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Restricted Earth Fault Operation

ANSI Designat ion

Often 87N or 64REF are used to

denote restricted earth fault

protection. The intent is that both

mean the same thing.

Which is the correct or better to

use may be open to interpretation.

To me, 87N is more appropriate as

REF is a differential type protection

(ANSI 87 – differential protection

relay). I think 64 is strictly more

relevant to detecting earth/ground

faults due to breakdown of 

insulation.

This was a question asked below

(in the discussion). If anyone has

any views, please join that thread.

Filed in [Power Systems] By Steven on March 25 th, 2012 

Restricted Earth Fault Protection

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.

Basic Operation

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 I would

prefer 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 these issues 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 technologyhave 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.

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  ► Electrical   ► Protection   ► Transformer    ► Relay

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Raman says:

7/27/2012 12:14 PM

Thank you. You have explained it in simple terms. Can you also Please clarify the following :

For a radial transformer feeder provided with REF protection on LV side, does the operation of 

REF relay be wired to trip the primary HV side or secondary LV side Breaker?

For parallel operation system, does operation of REF be connected to trip both primary and

secondary Breakers.?

It would be great if the breaker linkage is shown as part of the schematic.

Can you please clarify?

Thanks

Steven says:

7/27/2012 2:52 PM

REF is a fault in the winding and you need to isolate the transformer - trip the MV/HV side.

The REF is unit protection and restricted to one transformer. If you have two in parallel you

could isolate only the faulty transformer. Just make sure the other transformer can support

the full load.

I don't have access to my normal computer at the moment, but as soon as I do I will look at

updating the schematic.

Steven says:

1/26/2013 5:28 AM

Ugly's

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References,

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Black &

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Image now updated.

LYH says:

8/21/2012 2:10 AM

Further question: Is REF protection should be implemented on a small 6/0.4kV, 500kVA oil type

transformer?

Steven says:

8/24/2012 7:25 AM

There is a cost to REF and at some stage it becomes expensive in relation to the

transformer size. At 500 kVA, I would tend to think the cost may not be justified (unless the

transformer were particularly important).

It would be interesting if other site users have opinion on this and can add to the

discussion.

waji says:

9/26/2012 6:42 AM

Can you please explain how a transformer can detect an earth fault by using using phase CTs?

rajadurai says:

10/19/2012 11:33 AM

What class of cts used for REF and SEF protection of transformers

Amit says:

10/23/2012 4:52 AM

For REF protection, PS class CT are used for high accurecy and for SEF 5P10 type CTs

are used.

vishal says:

12/22/2012 6:25 AM

I want to just ask that how we can restrict the earth fault detection of a Particular Critical System

let us consider an alternator and what is the Logic Behind that?what are the things to keep in

mind and what is the difference between Overcurrent Earth Fault and REF ?How the CTs are

arranged in Differential Protection what is the logic behind the arrangement .

Daniel says:

1/5/2013 2:29 AM

Hi,

Is it a MUST that the knee point voltage, Vkp and excitation current, Imag of the Neutral CT and

phase CT to be the same?

if Vkp and Imag of both CT not the same, what will be the consequences on the Bias Differential

and REF protection?

 Anybody can advise me on these? Thanks

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black mamba says:

4/4/2013 2:45 PM

what's the difference between ANSI 87N and ANSI 64REF?

Is the polarity of connection samefor both protection schemes?

Steven says:

4/7/2013 12:27 AM

I think the intent is that both mean the same thing. Which is the correct or better to use

may be open to interpretation.

To me, 87N is more appropriate as REF is a differential type protection (ANSI 87 –

differential protection relay). I think 64 is strictly more relevant to detecting earth/ground

faults due to breakdown of insulation.

Skara says:

5/21/2013 4:03 PM

goodday gents,

kindly please assaist with what is the difference between high impendance REF and low

impedance REF

Steven says:

5/22/2013 5:37 AM

The primary difference is in the type of relay used. High impedance relays have a high

input impedance and draw very little current under normal conditions. Low impedance

relays are the opposite.

There is a lot of debate on which to use and some theory. My view is that low impedance is

preferred. It's simpler, there is more flexibility in the CTs and inputs and coupled with all the

functions of a modern numerical relay it eliminates a lot of problems that exist in high

impedance systems.

There is also a technical comparison on the two types here which should help you:

https://www.selinc.com/WorkArea/DownloadAsset.aspx?id=5562 

Freddie says:

5/30/2013 8:57 AM

What can cause 64REF to trip off a transformer during disturbances from the grid

Steven says:

6/2/2013 4:37 AM

It could be many things.

 A common reason could be unbalanced/incorrectly selected CTs causing false residual

current and nuisance tripping. Similarly, if your CTs are becoming saturated they canbecome unbalanced and give problems.

You should also make sure that the CT ratios, phase shift, logic, etc. are programmed

correctly in to the relay and that your polarities/wiring are fine.

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circuits/issues logically until you can narrow it down. If you are also able to get a recording

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SYAM says:

9/3/2013 12:23 PM

Hi Steven,

"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)".

Can you please explain why..?

I have a few queries also (it may sound stupid).

Query 1. I am bit confused between the operation of REF and Standby earth fault relay. Can

you help me in understanding with some diagrams or any links to refer.?

Query 2. In our area we use separate CTs for REF and SEF+ OC protection. i.e 4 or 5 nos PS

class CTs (3 in Phases 2 in Neutral) along with 4 no 5P10 class CT ( 3 in Phases 1 in Neutral

for SEF+OC. We use separate relays for REF, SEF, & OC For SEF we use Induction (disc type)

relay. My query is that is it possible to use only one type of (PS Class) CT for all the

protections..? Is it possible to use a single relay(if the relay is capable of) to incorporate all the

protections.?

Query 3.Can I use Class PS CT for OC or SEF protection or for metering purpose.?

Query 4. When a fault occurs between one phase and the ground will the fault current travel

back from the ground through the neutral to the fault point..? (using the concept that current

flows only in a closed circuit)

For faults outside the Zone SEF will operate, REF will not operate.

For Faults inside the Zone the REF will operate When an earth fault happens inside the zonewill the CT provided for SEF sense the fault current. (i mean when a current flows in the primary

(neutral) a proportional current in the secondary)?

Please clarify my doubts.It may sound silly or stupid but I would like to know why and how.

Thanks in advance

Regards

SYAM

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