IEEE C57.154-2012 pdf free download.IEEE Standard for the Design, Testing, and Application of Liquid-Immersed Distribution, Power, and Regulating Transformers Using High-Temperature Insulation Systems and Operating at Elevated Temperatures.
NOTE 1—Some of the limiting factors to be considered in determining the permissible maximum temperatures are: a) Aging of materials, such as cellulose-based insulation, which introduce moisture inside the transformer tank b) Velocity of the liquid in the cooling ducts, since long exposure of the liquid to high temperature will accelerate degradation c) Accelerated aging of the liquid and insulating materials due to catalytic action caused by the presence of bare copper and silver surfaces, which generate by-products such as particles and copper derivatives dissolved in insulating liquids d) Gas bubbles caused by overheated trapped moisture between the winding conductors and the conductor insulation NOTE 2—More general information on bubbling and an equation for calculating the approximate temperature at which bubble generation occurs can be found in a paper by Oommen [B37] 6 and in IEEE Std C57.91. While this information is based on cellulose-based insulation and mineral oil, the concepts are essentially the same for high-temperature materials. However, studies referenced in IEEE Std 1276 [B19] indicate that high-temperature insulation materials tend to have lower moisture content than cellulose-based insulation and, consequently, tend to initiate bubbling at much higher temperatures. This is also true for high-temperature insulating liquids with high moisture saturation levels, such as esters, in either high-temperature or conventional solid insulation materials. NOTE 3—Although design references in this standard refer to core form transformer windings, the design principles and guidelines can be transferred to shell form transformers. The typical layout of windings in shell form transformers is different from that shown in the examples, but it should have no influence on the recommended temperature limits for both solid insulation materials and liquids.
In full hybrid insulation windings, high-temperature insulation shall be used at least in lead exit areas, where cables connect with windings operating at temperatures higher than conventional. Frequently, these cables are connected directly to hot spot areas of the windings or hot spots are created in connection points. The selection of insulation material for the remaining length of the cable shall be based on its designed temperature gradient and may include conventional insulation materials. Similar to mixed hybrid insulation windings, the use of high-temperature insulation materials can be selective and limited to specific areas only. Even if the entire winding is conventional, the lead exits or entire lead cables can still be designed to operate at temperatures higher than conventional. In such cases the cable insulation shall be selected appropriate to the designed temperatures. 6.3 Bushings General requirements and test procedures for outdoor power apparatus bushings intended for use in liquid- immersed transformers and reactors shall be in accordance with IEEE Std C57.19.00. General requirements and test procedures for dc apparatus bushings intended for use in liquid-immersed converter transformers and smoothing reactors shall be in accordance with IEEE Std C57.19.03. The bushings defined in these standards are often a liquid-filled condenser type that shall be designed specifically for top liquid temperatures operating above a 65 °C temperature rise, or standard bushings shall be derated for the specific temperature of the application. The bushing manufacturer’s derating factors shall be applied for applications where the top liquid temperature rise exceeds 65 ° C. NOTE—IEEE Std C57.19.100 TM [B20] provides an example of derating calculations for top liquid temperature rises between 55 °C and 65 °C. 6.5 Tap-changer The general requirements for de-energized (DETC) and load tap changers (LTC) shall be in accordance with IEEE Std C57.131 for transformers operating at high-temperatures. However, special design considerations shall be necessary when the temperature of the liquid surrounding the tap-changer exceeds 105 ° C. This temperature limit shall be based on normal rated load and a maximum ambient temperature of 40° C. For a small increase in the insulating liquid temperature, derating a larger capacity switch shall be acceptable. For higher temperatures, a special switch designed for high environmental temperatures shall be required. See the reference paper by Dix and Hopkinson for more information on testing of switch contacts [B14]. Re-locating the switch to a cooler position in or on the transformer tank can also be an option.