If the source is really a 480 delta, all you should need is a 480 wye to anything delta to generate a ground reference.
Now whether that’s legal or not, the experts here will chime in. I’m not an NEC expert
Transformer for deriving neutral
- Thread starter 304sparky
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Now whether that’s legal or not, the experts here will chime in. I’m not an NEC expert
Are you saying nothing is connected to the primary of the transformer?
304sparky
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- West Virginia
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- Electrician
Yes sirAre you saying nothing is connected to the primary of the transformer?
MD Automation
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- Maryland
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- Engineer
If that's really the case - and the OP seems to confirm - the application of 480 VAC on a secondary designed for 208 VAC will drive each of the 3 secondary windings deep into magnetic saturation.Wait…
You put 480 on a 208 transformer??
The windings will be very cranky and if you're lucky the mains circuit protection will open before permanent damage is done.
winnie
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- Electric motor research
@304sparky , stop considering the use of a neutral deriving autotransformer for this feeder. Such a transformer really only makes sense when you have an ungrounded system which you want to ground.
Either run a neutral with your feeder if the source is a grounded wye system, or use a delta:wye isolating transformer to create a new system with a neutral. You will need a 480V:480/277V delta:wye transformer.
To everyone else still surprised by 'nothing connected to the primary' yep, that is a valid connection in rare circumstances. The wye is connected to the circuit, with X0 providing a derived neutral. The delta is there to circulate current internally and provide a path for unbalanced neutral current to draw from all 3 phases.
I've seen references to wye:delta:wye transformers where the unconnected delta provides the same low impedance neutral.
Jon
Either run a neutral with your feeder if the source is a grounded wye system, or use a delta:wye isolating transformer to create a new system with a neutral. You will need a 480V:480/277V delta:wye transformer.
To everyone else still surprised by 'nothing connected to the primary' yep, that is a valid connection in rare circumstances. The wye is connected to the circuit, with X0 providing a derived neutral. The delta is there to circulate current internally and provide a path for unbalanced neutral current to draw from all 3 phases.
I've seen references to wye:delta:wye transformers where the unconnected delta provides the same low impedance neutral.
Jon
kwired
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- NE Nebraska
Been mentioned a couple times, you have a coil(s) rated for 208 volts but you are applying 480 volts across them. That will result in high magnetic saturation from those coils resulting in higher current than the coil was designed to handle and overheating.
You need to connect to a 480/277 wye set of windings to keep from over saturation conditions. Then as mentioned the supply side details will matter some here. If this were a corner grounded supply, you still would have high amount of current flowing if you bond the wye point as the transformer would be trying to create 277 volts between two points but they are also bonded together even if it happens to be through several feet of equipment grounding conductor - that will also make high current flow through a portion of the total arrangement.
Just to clarify.. in the original post you stated a 480delta to 480wye transformer and then in Post 10 you stated the secondary was 208Y120 which ahs been pointed out would be a problem on a 480v source,.
In the end are you wanting a 277v neutral or a 120v neutral ??
In the end are you wanting a 277v neutral or a 120v neutral ??
304sparky
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- West Virginia
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- Electrician
277v neutral
jaggedben
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Yes in that case high current = dead short.
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- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Methinks you are very confused here. If you connected the Line and Load to the same side of the transformer, you were doing it wrong... That's not how you create a neutral.
wwhitney
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- Berkeley, CA
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- Retired
You can use a delta-wye transformer that way to derive a neutral. Not sure of the pros and cons of doing it that way vs a zig-zag transformer (which I haven't studied).
Cheers, Wayne
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- Wisconsin
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- PE (Retired) - Power Systems
Actually it might be, if the intent was to use a wye-delta configuration where the delta winding is left totally disconnected.
But this would be a transformer with a primary 480Y/277V winding, which does not appear to be the situation with the OP. I have had to do this a few times when data centers.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Zig-zag I am familiar with, I have never heard of using a D-Y transformer this way. Learned something new today I guess...
wwhitney
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- Berkeley, CA
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So what are the benefits of using a delta-wye in this way vs a zig-zag? And do you have to size the delta-wye transformer only for the maximum unbalanced L-N load, or for the maximum L-N load?
Cheers, Wayne
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Cost and availabilty.
Size for the current expected on the neutral.
For medium voltage, it is not uncommon to open the delta and insert some impedance and metering. It is about dealing with zero sequence currents.
JoesBagelandDeli
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- Location
- Chicago
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- E Eng
This sounds similar to conditions for an open-circuit test.
Rated voltage is applied to the secondary and the primary is left open.
The core becomes magnetized, but there is no load connected, so you can determine core losses.
I'm trying to understand paths of current and how the primary side plays a role and my head is starting to hurt...
Are you getting 277V between H-N?
Rated voltage is applied to the secondary and the primary is left open.
The core becomes magnetized, but there is no load connected, so you can determine core losses.
I'm trying to understand paths of current and how the primary side plays a role and my head is starting to hurt...
Are you getting 277V between H-N?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The Wye side is the primary, so yes you would get 277V Line-Neutral. The delta side is the secondary which is left unconnected.
Balanced L-L loading on the primary will cause minimal current in the delta secondary. As the primary L-N current increases so does the amount of current circulating in the delta. As long as this circulating current remains within the transformer rating everything works as intended and a 'neutral' has been created on the Wye side. The secondary delta can be broken and impedance and/or metering can be added for protection purposes such as GF monitoring.