PT 6002 Alternator Pt2 En

7/29/2019 PT 6002 Alternator Pt2 En

1/6

Alternator protection, part 2:Alternatives

Power topic #6002 Part 2 o 3 | Technical inormation rom Cummins Power Generation

Generators sets are commonly to be protectedrom the eects o overload conditions byequipment that is required by local codes andstandards. The standards generally do notstipulate what type o protection device thatis required, so a system designer can selectprotective devices based on the needs o theapplication and the preerences o the user.This document provides an overview o over-

load protection alternatives that are availableor generator sets.

Power topic #6002 Part 1 identifed requirements or

protection o alternators based on North American codes.

Inherent overcurrent protection

The US National Electrical Code Handbook (page606) notes that generator sets can be designed so thata short term overcurrent condition causes a collapseo the voltage on the output o an alternator. This limitsthe current and output kW o the alternator during an

overload or short circuit condition so that it can consid-ered inherently sel-protected.

In practice, this can be achieved with a shunt-typeexcitation system design. In this type o generatorexcitation system, both the sensing and power leads othe voltage regulator are connected to the output o thealternator. When an overload or short circuit occurs, thecollapsing voltage on the power supply to the voltageregulator eectively limits the power available to thealternator excitation system, thus causing alternatoroutput voltage to collapse until the overload is removedrom the system.

The weakness o this design is that shunt-type systemstypically utilize single phase sensing voltage regulators,so a single-phase ault or overload may not cause thesystem to collapse immediately. I a ault or overloadoccurred on a phase which is not used or sensing orregulation power, the overcurrent condition would bemaintained until the ault spread ar enough to aectthe power supply to the voltage regulator. By that time,alternator damage may have occurred.

Shunt-type excitation systems have other weaknesses

related to motor starting and coordination (discrimina-tion) on generator sets, so they are not oten specifed,particularly or large or critical systems. However, theirhistorical acceptance without other overcurrent protec-tion is a precedent or acceptance o other types oinherent protection or alternators.

Other inherent protection systems are available whichaddress the concerns stated here or shunt-type systems.

Circuit breaker protection

Molded case breakers with

thermal-magnetic trip units

Generator sets that are rated or operation at less than1000 volts, and 1200 amps and smaller are oten pro-vided with a molded case circuit breaker or alternatorprotection. The molded case breaker is usually providedwith a thermal-magnetic trip unit. The conventionalwisdom is that this device will protect the alternatorand the eeder connecting it to the frst level o distribu-tion, and these devices are oten accepted or alternatorprotection by authorities. However, it is difcult (i notimpossible) to design an eective alternator protectionsystem using this equipment.

> White paper

By Gary Olson, Technical Counsel


2/6

www.cumminspower.com

2007 Cummins Power Generation

Power topic #6002 Part 2 o 3 | Page 2

In FIGURE 1 you can see an illustration o a typicalgenerator set thermal damage curve that has beenoverlaid on a time overcurrent characteristic curve ora molded case circuit breaker with thermal-magnetictrip unit. A problem is apparent rom the frst glance atthe chart: The thermal damage curve o the alternatoroverlaps the trip curve o the circuit breaker. For properprotection the damage curve must be entirely to theright o the trip curve o the breaker, illustrating that orany current level, or any time duration, the breaker will

positively trip beore alternator damage will occur.

In general, a molded case breaker that is sized to carrythe ull output o a generator set on a continuous basiswill overlap the alternator thermal damage curve. Use oa larger breaker would make the situation even worse.

I the generator set is exposed to a bolted ault closeto the terminals o the generator set, the circuit breakermay trip in its instantaneous region as much as severalseconds beore it needs to trip to protect the alternator.Tripping in the instantaneous region o the breakercurve can also occur due to surge currents rom start-ing o large motors, or due to transormer magnetizing

current. In many cases, this would be considered anuisance trip, because there is no potential damageto any part o the system, but the breaker has clearedanyway. I this happens in the real world, the operatorwould probably re-adjust the trip curve o the gensetbreaker to delay tripping, thus making the trip curvemove urther to the right, and providing even lessprotection to the alternator.

