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  • 1. Arifianto, I.
    et al.
    Josue, F.
    Saers, R.
    Rosenlind, Johanna
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Suwarno,
    Investigation of transformer top-oil temperature considering external factors2012In: Condition Monitoring and Diagnosis (CMD), 2012 International Conference on, IEEE , 2012, p. 198-201Conference paper (Refereed)
    Abstract [en]

    Standard estimation of top-oil temperature uses a thermal model related to load changes and variation of ambient temperature. Attempts have been done to improve the accuracy of top-oil temperature calculations by introducing internal properties into the model i.e. oil viscosity and winding resistance. The interest of this paper is to investigate the effect of external factors on top-oil temperature by looking into the weather, i.e. wind velocity. The results are compared with measurements on a 63MVA-ONAF 55/140 kV transformer unit, which is operated in ONAN cooling mode. The unit is located in subarctic climate, and it is equipped with a monitoring system and a weather station.

  • 2.
    Babu, Sajeesh
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Reliability and Sensitivity Analysis of Substation and Feeder DesignConsidering Correlated Failures2017Conference paper (Refereed)
  • 3.
    Babu, Sajeesh
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Jürgensen, Jan Henning
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On the status of reliability studies involving primary and secondary equipment applied to power system2014In: 2014 International Conference on Probabilistic Methods Applied to Power Systems, PMAPS 2014 - Conference Proceedings, IEEE , 2014, p. 6960653-Conference paper (Refereed)
    Abstract [en]

    Growth in infrastructure and energy utilization consistently put forward the demand for added quality and quantity of electric power. Reliability concerns over power systems are widespread within its different associated divisions like 'primary' power system structure, protection system, control equipment, ICT (Information and Communication Technologies) etc. This paper is a review of the present status of practices regarding reliability analysis in these divisions and works towards collectively assessing some of the studies in the respective areas. The idea of integrating reliability analysis from the above areas is introduced along with pointing out the major challenges associated. A set of tools for operators to make use in these reliability evaluations and modelling are mentioned. The earlier attempts towards combined overall system reliability analysis are discussed and the approach in this regard with the help of 'control functions' is emphasised. The paper includes works dealing with theory, different methodologies and data associated with power system reliability.

  • 4.
    Babu, Sajeesh
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Shayesteh, Ebrahim
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Enarsson, Lars Einar
    Reliability Evaluation of Distribution Structures Considering the Presence of False Trips2018In: IEEE Transactions on Smart Grid, ISSN 1949-3053, E-ISSN 1949-3061Article in journal (Refereed)
    Abstract [en]

    This paper presents a method for modelling the different modes of failures in a substation and feeder architecture along with updating the possible false tripping scenarios in it. A traditional approach to collectively assess the failure modes using reliability block diagram is reviewed, and the method is updated to count in the unaccounted false tripping scenarios. A generalizable radial feeder branching structure is adopted and the effect of total feeder length and number of feeders from each busbar is examined and modelled with the help of the updated reliability block diagram. The modelled trends are also studied from real-world substation architectures. Thus, the analysis attains an improved estimation of the complex hidden failure probabilities combining theoretical and practical models.

  • 5.
    Babu, Sajeesh
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Jürgensen, Jan Henning
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wallnerström, Carl Johan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Tjernberg, Lina Bertling
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Analyses of Smart Grid Technologies and Solutions from a System Perspective2015In: Smart Grid Technologies - Asia (ISGT ASIA), 2015 IEEE Innovative, IEEE conference proceedings, 2015, p. 1-5, article id 7387089Conference paper (Refereed)
    Abstract [en]

    This paper consolidates the data, analysis andobservations from a case study conducted in cooperation withthe Smart Grid Gotland project. The analysis identifies howelectrical power consumption interacts with distributedelectricity generation such as wind and solar power andpresents how it correlates to weather data and smart gridsolutions. The analysis model developed based on the Gotlandnetwork is generic and hence can be functional in investigatingother power networks of different size, voltage level andstructures. The key observations from the study of smart gridsolutions such as dynamic load capacity and energy storagesolutions are specified. Based on the project, an overview offuture risks and opportunities of smart grid systems is presented.

  • 6.
    Babu, Sajeesh
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Shayesteh, Ebrahim
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Analysing Correlated Events in Power System Using Fault Statistics2016In: Probabilistic Methods Applied to Power Systems (PMAPS), 2016 International Conference on, IEEE, 2016Conference paper (Other academic)
    Abstract [en]

    Power system automation requires logical presumptions made on practical grids to correctly comprehend and manage complex and correlated faults occurring in real world systems. Traditional grid fault analysis methods lack in-depth understanding of these complex events and demand development of approaches that make use of available data to address this problem. Here, the traditional classification approach and challenges relating control equipment in power system are reviewed and a method observing the affected customers during faults along with grid design is discussed based on Swedish case study data. Various contrasting observations are made on the data recorded over two time periods to understand the trend developing over years. Moreover, it will be shown that the classification method also has potential in identifying weak spots in the grid when it comes to the reliability of control equipment.

  • 7.
    Bergerland, Sune
    et al.
    Karlstads El- och Stadsnät.
    Wallnerström, Carl Johan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Summary of the Swedish tariff regulation and impact of changes on investment strategies2015Conference paper (Refereed)
    Abstract [en]

    This paper evaluates how upcoming changes in the Swedish tariff regulation could affect distribution system operators (DSOs), with focus on reinvestment planning. This is done by general analyses as well as by authentic calculation examples of a real power distribution system. The paper describes the Swedish tariff regulation with expected changes, provides a summary of changes in Swedish laws and regulation affecting DSOs between 1996 and 2016, describes how a DSO at local distribution level conduct their reinvestments, illustrates economic calculation examples and finally presents analyses and conclude the results. Analysis results presented show that the outcome from the regulation is sensitive towards relatively small changes in WACC and age structure. The tariff cap allowed will however be significantly reduced for all tested scenarios. A reinvestment rate of in average~10 % regarding meters and IT and ~2.5 % regarding all other categories could be a rough guideline to meet the new incentives, but that could differ depending on the actual age structure of the DSO.

  • 8.
    Bertling, Lina
    et al.
    Chalmers.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Jensen, Jolanta
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Setreus, Johan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wallnerström, Carl Johan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    RADPOW development and documentation2008Report (Other (popular science, discussion, etc.))
    Abstract [en]

    This report summarizes the status of the computer program RADPOW. RADPOW is a program for Reliability Assessment of electrical Distribution POWer systems. It was developed at KTH School of Electrical Engineering, within the research program EKC and the research project on reliability of new electrical distribution systems. Further on, RADPOW has been used and further developed within the RCAM research group at KTH School of Electrical Engineering.

    This status report contains a brief description of the RADPOW_2006 version, the Loadflow module from the RADPOW_1999_PH version and a description of the work done in the resulting RADPOW_2007 version. This version now includes a tested load flow module and the ability to calculate the latest component importance indices developed within the RCAM research group. The source code for the program has also been restructured and commented in a more detailed level than before.

