IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 49, NO. 5, SEPTEMBER/OCTOBER 2013 1971

Innovative Differential Protection of PowerTransformers Using Low-Energy

Current SensorsLjubomir A. Kojovic, Martin T. Bishop, and Dharam Sharma

Abstract—Traditional differential protection systems are ap-plied on large power transformers using current transformers(CTs). However, because of high secondary currents (often exceed-ing 100 kARMS), differential protection systems for electric arcfurnace transformers have not been applied in the past due tothe lack of commercially available CTs. This paper will presentdifferential protection systems that have been in use for manyyears using Rogowski coil current sensors. These protection sys-tems use high-precision printed circuit board Rogowski coil cur-rent sensors. This paper reviews the characteristics, designs, andapplication of these Rogowski coil sensors for advanced protection,control, and metering systems with new multifunction relays.This paper compares performance characteristics of new solutionsbased on Rogowski coil sensors with the conventional differentialprotection systems based on CTs, demonstrating that the newsystems do not have the limitations of conventional technology.Operating experience from several site applications is included.

Index Terms—Electric arc furnace (EAF) transformer, relayprotection, Rogowski coil.

I. INTRODUCTION

ROGOWSKI coil current sensors are transformers thatoperate on the same principles as conventional iron-

core current transformers (CTs). The main difference betweenRogowski coils and CTs is that Rogowski coil windings arewound over an air core, instead of over an iron core. Asa result, Rogowski coil sensors are linear since the air corecannot saturate. However, the mutual coupling between theprimary conductor and the secondary winding in Rogowskicoils is much smaller than in CTs. Therefore, Rogowski coilsensor output power is small; thus, they cannot drive currentthrough a low-resistance burden such as a typical CT applica-tion. Rogowski coil current sensors can provide input signalsfor microprocessor-based devices that provide a high inputresistance, and these devices therefore measure voltage acrossthe Rogowski coil secondary output terminals.

Manuscript received November 3, 2009; accepted March 15, 2010. Date ofpublication June 3, 2013; date of current version September 16, 2013. Paper2009-METC-387, presented at the 2009 Industry Applications Society AnnualMeeting, Houston, TX, October 4–8, and approved for publication in theIEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Metals IndustryCommittee of the IEEE Industry Applications Society.

L. A. Kojovic is with Eaton Corporation, Franksville, WI 53126-0100 USA(e-mail: ljubomirAkojovic@eaton.com).

M. T. Bishop is with S&C Electric Company, Franklin, WI 53132-8663 USA(e-mail: Martin.Bishop@sandc.com).

D. Sharma is with Nucor-Yamato Steel Company, East Armorel, AR 72310USA (e-mail: Dsharma@nucor-yamato.com).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIA.2013.2264792

In general, Rogowski coil current sensors have performancecharacteristics that are favorable when compared with conven-tional CTs. These characteristics include high measurement ac-curacy and a wide operating current range, allowing the use ofthe same device for both metering and protection. In addition,Rogowski coil current sensors make protection schemes possi-ble that were not achievable using conventional CTs becauseof saturation, size, weight, and/or difficulty encountered whenattempting to install CTs around conductors that cannot beopened.

Rogowski coil current sensors can replace conventional CTsfor protection, metering, and control. Rogowski coil currentsensors have been applied at all voltage levels (low, medium,and high voltage). However, unlike CTs that produce secondarycurrent proportional to the primary current, Rogowski coilcurrent sensors produce an output voltage that is a scaled timederivative di(t)/dt of the primary current. Signal processing isrequired to extract the power frequency signal for phasor-basedprotective relays, and microprocessor-based equipment must bedesigned to accept these types of low-energy signals.

II. COMPARATIVE ANALYSIS

A. Rogowski Coil Current Sensor Principle of Operation

Conventional iron-core CTs are typically designed with ratedsecondary currents of 1 or 5 A, to drive a low-impedance burdenof several ohms. ANSI/IEEE Standard C57.13-2008 [1] speci-fies CT accuracy class for steady-state and symmetrical faultconditions. Accuracy class of the CT ratio error is specified tobe ±10% or better for fault currents up to 20 times the CT ratedcurrent and up to the standard burden (maximum ohm value ofburden that can be connected to the CT secondary). CTs aredesigned to meet this requirement. However, if a symmetricfault current exceeds 20 times the CT rated current or if the faultcurrent is smaller but contains dc offset (asymmetric current),the CT will saturate. The secondary current will be distortedand the current root mean square (RMS) value reduced. Tra-ditional Rogowski coil designs consist of a wire wound on anonmagnetic core (μr = 1). The coil is then placed aroundconductors whose currents are to be measured. New designsmay use printed circuit boards (PCBs) with imprinted windingson the board (see Fig. 1).

