In the deregulated electrical energy market, that strives towards higher system availability at lower costs, the reliability estimation of electrical components is becoming an increasingly important, as well as difficult, task. Indeed, utilities have to face on one hand the progressive aging of many power system devices and on the other hand the high-reliability of such devices, for which only a small number of lifetime values is observed. This paper gives theoretical and practical aids for the proper selection of reliability models for power system components. Firstly, the most adopted reliability models in the literature about electrical components are synthetically reviewed and the classical “direct reliability assessment” - i.e. reliability assessment via statistical fitting directly from in-service failure data of components – is illustrated, that is commonly used at the maintenance stage of components already at work. The properties of these models, as well as their practical consequences, are discussed and it is shown that direct fitting of failure data may result poor or uncertain due to the limited number of data. Thus, practical aids for reliability assessment can be given by the knowledge of the degradation mechanisms responsible for component aging and failure. Then, since insulation is the weakest part of most electrical devices (particularly in MV and HV systems), phenomenological and physical models developed over the years for the estimation of insulation aging and life are illustrated. Such aging and life models, when inserted in a probabilistic framework, lead to “physical reliability models” that are employed for the so-called “indirect reliability assessment”, very useful at the design stage of components such as cables, capacitors, transformers and electrical machines. Finally, the paper shows by means of numerical and graphical examples that seemingly similar reliability models can possess very different lifetime percentiles and hazard rates. This is a very practical consequence of the model selection which must be carefully accounted for, since it involves completely different maintenance actions and costs.
Theoretical and practical aids for the proper selection of reliability models for power system components
MAZZANTI, GIOVANNI
2008
Abstract
In the deregulated electrical energy market, that strives towards higher system availability at lower costs, the reliability estimation of electrical components is becoming an increasingly important, as well as difficult, task. Indeed, utilities have to face on one hand the progressive aging of many power system devices and on the other hand the high-reliability of such devices, for which only a small number of lifetime values is observed. This paper gives theoretical and practical aids for the proper selection of reliability models for power system components. Firstly, the most adopted reliability models in the literature about electrical components are synthetically reviewed and the classical “direct reliability assessment” - i.e. reliability assessment via statistical fitting directly from in-service failure data of components – is illustrated, that is commonly used at the maintenance stage of components already at work. The properties of these models, as well as their practical consequences, are discussed and it is shown that direct fitting of failure data may result poor or uncertain due to the limited number of data. Thus, practical aids for reliability assessment can be given by the knowledge of the degradation mechanisms responsible for component aging and failure. Then, since insulation is the weakest part of most electrical devices (particularly in MV and HV systems), phenomenological and physical models developed over the years for the estimation of insulation aging and life are illustrated. Such aging and life models, when inserted in a probabilistic framework, lead to “physical reliability models” that are employed for the so-called “indirect reliability assessment”, very useful at the design stage of components such as cables, capacitors, transformers and electrical machines. Finally, the paper shows by means of numerical and graphical examples that seemingly similar reliability models can possess very different lifetime percentiles and hazard rates. This is a very practical consequence of the model selection which must be carefully accounted for, since it involves completely different maintenance actions and costs.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
