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  • 1.
    Al-Jumaili, Mustafa
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Mahmood, Yasser Ahmed
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Karim, Ramin
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Assessment of railway frequency converter performance and data quality using the IEEE 762 Standard2014In: International Journal of Systems Assurance Engineering and Management, ISSN 0975-6809, E-ISSN 0976-4348, Vol. 5, no 1, p. 42694-Article in journal (Refereed)
    Abstract [en]

    Reliability, availability and maintainability analysis is one of the most important tools for measuring system performance. The performance of a traction power supply system (TPSS) can be measured using the data collected from frequency converters, as these converters constitute the main part of the TPSS. The quality of the collected data should be good enough to provide the correct and complete information necessary for assessment of frequency converter performance. Many methods can be used to assess the performance of converters such as neural networks, fuzzy logic and standards. The IEEE 762 Standard offers a methodology that can provide key performance indicators for power generation units. This standard has been chosen for its widespread acceptance and applicability. To be able to evaluate a converter’s performance, IEEE 762 indexes should be calculated using data such as the downtime, reserve shutdown hours and service hours. Therefore, the purpose of this study is to assess the performance of the Swedish TPSS frequency converters using IEEE 762, and to assess the quality of data by inspecting their compatibility with this standard. In this study, an application has been developed to generate the missing information and to calculate the indexes provided by the standard, in order to evaluate the power converters’ performance. A case with sample data is also discussed in this paper.

  • 2.
    Garmabaki, Amir
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Ahmadi, Alireza
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Mahmood, Yasser Ahmed
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Barabadi, Abbas
    Tromsø University.
    Reliability Modelling of Multiple Repairable Units2016In: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 32, no 7, p. 2329-2343Article in journal (Refereed)
    Abstract [en]

    This paper proposes a model selection framework for analysing the failure data of multiple repairable units when they are working in different operational and environmental conditions. The paper provides an approach for splitting the non-homogeneous failure data set into homogeneous groups, based on their failure patterns and statistical trend tests. In addition, when the population includes units with an inadequate amount of failure data, the analysts tend to exclude those units from the analysis. A procedure is presented for modelling the reliability of a multiple repairable units under the influence of such a group to prevent parameter estimation error. We illustrate the implementation of the proposed model by applying it on 12 frequency converters in the Swedish railway system. The results of the case study show that the reliability model of multiple repairable units within a large fleet may consist of a mixture of different stochastic models, i.e. the HPP/RP, TRP, NHPP and BPP. Therefore, relying only on a single model to represent the behaviour of the whole fleet may not be valid and may lead to wrong parameter estimation.

  • 3.
    Mahmood, Yasser Ahmed
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Availability Analysis of Frequency Converters in Electrified Railway Systems2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The increased interest in electrified railways can be explained by their huge capacity, high efficiency, and low pollution. Today’s electrified railway is expecting higher demands for electric power for increased speeds, increased traffic volume capacity, and heavier freight loads. This will in turn impose greater demands on railway infrastructure managers to increase the overall capacity of their railways without traffic disruption. The increasing demand for railway transportation services is having a significant effect on important stakeholder requirements, such as safety, punctuality, dependability, sustainability and costs. This in turn is affecting railway practices in the areas of operation, maintenance, and modernisation.The traction power supply system (TPSS) is one of the most important parts of the electrified railway system due to its responsibility for providing a continuous and adequate electrical power for electric railway vehicles and its great impact on the availability performance. The TPSS provides the railway with power either directly with low-frequency generators or indirectly via frequency converters (FCs). In the converter-based electrified railway network, frequency converter stations represent the generation system which converts the electric power from the three-phase 50 Hz public grid to electric power for the single-phase 16.7 Hz traction grid. Obviously, any operational problems in the FCs will cause an unavailability of traction power capacity and consequently entail operational problems for trains, resulting in speed reduction, train delays, or cancellations. Shortages of traction power may occur due to a loss of capacity or a shortage of reserve capacity. The installed capacity should be capable of meeting the system load in the case of unexpectedly large loads, in the event of one or more FC units not being in service due to forced outages or scheduled maintenance, or when a combination of these scenarios takes place. The purpose of the research for this thesis has been to develop methodologies and tools for availability evaluation and improvement of FCs and the traction power capacity, in order to prevent and mitigate outage events through optimal decisions for improvement. To fulfil the stated purpose, empirical data have been collected from reporting databases, interviews, and documents. Examples of the data gathered are train delay statistics, failure statistics, and electrical measurements. The data have been analysed on two different levels, the FC type level and the FC station level, as well as by applying theories related to RAM analysis. The study has addressed issues regarding availability performance as follows: improvement of data reporting system planning, reliability modelling, evaluation of availability performance, and, finally, identification of effective ways to reduce shortages of traction power. The analysis outcome has revealed the areas where resources must be invested for improvement to reduce and prevent loss of load due to capacity outages.

