.2495/EQ-V3-N4-266-276 (DOI)2-s2.0-85067793925 (Scopus ID)
Note

Validerad;2019;Nivå 1;2019-07-09 (johcin)

Available from: 2018-11-28 Created: 2018-11-28 Last updated: 2019-07-09Bibliographically approved

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Citation style
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