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Topic Title: Transformer Supply Characteristics V, Ze
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Created On: 04 September 2013 08:57 AM
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 04 September 2013 08:57 AM
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irnbru

Posts: 30
Joined: 23 April 2002

I have been tasked with installing a 11kV/433V 2000KVA 5.4% transformer and Main incomer LV board on a private network. When the time comes to document the "Supply Characteristics" do I

Give the nominal tapped voltage (433V) of transformer or do I measure off load volts (say 432.6V) and give that?

Given that the LV board will be in close proximity (10m) to the installed transformer and almost impossible to measure with a loop tester what do I give my Ze value as? If Ze is found by calculation what electrical info do I need from TX manufacturer and what formula do I use. A worked example would be nice.

Thanks
 05 September 2013 10:14 PM
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statter

Posts: 118
Joined: 06 February 2013

Normal to declare standard voltage e.g. 400V unless you are stating open circuit. You will need to measure Ze as it is the external part of the earth loop impedance. Normally there will be an earth rod or mat installation. You should be able to measure Zs too. Where is the star point earthier, in the t/f cable box or at the switchboard?
 07 September 2013 05:44 PM
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Snips

Posts: 26
Joined: 01 April 2012

Hello, this what I've used before, I'm not saying its correct but it seems to make sense.

So for example a 3 phase 11kV/400V Tx at 2000kVA with a percentage impedance of 5.4%.

So full load current, Ifl= 200000VA / (3 x 230V) = 2,899A per phase.
Or
A quick way for Full load current, Ifl= Tx kVA/0.69. Eg. 2000/0.69 = 2,899A

So loop impedance = (230V x (5.4/100))/2,899A = 0.0042849 ohms.

This is the loop impedance at the bars of the transformer so you will need to add the resistance of the cabling between the Tx and the LV panel.

I got given this calc from a project engineer whilst I was testing a large R+D site where there were 4 x 1500kVA Tx's where we had to calculate the ELI as the test kit we had on site were not accurate enough. So we used the tester to prove the earth fault path and the calculated value in the cert.

I'm sure someone on the forum will say I'm completely wrong but it seemed to work for me.

Hope it helps.
 09 September 2013 09:32 AM
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irnbru

Posts: 30
Joined: 23 April 2002

I assume the star point would be at the Tx.

Thanks for the calculations Snips most useful.
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