Transformer Mixup

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SG-1

Senior Member
45KVA / 3 Phase / 6900V to 230V / 60HZ / Delta-Delta CPT was believed to be ordered. Delta-delta was requested.

When the transformer was to be wired it was discovered that it was delta-wye. The transformer has an X0 terminal.

Ultimate customer will be using the secondary ungrounded.

The transformer will be used in an 83 throw-over scheme with an existing delta-delta transformer.

Are there any dangers or disadvantages to using this transformer's secondary ungrounded ( leave the X0 terminal unwired ) with the conditions discribed above ?

Thanks, in advance for all advice.
 

iwire

Moderator
Staff member
Location
Massachusetts
One of the utility engineers that frequents this site has mentioned the power company he works for only stocks wye type pad mount transformers to reduce inventory and they use them for either wye services or delta service by letting XO float.
 

iwire

Moderator
Staff member
Location
Massachusetts
delta_wye.jpg



With the delta trans you would have X1, X2 and X3

With the wye you would have X1, X2, X3 and XO with XO hitting the mid point.

In order to get ungrounded delta from the wye transformer you must leave the mid point floating.
 
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TOOL_5150

Senior Member
Location
bay area, ca
ok so X0 IS X4?

Im pretty sure I understand now. Basically, on the wye transformer you would let the center point float, as there is no center point on a delta transformer.

~Matt
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
I have heard though, that using a delta/wye like this causes unstable voltages. It may be an urban myth though, as I cannot see it being any different than using a wye with no line to XO loads.
 

zog

Senior Member
Location
Charlotte, NC
The transformer will be used in an 83 throw-over scheme with an existing delta-delta transformer.

.

That is a problem, they need to be the same, a 3-phase transformer can have its primary and secondary windings connected the same (delta-delta or wye-wye), or differently (delta-wye or wye-delta). It's important to remember that the secondary voltage waveforms are in phase with the primary waveforms when the primary and secondary windings are connected the same way. This condition is called ?no phase shift.? But when the primary and secondary windings are connected differently, the secondary voltage waveforms will differ from the corresponding primary voltage waveforms by 30 electrical degrees. This is called a phase shift. When two transformers are connected in parallel, their phase shifts must be identical; if not, a short circuit will occur when the transformers are energized.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
Yes, as far as line-line voltage goes, it would present itself as an ungrounded delta.

Would that hold true regardless of whether the XO was grounded or left to float?

No, if you ground XO you will not have ungrounded system.

How does grounding the system change the way voltage is presented to line-line voltages? Will anything work any differently? Will any line-line loads know the difference (whether the XO is grounded or floating)?
 

BJ Conner

Senior Member
Location
97006
45 KVA seems big for a control scheme.
I would have to see the whole schematic for the transfer scheme but if your sensing with an ungrounded 3 phase system there is the old problem with on phase going to ground.
 

hurk27

Senior Member
Deltas many times are specked because is gives a little redundancy for motor loads if a winding fails, if you loose a winding on a delta transformer the motor will still see a 3-phase supply (open delta), and this will not cause the motors to single phase, so I would check with the person who specked the transformer and find out if a Y is ok.


Edited to add: I didn't see page 2 when I made this post:roll:
 
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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
If this transformer is covered by the rules found in the NEC, you can't float XO and install it as an ungrounded system.
250.20(B) Alternating-Current Systems of 50 Volts to 1000 Volts. Alternating-current systems of 50 volts to 1000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions:
(1) Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts ...
 

SG-1

Senior Member
This transformer is not under the perview of the NEC, it is part of a manufactured assembly & going to another country.

The output is left for the customer to connect at the secondary breaker. We do not know what his loads are. We can see from the one line that the other existing transformer is an ungrounded delta.

I will post more later tonight.

Thanks so far...
 
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