generator protection typical schemes and l

8/10/2019 Generator Protection Typical Schemes and L

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Generator Protection Typical Schemes and L&T Solutions

1.0 With increasing complications in the power system, utility regulations , stress on cost reduction andtrend towards automation, Generator protection has become a high focus area. State of the art ,microcontroller based protection schemes from L&T offer a range of solutions to customers to addressthe basic protections and control requirements depending upon the size and plant requirements.

Generators - size less than 300 KVANormally these generators are controlled by MCCBs, which offer O/C and short circuit protections. It isadvisable to have following protections in addition to MCCB (Fig.1):

E/F protection (51N) : This will protect the generator from hazardous leakages and ensure operatorsafety. Many SEBs have already made E/F protection as mandatory. L&T Relay for this is MC12A.

3.0 Generators - size 300 to 1 MVAThere are two major differences when compared with the small machines considered in section 2.0.

a) IDMT Over current + E/F relay will be required addition to normal MCCB or ACB releases - sincethe generator may need shorter trip times for faults in the range 100% to 400% level. L&T RelayMC61A will be the ideal choice.

b) By virtue of larger power level, any faults inside the stator or fault between the neutral of themachine and the breaker terminals can reach very high intensity.

Such internal faults must be cleared instantaneously. Normal IDMT over current / E/F relays are notadequate to monitor this internal fault condition. A separate relay scheme is required to monitor this

internal fault status - otherwise the machine can circulate very high fault currents resulting in severedamage.

A high impedance differential relay scheme, with L&T relay SC14S (3 nos) is the best suited for thispurpose (Fig.2). If the neutral is formed inside the machine, the differential relay scheme will not bepossible - in this case a Restricted E/F scheme , with same SC14S relay (1 No.) is the solution. Careshould be taken to provide adequate no. of CTs as shown in the diagram.

c) Machines of this size are likely to have external controls for frequency and excitation - so that they


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by the thermal overload relay - since the phase currents will be well within limits. A two levelmonitoring for unbalance is preferred - first level for alarm and the second level for trip.

Loss of excitation(40) :

Loss of excitation(40) : When excitation is lost in a running generator, it will draw reactive power fromthe bus and get over heated. This condition is detected from the stator side CT inputs - by monitoringthe internal impedance level & position of the generator.

Reverse Power (32) :Generators of this size may operate in parallel with other sources, which maycause reverse power flow at certain times ( during synchronization or when there is a PF change dueto load / grid fluctuation or when there is a prime mover failure). When reverse power happens, thegenerator along with prime mover will undergo violent mechanical shock - hence reverse powerprotection is absolute must.

Under power (37) :It may not be economical to run generators below a certain load level. Thisprotection will monitor the forward power ..4 -4- delivered by the machine and give alarmwhen the level goes below a set point.

Under / Over Voltage (27 / 59 ) :This will protect the machine from abnormal voltage levels, particularlyduring synchronization and load throw off conditions.

Under / Over frequency (81) : This will protect the machine from abnormal frequency levels, particularlyduring synchronization and load throw off conditions. This will also help in load shedding schemes forthe generator.

Breaker Failure Protection : This protection detects the failure of breaker to open after receipt of tripsignal. Another trip contact is generated under breaker fail conditions , with which more drasticmeasures (like engine stoppage, opening of bus coupler etc) can be taken.

Multifunction relay IM3GVfrom L&T, as a single unit protects the generator for all above listed faults (a)

to (h)- giving at the same time the benefit of small panel space, simple wiring and user friendlyoperation.

Stator Earth fault (64S):L&T relay MC12A will do this job.

Differential Protection (87G) : This protection is very important - since the machines of this size have tobe protected for severe damages that may occur due to internal faults. Considering the large powerlevels, it is necessary to have a percentage biased, low impedance differential relay - L&T's relayMD32G is the ideal solution. MD32G has following advantages : - it provides percentage biaseddifferential protection with dual slope characteristics - it has a built in REF protection element (87N),which will monitor the generator for internal earth faults. - It has a built in O/C protection, as a back up

PT Fuse Failure Protection : This relay will detect any blowing of PT secondary fuse - and give a

contact which can be used to block the under voltage trip. L&T relay PTF03, with less than 7 msresponse, will be ideal for this purpose.

