Found this interesting article (entitled Minimizing UPS and Gen-Set Incompatibility) explaining about compatibility issues with UPS and gensets, primarily due to:
- Failure of the UPS to acquire and accept power from the gen-set,
- Instability of the gen-set frequency or voltage when powering the UPS,
- Inability of the UPS to synchronize its output to the gen-set,
- Inability to enable the operation of the maintenance bypass circuit,
- Excessive output voltage distortion, and
- Power transfer issues.
It provided an excellent list of recommended solutions. Excerpt of the recommended solutions are posted here:
- Gen-sets providing power to UPS loads should include an Automatic Voltage Regulator (AVR) immune to misoperation due to waveform distortion.
- When available, the gen-set should have a sub-transient reactance of 15% or less based on the standby rating of the unit. Lower reactance yields better performance, however, when the system is large, the short-circuit current available from the gen-set may be too large for the the switchgear you’re providing — particularly in utility paralleling situations.
- Specify UPS systems with input filters (especially with 6-pulse UPS units) to minimize voltage waveform distortion. Check the reverse reactive capability of the proposed gen-set to be sure the capacitive components of the filters will not disrupt the gen-set. A permanent reactive load bank will help the generator set maintain voltage regulation while the UPS is ramping on.
- Gen-sets will usually need isochronous governors to provide 50 Hz or 60 Hz output at all load levels. This enables the UPS output to be more effectively synchronized to the generator power and help avoid activating “bypass disabled” alarms.
- Gen-set sizing should be based on the total load applied by the UPS under worst-case conditions. This means that even though the UPS normally operates at a relatively low-load level, the gen-set should be sized for full output of the UPS. Must also consider the battery recharge rate, UPS power factor, and efficiency at full load. You should use a system design limit of no more than about 10% to 15% total harmonic distortion (THD).
- Specify a program transition (“open” time of 0.5 sec or more) for transfer switches feeding UPS loads. A separate isolation transformer serving the UPS will be useful in avoiding operational problems, provide better power quality to the load, and prevent ground-fault sensing problems.
Tags: Data Center, electrical, generator, UPS