Chapter 14. Loading and Thermal Performance

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Unformatted text preview: 14 Loading and Thermal Performance Robert F. Tillman Alabama Power Company 14.1 Design Criteria............................................................... 14-1 14.2 Nameplate Ratings......................................................... 14-2 14.3 Other Thermal Characteristics ..................................... 14-2 14.4 Thermal Profiles ............................................................ 14-3 14.5 Temperature Measurements.......................................... 14-3 14.6 Predicting Thermal Response....................................... 14-4 14.7 Load Cyclicality.............................................................. 14-6 14.8 Science of Transformer Loading................................... 14-7 14.9 Water in Transformers under Load ............................. 14-8 Dielectric Effects of Moisture in Oil . Bubble Evolution 14.10 Loading Recommendations.......................................... 14-9 14.1 Design Criteria ANSI standards collection for the C57 series sets forth the general requirements for the design of power transformers. With respect to loading, the requirements of concern are those that define the transfor- mers thermal characteristics. The average ambient temperature of the air in contact with the cooling equipment for a 24-hr period shall not exceed 30 8 C, and the maximum shall not exceed 40 8 C (IEEE, 1994). For example, a day with a high temperature of 35 8 C and a low of 25 8 C has an approximate average ambient temperature of 30 8 C. The average winding-temperature rise above ambient shall not exceed 65 8 C for a specified continuous-load current (IEEE, 1994). The industry uses this criterion rather than the more relevant hot-spot temperature rise because manufacturers can obtain it by simply measuring the resistance of the windings during temperature-rise tests. The manufacturer must guarantee to meet this requirement. The hottest-conductor temperature rise (hot spot) in the winding shall not exceed 80 8 C under these same conditions (IEEE, 1994). This 80 8 C limit is no assurance that the hot-spot rise is 15 8 C higher than the average winding-temperature rise. In the past, using the 15 8 C adder to determine the hot-spot rise produced very conservative test results. The greatest thermal degradation of cellulose insulation occurs at the location of the hottest spot. The hot-spot location is near the top of the high- or low-voltage winding. The most common reason for hot spots to occur is that these regions have higher localized eddy losses because the leakage flux fringes radially at the winding ends. Standards specify requirements for operation of the transformer above rated voltage. At no load, the voltage shall not exceed 110% (IEEE, 1994). At full load, the voltage shall not exceed 105% (IEEE, 1994)....
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Chapter 14. Loading and Thermal Performance - 14 Loading...

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