Because Rogowski coil designs use a nonmagnetic core tosupport the secondary windings, mutual coupling between theprimary and secondary windings is weak. Because of weak

0093-9994 © 2013 IEEE

1972 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 49, NO. 5, SEPTEMBER/OCTOBER 2013

Fig. 1. Rogowski coil.

coupling, to obtain quality current sensors, two main criteriamust be met when designing Rogowski coil current sensors.

1) The Rogowski coil output signal should be independentof the primary conductor position inside the coil loop.

2) The impact of nearby conductors that carry high currentson the Rogowski coil output signal should be minimal.

To satisfy the first criteria, mutual inductance M must havea constant value for any position of the primary conductorinside the coil loop. This can be achieved if the windings arewound on a core that has a constant cross section S and woundperpendicular on the middle line m (dashed line in Fig. 1) witha constant turn density n. Mutual inductance M is defined bythe following formula:

M = μ0 · n · S.

Here, μ0 is the permeability of air.The output voltage is proportional to the rate of change of the

measured current as given by the following formula:

νs = −Mdip(t)

dt.

Because the Rogowski coil primary and secondary windingsare weakly coupled to prevent the unwanted influence fromnearby conductors carrying high currents, Rogowski coils aredesigned with two wire loops connected in electrically oppositedirections. This cancels electromagnetic fields coming fromoutside the coil loop. One or both loops can consist of woundwire. If only one loop is constructed as a winding, then thesecond wire loop can be constructed by returning the wirethrough or near this winding. If both loops are constructed aswindings, then they must be wound in opposite directions. Thisway, the Rogowski coil output voltage induced by currents fromconductor(s) inside the sensor window will be doubled.

B. Equivalent Circuits

Fig. 2 shows the equivalent circuit of an iron-core CT.Magnetizing current Ie introduces amplitude error and phaseerror. Since the CT iron core has a nonlinear characteristic,it may saturate at high currents or when dc is present in theprimary current. When a CT saturates, the magnetizing currentwill increase, and the secondary current will decrease. This maynegatively impact relay performance such as delayed operation,nonoperation, or unwanted operation.

Fig. 2. CT equivalent circuit.

Fig. 3. Rogowski coil equivalent circuit.

Fig. 3 shows the equivalent circuit of a Rogowski coil. Thephase angle between the Rogowski coil primary current and thesecondary voltage is nearly 90◦ (displaced from 90◦ by a smallangle caused by the coil inductance Ls).

As the Rogowski coil signal is a scaled time derivative, i.e.,di(t)/dt of the primary current, signal processing is required toextract the power frequency signal for phasor-based protectiverelays. This may be achieved by integrating the Rogowski coiloutput signals or using nonintegrated Rogowski coil outputsignals in other signal processing techniques. Integration ofthe signals can be performed in the relay (by analog circuitryor by digital signal processing techniques) or immediatelyat the coil. To use the Rogowski coil nonintegrated analogsignal, it is necessary to perform the signal corrections for boththe magnitudes and phase angles. For phasor-based protectiverelaying applications, the Rogowski coil secondary signal mustbe scaled by magnitude and phase shifted for each frequency.

C. Rogowski Coil Current Sensor Designs

Rogowski coil current sensors may be designed with dif-ferent shapes such as round and oval. The sensors may beconstructed using rigid or flexible materials. Rogowski coilsensors can be designed as nonsplit style, or alternatively assplit-core construction that can be opened to assemble arounda conductor that cannot be opened. The cross-sectional shapeupon which the coil is formed is generally either circular orrectangular. Rigid Rogowski coil current sensors have higheraccuracy than the flexible style and may be designed usingPCBs as window (non-split-core) type or split-core type. PCBRogowski coils can be designed using one or two PCBs toimprint windings. Designs using one PCB have both windingsimprinted on the same PCB. Designs using two PCBs have onecoil imprinted on each PCB, with each imprinted coil wound inopposite directions.

KOJOVIC et al.: DIFFERENTIAL PROTECTION OF POWER TRANSFORMERS USING CURRENT SENSORS 1973

Fig. 4. Non-split-core style PCB Rogowski coil designs.

