Topics: What is REF (Restricted earth fault protection) on transformer?
on Power Supply
What is REF (Restricted earth fault protection) on transformer?
What is the use of REF( Restricted earth fault protection) on transformer?
08-22-2013 11:09 PM
Restricted earth protection which also known as Differential Earth protection by which a particular defined zone is protected (restricted).
For REF, the neutral point of the Transformer (star point) is required to be earthed . The REF composed from 4CTs, one to be installed on the Earthed-Neutral while the remaining 3no. to be for phases(R,Y,B).
The REF arranged as follows:
One side of the 4 CTs to be connected together and the other terminals of the phases Cts to be connected to the first terminal of their operated relays. The other terminals of the relays to be connected together to the earth relay & the other Neutral CT lead.
The current passing through the earth relay equal to the difference between the current of earthed-neutral & the sum (resultant) of the phases current (IR+IY+IB-IN)=0.
Therefore, there will be no current through the earth relay unless there a fault occurs within the protected zone (prior to Phases CTs).
08-22-2013 11:10 PM
The term "restricted" comes from the fact that you are only looking at a restricted zone as aptly described above and not the complete or total zone differential coverage.
08-22-2013 11:10 PM
Agree above, its purpose is to provide sensitive but fast operation for faults within the neutral grounded transformer winding, yet allow "through" neutral currents to flow for long enough for faults in downstream equipment to be detected and cleared by the downstream plants protection equipment.
08-22-2013 11:12 PM
I presume that the Restricted Earth Fault Protection corresponds to the Impedance Grounded Fault Protection in terms of US Standards. In this case the neutral of the transformer is grounded (earthed) thru an Impedance. The impedance may be a high resistance, low resistance, or a combination of resistance and inductance (Z). The purpose of this Impedance grounding (earthing through an impedance) is to limit or restrain the line to ground fault current to a certain threshold, to a calculated value desired in the relaying system for managing the fault current. Without the restraining impedance, the ground fault current on a bolted line to ground fault will be maximum.
Unlike the wye or the star transformer connection, the delta connection does not have a neutral point to ground. A delta connection will not generate a line to ground fault.
The protection of the winding of the star transformer on ground fault will be the tripping of the line circuit breakers through the sensing / actuation of the sensing CT or relay on the neutral grounding. In some cases, the design do not intend to trip the breaker but instead set an alarm indication that there is a ground fault. This is done on ground fault currents that are manageable / tolerable to operate if the magnitude is within a level that will not harm the transformer windings. Setting of the impedance or resistance value is accurately designed and coordinated with the the grounding (earth) resistance of the soil.
08-22-2013 11:13 PM
REF is the restricted earth fault protection. It is 'restricted' because a)it detects earth faults for a part/portion of the network. This part can be looked as a zone.It is a unit protection.It need not be graded or coordinated with any other protection.There need be no intentional delay for operation.
b)It is only for earth fault not for over-currents and short circuits or feed-backs in which there is no earth faults.Hence is restricted to earth faults only.
It is one of the beautiful protection.It is based on second law of Kirchoff. Thanks and respects to Kirchoff.
It can be applied to star or delta or double star or three winding, zig zag etc connected transformer.,generator windings. This protection is reliable if connection CT selection etc are properly done.It is advisable not to use CTs for REF and O/L in series.
08-22-2013 11:15 PM
I do agree with your comments included in the first portion.
Let me have this opportunity to highlight the following points to avoid any confusion to the reviewer that may raised in later stage regarding the caption subject.
To give a clear picture, It may be necessary at the beginning to differentiate between:-
a)- Restricted Earth Fault Protection / differential earth fault (ERF)
b)- Stand By Earth Fault (SEF).
c) – Balance Earth fault (BEF).
d) – Overall Differential Protection.
A)- Restricted Earth Fault Protection / differential earth fault (ERF)
i. 4 CTs are required – One in each phase and one in the Neutral (as explained in my previous comments).
ii. All the 4CTs have to be located at the secondary side (Star winding) where the neutral is available.
iii. The earth protection (repeat the earth protection) is limited to the defined zone (Internal – within the 4 CTs.).
