Restricted Earth Fault Relays for Transformer Protection
Restricted Earth Fault Relays for Transformer Protection
(OP)
Can anyone tell me how to select an appropriate setting for restricted earth fault protection relays used to protect low voltage distribution transformers?
Is there a rule of thumb method?
Is there a rule of thumb method?






RE: Restricted Earth Fault Relays for Transformer Protection
If you have long cables and/or small earth conductors, then your earth fault drops ! To how much ? Well it depends !
What you have to do is calculate the value of this earth fault current (using computer programs), and make sure that :
- your earth fault current is greater than the magnetic
- a restricted earth fault relay is used to protect the transformer. Set this realy to a value under the earth fault current.
RE: Restricted Earth Fault Relays for Transformer Protection
The relay is either a voltage operated device (eg Alstom MFAC14)or a current relay with an external setting resistor (eg Alstom MCAG14). 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. A practical setting is 50% of the CT knee point voltage, if full info is not available.
RE: Restricted Earth Fault Relays for Transformer Protection
It appears that more information is needed pertaining to your specific low voltage distribution trasformer application. The above reference can be found useful; especially,
(d) Restricted Earth Protection on page 181 involving useful components, e.g. stabilizing resistor, and transient blocking scheme.
10.4.1. Earth Fault Relays on page 387 providing additional more detailed explanation with a consideration of various ground faults. Notice that "Restricted Earth Protection" is an English term meaning the differential protection of transformers (or generators) against ground faults.
RE: Restricted Earth Fault Relays for Transformer Protection
RE: Restricted Earth Fault Relays for Transformer Protection
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.
RE: Restricted Earth Fault Relays for Transformer Protection
RE: Restricted Earth Fault Relays for Transformer Protection
Bung
RE: Restricted Earth Fault Relays for Transformer Protection
RE: Restricted Earth Fault Relays for Transformer Protection
I believe REF provides additional sensitivity over 87T alone, for faults close to the neutral point. See the GEC Protective Relay Application Guide.
RE: Restricted Earth Fault Relays for Transformer Protection
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.
RE: Restricted Earth Fault Relays for Transformer Protection
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
RE: Restricted Earth Fault Relays for Transformer Protection
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
RE: Restricted Earth Fault Relays for Transformer Protection
Could you possibly send me a copy of the calc sheet too. Many thanks in advance.
James
james.flanagan@wwp-london.co.uk
RE: Restricted Earth Fault Relays for Transformer Protection
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.
RE: Restricted Earth Fault Relays for Transformer Protection
Bung
RE: Restricted Earth Fault Relays for Transformer Protection
RE: Restricted Earth Fault Relays for Transformer Protection
my e.mail address is salim.haddabi@pdo.com.om
RE: Restricted Earth Fault Relays for Transformer Protection
Many Thanks
RE: Restricted Earth Fault Relays for Transformer Protection
http://www.geindustrial.com/pm/notes/ansinums.pdf but it's fairly crippled.
Start a new thread to continue this discussion.
RE: Restricted Earth Fault Relays for Transformer Protection
I am an electrical + Electronics engineers involved in Power System Protection.
I have a doubt on generator protection.
Where should we connect the protective relay for generator ( O/C+ E/F relay)? Should it be to the CT which is near the breaker or should we connect the three phase CT on the neutral side of the generator and connect the O/C + E/F relay to this ?
Many schemes from ABB, Alstom, GE show the relay on the breaker side. Siemens show the relay on the neutral side .
Can one you explain what is correc,why, and why the other is not correct. ?
Thanks
R.Seshadri
RE: Restricted Earth Fault Relays for Transformer Protection
RE: Restricted Earth Fault Relays for Transformer Protection
If you are looking for a flow of fault current from the CB side into the generator then connect the relay to CT's at the CB end. The thing is, if your generator can run in island mode all by itself an OC relay connected to CT's at the CB end will not be able to detect an internal generator fault as (neglecting contribution from in-service motors) there will be no flow of fault current for the relay to "see".
If you want the OC relay to be able to detect an internal generator fault and there is any chance that the generator can run in island mode all by itself, then connect the OC relay to CT's at the starpoint end. A relay so connected will also detect faults external to the generator.
Also, take care. The OC relay may need to have a voltage dependent characteristic, it depends on what you want it to do.
As to how a modern multi-function relay is configured, I concur with jbartos, it depends on the model and the manufacturer. I have just completed a generator protection relay installation using Alstom P343 relays. These relays utilise phase CT's at both the CB and the starpoint ends, with the CT's at the starpoint end being used for OC protection.
Alstom's less capable generator protection relay, for smaller machines that typically are not fitted with CT's at the starpoint is the P342. This relay has provision for CB end CT's only so (naturally) the OC protection will utilise CT's at the CB end.
So, what is correct? I think definately use CT's at the starpoint end if the generator can ever operate in island mode. You should also have a look through IEEE C37.102, a very useful standard, and have a think about exactly what protection you are trying to provide. Also, many manufacturers have excellent information on their websites and you should check these out.
Regards
RE: Restricted Earth Fault Relays for Transformer Protection
RE: Restricted Earth Fault Relays for Transformer Protection
A word of caution on using integrated multifunction relays - don't put all of your eggs in that one basket. Prudent relay engineering practice dictates that at least some of the functions should be provided (or duplicated) by separate devices. As an example, I typically provide separate ground relaying and backup overcurrent (or duplicate differential) protection when using a multifunction relay.
RE: Restricted Earth Fault Relays for Transformer Protection
A word of caution on using integrated multifunction relays - don't put all of your eggs in that one basket. Prudent relay engineering practice dictates that at least some of the functions should be provided (or duplicated) by separate devices. As an example, I typically provide separate ground relaying and backup overcurrent (or duplicate differential) protection when using a multifunction relay for generator protection.