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 Post subject: Short circuit calculation for split winding transformer
PostPosted: Tue Jun 25, 2013 1:59 pm 

Joined: Tue Jul 10, 2012 11:18 am
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Hi All,
a first step in arc flash is to calculate short circuit.
i have in my system a transformer with a split secondary winding winding
the rating of the transformer
Primary 3000KVA
Secondary 1: 1500kVA
Secondary 2: 1500kVA
ZI% = 6%
ZY%= 5.75
ZX%= 6.01
primary voltage 25000V
secondary 480V

My software (using ANSI / IEEE method) calculated the fault as
3000/(1.73*0.48*0.0575)

i think it should be calculated differently, and i should use 1500kV in my circulations for each side.

Please let me know if you share my opinion or maybe im completely wrong.
Should you have a link to a reliable source it would supper helpful.


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PostPosted: Fri Jun 28, 2013 6:43 am 
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I think 1500 kVA should have been used. The reason is that the bus attached to the secondary winding will only see the fault current that can be passed by the 1500 kVA winding.
What software are you using?

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PostPosted: Sun Jun 30, 2013 9:58 am 
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I assume this is a delta-delta / delta-wye transformer? Is it for a rectifier? Just curious. I used to perform these types of calculations for large DC rectifier traction power systems and this was common for cancellation of 5th and 7th harmonics. wbd nailed it with his answer.

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PostPosted: Mon Jul 01, 2013 8:59 am 
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Many of the larger short circuit programs have the capability of modeling a three winding transformer. I know that SKM PowerTools for windows will handle this.

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PostPosted: Mon Jul 01, 2013 10:16 am 
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Jim Phillips (brainfiller) wrote:
I assume this is a delta-delta / delta-wye transformer? Is it for a rectifier? Just curious. I used to perform these types of calculations for large DC rectifier traction power systems and this was common for cancellation of 5th and 7th harmonics. wbd nailed it with his answer.


I have seen grid-tie applications that use transformers like this. You have an inverter on each secondary and the primary ties to the grid.


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PostPosted: Mon Jul 01, 2013 10:34 am 
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From your description, it looks like the utility (25KV) is feeding your transformer (3MVA), which has secondary split winding 1.5MVA/0.480KV. If I were you, I would trace the utility company and get the utility information that contains the short circuit current contribution. Usually, they will have details of their short circuit currents already calculated for their entire electrical network and it will be in their database. Also remember to specify where they (utility) have measured their short circuit contributions when you shoot your RFI. Example, if they have measured in your switchboard/panelboard, and in the software, if you model that short circuit contribution at the primary side of the transformer, then because of the impedance of the transformer, your short circuit calculation will be altered thus affecting the short circuit contributions, incident energy and Hazard Risk Category. However, in the mean time, if you want to compile the program in software like SKM or EasyPower or ETAP, I would go with 1.5MVA/0.48KV. Then use +/-10% of your Z%. For X/R ratio you can use the default value as 8 or 12. Plug in the size and length of the cable from secondary side of your transformer to your power distribution panel (bus). Create three scenarios, %Z, +10%Z & -10%Z and calculate the short circuit on that bus. Now your bus and any protection devices connected to this bus must withstand this calculated short circuit current (highest) and you will have a clear idea of SC rating of bus and protection devices you need in your design.


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PostPosted: Tue Jul 02, 2013 4:46 am 
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EasyPower is another program that I know can handle 3 winding transformers.

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PostPosted: Tue Jul 02, 2013 8:32 am 
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wbd wrote:
I think 1500 kVA should have been used. The reason is that the bus attached to the secondary winding will only see the fault current that can be passed by the 1500 kVA winding.

????

The %Z is based on the KVA Base, and typically the KVA base is the full rating of the transformer. [ If the %z is based on the per winding KVA then 1500 Would be correct. Contact the XF mfg to verify which value is the base kva if the name plate isn't clear. ]


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PostPosted: Mon Jul 15, 2013 8:36 am 

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The impedance of a transformer is determined by short circuiting a winding (LV in this case Winding 1) and passing winding rated current and determining the input voltage. Thus each winding (1.5MVA) would have its impedance based on the above test. This should be confirmed by the certified test report for the transformer (as noted by JK above). With a three winding transformer there should be three impedance components H-X, H-Y and X-Y. Sequence networks are then needed to determine the fault currents.


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