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 Post subject: Generator decrement curve and short circuit study
PostPosted: Fri Mar 07, 2014 1:56 am 
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Joined: Fri Jan 03, 2014 6:57 am
Posts: 66
Location: the Netherlands
d day fine people,

I was trying to get some TCC curves for my boss when I noticed something weird.

The generator decrement curve is wrong I think or I just don’t understand what a decrement curve really is and I think its weird that the result off my IEC61363 calculation is so different then that of the comprehensive calculation. Ow and this is a situation where 1 of 6 parrallel generators is running and the running generator is roughly 1/6 off the total power.

I know arc flash uses the comprehensive short circuit study and I do a IEC61363 short circuit study but they should be reasonably close right?

The generator:
1636 kVA
690 V
Pf 0.8
FLA 1368 A
Other info can be seen in the image.

My IEC hand calculations say I should have a peak of 28.4kA and 10.8kA Iac at 0,5T,
SKM on IEC61363 mode says I have a peak of 28.4kA and 10.7kA Iac at 0,5T. so no problems their.
When I do a comprehensive systems study with SKM I get a Isc 3P of 11.3kA, but is this the maximum or effective or at what t does this current flow?

And in the image you see the generator curve mis the short circuit protection completely! In my mind the decrement curve is the actual dynamic short circuit current of the generator but if this is true then the main switch board is not protected against short circuit? The right purple line is the total and the left is the Iac. The total is about 18kA and the Iac is 9.8kA, but again this seems not logical if I compare those values to my calculated values.

And I’ll be honest the Ta, If and Ifg are the standard values of SKM I did not change them.

Edit:
I forgot to mention that the steady state short circuit current 5.6kA is.


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PostPosted: Tue Mar 11, 2014 1:17 am 

Joined: Sun Feb 14, 2010 9:28 pm
Posts: 21
Location: Queensland
Your parameters look reasonable for this machine, although Td' is short. I would have expected about 3 times this value. Assuming normal field forcing capability, the default IF, IFg are appropriate and the Ta is typical. Could you please explain with your modelling when you say that there are 6 equivalent machines in parallel, do we assume that they are all on-line ? With field forcing, steady state contribution from each machine of about 1.6kA appears to agree with your decrement curve, however, without other network details and fault contributions it is not easy to comment on how 5.6kA is arrived at. Certainly it would appear that the indicated protection curve may only be cable and not machine thermal protection. It would be worth referring to the protection philosophy document and protection design calculations for the installation. I refer you also to generator thermal capability requirements of the standards and excitation protection under field forcing conditions. These may be over-riding around the 10s mark.


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PostPosted: Tue Mar 11, 2014 7:06 am 
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Joined: Fri Jan 03, 2014 6:57 am
Posts: 66
Location: the Netherlands
Thank you GBISON,

In this situation the generator is the only one online the others are offline. The 5.6kA is the sustained short circuit current (effective) off the generator, so if the generator is the only power supply then this should be the current at the generator terminals right? So not 1.6kA but 5.6kA

And what I don’t understand is that at 0.632secondes the decrement curve is at about 2.5kA but according to my calculations it should be about 8kA

I really don’t understand what I am doing wrong because I am confident I did not make a mistake with the calculations conform the IEC61363 and I know the comprehensive way is more accurate but it can’t be such a big difference right?

Maybe I’m just thinking about this all wrong, just answer me this; in the image will the short circuit be shut down within a second or will it go on to roughly 150 seconds? Because the way I look at it, it will take about 150seconds before the protection device will do something.


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PostPosted: Tue Mar 11, 2014 6:21 pm 

Joined: Sun Feb 14, 2010 9:28 pm
Posts: 21
Location: Queensland
"The 5.6kA is the sustained short circuit current (effective) off the generator" - I am not sure where this comes from, but it is not born out by the decrement plot that you sent, which agrees with my plot for the same parameters. This plot shows 1.6kA. If the model is giving 5.6kA then I expect that there are other generating sources.
I have not applied IEC61363. Normally I am using IEC 60909 / AS3851.
I presume that the 0.632s is the I>> definite time as indicated for the GCB overcurrent element, presuming it sees a fault current above the ~4.5kA indicated in the plot. If not, the trip time will differ. Based on your plot, the over-current element will pick up ~150s at 1.6kA.
However, I would expect other generator protection will come into play before this time, since it is unlikely that the excitation train will sustain field forcing for this duration. More typically field forcing might be limited to 10s to prevent damage to rotor / exciter windings.
Without more detail it is difficult to comment.
What is the location of the arcing fault of concern? ie down-stream or up-stream of the generator breaker? If upstream, we rely on tripping of excitation and dissipating the field to minimise fault energy.
Does the generator have under-voltage protection? When supressing terminal voltages on a machine, active power export is suppressed and the machines accelerate. An out of step protection will typically also be deployed around 0.4s depending on machine moments of inertia and governor response times. You may wish to check other element of the protection scheme that may come into play in this case.


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PostPosted: Thu Mar 13, 2014 12:44 am 
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Joined: Fri Jan 03, 2014 6:57 am
Posts: 66
Location: the Netherlands
Dear GBISON,

Thanks again for your reply,

Quote:
"The 5.6kA is the sustained short circuit current (effective) off the generator" - I am not sure where this comes from, but it is not born out by the decrement plot that you sent, which agrees with my plot for the same parameters. This plot shows 1.6kA. If the model is giving 5.6kA then I expect that there are other generating sources.


See attached image. It is the only generator online, but if you calculate it with the IEC61363 using a long time (1 second or something) then the short circuit current will be equal to steady state:
A.C. component is ‘subtransient current + transient current + steady state current’, the subtrainsient and transient current will be 0 eventually so for the AC component the steady state is all that is left.
D.C. component will also be 0 eventually.

So to determine the current at a given time after subtransient, transient and dc is 0 you get the formula: Ik(t) = Square root(2) * Ikd

This I derived from the formula Ik(t) = Square root(2) * Iac + Idc

The fault location is downstream of the breaker, this is not really a concern (yet) because it at least trips the breaker in its I>> range. The actual problem is that if I use my hand calculations the short circuit protection works when there is a short circuit on the main switchboard but if I look at the TCC it suggests that it does not work.


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PostPosted: Sun Mar 16, 2014 2:55 am 

Joined: Sun Feb 14, 2010 9:28 pm
Posts: 21
Location: Queensland
Steady state
id = ( e / Xd ).( IF / IFg )
so for Ikd = 5.6kA we would need IF / IFg = 10.4
Do you have further detail on the excitation system that can confirm this?


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PostPosted: Mon Mar 24, 2014 6:17 am 

Joined: Tue Sep 28, 2010 1:51 pm
Posts: 2
I don't mean to hijack this thread, but..
In my experience it is difficult to obtain If/Ifg information from manufacturers. Instead it frequently seems necessary to adjust this value in the software to obtain the stated sustained fault current.

However, PTW arc flash calculations disregard this data and only utilize a simple step reduction that you enter manually in the study options for synchronous generators and motors.


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 Post subject: Re: Generator decrement curve and short circuit study
PostPosted: Thu Oct 02, 2014 6:49 am 

Joined: Fri Sep 19, 2014 12:57 pm
Posts: 4
can't see any attachments. should i be able to?


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