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  • Component reliability importance indices for maintenance optimization of electrical networks

    Patrik Hilber

    Licentiate thesis

    Royal Institute of Technology Stockholm, Sweden 2005

  • Akademisk avhandling som med tillstnd av Kungl Tekniska Hgskolan framlgges till offentlig granskning fr avlggande av teknologie licentiatexamen, onsdagen den 25 maj 2005 klockan 11:30, sal H1, Teknikringen 33, Kungl Tekniska Hgskolan, Stockholm. Granskare: Dr. Gerd Hovin Kjlle, SINTEF, Norge. Patrik Hilber, april 2005 Tryck: Universitetsservice US AB TRITA-ETS-2005-09 ISSN-1650-674X ISRN KTH/R-0504-SE ISBN 91-7178-055-6

  • i

    Abstract 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. Key words: Reliability Importance Index, Maintenance Optimization, Asset Management, Customer Interruption Cost, Reliability Centred Maintenance (RCM), Reliability Centered Asset Management (RCAM), Monte Carlo Simulation.

  • iii

    Preface This thesis was written as a part of the Ph.D. project Development of optimization methods for maintenance considering reliability and costs at the department of Electrical Engineering, division of Electrotechnical Design at the Royal Institute of Technology. This project was funded by the national research program EFFSYS and Fortum Distribution. I would like to thank the following persons for their contribution to this work: Professor Roland Eriksson and Dr. Lina Bertling, my supervisors, for the opportunity to carry out my Ph.D. studies at the department. I am also thankful to my supervisors and my colleague Tommie Lindquist for many interesting and animated discussions as well as for their invaluable knowledge within the field of Electrical Engineering. Bengt Hllgren for many absorbing discussions as well as for providing a connection to the reality of electrical networks. The staff at Kristinehamns Energi and especially Mohammad Bagherpour, for their patience with all my questions and requests of data. And not the least, the project steering committee for their knowledge, ideas and help.

  • v

    List of Papers

    I Monetary Importance of Component Reliability in Electrical Networks for Maintenance Optimization. Patrik Hilber and Lina Bertling. Proceedings 8th International Conference on Probabilistic Methods Applied to Power Systems, PMAPS 2004, Ames, USA.

    II Optimizing the replacement of overhead lines in rural distribution systems with

    respect to reliability and customer value. Patrik Hilber, Bengt Hllgren and Lina Bertling. Accepted to 18th International Conference & Exhibition on Electricity Distribution, CIRED 2005, Turin, Italy.

    III A Method for Extracting Reliability Importance Indices from Reliability

    Simulations of Electrical Networks. Patrik Hilber and Lina Bertling. Accepted to 15th Power Systems Computation Conference, PSCC 2005, Liege, Belgium.

  • vii

    Table of Content

    Abstract i

    Preface iii

    List of Papers v

    Table of Content vii

    1 Introduction 1

    1.1 Background 1 1.2 Related research at the department 1 1.3 Project objective 2 1.4 Scientific contribution 2 1.5 Thesis outline 2

    2 Asset management 5

    2.1 The aim of Asset Management 6 2.2 Risk and Asset Management 6 2.3 Literature study 7 2.4 Conclusions 10

    3 Maintenance optimization 11

    3.1 Classification of maintenance optimization 12 3.2 Maintenance of electrical networks 14 3.3 Prerequisites for Maintenance optimization of electrical networks 19

    4 Reliability calculation and modeling 21

    4.1 General simplifications and assumptions 22 4.2 Reliability assessment techniques 24 4.3 General component reliability importance indices 25 4.4 Importance index methods developed for electrical networks 29

    5 Component reliability importance indices 31

    5.1 Interruption cost index IH 31 5.2 Maintenance potential IMP 32

  • viii

    5.3 Simulation based index IM 32 5.4 Comparison of the indices IH, IMP and IM 34

    6 Case studies 35

    6.1 Customer interruption costs 35 6.2 Urban system - Birka 36 6.3 Rural system - Kristinehamn 40 6.4 Conclusions 47

    7 Closure 49

    7.1 Conclusions 49 7.2 Future work 49

    Appendix A: Reflections on customer interruption cost, a measure of network performance 53

    References 61

  • To my little Family

  • 1

    1 Introduction

    1.1 Background 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 becomes crucial while meeting demands from customers and regulators. One of the fundamental objectives is therefore to relate maintenance and reliability in an effective 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. Methods to support cost-effective maintenance strategies have been developed as presented in [1], which presents a generic quantitative method for developing RCM plans. The resulting technique involves the comparison of different strategies for maintenance and therefore the results provide not the optimal solution but the best solution from among the selected strategies. This thesis can therefore be seen as a direct extension of the method developed and constitutes a foundation for a maintenance optimization method. The question of finding an optimal maintenance solution is not new, see [2]-[3]. However, the relation between maintenance, reliability and costs is not completely solved and requirements from owners, authorities and customers, create needs and incentives for still newer methods to handle the maintenance in an effective and efficient way. Consequently, and with this focus a research program is in progress at the Department of Electrical Engineering, KTH [4]. This thesis constitutes a midway report of one Ph.D. project within the program.

    1.2 Related research at the department The Department of Electrical Engineering at KTH has built up specialist knowledge in the field of reliability modeling of power distribution systems. This has for example resulted in two published doctoral theses [1] and [5]. These Ph.D. projects had different focuses for evaluating the impact on the system reliability: preventive maintenance of components to reduce system failures, and automation of the system

  • 2

    to reduce the outage times due to component failures. The first project used results from earlier research at the department in the fields of cable insulation diagnostics. The present research project together with a project on component reliability modeling, with specific reference to maintenance [6], constitutes a logical continuation of the work at the department. The vision is to establish a comprehensive program in power system asset management, with a special focus on the effects of maintenance on reliability, at both component and system level. A description of the suggested research program on reliability centered asset management is presented in [4].

    1.3 Project objective The overall objective of the Ph. D. project, of which this thesis is a part, is to develop an optimization method for cost-effective maintenance plans for electric power networks. The resulting methods shall be applicable and hence comprehensive case studies using the developed methods are crucial means of achieving the objective.

    1.4 Scientific contribution The main result of the work is the formulation and derivation of three importance indices that provide a foundation for maintenance optimization of electrical networks. This is achieved by creating a connection between total network performance and the individual

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