480V Gen step up to 12470V

[YuriSarcas]

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.

 

[itsmoked]

No, Wye-Delta is the correct setup. As I understand it, you leave the Wye point un-grounded in your generator's case. The Wye configuration is to reduce unbalanced circulating currents from the generator's windings.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

No Keith, ideally there is always a delta on the generator side of the transformer. Exceptions exist, but always compromise the protection of the generator. The system side of the transformer could be either delta or wye as appropriate for the connected system and the system grounding. Since 12470V is rarely associated with an ungrounded delta system, it is highly likely that in this installation the high side of the transformer should have been a wye, solidly or resistance grounded as appropriate for the system. Unless the 12470V system has nothing connected phase to ground/neutral (single phase transformers or lightning arrestors), your delta connection risks damage due to voltage rise on the unfaulted phases during a single phase to ground fault.

 

[dpc]

There is not a single right answer to this. Generally, I agree with David Beach. It will depend on the generator grounding and the type of 12.47 kV distribution system and its grounding.

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.

 

[itsmoked]

I stand corrected. A wee bit confused... But corrected.

I shall endeavour to persevere.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

A common installation may be three 7200:480 volt transformers.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[YuriSarcas]

After somemore digging today one of the project managers said it has to do with the Utility. We have four Utility sources on this. I am not sure which are from the same substation or if they have all there own. He was told that since the Utility is coming in WYE that our close transition would not work correctly if our 12470 from the gens was Delta. We still only have the three phases of Utility coming into the swtichgear with no reference to the neutral.

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.

 

[davidbeach]

At this point you probably need to post a fairly detailed one-line diagram to improve the quality of possible answers.

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.

 

[YuriSarcas]

I will have to draw something up if I get some time today. I will post it later.

Thanks.

 

[Kiribanda]

YuriSarcas,

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.

 

[vic3fan]

The reason to install delta-wye transformers is because of the great advantage that offers: do not transmit the third harmonic, and multiples of 3 harmonics, from one side to another. Thus, your generator windings will be safe against influences from the system regarding those harmonics. True, there is a 30 (electrical) degrees shift between phase voltages, but this is not an issue.
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.

 

[ScottyUK]

The main reason big utility class generators have the delta on the generator side is to allow a high resistance grounding scheme to be employed, ensuring that a stator earth fault is limited to a few amps, typically only 5A or 10A on a modern machine. This limits damage caused by an earth fault to the hugely expensive stator core, usually meaning the difference between a rewind and either a restacked core & rewind or a new stator.


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