B Y C H A R L E S J . M O Z I N A & D O U G L A S C . M O O D Y

LECTRICAL GENERATION AT PAPER

mills is becoming an increasingly critical as-

set and should be well protected from faults,

system upsets and abnormal operating con-

ditions. The loss of a major generating unit for an extended

period would result in very costly replacement power and re-

pair costs for any paper mill. This article presents the reasons

why mill generator owners should consider upgrading the

electrical protection of their generators to meet todays stan-

dards. It specifically outlines the risks assumed in protection

areas where 20-year-old (and older) generator protection is

inadequate. The article relates the experience of completed

protection upgrade projects at MeadWestvacos Luke Mill

in Maryland, USA, where 1960-vintage electromechanical

relays were replaced with modern digital protection. This

article points out the advantages of this technology versus

conventional electromechanical protection and also dis-

cusses the unique application considerations in applying

digital protection.

Background

Contrary to popular belief, generators do experience short

circuits and abnormal electrical conditions. In many cases,

equipment damage due to these events can be reduced or

prevented by proper generator protection. As generators

become older, the likelihood for failure increases as insula-

tion begins to deteriorate. Generators, unlike some other 37

1077-2618/03/$17.002003 IEEE

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The ADVANTAGES of DIGITALversus ELECTROMECHANICAL RELAYING

for mill generator upgrade projects.

power system components, need to beprotected not only from short circuits,but also from abnormal operating condi-tions. Examples of such abnormal condi-tions are overexcitation, loss-of-field,and unbalanced currents. When sub-jected to these conditions, damage orcomplete failure can occur within sec-onds, thus requiring automatic detec-tion and tripping.

In the late 1980s, the IEEE Power Sys-tem Relay Committee first issuedANSI/IEEE C37.102, the guide for theprotection of synchronous generators [1].Most of the recommended practices applicable to indus-trial-sized generators have been incorporated in the latest re-vision of The IEEE Buff Book [2]. These industry guidesoutline current recommended practices for the protection ofgenerators and document the substantial changes that haveoccurred in generator protection over the last 20 years. Thesechanges fall into three broad categories: improved sensitiv-ity, new protection areas, and special protection applica-tions. These are the key functional areas that need to beaddressed when developing an upgrade program to bringgenerator protection up to current industry standards.

The protection at the Luke Mill is a typical example ofgenerator protection installed in the 1960s, employingelectromechanical relays that remained basically un-changed since they were commissioned decades ago. Theaging protection system provided minimal alarming withtrip data limited to relay flags that could be hastily resetwith a total loss of critical relay tripping information.Planned generator breaker upgrade projects for two13.8-kV, 40-MVA generators afforded the opportunity toeconomically install microprocessor-based digital relayingand move to modern generator protection. The upgrade onthe first unit was completed in 1996, and the second unitwas completed in 2001.

Significant improvements in protec-tion using multifunction digital relaysare described in the first sections of thisarticle. The protection system improve-ments applied by Luke Mill are discussedin detail with several benefits to millgeneration operations noted. The processof upgrading generator protection af-forded an excellent learning experiencefor mill personnel in understanding theprotection scheme, both from an engi-neering and operations standpoint. Thenew Luke Mill generator relays are lo-cated a considerable distance from the

plant control room. The communications capabilities ofthe newly installed relays were used to remotely display re-lay data to the control room for rapid operator analysis.

Areas of Protection Upgradeon Older Mill GeneratorsThe areas of upgrade on generator protection that is 20years old or more fall into three broad categories: Improved sensitivity and reduction of damage in

protection areas where older relaying does not pro-vide the level of detection required. Examples of pro-tection in this area are the following: stator ground and ground differential protection field ground fault protection dual-level loss-of-field protection negative sequence (unbalanced current) protec-

tion sensitive overexcitation protection.

New or additional protection in areas that 20 years agowere not perceived to be a problem but operating expe-riences have since proved otherwise. These areas are: inadvertent generator energizing sequential tripping oscillographic monitoring.

Application considerations of multifunctional digi-tal relays that are unique to generators.