I concur with your comment that the phase-connected CTs and relays used for the external OC protection including the external earth fault as overcurrent but a balancing current is supplied by the neutral CT when the faults is on part of the system other than the Transformer (external earth fault) i.e. restricts the operation of the protection to faults within the transformer protection Zone. Accordingly, I emphasized that an External earth fault will never protected (detected) by REF.
Note: the arrangement of REF is different from convention earth fault relay. The former requires 4CTS with 4 operated relays while the later requires 3 CTs with 3 operated relays (no feedback of leakage current). The later in line with Mr. Wei comments that the neutral can be any where and accordingly, this relay can be installed any where of the installtion.
B)- Stand By Earth Fault (SEF).
Nowadays the SEF relays is considered as the last line of defense in the protection scheme, it supplied by a SINGLE CT in the Transformer Neutral (Star point-Installed at the secondary) and accordingly detect all LV side faults.
C)- Balance Earth fault (BEF).
i. Installed in primary side (delta connection).
ii. Composed of 3 CTs residually connected. The current will be balanced even there was an earth fault on the star side.
iii. Therefore, the residually connected CTs will operate only if there was an earth fault on the delta winding.
D)- The Overall Differential Protection (Other than differential Earth Fault –REF).
The basis of these Schemes used for the overall differential protection for Big Transformers in Merz-Price system that relies for stability to through faults on the balance of input and output current in the unit to be protected.
It is composed of 3CTs at each side. The input and the output current of the Transformer is different but can be compensated for by having CTs of different ratios on the primary/secondary (as per the unit Ratio) sides, while the difference in the phase-angle can be overcome by connecting CTs at the primary side in Star and that at the secondary in delta.(opposite to the unit arrangement).
08-22-2013 11:16 PM
Restricted earth fault protection is provided in electrical power transformer for sensing internal earth fault of the transformer. In this scheme the CT secondary of each phase of electrical power transformer are connected together. Then common terminals are connected to the secondary of a Neutral Current Transformer or NCT. The CT or Current Transformer connected to the neutral of power transformer is called Neutral Current Transformer or Neutral CT or simply NCT. Whenever there is an unbalancing in between three phases of the power transformer, a resultant unbalance current flow through the close path connected to the common terminals of the CT secondaries. An unbalance current will also flow through the neutral of power transformer and hence there will be a secondary current in Neutral CT because of this unbalance neutral current. In Restricted Earth Fault scheme the common terminals of phase CTs are connected to the secondary of Neutral CT in such a manner that secondary unbalance current of phase CTs and the secondary current of Neutral CT will oppose each other. If these both electric currents are equal in amplitude there will not be any resultant current circulate through the said close path. The Restricted Earth Fault Relay is connected in this close path. Hence the relay will not response even there is an unbalancing in phase current of the power transformer.
08-22-2013 11:16 PM
Restricted or unrestricted earthfault protection is applicable for earthed systems.Unearthed systems are few and far and restricted to confined and closed areas which are in control and close supervision.
Hence there is no question of providing an REF protection for an un-earthed system.As mentioned earlier REF can be provided for a part of system upstream or downstream.delta or star or whichever configuration.
It is a balanced protection scheme CT selection, lead burden and calculations etc are involved.In some situations five CTs are essential instead of four.
In a larger perspective REF is not an earth fault relay at all.When ever an overall unbalance current flows REF acts.It is presumed this overall unbalance has happened because of earth fault.rEF relay acts in this fashion.(Note :this is not RYB unbalance in star but is balance of R+Y+B in delta.
08-22-2013 11:17 PM
With all the discussions about REF, I can say that I agree with Mr. Peter observation / comment.
I still remain confident on the excellent protection offered by the transformer differential relay 87T. It protects the transformer primary as well as the transformer secondary even on earth faults at any sides of the windings.
in addition to my comment I am still wondering about the discussion on the assertion of grounding the delta. At low voltage below 300 volts it may be acceptable but grounding the corner of the delta at voltage over 300 volts is unsafe and not in compliance with the US electrical code.
