There are important tradeoffs to be made when moving transformers from outdoor locations to inside a facility.
I saw a design for very large data center project a few years ago, that used twenty-four 3,000-kVA cast resin units located inside the facility, close-coupled to low voltage switchgear, in a wise "loadcenter" approach (with RC snubbers on the primary of every unit). That arrangement probably eliminated at least a million pounds of underground copper that would have been otherwise required to connect transformers to switchgear.
However, there are important tradeoffs to be made when moving transformers from outdoor locations to inside the facility. One obvious tradeoff is that indoor electrical rooms need to be enlarged to accommodate the physical space requirements of the transformers (which can be significant, especially if they include primary air switches).
Secondly, the heat from the losses of the transformers now is exhausted to inside the building, instead of simply being vented to outdoor air. In most cases, that heat will result in additional loading on the plant’s cooling system, which usually will greatly increase the magnitude of wasted energy. You have the waste heat due to losses rejected by the transformers inside the building, plus the energy consumed by the cooling system to remove that same waste heat from the building. So, efficiency becomes even more important when moving the transformers indoors.
The table below shows a comparison of four styles of transformers, with "typical" efficiencies, in the facility in the above example. Assumptions in the table are that transformers are running at average loading of 75% and that average cost of energy is $0.07 per kWH. The right column uses the Cooper FR3 Envirotemp HDC transformer as baseline, and shows the incremental cost of the other three types, using those assumptions.
Comparison of Transformer Types and Parasitic Wasted Energy
Cost of Wasted
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VPI or Cast Coil Dry, 115 C Rise
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Some readers will argue that there are designs of dry-type and cast coil transformers available with higher efficiencies than those typical values listed in the table. That’s true, but improving efficiency in any dry-type design almost always involves large increases in physical size and in initial cost (and often, involves large and difficult-to-manage increases in inrush current on energization).
The point is, that a liquid HDC transformer, with its average winding temperature operating at 55 C above ambient temperature, will always produce lower losses and less heat than a dry-type transformer with its windings running at an 80 C, 115 C, or 150 C temperature above ambient air temperature.