The REF protection provides fast protection for ground faults for wye windings intransformers. It provides very sensitive ground fault protection for low magnitude fault currents. These low levels ground fault will not be detected by 87 differential element. It is applied usually in resistance grounded systems when the ground fault current is limited and can be very small for detection by differential relay. If no REF, the fault would be cleared by time overcurrent 51G . Since you have a low level ground fault in the transformer winding you always want to trip quickly HV and LV to isolate the fault.for standard you can use : C37.91-2000 IEEE Guide for Protective Relay Applications to Power Transformers .

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REF Protection for HV/LV transformerDhritiman DasSenior Engineer, Electrical at Alstom GroupRecently, I have received a requirement for considering provision of tripping downstream LV breaker with REF of upstream HV/LV transformer. It is known to be standard to trip the upstream HV breaker with REF to protect the transformer from damage.Reason for this sort of requirement could not be ascertained. What is/are the probable reason(s)? Is it any new standard of protection that recommends this, or was it always there, but lesser known? Like Comment (23) Share Follow Reply Privately 5 months agoComments23 commentsJump to most recent comment SelvakumarSelvakumar SPower System Engineer at ABB Global Industries Ltd ChennaiI have understood REF as Restricted Earth Fault. If so, It is mandatory to trip both HV and LV side breakers to ensure the fault is isolated. Like Reply privately Flag as inappropriate 5 months ago DhritimanDhritiman DasSenior Engineer, Electrical at Alstom GroupYou have understood correctly.Since, our purpose is to protect the HV/LV transformer, it is of utmost importance to isolate it from upstream power and therefore de-energise the transformer under fault. Could you explain, further, the purpose of isolating the LV downstream? It would also be great of you to mention the IEC or any other standard which can be referred. Like Reply privately Flag as inappropriate 5 months ago AriesAries De La CruzPrincipal Engineer at Hyder Consultingthe Motor loads will also contribute to the fault, in my experience this is practiced in Qatar/Kahramaa, since the Utility provides the transformers, they prefer to minimize the internal damage to the TX. Even for smaller TX's (1000, 1600 kVA) they still require REF protection with HV and LV side trip. Like Reply privately Flag as inappropriate 16 days ago DhritimanDhritiman DasSenior Engineer, Electrical at Alstom GroupHi Aries,Thanks for the information about such practices. Till date I had not come across such requirement from any of the clients, therefore it was new for me.

To put it simply, Restricted Earth Fault is meant to protect the transformer windings at the quickest on the basis of fault occurring in the secondary side (sometimes inclusive of the distribution cable connected to secondary). The REF relay immediately trips the upstream HV breaker to protect the transformer from major damage.Please let me know, how tripping the LV side breaker can help achieve further better protection.Also, it was not possible not understand how small LV motor loads can contribute to the REF protection, which is 'restricted' within secondary winding of the distribution transformer? Like Reply privately Flag as inappropriate 15 days ago HalimHalim BensmaiaLead Electrical Discipline Engineer at Shell Energy CanadaThe REF protection provides fast protection for ground faults for wye windings intransformers. It provides very sensitive ground fault protection for low magnitude fault currents. These low levels ground fault will not be detected by 87 differential element. It is applied usually in resistance grounded systems when the ground fault current is limited and can be very small for detection by differential relay. If no REF, the fault would be cleared by time overcurrent 51G . Since you have a low level ground fault in the transformer winding you always want to trip quickly HV and LV to isolate the fault.for standard you can use : C37.91-2000 IEEE Guide for Protective Relay Applications to Power Transformers . Like Reply privately Flag as inappropriate 15 days ago AriesAries De La CruzPrincipal Engineer at Hyder Consulting

I think the requirement imposed by the service provider does not depend on the magnitude of the down stream fault contribution but on the demarcation between service provider and customer supplied equipment. in our case the service provider will supply the tx. customer will provide lv panel + protection. since the service provider dictates the requirement they will spec the maximum protection available even for the smallest tx's since the customer will be buying the additional relays and ct's. also depending on the occupancy served by the transformer, there are cases that we have 60 to 80% motor loads per tx, that will also have a significant fault contribution to the internal tx fault if you dont open the LV side breaker. Like Reply privately Flag as inappropriate 15 days ago AriesAries De La CruzPrincipal Engineer at Hyder Consulting

I think the requirement imposed by the service provider does not depend on the magnitude of the down stream fault contribution but on the demarcation between service provider and customer supplied equipment. in our case the service provider will supply the tx. customer will provide lv panel + protection. since the service provider dictates the requirement they will spec the maximum protection available even for the smallest tx's since the customer will be buying the additional relays and ct's. also depending on the occupancy served by the transformer, there are cases that we have 60 to 80% motor loads per tx, that will also have a significant fault contribution to the internal tx fault if you dont open the LV side breaker. Like Reply privately Flag as inappropriate 15 days ago AriesAries De La CruzPrincipal Engineer at Hyder Consulting