08-22-2013 11:18 PM
If you want to understand how it works, do the following experiment:
Take two CT's of the same size and ratio and parallel the primary side, P1 to P1 and P2 to P2. Take the S1 and connect S1 to S1 and S2 to S2. Now apply primary current and you will find that the currents will oppose each other and the result on the secondary side should be zero Bare in mind that in real life there is a transformer and/or NEC between the P2 ct's. In this case the REF circuit you have just built is stable with two sinusoidal waves opposing each other. Any outside earth faults will therefore balance as current flow will not change. If however there is a transformer or NEC fault or any fault in this circuit, current flow will change and current will be flowing in the secondary side of your CT. You can prove this with your experiment and swap one CT S1 and S2 side. The 87 Diff protection looking at 2 or 3 winding will be insensitive to any earth faults due to Bias correction. The relay will also have a function called zero sequence filtering. This will "block" earth faults in the DIFF circuit. We now need to protect the transformer against internal earth faults by means of the REF
08-22-2013 11:18 PM
In continuation of my earlier comments some of which have been disagreed in this discussion,I will add a few points.
electrical engineering has jumped forward from voltage operated Earth fault relays to current operated relays.We had earlier earth leakage circuit breakers.Now they are called Residual current circuit breakers.(RCCB). without changing the scheme or circuit.
08-22-2013 11:20 PM
Lets imagine a simple , single phase supply to your house.
You have plugged in an electric lawn mower.
You have earth leakage protection fitted.
In mowing the lawn, you cut the cable , and end up with your fingers across the supply and neutral. You are completing the circuit, and jumping around. A small current is passing to earth, and returning to the supply source via Mother Earth that you are standing on..
The Earth leakage, which only has a CT on the earth wire only sees a really small, less than 30 mA current, so does not trip. You die.
Now, if a REF was fitted, which is the modern day standard, instead of Earth Leakage. There is a CT on supply and neutral wire. At any moment in time, the sum of the currents should be mathematically zero.
Same lawn mower scenario, this time, supply wire is allowing 10 Amps to pass through, but return, Neutral, also with a CT fitted measures only 9.9 amps, as some current is flowing to earth through your body. The REF, calculates an imbalance in the arithmetic sum of the currents, so a trip is initiated, typically 30mA, 1/2 cycle, 10 ms.
The application of the REF is most useful, when the supply cable is outside of the "protective zone", since it detects a current imbalance.
Or as the device is described, restrictive to an protective zone.
I put "protective zone" in quotes so people can look up the definition in BS or IEC standards, as that will kick off another thread.earth
Leakage would be fine, if you were inside a factory, say with metal floors, and the metal floors are connected to the earth of your source, you are inside the "protective zone" so a leak to earth is easily detected.
If you are in the garden of the factory, not standing on the metal floor, and cause a short circuit, the earth leakage device is oblivious of your problem as current is not flowing through you back to the supply source.
So, I hope that explains simply the differences, without being able to draw it out.
08-22-2013 11:21 PM
The 87T transformer differential will protect the transformer on any fault within the zone of protection, that is between the primary CTs point of application up to the secondary CTs point of application. Any kind of fault L-L, 3 phase fault or L-G fault (earth fault) will trip the breaker because the principle of 87T is that the summation of current entering the primary zone must equal to the summation of current leaving the secondary. If there is an earth fault within the zone, then the summation of current leaving the secondary will not equal the summation of current entering the primary thus the differential relay tripping coil will trip the breaker. That is the best relay protection that you can offer to your transformer.
08-23-2013 09:13 PM
You guys have explained well about REF. I would like to add as below:
REF has special purpose to detect earth faults occuring within initial portion of winding from neutral side in star connected windings of the transformer.
