An enhanced method of providing sensitive bus fault protection

An Enhanced Method of Providing SensitiveBus Fault ProtectionNorman T.Stringer,Senior Member,IEEEAbstract—The application of bus fault protection in industrial facilities has been primarily limited to overcurrent relays located at the upstream breaker.This does not always provide the best protec-tion for bus faults.The protection sensitivity is limited because of the need to coordinate with other downstream overcurrent devices. This paper describes an enhanced method of applying sensitive bus fault protection using digital overcurrent relays.In addition,this method provides both downstream feeder breaker backup protec-tion and relay backup protection.Index Terms—Bus protection,feeder protection,overcurrent protection,zone interlocking.I.I NTRODUCTIONT HERE HA VE been many ways to provide bus protection over the years.These have included simple overcurrent relays,high-impedance differential,low-impedance differen-tial,partial differential,and the inclusion of the bus within the transformer differential zone.The application of bus protection within industrial facilities has been limited primarily to the use of overcurrent relays located on the bus breaker,as shown in Fig.1.Phase overcurrent relays applied in this manner provide ad-equate bus protection for faults with high-magnitude currents. However,the pickup setting and time delay of the bus relay must be selected such that adequate time is allowed to coordinate with the operation of downstream feeder relays.Bus phase relays are typically set above the total maximum setting of all the feeder relays combined.Therefore,for feeder faults with lower current magnitudes,the clearing time for the bus relay could be substan-tially longer.The more time delay allowed,the more damage to equipment that can occur.The time-overcurrent function for the bus and feeder relays would normally be set for approximately150%–175%of the maximum expected load current.The time-dial setting should be sufficient to provide a coordination-time interval(CTI)of at least0.3s between the feeder relay(having the highest setting) and the bus relay(at the point of maximum current magnitude for a feeder fault).An example time–current coordination curve for the example system is given in Fig.2.Paper ICPSD01–25,presented at the2001IEEE/IAS Industrial and Commercial Power Systems Technical Conference,New Orleans,LA,May 13–17,and approved for publication in the IEEE T RANSACTIONS ON I NDUSTRY A PPLICATIONS by the Power Systems Protection Committee of the IEEE Industry Applications Society.Manuscript submitted for review May17,2001 and released for publication February12,2003.The author is with Artwel Electric,Inc.,Sacramento,CA95834USA(e-mail: terry.stringer@).Digital Object Identifier10.1109/TIA.2003.810932Fig.1.Typical bus protection method for an industrialfacility.Fig.2.Typical coordination between feeder relay and main bus relay.Given a maximum feeder fault of16000A,the feeder(phase)relay should trip in about0.06s.Operation of the bus relaywould follow this by approximately0.3s,or a total of0.36s,should the feeder breaker fail to clear the fault.A substantialamount of damage could occur during this time.Furthermore,given an arcing ground fault,which may pro-duce less than50%on a medium-voltage system,a substantiallylonger clearing time can occur[1].Considering this,the clearing 0093-9994/03$17.00©2003IEEEtime may be as long as1.6s,given a failed feeder breaker op-eration.In addition,for low-magnitude bus faults the burden becomes achieving an appropriate setting that is sensitive enough to operate for minimum bus faults while being sufficiently high enough to allow for maximum loading[2].It may also be difficult to provide appropriate settings using conventional type overcurrent relays on power systems that employ impedance to limit available ground-fault currents.Several suggested modifications have been made over the last few years.However,in most cases,these methods provide only minimal improvement over existing schemes and are not ade-quate in all applications[2]–[4].This paper describes an enhanced method of providing sen-sitive bus protection.Advanced capabilities of digital relays are used to develop a simplified control logic scheme for the protec-tion of bus faults.In addition to the primary overcurrent protec-tion provided for bus faults,this scheme includes fast backup protection for the failure of either the feeder relay or feeder breaker.By using programmable contact features and hardwired interconnection between the bus and feeder relays,an enhanced bus protection scheme can be achieved.II.O VERVIEW OF E NHANCED B US P ROTECTIONOne of the many advantages of digital relays is the ability for the user to program input and output(I/O)contacts to perform various functions.This adds a tremendous flexibility for devel-oping enhanced protection schemes where traditional protection methods are otherwise limited.The enhanced bus protection scheme draws upon the features provided by digital relays to control the operation of protec-tive functions of the bus relay depending upon the action of the feeder relay.This scheme is based on a concept called zone or re-verse interlocking where the bus and feeder relays are hardwire connected[5].The zone interlocking allows the feeder relays to communicate the relative location of a fault to the bus relay, thereby providing direct control.Benefits gained by using this enhanced bus protection scheme include the following:1)more sensitive bus relay settings;2)closer coordination;3)faster bus clearing