  • 9.
    Colmenares, Juan
    et al.
    KTH, School of Electrical Engineering (EES), Electric power and energy systems.
    Sadik, Diane-Perle
    KTH, School of Electrical Engineering (EES), Electric power and energy systems.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electric power and energy systems.
    Reliability Analysis of a High-Efficiency SiC Three-Phase Inverter2016In: IEEE Journal of Emerging and Selected Topics in Power Electronics, ISSN 2168-6777, E-ISSN 2168-6785, Vol. 4, no 3, p. 996-1006Article in journal (Refereed)
    Abstract [en]

    Silicon carbide as an emerging technology offers potential benefits compared with the currently used silicon. One of these advantages is higher efficiency. If this is targeted, reducing the on-state losses is a possibility to achieve it. Parallel-connecting devices decrease the on-state resistance and therefore reduce the losses. Furthermore, increasing the amount of components such as parallel connection of devices introduces an undesired tradeoff between efficiency and reliability, since an increased component count increases the probability of failure. A reliability analysis has been performed on a three-phase inverter rated at 312 kVA, using parallel-connected power modules. This analysis shows that the gate voltage stress has a high impact on the reliability of the complete system. Decreasing the positive gate-source voltage could, therefore, increase the reliability of the system approximately three times without affecting the efficiency significantly. Moreover, adding redundancy in the system could also increase the mean time to failure by approximately five times.

  • 10.
    Colmenares, Juan
    et al.
    KTH, School of Electrical Engineering (EES), Electric power and energy systems.
    Sadik, Diane-Perle
    KTH, School of Electrical Engineering (EES), Electric power and energy systems.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electric power and energy systems.
    Reliability analysis of a high-efficiency SiC three-phase inverter for motor drive applications2016In: 2016 IEEE Applied Power Electronics Conference and Exposition (APEC), Institute of Electrical and Electronics Engineers (IEEE), 2016, p. 746-753, article id 7467955Conference paper (Refereed)
    Abstract [en]

    Silicon Carbide as an emerging technology offers potential benefits compared to the currently used Silicon. One of these advantages is higher efficiency. If this is targeted, reducing the on-state losses is a possibility to achieve it. Parallel-connecting devices decrease the on-state resistance and therefore reducing the losses. Furthermore, increasing the amount of components introduces an undesired tradeoff between efficiency and reliability. A reliability analysis has been performed on a three-phase inverter for motor drive applications rated at 312 kVA. This analysis has shown that the gate voltage stress determines the reliability of the complete system. Nevertheless, decreasing the positive gate-source voltage could increase the reliability of the system approximately 8 times without affecting the efficiency significantly. Moreover, adding redundancy in the system could also increase the mean time to failure approximately 5 times.

  • 11.
    Duvnjak Zarkovic, Sanja
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Shayesteh, Ebrahim
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    On the Security of Electricity Supply in Power Distribution Systems2018In: 2018 IEEE International Conference on Probabilistic Methods Applied to Power Systems (PMAPS), IEEE conference proceedings, 2018, article id 8440489Conference paper (Refereed)
    Abstract [en]

    Security of electricity supply has become a fundamental requirement for modern societies. However, attempts to define and evaluate security of supply have differed from one another. This paper reviews relevant studies in order to give a comprehensive explanation of the security of supply concept. The paper includes theory, assessment, methodology, regulations, data and practical issues associated with the security of supply and power system reliability. Special focus is given to the methodologies used for improving the reliability and security of supply in power distribution systems.

  • 12.
    Edström, Fredrik
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Rosenlind, Johanna
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Alvehag, Karin
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Influence of Ambient Temperature on Transformer Overloading During Cold Load Pickup2013In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 28, no 1, p. 153-161Article in journal (Refereed)
    Abstract [en]

    This paper proposes a method to investigate the socioeconomical aspects of transformer overloading during a cold load pickup (CLPU) in residential areas. The method uses customer damage functions to estimate the cost for their power interruption and a deterioration model to estimate the cost for transformer wear due to the CLPU. A thermodynamic model is implemented to estimate the peak and the duration of cold residential load. A stochastic differential equation is used to capture the volatility of the load and to estimate the probability for transformer overloading. In a numerical example, an optimal cold load pickup for a two-area system is demonstrated where transformer overloading is allowed. In this example, an ambient temperature threshold is identified, where transformer overloading is socioeconomically beneficial.

  • 13.
    Edström, Fredrik
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Rosenlind, Johanna
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Modeling Impact of Cold Load Pickup on Transformer Aging Using Ornstein-Uhlenbeck Process2012In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 27, no 2, p. 590-595Article in journal (Refereed)
    Abstract [en]

    Thermostatically controlled devices, such as air conditioners, heaters, and heat pumps may cause cold load pickup (CLPU) problems after a prolonged blackout. This causes an increased load on the power components in the electrical grid. The result is unpredictable aging and increased risk of failure. Quantifying this risk is crucial for efficient asset management for cost-intensive components such as the transformer. This paper presents a new approach to model the loading profile of a CLPU using stochastic differential equations. The realization of the loading profile is used to determine the aging of a transformer. Two models for the deterioration of transformer solid insulation represent the loss of life due to the CLPU. A comparison between two models for the aging of the solid insulation in the transformer is made in a case study. Due to the stochastic behavior of the load, there is a probability for loading the transformer above the recommended ratings, and this probability is estimated with Monte Carlo simulations.

  • 14.
    Ekstedt, Niklas
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Babu, Sajeesh
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Failure Rate Trends in an Aging Population - Monte Carlo Approach2015In: 23rd International Conference on Electricity Distribution - CIRED 2015, CIRED - Congrès International des Réseaux Electriques de Distribution, 2015Conference paper (Refereed)
    Abstract [en]

    This paper proposes a method to make future failure predictions from input data on population age distribution and failure rates, using a Monte Carlo approach. In contrast to many methods used today, the method in this paper is designed to address multiple properties and assumptions simultaneously, which makes the task complicated. For example, the component population is allowed to be divided into both age and different types. The time-dependent failure rates are defined separately for each individual type, can consist of a combination of multiple different failure rates for separate modes, and can be of practically any shape. Furthermore, a volatility measure for the failure rates is introduced and used to model the uncertainties in failure rate estimates. The method handles investment and reinvestment scenarios as well as different restoration models, such as replacing a failed component with a new component of a different type. As a part of the project, a stand-alone software tool was developed and presented in the paper. In the included case study, the method and the tool are shown to be useful when investigating reinvestment strategies to renew the population and decrease the expected number of future failures. The paper gives the reader useful information and understanding on how the problem of predicting the reliability of the future power system can be addressed and solved.

