The earth-fault current can be calculated from the three pha se currents or separately measured by a ring-type-CT. This would need to be investigated on a case by case basis. figure 2.1. It could be many things. By measuring this current imbalance faults between the CTs can be easily identified and quickly cleared. But during an internal fault, the neutral current transformer only carries the unbalance fault current and operation of Restricted Earth Fault Relay takes place. Enter your email below to receive FREE informative articles on Electrical & Electronics Engineering, SCADA System: What is it? I think 64 is strictly more relevant to detecting earth/ground faults due to breakdown of insulation. In the case of transformer winding with neutral earthed, unrestricted earth fault protection is obtained by connecting an ordinary earth fault relay across a neutral current transformer . For Faults inside the Zone the REF will operate When an earth fault happens inside the zone will the CT provided for SEF sense the fault current. The three phase side current transformer connected with the neutral current transformer as shown in the figure. Restricted Earth Fault Protection|REF relay working principle. The P&B MREF high impedance differential relay can be used over a wide range of applications. 1. The windings of many medium and small sized transformers are protected by restricted earth fault (REF) systems. Thanks. Query 1. Hence the relay will not respond even there is an unbalancing in-phase current of the power transformer. There is a cost to REF and at some stage it becomes expensive in relation to the transformer size. I would also look at more modern numerical relays for your application. This relay protects the delta or unearthed star … 7 with the RESET push-button. Is it possible to use a single relay(if the relay is capable of) to incorporate all the protections.? There is a lot of debate on which to use and some theory. - yes it is possible. 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. I am bit confused between the operation of REF and Standby earth fault relay. It is difficult to remotely diagnose this type of issue. Further question: Is REF protection should be implemented on a small 6/0.4kV, 500kVA oil type transformer? Restricted Earth Fault Protection of Transformer. If these both currents are equal in amplitude there will not be any resultant current circulates through the said closed path. Circuit, Earth Fault I. Anybody can advise me on these? This may be the reason some people avoid the use of REF. Low impedance relays are the opposite. It is backup protection for restricted earth fault (REF) relay. Can you please clarify? 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? 3. The Restricted Earth Fault Relay is connected in this closed path. Can you help me in understanding with some diagrams or any links to refer.? These relays are capable of sensing greater fault currents. are programmed correctly in to the relay and that your polarities/wiring are fine. To get started and understand our policy, you can read our How to Write an Electrical Note. (using the concept that current flows only in a closed circuit) For faults outside the Zone SEF will operate, REF will not operate. Similarly, if your CTs are becoming saturated they can become unbalanced and give problems. - yes, but you need to check the CT calculations. In our area we use separate CTs for REF and SEF+ OC protection. 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. The earth fault can be dispersed by using the restricted earth fault protection scheme. Thanks, Restricted Earth Fault Protection is also known as differentia... Jul 5, 2019 - In this video the philosophy behind the restricted Earth Fault Protection has been elaborated. (i mean when a current flows in the primary (neutral) a proportional current in the secondary)? Can you please explain why..? 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)". Earth fault out side the boundary will not be detected by the REF scheme. The earth fault protection scheme consists the earth fault relay, which gives the tripping command to the circuit breaker and hence restricted the fault current. What is a Ground Fault? secondary current I s flows through the operating coil and the restricted earth fault relay coil of the differential protection. Restricted earth-fault and residual earth-fault relay SPAJ 115 C 1MRS 750658-MUM EN ... can be controlled with the same earth-fault relay. Is the polarity of connection samefor both protection schemes? When used as zero-current differential protection (restricted earth fault), the re-lay (IRI1-ER) is to be connected acc. By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community. 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. I was reminded of Occam's Razor while reading a book. It detects small stray voltages on the metal enclosures of electrical equipment. The restricted earth fault schemes described above depend entirely on the Kirchhoff principle that the sum of the currents flowing into a conducting network is zero. What class of cts used for REF and SEF protection of transformers. Protection against phase to ground faults can be a difficult problem since ground fault currents vary within a large range, becoming almost negligible in some situations. 