times;4)feeder breaker and relay backup.The digital overcurrent relay used in this scheme includes typical protection features such as instantaneous(no intentional time delay—typicallyFig.4.Enhanced bus protection scheme.connection should be made between the contacts of these two relays.This will result in the pickup output contact of the feeder relay becoming the blocking input contact for the bus relay.The coordination between these relays can be very close.However,a slight delay is required on the bus relay instantaneous setting to allow for the closing of the feeder relay blocking contacts and the scan time of the blocking signal by the bus relay.So now,when a feeder fault occurs,both feeder and bus relays will sense the fault,pickup,and begin timing.The pickup op-eration of the feeder relay will provide a blocking operation for the bus relay,inhibiting its operation for the feeder fault.Should the fault remain for a given time period,the blocking signal will be over-ridden and the bus relay will provide operation to clear the fault.By setting both the instantaneous and time-overcur-rent functions on the feeder relay to provide a blocking signal,additional security is achieved in the case of a low-magnitude fault current.The blocking inputs on the relay may be programmed to honor the blocking request for as long as the blocking input is active,or to ignore the blocking request after a period of time.In this case,twice the breaker failure timer setting (TBf).For the enhanced bus protection scheme,the blocking inputs on the bus relay should be set to honor the blocking request for only a fixed time period by setting TBp and TBg to 2TBf.The variable TBf on each of the feeder relays should be set to a time delay equal to their breaker’s expected operating time after receiving a trip signal.For example,given a 5-cycle breaker,TBf should be set to 5cycles.Tbg and TBp should be set to 2TBf,or2andIare assignedto operate output contact R4.These contacts will close immedi-ately when the operating current exceeds the pickup value.As shown in Fig.5,the output contact R3of the feeder relay is then connected to the phase blocking input Bf on the bus relay.The phase blocking input variable Bf should be programmed to display “BfIO800A),the bus relay can now be set towithin 105%–110%of the total feeder loading,or about 1911A.The bus relay will pickup routinely for faults located on the feeder.However,because of the input blocking signal being re-ceived from the feeder relays,any tripping will be inhibited until the blocking signal is removed.This will eliminate any nuisance tripping while providing greater sensitivity.The time-overcurrent function for the feeder relays should be set for pickup and timing as you would normally (approximately 150%–175%of maximum expected feeder load current).Sim-ilarly,the instantaneous function of the feeder relays should be set as you would normally (approximately 80%of maximum expected fault current).Using the enhanced bus protection scheme,the time-overcur-rent settings for the bus relay can be set much closer to the feeder relays.In this example,the bus relay can be moved as much as 50%closer to the feeder relay setting.As indicated in Fig.6,the closer setting of the phase relays provide a much faster clearing time for damaging bus faults as well as low-magnitude arcing feeder faults.Similar advantages are achieved with the ground relays,although to maintain clarity these were not shown in Fig.6.Application Considerations:There are four areas that should be considered in the application of this protection scheme.These are as follows:•feeder faults;Fig.5.Feeder and bus relayinterconnection.Fig.6.Coordination with enhanced bus protection scheme.•bus faults;•feeder circuit breaker failure;•feeder relay failure.Feeder Faults:When a fault occurs on a feeder,both the feeder relay and the bus relay will pickup and begin timing.The pickup contact closure of the feeder relay will block the oper-ation of the output contact of the upstream bus relay,allowing the feeder relay to clear the fault.When the fault is cleared,both relays will reset to normal.Bus Faults:If a bus fault occurs,the bus relay will pickup and begin timing or trip on instantaneous based on fault cur-rent magnitude.The feeder relays will not pick up for the given condition,eliminating the possibility of a blocking signal being sent to the bus relay.Without a blocking signal,the bus relay will close its contact after the set time delay,tripping the bus breaker.The time delay for the bus relay can be set very short.Onlyenough time (typically,10ms for the output relay to close its contacts)is required for the pickup element of the feeder relay to operate for a feeder fault and block the operation of the Bus relay.Although this method may require the addition of a short time delay of the instantaneous function,it will still provide in-herently faster clearing of bus faults.For applications where a bus fault results in current being back-fed from the feeders,there are two adjustments that can be made to this protection scheme.First,for those applications which produce decaying fault current contribution (i.e.,from downstream motors),the time delay setting of the bus relay can be increased slightly.Typically,only a few cycles delay should be necessary to allow the current to fall below the relay’s pickup setting.The second application is where fault current contribution from a downstream feeder is the result of an alternate source or path,such as a downstream generator or a loop connection back to the primary bus.In either of these or other