Restricted Earth Fault Protection
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Earth Fault Protection...
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7/22/2015
Restricted Earth Fault Protection
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The windings of many medium and small sized transformers are protected by restricted earth fault (REF) systems. The illustration shows the principal of REF protection.
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Rectifier (/questions/view/question/id/586) 7. Per Unit & Faults (/questions/view/question/id/584) 8. relay testing (/questions/view/question/id/583)
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email address Subscribe Under normal conditions and by application of Kirchhoff’s laws the sum of currents in both current transformers (CTs) equals zero. If there is an earth fault between the CTs then some current will bypass the CT's and the sum of currents will not be zero. By measuring this current imbalance faults between the CTs can be easily identified and quickly cleared. Fault detection is confined to the zone between the two CTs hence the name 'Restricted Earth Fault'. REF protection is fast and can isolate winding faults extremely quickly, thereby limiting damage and consequent repair costs. If CTs are located on the transformer terminals only the winding is protected. However, quite often the secondary CT is placed in the distribution switchboard, thereby extending the protection zone to include the main cable. Without REF, faults in the transformer star secondary winding need to be detected on the primary of the transformer by the reflected current. As the winding fault position moves towards the neutral, the magnitude of the current seen on the primary rapidly decreases and could potentially not be detected (limiting the amount of winding which can be protected). As the magnitude of the currents remain relatively large on the secondary (particularly if solidly earthed), nearly the entire winding can be protected using REF. It should be remembered that the protection as illustrated covers only the secondary of the transformer. Sometimes REF protection is added to the primary as well (although if primary protection is required it may be preferable to consider full differential protection). As it is essential that the current in the CTs be balanced during normal conditions (and through faults), historically REF has been implemented using High Impedance Relays. CT's have also been specified as matched pairs and the impedance of leads/wires and interconnecting cables has had a large influence on the functioning of the relay. Measurement errors associated with mismatched CTs, saturation and varying lead resistance have been responsible for nuisance tripping and the system can be difficult to commission. This may be the reason some people avoid the use of REF. Recent advances in numerical relay technology have all but eliminated these issues, making the implementation of REF relatively easy, ensuring no nuisance tripping and simplifying commissioning. In the diagram 87N is the ANSI Device Number for restricted earth fault.
See Also ANSI (IEEE) Protective Device Numbering (http://myelectrical.com/notes/entryid/148/ansiieeeprotective devicenumbering)
Power Systems (http://myelectrical.com/notes?tag=power+systems), Protection (http://myelectrical.com/notes/tag/protection), REF (http://myelectrical.com/notes/tag/ref), Earth Fault (http://myelectrical.com/notes?tag=earth+fault)
More interesting Notes: Cables for MV Power Distribution Earthed versus Unearthed Systems (http://myelectrical.com/notes/entryid/226/cablesformvpowerdistributionearthedversusunearthedsystems) Fault Calculations Introduction (http://myelectrical.com/notes/entryid/192/faultcalculationsintroduction) How to refer fault levels across a transformer (http://myelectrical.com/notes/entryid/184/howtoreferfaultlevels acrossatransformer) Differential protection, the good old days (http://myelectrical.com/notes/entryid/90/differentialprotectionthegoodold days) Frame Leakage Protection (http://myelectrical.com/notes/entryid/218/frameleakageprotection) Steven McFadyen (/users/steven) Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author
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Pradeep Rath • a year ago
Dear Friends, I would like to have your opinion on the best protection engineering practices with regard to providing a separate independent relay for REF (64) or Differential (87) protection in addition to a comprehensive relay for all other protections. 4 △
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• Reply • Share › Mod > Pradeep Rath •
Steven McFadyen
a year ago
If you are using a good multifuntion numerical relay, which incorporates REF or differential, I would would be ok with that. Only reason to use an extra relay is if the funcion is not HOME (HTTP://MYELECTRICAL.COM/) NOTES (HTTP://MYELECTRICAL.COM/NOTES) incorporated (or there is some technical limitiation). 1 △
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QUESTIONS (HTTP://MYELECTRICAL.COM/QUESTIONS) Manish Naha • a year ago
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Can REF used effectively in cases where a neutral has been earthed through resistors,
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SUPPORT (HTTP://MYELECTRICAL.COM/SUPPORT) Steven McFadyen Mod > Manish Naha • a year ago
Yes. In fact I think it is recommended, although it will probably protect less of the winding then would be the case with a solidly earthed system.