Fig. 5. Split-core PCB Rogowski coil design.

Fig. 6. Split-core Rogowski coils in service applications.

Fig. 4 shows two designs of window-type PCB Rogowskicoils for indoor and outdoor applications that can be easilyinstalled around primary conductors similar to conventionalbushing-type CTs. Fig. 5 shows a split-core Rogowski coildesigned for installation around primary conductors withoutrequiring the opening of the primary conductors. Fig. 6 showssplit-core style Rogowski coils, one that monitors currents inmultiple conductors and one coil installed around a water-cooled conductor with a large cross section.

D. Interface to Relays

Conventional CTs require heavy-gauge secondary wires forinterconnection to relays and other metering and control equip-ment as illustrated in Fig. 7. The wire resistance adds to theCT burden and may negatively impact the CT transient re-sponse, causing CT saturation at high fault currents. In addition,terminal blocks are required; thus, the CT secondary can beshorted. Hazardous voltages can be generated when the CTsecondary circuit is opened while load current is flowing. OtherCT disadvantages include large size and weight. For example,Fig. 7 shows a 2000/5 A C 800 class CT connected to a relay.This CT has the core and winding height of 4 in and weighsabout 200 lb.

Rogowski coils may be connected to relays via twisted-pair shielded cables with connectors as illustrated in Fig. 8.

Fig. 7. CT connections to relays.

Fig. 8. Rogowski coil connections to relays.

Terminal blocks are not required since the coil output signalis a minimal voltage from the safety aspect, and this voltagedoes not increase when the secondary circuit is opened.

Fig. 8 shows an actual Rogowski coil current sensor with athickness and weight that are much smaller than a C800 classCT. This coil has the same size window as the CT shown inFig. 7 but can be applied to a significantly larger current rangethan the CT.

III. APPLICATIONS FOR RELAY PROTECTION

Rogowski coil current sensors may replace conventionalCTs for metering and protection. IEEE Std. C37.235-2007 [2]provides guidelines for the application of Rogowski coil sensorsused for protective relaying purposes.

A. Rogowski Coil-Based Relay Protection

Traditional differential protection schemes that use conven-tional CTs require stabilization for external faults or distur-bances that cause CT saturation because it is not feasible toavoid CT saturation under all circumstances. Even where CTsare of similar design and the leads between each set of CTs andthe differential relay are balanced, the CTs will not saturate tothe same degree at the same time because of remanent flux.

Fig. 9 illustrates differential current error caused by CT satu-ration. To avoid misoperation for through faults, the percentage-restrained differential element is typically designed with two ormore slope characteristics.

Rogowski coil-based solutions improve protection perfor-mance because they provide high dependability (sense andoperate for low in-zone fault currents) and provide high secu-rity for out-of-zone faults (exceeding 60 kA). The protection

1974 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 49, NO. 5, SEPTEMBER/OCTOBER 2013

Fig. 9. CT-based power transformer differential protection.

Fig. 10. Rogowski coil-based EAF transformer differential protection.

algorithms are simple since Rogowski coils do not saturate. Inaddition, multiple slopes are not required. Transformer inrushcurrents are determined using a current waveform recogni-tion algorithm. Protection settings can be at a lower currentthreshold compared with conventional solutions based on CTs.The load tap changer (LTC) position is also used by the relayto adaptively adjust the transformer ratio allowing the settingthreshold to be further reduced. Introduction of Rogowski coilcurrent sensors for metering and protection is a paradigm shiftin technology. Protection engineers had legitimate concerns thatnew sensors may have a significant impact on existing meteringand protection philosophy. To demonstrate that the change inparadigm is not a concern and that Rogowski coil-based pro-tection systems provide superior protection over conventionalCT-based protection systems, the first Rogowski coil-basedsystems were developed and applied for electric arc furnace(EAF) transformers. These critical units were not protectedusing differential protection in the past, due to the difficultyin designing conventional iron-core CTs for load currents of60 kA or more. The Rogowski coil protection system wasimplemented for the first time on two 90-MVA 34.5-/1-kVEAF transformers equipped with an LTC. Primary Rogowskicoil sensors were designed as non-split-core style. Because ofhigh secondary currents exceeding 60 kA, the EAF transformersecondary has a delta closure consisting of two water-cooledtubes per phase (9-in diameter each). Since the tubes cannotbe opened, the secondary side Rogowski coil sensors weredesigned in split-core styles as illustrated in Fig. 10.