Improved Sensitivity and Reduced Damage

Stator Grounding andGround Differential ProtectionWhen a generator stator ground fault is detected by protec-tive relays, the generator is shut down by tripping the gener-ator breaker, field breaker, and turbine. The systemcontribution to the fault will immediately be removed whenthe generator breaker trips, as illustrated in Figure 1. Thegenerator stator ground current, however, will continue toflow after the tripping. The generator short-circuit currentcannot be turned off instantaneously because of the storedenergy in the rotating machine. The flow of damaging gen-erator fault current will continue for several seconds after thegenerator has been tripped. This long decay time results inthe vast majority of the damage occurring after tripping [3].Reducing the decay time is very difficult; however, reducingthe fault current during the generator coast-down can bedone. As machines get older, the possibility of stator groundfaults increase. Reducing the damage, therefore, becomes a38

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Current

Igen Isystem

PowerSystem

Isystem

Igen current ground decay

Time (s)

GeneratorBreaker Trips

0

G

Generator ground-fault relay.

1

ELECTRICALGENERATION ATPAPER MILLS ISBECOMING ANINCREASINGLYCRITICAL ASSET.

major objective. Recent highcosts and long outages of mill gen-erators have caused engineers toponder the problem of reducingground fault damage during gen-erator coast-down. The mostpromising solution is called hy-brid generator grounding.

In mill applications, generatorsare directly connected to a bus thatservices the local load. Figure 2 il-lustrates this type of configuration.Hybrid grounding can be typicallyapplied to these types of generators.

The generator is both high- andlow-resistance grounded. Undernormal operating conditions, bothgenerator ground sources are oper-ated in parallel. For ground faultson the mill system, the ground faultcontribution from the generatorwill typically be almost entirelyfrom the low-resistance (200-400A) source. This provides the re-quired level of system ground cur-rent for proper mill ground relayoperation, allowing the generator tosupply the mill load when the util-ity system is unavailable (breakersA and B open). When there is aground fault in the generator statorwindings or associated bus connec-tion to the generator breaker, theground differential (87G) will oper-ate to initiate a unit shutdown. Aspart of the generator tripping, the ground interruption de-vice in series with the low resistance path is tripped, typi-cally reducing ground current to below the 10-A level. Thisgreatly reduces stator ground fault damage during the gen-erator coast-down. This is a relatively new idea and was re-cently implemented on one of the two Luke Mill generators.See [3] for more details on hybrid grounding.

Mill generators are generally grounded through a resis-tor in the generator neutral as described above. Sensitivedetection of stator ground faults can be substantially in-creased through the addition of an 87G ground differentialrelay that uses a product approach utilizing the followingequation. The relay operating characteristic is:

( )I IOPP O N= ,31 coswhere

31O is the residual current from the bus side CTs IN especially, electronic is the generator neutral cur-

rent is the phase angle between the currents.The scheme is illustrated in Figure 3. The use of digital

technology allows the scheme to be applied using the nor-mal complement of generator CTs without the need for aux-iliary CTs. The 87G was an upgrade area on the Luke Millgenerators that previously had only 87-phase differential 39

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Utility System

Typically200-400 A

R

A

Industrial System Bus

To OtherBus Sections

Mill Load

87G

51G

G

59NGround

InterruptingDevice Typically

200-400 A

Low-ResistanceGround

High-ResistanceGround(Typically LessThan 10 A)

R

B

Hybrid generator grounding.

2

3I0

( 3IO)INcos0

IN

87 - GeneratorDifferential87G - GeneratorGround Differential

51G - Neutral Overcurrent

51G

87 87G

G

R

Sensitive stator ground fault protection.

3

protection. This change substantially im-proved stator ground fault sensitivity.

Field Ground Fault DetectionThe field circuit of a generator is an un-grounded (typically 600 V) dc system, asshown in Figure 4. A single field groundfault will generally not affect the opera-tion of a generator, nor will it produceany immediate damaging effects. How-ever, the probability of a second groundfault occurring is greater after the firstground fault has established a groundreference. When a second ground faultoccurs, a portion of the field windingwill be short-circuited, thereby produc-ing unbalanced air-gap fluxes in the ma-

chine. These unbalanced fluxes produceunbalanced magnetic forces that result inmachine vibration. A field ground faultalso produces rotor iron heating from theshort-circuit currents. The tripping prac-tices within the industry for field groundrelaying are not well established. Someusers trip while others prefer to alarm,thereby risking a second ground faultand major damage before the first groundis cleared.

