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 Post subject: Problem with calculating Iarc for an Transformer (secondary side)
PostPosted: Mon Nov 08, 2010 1:43 am 

Joined: Fri Aug 20, 2010 1:16 am
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Location: Dordrecht the Netherlands
Hello,

I'm busy making arc flash calculations for my employee Dupont but i'm encountering a problem with the arc flash calculations for the secondary side of an transformar which is only protected on the primary side.
The problem is that we have an transformer of 2000kVA 13000/400V which is protected on the primary side with an S&C HV fuse of 80E. On the secondary side is no protection device placed and the transformer is directely connected to the (400V) main distribution board. The bolted fault current on the primary side is 19,62kA and the bolted fault current on the secondary side is 29,67kA.
Now i want to calculate the arc flash current on the secondary side (400V rail of the MDB) when i calculate this according the formula of the IEEE1584 (formula 1 P5.2 page 10) (<1kV) with te settings: (V=0,4kV, Gap= 32mm, arc in an box k=-0,097) than i find an Iarc of 13,15kA. But the software i'm usimg (SKM PTW v6518b2) calculates the value Iarc =11,18kA.
Can anybody explain what i'm doing wrong here?


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PostPosted: Mon Nov 08, 2010 5:42 am 
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bhazewindus wrote:
Hello,

I'm busy making arc flash calculations for my employee Dupont but i'm encountering a problem with the arc flash calculations for the secondary side of an transformar which is only protected on the primary side.
The problem is that we have an transformer of 2000kVA 13000/400V which is protected on the primary side with an S&C HV fuse of 80E. On the secondary side is no protection device placed and the transformer is directely connected to the (400V) main distribution board. The bolted fault current on the primary side is 19,62kA and the bolted fault current on the secondary side is 29,67kA.
Now i want to calculate the arc flash current on the secondary side (400V rail of the MDB) when i calculate this according the formula of the IEEE1584 (formula 1 P5.2 page 10) (<1kV) with te settings: (V=0,4kV, Gap= 32mm, arc in an box k=-0,097) than i find an Iarc of 13,15kA. But the software i'm usimg (SKM PTW v6518b2) calculates the value Iarc =11,18kA.
Can anybody explain what i'm doing wrong here?


Did SKM use the same gap? I believe it depends on how the equipment is designated i.e. panel / swgr etc.

The thing that jumped out at me right away was the bolted fault current. 29.67 kA (we use periods in the U.S.) seems quite low for a 2000 kVA transformer with a 400 V secondary. I would think it might be more like 40 to 50 kA - what is the impedance of the tranformer? The reason I brought this up - is it modeled correctly? I hand calculated a value close to what you had. Although sometimes this can all seem frustrating, you can really learn alot through these types of problems.

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PostPosted: Mon Nov 08, 2010 6:15 am 

Joined: Fri Aug 20, 2010 1:16 am
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Location: Dordrecht the Netherlands
brainfiller wrote:
Did SKM use the same gap? I believe it depends on how the equipment is designated i.e. panel / swgr etc.

The thing that jumped out at me right away was the bolted fault current. 29.67 kA (we use periods in the U.S.) seems quite low for a 2000 kVA transformer with a 400 V secondary. I would think it might be more like 40 to 50 kA - what is the impedance of the tranformer? The reason I brought this up - is it modeled correctly? I hand calculated a value close to what you had. Although sometimes this can all seem frustrating, you can really learn alot through these types of problems.


Jim, SKM used the same Gap swgr and 32mm, i have calculated the impedance of the transformer with (Uk=8,8% an Pcu = 15,63kW) so Z of the transformer is Z= 0,625+j7,012milli ohm. I'm learnig a lot indeed of these kind of problems, but this is the last problem i have to solve for comleting the studie and to understand the calculationmethod, but unfortunately i can't find the answer to this problem by my self :(
I have manually recalculated a lot of the results of SKM and they where all correct but this one.


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PostPosted: Mon Nov 08, 2010 6:38 am 
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Could your calcs and SKMs be assuming a different open vs box configuration? The difference in K values would approximately account for the difference in calculated arc current.


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PostPosted: Tue Nov 09, 2010 4:20 am 
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You also may want to check and see if you selected 60 or 50 hz for SKM.

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PostPosted: Tue Nov 09, 2010 2:30 pm 

Joined: Tue Nov 09, 2010 2:25 pm
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It seems to me that SKM is giving you 0.85*Iarc. One of the things that SKM does is that it will compare the incident energy of Iarc and 0.85*Iarc and reports the values for the higher incident energy.


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PostPosted: Wed Nov 10, 2010 10:04 am 
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MWagner wrote:
It seems to me that SKM is giving you 0.85*Iarc. One of the things that SKM does is that it will compare the incident energy of Iarc and 0.85*Iarc and reports the values for the higher incident energy.


Great catch! I never thought of the obvious.

To be sure, check the clearing time used with the lower value. It will likely be longer if it is using the 0.85.

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PostPosted: Fri Nov 12, 2010 5:31 am 

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MWagner wrote:
It seems to me that SKM is giving you 0.85*Iarc. One of the things that SKM does is that it will compare the incident energy of Iarc and 0.85*Iarc and reports the values for the higher incident energy.


:eek: Yep youre wright, i've forgot to calculate the 85% Iarc current, SKM shows the factor (*N3) in the arc flash report. This was the last issue in my study thanks ;)


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PostPosted: Fri Nov 12, 2010 10:59 am 

Joined: Fri Aug 20, 2010 1:16 am
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When you calculate the 85% Iarc do you have to calculate also the normalized energy [En] for the 85% Iarc to get the real energylevel? Or do you use the 100% [En] with the 100% Iarc


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