4.2 Rotor side protections :Generators of this size, will need rotor side protections listed below :

a) Rotor Excitation Under Current : This is a DC under current relay ( UBC/A/37), which will monitor theexcitation current.

b) Rotor Excitation U/V(80) :This is a DC under voltage relay (UBC /80), which will monitor rotor


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voltage.

c) Diode failure Relay : Brushless excitation systems will have rotor mounted diodes, which canbecome short or open during operation. Diode Failure relay (RHS) will monitor the condition of thesediodes , for both open circuit and short, and give alarm.

d) Rotor Earth Fault(64R) : Relay UBO/CR will monitor the rotor winding status for the Earth fault.

Please see Fig 3 for the scheme with relays as above.

5.0 Generators above 10 MVA

For large generators above 10 MVA size, the philosophy of main protection and back up protectionhas to be followed. In addition to the protections listed in Section 4.0, following extra protections are tobe considered:

a) 100% Earth Fault Protection :This will help in sensing earth faults close to neutral.

b) Inadvertent Breaker Closure : This will avoid closing of generator to bus during coasting to stop, orwhen stand still or before synchronism.

c) Under Impedance : This will be required as a back up protection for the whole system including thegenerator transformer and the associated transmission line. If the distance relay fails to pick for somereason, this under impedance function will pick up and save the generator.

d) Over Excitation :This will protect the generator from Over fluxing conditions.


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Please see Fig. 4 for the SLD.

6.0 Generator connected in parallel to grid : Whenever generators are running parallel to grid, acomprehensive Auto Synchronising & Grid Islanding Scheme will be required. This scheme will help insynchronizing the generator to the bus and opening the incomer breaker of the plant whenever there isa severe grid disturbance, thus protecting the generator from ill effects of disturbed grid. L&T RelaysSPM21, UFD34 , MW33 and MRP11 are ideal for this scheme. Please see Fig. 5.


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R.Seshadri

7.0 Generators connected in parallel on a common bus : Whenever more than one generator isoperating in parallel , it is necessary to see that the plant load is equally shared by the generators inparallel. If there is unequal sharing, there would severe hunting amongst the generators andeventually this will lead to cascaded tripping of all generators, causing a total black out. L&T relaysRRS (two relays per generator) provide the most effective, online load sharing system for generators

in parallel. Please see Fig 6 for a representative scheme of load sharing for three generators.

8.0 Summary : The sections above describe the actual requirements and various options available from


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L&T towards protection and management of generators of various sizes. For generators above 1 MVAsize, it is also recommended to have complete data acquisition & control through PC. L&T suppliesMSCOM software for this purpose. The relays like IM3GV, MG30, MD32T,UM30A can allcommunicate to a PC and provide all management data.

A comprehensive scheme as shown below will be the most desirable one.


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Home >Basics > Sizing Gen-Sets For Large Motor Starting

Sizing Gen-Sets For Large Motor Starting

Feb 1, 2000 Larry A. Bey, Cummins Onan Corp. | Electrical Construction and Maintenance

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Remember, an on-site engine-generator set is a limited source of power, both in horsepower available

from the engine, and kVA available from the generator. As such, it must be large enough to start as well
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The response of the generator excitation system;

The energy stored in the rotating inertia of the gen-set; and

The acceleration of the motor and its load.

You must consider all these factors for proper gen-set sizing. Here's a simple rule for estimating the size

of an engine-generator set for motor starting: 1kW of generator set rating per each 3/4 to 1 hp of motor

nameplate.

Let's look more closely at a motor start. Induction motors have typical starting characteristics. The curve

of motor current versus speed shows that during starting, the motor draws approximately six times its

full load current; this current remains high until the motor reaches about 80% of speed. This high inrush

current causes a dip in generator voltage. The electric power initially required by the motor (with the

motor at standstill) is about 150% of rated power. The power required by the motor peaks at about

300% of rated power and 80% of speed with full voltage applied. But, the generator set supplies less

than 300% power because starting voltage is lower than full voltage during acceleration, and becausethe generator set's rotating inertia transfers energy to the motor.

The motor must develop greater torque than required by the load. The motor's torque curve at full

voltage is above the load's torque curve. The difference between the torque developed by motor and

the torque required by the load determines the rate of acceleration. Since torque is proportional to

voltage, any reduction in voltage means a proportional reduction in torque.

