Beware Rotation Issues on Transformers

Published by Tom Shaughnessy, Shaughnessy Consulting Services

208 Jasper Way, San Marcos, CA 92078



Phase rotation is something that contractors and electricians routinely check during construction and installation of motors and UPS systems. In facilities with only one service, seldom does a rotation change require utility involvement. However, complications can and do arise when a facility has more than one utility service if steps were not taken to ensure the same primary phase rotation exists for each service. This is especially critical if each utility service supplies power to delta/wye transformers and if there are plans to connect the services together at some point.

Typically, the use of fast switching automatic transfer switches inside the facility will bring the primary rotation problems to light. Figure 1 shows waveforms associated with primary rotation problems. The blue and red traces reflect phase A – to-neutral voltages for two different services – 277 volts measured from phase to neutral. There is a 60 degree phase difference between the waveforms. The 60 degree phase difference develops because the primary rotation for one utility transformer leads by 30 degrees and the other lags by 30 degrees. The result is a 60 degree phase difference between the services. It is important to note that both services have the same secondary rotation.

Beware Rotation Issues on Transformers_Figure 1

Figure 1: Resulting out of phase voltage waveforms

Not only is the phase difference an issue, but when one measures from Phase A of one service to Phase A of the second service, where there should be little to no voltage differential when the primary rotation angles are correct, there is now significant voltage difference – 482 volts (black trace).

At this point there are no happy answers:

  • Force the transfer – bad things will probably happen such as damage to automatic transfer switch, blown breakers, possibly even damage to the utility transformers. At the very least, there will be blown fuses.
  • Install custom phase shifting transformers to one of the primary connections to the automatic transfer switch. This approach will be costly and require significant lead times for the phase shifting transformer.
  • Change the utility primary transformer leads to eliminate the 60 degree phase shift. This means a serious nighttime effort and a facility shutdown.

The moral of the story is that prior to construction detailed instructions must exist advising that the primary rotation for each planned service has to match. The same applies if an additional service is added.

Please feel free to leave a question in the comments section.

Published by PQBlog

Electrical Engineer

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