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Generator Sizing
Considerations
M. Scott Thomas Application Engineering Manager
2
Agenda
Generator Ratings Information Required for Sizing Motor Loads Non-linear Loads UPS and VFDs Sample Project
Question and Answer
3
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
4
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)
8
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
9
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)
10
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.
11
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
12
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
13
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
14
Example of Alternator Locked Rotor Curve
Instantaneous Voltage dip function of alternator reactance.
15
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:
16
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
17
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
18
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
19
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)
20
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
21
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
22
(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%
23
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
24
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
25
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
26
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
27
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
28
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
29
30
Hypothetical Standby System
Fire Pump
Soft
Start
Switchboard
31
Fire Pump
Soft
Start
Fire Pump, 50 HP Soft Start, No Bypass
UPS, 400 KVA
Egress Lights, 100 Amps
Miscellaneous, 100 KW
Switchboard
32
33
Fire Pump
Soft
Start
Switchboard
Any Code
Violations?
34
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
35
36
37
Fire Pump
Soft
Start
Switchboard
Fire Pump
Controller, W/Soft
Start and Bypass
3 1 2 4
OOPS! Add
Another Generator
38
Fire Pump
Soft
Start
Switchboard
Fire Pump
Controller, W/Soft
Start and Bypass
3 1 2 4
Try stepping
the motors
39
40
41
Fire Pump
Soft
Start
Switchboard
Fire Pump
Controller, W/Soft
Start and Bypass
3 1 2 4
Back To Single Generator
Not Advised
42
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
43
Questions?
Revised 4/24/12
44
And the problem here is?
45
Installation Review: Accessibility
46
Installation Review: Flow Through Design
47
And the problem here is?
48
Installation Review: Exhaust Flex
49
Installation Review: Outdoor Gensets
50
Whats wrong with this picture?
51
How many problems can you find on
this?
52
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
53
Application Considerations
Indoor
Clearances Airflow Sound Attenuation Fuel System Connections and Wiring
54
Application Considerations
Outdoor
Clearances Airflow Sound Attenuation Fuel System Connections and Wiring
55
Questions?
Revised 4/24/12