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kushal_1206
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Join Date: May 2012
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Why REF is Tripping Beyond Its Zone of Protection?

08/02/2012 4:23 AM

My question is regarding REF protection...in one of the instances an earth fault has taken place in the LV busbar zone..however the REF Relay has tripped the circuit.As far as I know REF is used for specific zone protection i.e. in this case it is meant to protect the region from the transformer to the incoming busduct...why has REF tripped if a fault is outside its zone of protection..any help would be highly appreciated...
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Pathfinder Tags: REF Restricted earth fault protection
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PWSlack
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#1

Re: Why REF is tripping beyond its zone of protection??

08/02/2012 5:19 AM

Clearly the premise that the REF is operating outside its zone of protection is wrong.

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kushal_1206
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#4
In reply to #1

Re: Why REF is tripping beyond its zone of protection??

08/02/2012 7:27 AM

Sorry I did not get the statement of the premise being wrong...can you elaborate
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PWSlack
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#5
In reply to #4

Re: Why REF is tripping beyond its zone of protection??

08/02/2012 7:36 AM

The assumption that the fault is outside the REF's zone of protection is invalid?

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satnat.2008
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#2

Re: Why REF is tripping beyond its zone of protection??

08/02/2012 6:00 AM

Kindly clear the following point :

Is it an high impedence REF circuit or low impedence REF Circuit .?

whether the fault has occured in the upstream end or down stream end?
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kushal_1206
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#3
In reply to #2

Re: Why REF is tripping beyond its zone of protection??

08/02/2012 7:25 AM

The protection is by means of low impedance numerical relays...and the fault has occurred downstream...I just basically want to know how should an REF react to an external fault....and are there any chances for e.g. incorrect polarity or use of different CT ratios etc. that shall cause it to trip on external faults
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PWSlack
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#6
In reply to #3

Re: Why REF is tripping beyond its zone of protection??

08/02/2012 7:37 AM

An earth fault would do it.

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satnat.2008
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#7
In reply to #3

Re: Why REF is tripping beyond its zone of protection??

08/02/2012 7:48 AM

Since it is a low impedence circuit , earth fault in the down stream will also cause a back flow of elctron in the transformer neutral . that might be reason of REF tripping .
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PWSlack
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#8
In reply to #7

Re: Why REF is tripping beyond its zone of protection??

08/02/2012 8:36 AM

Precisely. There is a current between either a phase or the neutral to earth. The REF is doing its job.

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kushal_1206
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#10
In reply to #7

Re: Why REF is tripping beyond its zone of protection??

08/03/2012 2:18 AM

Do you mean to say that low impedance numerical relays are highly unreliable when it comes to REF protection and that it shall always trip with faults outside its zone for e.g. somewhere in the downstream...or is there a way of using the numerical relays (Low impedance) and yet preventing the backflow of current to Xmer neutral??
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raghun
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#11
In reply to #10

Re: Why REF is tripping beyond its zone of protection??

08/03/2012 3:03 AM

Dear Kushal,

The issue can be cleared if REF protection stability test can be conducted. The REF protection, whether low impedance type or high impedance type, is a unit protection and shall be stable for out-of-zone faults. The reasons can be many:

  • One of the CTs not in circuit (could happen if the short across one of the CTs is not removed, this is the case typically with neutral CT)
  • CT polarity issues
  • CT ratio issues - in case of low impedance REF protection using numerical relays, the CT ratio mismatch can be corrceted by appropriate setting in the relay.
  • Multiple earthing of CT secondary wires - this is one of the common problems. All the 4-CTs should have earthing at one common point to avoid circulation currents.
  • In case of low impedance REF protection - pickup threshold and slope are important for stability and should follow the manufacturer's recommendation.

Once you find the cause, please provide feedback to the forum.

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kushal_1206
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#12
In reply to #11

Re: Why REF is tripping beyond its zone of protection??

08/03/2012 5:09 AM

Thanks a lot!! that makes things a lot clear..!
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muditmah
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#13
In reply to #12

Re: Why REF is tripping beyond its zone of protection??

08/03/2012 9:38 AM

we were disturbed by the same problem a while ago in our plant.

found out that nuetral CT was not in line. a short link was applied through it's terminals.
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3PhaseDesign-Craig
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#9

Re: Why REF is Tripping Beyond Its Zone of Protection?

08/02/2012 6:21 PM

I would check the CT positions and the location of the N Earth link to ensure it has been assembled correctly
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3PhaseDesign-Craig (1); kushal_1206 (4); muditmah (1); PWSlack (4); raghun (1); satnat.2008 (2)

Previous in Forum: How to Couple PL Carrier Across Two Phase Power   Next in Forum: Selection of Fast Tripping Relays

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