Power Transformers
4-1. General
a. Type. Step-up transformers for use with mainunits should be of the oil immersed type for outdoor operation, with a cooling system as described in paragraph4-3, suited to the location. General Corps of Engineers power transformer design practice is covered by Guide Specification for Civil Works Construction CW-16320.
b. Three-phase transformers. In the majority of applications, three-phase transformers should be used for generator step-up (GSU) applications for the following reasons:
(1) Higher efficiency than three single-phase units of equivalent capacity.
(2) Smaller space requirements.
(3) Lower installed cost.
(4) Lower probability of failure when properly protected by surge arresters, thermal devices, and oil preservation systems.
(5) Lower total weight.
(6) Reduction in weights and dimensions making larger capacities available within practical weight and size limitations.
c. EHV applications. In applications involving interconnection
to EHV (345 kV and above) systems, reliability and application considerations dictate the use of single-phase units due to lack of satisfactory industry experience with three-phase EHV GSU transformers. The basic switching provisions discussed in Chapter 2 describe the low-voltage switching scheme used with EHV transformers.
d. Transformer features. Regardless of winding configuration, for any given voltage and kVA rating, with normal temperature rise, the following features should be analyzed for their effect on transformer life cycle costs:
(1) Type of cooling.
(2) Insulation level of high-voltage winding.
(3) Departure from normal design impedance. Examples of typical transformer studies which should be performed are contained in Appendix B of this manual.
e. Transformer construction. There are two types of construction used for GSU transformers. These are the core form type and the shell form type. Core form transformers generally are supplied by manufacturers for lower voltage and lower MVA ratings. The core form unit is adaptable to a wide range of design parameters, is economical to manufacture, but generally has a low kVA-toweight
ratio. Typical HV ranges are 230 kV and less and 75 MVA and less. Shell form transformers have a high kVA-to-weight ratio and find favor on EHV and high MVA applications. They have better short-circuit strength characteristics, are less immune to transit damage, but have a more labor-intensive manufacturing process. Both
forms of construction are permitted by Corps’ transformer guide specifications.
4-2. Rating
The full load kVA rating of the step-up transformer should
be at least equal to the maximum kVA rating of the generator
or generators with which they are associated.
Where transformers with auxiliary cooling facilities have dual or triple kVA ratings, the maximum transformer rating should match the maximum generator rating.
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