The existing practice within the in-dustry has been to use dc voltage relayingto detect field ground faults. These volt-age schemes have been prone to false op-erationespecially during start-up.Unit operators routinely reset the alarmand continue with start-up procedures. If

a persistent alarm occurred, operators attempted to locatethe problem.

A more secure field ground relay is desirable if auto-matic tripping is being considered. Such a relay is shown inFigure 5 and uses an injection principle. This principle hasbeen widely used in Europe with great success, but, untilrecently, it was not available in a multifunction relay. As il-lustrated in Figure 5, a 15-V square wave signal is injectedinto the field through a coupling network. The return sig-nal waveform is modified due to field winding capacitance.The injection frequency setting is adjusted (0.1 to 1.0 Hz)to compensate for field winding capacitance. From the in-put and return voltage signals, the relay calculates the fieldinsulation resistance.

The injection scheme provides a major improvementover traditional voltage schemes in terms of both sensitivityas well as security. In addition, digital relays can providereal-time monitoring of field insulation resistance, so deteri-oration with time can be monitored. The scheme can also de-tect grounds on an offline generator, allowing the operator

to determine if the field circuitry isfree of a ground before start-up.This 64F protection was an upgradearea on the Luke Mill generators.An added benefit of the injectionscheme described above is that itoperates at a low voltage (15 V)compared to the scheme it replaced(120 V), thus improving operatorsafety when changing brushes withthe unit online.

Dual-LevelLoss-of-Field ProtectionPartial or total loss-of-field on asynchronous generator is detri-mental to both the generator andthe power system to which it isconnected. The condition mustbe quickly detected and the gen-erator isolated from the system toavoid generator damage. Whenthe generator loses its field, it op-40

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Exciter

Brush

Field

FieldBreaker

BrushGrounding

Brush

Basic generator field circuit.

4

DigitalProtective

Relay

Processor

Field GroundDetection

SquarewaveGenerator

MeasuringCircuit

InjectedSignal

ReturnSignal

CouplingNetwork

C

C

R

R

+

Gen.Rotor

Ground

MachineFrameGround

R

Field ground protection using an injection voltage signal.

5

A MORE SECUREFIELD GROUND

RELAY ISDESIRABLE IFAUTOMATICTRIPPING IS

BEINGCONSIDERED.

erates as an induction generator, causing rotor heating.A loss-of-field condition that is not detected can have adevastating impact on the mill system by causing a lossof reactive power support as well as creating a substan-tial reactive power drain. If this is not quickly detectedon large generators, this condition can trigger a millvoltage collapse.

The most widely applied method for detecting a gener-ator loss-of-field is the use of an impedance measuring re-lay . The loss-of- f ie ld re laymeasures the impedance as viewedfrom the machine terminals, and itoperates when the impedance fallsinside the circular characteristic.The relay ohmic characteristic isoffset from the origin by one-halfof the direct axis transientreactance ( X d 2) to preventmisoperation during system dis-turbances and other fault condi-tions. The diameter of the circle isadjusted to be equal to the directaxis synchronous generatorreactance. A time delay is used toprovide security against falsetrippings on stable power swings.This time delay increases the oper-ating time of the relay, whichmeans that the Mvars drawn by thegenerator persist for a longer time,making the mill system more sus-ceptible to severe voltage dips. Many users have upgradedto modern two-zone impedance relays to enhance protec-tion. This scheme is shown in Figure 6. The innersmall-impedance circle is set to trip with only a few cyclesdelay and is within the impedance locus trajectory for mostloss-of-field events. The fast operation of the inner imped-ance unit quickly detects a loss-of-field condition. Thiswas an upgrade area on the Luke Mill generators, where thefast tripping by the inner circle characteristics prevented amill shutdown, which is described in later in this article.

