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 Post subject: CT saturation
PostPosted: Wed Mar 06, 2013 5:46 am 
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Location: Quebec, Canada
Hi guys

I'd like to know your opinion on this:

In an arc flash study, should we always check if CT's saturate at the calculated arcing current?


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PostPosted: Wed Mar 06, 2013 2:45 pm 
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IF saturation was evaluated for the bolted case and it was OK i.e. not much error, then I believe it should also be OK for the lower arcing current which should cause less saturation. The only case I could think of that might be a problem is if the arcing current is close to the instantaneous setting and saturation IS a problem (error) at the bolted current level - it then might be a problem at the arcing current level if it is close the IP setting.

However, for voltages above 15 kV, the calculation procedure is to assume the arcing current is the same as the bolted current.

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PostPosted: Wed Mar 06, 2013 3:16 pm 
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brainfiller wrote:
IF saturation was evaluated for the bolted case and it was OK i.e. not much error, then I believe it should also be OK for the lower arcing current which should cause less saturation. The only case I could think of that might be a problem is if the arcing current is close to the instantaneous setting and saturation IS a problem (error) at the bolted current level - it then might be a problem at the arcing current level if it is close the IP setting.

However, for voltages above 15 kV, the calculation procedure is to assume the arcing current is the same as the bolted current.


Maybe my question should have been: Should saturation always be evaluated to validate trip time used in energy calculations?

Said this way, I guess it's hard to answer no, but i'd like to know what you guys think.

Thanks again!


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PostPosted: Wed Mar 06, 2013 4:20 pm 
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In a properly designed protective relay system the available fault current is taken into consideration when determining the class of CT to be used in a circuit. The one case that is hard to evaluate for saturation is a fault very close to the CT. This may cause the CT to saturate where a fault further down the line would not have the same potential reaction.


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PostPosted: Thu Mar 07, 2013 7:34 am 
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I agree with the comments above, but the whole point is do you guys usually consider the CT's properly designed? Or should it be a typical step in an arc flash study? I'm thinking of studies done on existing systems.

I will say it first: I do not evaluate CT saturation for every relay in an arc flash study. I only do it when the CT is «suspect» (small ratio or C100 CT with high fault current, etc).


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PostPosted: Mon Mar 18, 2013 7:29 am 
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hello again mister JPEG
knowing the CT saturation level is one thing, you also need the know the impedence of the circuit...if you just feed the relay in the breaker door it's not a big issue..

but if you do a differential protection to a relay at the other end of the plant it could change

or like i have seem last week...the ct's go into a motor exciter to meter and finally the protective relay...the CT were C400 so probably no worry...but lower like you said could have been an issue

but nothing can confirm like testing it for real...normally i just hook my test set to the secondary of the CT's and inject a current to make sure the relay trip in short circuit...if the ct's saturate it won't trip


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