Fig. 11. EAF single-line diagram.

Fig. 12. RMS values of EAF currents during one heat cycle (averagedduring 1 s).

The operation of an EAF transformer is significantly dif-ferent when compared with a utility power transformer ofcomparable size. These differences present many challengesto the protection system designer for developing a successfuldifferential protection system. To explain why it is difficult (orimpossible) to apply differential protection on EAF transform-ers using conventional iron-core CTs, a typical EAF operationpowered by a 25-MVA transformer was presented.

The electrical system single-line diagram is shown in Fig. 11.The current in RMS ampere values are shown in Fig. 12. TheRMS values are averaged over 1 s of recording.

In the routine operation of the furnace, a heat cycle startswith charging the furnace with a cold scrap. The heat cy-cle starts when the electrodes are lowered into the scrap(“bore-in” phase) starting the arc. This causes a short circuit thatmomentarily develops very high currents resulting in excessiveforces that blow the scrap away from the electrodes, sometimes

KOJOVIC et al.: DIFFERENTIAL PROTECTION OF POWER TRANSFORMERS USING CURRENT SENSORS 1975

Fig. 13. Max and min values of EAF currents during one heat cycle (averagedduring 1 s).

interrupting the electric arc. Then, the arc quickly reignites.This on-off process can last for several minutes. During thisperiod, current magnitudes rapidly and chaotically change fromlow to high values. After 5–10 min, arc stability improves, butthere is still a high degree of current variation, as comparedwith current variation that a utility power transformer mayexperience. The next stage of the cycle is referred to as theearly melt period. To optimize the melting process, the EAFregulator may send a command to change the EAF transformertap position. In a heat cycle, there is usually more than one scrapcharge in order to fill the furnace, and each one results in an off-on cycle.

Fig. 13 shows RMS values of EAF currents during one heatcycle that includes three heating periods for the steel rechargingor tapping the furnace. Three periods of current interruptionsare intentional and are required for the steel recharging andtapping the furnace at the end of the heat. Fig. 13 showsminimum and maximum current values to better depict chaoticcurrent changes. Currents rapidly change from low values toover 90 kA, i.e., up and down.

In summary, the following challenges exist to design reliabledifferential protection for EAF transformers.

1) The first challenge is to provide high-protection systemsecurity because EAF transformers are subject to frequentenergization. There can be 70–100 energizations per dayfor a typical furnace operation. Even a small percentageof failure to properly identify inrush conditions can resultin many nuisance operations.

2) The second challenge is to maintain the scheme sensitiv-ity to the frequent operation of on-load tap changers. EAFtransformers might have 20 to 30 taps with a voltage vari-ation of 800–1400 V line-to-line on the secondary side.This may result in a mismatch of primary to secondarycurrent of 30% for a relay set at a fixed ratio setting at themidpoint of the range. To provide a sensitive differentialprotection scheme that can detect low current in-zonefaults, the relay must be able to adapt to changing tapsduring transformer operation.

Fig. 14. EAF primary currents (A, B, and C phases) recorded by a transientrecorder.

3) The third challenge is to provide reliable operation ofthe scheme for distorted current waveforms during thearcing process in an EAF. The nonlinear characteristicof the arc, plus the erratic nature of the continuity ofthe arc in the scrap, results in high percentages of lowerorder harmonics. A differential relay that uses harmoniccontent to determine an inrush condition may have asetting that will block the relay operation for a secondaryarcing fault in the zone of protection.

4) The fourth challenge is to provide a reliable relay protec-tion system operation in severe environmental conditionsthat include dust, vibration, and extremes of temperatureand humidity. EAF dust has high iron content and isconductive in concentrated amounts. This dust is some-times the cause of short circuits on the EAF transformersecondary terminals. To prevent dust penetration intoEAF transformer vaults, air-handling systems keep pos-itive pressure in the vault to minimize dust penetration.Despite all efforts to avoid dust penetration into the vault,dust cannot be completely prevented. Temperature in thevault can be low during winter time and high (over 100◦)in the summer, since the air for air-handling systemsusually comes from outside the building (at prevailingrelative humidity). In addition, EAF transformers and thewhole building are exposed to high vibration from theoperation of the EAF furnace.