  • 15.
    Ekstedt, Niklas
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Categorization and Review of Failure Rate Factors Used in Power Systems2014In: 2014 International Conference on Probabilistic Methods Applied to Power Systems, PMAPS 2014 - Conference Proceedings, 2014, IEEE Press, 2014Conference paper (Refereed)
    Abstract [en]

    To evaluate the reliability of power systems, good estimates of the failure rates of the included components are needed. Better predictions can be performed if relevant factors that affect the failure rates are used, and an increasing number of models that include different types of factors have been presented recently. This paper proposes a categorization of failure rate factors into seven categories, based on the type of information for the factors. The categorization can be used to map future studies in the context of similar work.  Furthermore, the paper presents a review of a number of publications that uses different factors to model the failure rate of different power system components. The failure rate factors used in the reviewed publications are categorized into the proposed seven categories and a comprehensive summary table is included. The used models and methods to estimate the failure rate in the reviewed publications are also noted in the summary table.

  • 16.
    Ekstedt, Niklas
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wallnerström, Carl Johan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Babu, Sajeesh
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Westerlund, Per
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Jürgensen, Jan Henning
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Lindquist, Tommie
    Svenska kraftnät.
    Reliability Data: A Review of Importance, Use, and Availability2014Conference paper (Refereed)
    Abstract [en]

    For reliability studies of power distribution systems availability and collection of data on reliability is a key aspect. The acquirement of data can be challenging, because it endures effort and experience to know where to obtain accessible types of data. This paper gives the reader a guide to why input data to reliability analyses and asset management are useful, which data that can be obtained, and how to access the different types of data. Also, how to measure data accurately and the quality needed are discussed in the paper.

    After a general discussion on the benefits of data, we discuss the importance of knowing exactly what the data are measurements of. Furthermore, we argue that data from different contexts, even if seemingly similar, should be used with care. We also state and explain that the amount data restrict the type of analysis that can be conducted. The paper continues with a description of some examples of (to different degrees) open accessible data. Nationally collected reliability data from Swedish utilities, reported to authorities and interest organizations, are described and discussed. We discuss how Swedish weather data, which recently have become free and open, enable more studies on the weather related reliability effects, and some existing test systems are mentioned. A section follows that describes how failure and condition data are typically stored and utilized internally in organizations. Finally, we conclude that the paper is a potential guide and inspiration for anyone planning to conduct a reliability study in the future.

  • 17.
    Estanqueiro, Ana
    et al.
    National Laboratory on Energy and Geology (LNEG).
    Ahlrot, Claes
    E.ON AB.
    Duque, Joaquim
    National Laboratory on Energy and Geology (LNEG).
    Santos, Duarte
    National Laboratory on Energy and Geology (LNEG).
    Gentle, Jake P.
    Idaho National Laboratory (INL).
    Abboud, Alexander W.
    Idaho National Laboratory (INL).
    Morozovska, Kateryna
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Söder, Lennart
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Kanefendt, Thomas
    Fraunhofer IEE.
    DLR use for optimization of network design withvery large wind (and VRE) penetration2018Conference paper (Refereed)
    Abstract [en]

    Due to the stochastic nature of wind and clouds,the integration of wind and PV generation in the powersystem poses serious challenges to the long-term planning oftransmission systems. Grid reinforcements always involverelevant direct costs while the average load factor of the windand solar PV dedicated transmission lines is usually low.Additionally, in very windy sites, the same high windresource that produces large amounts of wind generation andmay congest the transmission lines transporting it to distantconsumption centres may also have a beneficial effect inincreasing the transmission capacity of those lines. In fact, theoccurrence of wind not only contributes to the loading of theconnecting line, but also increases the line capacity, via theconvective cooling of the cables - one of the main heattransfer mechanisms in conductor heat balance; in otherwords, higher winds speeds contribute to faster cooling ofconductor and therefore higher conductor’s capacitypotential. In this paper the existing methodologies tocharacterize those thermal effects in electrical cables - usuallyreferred as dynamic line rating (DLR) - are applied to severalIEA Task 25 countries case studies to characterize thetechnical value of the dynamic operation of thermallycongested lines, as well as its potential economic benefits.

  • 18.
    Fazlagic, Berina
    et al.
    KTH.
    Wallnerström, C. J.
    Shayesteh, Ebrahim
    KTH.
    Hilber, Patrik
    KTH.
    Development of the utilisation and smart grid incentive scheme within the Swedish revenue cap regulation2017In: CIRED - Open Access Proceedings Journal, Institution of Engineering and Technology, 2017, Vol. 2017, no 1, p. 2696-2699, article id 1Conference paper (Refereed)
    Abstract [en]

    This study provides a summary on how Swedish distribution system operators (DSO) are regulated after a revenue cap model, and describes a potential development on the current utilization incentive scheme within this regulation. The analyses are based on data from a Swedish DSO, which have been elaborated with the use of demand response program. The outcome of the demand response simulation has in a later step been applied to calculate the incentive in the revenue cap regulation. Two different calculation approaches are used and compared in order to calculate the load factor in the revenue cap regulation. The results of the case study show that by applying a weighted daily load factor, the DSO in the case study can receive ~3% additional economic income compared to applying an average daily load factor in the incentive calculation. The motivation behind applying weighted load factor is to prioritize days with high energy consumption since those days have more impact on the costs. Most important, the analysis display that replacing the average load factor with a weighted load factor have a non-negligible impact on the incentive calculation and hence if the change fulfill its purpose enough.

  • 19.
    Giesecke, Oskar
    et al.
    KTH, School of Electrical Engineering (EES).
    Karlsson, Rikard
    KTH, School of Electrical Engineering (EES).
    Morozovska, Kateryna
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Reliability study of two offshore wind farm topologies: Radial and ring connection2016In: PROCEEDINGS 15th Wind Integration Workshop: International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as on Transmission Networks for Offshore Wind Power Plants / [ed] Uta Betancourt / Thomas Ackermann, Darmstadt: Energynautics GmbH, 2016Conference paper (Refereed)
    Abstract [en]

    Reliable electrical infrastructure in offshore wind farms (OWF) is a key to maintenance cost minimization. Due to the difficult environment and distance to shore reliability is crucial for the feasibility of the OWF. In this study, two different topologies of OWF designs were investigated and compared. An algorithm was established to estimate reliability and economic profit of the two systems. The two topologies under investigation were the radial and ring system configuration. A radial system has in general less components, but the ring configuration provides redundancy. For the particular case studied, with its assumptions, a threshold of 18 turbines was identified. From this level and above the ring configuration is beneficial.

  • 20.
    Grahn, Pia
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Rosenlind, Johanna
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Alvehag, Karin
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Söder, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    A method for evaluating the impact of electric vehicle charging on transformer hotspot temperature2011In: 2011 2nd IEEE PES International Conference and Exhibition on Innovative Smart Grid Technologies (ISGT Europe), 2011Conference paper (Refereed)
    Abstract [en]

    The expected increasing market share of electric vehicles is a response to the combination of new technological developments, governmental financial control, and an attitude shift of residents to a more environmentally friendly lifestyle. The expected capacity required for charging, imposes changes in the load to the already existing components in the electric power grid. In order to continue managing these existing assets efficiently during this load change, it is important to evaluate the impact imposed by the battery charging.

