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  • 1.
    Eriksson, Roland
    et al.
    KTH, Superseded Departments, Electrical Systems.
    Lindquist, Tommie
    KTH, Superseded Departments, Electrical Systems.
    Bertling, Lina
    KTH, Superseded Departments, Electrical Systems.
    Reliability modelling of aged XLPE cables2003In: Nordic Insulation Symposium (Nord-Is), 2003, p. 216-224Conference paper (Refereed)
  • 2.
    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.

  • 3.
    Lindquist, Tommie
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Estimating the accuracy of thermography of disconnector contacts in the field2008Report (Other academic)
    Abstract [en]

    This report summarises the results from an investigation into the accuracy of condition assessment of electrical contacts via thermography measurements in the field. The investigation was carried out in Ludvika, Sweden at the Swedish Transmission Research Institute (STRI) outdoor test site during the autumn of 2007. The investigation was performed as a part of the PhD project Modelling of power equipment reliability with special reference to the impact of maintenance carried out at the RCAM research group at the KTH School of Electrical Engineering.

    Two naturally aged disconnector poles of the make ABB NSA 400/3150 was loaded up to their nominal load in three stages and the contact temperature rises were measured both using thermography and thermocouples attached to the contacts. The thermography measurements were carried out by am experienced technician from Vattenfall Service. After the first set of measurements some contacts were subjected to standard maintenance and the measurements were repeated.

    The investigation resulted in statistical distribution for the errors related to the estimation of contact temperature rise at nominal load based on thermography measurements. These error distributions were used to simulate the accuracy of the estimations. Furthermore the investigation resulted in two linear regression models of the relationship between contact resistance and temperature rise as well as the effect of maintenance on contact temperature rise.

    A conclusion from the measurements was that statistical models should be used to estimate the effect of maintenance on the contact condition as well as estimating the contact condition from resistance measurements.

  • 4.
    Lindquist, Tommie
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On reliability and maintenance modelling of ageing equipment in electric power systems2008Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Maintenance optimisation is essential to achieve cost-efficiency, availability and reliability of supply in electric power systems. The process of maintenance optimisation requires information about the costs of preventive and corrective maintenance, as well as the costs of failures borne by both electricity suppliers and customers. To calculate expected costs, information is needed about equipment reliability characteristics and the way in which maintenance affects equipment reliability. The aim of this Ph.D. work has been to develop equipment reliability models taking the effect of maintenance into account.

    The research has focussed on the interrelated areas of condition estimation, reliability modelling and maintenance modelling, which have been investigated in a number of case studies. In the area of condition estimation two methods to quantitatively estimate the condition of disconnector contacts have been developed, which utilise results from infrared thermography inspections and contact resistance measurements. The accuracy of these methods were investigated in two case studies. Reliability models have been developed and implemented for SF6 circuit-breakers, disconnector contacts and XLPE cables in three separate case studies. These models were formulated using both empirical and physical modelling approaches. To improve confidence in such models a Bayesian statistical method incorporating information from the equipment design process was also developed. This method was illustrated in a case study of SF6 circuit-breaker operating rods. Methods for quantifying the effect of maintenance on equipment condition and reliability have been investigated in case studies on disconnector contacts and SF6 circuit-breakers. The input required by these methods are condition measurements and historical failure and maintenance data, respectively.

    This research has demonstrated that the effect of maintenance on power system equipment may be quantified using available data. However, realising the full potential of these methods requires the gathering and utilisation of failure and maintenance data as well as condition measurements to be improved.

  • 5.
    Lindquist, Tommie
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On reliability modelling of ageing equipment in electric power systems with regard to the effect of maintenance2005Licentiate thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Power system maintenance optimisation involves obtaining the minimum total costs, including preventive and corrective maintenance costs and the cost of failures for both supplier and customer. To calculate the cost of failure, information is needed about the equipment reliability characteristics. It is also necessary to know how maintenance affects component reliability. The aim of the work leading up to this thesis has been to develop reliability models that include the effect of maintenance.

    Three case studies have been carried out for different types of power system components using three distinct methods. In the first study the reliability of the first generation XLPE cables was modelled with respect to failures caused by water treeing using load-strength modelling. The model was based on assumptions of the ageing process and the distribution system characteristics. This study showed that it is possible to and overvoltage and insulation characteristics that can be fitted to agree with failure statistics for water tree ageing in XLPE cables. The second case study included a study of all circuit breaker failures in the Swedish transmission grid during the period from 1 January 1999 to 30 June 2003. In a subsequent investigation a method to combine information from the design process with maintenance records and failure statistics was employed using Bayesian methods. The resulting reliability model is continuously updated as more failure and maintenance data

    becomes available. This case study showed that it is possible to develop reliability models for components that have not yet failed by utilising information from the design process and right-censored observations from inspections. Finally, in the third case study a quantitative method for establishing the condition of disconnector contacts by the use of thermography was developed. Two sets of measurements on disconnector contacts in the Swedish transmission

    grid were carried out to establish the accuracy of the method. By utilising the results from the measurements estimates of the statistical distributions of the error sources were produced.