1000

100

10

1

0.1

0.0110

20

50

100

200

500

1000

2000

5000

10000

20000

50000

100000

ALTERNATOR THERMAL DAMAGE CURVE

AND MOLDED CASE BREAKER TRIP CURVE

Current in Amperes at 480 Volts

TimeinSeconds

GE 800A Molded Case

Circuit Breaker

Generator

FLA, 752A

Maximum

Instantaneous

Trip Setting (8X)

Minimum

Instantaneous

Trip Setting (3X)

Generator

Damage

Curve

FIGURE 1 At current levels less than 2000 amps the molded case

breaker cannot provide protection or the alternator.

North American requirements for generator

(alternator) protection

The 2005 US National Electric Code (NEC), NFPA 70,makes the ollowing reerences to generator protection:

240.21 Location in circuit: (G)Conductors from

Generator Terminals. Conductors rom generator ter-minals that meet the size requirement in Section 445-13shall be permitted to be protected against overload bythe generator overload protective device(s) required bySection 445-12.

445.12 Overcurrent protection: (A)Constant VoltageGenerators. Constant voltage generators, except ACexciters, shall be protected rom overloads by inherentdesign, circuit breakers, uses, or other acceptableovercurrent protective means suitable or the conditionso use. Exception to (A) through (E): Where deemed by the

Reprinted with permission rom NFPA 70HB-2008, National Electric Code Handbook 2008, National Fire Protection Association, Quincy, MA 02169. This

reprinted material is not the complete and ofcial position o the NFPA on the reerenced subject, which is represented only by the standard in its entirety.

authority having jurisdiction, a generator is vital to the operationof an electrical system and the generator should operate tofailure to prevent a greater hazard to persons. The overload

sensing device(s) shall be permitted to be connected to an an-nunciator or alarm supervised by authorized personnel insteadof interrupting the generator circuit.

445.13 Ampacity o conductors: The ampacity othe conductors rom the generator terminals to the frstdistribution device(s) containing overcurrent protectionshall not be less than 115 percent o the nameplatecurrent rating o the generator. It shall be permitted tosize the neutral conductors in accordance with Section220.61. Conductors that must carry ground aultcurrents shall not be smaller than required by Section250.24(C) Exception: Where the design and operation of the

generator prevent overloading, the ampacity of the conductorsshall not be less than 100 percent of the nameplate currentrating.


3/6




I a low level overcurrent condition occurs the alterna-tor will be damaged long beore the breaker trips toprotect the alternator. For example (see FIGURE 1), witha 1000 amp load, the alternator will be damaged ater300 seconds, but the breaker would not trip until thecondition had lasted at least 1000 seconds.

Clearly, a molded case breaker with a thermal-mag-netic trip, sized or the ull output o the genset, doesnot provide eective alternator protection under allpotential ailure modes and conditions. There is anotheracet to the problem, and that is the reliability o serviceto loads when using molded case breakers.

Molded case breakers with thermal-magnetic trip unitsare generally not continuously rated devices. Unlessa breaker is specifcally listed and labeled to carry a100% rating, it will carry only 80% o its nameplaterating on a continuous basis. Use o a breaker that isequal in rating to the generator output rating can limit

the ability o the generator set to operate at high currentlevels (which it is designed to do) or extended periods

o time (particularly at high ambient temperatures). Ithe generator set is operating at a high load level, themolded case breaker can trip in the 80-100% currentrange, especially in high ambient conditions, causingan unnecessary interruption o service to critical loads.

