06 AET - Genset Sizing

8/18/2019 06 AET - Genset Sizing

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Genset Sizing


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Required Information

Duty rating to be applied

Frequency (50Hz/60Hz)

Nominal Voltage

Climatic Conditions

Voltage and Frequency dip requirement

Load parameters

Mandatory load steps or sequence of load step, if they exist

Number of genset in parallel

Sizing software(Power-Suite)


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Motor Load

AC electric motors represent inductive loads withlagging power factors

Different types of motors have different starting

characteristics.

Starting characteristics may drive the need for a

larger or smaller generator.

Starting an electric motor can create voltage dips in

excess of 40% if the genset is not properly sized.

Cummins Confidential


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Inductive Load: kW vs. KVars

Inductive loads need two kinds of power to function properly:

– Active power (kW) – actually performs the work

– Reactive power (kvar) – maintains the electromagnetic fields

Engines Produce kW--Fuel Rate Controls

Alternators Make kVAR--Excitation Controls

kW Dem anded b y Load

kVa r Dem anded b y Lo ad

kW = engine

k V A

R =

a l t e r n

a t o rppa r en t pow e r


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Voltage Response at Constant Speed

Voltage

requency

Generator Transient Response to StepLoads

Alternator Must Support kVA:

– Limit Initial Dip

– Recover to Acceptable Voltage(Motor Torque Proportional to

Voltage2

)

Engine Must Support kW (Similarto Voltage Response)

Dynamic Response Combination ofFrequency and Voltage


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We are going to create a project with PowerSuite with thefollowing criterias:

50Hz

400V

Min rated capacity 30%

18% Voltage Dip, 5% Frequency Dip

25C

150m


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Electrical Motors

UPS

Lighting

Regenerative loads

Miscellaneous load

Differents types of loads


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Apply Alternative Starting Methods on HighInertia Loads With Caution

Motor Torque Proportional to

Appl ied (Vol tage)2 Okay for low inertia applications.

Watch out with high loads!


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Typical 3 Ø Induction MotorCharacteristics

Direct On-Line (DOL) Start

High Locked Rotor Currentand KVA

Torque Shown for 100%

Rated Voltage (sol id red line) Starting (instantaneous) PF

0.2 to 0.4 for 3 Ø Motors

Higher StartingRequirements for HighEfficiency 3Ø and for 1Ø

Treat Synchronous MotorsLike Induction Motors

P E

R C E N T T O R Q U E , P O W

E R , C U R R E N T

PERCENT MOTOR RATED SPEED

20 40 60 80 100

100 0.2

1.0

P O W E R F A C T O R ( L

A G G I N G )

600

Current

Power Factor

Load Torque

Motor Torque

Power

0


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Let’s enter the following load with direct on line starting

method in Powersuite:

Motor Running Power (Electrical) = 300kWe, 350kVA

Starting kVA = 6 x 350kVA = 2100kVA

Starting kW = 2100kVA x 0.3 p.f. = 630kW


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Star-delta Starting…

Reduced Starting Impact

Watch out for the transition(kVA)

Potential start failure ifalternator cannot sustainvoltage in transit ion.

Typical kVA demand of motor…

Typical kW demand of motor…


12/27Cummins Confidential

Transit ion on Reduced Voltage Starters

Same as DOLTrue Reduced

Voltage

Transition

too Early


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We will now enter the following load with star/delta startingmethod in PowerSuite:

Motor Running Power (Electrical) = 300kWe, 350kVA

Starting kVA = 3 x 350kVA = 1050kVA

Starting kW = 1050kVA x 0.3 p.f. = 315kW


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Auto TransformerThis method uses transformer action to reduce the voltage applied to the motor

and current seen by the supply. An improved torque/amp ratio is achieved and

starting current is typically 3 x FLC, depending on the voltage tapping selected.Normally the voltage is applied to the motor in voltage steps through the T/F

taps being selected through contactors. Typical tappings are 50%, 70%,

followed by full voltage being applied to the motor . The major disadvantages

are size & cost. Mechanical snatch at switch on is not controllable and may

cause problems. Once tappings have

been selected, it may be necessary tochange them according to changes in

load parameters.

Advantages

• Simple operation

Disadvantages

• Poor controllability

• Bulky

• Very Expensive


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Soft Starters Limit the starting current by limiting the amount of

starting torque. Current distortion has to be considered for the non

linear loads.

Advantages: Less mechanical stress, reduced startcurrent, improved control of acceleration and

deceleration

Disadvantages:

Size & Cost


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Uninterruptible Power Supply

UPS can produce high levels of

current distortion.

Excessive generator voltage

distortion

Increased Power SystemComponent losses and heating

Harmonics can be reduced by

filters on the UPS.


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(Or 18 Pulse)

Cummins Confidential

Phase Controlled Rectif iers: Voltage andCurrent Distortion

3 1


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Let’s enter the following UPS Load in PowerSuite

Rated Output= 200kVA

Rectifier= 12 Pulses


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Lighting Loads

Fluorescent & Discharge

Both are non-linear loads which may need to be

considered when sizing your generator

Incandescent

Defined as a general load for sizing purposes


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Let’s enter the following light ing load in PowerSuite

Light Type : Fluorescent

Running KVA = 50kVA


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Regenerative Load

Reverse power needs to be considered.

http://upload.wikimedia.org/wikipedia/commons/5/55/Gondola_car_20060409.jpg


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Regenerative Load

Stage 1:-

Genset Supplies Site Loads

Stage 2:-

Lift Motor Produces

Reverse Power 200kW

100kW Load Bank Removes

Risk of Reverse Power


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Miscellaneous Loads

If the load is so specific that you can not find its kind in PowerSuite,you can create your own kind of load


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Let’s enter the following load in PowerSuite

Running & Starting = 100 kW

Running Power factor= 0.95

Starting Power factor= 0.50


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We are going to size the genset with a single load step and the load weentered:

(In case a single genset is not enough to start the loads with the criteriasit is possible to add more gensets)


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Size of genset optimization

Step Starting Sequence

Step Loading the Generator

• Apply Load in Increments

• Apply Large Motor Loads First

• Limit Voltage and Frequency dip

• Ultimately reduce genset size

Let’s now optimise the genset size


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Questions?

27

Thank You

Documents

06 AET - Genset Sizing