  • 21.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Cable Segment Replacement Optimization2014In: Reliability Modeling and Analysis of Smart Power Systems / [ed] Rajesh Karki, Roy Billinton, Ajit Kumar Verma, New Delhi Heidelberg New York Dordrecht London: Springer London, 2014, 1, p. 195-202Chapter in book (Refereed)
  • 22.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Component reliability importance indices for maintenance optimization of electrical networks2005Licentiate thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Maximum asset performance is one of the major goals for electric power system managers. To reach this goal minimal life cycle cost and maintenance optimization become crucial while meeting demands from customers and regulators. One of the fundamental objectives is therefore to relate maintenance and reliability in an efficiently and effectively way, which is the aim of several maintenance methods such as the Reliability Centered Maintenance method (RCM). Furthermore, this necessitates the determination of the optimal balance between preventive and corrective maintenance to obtain the lowest total cost.

    This thesis proposes methods for defining the importance of individual components in a network with respect to total interruption cost. This is a first step in obtaining an optimal maintenance solution. Since the methods consider several customer nodes simultaneously, they are especially suitable for network structures that serve many purposes/customers e.g. transmission and distribution networks with more than one load point. The major results are three component reliability importance indices, which are applied in two case studies. The first case study is based on a network in the Stockholm area. The second case study is performed for one overhead line system in the rural parts of Kristinehamn. The application studies demonstrate that the indices are possible to implement for existing electrical networks and that they can be used for maintenance prioritization. Consequently these indices constitute a first step in the overall objective of a maintenance optimization method.

    The computations of the indices are performed both with analytical and simulation based techniques. Furthermore, the indices can be used to calculate the component contribution to the total system interruption cost. The approach developed for the importance indices can be utilized in any multi-state network that can be measured with one performance indicator.

  • 23.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Maintenance optimization for power distribution systems2008Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Maximum asset performance is one of the major goals for electric power distribution system operators (DSOs). To reach this goal minimal life cycle cost and maintenance optimization become crucial while meeting demands from customers and regulators. One of the fundamental objectives is therefore to relate maintenance and reliability in an efficient and effective way. Furthermore, this necessitates the determination of the optimal balance between pre¬ventive and corrective maintenance, which is the main problem addressed in the thesis.

    The balance between preventive and corrective maintenance is approached as a multiobjective optimization problem, with the customer interruption costs on one hand and the maintenance budget of the DSO on the other. Solutions are obtained with meta-heuristics, developed for the specific problem, as well as with an Evolutionary Particle Swarm Optimization algorithm. The methods deliver a Pareto border, a set of several solutions, which the operator can choose between, depending on preferences. The optimization is built on component reliability importance indices, developed specifically for power systems. One vital aspect of the indices is that they work with several supply and load points simultaneously, addressing the multistate-reliability of power systems. For the computation of the indices both analytical and simulation based techniques are used. The indices constitute the connection between component reliability performance and system performance and so enable the maintenance optimization.

    The developed methods have been tested and improved in two case studies, based on real systems and data, proving the methods’ usefulness and showing that they are ready to be applied to power distribution systems. It is in addition noted that the methods could, with some modifications, be applied to other types of infrastructures. However, in order to perform the optimization, a reliability model of the studied power system is required, as well as estimates on effects of maintenance actions (changes in failure rate) and their related costs. Given this, a generally decreased level of total maintenance cost and a better system reliability performance can be given to the DSO and customers respectively. This is achieved by focusing the preventive maintenance to components with a high potential for improvement from system perspective.

  • 24.
    Hilber, Patrik
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Bertling, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A Method for Extracting Reliability Importance Indices from Reliability Simulations of Electrical Networks2005In: 15th Power Systems Computation Conference, PSCC 2005, Power Systems Computation Conference ( PSCC ) , 2005Conference paper (Refereed)
    Abstract [en]

    This paper proposes a reliability importance index that is possible to extract from existing reliability simulations at a low additional computational cost. The index utilizes the concept of reliability worth as a measure of system reliability in order to establish the importance of components in systems with several load points. Since the method is based on simulations, it is suitable for large networks with a high level of detail. The index can be used as decision support for asset management, for example where maintenance actions will become most beneficial. The index is evaluated against the background of a number of analytically calculated indices. Furthermore, the index is applied to a network in the Stockholm area. The conclusion of the paper is that the proposed simulation based importance index provides a means of improving analysis of electrical network reliability.

  • 25.
    Hilber, Patrik
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Bertling, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Component Reliability Importance Indices for Electrical Networks2007In: 2007 CONFERENCE PROCEEDINGS IPEC, VOLS 1-3, NEW YORK: IEEE , 2007, p. 257-263Conference paper (Refereed)
    Abstract [en]

    This paper presents three new component reliability importance indices, developed for electrical networks. Component reliability importance indices are useful for prioritization of components as part of a system, especially when considering maintenance activities and/or investment in new equipment. Traditional component reliability importance indices were developed for systems with one input and one output point, which not captures the general function of electrical networks. The proposed indices utilize the concept of reliability worth as a single measure of system reliability in order to establish the importance of the components. This single measure of reliability works over several supply and load points and does for example enable comparisons between components decicated to different load points as well as components from different systems. Furthermore this paper presents data from application studies, which shows on some of the possibilities with the indices. The conclusion of the paper is that the proposed indices provide means of improving analysis of electrical network reliability.

  • 26.
    Hilber, Patrik
    et al.
    KTH, Superseded Departments, Electrical Systems.
    Bertling, Lina
    KTH, Superseded Departments, Electrical Systems.
    Monetary Importance of Component Reliability in Electrical Networks for Maintenance Optimization2004In: 8th International Conference on Probabilistic Methods Applied to Power Systems: Iowa State Univ, Ames, IA, SEP 12-16, 2004, 2004, p. 150-155Conference paper (Refereed)
    Abstract [en]

    This paper proposes a method for defining the importance of individual components in a network with respect to total interruption cost. Since the method considers several customer nodes simultaneously, it is especially suitable for network structures that serve many purposes/customers e.g. transmission and distribution networks with more than one load point. The major result is an importance index, I-H, for every component. This index can be used for the evaluation of maintenance actions. Furthermore, the importance index is used to calculate the component contribution to the total system interruption cost. Finally, the goal to reach an optimal maintenance plan is outlined. The methodology is presented together with an application study for an electrical distribution system in-the Stockholm area.