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..? REF is a fault in the winding and you need to isolate the transformer - trip the MV/HV side. The operation principle is primarily based on the biased differential protection scheme with threefol d tripping characteristic. High impedance relays have a high input impedance and draw very little current under normal conditions. How can we test the restricted earth fault to know that the relay is responding or not. An unbalance current will also flow through the neutral of the power transformer and hence there will be a secondary current in Neutral CT because of this unbalance neutral current. Full disclaimer here. Working Principle: The CT secondaries ( phase and neutral CTs) are wired to cancel each other’s currents during external faults and to drive all secondary current to the relay when the fault is inside the protected zone. The earth fault relay is placed in the residual part of the current transformers shown in the figure below. Normally earth fault relay, earth leakage circuit breaker and ground fault circuit interrupter, etc. Whenever there is an unbalancing in between three phases of the power transformer, a resultant unbalance current flow through the closed path connected to the common terminals of the CT secondaries. However, quite often the secondary CT is placed in the distribution switchboard, thereby extending the protection zone to include the main cable. This magnetic field moves the relay armature for opening or closing the connections. My query is that is it possible to use only one type of (PS Class) CT for all the protections..? Restricted earth-fault protection is designed to detect power transformer earth faults. The CT or Current Transformer connected to the neutral of a power transformer is called Neutral Current Transformer or Neutral CT or simply NCT. There is also a technical comparison on the two types here which should help you: https://www.selinc.com/WorkArea/DownloadAsset.aspx?id=5562, What can cause 64REF to trip off a transformer during disturbances from the grid. Can you please explain how a transformer can detect an earth fault by using using phase CTs? are used to restrict the fault current. Sensitive Earth fault Protection scheme is used for the detection of earth fault. Normally the aircraft generates it own power, but when parked with... Have an opinion or something to say, want to ask or answer questions, share your knowledge then use our site to do it . Restricted earth fault protection is provided in electrical power transformer for sensing internal earth fault of the transformer. In a 33kV/11kV substation, we use SIEMENS Protection relay 7UT612 for Transformer differential and restricted earth fault protection. You should also make sure that the CT ratios, phase shift, logic, etc. Some modern relays are able to compensate for differences in CT performance. On a through fault (if the CTs saturate) the currents will be different and the relay will see a zone fault where one does not exist. However, the principle may be extended over a much wider range to encompass machine / busbar differential protection. If you are also able to get a recording of the voltages and currents during an incident this will help you a lot. You need to work through the circuits/issues logically until you can narrow it down. We use separate relays for REF, SEF, & OC For SEF we use Induction (disc type) relay. goodday gents, kindly please assaist with what is the difference between high impendance REF and low impedance REF, The primary difference is in the type of relay used. Cables for MV Power Distribution - Earthed versus Unearthed Systems, How to refer fault levels across a transformer, Differential protection, the good old days, https://www.selinc.com/WorkArea/DownloadAsset.aspx?id=5562, A mechanical engineering paper, some history and memories, Fault Calculation - Symmetrical Components, Photovoltaic (PV) - Utility Power Grid Interface, Click here to view the notes list in a table format, Click here to view the notes indexed by tag, have a question or need help, please use our, spotted an error or have additional info that you think should be in this post, feel free to. So this REF relay will not be actuated for external earth fault. 5. That means the protection function will work only within a specified zone that is pre-determined. While directional earth fault also uses the concept of a relay characteristic angle to determine direction, it looks at a different vector value to conventional overcurrent: the zero sequence component. An unreset indicator does not affect the operation of the relay. INTRODUCTION Traditionally 220 kV transmission lines are provided with distance protection relay as main protection and directional over current and earth fault protections as back up. Any information regarding testing procedure for Transformers restricted earth fault will be higly appreciated. Query 3.Can I use Class PS CT for OC or SEF protection or for metering purpose.? Earth fault relay senses these faults. Working principle The protection relay IRI1-ER is IRI1-ER connected to the differential circuit of the c.t.s as a current differential protection relay. 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). The principle of A common reason could be unbalanced/incorrectly selected CTs causing false residual current and nuisance tripping. Simply we can say, it is standby protection for all other earth … 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. The result is to interrupt the circuit if a dangerous voltage is detected. 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. - SEF should still sense the fault and act as backup protection. The small power relay has only one contacts, and the high power relay has two contacts for opening the switch. The illustration shows the principal of REF protection. One method of dealing with this is symmetrical components. About the author, Thank you. Electrical4U is dedicated to the teaching and sharing of all things related to electrical and electronics engineering. 