480V vs. 12470V Gen Sets

[VTer]

We have a client with a facility containing (3) emergency/standby generators at 2000ekW/ 2500kVA each. The gen sets are 480V and then stepped up (3-delta-wye xfmrs) to 12,470V on a common bus(paralleled) with ties for distribution. (3ph, 3W)

Due to recent expansion, the facility requires two (2) additional gen sets to be tied to the same 12,470V switchgear. These gens are already pre ordered and will have the same power capability but will produce 12,470V directly without any step-up transformers. Since I am a recent graduate (lack much needed experience) and just assisting on the job, I was wondering if there is any possible concerns that I should be aware of with this set up (advantages/ disadvantages of 480V vs. 12470V gen sets). Since the job is in the very early stage of design, I am not concerned with synchronizing equipment, relay protection and and anything external to these units. I would only like to know the advantages/ disadvantages of 480V vs 12,470V unit, and why one would be used vs. the other in this scenario?


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[redfurry]

There are some ramifications to the system because of transformation.

If you have a step-up from 480V the transformer will help to limit fault infeeds from the generation (increase impedances). With a generator connected directly to the 12470V system the fault levels will be much higher.
Also the delta-wye transformer will isolate the zero sequence of the generation from the system (again reducing ground fault levels).



Ian Dromey -

http://www.dromeydesign.com

 

[VTer]

redfurry

I agree with the fault limiting benefit, but I wonder what is more cost effective.

a) To limit fault levels with the xfmrs and lower Isc rated gear?
Or
b) Just get the gear rated to withstand higher Isc without the xfmrs?
I would assume b would be a better choice


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[edison123]

I would advise using a trafo between the generator and the grid. I have seen some nasty failures on generators connected directly to the grid. In one case, there was even a 11 KV/11 KV trafo used to prevent frequent generator winding failures.

The surges in the grid affect the generator more seriously than the trafos, which act as a sort of barrier.

 

[slavag]

Hi.
Cost effective????
VTer, we had a lot of threads here on the topic.
All possible, of course possible connect new gen directly to bus and it's common practic too.
I like recommendation of Edison, it's reduce cost of swg, fault level, problems with 3-rd harmonic current circulation, protection issue.
If you would like your b choice, search in the Forum "Hybrid grounding of generators"
Good Luck.
Slava

 

[wbd]

"the facility requires two (2) additional gen sets to be tied to the same 12,470V switchgear."

What is the existing short circuit rating of this switchgear? Will adding 2 more generators exceed the switchgear rating?

 

[VTer]

Thanks for your responses,

wbd
I still need to find out what the existing switchgear Isc rating is. This info was not yet provided to us, and we have not surveyed the facility yet. As we just got the job, we were only provided scope of work that lists requirements for (2) additional gen sets paralleled to the existing (3), (including the info provided in the original post)

What struck me is the fact that they (the facility) have already placed the purchase order for the 12470V units for “whatever” reason and I was trying to figure out why someone would make a decision to go with 12470V rather than 480V as it is in the existing conditions. If the existing system has performed adequately (which I assume it did), it just makes sense to have the two new units similar to the existing ones. There obviously is some logic behind the decision to order 12470V unit rather than 480V, and I want to figure out if that could be a engineering based decision or something else . Based on the responses the 480V w/ step-up xfmr seems like a better system? Any suggestions why 12470V units in this case could be a better choice would be appreciated.


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[alehman]

I can't think of any reason 12.47kV units would be better in this application. They probably cost more than a 480 unit with a transf.

 

[edison123]

Probably they went for 12.47 KV to reduce the current and to eliminate large bus ducts and large ampacity switchgear needed at 480 V (where the current is 3000 Amps).

Of course, it would have been better if they went for a 7.1 KV generator with 12.47 KV trafo.

 

[ScottyUK]

Another reason why an LV machine with transformer is a better solution: at 12.47kV they will get PD activty on the generator windings. At 480V that won't happen.

I agree with alehman, I can't think of any signficant reason to use an HV machine, and many reasons why an LV machine with a transformer would be a better design.

edison,

Why bother with an LV breaker if the generator is close coupled to the transformer? Treat the LV generator and the transformer as an HV generator block. Bus duct is expensive but with good layout design this setup would require 20' or so per machine and the technical merits of the LV + transformer solution would make it worth a slightly higher capital cost, assuming that the cost difference between the LV and HV machine doesn't already cover it.


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If we learn from our mistakes I'm getting a great education!

 

[edison123]

scotty

Use of breaker on the LV side would depend of local rules & regulations, I guess. In India, it is mandatory for co-generation companies to have breakers on either side of the trafo. I have seen 415 V, 3 MW beasts with two 3000 amps breakers in connected parallel and topped with air-conditioned breaker panel to avoid over-heating !!!

But big power plants producing the same amps but at a higher voltage (typically 11 KV) do not require breakers on the trafo primary. Stragneg but true. Them is the rules here.

But generally, don't the generator-to-trafo bus runs exceed 20 feet ?

I strongly would recommend using a trafo in between the genny and the grid mainly to protect the genny from line surges. The impulse voltage withstand ability of trafo is much higher than the genny.

 

[slavag]

Good Morning All.
Generator Breaker is cost issue only.
Is always good solution, but for big generator is very expensive.
Next point is UAT connection, is it's connect between gen and trafo is prefered gen CB, but in old application isn't used.
Don't forgot, lot of industrial application used direct connection of MV generation to bus.
According to OP, gen is 2.5MVA, isn't big industrial gen, possible order LV gen with step-up, possible order 3.3kV gen with step-up trafo, this solution reduce conductor size.
Best Regards.
Slava

 

[itsmoked]

edison; Are you in India?!??

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Yes I am, Keith. I did my master's in US though

You know, about 5 minutes back, I saw the geography of eng-tips members button (at the left bottom corner) for the first time and was pleasantly surprised to see that next to the yankees, indians were the second in membership.

 

[ScottyUK]

Hi edison,

I'll bow to your knowledge of Indian codes. For a 2.5MVA set and step-up transformer I would think 20' is a reasonable estimate for a duct run from an indoor genny through a wall to an external transformer.


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If we learn from our mistakes I'm getting a great education!

 

[EEJaime]

Gentlemen,
Does the fact that this system is an emergency standby system affect your responses to this as there will be no direct utility connection? I assume that the system only operates in case of utility failure so the concerns about the effects of being "connected directly to the grid" would not come into play.

If the units have already been purchased, then what needs to happen is for the existing switchgear to be analized to determine that it has the necessary withstand and current ratings, and then address the synchronization issues, but it seems that the system should be workable if those are not problems.

I may be missing something, but we've got HV standby systems for many hospitals, colleges, etc..., without any problems over the years.

Good day.