A properly sized generator set will support the high starting kVA requirements of the motor, and

maintain sufficient output voltage for the motor, so it can develop adequate torque to accelerate the

load to rated speed.

All standby gen-sets use synchronous generators with exciters. Many are available with permanent

magnet generator (PMG) excitation systems. The PMG provides excitation power independent of the

generator terminal voltage. As such, it can maintain full excitation:even during transient loading, such as

motor starting. Full excitation power results in a less extensive voltage dip and improved recovery times.

Using reduced-voltage starting. Though a voltage dip often causes various problems, a controlled

reduction in voltage at motor terminals can be beneficial, but only when reduction in motor torque is

acceptable. Reducing motor starting kVA can reduce the required size of the gen-set, reduce the voltage

dip, and provide a softer start for the motor loads. When sizing gen-sets, you must first determine the

acceptable level of motor torque required during starting, or the loads will accelerate slowly, or even failto reach full speed:ultimately causing motor damage.

Using solid-state starters. Solid-state starters can adjust the starting torque, acceleration ramp time, and

current limit for controlled acceleration of a motor when it starts. For the purpose of sizing a gen-set,

the current limit adjustment reduces the inrush current and may be used to reduce the starting kW and

kVA requirement on the generator. The range of available current limit settings is typically from 150% to

600% of full-load current. A 600% current limit setting on the solid-state starter results in a gen-set


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sizing that's the same as an across-the-line starting. A 300% current limit setting reduces starting kVA by

50%.

Use of the current limit setting also reduces motor torque available to the load. From a gen-set sizing

perspective, an extended acceleration ramp time and low current limit setting (if appropriate for the

motor and the mechanical load) would result in the least voltage and frequency excursions.

One downside to using solid-state motor starters is their integral SCRs (silicon-controlled rectifiers) will

cause voltage distortion. To compensate, you'll have to oversize the generator. The recommendation:

two times the running kW load, except where you're using an automatic bypass. If the solid-state starter

does have an automatic bypass, the SCRs are only in the circuit during starting. Once the motor is

running, the bypass contactor closes and shunts the SCRs. In this case, you can ignore the voltage

distortion during starting, and you don't have to add generator capacity.

VFDs require larger generators. All versions of variable frequency drives (VFDs) are current limiting and

reduce starting kW and kVA. The current drawn by these drives is nonlinear (having harmonics), which

causes a distorted voltage drop across the reactance of the generator. Since VFDs are nonlinear, you

must include an additional generator capacity sizing factor to keep voltage distortion to a reasonable

level of approximately 15% total harmonic distortion (THD) or less. The larger the generator, the greater

the reduction in impedance of the power source (generator), which in turn, reduces the effects caused

by harmonic current distortion.

For six-pulse VFDs, a typical generator sizing factor would be twice the running kW of the drive. This

offsets any reduction in starting kW and kVA. If it is the pulse width modulated (PWM) type (or includes

an input filter to limit current distortion to less than 10%), then you can reduce the sizing factor down to

1.4 times the running kW of the drive.

Using a step starting sequence. The starting sequence of loads can have a significant effect on the size of

a gen-set. One commonly used approach is to assume all connected loads will start in a single step. This

results in the largest gen-set selection. Unless you do something to add load incrementally (such as

multiple transfer switches with staggered time delays, or a step load controller), then you should use a

single-step load for sizing purposes.

In multiple step applications, you start the largest motor first, to minimize the gen-set size. Once placing

all loads on line with the gen-set, you can stop and start load equipment with automatic controls. Here,

you'll have to size the gen-set by assuming the largest motor starts last, with all other connected loads

already on line.

Examples of sizing gen-sets. You can size a gen-set with manual calculations (using a worksheet) or with

PC software available from most major gen-set manufacturers. The basic process is the same. It's always

best to use actual data (if known).

If this information isn't available, using PC software is the best option, since much of the required

information on typical load characteristics is available as default information. If you use the manual


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sizing procedure, it should result in a recovery voltage of at least 90% of rated voltage and a starting

instantaneous voltage dip of approximately 20% to 40%.

The instantaneous voltage dip and frequency dip will likely vary from manufacturer to manufacturer,

based on equal ratings of gen-sets. For a closer estimation of transient (starting instantaneous voltage)

performance, use the manufacturer's sizing software.

Using the manual sizing procedure.

