IEEE Std C57.12.00 pdf free download
IEEE Std C57.12.00 pdf free download.Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers.
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 for 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
Transformers 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 [B12] provides information on recommended practices.
4.3.2 Insulation at high altitude
The dielectric strength of transformers that depend in whole or partly upon air for insulation decreases as the altitude increases due to the effect of decreased air density. When specified, transformers shall be designed with larger air spacing between terminals using the correction factors of Table I to obtain adequate air dielectric strength at altitudes above 1000 m (3300 Ii).
4.3.2.1 Insulation level
The minimum insulation necessary at the required altitude can be obtained by dividing the standard insulation level at 1000 m (3300 fi) by the appropriate correction factor from Table 1.
4.3.2.2 Bushings
Bushings with additional length or arcing distance shall be furnished when necessary for operation above 1000 m (3300 fi).
5.3.2.2 Dissimilar single-phase transformers
Two single-phase transformers arc assembled in an enclosure, and permanently interconnected, with the
following characteristics:
a) Performance characteristics shall be based on bank operation of three-phase to two-phase transformation or vice versa.
b) The single-phase transformers may not be identical or interchangeable.
5.3.2.3 Three-legged core
Another arrangement uses a three-legged core with main and teaser coil assemblies located on the two outer legs.
and a center leg that has no coil assembly. This provides a common magnetic circuit for the two outer legs.
5.3.2.4 Identical single-phase transformers
When specified. two identical single-phase transformers shall be furnished.
a) The kVA rating of each transformer shall be half the bank output required. The rating of the individual units shall agree with the ratings established for single-phase transformers.
b) Performance characteristics (except heating) shall be based on single-phase operation.
c) The temperature rise shall be based on delivering the required bank capacity when transforming from three-phase to two-phase or from two-phase to three-phase, as specified.
d) Transformers shall be interchangeable as main and teaser.
e) Regulating taps are not usually supplied on transformers for three-phase to two-phase or from twophase to three-phase service. When taps are required, the teaser tap shall be 86.6% of the mean regulating taps (used here, mean refers to the midpoint of the range of regulating taps).
5.10 Insulation levels
Transformers shall be designed to provide coordinated low-frequency and impulse insulation levels on line terminals and low-frequency insulation levels on neutral terminals. The primary identity of a set of coordinated levels shall be its maximum system voltage and basic lightning impulse insulation level (BIL). BIL will be selected dependent on the degree of exposure of the transformer and characteristics of the over-voltage protection system.
IEEE Std C57.12.00 pdf download.
