Case study: Expanding transformers

A common occurrence in electrical distribution systems using medium voltage as the primary distribution voltage is double-transforming the loads. In other words, two transformers are used in series to achieve the desired usage voltage. Passing 120/208-V loads through two transformers can result in significant compounded power losses on the system that increase utility costs for the facility.

There are times when this strategy makes sense. For example, in manufacturing facilities, the bulk of the loads may be lighting, mechanical, and large equipment on the 277/480-V distribution system and only small pockets of 120/208 V for receptacles. The power losses from the various small 480- to 120/208-V transformers will have little impact on the overall building’s power usage.

In some facilities, the 120/208-V loads can represent a significant portion of the overall building usage. In this example, NV5 consultants were contracted to study the expansion of an indoor plant-cultivation facility. The facility is served from the utility company with one 12.47-kV service. The medium-voltage service switchgear feeds one 2,500-kVA, 12.47-kV to 277/480-V outdoor oil-filled transformer and includes a spare feeder for future expansion.

In a review of the existing facility, the engineering team discovered the 2,500-kVA transformer serves the facility’s general lighting, mechanical systems—including the chiller plant, air handling units, and exhaust fans—and two step-down transformers. The two transformers are located adjacent to the 2,500-kVA transformer and are each rated 1,000 kVA. The 1,000-kVA transformers step down the voltage to 120/208 V for grow lights, small equipment, and general convenience receptacles. The existing loads are as follows:

277/480 V: 1,724 kVA

120/208 V transformer #1: 510 kVA

120/208 V transformer #2: 394 kVA

Total building load = 2,628 kVA

About 900 kVA of the building load passes through two transformers. Those loads will incur losses in the 1,000-kVA transformers due to core and winding losses. These losses are additive to the building loads, increasing the demand on the 2,500-kVA transformers. The losses from the two 1,000-kVA transformers are now compounded and result in additional demand on the utility service.

A better solution would have been to provide 12.47-kV to 120/208-V transformers and avoid the compounded transformer losses that increase demand on the utility and result in higher electricity costs on the facility. Obviously, the potential energy savings would not offset the construction costs to implement this solution for the existing equipment. However, the engineering team recommended this solution to the client for the expansion using the spare feeder breaker in the medium-voltage service switchgear.

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