Step 1: Gather information. You'll need to know the following for each motor load:

Nameplate hp,

Running kilowatts (RkW),

Running kilovolt-amperes (RkVA),

Running motor power factor (PF),

Starting motor PF, and

Locked rotor kVA/hp.

You can use the following equation to calculate RkW and RkVA for motors: RkW = [(Nameplate hp) x

(0.746kW/hp)] / Efficiency (eq. 1)

RkVA = RkW / Running motor PF (eq. 2)

To calculate starting kilovolt-amperes (SkVA) and starting kilowatts (SkW) for motors, use these

equations:

SkVA = (Nameplate hp) x (Locked rotor kVA/hp) (eq. 3)

SkW = (SkVA) x (Starting motor PF) (eq. 4)

Step 2: Total the RkW, RkVA, SkW, and SkVA numbers for all the loads.

Step 3: Select the gen-set by comparing the RkW, RkVA, SkW, and SkVA to the ratings on the

manufacturer's specification sheets (after appropriate derating for ambient temperature and altitude).

Example One calculation. Determine gen-set size for three loads started across-the-line in a single step.

Here's pertinent information:

Two 200 hp motors, Code G, 92% running efficiency, 0.25 starting PF, 0.91 running PF.

Total 100kVA of fluorescent lighting, starting PF of 0.95, and running PF of 0.95 (Note: We're using the

terms starting and running PF for the lighting load here for clarification when adding the motor loads.

Actually, the ballast for the lighting load has a constant PF of 0.95.)


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Step 1: Information gathering and calculations. 200 HP motor:

RkW = (200 hp x 0.746 kW/hp) / 0.92 = 162.2kW

RkVA = 162.2kW / 0.91 PF = 178.2kVA

SkVA = 200 hp x 5.9 kVA/hp41180kVA

SkW = 1180kVA x 0.25 PF = 295kW

Florescent Lighting:

RkW = 100kVA x 0.95 PF = 95kW

RkVA = 100kVA

SkVA = 100kVA

SkW = 100kVA x 0.95 PF = 95kW

Step 2: Totals.

Load.......... | RkW | RkVA | SkW | SkVA

200hp Motor | 162.2 | 178.2 | 295 | 1180

200hp Motor | 162.2 | 178.2 | 295 | 1180

Lighting....... | 95.... | 100... | 95. | 100

Totals (kVA). | 420... | 457.. | 685 | 2460

Step 3: Selection. At a minimum, you'll have to size the gen-set to supply the maximum starting (surge)

demands and the steady-state running loads of the connected load equipment. In this example (using

one manufacturer's published data), you would select a 750kW generator set with 2944 SkVA available

at 90% recovery voltage to supply the total load SkVA of 2460. The load totals for RkW, RkVA, and SkW

are well within the rating of the 750kW (938kVA) gen-set you selected. The running kilowatt load of

420kW is 56% of the 750kW gen-set standby rating.

Example Two calculation. Assume you have the same three loads as in Example One, but now you're

using an autotransformer type reduced voltage starter that is set at the 65% starting voltage to start the

two motors. This tap setting will reduce the starting kVA by the square of the voltage (0.65)squared, or

0.42 times the starting kVA.

Step 1: Calculations

200 HP motor:

RkW = (200 hp x 0.746 kW/hp) / 0.92 = 162.2kW


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RkVA = 162.2kW / 0.91 PF = 178.2kVA

SkVA = 200 hp x 5.9 kVA/hp = 1180 x (0.65)squared = 495kVA

SkW = 495kVA x 0.25 PF = 124kW

Florescent Lighting:

RkW = 100kVA x 0.95 PF = 95kW

RkVA = 100kVA

SkVA = 100kVA

SkW = 100kVA x 0.95 PF = 95kW

Step 2: Totals

Load.......... | RkW.. | RkVA | SkW | SkVA

200hp Motor | 162.2 | 178.2. | 124. | 495

200hp Motor | 162.2 | 178.2. | 124. | 495

Lighting...... | 95..... | 100... | 95... | 100

Totals (kVA) | 420... | 457... | 343. | 1090

Step 3: Selection. Using one manufacturer's published data, you would select a 450kW gen-set to supply

the required starting kVA. The running kilowatt load of 420kW is 93% of the gen-set's standby rating. So,

if you want a margin for future load additions, you would select a 500kW gen-set running at 84% of

rated standby power.