Some designers attempt to address the derating

problem by application o oversized breakers or byuse o continuously rated breakers. Note that a largercapacity breaker would probably trip later, makingprotection less eective or the alternator. It also wouldrequire larger generator eeder conductors that addcost to the installation because the eeder conductorsize and the breaker must be coordinated to protectthe eeder. Any continuously rated breaker would stillneed to be matched to the thermal damage curve o thealternator in order to protect the alternator through alloverload conditions.

Another potential solution is to use multiple smaller

breakers to eed smaller transer devices or systemloads. I a molded case breaker with thermal-magnetictrips is rated or approximately hal the rating o thealternator, it will generally protect the machine throughall overcurrent levels and durations. (The designershould veriy this or each application by comparison othe breaker trip curves and alternator thermal damagecurve.) This strategy can be used where the loads ona generator set can be split and ed to multiple circuitsdirectly rom the generator set.

However, in general we can say that the most com-monly specifed and applied protective system or low

voltage generator sets (molded case/thermal-magneticbreakers) will not provide necessary protection underall circumstances, and will subject the power systemto nuisance power ailures due to unnecessary breakertripping.

An alternative that can be investigated is the use ogenerator type circuit breakers. These breakers areavailable rom a ew manuacturers, and allow bettercoordination o the genset thermal damage curve bymodiying the characteristic trip curve to be more likethe genset curve.

Molded case (and other) breakers

with solid state trip unitsBetter alternator protection can also be achieved by useo breakers with more capable trip units. In FIGURE 2an 800 amp insulated case breaker with solid state tripis overlaid on the alternator thermal damage curve. Inthis case the breaker characteristic curve shape moreclosely resembles the thermal damage curve o the

1

1000

100

10

0.1

0.0110

20

50

100

200

500

1000

2000

5000

10000

20000

50000

100000

ALTERNATOR PROTECTION

USING A MOLDED CASE BREAKER WITH SOLID STATE TRIP UNIT

Current in Amperes at 480 Volts

TimeinSeconds

Potential

Problem Area

Generator

FLA, 752A

GE 800A RMS-9

Programmer

Generator

Damage

Curve

FIGURE 2 This breaker provides better protection than one with a

thermal-magnetic trip unit.


4/6




alternator and it alls almost completely to the let o thealternator damage curve, so much better protection isprovided to the generator set. There is the potential ordamage due to a sustained low level overload conditionor an extended period o time.

Insulated case breakers are not oten applied on

generator sets since they are considerably moreexpensive than molded case breakers, and may be toolarge or ragile or direct mounting on many generatorsets. Most molded case breakers are available withsolid state trip units (usually higher ampacities). Theseallow the designer to more closely match the breakeroperation characteristic to the alternator provided.However, they do need to be adjusted or each alterna-tor, and they may not provide complete protection insome operating regions, or may be misadjusted inthe feld due to what are perceived by an operator asnuisance trips.

All breakers that include instantaneous trips aresusceptible to nuisance tripping under some surge loadconditions on a generator set. (Tripping that occurs dueto peak inrush current which is less than the thermaldamage limit o the alternator, but higher than theinstantaneous pickup setting.)

A fnal point: The natural reaction to a breaker trip(and generator shutdown) by most operators wouldbe to reset the breaker and immediately restart themachine. I the ault were still present on the machine,the machine could be damaged on the second opera-tion. This is because the breaker would theoreticallytake the same time to clear as on initial trip, but sincethe machine is at higher internal temperatures at thestart o the ault, it would reach a higher than expectedtemperature, potentially damaging the machine. Thisproblem is present in all breakers, and most overcur-rent relays.

Overcurrent protective relaysSpecifc overcurrent protective relays could be providedor alternator protection. These are oten used oralternators operating at over 1000 volts AC. Relaysare available in the marketplace that will provide goodprotection or the alternator, and may include overcur-rent, dierential overcurrent, zero or negative sequencedevices. The drawback to these devices is cost, bothin terms o the design (device selection and settingsdecisions on the part o the system designer) andinstallation o the relay(s) selected. For small generatorsets the cost o a installed utility grade relay couldapproach the cost o the generator set! Installation cost

increases may be signifcant, since many protectivedevices are not designed or mounting or long termoperation on a generator set due to vibration or harshtemperature extremes that are common on a generatorset. Depending on the devices chosen, maintenancecosts may be increased due to the need or calibrationand other maintenance on the relays compared to otherprotective devices.

