Generator GF current w/ low zero sequence impedance

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elec_eng

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I am paralleling (3)-2MW, 277/480V generators. Due to the low zero sequence impedance, my ground fault current at the paralleling switchgear is way higher than 3-ph fault current. Per my calculation, I have

Three phase fault current: 63,483A (x?d=0.1421) (21,161amp x 3)
Ground fault current: 80,454A (x0=0.0336) (26,818amp x 3)

That being said, I have two questions.

1. Can I just provide 100K AIC CBs (instead of 65K AIC) for higher ground fault current and call it a day? Is 100k AIC CB is rated for 80k ground fault? Isn?t the 100k rating is based on the L-L rating?

2. How about adding a small reactance at the generator so that the ground fault current can be reduced to the 3-ph fault current level? Has anyone done this before? Can it be done by generator mfgrs? Or is this something that has to be done in the field?
 
Yes, you can add grounding reactor(s). You can add individual ones or a single one sized properly. Make sure the generators are all the same pitch.

Low impedance or high impedance should work fine.
 
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Kingpb,

Thanks for your comments.

kingpb said:
Low impedance or high impedance should work fine.
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Can you elaborate what you meant by low or high impedance? I just want to make sure that you are not talking about low or high resistance grounding syst4m. I need to maintain solidly grounded system since I have the L-N loads from the generator.

kingpb said:
You can add individual ones or a single one sized properly.
Click to expand...
Can you explain little bit further how can I get there? Is this something that gen mfgr can provide?

How about simply providing 100K AIC rated cbs? Will that work as well?
 
Does anyone know if 100K AIC rated CB is reated for 80KA ground fault current? My guess is not but I am not sure.
 
Something you need to verify is the reactance values being used for your calcs. Many people do not realize that the reactance given can be on the machine alternator base, NOT on the generator base. If so, you need to convert to the correct base; this could alter the short circuit value.

Next, the cable in between the generator terminals and the breaker needs to be considered as this could also reduce the current at the breaker.

Finally, if you really need to serve L-N loads directly from the generators, you will have to solidly ground the generators. I would recommend doing high reactance ground on gens, and then utilize a 480-480Y/277V transformer to get neutral for the L-N loads.

NEMA standards do not require standard gens to be braced to handle mechanical stresses due to L-G faults greater than the three-phase fault at the terminals of generator therefore the neutrals should not be connected without some limiting impedance. For a generator applied to a 4-wire emergency circuit application, generators are designed with sufficient bracing but manufacturer needs to know this. A cost comparison would need to be made for extra cost of gen versus cost of transformer to derive neutral.

I don't think the increased rating of breaker is your answer.
 
I guess I don't get all of your comments since some of them are over my head. What I am confused is this is not the first time we paralleded multiple generators and this issue had never came up. So, the question is have we done this wrong all this time? we outinley paralleled generators with solidly grounded system without any issues. We never had to have high reactance ground on gens and utilize 480-480/277 xfmr to get neutral L-N loads. What am I missing?
 
elec_eng said:
I guess I don't get all of your comments since some of them are over my head. What I am confused is this is not the first time we paralleded multiple generators and this issue had never came up. So, the question is have we done this wrong all this time? we outinley paralleled generators with solidly grounded system without any issues. We never had to have high reactance ground on gens and utilize 480-480/277 xfmr to get neutral L-N loads. What am I missing?
Click to expand...

Q: Have you told the DEG supplier that the units are going to be hooked up in parallel and will also be feeding L-N loads?
If the answer to that question is yes, than have the manufacturer verify that the units were designed and built with sufficient bracing to withstand the high fault currents that they will see.

