DG Step-Up Transformer Winding Configuration

[hibb1]

I need some comments and/or standards to refer to regarding wye-delta choices for a step-up transformer for diesel generators.
I have (3) 1.5 MW DG Units (with a resistor ground) generating at 4160V. We need to get this connected to the 12.47kV grid, so we will install a step-up transformer (5MVA) to do this. Do I choose Wye grounded or Delta for the Generator side of the new transformer? The 12.47kV winding will be wye-grounded. Thanks.

 

[davidbeach]

The best, for the generator, is delta on the generator side.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[stevenal]

David,
How is this best for the generator? The grounded wye-delta will provide a ground source on the 12.47kV system, possibly compromising protection.

See IEEE 1547.2.

 

[hibb1]

Thanks for your comment. Is delta better due to no line-ground fault levels at the genset, or are there voltage stability (or harmonics) consideration in having a delta?
These gensets were installed in the 80's and were connected directly to our 4160V wye system.

 

[hibb1]

I can accommodate for the grounded wye 12.47kV on the transformer. I'm also interested in making sure that I can run these units islanded and push a limited amount of MW out to the 12.47kV grid if needed.

 

[Palletjack]

Delta suppresses (provides a closed path) triplens and maintains balanced voltages.

 

[waross]

The delta facing the generator as David suggests better distributes unbalanced loads on the generator.
Consider a delta/wye with a single phase load line to neutral on the wye side.
That will become a line to line load on the delta.
One generator phase winding will contribute directly to the current and the other two phase windings will contribute an equal amount acting as an open delta.
With a wye/wye connection, the maximum single phase line to neutral load is 1/3 of the generator rating.
With a delta/wye connection, the maximum single phase line to neutral load is 2/3 of the generator rating.
With the current distributed across all three phase, the voltage balance with unbalanced loads is much better as well.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[slavag]

Delta on the generator side.

 

[hibb1]

I really appreciate the comments. The balancing on the loads on the delta will help out. On the existing connection straight to the 4160V wye grid, we've seen a lot of unbalanced load current on the generators. The unbalanced loads on the genset makes it difficult to apply all of the relay elements that I want to use.

 

[waross]

Unbalanced loads cause increased rotor heating. This may eventually lead to rotor failure. The increased heating is less with a delta connection facing the generator.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Kiribanda]

Since you are going to be an IPP (Independent Power Producer) to export power to grid,
when your gen or generators are sync to the 12.47kV grid, the 12.47kV system should still be
effectively grounded. To fulfill that Utility requirement, your step-up transformer shall be
WYE Grounded on 12.47kV side & delta on 4.16kV side.

 

[waross]

You may consider some type of restricted grounding on the delta or at least ground detection.
Differential protection will cover a lot of possible issues.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!