Negative Sequence(Unbalanced Current) ProtectionThere are a number of system conditions that can cause un-balanced three-phase currents in a generator. These systemconditions produce negative sequence components of cur-rent that induce a double-frequency (120 Hz) current onthe surface of the rotor. The skin effect of the double-fre-quency rotor current causes it to be forced into the rotorsurface, causing excessive rotor temperatures in a very shorttime. The general flow of this current in a cylindrical ma-chine rotor is shown in Figure 7. The current flows acrossthe metal-to-metal contact of the retaining rings to the ro-tor forging wedges. Because of the skin effect, only a verysmall portion of this current flows in the field windings.Excessive negative sequence heating beyond rotor thermallimits results in failure. These limits are based on the fol-lowing equation, for a given generator:

K I t= 22 ,

where K is the constant depending on generator design and

size t is the time in seconds I2 is the root-mean-square (rms) value of a negative

sequence current per unit of generator rating.The continuous unbalanced current capability of a gen-

erator is defined in [5] and shown in Table 1. For open-con-ductor or open-breaker pole conditions, the negative

41

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RetainingRing

WedgeLockingRing

FieldWinding

Unbalance currents in the rotor surface.

7

TABLE 1. CONTINUOUS UNBALANCED CURRENTCAPABILITY OF A GENERATOR.

Type of Generator:Cylindrical rotor

Permissible I2(percent of stator rating)

Indirectly cooled 10

Directly cooledto 960 MVA 8

Impedance Trajectoryon Loss-of-Field

Machine CapabitlityMinimum Exciter Limit

Light LoadHeavy Load

Xd

R

X d2

1.0 p.u.

+R

X

+X

Modern loss-of-field using two-zone offset impedance method.

6

sequence relay is usually the only protection. The low mag-nitude of negative sequence currents created by this type ofevent (typically 10-33% of stator rating) prevents otherfault relays from providing protections. For electrome-chanical negative sequence relays, the minimum pickupcan be set to only about 60% of the stator-rated current

sensitivity. Thus, these relays will provide no protection foropen-phase or open-generator breaker pole conditions thatare frequent negative-sequence events within the industry.The sensitivity of negative-sequence static or digital relaysis required. Almost all generators that are over 20 years oldare protected with electromechanical negative-sequencerelays, which make this an important upgrade area. Thiswas an upgrade area on the Luke Mill generators.

Sensitive Overexcitation Volts-per-Hertz ProtectionOverexcitation, or volts-per-hertz, relaying is used to protectgenerators from excessive magnetic-flux density levels. Highflux-density levels result from an overexcitation of the genera-tor. At high flux levels, the magnetic iron paths designed tocarry the normal flux saturate and flux begins to flow in leak-age paths not designed to carry it. These resulting fields areproportional to voltage and inversely proportional to fre-quency. Hence, high flux-density levels (and overexcitation)will result from overvoltage, underfrequency, or a combina-tion of both. Although overexcitation protection has beenrecommended by manufacturers for many years, it is not in-stalled on many mill generators that rely solely on overvoltageprotection. ANSI/IEEE Standard C50.13 has established1.05% (generator base voltage) volts-per-hertz limits for con-tinuous operation. For values above these levels, generatorshave short-time operating limits.

Damage due to excessive volts-per-hertz operation mostfrequently occurs when the unit is offline prior to synchro-nization. The potential for overexcitation of the generatordramatically increases if the operators manually preparethe unit for synchronization. This is particularly true if theoverexcitation alarms are inadequate or if the voltage trans-former (VT) has an open circuit due to an improper connec-tion. Modern digital relays provide improved protectionusing both definite-time as well as inverse-time character-istics to closely match the short-time overexcitation char-acteristics of a generator. Volts-per-hertz protection wasadded to the Luke Mill generators, and it immediately paiddividends by detecting a recurring volts-per-hertz condi-tion during generator shutdown.

New or Additional Protection Areas

Inadvertent Generator EnergizingInadvertent or accidental energizing of synchronous gener-ators has been a particular problem within the industry inrecent years. A number of machines have been damaged or,in some cases, completely destroyed when they were acci-dentally energized while offline. The frequency of these oc-currences has prompted the industry to recommend thatthe problem be addressed through dedicated protective re-lay schemes. Operating errors, control-circuit malfunc-tions, or a combination of these causes, have resulted ingenerators becoming accidentally energized while offline.In industrial applications, the major cause of inadvertentenergization of generators is the closing of the generatorbreaker through the mechanical close/trip control at thebreaker itself, thereby defeating the electrical interlocks.

Due to the severe limitation of conventional generatorrelaying to detect inadvertent energizing, dedicated pro-tection schemes have been developed and installed. Unlikeconventional protection schemes that provide protection42

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(b)

50Overcurrent

I p.u.>

27Undervoltage*

V p.u.