  • 4.
    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

  • 5.
    Mahmood, Yasser Ahmed
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Ahmadi, Alireza
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Karim, Ramin
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Kumar, Uday
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Verma, Ajint
    Stord/Haugesund University College, Haugesund.
    Fransson, Niklas
    Trafikverket.
    Comparison of frequency converter outages: a case study on Swedish TPS system2012Conference paper (Refereed)
    Abstract [en]

    The purpose of this paper is to survey the unavailability of the two main types of converters used in the Swedish traction power supply (TPS) system, i.e. the rotary and the static converter. The number of outages and the outage rate has been used to analyze and compare the unavailability of rotary and static converters. The mean cumulative function (MCF) has been used to analyze the number of outages and the unavailability trends, while the forced outage rate (FOR) concept has been used to analyze the outage rates. The study shows that the outages due to converter failure occur at a constant rate by calendar time at most of the converter stations, while very few stations have an increasing or a decreasing rate. It has also been found that the static converters have a higher number of outages and a higher outage rate by calendar time compared to the rotary converter types. The results of the study show that combining the number of outages and the forced outage rate gives a better view of the converters’ performance, and provides support for the maintenance decision. In fact, using one of them alone does not reflect reality. Comparing these two indexes is also helpful in identifying the areas where extra resources are needed to improve maintenance planning and where improvements can reduce the outage in the TPS system.

  • 6.
    Mahmood, Yasser Ahmed
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    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.
    Identification of Frequency Converter Models For Availability Improvement2013In: International Journal of Electrical Engineering, ISSN 1812-3031, Vol. 20, no 4Article in journal (Refereed)
  • 7.
    Mahmood, Yasser Ahmed
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Ahmadi, Alireza
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Verma, Ajint
    Stord/Haugesund University College.
    Kumar, Uday
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Fuzzy fault tree analysis: a review of concept and application2013In: International Journal of Systems Assurance Engineering and Management, ISSN 0975-6809, E-ISSN 0976-4348, Vol. 4, no 1, p. 19-32Article in journal (Refereed)
    Abstract [en]

    Fault tree analysis (FTA) is a widely used method for analyzing a system’s failure logic and calculating overall reliability. However, application of conventional FTA has some shortcomings, e.g. in handling the uncertainties, allowing the use of linguistic variables, and integrating human error in failure logic model. Hence, Fuzzy set theory has been proposed to overcome the limitation of conventional FTA. Fuzzy logic provides a framework whereby basic notions such as similarity, uncertainty and preference can be modeled effectively.The aim of this paper is to present a review of the concept of fuzzy theory with fault tree analysis and their applications since 1981, to reflect the current status of Fuzzy Fault Tree Analysis (FFTA) methodologies, their strengths, weaknesses, and their applications. This paper explains the fundamentals of fuzzy theory and describes application of fuzzy importance for using FFTA. The concept of the failure possibility and uncertainty analysis by using FFTA is discussed, and concludes with discussion on the application of FFTA in different fields. The review reveals the effectiveness of the FFTA in comparison with conventional FTA, when there is inadequate amount of accurate reliability oriented information.

  • 8.
    Mahmood, Yasser Ahmed
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Ahmadi, Alireza
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Verma, Ajit
    University of Haugesund.
    Identifying the critical of frequency converter models2013In: 10th International Conference on Condition Monitoring and Machinery Failure Prevention Technologies 2013, CM 2013 and MFPT 2013, 2013, p. 784-792Conference paper (Refereed)
    Abstract [en]

    The Swedish traction power supply system (TPSS) has approximately 134 frequency converters; there are two main types, static and rotary, with ten different models. Previous work performed a Reliability, Availability, and Maintainability (RAM) analysis of the converters to estimate their performance. The aim of this article is to use these RAM components to rank the converter models and identify the models that need more attention to improve overall operational availability. The study finds that the performance factor aggregated from IEEE Std 762 indexes can rank the converters. Furthermore, the paper has been found that the rotary models Q48 and TGTO have a low performance from RAM point of view.

