Generator Set Transient Performance
- 1. Generator Set Transient Response How good should it be? How good is good enough?
- 2. 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 ACT PWR EXC ENGINE
- 3. Transient Performance The Energy Transfer Process 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
- 4. 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 ECM GOV MPU Fuel to Engine PWR to LOADS EXC ENGINE
- 5. 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)
- 6. 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
- 7. 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 %VOLTAGE
- 8. Transient Performance Genset Database Development
- 11. 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.
- 12. Transient Performance Engine Fueling and Speed during Transient LOAD APPLIED FUEL LIMITED STABLE AT FULL 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)
- 13. Transient Performance Alternator Response Capability It takes time to build voltage in a machine after load is applied.
- 14. 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
- 15. 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 verify installation capability.