Back Feeding a single phase pole top distribution transformer

[ffont]

We have a 24940GRDY system, (14.4kV L-G) and in order to avoid installing a MV single phase metering unit to feed a small 37.5kVA single phaseload requiring 2400V, we plan to use some units we have in stock, ie a step down 100kVA 14,4kV-120/240, and connect its 240V to the 240V LV of a 75kVA 2400-120/240V. We would insert a standard single phase LV metering unit at the connection point between the 2 LV .

Is there any problem associated with this? Many thanks!

 

[cranky108]

Lets see, you are going to bill the customer for the losses in the 75kVA 2400-120/240V transformer? What could be wrong with that?

We do use back to back transformers to convert 6.9 kV phase to phase, to 6.9 kV phase to neutral to feed a remote microwave repeter. The customer is primary metered, and owns the line from our back to back transformers to there site. Before anyone asks, the 6.9 kV phase to phase is just two phases of a legecy delta system that serves a small hydro plant, and a few care taker houses for a water collection system.

So back to back transformers are not bad. Just uncommon. and difficult to protect.

 

[alehman]

In the U.S., railroads commonly use pole-top transformers as step-up to feed distribution along tracks in remote areas. Some of these circuits are fairly long (up to 30 or 40 miles) and I'm sure the transformer losses are small compared to other losses.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[Mbrooke]

I have never seen it done with a 120/240 secondary, but I see it done all the time with 'Jumbo' units. A legacy line waiting to be converted one day running at 2.4 or 4.8 will use a 7, 13.2, 16kv ect to 2.4/4.8 kv step down transformer to step up voltage for a new underground installation. 4.8 overhead to 13.2 underground. When the line is upgraded the unit is just removed and the overhead connects directly to the underground riser. So far flawless that I know of.

My theory is that if it works for these it should work with 120/240 units. However, I can possibly see inrush being much higher both because the unit will be magnetized from the secondary winding in addition to 2 transformers in series as appose to one. Also leave the secondary of the unit being back fed floating.