The Earthfault current inside a transformer is function of induced voltage and the % of winding coverage from neutral. The emf in initial 20-30% of star connected winding will drive eartfault current proportional to the induced emf. usually it lies in undetectable range of other earthfault relays and differential relays, which do not have enough sensitivity to detect these eartfaults near neutral.
Therefore the name comes Restricted earth fault relay. It covers the star connected winding of the transformer (unit protection). REF has to be stable against all the earthfaults external th the transformer.
08-23-2013 09:14 PM
Mr.Badiya, I fully concur with your comments.
I do not want to deviate the discussions from the track, but I emphasize that (RCCB -Residual Current Circuit Breaker) is totally different from REF .RCCB mainly used against in direct Contact as far earth leakage is conserned. To avoid confusion the following clarification may be necessary:
The ELCB is defined as Earth Leakage circuit Breaker which is totally different from the Residual Current Device “RCD”. RCD is made in accordance to BS 4293 and its Operation principle based on “Out of balance, while ELCB operation based on “voltage” .
By virtue of the RCD nature & principle of operation, Phase – neutral fault after RCD will not detected and will appear as load and also to mitigate the number of split circuits that require indirect contact protection and RCBO/RCCB can be used which is a simply a combination of MCB and RCD which gives protection against indirect contact, earth & O/C .
RCD/RCCB is not recommended to be used where PEN conductor is in use on the load side of RCDRCCB, while it is recommended where the value of the Earth Loop Impedance is insufficient to achieve the prescribed disconnection time.
08-24-2013 09:31 PM
Standby earth-fault relay was originally utilized for Neutral Earthing Resistor protection. Rated at Traf. Full Load current for a duration of 30s.The relay configuration and application are explained in my previous comments .
This relay becomes very important for Systems (22/0.4, 11/0.4 kV) where outgoing feeders at LV Switchgear are protected only by HRC fuses and have no earth -fault protection whatsover,mostely it is along time relay, depends on the protection policy.
This relay mainly is used in industry where protection by fuses for Electrical drives is predominant or for public distribution system using Fused DB/FP/ST for domestic purposes.Although it's CT (Single) has to be installed at the Earthed-neutral of the Traf secondary side, actuation of the tripping will be on the MV side. It is an standard term!!!?
I believe that one of the outcome of such discussions to familiarize / normalize standards as possible.
In such systems that I am taking about, let us imagine:-
a. 22 or 11 kV MV Network.
b. Distribution Substations (MV switchgear +Transformer(s) +LV Switchgears (Fuses Board and all substation auxilary equipment).
c. Down Stream System (LV), which in this case might be feeding Feeder Pillars (FP), Feeder pillar feeding service turret (ST).
d. From either FP or ST consumer MDB/DB to be connected.
From LV SG up to the ST all the protective devices will be HRC fuses while at the consumer MDB/DB a (MCCB + RCD) are to be availaible as per BS7671.
The transformer Neutral should be solidy earthed where a single CT to be installed for the SBEF as explained before.
An earth Rods/bit to be provided at the consumer entry boint where the earth terminal in his DB to be connected to earth Electrode and resistance is recommended to be not more than 10 ohms as per BS 6551 BS 4730 (as TT system). The armour of the LV Cables acting as seperate earth conductors and will be also connected to the aformentioned earthing terminal at the DB., via cable gland.
the earthing system become TN-S-TT and equipotential bonding can be achieved for all the consumer exposed conductive parts as well as lightning protection and in addition to earthing protection of LV system from LV SWG up to consumer entry point while from DB and downstream the RCD/RCCB will do.Accordingly, it may be defined as standby earth fault.
03-16-2015 01:16 AM
Could you all please explain me REF( Restricted earth fault protection) will work in star connection without neutral CT in Low Impedance Mode?
And is REF necessay for all rating of transformers ?
10-11-2015 12:53 PM
Sir how REF protection applied to delta winding of a power transformer with grounding transformer connected to it in zigzag
02-12-2016 02:41 AM
What should be the CT rating for REF trotection.
We have REF in a transformer where the CT rating is less the nominal current.
Is it right?