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jmd
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Join Date: Aug 2008
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Balanced Earth Fault Protection

05/14/2011 6:08 PM

Could someone pliz explain to me the theory behind BEF protection especially when applied to a delta winding of a power transformer. My understanding is that the BEF relay responds to the residual current detected by the three line CTs connected in parallel. This current is apparently zero sequence and it can only flow when there is a system connection to earth at two or more points and a potential difference between the earthed points. Now with an unearthed delta winding of a transformer, how are we gonna have to get the earth current flowing when there is no earth connection upstream?

Thanks in advance.

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hien.nguyenquoc
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#1

Re: Balanced Earth Fault Protection

05/14/2011 11:44 PM

I will develop more later since I am in a cafeteria at the moment. But the basic simple rule is that all three phases shall carry the same ammount current. If there is a change in term of current pending on how many percent you like to set, the relay will activate.

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hentoyk
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#2
In reply to #1

Re: Balanced Earth Fault Protection

05/15/2011 12:01 AM

What suppose to be balance is your 3-phase loads. Each phase has its earth-fault protection system depends how many percent you need to set to kick-off the eart-fault relay to shut down your equipement ... let say transformer/s sysmetrically in case there is a current leakeage occur in one of 3-phase will trip the 3-phase lines to avoid further escalation of damages to the system.

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jmd
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#5
In reply to #1

Re: Balanced Earth Fault Protection

05/15/2011 6:33 PM

My understanding is that BEF will not operate on the basis of an unbalance in load current regardless of the magnitude of unbalance. It will only operate when there is an earth fault in any of the three phases.

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nairck
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#3

Re: Balanced Earth Fault Protection

05/15/2011 8:49 AM

One pole of delta system is earthed .any EF on other pole or winding turns will result in circulation.I twill be detected.OK

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electricalexpert65
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#4

Re: Balanced Earth Fault Protection

05/15/2011 10:07 AM

Any earth fault - balanced or unbalanced - will work only and only if the equipment is connected to an earthed source. In such a case, even any single earth fault will be detected.
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jmd
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#6

Re: Balanced Earth Fault Protection

05/15/2011 6:44 PM

Electricalexpert65 - I agree with you, I would then be correct to say that corner delta transformers don't have an earth connection at source!

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nairck
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#7
In reply to #6

Re: Balanced Earth Fault Protection

05/15/2011 9:42 PM

Resistance earthig on one pole.Leakage potential feeding I, will be used totrip.

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hien.nguyenquoc
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#8
In reply to #6

Re: Balanced Earth Fault Protection

05/16/2011 11:53 PM

jmd:

What do you mean by that?

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electricalexpert65 (1); hentoyk (1); hien.nguyenquoc (2); jmd (2); nairck (2)

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