Sidebar: Here's What Causes Dip in Starting Voltage

When you start a motor across-the-line with a gen-set, the motor represents a low impedance load

while at locked rotor or stalled condition. This causes a high inrush current. The high motor inrush

current (I ms) flows through the generator armature windings and is affected by the reactance. This

causes a drop in generator voltage. Impedance controls the flow of current in AC circuits. But, the

generator armature reactance is such a large part of its total impedance that resistance is ignored.

The generator terminal voltage drops instantaneously when the motor starter contacts close at time

t40, as a function of the subtransient reactance (X"d). Generally, the larger the generator, the lower its

reactance. So, one way to minimize the instantaneous voltage dip is to increase the generator size.


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The generator terminal voltage may drop further, depending on response of the generator's automatic

voltage regulator and the power capability of the excitation system. (Most gen-set automatic voltage

regulators include underfrequency protection.)

During momentary overloads, the engine speed may also dip. If it does, the automatic voltage regulator

reduces excitation power to the main field, which lowers the generator terminal voltage. This, in turn,reduces the load on the engine, allowing it to recover to rated speed. Typically, a maximum generator

terminal voltage dip of 30% will not cause coils to drop out. (This allows for approximately 5% additional

voltage drop in the conductors between the generator and the motor).

Although the voltage dip, due to under frequency protection, may extend the voltage recovery time, it

also allows the engine to be sized closer to the steady-state running load rather than starting load. This

is particularly important with diesel engines, which should not run for an extended duration at less than

30% of rated load. (Extended light-load operation of a diesel engine can result in the accumulation of

unburned fuel in the exhaust system, due to incomplete combustion from low combustion

temperatures, called wet stacking. Light load operation can also result in engine damage from fuel and

water contaminating lubricating oil.)

After the initial voltage dip, it's important the generator restore voltage to a minimum of 90%-rated

value while supplying the motor starting kVA. At least 90% recovery voltage is necessary for the motor

to develop adequate torque to accelerate its load to rated speed.

A motor starting a high starting torque load, such as a loaded compressor, requires higher recovery

voltage than one starting an unloaded compressor. As the motor comes up to speed, the voltage will

rise, as the starting kVA input decreases. Once the motor is up to speed, the voltage should return to

rated value, if the gen-set is sized properly.

Sidebar: How Inertia Affects Gen-Set Sizing

The moment of inertia of a rotating mass offers resistance to acceleration. The load connected to the

motor shaft has its moment of inertia, and in practical situations for specific equipment, this may or may

not be available information.

Fortunately, for the purpose of sizing a gen-set, or more specifically to determine the engine power

needed to start and accelerate a rotating motor load, the motor load's moment of inertia need only bebroadly categorized as low or high inertia.

High inertia loads are characterized by high breakaway torque requiring prolonged acceleration. Low

inertia loads are characterized by low starting torque at standstill, with increasing torque as motor

speed increases resulting in rapid acceleration to rated speed.


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Starting low inertia loads will reduce the normal starting kW needed. Look for more information on this

is in the sample calculations within this article.

Sidebar: Examples of High and Low Inertia

High inertia loads include:

Single- and multi-cylinder pumps

Single -and multi-cylinder compressors without unloading valves

Crushers

Hydraulic elevators without unloading valves

Low inertia loads include:

Fans, centrifugal and blower

Compressors starting unloaded

Centrifugal pumps

Motor-generator elevators

Note: Pumps starting into high head pressure and large diameter fans or fans starting into high

restriction areas should be classified as high inertia loads.

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Lee Chee Seng (not verified)

on Jul 4, 2012

Dear Sirs, I appreciate if someone could enlighten me by providing a recommendation of a suitable

genset size for an exercise as follows:- What would be the right size of genset for starting a 220KW

water pump motor by auto-trans method with a voltage dip limitation of not more than 15%. Thank you

for assisting. Best regards / Lee Chee Seng

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Anonymous (not verified)

on Jul 19, 2012

very helpfull Many diesel Generator Sales people are blank about the start up power of motors. And ambattling to select the correct size of generator.


