IEEE C57.12.01:2020 pdf free download – IEEE Standard for General Requirements for Dry-Type Distribution and Power Transformers
These factors,in combination with time and temperature,may increase the rate of thermal degradation ofmaterials and contribute to early failure.The winding temperature-rise limits and insulation-system materialsfor dry-type transformers are so chosen that the transformers will have a satisfactory life under usual operatingconditions based on insulation-system thermal evaluation.Unusual loading conditions are discussed in 4.2.3.
5.11.2 Classification of insulation systems
5.11.2.1 General
The duration of the service life of a transformer is predominately defined by the thermal aging of the insulationsystem.Experience has shown that the thermal aging characteristics of a complete insulation system can bedifferent than thermal aging characteristics of each component in the system.For example, some componentshave ratings lower than the temperature rating of the complete insulation system.To assure satisfactory servicelife, insulation systems shall be evaluated by service experience or accelerated-life tests.Accelerated-life testsare used to shorten the evaluation period required before insulation systems using new or a combination ofnew and existing insulating materials can be used with confidence. Tests on complete insulation systems arenecessary to confirm the performance of materials for their specific functions in the transformer. Insulation-system testing for dry-type transformers shall be conducted in accordance with IEEE Std C57.12.60.
5.11.2.2 Insulating materials
Insulating materials are the variety of the dielectric materials that are used in the manufacturing of dry typetransformers. These materials include products in the sheet form(papers, boards, or films), products appliedin liquid form(impregnating resins, casting resins, wire enamels or adhesives) as well as other componentsproduced to perform a specific function in the transformer(tapes, sleeving, etc.).Each of these materials hasunique characteristics (thermal index, electrical and mechanical properties) which make them appropriate forthe specific location used in the transformer design.
5.11.2.3 Insulation system
A transformer insulation system is a combination of insulating materials and insulation componentsmanufactured from these insulating materials arranged to create the structure of the transformer. The majorfunctions of the transformer insulation system are to prevent dielectric breakdown from an electric stress andto provide a mechanical support to the transformer elements(windings, core, frame, etc.) which maintains theability of the transformer to withstand these stresses over the life of the transformer when subjected to thermal,mechanical and environmental forces.
5.11.2.4 System limiting temperature
Limiting system hottest-spot temperatures and associated maximum winding-temperature rises are describedin 5.11.3 and are approved only when used in the insulation of apparatus within the scope of this standard.These temperatures should not be confused with the values used for the identification and classification of thematerials themselves.
The electrical and mechanical properties of the insulated winding shall not be impaired by the application ofthe hottest-spot temperature permitted for the specific insulation system.The word impaired is used here inthe sense of causing any change that could disqualify the insulating material from continuously performing itsintended function, whether it is creepage spacing, mechanical support, or dielectric barrier action.
5.11.3 Limits of temperature rise for continuously rated transformers
Hottest-spot temperature rise above the ambient temperature shall not exceed the limits given in Table 7.Theaverage winding-temperature rises above the ambient temperature, when measured by the resistance methodand tested in accordance with the applicable provisions of IEEE Std C57.12.91, shall not exceed the valuesgiven in Table 716.The hottest-spot temperature rise shall be determined by calculation or from temperaturetest data.