  • 27.
    Hilber, Patrik
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Bertling, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Multiobjective Maintenance Policy for a Distribution System an Application Study2008In: 10th International Conference On Probabilistic Methods Applied To Power Systems, 2008, p. 229-234Conference paper (Refereed)
    Abstract [en]

    Maintenance managers of today's electrical distribution systems are faced with the problem of where their maintenance resources will be utilized best. In previous work the authors have developed a method for maintenance policy optimization for electrical networks. This paper presents results from an application study performed for a distribution system, including both urban and rural parts, for this method. A basic optimization is performed in order to find the right level of maintenance for the involved components with respect to customer interruption costs and maintenance budget. For the rural parts of the system the conclusion is that intensified tree trimmings is the most beneficial maintenance activity. While for the urban parts the maintenance should be focused towards customer intense areas. Furthermore, it becomes clear, especially for the urban part of the system, that the operation mode of the network becomes important. The method presented helps the managers to prioritize their maintenance to the places where it will result in the best system performance.

  • 28.
    Hilber, Patrik
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Bertling, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hällgren, Bengt
    Karlstads Universitet.
    Effects of Correlation between Failures and Power Consumption on Customer Interruption Cost2006In: 2006 International Conference on Probabilistic Methods Applied to Power Systems, Vols 1 and 2: 9th International Conference on Probabilistic Methods Applied to Power Systems. KTH, Stockholm, SWEDEN. JUN 11-15, 2006, 2006, p. 1120-1123Conference paper (Refereed)
    Abstract [en]

    This paper evaluates the correlation between failures and power consumption for a distribution systems operator. This is done in order to scrutinize whether the commonly used assumptions of constant failure rate and constant power consumption is reasonable to use for reliability calculations. The studied entity is energy not supplied, which is assumed to be a good estimate of how customer interruption costs are affected. Three different aspects are studied with respect to energy not supplied; seasonal variations, daily variations and repair time variations (as a function of the hour of day). The conclusion is that by using constant failure rates, repair rates and power consumption the approximation of customer costs becomes somewhat low, i.e. by 7% for the studied case. This result indicates that the assumptions of constant failure rates, repair rates and power consumption are quite sufficient for at least the actual case study. Le. since this error probably is significantly smaller than other types of errors, for example customer outage costs estimates. Nevertheless, having performed these calculations the current results should be applied to further modeling of the studied network.

  • 29.
    Hilber, Patrik
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hällgren, Bengt
    Karlstads Universitet.
    Bertling, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Optimizing the replacement of overhead lines in rural distribution systems with respect to reliability and customer value2005In: Eighteenth International Conference and Exhibition on Electricity Distribution, CIRED 2005, 2005, p. 127-131Conference paper (Refereed)
    Abstract [en]

    In this paper we present a method for establishing the value of a network's components from a reliability worth perspective. The method can be applied to a general distribution system i.e. both for radial and meshed network systems. Moreover, this paper shows results from an application study for a rural network system that is dominated by overhead lines. The purpose of the study was to establish the value of a secondary feeding point. Further, the method is used to establish the best replacement strategy for the concerned overhead lines.

  • 30.
    Hilber, Patrik
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Miranda, Vladimiro
    Faculty of Engineering, University of Porto.
    Matos, Manuel A.
    Faculty of Engineering, University of Porto.
    Bertling, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Multiobjective Optimization Applied to Maintenance Policy for Electrical Networks2007In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 22, no 4, p. 1675-1682Article in journal (Refereed)
    Abstract [en]

    A major goal for managers of electric power networks is maximum asset performance. Minimal life cycle cost and maintenance optimization becomes crucial in reaching this goal, while meeting demands from customers and regulators. This necessitates the determination of the optimal balance between preventive and corrective maintenance in order to obtain the lowest total cost.

    The approach of this paper is to study the problem of balance between preventive and corrective maintenance as a multiobjective optimization problem, with customer interruptions on one hand and the maintenance budget of the network operator on the other. The problem is solved with meta-heuristics developed for the specific problem, in conjunction with an evolutionary particle swarm optimization algorithm.

    The maintenance optimization is applied in a case study to an urban distribution system in Stockholm, Sweden. Despite a general decreased level of maintenance (lower total maintenance cost), better network performance can be offered to the customers. This is achieved by focusing the preventive maintenance on components with a high potential for improvements. Besides this, this paper displays the value of introducing more maintenance alternatives for every component and choosing the right level of maintenance for the components with respect to network performance.

  • 31.
    Hilber, Patrik
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wallnerström, Carl Johan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Rosenlind, Johanna
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Babu, Sajeesh
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Westerlund, Per
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Benefits of Reliability Centred Asset Management2013In: 22nd International Conference and Exhibition on Electricity Distribution (CIRED 2013), 2013, p. 0668-Conference paper (Refereed)
    Abstract [en]

    This paper provides an overview of risk and reliability assessment techniques, some which are available for distribution system operators, and others that are in the process of development. The main contribution of this paper is showing the possibilities and benefits of detailed risk and reliability analysis. Six samples of findings from research developed over the last decade within the RCAM group (Reliability Centred Asset Management) at the Royal Institute of Technology, Stockholm Sweden, are presented. The research is directly associated with risk and asset management applied to power systems. The first three examples are within developed research, followed by three areas where great potential is seen: 1) The value of accurate thermal models of transformers; 2) The impact of tariff regulation on asset management decisions; 3) Detailed interruption studies; 4) Dynamic rating; 5) Combined risk and reliability analysis of primary equipment and control equipment; 6) Systematic diagnostic measures for asset management.

  • 32.
    Hilber, Patrik
    et al.
    KTH, School of Electrical Engineering (EES).
    Wallnerström, Carl Johan
    KTH, School of Electrical Engineering (EES).
    Rosenlind, Johanna
    KTH, School of Electrical Engineering (EES).
    Setréus, Johan
    KTH, School of Electrical Engineering (EES).
    Schönborg, Niclas
    Svenska Kraftnät.
    Risk analysis for power systems: overview and potential benefits2010Conference paper (Refereed)
    Abstract [en]

    This paper provides a mapping and sample of recently developed risk assessment techniques that are available for the distribution system operator. Three estimates on the value of more detailed risk analysis are desccribed. I.e. component reliability importance indices can be used to divversify the maintenance efforts, gaining better expected system performance at no cost. Furthermore, components that are assumed to be relatively harmless (based on average values) are identified as critical for longer interruptions. Finally it is shown that losses in a transformer are critical in the decision on transformer lifetime.

