transformer delta to wye windings or not

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augie47

augie47

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Did you read the cut sheet you provided. It states primary 480v, secondary 208Y120
Siemens Low Voltage Low Voltage Distribution Dry-Type Transformers. 3-Phase transformer. Primary voltage: (480V), secondary voltage: (208Y/120



(It could be wired in reverse if allowed by manufacturer)
 
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wwhitney

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electrofelon said:
One other comment: a delta-wye will always be an SDS.
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What about a delta-wye supplied by a wye system, with a neutral from the supply brought to the transformer and connected to the secondary wye neutral? Seems to me that would both be allowed and not an SDS.

When the source wye system has a grounded neutral, this is the identical connectivity as an SDS. The source wye neutral and secondary wye neutral are simply directly connected, rather than connected via the GES.

Cheers, Wayne
 
electrofelon

electrofelon

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wwhitney said:
What about a delta-wye supplied by a wye system, with a neutral from the supply brought to the transformer and connected to the secondary wye neutral? Seems to me that would both be allowed and not an SDS.

When the source wye system has a grounded neutral, this is the identical connectivity as an SDS. The source wye neutral and secondary wye neutral are simply directly connected, rather than connected via the GES.

Cheers, Wayne
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Whispering in ear: this is a sting thread, lets try and keep it simple........
 
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GoldDigger

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electrofelon said:
One other comment: a delta-wye will always be an SDS. A wye-wye can be wired either as an SDS or not - It's not automatically one or the other. Wye-wyes are extremely rare in <600V systems.
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It's not exactly common, and maybe has no goo purpose, but a corner grounded delta and a center grounded wye can be wired as an SDS simply by connecting the two grounded conductors.
An ungrounded delta supply or a delta source derived from a wye source by not running the center point conductor would indeed have to be an SDS.
If the source neutral is present you can still wire non-SDS even with a delta primary.
 
electrofelon

electrofelon

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hhsting said:
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Awwwww 🤗......feel better now?

In practice these Transformers discussed will always be wired as an SDS. These discussions about making them non SDS are pretty much just academic. About the only Transformers you will see that are not SDS are buck boost Transformers and auto Transformers like this:

store.maddoxtransformer.com

3-Phase Encapsulated Autotransformer 600-480-400-240-208v

For larger kVA sizes of this design click here. Voltages: 600-480-400-240-208v This autotransformer is designed to convert any 2 of the voltages listed above in order to run equipment such as solar panels, motors, CNC machines, welders, conveyor systems, pumps, printing equipment, fabrication...
store.maddoxtransformer.com store.maddoxtransformer.com
 
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Carultch

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hhsting said:
In the link below and snapshot would the transformer be considered separately derived system and has primary windings delta and secondary windings wye? Would any one here tell? I just need to know if NEC 2017 Section 250.30 applies or not
Thanks

Click to expand...

Here's how to know whether it is an SDS or not:

Take the transformer, before you energize it, and before you ground its two sides. Imagine taking an ohm meter across the primary and secondary sides. Do you get continuity? If you don't get continuity, it means the secondary is a separately derived system from the primary. If you do get continuity, it means you don't have a separately derived system.

Since an ohm meter uses DC as its sample test voltage, the transformer windings have continuty across both ends of each coil. But there will not be continuity across the magnetic coupling, between two interacting coils. The transformer coils are laminated in an insulating material, to stop current from taking a shortcut and defeating the intent of the turns ratio. The two interacting coils, only interact by magnetism, using Faraday's law and Ampere's law, to carry power between them. They can only transmit a change of voltage, and cannot transmit a steady voltage. Therefore a DC test voltage of your ohm meter, will not get continuity between two interacting coils.

An autotransformer will give you continuity between primary and secondary of any given coil, since the primary and secondary are part of the same coil in each phase. But a conventional transformer will not, as it only has magnetically interacting coils to carry the power.
 
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wwhitney

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Carultch said:
Take the transformer, before you energize it, and before you ground its two sides. Imagine taking an ohm meter across the primary and secondary sides. Do you get continuity? If you don't get continuity, it means the secondary is a separately derived system from the primary.
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Right, but if both sides are earthed to a common GES, then you'll get continuity after you do the grounding. So I've never been clear on why this distinction gets so much attention. Is it just a matter of "we don't want to leave an SDS unintentionally unearthed, so there are a bunch of rules about how to do the grounding?"

Cheers, Wayne
 
ActionDave

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wwhitney said:
Right, but if both sides are earthed to a common GES, then you'll get continuity after you do the grounding. So I've never been clear on why this distinction gets so much attention. Is it just a matter of "we don't want to leave an SDS unintentionally unearthed, so there are a bunch of rules about how to do the grounding?"

Cheers, Wayne
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I've wondered about this for a long time and never got a good answer either.
 
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paulengr

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Since delta-delta and wye-wye have no phase shift ground faults are passed through. How can this be considered SDS? Even if you don’t specifically ground it, it still passes things through. An ungrounded wye-wye for instance still has the same phase voltages except for the transformer ratio and a similar situation exists for phase currents with delta-delta.
 
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Carultch

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paulengr said:
Since delta-delta and wye-wye have no phase shift ground faults are passed through. How can this be considered SDS? Even if you don’t specifically ground it, it still passes things through. An ungrounded wye-wye for instance still has the same phase voltages except for the transformer ratio and a similar situation exists for phase currents with delta-delta.
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The differential voltages are the same (except for the transformer ratio) between pairs of corresponding points (phase and neutral) within the system, but the absolute is not guaranteed to be the same, until you make your grounding connection.

Two sides of a separately derived system could be at wildly different voltages to ground, if you don't establish grounding of the secondary side of the system. The whole idea of why SDS's get their own section on grounding and bonding, is due to the need to establish a new grounding system for the SDS, in order to prevent this from happening.
 
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