B. Experience With Rogowski Coil-BasedDifferential Protection

The Rogowski coil sensor-based differential protection sys-tems for EAF transformer protection have demonstrated supe-rior performance since installation (8 years as of this writing).Rogowski coils are linear and accurately reproduce primarycurrents. The coil output signals of EAF currents recordedduring a heat cycle by a transient recorder are shown in Fig. 14.Another snapshot of EAF currents derived by the relay isshown in Fig. 15. Although the relay sampling rate is muchsmaller than the transient recorder, the current waveforms aredetailed enough to properly represent waveform distortion andharmonic content. Fig. 16 demonstrates that the coil outputsignals of EAF primary and secondary currents recorded by a

1976 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 49, NO. 5, SEPTEMBER/OCTOBER 2013

Fig. 15. EAF primary currents (A, B, and C phases) derived by the relay.

Fig. 16. EAF A-phase primary and secondary currents recorded by a transientrecorder (referred to the primary).

Fig. 17. EAF A-phase primary and secondary currents derived by the relay(referred to the primary).

transient recorder match very well when adjusted for the EAFtransformer turns ratio. Fig. 17 demonstrates that the coil outputsignals of EAF primary and secondary currents derived bythe relay also match very well to accurately derive differentialcurrents.

Detection of Inrush Currents: Traditionally, the second har-monic restraint method was used to avoid unwanted opera-tions when energizing a power transformer. This methodcannot reliably provide restraint signals for EAF trans-formers since there is the possibility of a low-voltage arc-ing fault condition within the zone of protection that willresult in current waveforms that are not that much different

than the arcing in the furnace just outside of the zone ofprotection. The operation of an EAF results in harmoniccurrents due to the erratic nature of the arc. Applying asecond harmonic restraint method could require setting asecond harmonic inrush restraint element at a higher levelto avoid blocking when a secondary arcing fault occurredin the zone. A higher setting makes the identification ofinrush conditions less reliable. In addition, inrush currentscan be combined with load currents that can reduce theamount of second harmonics derived by the relay. Asa result, the relay will not be blocked and a nuisanceoperation of differential protection may result.

A novel transformer inrush current detector was de-veloped to detect characteristic current waveforms duringtransformer energizing. This algorithm has been imple-mented in all of the Rogowski coil-based EAF transformerprotection applications.

Backup Ground Fault Element: Many EAF facilities applyground overcurrent elements on conventional CT-basedrelays at the vault vacuum switch and/or at the substationcircuit breaker. In one application, the plant was experi-encing nuisance operations of the instantaneous groundelement (50N) that was using three iron-core CTs in thethree phases. The EAF transformer was connected in Deltaon the primary side; thus, there should not be residualcurrent flowing on the primary circuit from the transformer.The 50N device was set at 2000 primary amperes, and therewere a number of nuisance operations during transformerenergizing; thus, the operator disabled this protection func-tion. The cause of the relay nuisance operation was the CTsaturation during transformer energization, producing falsesecondary residual currents.

The Rogowski coil sensor-based primary side backup groundelement can be set to operate at low fault currents, eliminatingthe nuisance operation. In the same application, the groundelement was set to 500 primary amperes, which is 25% of theCT-based ground relay setting that caused nuisance operations.Rogowski coil-based ground fault protection did not experienceany nuisance operation during transformer energizing or oper-ation since the protection system installation. This protectionfunction provides backup for the differential (87-1) element inthe event of a line-to-ground fault event during energizationwhen an inrush detector might be blocking the 87-1 element.

IV. OPERATING EXPERIENCE

Reliable performance of the power transformer protectionmust be preserved for both in-zone faults (defined as the schemedependability) and external (out-of-zone) faults (defined as thescheme security).Dependability: For EAF transformers, faults at the transformer

secondary have current magnitudes that are very muchthe same as the load currents. Therefore, only differentialprotection schemes can provide reliable fault identificationand protection.

Security: For EAF transformers, load currents can exceed100 kARMS, currents are very unbalanced and containsignificant amount of harmonics. Therefore, for EAF

KOJOVIC et al.: DIFFERENTIAL PROTECTION OF POWER TRANSFORMERS USING CURRENT SENSORS 1977

Fig. 18. Idea relay event recording of the fault (relay operated in one-halfcycle, circuit breaker operated in 4 cycles).

transformers, normal operation can be considered as sim-ilar to permanent out-of-zone faults for a conventionalpower transformer. In addition, inrush currents must bereliably detected to provide the scheme security. This isparticularly important for an EAF application with perhaps100 energization inrush events per day.

