480V Gen step up to 12470V
480V Gen step up to 12470V
(OP)
In my searches through the internet to find an answer to my question I came across this website so I am brand spanking new to this site. Here is my question and hopefully someone here with a more transformer knowledge than me can shed some light.
My application:
I have a 480V generator three phase with no nuetral. This currently goes to a Dry Type Delta-Delta step up transformer to 12470V. This then goes to the PCS gear and feeds the distribution. I have been told that this Delta-Delta transformer is wrong and needs to be a Wye-Delta transformer. My first question is why is it wrong? My second question is why would it be a Wye-Delta and not a Delta-Wye? Since there is no nuetral on the generator I would think we would bring in the three phases to Delta and then take the Wye to the switchgear for all the distribution. The switchgear is three phase with no nuetral but the ground reference out of the Wye transformer would be the same ground reference through out the facility.
Any thoughts? Any knowledge I gain from this is more than I had before. Thanks.
My application:
I have a 480V generator three phase with no nuetral. This currently goes to a Dry Type Delta-Delta step up transformer to 12470V. This then goes to the PCS gear and feeds the distribution. I have been told that this Delta-Delta transformer is wrong and needs to be a Wye-Delta transformer. My first question is why is it wrong? My second question is why would it be a Wye-Delta and not a Delta-Wye? Since there is no nuetral on the generator I would think we would bring in the three phases to Delta and then take the Wye to the switchgear for all the distribution. The switchgear is three phase with no nuetral but the ground reference out of the Wye transformer would be the same ground reference through out the facility.
Any thoughts? Any knowledge I gain from this is more than I had before. Thanks.






RE: 480V Gen step up to 12470V
Keith Cress
kcress - http://www.flaminsystems.com
RE: 480V Gen step up to 12470V
RE: 480V Gen step up to 12470V
Normally, you would want a delta on the generator side, with local grounding at the generator. This should work for you since you say there is no neutral connection at the generators. But if you have other 480 V loads on this system and the generators are off-line for any extended period of time, you may want a more reliable grounding method for the 480 v system.
The 12.47 kV winding should generally be grounded wye when there is a local generation source as you have described. But it could be delta if the 12.47 kV system is ungrounded or there is another good ground source such as a grounding transformer at the same facility.
If someone is telling you the delta-delta is "wrong", I would asked them for their reasoning.
RE: 480V Gen step up to 12470V
I shall endeavour to persevere.
Keith Cress
kcress - http://www.flaminsystems.com
RE: 480V Gen step up to 12470V
The 480 V windings would be start connected and fed by the generator(s). The 480 ground point may be the star point of the generator windings. The 7200 V windings would be grounded star connected and develop your 12,470 V distribution voltage.
I have a client with a similar system. 7620/13,200 on the high side but the same voltage class, the same rules and the same equipment.
One of the reasons for the delta connection facing the generator is that a heavy single phase current will be fed by all three generator windings with a delta connected transformer.
As I have mentioned, a wye connection is common on the 7200/12470 side. A fault from phase "A" to ground on the high side will be transposed by a delta low side to a phase to phase current on the generator. 50% of the current will be supplied by "A" phase and 50% of the current will be supplied by "B" and "C" phases.
With a wye/wye connection an "A" phase to ground fault on the high side would be fed entirely by the "A" phase winding on the generator.
A wye connection on the high side allows easy grounding, either direct or high impedance.
Hope this helps.
Bill
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"Why not the best?"
Jimmy Carter
RE: 480V Gen step up to 12470V
Maybe it has something do with the circulating currents when we try to load or unload utility in close transition? Does this make sense to any of you?
Thank you all for your responses. I have had a chance to look at this site and I am upset that I have not found it sooner. I love the information that it has and the ability to toss around ideas.
RE: 480V Gen step up to 12470V
Just having a wye source from the utility does not inherently require a wye source from high side of the generator transformer, but for a specific site with specific conditions it might be a good thing. Tough to say without the specifics.
RE: 480V Gen step up to 12470V
Thanks.
RE: 480V Gen step up to 12470V
In my opinion,if your 12470 V side is owned by the Utility, then you should have always WYE on Utility side. That is to make sure that when your generator is on line the Utility is always effectively grounded for a ground fault on 12470 V Utility side.
Having said that, then the 480 V side could be either again WYE or DELTA. Since your generator is 480 V DELTA, it is essential to have WYE on 480 V side to implement a ground fault protection scheme.
If your code allows an isolated 480 V network, then you can have DELTA on the 480 V side. But then, your 480 V network becomes DELTA-DELTA which is an isolated system. In that case you should have all your electrical components suitable for a 480 V isolated system e.g. 133% insulated cables, single rated CBs etc etc.
Based on above, I will have the transfomer with
12470 V side WYE(Grounded)
480 V WYE(Grounded)
Hope this helps.
RE: 480V Gen step up to 12470V
So, using the delta connection of the transformer on the LV side, you will need a grounding (zig-zag) transformer to detect a ground fault, but you also can use for this purpose three potential transformers connected in open delta configuration. The probability of having a ground fault on the LV side should be much lower than the probability of a ground fault on the transmission side, so a wye-grounded connection is more beneficial to be placed on the HV side.
RE: 480V Gen step up to 12470V
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