  • 9.
    Mahmood, Yasser Ahmed
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Ahmadi, Alireza
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Verma, Ajit
    Karim, Ramin
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Kumar, Uday
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Availability and reliability performance analysis of traction frequency converters: a case study2013In: International Review of Electrical Engineering, ISSN 1827-6660, Vol. 8, no 4, p. 1231-1242Article in journal (Refereed)
    Abstract [en]

    The frequency converter is one of the main parts of the traction power supply system (TPSS) and converts adequate traction power from the power company to the electric vehicle. In the Swedish railway network, this system comprises around 133 frequency converters with a total capacity reach of 1,295 MVA and covers around 9,543 km of electrified railway. The aim of this article is to evaluate and compare the reliability, availability and maintainability (RAM) performances of the ten models of converters used in the Swedish railway system. The key performance indicators introduced by the IEEE Std 762 methodology have been used to measure and compare the RAM performance of the converters. Moreover, the mean cumulative function (MCF) has also been used for the monitoring and comparison of the field reliability of converters versus the operating time, capacity factor and converted power. The study shows that, in general, both the static and the rotary types have a high level of availability ranging from 97%-99%. The results also show that the TGTO model (a static type) has the lowest RAM performance among all the models. In addition, it has been found that using IEEE Std 762 or the MCF alone does not always provide a complete answer. Therefore, the study suggests using both the IEEE Std 762 and the MCF methodology to arrive at a more realistic result. The study has shown that the MCF based on the capacity factor gives a good result compared with the results obtained using the MCF based on the operating time and on the converted power as a measure of the usage intensity.

  • 10.
    Mahmood, Yasser Ahmed
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Ahmadi, Alireza
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Verma, Ajit Kumar
    Department of Electrical Engineering, Indian Institute of Technology, Bombay, International Institute of Information Technology, P-14, Pune Infotech Park, Phase-1, Hinjawadi.
    Evaluation and Selection for Availability Improvement of Frequency Converters in Electrified Railway2014In: International Journal of Power and Energy Systems, ISSN 1078-3466, E-ISSN 1710-2243, Vol. 34, no 4, p. 162-172Article in journal (Refereed)
    Abstract [en]

    Railway traction power supply systems (TPSSs) that work at low frequencies contain frequency converters (FCs) that convert the adequate power from a public grid to a traction grid. Availability and system importance analysis is a key aspect of overall system performance. It identifies the weakest areas of a system and indicates where resources must be invested to achieve maximum improvement. Availability analysis of FC types is crucial to reduce outages of traction power that lead to traffic delays and cancellations due to unavailability of FCs. The aim of this study is to rank FC types on the basis of priority of needs for reliability and maintainability improvement so that the availability can be enhanced. The proposed approach involves collection of outage data by calculating performance measures, and then these measures are compared via a comparative chart. Thereafter, opinions of experts are collected and aggregated through linguistic variables, and the FC types are then identified using analytic hierarchy process. Reliability, availability and maintainability (RAM) have been used as the criteria to identify the FC types. In addition to power sharing, capacity and service factors are considered to be productivity measures. Probabilistic performance indexes are used to measure availability, maintainability and productivity, whereas the mean number of outages is used as a reliability index.

  • 11.
    Mahmood, Yasser Ahmed
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Garmabaki, Amir
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Ahmadi, Alireza
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Verma, Ajit Kumar
    Western Norway University of Applied Sciences.
    Reliability model for frequency converter in electrified railway2018In: International Journal of Electrical Power & Energy Systems, ISSN 0142-0615, E-ISSN 1879-3517, Vol. 94, p. 385-392Article in journal (Refereed)
    Abstract [en]

    Reliability analysis of frequency converters based on failures and outages reports constitute an important basis for asset performance and management. Two- and four-state reliability models that recognize the operating characteristics of base load units and peaking units are presented and compared in this study. In this study, a four-state model is modified to a three-state model by combining the ‘needed’ and ‘not-needed’ forced-out states. Moreover, the transitions in the three-state model for power frequency converter have been designed according to real operational data. An outage-reporting database modelled considering IEEE STD 762 is presented and compared with the existing failure-reporting database of the case considered here. Furthermore, a method to extract information missing in the failure-reporting database by electrical readings is proposed to meet the requirements of the outage-reporting database. The study found that the results of indexes based on the IEEE four-state model are not reasonable for the frequency converter given their differences with the gas-turbine results under operational conditions. The forced outage rates and availability factors of twelve actual traction frequency converters of Swedish railways network are presented to validate the modified model.

  • 12.
    Mahmood, Yasser Ahmed
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Karim, Ramin
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Al-Jumaili, Mustafa
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Operation, Maintenance and Acoustics.
    Assessment of reliability data for traction frequency converters using IEEE Std 762: a study at Swedish railway2012In: Proceedings of the 2nd International Workshop & Congress on eMaintenance: Dec 12-14 Luleå, Sweden : eMaintenace: trends in technologies and methodologies, challenges, possibilities and applications / [ed] Ramin Karim; Aditya Parida; Uday Kumar, Luleå: Luleå tekniska universitet, 2012, p. 171-174Conference paper (Refereed)
1 - 12 of 12
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