generator load

8/7/2019 generator load

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While most sizing exercises are best donewith sizing programs or with the help o amanuacturer’s representative, it is still impor-tant to understand the actors that aect theoperation o your generator set so you canbe condent you have the right equipmentor your application.

Project parameters

The rst step in sizing and selecting a generator set isto establish project parameters.

• Minimum generator set load/capacity: Runninga generator set under light load can lead to enginedamage, reducing reliability. Cummins PowerGeneration does not recommend running generatorsets at less than 30 percent o rated load. Loadbanks should supplement the regular loads whenloading alls below the recommended value.

• Maximum allowable step voltage dip (starting andrunning): As you reduce the maximum allowablestep voltage dip during initial startup, when loads

cycle under automatic controls or when high peakloads operate, you need to increase the size o thegenerator set specied. Choosing lower allowablevoltage dip requires a larger generator set.

• Maximum allowable step frequency dip: As youreduce the maximum allowable requency dip, youincrease the size o the generator set needed.

• Altitude and temperature: Based on the sitelocation, the size o the generator set must increaseor a given level o perormance as altitude andambient temperature rise.

• Duty cycle: Generator set size is also infuenced bywhether the application is or standby power, primepower or utility paralleling. Standby power systemsgenerally have no overload capability. Prime powersystems generally have a minimum o 10 percentoverload capacity. Generator sets that are intended tooperate extended hours at steady constant load shouldnot be operated in excess o the continuous rating.

• Fuel: The preerence or gas, diesel, or LP will aectgenerator set choices. Oten, generator sets running

on gas or LP must be oversized due to derating.Emergency systems typically must be supplied byuel stored locally.

• Phase: Select either single- or three-phase. Thethree-phase selection permits single-phase loadsbut the assumption is that the single-phase loadswill be balanced across the three phases.

• Frequency: Either 50 Hz or 60 Hz.

• Voltage: Voltage choices are usually a unction oselected requency.

> White paper

By Jim Iverson, Senior Applications Engineer

How to size a genset:Proper generator set sizing requiresanalysis o parameters and loads

Power topic #7007 | Technical inormation rom Cummins Power Generation

Four 1500 kW Cummins Power

Generation generator sets pro-vide backup power to the main

terminal at Minneapolis-St. Paul

International Airport.



www.cumminspower.com

© 2007 | Cummins Power Generation

• Single-phase loads and load imbalance: Singlephase loads should be distributed as evenly aspossible between the three phases o a three-phasegenerator set in order to ully utilize generator setcapacity and limit voltage imbalance.

• Peak loads: Peak loads are caused by loads that

cycle on and osuch as welding equipment,medical imaging equipment, or motors. Takingcyclic loads into account can signicantly increasethe size o the recommended generator set despitepainstaking eorts to place loads in a step startingsequence.

• Motor loads: Calculating specic motor loads isbest handled by sizing sotware which will converttypes o motors into load starting and runningrequirements. For this discussion, however, it issucient to broadly characterize loads as high-inertia or as low-inertia loads or the purpose o

determining engine power needed to start andaccelerate motor loads.

• Low-inertia loads include ans and centriugalblowers, rotary compressors, rotary and centriugalpumps.

• High-inertia loads include elevators, single- andmulti-cylinder pumps, single- and multi-cylindercompressors, rock crushers, and conveyors.

• Motors over 50 HP: A large motor startedacross the line with a generator set represents alow-impedance load while at locked rotor or initial

stalled condition. The result is a high inrush current,typically six times the motor rated (running) current.The high inrush current causes generator voltagedip which can aect other systems. The manner inwhich generator voltage recovers rom this dip isa unction o the relative sizes o the generator, themotor, engine power (kW capacity) and generatorexcitation orcing capability. Depending on theseverity o the load, the generator should be sizedto recover to rated voltage within a ew seconds, inot cycles. Various types o reduced-voltage motorstarters are available to reduce the starting kVA o amotor in applications where reduced motor torque

is acceptable. Reducing motor starting kVA canreduce the voltage dip, the size o the generator set,and provide a soter mechanical start. However,these starting methods should only be applied tolow-inertia motor loads unless it can be determinedthat the motor will produce adequate acceleratingtorque during starting.

Loads

The next and most important step in sizing a generatorset is to identiy every type and size o load the genera-tor set will power. In general, when non-linear loads arepresent, it may be necessary to oversize the alternator.

Following is a general discussion o how various loadsand electrical actors aect the sizing o generator sets.

• Power factor (PF): The inductances andcapacitances in AC load circuits cause the pointat which the sinusoidal current wave passesthrough zero to lag or lead the point at which thevoltage wave passes through zero. Capacitiveloads, overexcited synchronous motors, etc. causeleading power actor, where current leads voltage(FIGURE 1). Lagging power actor, where currentlags voltage, is more generally the case and isa result o the inductance o the circuit. PowerFactor is the ratio o kW to kVA and is expressedas a decimal gure (0.8) or as a percentage (80%).Three-phase generator sets are rated or 0.8 PFloads and single-phase generator sets or 1.0PF loads. Lower PFs require larger alternators orgenerator sets to properly serve the load. Cautionshould be used whenever applying generator setsto leading power actor loads. Only slightly leadingpower actor can cause generator sets to losevoltage control.