  • 33.
    Hilber, Patrik
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wallnerström, Carl Johan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Setreus, Johan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Isenberg, Anna
    Potentiell användning av standardkostnader i regleringen av elnätsföretagens löpande påverkbara kostnader2010Report (Other academic)
    Abstract [sv]

    Potentiell användning av standardkostnader i regleringen av elnätsföretagens löpande påverkbara kostnader har utretts på uppdrag av Energimarknadsinspektionen (EI). Specifikt om mått baserade på Ekm kan användas i regleringen och hur dessa mått bör korrigeras. Ekm (Ekvivalent ledningslängd) är en standardkostnadsmodell där olika entiteter (t.ex. mängd av en komponentkategori) av elnät tilldelas en kvot för årlig drift- och underhållskostnad i förhållande till 1 km 0,4 kV luftledning. Projektets huvudslutsats är att existerande Ekm, tillhandahållna av Svensk Energi, inte kan användas utan rigorösa kontroller och omfattande utveckling, vilket entydigt styrks av utförda analyser. Emellertid finns det en potential att använda Ekm-liknande mått i kommande reglering. Vid ett eventuellt införande rekommenderas följande: 

    • Preliminära Ekm-liknande tal implementeras under första tillsynsperiod för en lägre andel av löpande påverkbara kostnader än ursprunglig plan (25%), förslagsvis så att de medel som omfördelas i genomsnitt blir väsentligt lägre än de medel som omfördelas på grund av kvalitetsregleringen. Noggrann utvärdering görs fortlöpande, vars resultat används för revidering av Ekm till nästkommande tillsynsperiod.
    • Detaljerad analys och utveckling av Ekm liknande tal; förslag på handlingsplan tillhandahålls i appendix.

    Två enkäter har utformats, huvudenkäten har skickats ut till samtliga nätägare i Sverige. Svarsfrekvensen på denna blev 46 % (motsvarande ca 75 % av kundunderlaget). Uppföljande enkät skickades till de som angivit intresse för fördjupat deltagande (12st). Utöver enkäter bygger studien på material från EBR (del av Svensk Energi) och EI samt diskussioner med dessa och nätägare. Trots mängden empiriskt material har inga konkreta förslag på Ekm liknande mått kunnat tillhandahållas, dels på grund av stora variationer mellan svar och analyser och dels på grund av låg svarsfrekvens i enkäten för de delar som avser konkreta kostnader. I vissa fall ger enkätsvaren och analyserna indikationer på storleksordning för Ekm tal och hur dessa bör justeras. Ytterligare en osäkerhetsfaktor är huruvida nuvarande indelning är lämplig, dvs. om fler eller färre objektiva förutsättningar, t.ex. terräng, bör tas i beaktning. Detta är kopplat till den viktiga frågan om att kunna göra en analys av utfallet av ett införande, innan implementering (hur kommer intäkterna omfördelas). Detta har ej kunnat göras då företagen ännu ej rapporterat in sina nät-tillgångar. Studiens viktigaste resultat:

    • Möjliggörande för samtliga parter att bidra med synpunkter innan eventuell skarp implementering. 
    • Inventering nuvarande förutsättningar avseende Ekm.
    • Indikation på hur vissa mått bör modifieras.
    • Metoder för utvärdering och bestämmande av nya mått. 
    • Ökad medvetenhet och kunskap hos myndighet och nätföretag. 
    • Handlingsplan för fortsatt arbete.
  • 34.
    Hilber, Patrik
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Westerlund, Per
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Lindquist, Tommie
    The reliability chain2015Conference paper (Refereed)
    Abstract [en]

    This is a conceptual paper introducing the novel reliability chain as a holistic framework for the asset management process. The main objective of the reliability chain is to give a tool for communication regarding asset management decisions and to visualize the different areas that are important for efficient decisions.

     

    The reliability chain describes the process from data on component and system level to asset management decisions, via steps of information handling. Furthermore a brief outline on how to utilize the concept in order to categorize different asset management strategies is provided.

     

    A main challenge in many maintenance and asset management processes is the risk of putting too much attention to a limited part of the process. One of the main conclusions from the paper is that significant efforts have to be spent on the transitions between the different links in the reliability chain in order to reach an efficient and well-organized asset management.

  • 35. Josue, Fretz
    et al.
    Arifianto, I.
    Saers, R.
    Rosenlind, Johanna
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Suwarno,
    Transformer hot-spot temperature estimation for short-time dynamic loading2012In: Proceedings of 2012 IEEE International Conference on Condition Monitoring and Diagnosis, CMD 2012, IEEE , 2012, p. 217-220Conference paper (Refereed)
    Abstract [en]

    On the transformer, the effect of thermal stress is aging of the solid insulation. Excessive thermal stress could damage the solid insulation. To avoid this, transformer condition monitoring systems could use thermal models to forecast the operating temperatures during dynamic loading. There are several thermal models of varying complexity, including the thermal models stipulated by the IEC and IEEE standards. In this paper, the thermal model which is referred to by IEC Std. 60076-7 as the differential method, is modified, so it accounts for the oil viscosity dependence on temperature. The model is validated using hot-spot temperature measurements from a 40-MVA transformer, OFAF-cooled, 21/115 kV which is located in the subarctic climate region.

  • 36.
    Jürgensen, Jan Henning
    KTH, School of Electrical Engineering and Computer Science (EECS).
    Hilber, Patrik
    KTH, School of Electrical Engineering and Computer Science (EECS). KTH Royal Institute of Technology.
    Estimation of Individual Failure Rates for Power System Components Based on Risk Functions2019In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 34, no 4, p. 1599-1607Article in journal (Refereed)
    Abstract [en]

    The failure rate is essential in power system reliability assessment and thus far, it has been commonly assumed as constant. This is a basic approach that delivers reasonable results. However, this approach neglects the heterogeneity in component populations, which has a negative impact on the accuracy of the failure rate. This paper proposes a method based on risk functions, which describes the risk behavior of condition measurements over time, to compute individual failure rates within populations. The method is applied to a population of 12 power transformers on transmission level. The computed individual failure rates depict the impact of maintenance and that power transformers with long operation times have a higher failure rate. Moreover, this paper presents a procedure based on the proposed approach to forecast failure rates. Finally, the individual failure rates are calculated over a specified prediction horizon and depicted with a 95% confidence interval.

  • 37.
    Jürgensen, Jan Henning
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Brodersson, Anna Lilly
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    TOWARDS HEALTH ASSESSMENT: FAILURE ANALYSIS AND RECOMMENDATION OF CONDITION MONITORING TECHNIQUES FOR LARGE DISCONNECTOR POPULATIONS2016In: CIRED Workshop - Helsinki 14-15 June 2016, Helsinki, 2016, p. 4-Conference paper (Refereed)
    Abstract [en]

    Condition monitoring of power system equipment is an important part of asset management. Hence, health indices were developed to describe the equipment condition in a linguistic form with the obtained data. The development of health indices requires knowledge about the equipment population under investigation to consider all important factors. Therefore, this paper investigates the failure data of a large disconnector population to identify population characteristics such as failure modes and failure locations. The analysis showed that the functions maneuverability and current carrying are essential to monitor. Moreover, this paper discusses condition monitoring techniques for disconnector and their applicability in large populations. The paper concludes that even without cost intensive investments in condition measurements or higher preventive maintenance costs, a condition evaluation can be performed.