Another solution

Cummins PowerCommand generator sets withAmpSentry protective relay provide the alternator

protection that is required by codes and standards andby acility owners desiring good alternator protection.Because AmpSentry protective relay is specifcallydesigned or alternator protection, it can providenecessary protection without the compromises andlimitations that are necessary with other approaches.(FIGURE 3)

FIGURE 3 PowerCommand generator sets include an overcurrentand short circuit protection algorithm that is designed specifcally or

alternator protection. The I2t portion o the curve is matched to the

alternator provided on the genset.

1000

100

10

1

0.1

0.01

0

1

0.05

0

3

1

0

10

0

AMPSENTRY PROTECTIVE RELAY WITH

DECREMENT CURVE AND ALTERNATOR THERMAL DAMAGE CURVE

Current in Amp (Times Rated)

TimeinSeconds

Molded Case Circuit Breaker

With Thermal-Magnetic Trip

Alternator Thermal

Damage Curve

Amsentry Protective Relay


5/6



AmpSentry protective relay function

Overload conditions

PowerCommand generator set controls continuouslymonitor the output current and voltage on each phaseo the alternator. On sensing a current level o morethan 110% o the steady state rating o the generator

set or more than 60 seconds on any phase, an alarmcondition is initiated to alert the operator that an abnor-mal condition is present on the machine. The controlalso logs the time and nature o the ault condition.

I the overload continues, the control continuouslymonitors the current level and duration and perormsI2t calculations to determine when the overload hasreached a point that it may damage the machine. Priorto reaching that point, the excitation system is switchedo, thus protecting the alternator and system loadsrom the aects o the condition. Depending on theversion o the PowerCommand control provided, the

machine may or may not operate or a short time (withexcitation switched o) to allow the engine and alterna-tor to cooldown prior to being shut down. This reducesthe risk o damage to the engine and presents nodanger to the installation since the alternator excitationsystem is switched o.

Short circuit conditions

Under 3-phase short circuit conditions, the operationo the control is slightly dierent than operation underovercurrent conditions. Again, the PowerCommandcontrol continuously monitors the output current andvoltage on all phases o the machine. I the control

senses current on any phase greater than 3 timesrated, it will switch to current regulation mode. Thecontrol will adjust the excitation system output to thefeld o the alternator based on sensed current levelrather than sensed voltage level. The control, using apermanent magnet excitation support system providedwith the alternator, will regulate the output current to300% o the rated output current. The 300% currentlevel is intended to be sufcient to clear downstreamovercurrent protective devices. The generator set mayalso enter this state during starting o motors that arelarge relative to the size o the alternator. When the aultclears (or the motor starts), the control returns to voltage

regulation mode, and sotly ramps the system voltageback to normal levels, without a voltage overshoot.

The overcurrent protective unctions o an AmpSentryprotective relay does not completely reset ater theault is cleared. The control system remembers theovercurrent energy expended by the machine, and thetime since clearing. It will progressively move the trip

curve let to maintain protection or the alternator i it isnot ully cooled down ater a ault.

AmpSentry protective relay protections in PowerCommandcontrols eectively eliminates the need or a main genera-tor set circuit breaker. AmpSentry provides both positivealternator protection and protection or the eeder con-

necting the generator set to the next level o downstreamprotective devices, as long as the eeder is sized tooperate under the ull rated output o the generator set.