Power topic #7007 |

FIGURE 1 – A typical alternator curve o reactive power (kVAR)capability. A reasonable guideline is that a generator set can carry

up to 10 percent o its rated kVAR capability in leading power actor

loads without being damaged or losing control o output voltage.The most common sources o leading power actor are lightly loaded

UPS systems with input lters and power actor correction devices

or motors. Loading the generator set with lagging power actorloads prior to the leading power actor loads can improve stability.

1.0

0.8 0.8

1.0 PF0.8 PF 0.8 PF

0.6 0.4 0.2 0 0.2 0.4 1.00.6

0.6

0.8

0.4

0.2

0.0

1.0

0.6

0.8

0.4

0.2

0.0

STEADY STATE ALTERNATOR REACTIVE POWER CAPABILITY CURVE

Per Unit kVARImport (Leading) Export (Lagging)

1.0

Per Unit kW

UnstableVoltage Region

AcceptableSteady State

Operating Region

RoterOverheating

Region




© 2007 | Cummins Power Generation

Power topic #7007 |

• Battery charger loads: A battery charger is anon-linear load requiring an oversized alternatorbased on the number o rectiers (pulses)up to2.5 times the steady-state running load or three-pulse; to 1.15 times the steady-state running loador 12-pulse. These loads are typically ound intelecommunications systems.

• Medical imaging loads: These include CAT scan,MRI, and X-ray equipment. The generator set shouldbe sized to limit the voltage dip to 10 percent when

the medical imaging equipment is operated with allother loads running in order to protect image quality.

• Lighting loads: In addition to lamp wattages, ballastwattages and starting and running power actorsshould be considered.

• Regenerative loads: For loads such as elevators,cranes and hoists, the power source is oten reliedupon or absorbing power during braking. That isusually not a problem when the utility is supplyingpower because it can be considered as an innitepower source with many loads. A generator set,in comparison, is able to absorb ar less power,

especially with no other loads connected. Generally,the regeneration problem can be solved by makingsure there are other connected loads whichcan absorb the regenerative power. Excessiveregenerative load can cause a generator set toover-speed and shut down.

• Variable frequency drive (VFD) motors: Variablerequency drives (or variable speed) are non-linearloads, which are used to control the speed oinduction motors, induce distortion in generatoroutput voltage. Larger alternators are required toprevent overheating due to the harmonic currentsinduced by the VFD and to lower system voltagedistortion by lowering alternator reactance. Forexample, VFD loads on a generator must be less thanapproximately 50 percent o generator capacity tolimit total harmonic distortion to less than 15 percent.

Other loads

• Uninterruptible power supply (UPS) loads: AUPS system uses silicon controlled rectiers orother static devices to convert AC voltage to DCvoltage or charging storage batteries and areanother type o non-linear load. Larger alternatorsare required to prevent overheating due to theharmonic currents induced by the rectiers and tolimit system voltage distortion by lowering alternatorreactance. Past problems o incompatibility betweengenerator sets and static UPS devices lead to many

misconceptions about sizing generator sets orthis type o load. Most UPS manuacturers haveaddressed these issues and it is now more costeective to require UPS devices to be compatiblewith the generator set than to signicantly oversizethe generator or the UPS. Use the ull nameplaterating o the UPS or determining load to allowsucient capacity or generator set battery chargingand accommodating ull UPS load capacity.

Sizing sotware, such as Cummins Power Generation’sGenSize, greatly simplies this process and allowsanalysis o alternative to optimize the generator size.Knowing manual calculation methods helps theengineer understand how each actor contributes to acorrectly-sized genset.

Many actors infuence generator set sizing, makingit time consuming to manually calculate requiredgenerator capacity. Parameters that determine therequired generator set size include minimum generatorset load; maximum allowable step voltage dip and steprequency dip; altitude and temperature; duty cycle,uel, phase, requency and voltage.



Power topic #7007 | Page


© 2007 Cummins Power Generation Inc. All rights reserved.

Cummins Power Generation and Cummins are registered

trademarks o Cummins Inc. “Our energy working or you.”

is a trademark o Cummins Power Generation.

PT-7007 (03/07) Formerly PT-305

Load step sequencing

In many applications the generator set is sized to pickup all loads in one step. In some applications it isadvantageous to start up the loads which cause thelargest starting surge rst and then the rest in multiplestepsthe “largest motor rst” rule. Codes may requiresequenced load starting to start emergency and liesaety loads within as little as ten seconds, whileallowing other loads longer periods o time. In general,

sequenced startup allows the smallest generator set inrelation to the steady state load. When cycling motorloads exist, it will still be necessary to size the genera-tor set to start the largest cycling motor last, with allother loads connected.

Future needs

One last step in the sizing equation has to do withuture needs. Power use is not xed and tends to growover time. Thereore, any generator set sizing exercise

needs to take system expansion into consideration.Even with sophisticated sotware solutions, the naldecision on generator set size needs to be temperedwith judgment. And, the more you know about theparameters that aect sizing, the better that judgmentwill be.

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

Controls, and Technical Marketing & Sales. Jim providestechnical direction to Commercial Marketing & Sales,contributes to domestic industry codes and standardsdevelopment, oers sales and service training, oerstechnical input or published literature and sotware,publishes technical papers on industry topics, andprovides application engineering support to customers.

About the author

Jim Iverson is a senior applications engi-neer or Cummins Power Generation, withan M.S. in Engineering Science, and aB.S. in Electrical Engineering. Since 1976,he has managed Transer Switch Design,Systems Engineering, Switchgear &

Two 1 MW generator sets provide

ull redundancy to three power

branches at Hancock Hospital,Greeneld, Indiana.

generator load

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