A. Scheme Dependability

The first differential protection system was put in service in2004. Other systems have followed since the first installation.Over 12 fault events have been experienced either in test modeor in actual operation at the variety of installations. A summaryof several fault events is discussed here.

Fault Event 1: On March 28, 2007, a fault occurred at theEAF transformer secondary terminals caused by waterleaking from the roof in the EAF transformer vault. Thewater leak caused a phase-to-phase fault in the secondarybus, quickly escalating and eventually leading to a shortcircuit on the 34.5-kV side of the transformer. The transferof the fault from secondary to primary was due to theproximity of the terminals and the plasma burst created bythe secondary fault current, estimated to be approximately250 kA. The fault was detected by the differential relayprotection, and the relay closed its output contact withinone-half cycle, initiating circuit breaker operation. Thecircuit breaker operated in 4 cycles. Fig. 18 shows the eventrecord captured by the relay. Because of this fast relayoperation, the EAF transformer was not damaged. Aftercleaning the affected area, the transformer was energizedin less than 6 hours after the fault, and the production ofsteel continued (see Fig. 19). The effectiveness of the EAFtransformer differential protection based on Rogowski coilcurrent sensors in preventing severe damage can be prop-erly understood by comparing this fault event with a similar

Fig. 19. EAF secondary terminals flashover (fast tripping, resulting in mini-mal damage).

event that occurred in 2002 when EAF transformers werenot equipped with differential protection. At that time, theinstantaneous and time-delayed overcurrent relays in thesubstation were used to detect a fault condition in theEAF vault. The time-delayed relays resulted in longer faultevents due to settings that minimized nuisance operationsfor inrush and bore-in current magnitudes. Because oflonger exposure to the electric arc caused by the fault, theEAF transformer had to be replaced, resulting in severalmillion dollars in repair and lost production costs.

Fault Event 2: On April 4, 2007, a second flashover was experi-enced in the Delta closure assembly in the EAF vault. The87-1 element detected the fault and closed the trip contactsin approximately 2 cycles. The fault occurred in the clamp-ing assembly where the flexible bus sections are clampedto the delta closure. The fast tripping resulted in minimaldamage to the bus bars, but there was some resultingblackening of the insulating boards between the energizedbus bars. At the discretion of the melt shop maintenancepersonnel, the clamping assembly was disassembled, andnew insulation boards were used in the assembly. The out-age lasted about 12 hours, to do the required replacementwork. The flashover was blamed on contamination fromdust settling on the clamping assembly.

Fault Event 3: On January 21, 2007, there was a clampfailure on one of the phase conductors connected to thevacuum switch serving the EAF at the plant. The looseconductor started arcing and eventually flashed over phase-to-ground and, finally, developed into a multiple phasefault. The 87-1 element operated with some delay due toharmonic content in the fault current. The phase currentsreflected in the event file had magnitudes of approximately21–25 kA. The system serving the vault is solidly groundedin the substation; thus, the residual current is also approx-imately 20 kA during the fault. The instantaneous groundovercurrent relay on the substation feeder circuit breakerdetected the fault, and cleared it before the differentialelement operated.

Fault Event 4: On August 7, 2007, there was a failure inthe diverter in the LTC tank on the EAF transformer.The overheating resulted in pressure in the LTC partof the tank, and ultimately, a pressure relief deviceoperated. The hot carbonized oil sprayed into the primaryenergized bus and resulted in a flashover on the primary

1978 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 49, NO. 5, SEPTEMBER/OCTOBER 2013

34.5-kV side of the transformer. The operation of thepressure relief device automatically issued a trip signal tothe substation circuit breaker. Prior to the fault event, thesecondary current flowing out of the vault was between 102and 126 kA, and the primary currents were approximately3200–4000 amperes per phase. The differential elementswere balanced in this period of operation despite the veryhigh currents flowing through the zone of protection. The87-1 element operated in slightly less than 2 cycles afterthe fault was initiated.

B. Scheme Security

The new implementation is running on all Rogowski coil-based transformer differential applications, and to date, allschemes preserved security without nuisance operations onenergization or during heat cycles. At the time of this writing,there have been over 450 000 energization and heat cycles onthe multiple EAF systems in operation.