Q: How are you paralleling 3 x 2MW machines, at 480V; i.e. current is around 9000A. I am not aware of any 480V gear capable of that high of a current rating. In most cases I think I would do 4160V L-L voltage, utilize HRG on gens, and then 4160-480Y/277V transformation, qty as needed to distribute power.
 
kingpb said:
Q: Have you told the DEG supplier that the units are going to be hooked up in parallel and will also be feeding L-N loads?
If the answer to that question is yes, than have the manufacturer verify that the units were designed and built with sufficient bracing to withstand the high fault currents that they will see.
Click to expand...
Yes. They are aware of that. So, what I am taking from this is that the high ground fault is the issue of bracing at the generator, not at the circuit breaker? Is that correct? So, 100KAIC breaker is good for this application?

kingpb said:
Q: How are you paralleling 3 x 2MW machines, at 480V; i.e. current is around 9000A. I am not aware of any 480V gear capable of that high of a current rating. In most cases I think I would do 4160V L-L voltage, utilize HRG on gens, and then 4160-480Y/277V transformation, qty as needed to distribute power.
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I think the paralleling switchgear can be go up to 10,000A bus. However, I am only providing 6000A bus by locating the generator at the end of the switchgear line. The bus will not see more than 6000A. My first choice was 4160V with NRG but the owner wanted to have 480V system.
 
elec_eng said:
I am paralleling (3)-2MW, 277/480V generators. Due to the low zero sequence impedance, my ground fault current at the paralleling switchgear is way higher than 3-ph fault current. Per my calculation, I have

Three phase fault current: 63,483A (x?d=0.1421) (21,161amp x 3)
Ground fault current: 80,454A (x0=0.0336) (26,818amp x 3)

That being said, I have two questions.

1. Can I just provide 100K AIC CBs (instead of 65K AIC) for higher ground fault current and call it a day? Is 100k AIC CB is rated for 80k ground fault? Isn?t the 100k rating is based on the L-L rating?

2. How about adding a small reactance at the generator so that the ground fault current can be reduced to the 3-ph fault current level? Has anyone done this before? Can it be done by generator mfgrs? Or is this something that has to be done in the field?
Click to expand...

Something is wrong with your calculations, IMO. Unless there are other contributions (you didn't specify), your kA figure is higher than what I got from your data.

Lemme try a quick calc: 1/0.1421 = 7.037pu; 7.037 x 2MVA = 14.07 SCMVA for each generator and you have an available SC MVA of = 3 x 14.07 = 42.22 system SCMVA; or a three-phase short circuit of 50.79 kA! Also, assuming your negative sequence impedance is equal to your positive sequence impedance and your 0.0336pu zero seq. impedance, I got an SLG value of 68 kA! IMHO, these values could drop if you punch in the cable impedances. Please check again.
 
topgone said:
Something is wrong with your calculations, IMO. Unless there are other contributions (you didn't specify), your kA figure is higher than what I got from your data.

Lemme try a quick calc: 1/0.1421 = 7.037pu; 7.037 x 2MVA = 14.07 SCMVA for each generator and you have an available SC MVA of = 3 x 14.07 = 42.22 system SCMVA; or a three-phase short circuit of 50.79 kA!
Click to expand...

I think you should've used 2.5MVA instead of 2MVA? Then you should get 52.78 MVA or 63.48kA.


topgone said:
Also, assuming your negative sequence impedance is equal to your positive sequence impedance and your 0.0336pu zero seq. impedance, I got an SLG value of 68 kA! IMHO, these values could drop if you punch in the cable impedances. Please check again.
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x2=0.1606

Since they are PU values, I converted them to ohms. Isc= (E * sq root(3)) / (x"d + x2 + x0)
 
Attached is the GE CB AIC rating chart. It shows the 3-pole rating and 1-pole rating. Based on this, is it safe to assume that 100KAIC rated CB is not rated for 80,000A Ground fault current? I am trying to find this out from GE but hasn't received any response yet.
 

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I modeled it in ETAP and get 3ph fault of 21.15KA and SLG fault of 27.3kA individually; or 63.45kA and 81.9kA, respectively for total gen contribution.

I think your numbers are good. You just need to have manufacturer tell you that they have designed the gen for this high of a SLG fault current.
 
kingpb said:
I modeled it in ETAP and get 3ph fault of 21.15KA and SLG fault of 27.3kA individually; or 63.45kA and 81.9kA, respectively for total gen contribution.

I think your numbers are good. You just need to have manufacturer tell you that they have designed the gen for this high of a SLG fault current.
Click to expand...

kingpb,

My concern was never a generator. It was the AIC rating of CB. Can the CB handle such a high SLG ground fault current? Is the CB rated for that?
 
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