Transformer connection configuration

[patm72]

I am wondering about which transformer configuration to use in a 3-phase low voltage system, depending on the 600V feeder configurations below. Both are possible, although I would expect the case 1 to be most probable. The primary of the new transformer has to be 600V, and the secondary 480V, so merely a little step down (the 480V side needs a wye connection).

Case 1) The 600V on the primary comes from an upstream transformer with its secondary wye-grounded.

Case 2) The 600V on the primary comes from an upstream transformer with its secondary delta-connected.

In other words, should the configuration on the primary side of a transformer be the same as the secondary side of its source transformer? I don’t think it matters much, but just to have some opinions.

Thank you.

 

[waross]

The most common configuration is a delta:wye transformer, 600 Volts delta to 480 Volts wye.
Another configuration that I have seen used successfully is a wye auto-transformer connection.
The optimum transformers for a wye auto-transformer from 600 Volts to 480 Volts is a transformer rated for 277 Volts to 70 Volts.
It is often more economical to use "Off the shelf" 480 Volt to 120 Volt transformers.
The wye auto-transformer often has better voltage regulation than a two winding transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

I have seen both cases with secondary MVs of transmission step down transformers.

Popular connection for your transformer ( normally 11 kV distribution transformers) will be delta/star as waross mentioned. Then with case 2, you may require a grounding transformer on 600 V side. Since the voltage ratio 600:480 is nearby, optimum solution will be an auto-transformer with neutral grounded, comfortable with both cases mentioned by you, not requiring grounding transformers also.

 

[mvcjr]

Just wondering...

Case 2 would work well with Y autotranformers? The OP mentioned that the 480-V side needs a wye connection connection so I presume the loads would be a combination of L-N and L-L types. Is there no possibility of neutral shift when the L-N loads are not properly balanced?

Pardon my ignorance...

 

[waross]

600 Volts delta source implies an ungrounded source. It may be corner grounded but that won't help you.
Re-reading my previous post, I appologise.
I did not make it clear that the autotransformer solution is only applicable to a wye grounded source.
If a wye transformer is connected to a delta source, unbalanced loads may cause a neutral shift as suggested by mvcjr.
Grounding the wye point will not change the neutral shift. It will change the reference from the geometric neutral to the shifted neutral.
There will be the same shift in relation to the line to neutral voltages.
Grounding the wye point of a wye transformer fed from an ungrounded delta will introduce a ground to an ungrounded system That may interfere with the ground fault protection of the delta source.
The owner of the source may object to a ground on his system.
A zig-zag or a wye-delta transformer may be used to develop a wye point for a wye-wye transformer. This will allow you to avoid a neutral shift while avoiding a ground on the ungrounded delta system.
But A grounding transformer whether used as a grounding transformer or to generate a true floating neutral is normally sized to carry ground fault currents that are limited by a grounding resistor, or in the case of a floating neutral, sized to carry any unbalanced loads.
In the case of an autotransformer serving line to neutral loads, codes and good practice require the neutral to be solidly grounded.
This gives rise to the three points of;
1. Adding a ground to an ungrounded system and,
2. Transformer sizing. The grounding transformer must now be sized to carry ground fault current limited by three times the source impedance. This may be an issue depending on the size of the 600 Volt delta source transformer.
3. Adding a solid ground to a system that may be resistance grounded.
Given the issues raised by connecting a wye transformer to an ungrounded delta source transformer, I strongly suggest a delta-wye transformer for an ungrounded delta source.




Bill
--------------------
"Why not the best?"
Jimmy Carter