V. CONCLUSION

This paper has presented novel relay protection systemsthat are the first differential protection systems applied toEAF transformers in the United States and quite possibly inthe world. The protection systems consist of high-precisionPCB Rogowski coil current sensors and multifunction relays.Operating experience over many years has confirmed superiorperformance of the protection solution.

Scheme dependability: In all fault events that occurred sincethe protection implementation faults were cleared fast, resultingin minimal damage to equipment despite high fault currentmagnitudes (250 kA). Production resumed within hours, savingsubstantial time and money.

Scheme security has been preserved for an extraordinarynumber of EAF transformer energizing and heat cycle opera-tions that are over 450 000 energizations over many differentapplications. Today, there are a number of differential pro-tection schemes developed and implemented for protection ofpower transformers, lines, cables, and busbars.

Note: Since this paper was first published in 2009, severalin-zone faults occurred, and in all these cases, EAF differentialprotection operated fasts. As a result, only minimal damagewas caused to equipment despite high fault current magnitudes.Production continued within several hours, after cleanup of thearcing debris.

As of now, EAF transformers exceeded over 1 000 000 ener-gizing and heat cycle operations since installation of the firstsystem without unwanted operation (preserving high securityof the protection systems).

REFERENCES

[1] Requirements for Instrument Transformers, IEEE Std. C57.13-2008, 2008.[2] IEEE Guide for the Application of Rogowski Coils Used for Protective

Relaying Purposes, IEEE Std. C37.235-2007, 2008.[3] Lj. A. Kojovic, “Rogowski coil performance characteristics for advanced

relay protection,” in Proc. 9th Int. Conf. DPSP, Glasgow, U.K., Mar. 17–20,2008, pp. 406–411.

[4] Lj. A. Kojovic, M. T. Bishop, D. Sharma, and C. Birkbeck, “Application ofdifferential protection on electric arc furnace and substation transformers,”presented at the Iron Steel Technology Conf. Expo., Pittsburgh, PA, USA,May 5–8, 2008, Paper PR-353-080.

Ljubomir A. Kojovic received the Ph.D. degreein electric power systems from the University ofSarajevo, Sarajevo, Bosnia and Herzegovina.

He is currently a Technical Fellow with theThomas A. Edison Technical Center, Eaton Cor-poration, Franksville, WI, USA. He is an AdjunctAssistant Professor with Michigan TechnologicalUniversity, Houghton, MI, USA. He is the holderof 15 U.S. patents and is the author of more than200 technical publications.

Dr. Kojovic is a Registered Professional Engineerin the State of Wisconsin. He is a Senior Member of the IEEE Power Engineer-ing Society, a member of the IEEE Power System Protection Committee, anda member of the International Council on Large Electric Systems, ConferenceInternationale des Grands Réseaux Électriques. He is a Technical Advisor forthe U.S. National Committee at the Technical Committee TC-38 InstrumentTransformers of the International Electrotechnical Commission.

Martin T. Bishop received the B.S. and M.Eng.degrees in electric power engineering from Rensse-laer Polytechnic Institute, Troy, NY, USA. He alsoreceived the M.B.A. degree from the Keller GraduateSchool of Management.

His first position was with Westinghouse ElectricCorporation, Pittsburgh, PA, USA, in the AdvancedSystems Technology Division where he worked on avariety of R&D projects including relay developmentfor high-impedance fault detection. He worked forapproximately 20 years in the Systems Engineering

Department at Cooper Power Systems where he supervised a group thatperformed industrial studies, power quality measurements, and designed filtersystems for large industrial plants. He also participated in the development,application, and marketing of new relay protection systems using Rogowski coilcurrent sensors. In 2010, he joined S&C Electric Company, Franklin, WI, USA,as a Senior Marketing Manager within the Strategic Solutions Business Unit.His group works on application studies for the marketing of the DSTATCOMand Energy Storage projects, as well as distribution automation applications.

Dharam Sharma received the B.Tech. degree inelectrical engineering from the Indian Institute ofTechnology Bombay, Mumbai, India, in 1970.

He has a combined experience of 17 years in threereputable Canadian Steel companies (i.e., Ivaco,Gerdau, and IPSCO). In 1991, he moved to theUSA when he joined Nucor-Yamato Steel Company,Blytheville, AR, where he is currently a Staff En-gineer with the Electrical Maintenance Department.His experience is in the fields of electric arc fur-nace technology, power distribution, HV substation,

power quality, and static var compensator systems.