Generator Sizing

Considerations

M. Scott Thomas Application Engineering Manager

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Agenda

Generator Ratings Information Required for Sizing Motor Loads Non-linear Loads UPS and VFDs Sample Project

Question and Answer

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ISO 8528 - 5 Part 1 defines these ratings:

Emergency Standby power (ESP)

Limited Time Prime Power ( LTP)

Unlimited Time Prime Power (PRP)

Continuous Operating Power (COP)

Any manufacturer can go above and beyond

the ISO ratings definitions

Definitions

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Emergency Standby Power

ESP

For supplying emergency power for the duration of a

utility power failure.

Not to exceed 200 hrs/yr

Average load factor of 70% of the standby rating

over 24 hour period

No negotiated outage operations.

t1 ... t1 + t2 + t3 + ...+tn = 24

hours

Time

Power

t2 t3 t4 tn-1 tn

100%

70% AVG

=11 + 22 + 33 ++

1 + 2 + 3 ++ = =1

=1

5

t1

1 Year

Time

Power t1

Shutdown for

maintenance

Limited Time LTP:

limited number of hours 500 h/year

Non-Variable Load

Applications at power

levels never to exceed the

Prime Power rating.

Any operation exceeding 500 hours per year should

use the Continuous Power

Rating

Prime Power

500 hr/yr max

t1

6

t1 t5

24 Hours/365 days a year

Time

Power

t2 t3 t4 t6 t7

70%

100% Unlimited Time PRP: (8760 hr/yr less service)

Unlimited number of hours per year

Variable load

not to exceed 70% average of the PP

rating during any

operating period of 24

hours.

Prime Power

AVG

=11 + 22 + 33 ++

1 + 2 + 3 ++ = =1

=1

7

t1 t1

1 Year

Time

Power

t1 Shutdown for

maintenance

COP

Applicable for supplying

utility power at a constant

100% load for an unlimited

number of hours per year.

No overload capability is

available for this rating.

Continuous Power

(8760 hr/yr., less service)

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Stand by rating 2500 kW

maximum average in a 24h period as per ISO 8528

(70%) 1750 KW

200h/year

Prime Power Rating 2336 kW

maximum average in a 24h period as per ISO 8528 (70%):

1635 kW

Limited time Prime Power 2336 kW

500h/year with non-variable load

Continuous Operating Power 1951 kW

Rating Example 2500 DQLC

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Sizing: Required Information

Duty rating to be applied Frequency (50Hz/60Hz) Nominal Voltage Location - Climatic conditions (Altitude and

Temperature)

Voltage and frequency dip limitations (May be an important factor on the genset size)

Load parameters Mandatory load steps, when they exist Sizing software (Cummins PowerSuite)

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Motor Load

AC electric motors represent inductive loads with lagging 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.

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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 Demanded by Load

kVar Demanded by Load

kW = engine

kV

AR

=

alt

ern

ato

r

Apparent power

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Fundamental Criteria for Motor Starting

Sufficient LRKVA at the instantaneous voltage dip for inrush current

LRKVA at the maximum permissible instantaneous voltage dip should considered as the first step for motor starting

Sufficient genset torque and power the torque available from the genset must exceed the

torque required by the motor load

Sufficient alternator excitation system strength excitation system strength and adequate response to

accelerate the motor and return it to operational

voltage and speed

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Generator Transient Response to Step Loads

Alternator Must Support kVA: Limit instantaneous Vdip

Recover to acceptable voltage (motor torque proportional to voltage2)

Engine must support kW (similar to voltage response)

Dynamic Response Combination of Frequency and Voltage

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Example of Alternator Locked Rotor Curve

Instantaneous Voltage dip function of alternator reactance.

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Can Run What We Can Start (If Voltage Dip Acceptable)

2 KW Per HP DOL Generally OK

Optimize Toward 1 kW Per HP

Apply Loads in Multiple Steps

Start Large Motor Loads First

Use Alternative Starting Method

Analyze Cyclic Loads

Motor Starting Rules of

Thumb:

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Use Multiple ATS or Feeders

Power Command Gen-set

Essential

Loads

Utility

400 Amps

ATS

Non-essential

Loads 400 Amps

ATS

Equipment

Loads 400 Amps

ATS 2

3

1

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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 start current

Active speed control

Disadvantages: Size & cost

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Optimizing Generator Set Size

Step Loading the Generator

Apply Load in Increments

Apply Large Motor Loads First

Limit Voltage and Frequency dip

Ultimately reduce gen-set size

Step Starting Sequence

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Motor Starting Recommendations

Apply Load in Multiple Steps: Each Step Less Than Rated Load

Multiple Transfer Switches

Motor Control Center

Time Delay Relays Load Controls

Allow Recovery Between Steps (2-3 Sec)

Start Larger Loads First (After Critical Loads)

Apply Alternative Starting Methods: Soft Starter or VFD

Load Walk-in (Ramping)

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Motor Starting Suggested

Specifications

Maximum Allowable Voltage Dip:

20% to 35% With Maximum Step Load (15-20% Water Wastewater)

