On reliability modelling of ageing equipment in electric power systems with regard to the effect of maintenance

University dissertation from Stockholm : KTH

Abstract: 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 databecomes 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 transmissiongrid 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.