    The results from the case studies show that the lack of detailed, high-quality data remains a critical problem when modelling reliability of power system equipment, even when using methods that require a minimum of data.

  • 6.
    Lindquist, Tommie
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Bertling, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A method for calculating disconnector contact availability as a function of thermography inspection intervals and load current2007In: CIGRE Symposium Osaka 2007: System Development and Asset Management under Restructuring, 2007Conference paper (Refereed)
    Abstract [en]

    This paper presents a practical method of quantifying and evaluating changes to existing thermography inspection schemes for disconnector contacts. The inspection interval availability is used to calculate disconnector contact average maintenance costs. Any changes to existing thermography schemes regarding load or inspection intervals are subsequently evaluated using these maintenance costs. The proposed method is illustrated by a case study of three disconnectors in the Swedish 400-220kV transmission system. The three disconnectors represent different typical load characteristics. The result of the case study was that the new thermography inspection scheme would reduce the total average maintenance cost by a third, for the three disconnectors. In the study, the large reduction in maintenance cost was due to the specific load characteristics of the disconnectors. When applying the method to a larger disconnector population the cost reduction would depend on the actual loading characteristics of all disconnectors. The aim of the case study is to illustrate the usefulness of the proposed method when it comes to quantifying the effect of changes in thermography inspection schemes. The only failure mode considered in this paper is contact over-heating. A disconnector contact that is found to be failed is assumed to be replaced by a new identical contact. Since the exact time of a hidden failure is unknown, the exponential distribution is used for the time to failure in order to adapt the model to the amount of information available to the network owner. The proposed method provides a practical tool to quantify and evaluate the effect of different changes to the inspection scheme with respect to maintenance costs and availability.

  • 7.
    Lindquist, Tommie
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Bertling, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hazard rate estimation for high-voltage contacts using infrared thermography2008In: Proceedings. Annual Reliability and Maintainability Symposium, ISSN 0149-144X, p. 233-239Article in journal (Refereed)
    Abstract [en]

    Infrared thermography is the most common way to assess the condition of high-voltage electrical contacts in electric distribution and transmission systems. However, thermography has one major drawback as contacts carrying low or no load may not be assessed.

    This paper proposes a method to save and make use of thermography measurements to estimate statistical distribution parameters for the time to failure for a population of electrical contacts. The statistical distributions may then be used to support maintenance decisions for the sometimes large proportion of contacts that may not be assessed directly due to low load. As the measurement results accumulate over the years more accurate predictions can be made.

    The proposed method is illustrated using three test cases applied to a population of high-voltage disconnectors with randomly generated thermography measurements.

    The main conclusion of the paper is that using the proposed method will provide maintenance decision support for high-voltage apparatus that may not be assessed directly by thermography.

  • 8.
    Lindquist, Tommie
    et al.
    KTH, Superseded Departments, Electrical Systems.
    Bertling, Lina
    KTH, Superseded Departments, Electrical Systems.
    Eriksson, Roland
    KTH, Superseded Departments, Electrical Systems.
    A feasibility study for probabilistic modeling of aging in circuit breakers for maintenance optimization2004In: 2004 INTERNATIONAL CONFERENCE ON PROBABILISTIC METHODS APPLIED TO POWER SYSTEMS, NEW YORK: IEEE , 2004, p. 1020-1025Conference paper (Refereed)
    Abstract [en]

    To support the maintenance optimization carried out within the framework of the reliability-centered asset management (RCAM) method this paper presents a feasibility study for the probabilistic modeling of the aging of circuit-breakers (CB). The aim of the study is to investigate the possibilities of the development of a reliability model for a CB, using failure data and maintenance records. A study of such statistics was carried out on all 565 CBS in the Swedish National Grid (transmission level) during a time period ranging from 1999-01-01 to 2003-06-30. A total of 53 CB failures were identified and studied with respect to faulty subcomponent, CB function, number of operating cycles to failure and time in operation to failure. Results from this study are presented in this paper.