Include loads and steps in specifications

Require sizing reports to be submitted

Caution: Sensitive Loads Not On UPS

Alternator

Minimum Motor Starting kVA

Alternator Rating (KW/KVA)

Temperature Rise

PMG Excitation

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1. Increased Power System Component Losses and Heating

2. Objectionable Neutral Current in 3, 4 Wire Circuits

3. Excessive Generator Voltage Distortion

4. Control Interaction/Instability

Impact of Nonlinear Load Induced

Harmonics

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(Or 18 Pulse)

Current

THD(%)

Voltage

THD(%)

SCR = 20

Voltage

THD(%)

SCR = 100

Phase Controlled Rectifiers: Voltage and

Current Distortion

3 1

Need to reduce impedance by over-sizing alternator:

o 6 pulse rectifier on a UPS 40%

o 12 pulse rectifier 15%

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Leading Power Factor Loss of Voltage Control

Avoid Power Factor Correction or Lightly Loaded UPS/VFD Input Filters when on

generator; Capacitance Raises Generator

Output Voltage

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VFD Load Issues

Many Legacy 6 Pulse SCR Drives Require 100% Oversized Alternator

Severe Voltage Notching

Many Incorporate PWM Inverters: 40% Oversized Alternator

Complicated Controls, Potential Incompatibility Many Protective Shutdowns Process Restart Following Shutdown

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Static UPS Recommendations

Alternator Oversized 15 40% Depending On Rectifier Design

Isochronous Governor Required Size for UPS Nameplate Rating

Battery Charging Current Limit

Future Growth

Consider All Alternatives and Associated Cost Tradeoffs

Larger Generator

Higher Technology UPS

Filters

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VFD Recommendations

Oversize alternator or specify low temp rise alternator Many Legacy 6 Pulse SCR Drives Require 100% Oversized

Alternator

Many Incorporate PWM Inverters: 40% Oversized Alternator

Avoid sudden load changes step loads Reasonable ramp time. Disable PF correction capacitors Avoid low cost AVRs and specify PMG exciters Use VFDs with wide voltage and frequency tolerance

island mode

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Generator: Low Reactance Alternator (< 12% Sub-

transient)

Low Temperature Rise Alternator

Isochronous Governor

PMG Excitation

UPS/ VFD: Generator Compatible

Specify Adjustable Walk-in

Avoid Leading PF Input Filters

Suggested Specifications Related to

Nonlinear Loads

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Sample Project

Identify Generator Type Fuel Location Power Requirements

Define Load Requirements Code Considerations Facility Needs Electrical Characteristics

Optimize Generator Performance Understand Recommendations Consider Tradeoffs Specifications

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Hypothetical Standby System

Fire Pump

Soft

Start

Switchboard

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Fire Pump

Soft

Start

Fire Pump, 50 HP Soft Start, No Bypass

UPS, 400 KVA

Egress Lights, 100 Amps

Miscellaneous, 100 KW

Switchboard

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Fire Pump

Soft

Start

Switchboard

Any Code

Violations?

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Fire Pump

Soft

Start

Switchboard

Fire Pump

Controller, W/Soft

Start and Bypass

3 1 2 4

M1 200 HP DOL

M2 175 HP DOL

M3 600 HP VFD M4 600 HP VFD

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36

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Fire Pump

Soft

Start

Switchboard

Fire Pump

Controller, W/Soft

Start and Bypass

3 1 2 4

OOPS! Add

Another Generator

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Fire Pump

Soft

Start

Switchboard

Fire Pump

Controller, W/Soft

Start and Bypass

3 1 2 4

Try stepping

the motors

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40

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Fire Pump

Soft

Start

Switchboard

Fire Pump

Controller, W/Soft

Start and Bypass

3 1 2 4

Back To Single Generator

Not Advised

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Capital Cost Comparison

2500 kW $970,000 480 VAC, Sound Enclosure, 24 Hour Tank

(2) 1750 kW, Paralleling Switchgear $1,600,000

480 VAC, Sound Enclosure, 24 Hour Tank

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Questions?

Revised 4/24/12

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And the problem here is?

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Installation Review: Accessibility

46

Installation Review: Flow Through Design

47

And the problem here is?

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Installation Review: Exhaust Flex

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Installation Review: Outdoor Gensets

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Whats wrong with this picture?

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How many problems can you find on

this?

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Indoor vs Outdoor Sets

Outdoor Lower Cost

Security

Ease of Monitoring

Fuel Heating (prevent waxing)

Battery Heating

Coolant Heater Changes

Anti-Condensation Heaters for Alternator and Controls

Noise

Aesthetics

Proximity to Facilities

Indoor More stable environment

More difficult service access

Air flow issues

Noise

Still need coolant heaters

May need Anti-Condensation Heaters for Alternator and Controls

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Application Considerations

Indoor

Clearances Airflow Sound Attenuation Fuel System Connections and Wiring

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Application Considerations

Outdoor

Clearances Airflow Sound Attenuation Fuel System Connections and Wiring

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Questions?

Revised 4/24/12