Transformer case study

Electrical engineers must know when and where to specify dry-type transformers. Energy efficiency guides play a vital role in this process.


Figure 1: Shown above is a typical dry-type distribution transformer. All graphics courtesy: RTM Engineering ConsultantsDuring a recent higher education renovation project completed in 2016, the transformer selection and installation processes were closely studied. The project consisted of a new floor remodel, in which it was necessary to tie into a new switchboard that had been implemented as part of a previous project. The facility was in the process of swinging all loads over to a new 480 V switchboard and decommissioning the existing one. While this process of transferring loads over to the new switchboard was being handled floor by floor, only the first-floor integration is described

A single 480 V feed was to be extended up from the basement to the first floor. The design implemented one 480 V distribution switchboard to handle HVAC loads and a single dry-type transformer to supply the 208 V distribution system. For this project, RTM Engineering Consultants closely analyzed loads, compared costs, evaluated any harmonics that would affect the performance of the transformer, and considered current standards in the selection process.

The first consideration when moving forward with the sizing of the transformer was to review the loads for the first-floor space. This higher education facility is largely composed of HVAC, computer, and lighting loads. The HVAC equipment would be fed from the 480 V distribution section, and the effects of variable frequency drives would not be directly connected to the transformer 208 V secondary. The space would have computer loading as well as LED lighting throughout. Other loads that were considered included audio/video equipment, such as flat-screen TVs and projectors.

Calculations began with having in-depth conversations with the end user to determine the exact planned electrical load. After a list of equipment had been obtained, loads could be reviewed and calculated according to NFPA 70-2008: National Electrical Code (NEC). The total connected loading on the 208 V system was calculated at 398 kVA. At first glance, the loading appeared to fit nicely with a 500-kVA transformer. Demand calculations were then performed based on NEC allowances to determine the demand load. The electrical demand for the first-floor space was calculated at 223 kVA.

Following the connected and demand calculations, the actual transformer construction and performance was reviewed. If a 500-kVA transformer was to be selected, the manufacturer design performance was optimized at 35% loading. Because the actual target for the transformer was calculated at 175 kVA, the 500-kVA option would fit nicely with the calculated demand load.

At this point, the transformer-sizing selection was approached. For one last comparison, an alternative transformer selection was reviewed. If a 333-kVA transformer was installed in lieu of the 500-kVA transformer, what would be the tradeoffs?

Figure 2: This diagram is of a typical transformer core and shows the primary and secondary windings with associated magnetic flux.The concern with the 333-kVA transformer is twofold from a performance standpoint. First, if there is a situation where the true load approaches the connected load—which was very feasible for this facility—the 333-kVA transformer would be overloaded, degrading the performance, efficiency, and life of the transformer by operating outside of normal operating conditions. Secondly, the calculated demand load was much closer to the optimal-performance target of the 500-kVA transformer than the 333-kVA transformer. That being said, under normal operating conditions, using the 500-kVA transformer would be more efficient.

Following the load analysis, it made sense to review the need for a K-factor-rated transformer. Given the study of the loads and the available harmonics present in the electrical power distribution system, a K-4 factor was deemed to be the best possible choice to reduce and mitigate the heating effects harmonics would have on the transformer.

The final step in the process involved taking the transformer-selection data and comparing the efficiency of insulation and windings, which helped to select the best transformer for the application. These are all discussions that should be planned with your owner/client so that they are informed and onboard with the equipment selection. Working closely with an equipment vendor to obtain pricing and run cost analysis will also help your owner/client make the best possible decision for their facility.

Matt Zega is an associate with RTM Engineering Consultants. He specializes in electrical power distribution for commercial, industrial, and health care facilities.

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