  • 9.
    Lindquist, Tommie
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Bertling, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Eriksson, Roland
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A method for age modeling of power system components based on experiences from the design process with the purpose of maintenance optimization2005In: Proceedings. Annual Reliability and Maintainability Symposium, ISSN 0149-144X, p. 82-88Article in journal (Refereed)
    Abstract [en]

    This paper introduces a first step towards establishing the effect of maintenance on component reliability. The concept of relative age is introduced and defined as well as a method to establish this relative age of electric power system components for the purpose of maintenance optimization. The proposed method is demonstrated by modeling the relative age of a sub-component for a circuit breaker (CB). The modeling is based on the results from a major CB reliability study and a survey of all CB failures in the Swedish National Grid during the period from January 1(st), 1999 to July 31(st), 2003. The results from this application study show that it is possible to model the reliability of power system components by using results from development tests and calculations from the design process along with failure and maintenance data.

  • 10.
    Lindquist, Tommie
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Bertling, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Eriksson, Roland
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A practical method for benefit/cost analysis of preventive maintenance tasks in power system equipment2007In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208Article in journal (Other academic)
  • 11.
    Lindquist, Tommie
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Bertling, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Eriksson, Roland
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Circuit breaker failure data and reliability modelling2008In: IET Generation, Transmission and Distribution, ISSN 1751-8687, Vol. 2, no 6, p. 813-820Article in journal (Refereed)
    Abstract [en]

    The results from an extensive study of SF6 and minimum-oil circuit breaker failures in the Swedish and Finnish transmission systems are presented. The study includes 1546 breakers with a total operating history of 16 384 years. In the study, a detailed analysis of a specific SF6 type of breaker revealed which parts caused the failures and an estimation of their hazard rates was made. A model of complete reliability for this type of breaker is presented, illustrated by presenting the operating and maintenance history of a shunt reactor breaker in the Swedish transmission system.

  • 12.
    Lindquist, Tommie
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Bertling, Lina
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Eriksson, Roland
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Estimation of disconnector contact condition for modelling the effect of maintenance and ageing2005In: 2005 IEEE Russia Power Tech, PowerTech, 2005Conference paper (Refereed)
    Abstract [en]

    In order to optimize the maintenance of electric power equipment, models of equipment reliability as a function of age and maintenance are necessary. As a first step towards the development of these kinds of models, this paper proposes a quantitative method for establishing the condition of disconnector contacts by the use of thermography, which is a well-established diagnostic practice used in preventive maintenance programmes. Results from two sets of thermography measurements carried out in the Swedish transmission grid are presented. Statistical distributions for the uncertainties when using the method are estimated from the measurements. A method for converting temperature rises measured at non steady state loading conditions to a corresponding steady state temperature rise have been devised using a simple thermal model. The probability of sending an unaged contact to maintenance is estimated using Monte Carlo simulations based on the statistical distributions of the uncertainties. Finally, it is shown that by using the method proposed in this paper it is possible to estimate the contact condition of disconnectors, including Monte Carlo simulated confidence intervals.

  • 13. Montgomery, N.
    et al.
    Lindquist, Tommie
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Garnero, M. -A
    Chevalier, R.
    Jardine, A. K. S.
    Reliability functions and optimal decisions using condition data for EDF primary pumps2006Conference paper (Refereed)
    Abstract [en]

    There are many examples in which proportional hazards modelling (PHM) is used to accurately model the effects of the operating environment on an item's lifetime. Using such techniques it is possible to find an economically optimal moment of replacement based on condition monitoring data. However, in a nuclear power plant, the problem is somewhat different as it is not possible to stop the process and perform preventive maintenance at the most economical moment in time. The problem of interest in such cases has more to do with whether the item will last until the next scheduled plant shutdown or not. This paper presents some developed and implemented theory to calculate the conditional reliability function of an item given the current equipment age and the current values of the condition-monitoring variables. A case study of a shaft seal for a reactor cooling pump (RCP) using data from several French nuclear power plants operated by the Electricité de France (EDF) is also presented.

  • 14.
    Westerlund, Per
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Lindquist, Tommie
    Svenska kraftnät.
    Condition monitoring of the disconnectors in the electric power transmission grid with temperature sensors2013In: Conference proceedings of 3rd International Conference on Maintenance Performance Measurement and Management, Lappeenranta: Lappeenranta University of Technology , 2013Conference paper (Refereed)
    Abstract [en]

    In the electrical substations there are disconnectors, which can be opened to disconnect equipment from high voltage parts. As the disconnectors are normally closed and transfer current owing to their often critical positions in the power grid, it is difficult to maintain them. The purpose is to develop a new online method to monitor disconnectors and a new theory for their ageing, in order to schedule the maintenance better.

    Increasing resistance in the contacts could cause high temperatures and eventually failure. Nine IR temperature sensors have been placed on each of six disconnectors together with reference sensors. After cleaning and interpolation of the data, two regressions are made, first between a sensor at a contact and the reference sensor, then adding the square of the current. The method is promising since it explains most of the variation in temperature with a sensor dependent parameter.

     

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