Generator Set Transient Performance

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


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


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


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)


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


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


Genset Database Development


Frequency Recovery


Voltage Recovery


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.


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)


Alternator Response Capability

� It takes time to build voltage in a machine after load is applied.


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


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.

Engineering

Generator Set Transient Performance