Throughout this document another signifcant assump-tion has been made: That the engine has sufcienthorsepower available to drive the alternator at rated speedduring the overload or short circuit condition. It is possiblethat under some ault conditions the horsepower load onthe engine will be sufciently large to cause the enginespeed to drop or even collapse. PowerCommand gensetswith AmpSentry protective relay include a kW overloadunction and under requency protection to respond to

these conditions.

Medium voltage applications

There are no specifc special protection requirementsor medium voltage applications versus low voltageapplications. Medium voltage alternators must meet thesame general protection requirements as low voltagemachinesthey must be protected.

Several IEEE documents provide recommendations orsystem design or medium voltage alternators. Theseusually incorporate many more protective unctionsthan are commonly provided low voltage machines.

In addition, the IEEE recommendations oten includeneutral grounding resistors or medium voltage genera-tor sets. The grounding resistor will limit line to groundault magnitude and limit the magnitude o over voltagethat occurs on a ground ault, so that there is more timeto clear downstream aults beore alternator damageand less probability o damage to the alternator due tothe over voltage condition.

Generally, system designs utilize Overcurrent and otherprotective relays to provide alternator protection ratherthan depending on a circuit breaker trip unit.

Cummins generator sets that incorporate AmpSentry

protective relay unctions provide the necessary overcur-rent unctions that are coordinated with the alternatorthermal damage curve, other protective unctions, andault current regulation capability that limits ground aultcurrent without impeding coordination and also preventthe over voltage condition that will occur on a groundault due to overexcitation in the system.



6/6

References

ANSI/NFPA 70, National Electrical Code, 2005.

ANSI/NFPA 99, Health Care Facilities,

ANSI/NFPA 110, Generator Sets for Emergency and Standby

Power Applications.

ANSI/IEEE Std 242-2001, IEEE Recommended Practice for

Protection and Coordination o Industrial and Commercial Power

Systems (Bu Book), Section 12.

Baker, David S., Generator Backup Protection,

IEEE Transactions on Industry Applications, Vol. 1A-18, No. 6,November-December 1982.

Canadian Standards Association, C282,

Emergency Electrical Power Supply or Buildings 2000.

Earley, Mark W., Murray, Richard H., and Caloggerro, John M.,

National Electrical Code Handbook, National Fire ProtectionAssociation, Fith Edition.

R-1053, PowerCommand AmpSentry Time Overcurrent

Characteristic Curve, Cummins Power Generation, 2007.

For additional technical support, please contact yourlocal Cummins Power Generation distributor. To locateyour distributor, visit www.cumminspower.com.

Suggested protection specifcations:

Alternator shall be protected per the requirements o NFPA 70 section445.12. The protection provided shall be coordinated with the thermal

damage curve o the alternator. Damage curve and protection curve

shall be submitted to veriy perormance.

The protection shall allow operation o the generator set continuously

at its rated output.

The protection equipment provided shall be 3rd party certifed to

veriy perormance.

Generator set shall be provided with individual phase line to neutral

over voltage protection that is adjustable and set to operate at morethat 110% o nominal voltage or more than 0.5 seconds.

He has been employed by Cummins Power Generationor more than 25 years in various engineering andmanagement roles. His current responsibilities includeresearch relating to on-site power applications, technicalproduct support or on-site power system equipment,and contributing to codes and standards groups.He also manages an engineering group dedicated todevelopment o next generation power system designs.

About the author

Gary Olson graduated rom Iowa StateUniversity with a Bachelor o ScienceDegree in Mechanical Engineering in1977, and graduated rom the College oSt. Thomas with a Master o Business

Administration degree in 1982.



2009 Cummins Power Generation Inc. All rights reserved. Cummins Power Generation

and Cummins are registered trademarks o Cummins Inc. PowerCommand is a registered

trademark o Cummins Power Generation. AmpSentr y and Our energy working or

you. are trademarks o Cummins Power Generation. Other company, product, or service

names may be trademarks or service marks o others.

PT-6002; Part 2 o 3 (1/09)

Documents

PT 6002 Alternator Pt2 En