Use of Y-Y Transformer in replace of D-Y 2

[Humble2000]

Customer has a D-Y transformer 27.6Kv-600/347 Volt.
He wants to replace it. I have a Y-Y transformer that I can offer him, with the same voltage rating. Would this cause any issues?
Is there some other factors that I need to be concerened about?

Thanks

 

[davidbeach]

The high side ground fault protection will need to be thoroughly reexamined.

 

[waross]

In addition to davidbeach's post;
A delta primary does a better job of distributing unbalanced currents than a wye primary. If your load is balanced, no problem. If you have unbalanced phase loading, your voltage drops on the primary lines will be greater with a wye primary than with a delta primary.
Also, unbalanced secondary loading will result in primary neutral currents which may be an issue with your primary ground fault protection. This is only part of the reason for davidbeaches's recommendation.
respectfully

 

[rcw retired EE]

Is this the only transformer on the customers system? If this unit's 600V side parallels or connects to any other transformer's 600V winding, the phasing will be off.

If there is differential overcurent protection on the trasnformer, the CT wiring or relay settings may need to change.

 

[Humble2000]

The primary side is fed through hydro lines. I am not sure what type of ground fault protection they have on their system.I think it's only protected through fuses, which will be the same for both systems.
Waross,
Could you please explain how the unbalancing is better distrubuted by Delta transformer on the high side?

Yes, this is the only transformer at the customer site.

 

[waross]

Suppose we have a 100 amp load on "A" phase at 347 volts.
With a Y-Y transformer we will have 2.18 Amps on "A" phase of the primary and 2.18 Amps on the primary neutral.(100A x 347V/1595V)
we will have zero amps on "B" phase and "C" phase of the primary.
With a D-Y transformer we will have 1.26 Amps on "A" phase and 1.26 Amps on "B" phase. (100A x 347V/27600V)
We will have zero amps on "C" phase and on the neutral.

A line to neutral fault or a line to ground fault on the secondary (347V) will reflect as a line to line current on the primary of the D-Y transformer.
A line to neutral fault or a line to ground fault on the secondary (347V) will reflect as a line to neutral current on the primary of the Y-Y transformer.
Because of Multiple Protective Earthing on the primary, a line to neutral fault on the secondary of the Y-Y transformer may appear as a ground fault to the utility's ground fault relays on the primary.
Hence davidbeaches advice re. high side ground fault protection.
I suggest calling the engineering department of the utility for their comments, as there may be implications to their protective relay scheme.
Please let us know how it goes.
respectfully

 

[jghrist]

Two other differences between Delta-Wye and Wye-Wye that may be significant if the transformer is protected by fuses as you think.

If one fuse blows on the high side of a Wye-Wye transformer, the low side will have normal voltage on two phases and zero on the other. On a Delta-Wye transformer, the low side will have half voltage on two phases (same angle) and full voltage on the other (180° out of phase). The half voltage situation on two phases may be a more serious condition for motors on the secondary than losing one phase completely, depending on what type of phase loss protection is employed.

If the transformer is connected by cable from the fuse location to the transformer, there will be a much higher probability of ferroresonance with a Delta-Wye transformer than with a Grd Wye-Grd Wye transformer if one or two phases are open.

 

[redox]

If I may ask:

Will there be any differences if a Wye-wye transformer is used, but the primary neutral is not grounded?

Thanx

 

[davidbeach]

If you don't ground the primary neutral you will solve the ground fault issue, but at the cost of secondary voltage unbalance, possibly severe enough to damage equipment. The load will cause the floating wye point on the primary to find its own voltage relative to the three phase voltages. Each secondary voltage will then have the same per unit voltage line-to-neutral as the primary.