I think the requirement imposed by the service provider does not depend on the magnitude of the down stream fault contribution but on the demarcation between service provider and customer supplied equipment. in our case the service provider will supply the tx. customer will provide lv panel + protection. since the service provider dictates the requirement they will spec the maximum protection available even for the smallest tx's since the customer will be buying the additional relays and ct's. also depending on the occupancy served by the transformer, there are cases that we have 60 to 80% motor loads per tx, that will also have a significant fault contribution to the internal tx fault if you dont open the LV side breaker. Like Reply privately Flag as inappropriate 15 days ago AriesAries De La CruzPrincipal Engineer at Hyder Consulting

I think the requirement imposed by the service provider does not depend on the magnitude of the down stream fault contribution but on the demarcation between service provider and customer supplied equipment. in our case the service provider will supply the tx. customer will provide lv panel + protection. since the service provider dictates the requirement they will spec the maximum protection available even for the smallest tx's since the customer will be buying the additional relays and ct's. also depending on the occupancy served by the transformer, there are cases that we have 60 to 80% motor loads per tx, that will also have a significant fault contribution to the internal tx fault if you dont open the LV side breaker. Like Reply privately Flag as inappropriate 15 days ago HalimHalim BensmaiaLead Electrical Discipline Engineer at Shell Energy CanadaMake sure you are talking about ground fault and not other phase faults. You must be able to provide settings for 87GD also. Then check the sensitivity and other factors affecting this protection before talking about significant fault contribution for transformer internal faults. You can also discuss with and without 87 (differential) since this protection is differential but operate with a slope! Like Reply privately Flag as inappropriate 14 days ago HalimHalim BensmaiaLead Electrical Discipline Engineer at Shell Energy Canada...and operate with a slope and must be sensitive for low level GF. Like Reply privately Flag as inappropriate 14 days ago HalimHalim BensmaiaLead Electrical Discipline Engineer at Shell Energy CanadaThis protection can be unstable for LR grounded system. Using digital relay you need to select appropriate slope and figure out the sensitivity. Transformer inrush and external fault can have also have an impact on this protection. Like Reply privately Flag as inappropriate 13 days ago Pablo T.Pablo T. Lazo, REEPMT Electrical Engineer at Saudi AramcoTop ContributorA simple overcurrent and earth fault relay will not provide adequate protection for winding earth faults. Even with a biased differential relay, the biasing de-sensitises the relay such that it is not effective for certain earth faults within the winding. This is especially so if the transformer is resistance or impedance earthed, where the current available on an internal fault is disproportionally low. In these circumstances it is often necessary to add some form of separate earth fault protection. The degree of earth fault protection is very much improved by the application of REF-systems. Both windings of a transformer can be protected separately with restricted earth fault, thereby providing high speed protection against earth faults over virtually the whole of the transformer winding.

Anything which gets a fault off the system is generally a good thing. REF is a good relay to protect against an internal short close to the neutral point which would probably not be seen by an overcurrent relay, and is effective for any in-zone fault. Today it is a fairly cheap protection function which provides rapid clearance for an in-zone fault. What is the capacity/rating of the transformer in question, and how important is the load it feeds?

Oh and hey, aren't you working in Alstom Group? this is a potential revenue to your company! Like Reply privately Flag as inappropriate 5 days ago HalimHalim BensmaiaLead Electrical Discipline Engineer at Shell Energy CanadaREF scheme sensitivity is a problem only on star windings with resistance grounding because the fault current is a function of fault position, phase-to-neutral voltage, and earthing resistance value. For faults close to neutral, the fault current is very small.Most REF relays use an operating and a restraint current. There are difference between relays to determine the restraint quantities and in the detection algorithms and supervision used for each relay to avoid nuisance tripping.As Pablo mentionned before, it will be excellent to understand and/or discuss the algorithm used in the Alstom relays for REF in order to determine the relay setting and provide transformer data. if REF is set correctly, then you will trip primary and secondary CBs for low level ground fault only! Like Reply privately Flag as inappropriate 2 days ago Pablo T.Pablo T. Lazo, REEPMT Electrical Engineer at Saudi AramcoTop ContributorThe restricted earth fault REF relays provide more sensitive protection for transformers internal faults to ground as Halim already pointed out in his post earlier.

