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alehman (Electrical) (OP)
23 Oct 03 22:31
I have recently come across an old installation (1930) with a 400kVA, 240 volt 1-phase, 2-wire transformer being used to supply a 120/240V single phase system.

A separate small 120/240V transformer (no nameplate) with a center-tapped (3-wire) winding is connected to the system and the center leg used to create a neutral and is grounded. The "grounding" transformer may be somehting in the range of 50KVA to judge from the size of the tank. It has no primary connection. Can this be a valid method to obtain a neutral? Is there such a thing as a single-phase grounding transformer?
jbartos (Electrical)
23 Oct 03 22:43
Suggestion marked ///\\\
I have recently come across an old installation (1930) with a 400kVA, 240 volt 1-phase, 2-wire transformer being used to supply a 120/240V single phase system.
///Please, clarify. 2-wire transformer 1phase cannot deliver 120/240V. It would have to be 3-wire 1phase transformer to deliver 120/240V.\\\
A separate small 120/240V transformer (no nameplate) with a center-tapped (3-wire) winding is connected to the system and the center leg used to create a neutral and is grounded. The "grounding" transformer may be somehting in the range of 50KVA to judge from the size of the tank. It has no primary connection. Can this be a valid method to obtain a neutral?
///Yes, however the neutral rating is limited by the 50kVA transformer not by 400kVA transformer.\\\
 Is there such a thing as a single-phase grounding transformer?
///Yes, the above mentioned one provides grounding. However, the transformer is single phase three wire type, not the single phase two wire type.\\\
Helpful Member!  alehman (Electrical) (OP)
26 Oct 03 22:36
Thank you. This follows my thought process. There is a significant voltage unbalance (app. 4V), and the operating engineer was questioning the validity of the arrangement. We need to determine the grounding transformer rating.
mvcjr (Electrical)
26 Oct 03 22:55
Your 120-volt loads will determine the capacity of the grounding transformer.

It's also a good thing to try to balance the loads on both sides of the center tap.

God bless!
jbartos (Electrical)
27 Oct 03 20:11
Suggestion: It would be better to obtain a new transformer with its proper nameplate.
It is difficult to accurately establish the transformer parameter from scratch. To stay on the safe side, it would probably be very underrated.
busbar (Electrical)
29 Oct 03 20:01

Another aspect of neutral derivation is that during a fault the autotransformer should have a short-time rating that will withstand ground overcurrent persisting in the interval the fault or overload occurs and protective device operates to relieve the overcurrent.  The best bet may be to compare existing to commercially available devices in modern catalogs.  

Also, there should be no independent overcurrent device that is capable of isolating the autotransformer and its associated neutral point from the rest of the circuit it serves.  See 99NEC450-4.  Although 450-5 does not directly apply to your situation for 1ΓΈ 3w, the concept is also applicable.
  
alehman (Electrical) (OP)
29 Oct 03 22:21
Thanks for the excellent comments. There is no OC protection on the grounding transformer. It would be a good idea to replace it (as well as the whole system). I was just brought on by the new owner of this old building. I hope to convince him to do some upgrades soon.

Just to keep things interesting and give you an idea of what I'm facing, today I found some temporary jumper leads on the back side of a CT test block for the primary (13kV) breaker. They effectively short out the CT's. The maintenance guy said some electrician had put them there several years ago because the breaker kept tripping!! To make it even worse, there's no seondary main breaker. The transformer directly feeds the main bus of an old slate switchboard with numerous fused knife switches which feed panels throughout the building. I had my company's insurer review my letter to the owner before sending it today.
busbar (Electrical)
29 Oct 03 23:01

Weirdly, with the two-wire secondary, the {operational/unbypassed} primary-side overcurrent device would protect the transformer primary and secondary, but that changed with addition of the grounding autotransformer.

[alehman, I think I know that slate live-front switchboard.  See http://memory.loc.gov/pnp/habshaer/pa/pa2600/pa2688/pho... ]
 
alehman (Electrical) (OP)
29 Oct 03 23:07
That's it! Even the big breakers on the top look the same.
Good point about OC protection through the transformer.

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