Anonymous (not verified)

on Nov 26, 2013

I did it with an Woodward easygen-3500 controller using the ramp up feature


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UNDERSTANDING GENERATOR SET RATINGS

Generator set ratings may seem complex, but their basic purpose is simple: fit the customersneeds at the optimum reliability, performance, and cost. An improper rating means acustomer either buys more capacity than needed or risks shorter life to overhaul, morerepairs, and more downtime. Read more to learn how smart rating choices combined with thelatest in onsite power generation systems can help engineers match equipment to their

customers applications for optimal long-term performance.

UNDERSTANDING EQUIPMENT APPLICATIONS AND CAPABILITIES

Available ratings have changed in recent years, and more sophisticated switchgear can beintegrated with generator sets. That means more flexibility to specify generating systems thatclosely match users requirements. The key to choosing the right ra ting is to understand theapplication in detail. That means knowing the type of duty and answering several keyquestions:

What is the average load factor?

What is the maximum required load?

How many hours per year will the generator sets run? Will the generator sets be run isolated from or in parallel with the utility?

RATINGS DEFINED

Caterpillar defines five basic generator set ratings:

Standby

Emergency Standby Power (ESP)

Mission Critical Standby

Prime

Continuous

Cat generator set ratings differ in certain respects from those defined by the industrystandard ISO 8528-1. Basic descriptions of the Caterpillar genset ratings are offered below.

STANDBY RATING

In this application, the generator set is capable of providing emergency backup power at thenameplate rating for the duration of an outage.

The average load factor of a Standby rated generator set should be no more than 70% of thenameplate rating and applied to varying loads. A Standby generator set can run for amaximum of 500 hours per year. The normal standby rating is not for use in utility paralleling

applications. For example, a 3 MW standby rated generator set will provide power for theduration of an outage. It should be run for up to 500 hours per year and have an averageload factor of 2.1 MW.

EMERGENCY STANDBY POWER (ESP) RATING

The ESP rating differs from the Standby rating only in the number of running hours allowedper year. ESP ratings allow a maximum running time of 200 hours per year at a 70% averageload factor with varying load.

CALL NOW TO LEARN MORE

Neal Reinhardt865-595-1034 or 865-771-1036Send me a message

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JOIN OUR EMAIL LISTSign up to receive Stowersnews, expert advice andtips, and specials in youremail inbox.CATERPILLAR GENERATORSET RATINGS GUIDEFor Caterpillar diesel andgas-powered generatorsets, 60Hz and 50Hz. (PDF672KB)CATERPILLAR GENERATORSET RATINGS DEFINITIONS

Get more details about theratings definitions forCaterpillar diesel generatorsets. (PDF 28KB)POWER SYSTEMS WHITEPAPER: A HOLISTIC VIEW ONGENERATOR SET RATINGSLearn more about thefactors to be consideredwhen selecting a particularrating for a givenapplication. (PDF 80KB)CATERPILLAR GENERATORSET SPECIFICATION SHEETSView specification sheets

for your diesel generatorset applications at Cat'swebsite.CONTACT A SALES REPFind a Power Systemsrental application specialistby entering your ZIP code.
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MISSION CRITICAL STANDBY RATING

In this application, the generator set is capable of providing emergency backup power at thenameplate rating for the duration of an outage. The average load factor of a mission criticalstandby rated generator set should be no more than 85% of the nameplate rating withvarying loads. A mission critical standby generator set can run for a maximum of 500 hoursper year.

Typical peak demand is 100% of the rating for maximum of 5% of the operating time. Themission critical standby rating is not for use in utility paralleling applications. For example, a 3MW mission critical standby rated generator set will provide power for the duration of anoutage. It should be run for up to 500 hours per year and have an average load factor of upto 2.55 MW.

PRIME RATING

In this application, the generator set is capable of providing power to a varying load for anunlimited number of hours per year. A Prime rated generator set is capable of providing fullnameplate rating for a period of time, but must have an average load factor of no more than70% of the Prime rating. Ten percent overload is allowed for emergencies for a maximum of

one hour in 12, and for no more than 25 hours per year. The standard prime rating is for usein either utility paralleling or isolated applications.

For example, a 2.7 MW rated unit may provide the full nameplate rating for a short duration,but should have a maximum average load of 1.89 MW (not including generator set non-running time per ISO8528-1). The generator set can also provide 3 MW of power inemergencies as defined above. An example of the Prime rating is shown in Chart 3.