similar situ-ations,this same protection scheme may be applied by simply utilizing directional sensing on both the feeder and bus relays.This will limit the operation of the relays for faults in only the downstream direction.Also,applying directional sensing elim-inates the need for additional time delay on the bus relay as de-scribed above.The scheme shown here is for a radial system,but this method can be applied equally to a dual-source system with a normally open tie breaker with little complication.When used on a dual-source system with a normally closed tie breaker,there are complications which are solved by adding direction-ality to the tie breaker relay [5].Feeder Circuit Breaker Failure:Upon a feeder fault,the feeder relay will attempt to trip the breaker.Should the feeder breaker fail to open and clear the fault,the breaker failure function of the relay will be initiated.After the set time delay TBg,the pickup element of the feeder relay will automatically drop out.This removes the blocking input signal from the bus relay,allowing it to trip.As noted previously,the blocking input signal to the bus relay inhibits only the closing of the output contacts,not the pickup or timing functions.Therefore,when the blocking signal is removed,the output contacts of the bus relay will close instantaneously,tripping the breaker.No further time delay is imposed in the circuit.If the breaker failure function does not operate in the feeder relay,or if the blocking circuit connection is shorted to the bus relay,the blocking signal would remain.However,the bus relay will ignore the blocking request after twice the breaker oper-ating time TBg,thereby allowing the bus relay to trip. Feeder Relay Failure:If a feeder relay fails to operate for a feeder fault,the blocking signal output will most likely not be functioning as well.Without receiving a blocking signal,the bus relay will close its output contacts after the programmed time delay,tripping its circuit breaker.Similarly,only enough time delay is required for the pickup element of the feeder relay to operate for a feeder fault and block the operation of the bus relay. Therefore,there is no additional coordination time required.As a result,the time delay can be set to a very minimal value.This scheme provides fast,effective backup protection for failure of the feeder relays.IV.C ONCLUSIONThe enhanced bus protection scheme described in this paper provides faster clearing for bus faults than traditional overcur-rent protection methods.By providing a hardwire connection between the feeder and bus relays,closer coordination can be ing programmable I/O contacts of a digital over-current relay,the feeder relay can be set to provide a blocking signal to inhibit the operation of the bus relay.As a result,the bus relay may be programmed for a lower pickup setting than you would normally consider.A lower pickup setting translates to improved sensitivity for bus faults.This comes with the added benefit of backup protection for the feeder relay or for a breaker failure condition.The enhanced bus protection allows closer coordination time intervals between the bus and feeder relays without problems of nuisance tripping.Additional time delays of typical backup pro-tection schemes are substantially reduced or eliminated.These advantages are gained without the need for additional relays.This scheme is not necessarily limited to bus and feeder appli-cations.The protection enhancements gained with this method may be considered for other applications where an upstream overcurrent relay must coordinate with a downstream overcur-rent relay.The only caveat is to ensure that the coordination of other protection devices would not be affected by applying this scheme.R EFERENCES[1]J.R.Dunki-Jacobs,“The escalating arcing ground-fault phenomenon,”IEEE Trans.Ind.Applicat.,vol.IA-22,pp.1156–1161,Nov./Dec.1986.[2] A.F.Elneweihi and E.O.Schweitzer,“Improved sensitivity and securityfor distribution bus and feeder relays,”in Proc.1992Pennsylvania Elec-tric Association Relay Committee Meeting,Hershey,PA,Sept.22–23, 1992,pp.1–33.[3]M.W.Feltis,G.W.Scheer,and E.O.Schweitzer,“A fresh look at dis-tribution protection,”in Proc.2nd Int.Symp.Distribution and Demand Side Management,uderdale,FL,Jan.13–15,1992,pp.1–10.[4]T.Rick,A.T.Giuliante,and G.Jay,“Improvements in distribution feederprotective relaying,”in Proc.46th Annu.Conf.Protective Relay Engi-neers,College Station,TX,Apr.12–14,1993,pp.1–7.[5] B.Bailey and D.Jakominich,“Static digital overcurrent relay permitenhanced protection,”in Proc.1998I&CPS Tech.Conf.,p.25.Norman T.Stringer(M’82–SM’95)received theB.S.E.E.degree from the University of Texas atArlington in1982and the M.B.A.EngineeringManagement degree from the University of Dallas,Dallas,TX,in1985.He has been in the power system protection fieldfor over20years with companies such as TXU,ABBPower T&D,Brown&Root,Basler Electric Com-pany,and Cooper Power Systems.In July2000,hejoined Artwel Electric,Inc.,Sacramento,CA,as aSales Engineer for the Northern California industrial market.He has authored or coauthored numerous technical papers,articles,tuto-rials,and training guides.He is also the chapter author of“Transformer Protec-tion”in the Encyclopedia for Electrical and Electronic Engineers(New York: Wiley,1999).Mr.Stringer is a Senior Member of the IEEE Power Engineering and IEEE Industry Applications(IAS)Societies.He is actively involved in the Industrial and Commercial Power Systems Department of the IAS,where he has served on several committees.He is a Registered Professional Engineer in the State of Texas.。