Reliability Assessment for Elements of Transmission and Transformation Systems
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4.1 Calculating reliability indices of substations based on the overload capability of the transformers The reliability indices for a generating system take into account the load and the available capacity of the system (the total rated power of all available generation units). The analytical models for determining the reliability indices of the generation system are relatively simple. This is true, especially when the load model is represented by the load duration curve i.e., without considering the load variation in time. Moreover, the load duration curve can be represented by the daily peak loads. The load representation by the load duration curve is justified theoretically for studies of power generation. The reason for this is that the load capacity of the generation units is practically close to their rated power. When transformers are considered, the situation is different. The load capacity of transformers is determined by the hottest spot temperature of the insulation and strongly depends on the daily load curve of the transformer. The temperature has strong influence on the life expectancy of the transformer. Thus, the load duration curve should not be used for the calculation of reliability indices of substations when the real load capacity of the transformers is considered. For transformers, designed in accordance with IEC standards principles (IEC 60354/1991, IEEE Std. C57. 91-1995 and IEC 60076-7/2005) the socalled “rated” life expectancy is obtained when the transformer is operated so that the hottest point temperature of the insulation is equal to 98°C. Thus two possible operational conditions may be defined: 1. Normal operation of the transformer. In this case, the permissible load level is determined so that the lifetime of the transformer is equal to its “rated” usage period, i.e., the usage period acquired at a constant temperature of 98°C. 2. Transformer operation under emergency conditions. Such a case is, for example, a forced outage of one of the transformers. In this case, the remaining transformers may be overloaded and operating at temperatures above 98°C. In any case, the load capacity of the transformer is limited in such a way, that the insulation hottest spot temperature should not exceed 140°C D. Elmakias: Reliability Assessment for Elements of Transmission and Transformation Systems, Studies in Computational Intelligence (SCI) 111, 239–372 (2008) www.springerlink.com © Springer-Verlag Berlin Heidelberg 2008
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New Computational Methods in Power System Reliability
and the oil temperature in the upper part of the transformer should not increase above 115°C (for transformer design according to IEC standards). This means that if reliability indices are to be determined by a probabilistic approach, they should be calculated in relation to the hottest spot temperature of the insulation. At normal operation conditions, the hottest spot temperature of the insulation and the rate by which the transformer life time is determined, depend on the load curve and the ambient temperature. In t
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