LOAD MANAGEMENT RATING

The Load Management rating is simply a special application of the Prime rating. A Primerated generator applied under load management guidelines allows for a Prime ratedgenerator set to be used in parallel with the utility.

A Prime rated generator set under load management guidelines can run for a maximum of

500 hours per year. This generator set has the same nameplate rating as a Prime rated unit,but allows for an average load factor of up to 100%. The Prime rating with load managementguidelines does not allow for a 10% overload capability. For example, these guidelines statethat a 2.7 MW unit (same nameplate rating as the Prime rated unit) can be run at 2.7 MW fora maximum of 500 hours.

There are two basic load management practices: base loadingand peak shaving. In baseloading, the generator set operates at a fixed kW output, and the utility provides power forany peaks above that level. In this scenario the end user may export power to the grid if morepower is being generated then is required by the facility loads, and the appropriateagreements with the utility are in place.

CONTINUOUS RATING

In this application, the generator set is able to provide power to a non-varying load for anunlimited number of hours per year. The average power output of the generator set is 70% to100% of the rating. The rating is designed to provide 100% of the rating for 100% of theoperating hours.

Typical Continuous rating applications include base loading in parallel with the utility andcogeneration operations. An example of a Continuous rated generator set is shown in Chart6.

PERFORMING DUAL DUTY


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SKVA AND MOTOR STARTING

Generator sets are typically evaluated on the basis of their ability to start electric motorsand accelerate them to rated speed. Although there is a generally accepted standard forthis measurement, some manufacturers refer to a standard based on sustained voltagedip, while other industry leaders measure the instantaneous voltage dip. Since there isno direct relationship between sustained and instantaneous voltage dip, a comparison

between the two may be misleading. Below is an explanation.

MOTOR STARTING CAPABILITY

Generator sets are typically evaluated on the basis of their ability to start electric motors and accelerate them to ratedspeed. Regardless of whether the motor has a load on it or not, starting it requires kVA far in excess of the motorsnormal running kVA demand. The motor starting capability, measured in terms of starting kilovolt-amperes (sKVA), isan indication of a generator sets ability to start motors with minimal drop in voltage. When a generator is coupled to asynchronous driver (i.e., a nonslip motor tied to utility power), a generator voltage dip of 30% maximum is generallyacceptable (20% in many applications). The voltage will recover to within 2% of normal voltage as soon as the motor hasaccelerated to rated speed normally about one second, or 60 electrical cycles. Due to engine lug, diesel-poweredrecovery is longer but still occurs within seconds. The higher a generators sKVA for a given voltage dip, the better thegenerator for motor starting.

SUSTAINED VOLTAGE DIP VERSUS INSTANTANEOUS VOLTAGE DIP

Although there is a generally accepted standard for how motor starting capability is measured, some manufacturers referto a sustained voltage dipvalue, which they explain as being the undervoltage which occurs for some period after motorstarting commences. However, this value fails to quantify the absolute instantaneous voltage dip experienced duringmotor starting. Further, the method of calculating the value of sustained dip is largely undefined and does not account forthe fact modern voltage regulators will most always return a generator to rated voltage after motor starting (assuming theengine can overcome the load).

Other industry leaders measure the minimum voltage immediately after the motor is switched on, known as theinstantaneous voltage dip. This method assures that the actual absolute value of voltage dip is presented andgenerator starting capability is truly understood.

SIMPLE COMPARISONS MAY BE MISLEADING

This issue is further complicated by the fact that there is not likely to be a direct relationship between a sustained andinstantaneous voltage dip, so a simple comparison between the two may be misleading. The difference in themethodology used for presenting motor starting capability between various manufacturers of generator sets must beconsidered when developing specifications. Simply specifying an sKVA value may no longer be enough to precludemisapplication in the field. Therefore absolute instantaneous voltage dip valuesshould be utilized to ensure adequatesupport of sensitive loads.


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neutral grounding in parallel connected gensets

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11-02-2014 12:22 AM

hi all,

There is 4nos. 3.3kV, 2000kVA gensets which is parallel connected. all of its neutrals are grounded individually

without NGR, and each gensets are provided with e/f protection by neutral cts in the neutral to ground cable. but i

seen most of the systems are usually grounding only one geset neutral. my question if only genset is grounded, how

the rest of the gensets will be protected from earth fault, please share you valuable knowledge. see the attached

sketch for the present system design. thanks.
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