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Dattatraya Joshi • 3 months ago
Sir, I was told that I need to have a 3pole ( and not 4 pole) main incomer on LV switchboard to incorporate REF. The neutral needs to be directly connected to main BUS of LV board. is that right? can not I use a 4P ACB on LV side?What if I have to use REF on a DG? DO i need 3 Pole breaker only for isolation?
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Steven McFadyen
3 months ago
For incoming supplies (and those from separately derived sources), I would use a 4pole. That way you completely isolate the system. 1 △
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Lanz Nellas • 3 months ago
explain why fault protection is essential? i need your suggestion
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Juancer Simangunsong • 3 months ago
sir can you explain about REF and SEF/SBEF relay......and the picture
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Siva Prakash • 5 months ago
will REF protect 100% of its zone
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Steven McFadyen
Mod > Siva Prakash •
5 months ago
It will protect everything within the zone between two sets of CT. If you tried, you could possibly make faults (for example in a winding very close to the neutral), which would be below the relaying sensitivity.
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Mohamad Fazdhli Hassan • 6 months ago
The concept for the above REF is based on current differential. However our Siemens REF relay (called high impedance relay) uses voltage elements as the setting only. Can you clarify. Is it lookig at the same thing ?
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Steven McFadyen
Mod > Mohamad Fazdhli Hassan •
6 months ago
I think it is looking at something different. If you can post the model number of the relay, I can maybe give a better answer.
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Mohamad Fazdhli Hassan > Steven McFadyen • 6 months ago
Thanks The relay is Siemens 7PG23 5B3. 1 △
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Steven McFadyen
Mod > Mohamad Fazdhli Hassan •
6 months ago
Sorry, my misread, I thought you was taking a voltage measurement to the relay.
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The principal is as still as shown in the note. However, to take care of any spill current (due to CT mismatch, saturation on faults, etc.), the relay impedance is increased (hence high impedance). When you work out the theory, it shows that you can use voltage measurement across the relay for sensing any fault. This is what is happening in your relay. The best way, for you to understand this, is to download the NPAG from Alstom. The guide has a section on high impedance differential protection, which explains all the theory. If you also download, the relay manual, it explains how to select the correct setting. 1 △
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Naren • 6 months ago
what is the connection if one side of transformer is delta connected? i mean how can you introduce neutral point?
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Steven McFadyen
Mod > Naren •
6 months ago
You would need to use earthing transformers somewhere on the network to introduce a path for the earth fault. Depending on the situation, it may be better to consider full differential protection.
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Satinder Pal Singh • 8 months ago
If we put the Neutral CT in Neutral busbar link of distribution panel for Ref protection Instead of puting the Neutral CT in the transformer where star point are grounded. In this condition my Ref relay will work or not??
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Steven McFadyen
Mod > Satinder Pal Singh •
8 months ago
It is not a neutral CT, but summation CT of all phase currents. If you wanted to place this in the distribution panel, it would still work. The zone of protection would be extended up to the panel.
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Satinder Pal Singh > Steven McFadyen • 8 months ago
Respcted steven sir, Thank you for suggestion.
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Deepak Roy • a year ago
Which is better for REF protection. 64REF relay or 87N relay
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Steven McFadyen
Mod > Deepak Roy •
8 months ago
I think they are taken to mean the same thing. There is a question on the site about this at: http://myelectrical.com/questi...
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Manish Naha • a year ago
Thanks Steven ,I also agree that in case fault is very near to Neutral point the sesitivity will be low.
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Mihir Velani • a year ago
Why is it not possible to protect the whole generator winding with a simple balanced earth fault protection?
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Steven McFadyen
Mod > Mihir Velani •
a year ago
It's a case of detecting the low magnitude of fault current with everything else going on. For faults close the neutral, the voltage across the fault will be approaching zero and the fault current limited. You need a sensitive detection. You also want some to activate quickly so the fault is not long on the system.
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