Restricted Earth Fault Relays for Transformer Protection 5

[smithm]

Can anyone tell me how to select an appropriate setting for restricted earth fault protection relays used to protect low voltage distribution transformers?<br>
<br>
Is there a rule of thumb method?<br>
<br>

 

[bfuchs]

In TN networks (where the Earth conductor is linked to the transformer's star point earting connection), the earth fault current is only limited by the impedance of the phase and earth conductor. This current is usually high, and normally the instantaneous (aka magnetic) setting of the circuit breaker should take care of the problem.<br>
<br>
If you have long cables and/or small earth conductors, then your earth fault drops ! To how much ? Well it depends !<br>
What you have to do is calculate the value of this earth fault current (using computer programs), and make sure that :<br>
- your earth fault current is greater than the magnetic<br>
- a restricted earth fault relay is used to protect the transformer. Set this realy to a value under the earth fault current.<br>

 

[peterb]

Restricted earth fault protection is a differential scheme, with CTs on the transformer neutral and phase leads.&nbsp;&nbsp;The phase CTs are connected together in parallel & then paralleled with the neutral CT.&nbsp;&nbsp;The resticted earth fault (REF) relay is connected across the paralled CT circuits.<br><br>The relay is either a voltage operated device (eg Alstom MFAC14)or a current relay with an external setting resistor (eg Alstom MCAG14).&nbsp;&nbsp;In either case, the scheme is set at a voltage based on the maximum voltage that is developed at the relay for an external fault.&nbsp;&nbsp;A practical setting is 50% of the CT knee point voltage, if full info is not available.

 

[bartos]

Reference: A. R. van C. Warrington &quot;Protective Relays Their Theory and Practice.&quot; Volume One, 1971<br>It appears that more information is needed pertaining to your specific low voltage distribution trasformer application. The above reference can be found useful; especially, <br>(d) Restricted Earth Protection on page 181 involving useful components, e.g. stabilizing resistor, and transient blocking scheme.<br>10.4.1. Earth Fault Relays on page 387 providing additional more detailed explanation with a consideration of various ground faults. Notice that &quot;Restricted Earth Protection&quot; is an English term meaning the differential protection of transformers (or generators) against ground faults.

 

[Jiz]

What is restricted earth fault? And how does it occur?

 

[marmic]

calculate the fault current ...set voltage obtained by multiplying the secondary fault current by the CT and lead resistances sum.
Compare the result obtained to the Magnetising Characterictic of CT , if the knee point is higher than twice the value obtained than the setting should be done according to the calculation. if the knee point is lower than twice the value obtained than the setting must be lowered such that it is less than halve the knee point value.

 

[gordonl]

Restricted earth fault is not something that happens, it is a way of looking at earth (ground) faults. As previously mentioned, it is a method to quickly detect a ground fault in a zone, usually a transformer. The ground current into the transformer is compared to the ground current out of the transformer. If the ground current is going in and not out, and is above the setting of the restricted earth fault relay, the transformer is isolated.

 

[Bung]

If the transformer already has a full differential scheme and the neutral is directly earthed, there isn't much value in REF (IMHO- but I'm open to being convinced otherwise!). You have to pick a setting which is not so fine you get nuisance tripping , which means the sensitivity you end up with is not thaht much different from the full diff scheme. REF is good where you have earth fault current limiting (star earthed through resistors or (horrors) a reactor), and so the the diff does not &quot;see&quot; very far into the winding for earth faults.

Bung

 

[vijay12]

Can some one give a Typical Calculation sheet for REF setting ? This will help me in setting my relay.

 

[stevenal]

Bung

I believe REF provides additional sensitivity over 87T alone, for faults close to the neutral point. See the GEC Protective Relay Application Guide.

 

[Bung]

Stevenal

re additional sensitivity of REF over standard 87T:

Yes, I thought so to, but when I did some detailed calcs (based on GEC PRAG and my notes from GEC APPS course) I discovered it was not giving me much extra cover (with our solidly earthed netrals on Dyn1 transformers). The difficulty lies in finding a setting current low enough to be useful, but not so low it trips every time a passing fly sneezes. It also adds to the expense of the installation(okay, its chickenfeed in the grand scheme of things) by requiring an extra CT core on the trannie neutral, more panel wiring, more scope for 'finger troubles' and all to protect a few percent more at the bottom of the winding.

 

[ADN]

vijay12,

If u wnat I can send u a sheet in excel which I use for REF calculations, with all the formulae builtin. However I would need ur email.

ADN

 

[smithm]

ADN,

I would be most grateful if you could send me a copy of your excel sheet for REF calculations. This would probably answer my orginial thread!

matt_smith1@yahoo.co.uk

 

[Jiz]

ADN,

Could you possibly send me a copy of the calc sheet too. Many thanks in advance.

James

james.flanagan@wwp-london.co.uk

 

[stevenal]

Bung,

A good paper dealing with REF on solidly grounded wye windings, and ground faults near the neutral point can be found at

http://www.selinc.com/techpprs/6092.pdf

I've employed REF on both resistive and solidly grounded wye windings. Neither sneezes nor through faults have caused it to trip.

 

[Bung]

thanks Stevenal. Looks like I might have to have another think about it!

Bung

 

[61]

where i can have some solid stuff related to reactive power compensation on net... can anyboday help me.. for the same.

 

[salim02]

I would be most grateful if you could send me a copy of your excel sheet for REF calculations. This would probably answer my orginial thread
my e.mail address is salim.haddabi@pdo.com.om

 

[salim02]

Can Anybody refer me to a web site to get the ANSI relay numbering.
Many Thanks

 

[busbar]

http://standards.ieee.org/reading/ieee/std_public/description/switchgear/C37.100-1992_desc.html

describes the standard.

http://www.geindustrial.com/pm/notes/ansinums.pdf

but it's fairly crippled.

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