IEEE 57.12.00-2015 pdf free.IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers.
For multi-winding transformers or autotransformcrs, applies only to specific loading conditions used as ihe basis of design. These loading conditions involve simultaneous coordinaiion of kVA input and output. load power flictors. and winding voltagc combinations [see item j) of 4.3.3j. Dificrences in loading and voltage regulation for various output windings may prevent simultaneous achievement of 105% voltage on all output terminals. In no case shall thc kVA outputs specified for any loading condition require continuous loading of any input winding in excess of its rating. Maximum continuous transformer operating voltage
The maximum continuous transtbrnier operating voltage should not exceed the levels specified in ANSI CM. I. System conditions may require voltage transformation ratios involving tap voltages higher than the maximum system voltage tir regulation purposes, however, the appropriate maximum system voltage should be observed under operating conditions.
4.1.7 Outdoor operation
Unless otherwise specified. transformers shall be suitable for outdoor operation.
4.1.8 Step-down operation
Unless otherwise specified, transformers shall be designed for step-down operation. Generator step-up transformers
Transformers idcniified as generator step-up transformers shall be designed for step-up operation. System tie transformers
lranstormers identified as system tie transformers or autotranstormers shall he designed ror step-down operation, or step-up operation, or both, as specified by the user.
4.1.9 Tank or enclosure finish
Temperature limits and tests shall be based on the use of a nonmetallic pigment surface paint finish. It should he noted that metallic-flake paints, such as aluminum and tine. have properties that increase the temperature rise of transtormcrs, except in direct sunlight.
4.2 Loading at other-than-rated conditions
IEEE Std C57.91 [B12] provides guidance for loading at other-than-rated conditions including the fllowing:
a) Ambient temperatures higher or lower than the basis of rating
b) Short-time loading in excess of nameplate kVA with normal life expectancy
c) Loading that results in reduced life expectancy
IEEE Std C57.91 [11121 is an IEEE guide. and as such, it does not specify mandatory requirements. It provides the best known general information for loading of transformers under various conditions based on typical winding insulation systems. It is based upon the best engineering information available at the time of preparation. The guide discusses limitations of ancillary components other than windings that may limit the capability of transftrmcrs. When specified, ancillary components and other construction liatures (such as cables, bushings. tap changers. insulating liquid expansion space. etc.) shall be supplied in a manner that will not limit the loading to less than the specified capability of the windings.
4.3 Unusual service conditions
Conditions other than those described in 4.1 are considered unusual service conditions and, when prevalent, should be brought to the attention of those responsible fir the design and application of the apparatus. Examples of some of these conditions are listed in 4.3.3.
4.3.1 Unusual ambient temperature and altitude conditions
Transfbrmers may be used at higher or lower ambient temperatures or at higher altitudes than those specified in 4.1.3, but special consideration should be given to these applications. IEEE Std C57.91 jBl2] provides inlrmation on recommended practices.
4.3.2 Insulation at high altitude
The dielectric strength of’ transtrmers that depend in whole or partly upon air for insulation decreases as the altitude increases due to the etTect of decreased air density, When specified. transtirmers shall be designed with larger air spacing between terminals using the correction facors of Table I to obtain adequate air dielectric strength at altitudes above 1000 m (3300 ft). Insulation level
The minimum insulation necessary at the required altitude can be obtained by dividing the standard insulation level at 1000 m (3300 (1) by the appropriate correction factor from Table I. Bushings
Bushings with additional length or arcing distance shall be furnished when necessary fbr operation above 1000 m (3300 ft).IEEE 57.12.00 pdf download.