Delta Wye Transfomrer Grounding 3

[HamidEle]

We have a Generator Delta Wye step-up transformer ( 4160V/27600V) to be connected to the Utility. The 27,600V Utility Side is normally solid grounded system, but not in the rural areas. So it is likely that the 27600V line around the Delta winding side of the transformer see the transient overvoltages during the ground faults. One solution is to add a Zig-zag transformer to mitigate this overvoltage. But it would be very expensive to do that. How successful it would be, i don't know. Are there other solutions?

Thanks in advance.

 

[waross]

Is it too late to order a transformer that is delta on the generator side?? This will do a much better job of distributing any unbalanced loads on the generator and give you a good ground point on the wye utility side.
Can you tell us your location?

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

 

[dpc]

Are you saying the 27.6 kV winding on the transformer connects to the utility system and is a delta winding? Generally not a good idea.

But the delta winding will insulated for full line-to-line voltage and the surge arresters on the transformer will protect it from transients.

If you add a grounding transformer, you will have a grounded system. If that is what is wanted, the step-up transformer was not specified correctly.

 

[HamidEle]

The 27.6KV utility system is solidly grounded. But the transformer was supposed to be specified as WYE. That's why a neutral needs to be created by Zig-zag transformer.

 

[dpc]

If you want to add a grounding transformer, it is not necessary to use a zig-zag configuration. A standard delta-wye transformer will work perfectly fine and will be much easier to find.

 

[HamidEle]

There seeems to be a lot of ways of creating a neutral for Delta winding transformer, such as 3 equal resistors, zig-zag transformer, T connected Autotransformer.

 

[burnt2x]

As always, delta side(low) of the step-up transformer will be on the generator side and the Wye (High) side will on the side being supplied. The high-side gets its ground reference by grounding the wye-point. The low-side of the transformer gets its ground reference from the generator's grounded neutral point. Did I read your post right?

 

[dpc]

As always, delta side(low) of the step-up transformer will be on the generator side and the Wye (High) side will on the side being supplied.

Not always. It's the most common way, and generally the best way, but there are exceptions.

HamidEle - do you have an actual question?

 

[HamidEle]

I have to explain this in more details.
In the event of ground faults, if the utilitiy CB trips before the generator CB, the 27600V system would become ungrounded, so overvoltage issue would occur. The solutions would be to have the utility CB trips after Genset CB or add a Zig-zag transformer to create a neutral on the generator transformer Delta side. Now the utility accepted to delay their settings. So we have had this issue solved.

Thank you guys.

 

[QBplanner]

Hi:
This is a cheap way to solve the O/V problems but not always teh case. In the system I work, there are cases that the utility side can not delay tripping which may cause other generators O.O.S. due to the delay tripping.


But good to know in your case problem solved.

 

[rockman7892]

There have been many threads similar to this involving transformer connections and grounding between generators and utilities.

What is a good source to gain a better understanding of these connections for different scenarios?

 

[jghrist]

What is a good source to gain a better understanding of these connections for different scenarios?

Try

http://www.beckwithelectric.com/powerlines/powerlines-37.html#4

 

[HamidEle]

OBPlanner,

Then what is the solution in your case when the utility cann't delay tripping?

 

[slavag]

Hi.
dpc:
"If you want to add a grounding transformer, it is not necessary to use a zig-zag configuration. A standard delta-wye transformer will work perfectly fine and will be much easier to find."

Dave, where I can find some documentation for this type of connection and calculation of size of delta-wye Xfr.
For many years, we used zig-zag, maybe need some new solution , I would like check your option .

Best Regards.
Slava

 

[rockman7892]

jghrist

Thanks for the link it was very informative.

So for a Delta-wye(grounded) transformer, with a ground fault on the secondary I understand that this zero sequence current will not travel through transformer is this correct? Therefore ground fault on secondary will only show up as an overcurrent on primary delta side? Does zero sequence current not flow through all delta windings?

It was mentioned above that the delta side(low) of the step-up transformer will be on the generator side and the Wye (High) side will on the side being supplied. The high-side gets its ground reference by grounding the wye-point. The low-side of the transformer gets its ground reference from the generator's grounded neutral point. What if there is a ground fault on the loads in the IPP's faciliy when recieveing power from the utility? Wont the reference be the ungrounded delta secondary of the transformer and thus not cause ground fault but rather an overvoltage?

 

[davidbeach]

Slavag, the zig-zag seems to be the common European solution but is extremely rare in North America. I wouldn't suggest that a European avoid the zig-zag, but I also wouldn't recommend it to someone in North America. One of those regional preference things.

 

[jghrist]

So for a Delta-wye(grounded) transformer, with a ground fault on the secondary I understand that this zero sequence current will not travel through transformer is this correct? Therefore ground fault on secondary will only show up as an overcurrent on primary delta side? Does zero sequence current not flow through all delta windings?

Ground fault on the wye grounded secondary will show up as a Ø-Ø fault on the primary.

It was mentioned above that the delta side(low) of the step-up transformer will be on the generator side and the Wye (High) side will on the side being supplied. The high-side gets its ground reference by grounding the wye-point. The low-side of the transformer gets its ground reference from the generator's grounded neutral point. What if there is a ground fault on the loads in the IPP's faciliy when recieveing power from the utility? Wont the reference be the ungrounded delta secondary of the transformer and thus not cause ground fault but rather an overvoltage?

That's the problem with this connection. If the generators are not running, then there is no ground source. To serve IPP load without generation, you would need a separate grounding transformer.

 

[dpc]

Slava,

The best resource I know of if this book:

http://www.groundingdesignbook.com/index.html

The sizing of the transformer will depend on the type of grounding and the relaying. For a resistance grounded system with a 10 sec resistor rating, we generally assume the transformer kVA rating needs to be 10% of the line-ground fault kVA if using standard delta-wye transformers.

Hope that helps,

Dave

 

[rockman7892]

It was mentioned above that it is best for the utility side to be a wye with the "wye" point connected to ground, and the secondary generator side to be delta.

If the utility side wye is connected to ground couldn't there be an issue with overheating the wye point for unbalancd loads on the secondary delta when operating from the utiltiy source? Couldn't there also be an issue with false ground current returing on this wye grounded connection for unbalanced delta secondary? I always thought it was best to leave this wye primary floating for such reasons? Leaving this floating however would then create an ungrounded situation when operating with the generator as the source.

 

[HamidEle]

My understanding, the tripping on the phase faults would disconnect the neutral of the transformer as well, since the power source from the transformer would be disconnected.