single phase grounding transformer?
single phase grounding transformer?
(OP)
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?
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?






RE: single phase grounding transformer?
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.\\\
RE: single phase grounding transformer?
RE: single phase grounding transformer?
It's also a good thing to try to balance the loads on both sides of the center tap.
God bless!
RE: single phase grounding transformer?
It is difficult to accurately establish the transformer parameter from scratch. To stay on the safe side, it would probably be very underrated.
RE: single phase grounding transformer?
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 1ø 3w, the concept is also applicable.
RE: single phase grounding transformer?
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.
RE: single phase grounding transformer?
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... ]
RE: single phase grounding transformer?
Good point about OC protection through the transformer.