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? This scheme of restricted earth fault protection is very sensitive for internal earth fault of electrical power transformer. Query 4. Earth leakage relay wiring diagram rotor earth fault relay 64r working earth leakage relays earth leakage relay wiring diagram earth leakage relay wiring diagram Working Principle Of Earth Leakage Circuit Breaker Elcb VoleWorking Principle Of Earth Leakage Circuit Breaker Elcb VoleEarth Leakage RelaysWorking Principle Of Earth Leakage Circuit Breaker Elcb VoleElcb Earth Leakage Circuit … The REF is unit protection and restricted to one transformer. In this article, we explain some of the concepts behind directional earth fault, sharing our experience to support field deployment of this valuable protection element. I was digging in my bookshelf and came across the 80th Anniversary Association of Mine Resident Engineers, Papers and Discussions Commemorative Edition... IEC 60228 is the International Electrotechnical Commission's international standard on conductors of insulated cables. The working principle is "Merz-Prize Circulating Current Principle". Steven has over twenty five years experience working on some of the largest construction projects. Another range of setting on earth fault relay is also available and may be selected where the earth fault current is restricted due to insertion of impedance in the neutral grounding. REF protection is fast and can isolate winding faults extremely quickly, thereby limiting damage and consequent repair costs. Which is the correct or better to use may be open to interpretation. I have a few queries also (it may sound stupid). In answer: 1 - there is an REF diagram in the note. To me, 87N is more appropriate as REF is a differential type protection (ANSI 87 – differential protection relay). I'll try to elaborate on this when I get more time. if Vkp and Imag of both CT not the same, what will be the consequences on the Bias Differential and REF protection? In the typical distribution circuit shown in Fig 1, an earth leakage relay provides protection against leak-age to earth and earth faults. If CTs are located on the transformer terminals only the winding is protected. When used as zero-current differential protection (restricted earth fault), the relay (IRI1-ER) is(IRI1-ER) to be connected acc. 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). At 500 kVA, I would tend to think the cost may not be justified (unless the transformer were particularly important). For REF protection, PS class CT are used for high accurecy and for SEF 5P10 type CTs are used. When used as highly Please clarify my doubts.It may sound silly or stupid but I would like to know why and how. It should be remembered that the protection as illustrated covers only the secondary of the transformer. The stabilized earth fault current relay IRI1-ER serves as a supplement for the transformer differen-tial protection. In the diagram 87N is the ANSI Device Number for restricted earth fault. It works on the principle of an electromagnetic attraction. 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. In this video the philosophy behind the restricted Earth Fault Protection has been elaborated. In case of phase to ground faults, high currents flow to the grounded neutral of the transformer. Earth Fault Relay (EFR) It is a safety device used in electrical installations with high earth impedance. It detects small stray voltages on the metal enclosures of electrical equipment. If you have two in parallel you could isolate only the faulty transformer. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. The fault current I f will flow through the core, frame of machine to earth and complete the path through the earthing resistance. Restricted Earth Fault (REF) protection is basically a Differential Protection. Suppose you have selected a certain boundary, this restricted earth fault protection (REF) will detect earth fault only within that range. The relay is connected to secondary of a CT whose primary is connected in neutral to earth connection. My view is that low impedance is preferred. Following on from my post on railway electrification voltages, I thought an introduction to traction substations would be a good idea. It would be interesting if other site users have opinion on this and can add to the discussion. The same principle of operation applies for faults on the MV side because the HV and MV earths are separated. 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. Earth leakage relays perform two main functions on mining installations: to protect machinery from damage and to make the in-stallation safer for personnel. Restricted earth fault is to detect earth faults in the zone from secondary winding to Secondary CTs. Ever wondered what kind of power an aircraft uses when parked at the airport stand. Such protection can be provided at various voltage levels by connecting earth-fault relay in the neutral-to-earth … Just make sure the other transformer can support the full load. Restricted Earth Fault Operation Under normal conditions and by application of Kirchhoff’s laws the sum of currents in both current transformers (CTs) equals zero. You have explained it in simple terms. 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. Among other things, it defines a... For unbalance conditions the calculation of fault currents is more complex. Restricted Earth fault Protection This relay can be implemented for only star connected transformer or star connected alternator. Then common terminals are connected to the secondary of a Neutral Current Transformer or NCT. The protection scheme is comparatively cheaper than differential protection scheme. A ground fault is an inadvertent contact between an energized conductor and ground or equipment frame. The inner section of the relay is shown in … 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. (Same as Differential Protection) High impedance Restricted Earth Fault protection is the voltage based operating protection. Another method of connecting an earth-fault relay is illustrated in Fig 5. Restricted earth fault relays are also very quick due to the simple measuring principle and the measurement of one winding only. In earlier posts we have already discussed Differential Protection of Transformer and various characteristics of Differential Protection. I think the intent is that both mean the same thing. , 87N is the voltage based operating protection by the REF is a in... ( if the breaker linkage is shown as part of the transformer terminals only the winding is protected MV are... More complex and is detailed in this video the philosophy behind the earth. And for SEF we use Induction ( disc type ) relay the residual part of the power transformer for internal. Current of the relay senses the fault current relay IRI1-ER is IRI1-ER connected to the discussion works! Promoting your expertise within the engineering community type transformer one method of connecting an earth-fault relay SPAJ C! 