  • 38.
    Jürgensen, Jan Henning
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Brodersson, Anna Lilly
    Vattenfall Eldistribution AB.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nordström, Lars
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    The Proportional Hazard Model and the Modelling of Recurrent Failure Data: Analysis of a Disconnector Population in Sweden2017In: 2017 Cigré SC B3 (Substations) Colloquium, 18 - 20 September 2017, Recife, Brazil, 2017, p. 1-8Conference paper (Refereed)
    Abstract [en]

    Failure rate estimation is an important tool for planning and operating decision making in asset management of the power system. Moreover, the knowledge of how different explanatory variables impact the failure rate of the power system equipment is crucial for substation design. This study investigates 2191 work orders of 1626 non-current breaking disconnectors with 344 major failures. In particular, this paper analyses the disconnector failure data regarding recurrent failure data. Since the original PHM cannot handle recurrent event data, different extensions were developed such as the Andersen-Gill (AG), Prentice, Williams and Peterson (PWP), and the Wei, Lin, and Weissfeld (WLW) model. These models are applied to the disconnector dataset with 140 recurrent time-to-failure processes. The explanatory variables age at admission, remote control, preventive maintenance, and voltage level are assessed. The results show that preventive maintenance has a significant and positive impact on the recurrences with all tested methods. Also remote control, voltage level, and age are significant covariates. Compared to the single failure study previously conducted, where age had no significance, age is significant when assessing the recurrent failure which is the most critical difference to the analysis without recurrences.

  • 39.
    Jürgensen, Jan Henning
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Brodersson, Anna Lilly
    Vattenfall Eldistribution AB.
    Nordström, Lars
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Impact Assessment of Remote Control and Preventive Maintenance on the Failure Rate of a Disconnector Population2018In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 33, no 4, p. 1501-1509Article in journal (Refereed)
    Abstract [en]

    This paper presents the impact of different explanatory variables such as remote control availability and conducted preventive maintenance, among others, on failure statistics of a disconnector population in Sweden using the proportional hazard model. To do so, 2191 work orders were analysed which included 1626 disconnectors and 278 major failures. Here, the results show that the remote control availability for disconnectors - an example of such Smart Grid technology - has a negative effect on the failure rate, whereas preventive maintenance has a positive impact. It is also shown that the disconnector age is not significant and that certain disconnector types have a significant and positive correlation towards failures when compared to other disconnector types. The results increase the understanding of disconnector failures to improve asset management.

  • 40.
    Jürgensen, Jan Henning
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nordström, Lars
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A Review and Discussion of Failure Rate Heterogeneity in Power System Reliability Assessment2016Conference paper (Refereed)
    Abstract [en]

    The failure rate is a reliability measure which isused for planning and operation of the power system. Thus far, average or experience based failure rates were applied to power system equipment due to their straightforward implementation. However, this approach limits the accuracy of the gained resultsand neglects the important differentiation between populationand individual failure rates. Hence, this paper discusses and demonstrates the necessity to distinguish between populationand individual failure rates and reviews the existing literature offailure rate estimation within the power system domain. The literature is categorized into statistical data driven approaches and failure rate modelling with focus on different criteria whichcan be used to describe the heterogeneity within populations. The review reveals that the environmental impact was modelled predominantly.

  • 41.
    Jürgensen, Jan Henning
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nordström, Lars
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A Review and Discussion of Failure Rate Heterogeneity in Power System Reliability Assessment2016Manuscript (preprint) (Other academic)
    Abstract [en]

    The failure rate is a reliability measure which isused for planning and operation of the power system. Thus far, average or experience based failure rates were applied to power system equipment due to their straightforward implementation. However, this approach limits the accuracy of the gained resultsand neglects the important differentiation between populationand individual failure rates. Hence, this paper discusses and demonstrates the necessity to distinguish between populationand individual failure rates and reviews the existing literature offailure rate estimation within the power system domain. The literature is categorized into statistical data driven approaches and failure rate modelling with focus on different criteria whichcan be used to describe the heterogeneity within populations. The review reveals that the environmental impact was modelled predominantly.

  • 42.
    Jürgensen, Jan Henning
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nordström, Lars
    KTH, School of Electrical Engineering (EES), Industrial Information and Control Systems.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A Scorecard Approach to Track Reliability Performance of Distribution System Operators2015In: CIRED 2015: 23rd International Conference on Electricity Distribution, CIRED - Congrès International des Réseaux Electriques de Distribution, 2015Conference paper (Refereed)
    Abstract [en]

    Firstly, this paper reviews the current process of reporting reliability data in Sweden. Limitations of reliability indices such as SAIDI and SAIFI are discussed and the need for more reliability measures is stated. The paper suggests the introduction of a reliability performance scorecard to analyse reliability measures in an organized system under different aspects. Furthermore, a set of measurements is provided that can be used to assess a utility’s reliability performance. The use of the scorecard is discussed and its applicability for implementing and tracking regulations. This would result in a better policy-making and a decreased pressure on electric utilities due to a higher understanding of what companies invest to achieve a reliable supply of energy.

  • 43.
    Jürgensen, Jan Henning
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Nordström, Lars
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Estimation of Individual Failure Rates for Power System Components based on Risk Functions2018Manuscript (preprint) (Other academic)
    Abstract [en]

    The failure rate is essential in power system reliability assessment and thus far it has been commonly assumed as constant. This is a basic approach that delivers reasonable results. However, this approach neglects the heterogeneity in component populations which has a negative impact on the accuracy of the failure rate. This paper proposes a method based on risk functions, which describes the risk behaviour of condition measurements over time, to compute individual failure rates within populations. The method is applied to a population of 12 power transformers on transmission level. The computed individual failure rates depict the impact of maintenance and that power transformers with long operation times have a higher failure rate. Moreover, the paper presents a procedure based on the proposed approach to forecast failure rates. Finally, the individual failure rates are calculated over a specified prediction horizon and depicted with a 95\% confidence interval.

  • 44.
    Jürgensen, Jan Henning
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nordström, Lars
    KTH, School of Electrical Engineering (EES), Electric power and energy systems.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Individual failure rates for transformers within a population based on diagnostic measures2016In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 141, p. 354-362Article in journal (Refereed)
    Abstract [en]

    The high monetary value of a transformer has placed the transformer life-time optimization into the focus of asset management. The average failure rate has created reasonable results within reliability modeling, however, it cannot reflect the probability of failure for an individual transformer. In this paper, a method is introduced to calculate individual failure rates for a transformer population based on failure statistics and diagnostic measurements such as dissolved gas, and 2-furfuraldehyde analysis. The method is applicable to all types of components and the comprehensibility makes it effective for practical implementation. The results are evaluated against two health indices based on a weight factor and fuzzy logic. It can be observed that the presented individual failure rates are plausible representatives of the transformer's probability of failure. Therefore, the results can also be utilized for asset management decision-making.