Basically, restricted earth fault protection is a differential scheme, with CT's on the transformer neutral and phase leads. The phase CT's 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 or a current relay with an external setting resistor. 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. Heavy external faults may trip the REF relay because of CT's asymmetries. To avoid this, additional choke, capacitor and short time delay (relay) 1 to 2 seconds are needed to mitigate the effect of harmonics and DC component. An stabilizing resistor is connected in series with the relay. See page 181 of A. R. van C. Warrington "Protective Relays Their Theory and Practice" Volume I., 1971. Like Reply privately Flag as inappropriate 2 days ago DhritimanDhritiman DasSenior Engineer, Electrical at Alstom GroupFriends,Thank you for your contribution so far.My question however is still there - what is the additional advantage of tripping the downstream LV side breaker when we achieve max. protection by fast REF actuated trip in upstream HV side breaker?2nd question: What are chances of nuisance tripping due to REF relay? Like Reply privately Flag as inappropriate 2 days ago HalimHalim BensmaiaLead Electrical Discipline Engineer at Shell Energy Canada1)

In order to limit damage in transformer, we need to trip both primary and secondary. It is not a matter of choice, with advantage or disadvantage but necessary to isolate the transformer since the available energy (kw-cycle) feeding the fault can cause a lot of damage. Several publications describing the level of damage for HR systems are available for you to read (for generators or transformers).

2)Depending on the method of transformer grounding and fault location, some transformer ground faults result in only a small increase in phase current, which transformer differential protection may not detect. Also, the amount of current in the neutral may be sufficient to detect most or all ground faults, depending on the grounding method. By connecting an REF relay to CTs installed in correct locations on the transformer, we can use REF protection to complete differential protection in detecting transformer ground faults (this was explained before). REF protection can be high-impedance REF or low impedanceREF relays. You need to verify yourself (I am sure that Alstom has documents covering REF protection for transformer). You need to understand the dfference between HI and LI REF . Modern numerical relays relly on LI and use detection algorithms and supervision functions with logic (harmonics, saturation, zero sequence, positive sequence ect...) to trip correctly and as explained before you have operating and restrained currents. This is not the case with HI .REF Nuisance tripping are caused by CTs saturation, external fault, and poor relay settings. You may have also inrush conditions if you did not include supervision of second harmonics in your relay logic and checked with the manufacturers. Like Reply privately Flag as inappropriate 1 day ago HalimHalim BensmaiaLead Electrical Discipline Engineer at Shell Energy CanadaDepending on your CTs (phase and neutral) you may only use LI REF protection. It is better to check your alstom relay. Like Reply privately Flag as inappropriate 1 day ago Pablo T.Pablo T. Lazo, REEPMT Electrical Engineer at Saudi AramcoTop ContributorHello Dhritiman, before we answer your two (2) questions below:

1.) What is the additional advantage of tripping the downstream LV side breaker when we achieve max. protection by fast REF actuated trip in upstream HV side breaker?

2.) What are chances of nuisance tripping due to REF relay?

In your original post, you've mentioned restricted earth fault (REF) protection only. Do you have overcurrent or differential protection or other protection relay? If so, what do these trip? Where are your overcurrent and differential relay located, is it in the source/primary of the transformer, or in the secondary side of the transformer. What is the substation configuration, is it radial feed or double ended substation with secondary selective main-tie-main. Like Reply privately Flag as inappropriate 1 day ago RameshRamesh BManaging Director at Interface Electronics Pvt LtdIf the motor loads are present on LT side,there will be current flow from these motors to feed the fault, although the HT side is tripped.It is to cut off the source feeding the fault, therefore both HT and LT breakers to be tripped. Like Reply privately Flag as inappropriate 23 hours ago HalimHalim BensmaiaLead Electrical Discipline Engineer at Shell Energy CanadaWhy are we still discussing tripping the primary circuit breaker?

Transformer has usually protective functions @ primary side and secondary side (ex : 50, 51, REF (87GD), 87...) . Some of this protection such as 50 @ primary side is a back-up for differential protection 87T and 51 @ primary side looks at secondary thru faults, 51 at the secondary provides transformer OL protection. Most of this protection trips PRIMARY and SECONDARY circuit breaker. We are also discussing ZONE protection . "Restricted" ground protection is same as saying we are looking at a "ZONE" protection (transformer zone).We also usually have back-feed from low voltage. Poor engineering or poor judgement only limit tripping or discuss tripping at the primary transformer side only.If we are using restricted (REF) already means we do not have enough GF current and differential protection can not isolate low level ground fault near the neutral. We usually use differential protection and restricted ground fault (if required) for large transformer. The size and protective requirements will be imposed or required by the client . Like Reply privately Flag as inappropriate 22 hours ago HalimHalim BensmaiaLead Electrical Discipline Engineer at Shell Energy Canada@PabloCan you please clarify further the relation between "REF nuisance tripping" and "substation configurations"? Like Reply privately Flag as inappropriate 21 hours ago Pablo T.Pablo T. Lazo, REEPMT Electrical Engineer at Saudi AramcoTop Contributor@ HalimLet Dhritiman Das to provide us more info on his questions so the speculations can come to an end.