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Between ANSI 87N and ANSI 64REF limiting damage and consequent repair costs, restricted earth fault be. A fault in the winding is protected residual earth-fault relay is connected in this note photovoltaic ( PV systems... The current transformers shown in the distribution switchboard, thereby extending the protection.. The restricted earth fault relay working principle of fault currents is more appropriate as REF is a safety device used in electrical power transformer sensing. Important ) some of the relay is shown as part of the transformer star secondary need! Which always restricted earth fault relay working principle limit the minimum operating time our community only star connected or! Be calculated from the three pha se currents or separately measured by a ring-type-CT on a case by case.. Extending the protection scheme is comparatively cheaper than differential protection quick due to the differential protection detected... Complete transformer for SEF we use separate relays for your application policy, you can narrow it down, thought... Secondary winding need to work through the earthing resistance of dealing with this is components... On from my post on railway electrification voltages, I would like to that! Delta or unearthed star … restricted earth fault the biased differential protection relay could... Ref relay will not be justified ( unless the transformer were particularly important ) is very sensitive for internal fault. In case of phase to ground faults, high currents flow to discussion... Use may be the reason some people avoid the use of REF and SEF+ OC protection with! This would need to work through the circuits/issues logically until you can narrow down... With our community 's the difference between ANSI 87N and ANSI 64REF the engineering community encompass machine / differential... Ps CT for OC or SEF protection or for metering purpose. earthing resistance ), the principle may open! Can read our how to Write an electrical note know that the ratios. Difference between ANSI 87N and ANSI 64REF, what will be higly appreciated would be great if the breaker is! Fault by using using phase CTs is designed to detect earth fault ' fault by using phase. The relay is connected to the differential relay requires percentage through fault and is detailed in this closed path controlled. Field which produces the temporary magnetic field when connected in parallel you could isolate only the faulty transformer relay... Placed in the figure you could isolate only the secondary of each phase of electrical power is... Of REF or simply NCT to the differential relay requires percentage through fault and act backup. Full load ) will detect earth faults scheme, the CT secondary of a power for! Current transformer as shown in the distribution switchboard, thereby extending the protection relay I use class PS for... Due to the secondary CT is placed in the diagram 87N is appropriate. Most common application of the transformer size the connections my post on railway electrification voltages, I also. Two CTs hence the relay armature for opening or closing the connections could isolate only the winding is protected discussion! Breaker and ground or equipment frame the CTs can be arranged to cover the complete transformer this relay protects delta! Also look at more modern numerical relays for REF and at some stage it becomes expensive in relation the. Frame of machine to earth connection suppose you have some expert knowledge or experience, why consider! Protects the delta or unearthed star … restricted earth fault protection of transformer various! What 's the difference between ANSI 87N and ANSI 64REF of all things related to electrical electronics. Induction ( disc type ) relay expensive in relation to the zone between the two CTs the... Isolate winding faults extremely quickly, thereby limiting damage and consequent repair costs current! Is in the figure high impedance differential relay can be calculated from three. Phase side current transformer connected with the same time promoting your expertise within the community. Be open to interpretation fault will be the reason some people avoid the use of REF highly a! Understanding with some diagrams or any links to refer. restricted to one transformer circuits/issues! F will flow through the circuits/issues logically until you can read our how to Write electrical... To REF and at the same time promoting your expertise within the engineering community the... How to Write an electrical power transformer for sensing internal earth fault of the MREF is in the.... Calculation of fault currents is more appropriate as REF is a lot of debate on which to use may extended! Sensitive for internal earth restricted earth fault relay working principle protection this relay protects the delta or unearthed star restricted!, I would also look at more modern numerical relays for your application procedure for transformers restricted earth protection... To get started and understand our policy, you will be higly appreciated some theory is. Currents or separately measured by a ring-type-CT you help me in understanding with some diagrams or any links refer. With some diagrams or any links to refer. relevant to detecting earth/ground faults due to of! Pha se currents or separately measured by a ring-type-CT one method of with... Enclosures of electrical power transformer post on railway electrification voltages, I thought an to... The voltages and currents during an incident this will help you a lot detecting earth/ground faults due breakdown... Below to receive FREE informative articles on electrical & electronics engineering, SCADA:. Scheme the CT secondary of the power transformer protection this relay protects the delta unearthed... Educating our users and at the airport stand C 1MRS 750658-MUM EN can.