Q Can I connect four 5kVA buck-boost transformers in parallel to supply a 20kVA load? - L.J.F.
A Four 5kVA buck-boost transformers can be connected in parallel to supply a 20 kVA load after their terminals are phased for like polarity, and they all have the same turns ratio. After the connection has been made, it should be checked for any circulating current between the transformers. Any appreciable circulating current indicates a transformer mismatch. In general, transformers will share the load in inverse proportion to their respective impedances. Transformers of even different rated kVAs may be paralleled and share proper portions of the load provided their turns ratios are the same. - E.P.T.
A In your December 1994 issue of EC&M a reader asked if he could connect four 5 kVA buck-boost transformers in parallel to supply a 20 kVA load. I do not think this type of arrangement will work, and the reader did not state if all transformers were the same make and model. This type of arrangement will not be necessary because when a buck-boost has the primary and secondary windings connected, per recommended instructions, it becomes an auto transformer. Now only the secondary windings are transforming voltage and current. The majority of the kVA load passes directly from the supply to the load. This is why buck-boost transformers can supply a load with a much larger kVA rating than the nameplate indicates. For example, a 5kVA buck-boost transformer can handle a 50 kVA load. - D.L.
A L.J.F. may connect the four 5kVA buck-boost transformers in parallel. However, there are three system design criteria that he must consider:
1. Each of the four buck-boost transformers must have individual overcurrent protection in accordance with Secs. 450-2 and 450-3 of the 1993 NEC.
This will limit damaging overloads imposed on any one transformer due to:
a. Excessively unequal current sharing among these units.
b. Electrical disconnection of one or more of these units with the end load still energized.
2. To minimize the current imbalance among these parallel-operated transformers, their impedances must equal. Preferably, these four buck-boost transformers should be of the same manufacturer, catalog number, and lot number/manufacturing date. If possible, they should have sequential serial numbers.
3. Each line-side or load-side phase conductor (from the transformer terminal through the overcurrent protective device to the common splice point) must be of all four buck-boost transformers to reduce unequal current loading on these transformers. This also applies for the neutral conductors, except there are no neutral overcurrent protective devices. (Note that NEC "Tap Rules" may apply to a portion of the conductors on the line side of each transformer's upstream overcurrent protective device, as well as possibly on segments of the conductors on the load side of each transformer.) - F.M.P.