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smithm (Electrical)
21 Mar 00 13:38
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?

Helpful Member!  bfuchs (Electrical)
28 Mar 00 10:06
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.

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.
Helpful Member!  peterb (Electrical)
3 Jul 00 13:54
Restricted earth fault protection is a differential scheme, with CTs on the transformer neutral and phase leads.  The phase CTs are connected together in parallel & then paralleled with the neutral CT.  The resticted earth fault (REF) relay is connected across the paralled CT circuits.

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.
bartos (Electrical)
3 Jul 00 20:12
Reference: A. R. van C. Warrington "Protective Relays Their Theory and Practice." Volume One, 1971
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.
Jiz (Bioengineer)
26 Feb 02 11:28
What is restricted earth fault? And how does it occur?
Helpful Member!  marmic (Electrical)
27 Feb 02 8:55
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.
Helpful Member!  gordonl (Electrical)
27 Feb 02 9:12
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 (Electrical)
4 Mar 02 17:29
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 "see" very far into the winding for earth faults.

Bung
vijay12 (Electrical)
5 Mar 02 3:30
Can some one give a Typical Calculation sheet for REF setting ? This will help me in setting my relay.
stevenal (Electrical)
5 Mar 02 15:28
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 (Electrical)
5 Mar 02 18:56
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.



Helpful Member!  ADN (Electrical)
7 Mar 02 1:02
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 (Electrical)
7 Mar 02 4:10
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 (Bioengineer)
7 Mar 02 9:32
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 (Electrical)
8 Mar 02 16:24
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 (Electrical)
11 Mar 02 23:46
thanks Stevenal.  Looks like I might have to have another think about it!

Bung
61 (Electrical)
12 Mar 02 2:06
where i can have some solid stuff related to reactive power compensation on net... can anyboday help me.. for the same.
salim02 (Electrical)
6 Apr 02 7:52
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 (Electrical)
7 Apr 02 3:47
Can Anybody refer me to a web site to get the ANSI relay numbering.
Many Thanks
busbar (Electrical)
7 Apr 02 12:46
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.

Start a new thread to continue this discussion.
Guest (Visitor)
5 May 02 0:43
Dear Friends,

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
jbartos (Electrical)
5 May 02 1:15
Suggestion: With the advent of integrated protective relaying, the integrated relay processor, which has its inputs from CTs and PTs, is the clue. The processor programming and internal design will probably be kept proprietary. Siemens integrated protective relays tend to be an example; however, others follow closely.
bigamp (Electrical)
5 May 02 4:29
It depends on what you want the relay to do.

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
busbar (Electrical)
5 May 02 12:28
(j)bartos' Jul 3, 2000 entry bears investigation, and Warrington is an indispensable text to have for protective relaying.  
  
  
peterb (Electrical)
5 May 02 21:17
Bigamp is correct in suggesting the neutral end CTs for islanded generators.  Another factor to take into account is that the neutral end CT location provides overcurrent protection before the generator is synchronized to the grid.  This is the preferred location for larger unit- connected generators.
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.
peterb (Electrical)
5 May 02 21:17
Bigamp is correct in suggesting the neutral end CTs for islanded generators.  Another factor to take into account is that the neutral end CT location provides overcurrent protection before the generator is synchronized to the grid.  This is the preferred location for larger unit- connected generators.
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.

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