IEEE C57.12.56-1986 pdf free.IEEE Standard Test Procedure for Thermal Evaluation of lnsulation Systems for Ventilated Dry-Type Power and Distribution Transformers.
IEEE C57.12.56  methods are developed to provide a means for evaluating insulation systems as a function of thermal agingand are an extension of AIEE 65-1956,°Thermal Evaluation of Ventilated Dry-Type Power and DistributionTransformers.
One method is based on retention of a dielectric withstand voltage equal to a percentage of the initial 5060 H.dielectric withstand c capability of the test sample.
The second method is based on the retention of the basic impulse insulation level’ by impulse testing, or by related[10] 50/60 Hz voltage withstand capability tests on models.See 3.7.
The intent of these test methods is to have each component of the insulation system tested under conditions that are asnearly as possible the same as those in the actual transformer. Thus, each of the components is evaluated in accordancewith its actual function.
The primary aging factors shall be temperature and time. Although the primary aging factors are temperature and time,the criterion of failure of these high-voltage insulation systems is assumed to be voltage related to the initial dielectricstrength or to the rated basic impulse voltage level.Therefore, the time to failure of the system is determined during the accelerated thermal aging by its ability to withstand prescribed proof test voltages applied after each thermal agingcycle. See 3.7.
The Arrhenius relationship is the theoretical basis for this standard. Insulating systems which contain a largepercentage of inorganic material may not lend themselves to complete thermal evaluation by techniques based on theArrhenius relationship.
Test methods specified in this test procedure are of an accelerated nature. Hence, an Arrhenius extrapolation of thetime to failure obtained at the test temperatures (log of life versus 1/absolute temperature) is required to obtain thetemperature classification for normal operation. As the conditions of this accelerated testing are unusually severe,extrapolation of the data will indicate a shorter time to failure than will be obtained in actual service use. Due to thelack of a universally accepted standard transformer insulation system, it is not possible to compare insulation systemswith a standard system by way of the Arrhenius approach. It is expected that with the extended use of this testprocedure such a comparison can be made.Until then, a reference time of 40 000 h to failure shall be used as aminimum acceptable basis for establishing a temperature classification. See 4.1(3)(d).
To ensure that valid results are obtained, free of bias and suitable for comparative studies, the test data shall be reducedstatistically and the results reported according to Section 4.Tests shall be carried out in accordance with Method I described in 3.1.l except that Method 2 described in 3.1.2 maybe used where applicable.IEEE C57.12.56 pdf download.