Generator Set Transient Response
How good should it be?How good is good enough?
Transient Performance
GenSet Performance
� Voltage Level is More Consistent than Utility
� Frequency Control is Poor Relative to Utility
� Load Changes Result in Frequency & Voltage Change
AVR
GOV MPU
ACTPWR EXC
ENGINE
Transient Performance
The Energy Transfer Process
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� Engine Provides HP to Rotate Alternator� Alternator “Converts” Mechanical to Electrical Energy� Motor “Re-converts” Electrical to Mechanical Energy to Rotate
Load� To start the motor (or any other load), engine must provide
HP(k/W)/torque needed, and alternator must provide Vars
Transient Performance
GenSet Transient Performance
� Engine slows and voltage drops until fuel rate and/or excitation level increases– Governor and AVR can’t
predict change in load, so,
– Load change = V, Hz change
� Magnitude of changes depend on:– Size of load relative to
genset– How fast fuel rate and
excitation can be changed– AVR V/Hz function
AVR
ECMGOV
MPU
Fuel toEngine PWR to LOADS
EXC
ENGINE
Transient Performance
What Does the Load Need?
� Resistive Loads– Sufficient kW to pick up load and recover speed
� Motor Starting:– Initial voltage dip not more than 35% (less is better)– Voltage recovery during acceleration (min. 90% per NEMA)
• Provides torque necessary to accelerate load
– Sufficient kW to operate load
� UPS– Will drop out on any nearly transient condition
• Typically depends on voltage range (± 10%), frequency range (± 3 Hz), AND slew rate (0.5 – 1.0 Hz/sec)
Transient Performance
NFPA 110 Prototype Test Report
� Report documents compliance to NFPA 110 for Emergency power systems
� NFPA 110 compliance requires 0.8 PF test and allows 1.0 in field
� NFPA 110 has no performance requirements other than to recover
� 480V machine
Transient Performance
What’s the voltage dip?
� 18%, right?
� Nope! This is alternator only!
VOLTAGE ROLLOFF
0
20
40
60
80
100
120
45 50 55 60 65
FREQUENCY%
VO
LTA
GE
Transient Performance
Genset Database Development
Transient Performance
Frequency Recovery
Transient Performance
Voltage Recovery
Transient Performance
160 VAC
4.2 Hz
� Figure 3: 2000kW generator set with 2000kW 0.8 power factor load applied. 33% voltage dip,
215 VAC
8.4 Hz
� Figure 4: Same generator set as Figure 3, but with 1.0 power factor load. 45% voltage dip.
Transient Performance
Engine Fueling and Speed during TransientLOAD APPLIED
FUELLIMITED
STABLE ATFULL LOAD
� Engine begins to respond to speed change within 20 mS.
� Stable recovery ~4 sec
� Limited by turbo lag (ability to get air into engine)
Transient Performance
Alternator Response Capability
� It takes time to build voltage in a machine after load is applied.
Transient Performance
Slew Rate
� Typical allowable slew rates are very low relative to genset capability
� Genset frequency slew rated depends on load magnitude� Any load step may cause bypass to drop out or rectifier to switch
off� Most UPS ramps on: good for genset and system performance
4.2 Hz
0.5 Hz/Sec
Transient Performance
Summary and Conclusions
� Performance of a genset depends on inertia in genset, energy stored in field (indicated by X”d), AVR performance, and engine fuel/air induction system performance
� There is some variation from machine to machine� Follow NFPA110 level 1: full load test at rated load and power
factor to demonstrate recovery; spec X”d; spec digital AVR and recovery kVA (to 90% voltage); spec voltage dip and recovery at 0.8 PF (and 1.0PF if desired to test in field)
� Best practice is to factory test gensets at 0.8PF and 1.0PF, and request strip chart recordings of performance at 25%, 50%, 75% and 100% on and off load tests.
� Test gensets at site with 1.0PF testing at the jobsite to verifyinstallation capability.