ETEP z Novel Modelling of Residual Operating Time of Transformer Oil M.A.A. Wahab, M.M. Hamada, A.G. Zeitoun, G. Ismail Abstract zyxwvutsrq This paper presents techniques developed to accurately model the characteristics zyx of transformer oil for the pur- pose of predicting the effect of aging on these characteristics. Aging causes some of the transformer oil charac- teristics to violate the internationally specified limits. The purpose of this simulation is to develop alternative techniques to predict the operating time a f e r which these characteristics would violate the limits. The results ob- tained from monitoring of twenty in-service power transformersfor long period of operating time up to ten years have been implemented in developing the proposed models. The physical, chemical and electrical characteristics have been determined periodically by internationally specijied testing methods. The patterns of violation sequence of the standard limits, against operating time, by different transformer oil characteristics have been revealed and the most common pattern has been determined. The definition of residual operating time (trot) of the different transformer oil characteristics has been introduced. The choice of transformer oil breakdown voltage trot to rep- resent that of the transformer oil characteristics has been justified. Modelling of trot as a function of transformer oil breakdown voltage, total acidity and water content by multiple-linear regression has been proposed and ve- rified. zyxwvutsrqp Also, polynomial regression model of trot as a function only of transformer oil breakdown voltage has been given. The accuracy and applicability of these models and the difierent modelling techniques have been discussed and proved. 1 Introduction zyxwvuts The reliability of an electric power system depends strongly on those of its components. Transformer insu- lation has a great impact on its reliability. Transformer oil has proved to be very reliable for the last decades. However it is affected by normal and faulted operating conditions. Major faults in power transformers can cause not only catastrophic damages within the unit but also interruption of electricity supply resulting in large economic loss [l]. The new trend of investigation of transformer insulation is the development of diagnos- tic, measurement and monitoring techniques [2-6]. The aim of these techniques is to prevent insulation deteri- oration and to eliminate unexpected transformer outag- es. International Electrotechnical Commission (IEC) and similar bodies put forward regulations on the levels of acceptance of the oil characteristics in the form of standard specifications. The in-service operating time after which some or all characteristicsviolate the speci- fied limits will be referred to as the residual operating time (trot). The prediction of trot is of vital importance to plan adequate maintenance schedule and to prevent unpredicted outage of several transformers, which in turn increases the reliability of electric power supply. The occurrence of a disturbance in the transformer can be detected by the deviation of consecutive predicted residual operating times from the general trend. The purpose of this paper is to introduce a model for the prediction of trot for transformer oil character- istics. The patterns of the violation sequence of the different transformer oil characteristics with operating time are revealed depending on the monitoring results and discussed from the viewpoint of their affects on the operation of the transformers. According to these pat- terns, the transformer oil characteristic with longest op- erating time has been determined. The trot has been for- mulated by treating the results obtained from monitor- ing all the transformers as those of a single equivalent transformer. This concept was first introduced by the authors in a previous publication zyx [7]. The contribution of this study can be summarized in the following points: Proposal and verification of the violation sequence patterns of the standard specification limits for the different transformer oil characteristics. Introduction and definition of trot for the different transformer oil characteristics. Determination of the descriptive characteristic of the transformer oil deterioration. Development of novel models to predict the trot of transformer oil characteristics. Justifying the applicability and validity of trot models by implementing them on predicting trans- former oil characteristics. The results of these transformers don’t have to be considered in devel- oping the models. Experimentation The procedures of determining the breakdown voltage, total acidity, water content, ash content, vis- cosity and flash point are given in [8-131 respectively. ETEP Vol. zyxwvutsrq 13, No. 4, JulyIAugust 2003 219