  • 45.
    Jürgensen, Jan Henning
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Nordström, Lars
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Hilber, Patrik
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Brodersson, Anna Lilly
    Vattenfall Eldistribution AB.
    Andreasson, Elin
    Vattenfall Eldistribution AB.
    Assessment of Explanatory Variables on the Failure Rate of Circuit Breakers Using the Proportional Hazard Model2018In: 2018 POWER SYSTEMS COMPUTATION CONFERENCE (PSCC), Dublin, Ireland: IEEE conference proceedings, 2018, article id 8442567Conference paper (Refereed)
    Abstract [en]

    This paper utilises the proportional hazard model to understand and quantify the impact of explanatory variables on the failure rate of circuit breakers (CB). Particularly, 4496 work orders with 2622 high voltage CBs are investigated with an occurrence of 281 major failures. Different explanatory variables such as CB type, manufacturer, preventive maintenance (PM), and others are gathered to quantify their significance and magnitude of their effect. The results present that PM has a positive impact, the number of operations within the last year a negative impact, and age has a small but negative impact on the failure rate. The CB type is not significant in all analyses which can be explained by examining the PM and age of these CB types. This paper contributes to the understanding of how explantatory variables impact the failure rate which is essential for power system asset management.

  • 46.
    Jürgensen, Jan Henning
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nordström, Lars
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Brodersson, Anna Lilly
    Vattenfall Eldistribution AB.
    Andreasson, Elin
    Vattenfall Eldistribution AB.
    Goel, Lalit
    Nanyang Technological University Singapore.
    Modelling of Recurrent Circuit Breaker Failures with Regression Models for Count Data2018Conference paper (Refereed)
    Abstract [en]

    High voltage circuit breaker (CB) are of fundamental importance to protect and operate the power system. To improve their performance and to better predict failures, it is necessary to understand the effect of covariates such as preventive maintenance, age, voltage level, and the CB type. A straightforward approach is to investigate recurrent failures with regression models for count data. In this paper, several regression models are developed to estimate the impact of the aforementioned covariates to predict the recurrence of failures. The results show that age has a significant and negative impact, preventive maintenance before the first failure has a positive impact, and that the voltage level has a negative impact. Moreover, the Poisson, Negative Binomial, and zero-inflated models are compared. The comparison shows that the Negative Binomial model has the best fit to the studied recurrent failure data.

  • 47.
    Jürgensen, Jan Henning
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Scheutz Godin, Axel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Health index as condition estimator for power system equipment: A critical discussion and case study2017In: CIRED - Congrès International des Réseaux Electriques de Distribution, 2017., Institution of Engineering and Technology, 2017, Vol. 2017, p. 202-205, article id 1Conference paper (Refereed)
    Abstract [en]

    Over the past decade, the health index has become an increasingly popular asset management tool in utilities. The health index as a condition indicator can improve the decision making process. However, it also has challenges which need to be considered during development and implementation. This paper addresses the advantages and disadvantages of the health index as a condition indicator in a critical discussion. Moreover, a case study is presented where a health index is calculated for three transmission power transformers. The case study illustrates that age and the load factor included in the health index calculation lead to an immoderately high health index for the transformers T2 and T3. Thus, the paper ageing of the transformer windings are used instead which results in a plausible condition representation of all three transformers. The case study also demonstrates that the observation of condition trends over time is lost if the health index is transformed into a linguistic expression.

  • 48.
    Mashad Nemati, Hassan
    et al.
    Halmstad University.
    Pinheiro Sant'Anna, Anita
    Halmstad University.
    Nowaczyk, Sławomir
    Halmstad University.
    Jürgensen, Jan Henning
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Reliability Evaluation of Power Cables Considering the Restoration Characteristic2019In: International Journal of Electrical Power & Energy Systems, ISSN 0142-0615, E-ISSN 1879-3517, Vol. 105, p. 622-631Article in journal (Refereed)
    Abstract [en]

    In this paper Weibull parametric proportional hazard model (PHM) is used to estimate the failure rate of every individual cable based on its age and a set of explanatory factors. The required information for the proposed method is obtained by exploiting available historical cable inventory and failure data. This data-driven method does not require any additional measurements on the cables, and allows the cables to be ranked for maintenance prioritization and repair actions.

    Furthermore, the results of reliability analysis of power cables are compared when the cables are considered as repairable or non-repairable components. The paper demonstrates that the methods which estimate the time-to-the-first failure (for non-repairable components) lead to incorrect conclusions about reliability of repairable power cables.

    The proposed method is used to evaluate the failure rate of each individual Paper Insulated Lead Cover (PILC) underground cables in a distribution grid in the south of Sweden.

  • 49.
    Mazidi, Peyman
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems. Comillas Pontifical University, Spain.
    Bobi, Miguel A. Sanz
    Shayesteh, Ebrahim
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Impact of health indicators on maintenance management and operation of power systems2017In: Journal of Risk and Reliability, ISSN 1748-006X, E-ISSN 1748-0078, Vol. 231, no 6, p. 716-731Article in journal (Refereed)
    Abstract [en]

    This article proposes a maintenance management and risk reduction approach. The approach introduces two reliability-based indexes called condition indicator and risk indicator. Condition indicator is a unit-less parameter that comes directly from monitored condition of a component and converts the categorical condition into a numerical value. Risk indicator in megawatt represents the risk imposed by the health of a component onto the system. To demonstrate application of the indicators, they are implemented through an hourly network constraint unit commitment problem and applied in a test system where the analysis of impact of condition of the generators to the operation is the new contribution. The results demonstrate how addition of such indicators will impact the operation of the grid and maintenance scheduling. The results show the benefit for the system operator as the overall failure risk in the system is taken into account, and the benefit for the asset owner as the direct impact of the maintenance to be carried out can be investigated. Two of the main outcomes of the maintenance management and risk reduction approach are as follows: asset owners can analyze their maintenance strategies and evaluate their impacts in the maintenance scheduling, and system operators can operate the grid with higher security and lower risk of failure.

  • 50.
    Morozovska, Kateryna
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Risk analysis of wind farm connection to existing grids using dynamic line rating2018In: 2018 International Conference on Probabilistic Methods Applied to Power Systems, PMAPS 2018 - Proceedings / [ed] Milorad Papic, IEEE conference proceedings, 2018Conference paper (Refereed)
    Abstract [en]

    Dynamic Line Rating (DLR) is a technology that allows extending capacity of existing power lines for transportation higher electrical power capacity from production site to the customer. Environmental conditions such as air temperature and wind speed have significant impact on the heat balance of the power line, therefore by monitoring the weather parameters one can dynamically vary the capacity of power line. Brief analysis of the wind speed and direction effects on the cooling of overhead lines and principles of wind power generation provide a background for the analysis of wind power connection to the existing power lines by means of Dynamic Line Rating.

    This paper presents the case study of wind farm connection to the existing power line by application of DLR. Statistical weather data and information about system properties give an input parameters for the risk analysis of the dynamic rating application and highlight benefits of using DLR. The results show a potential to expand the transportation capacity of the line by injecting more wind power. Probabilistic analysis of the historical weather data and reliability analysis of the system show that line capacity can be expanded with more wind power without affecting availability of the connection.

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