UPS and Generators – How Long Can They Run For?
UPS Battery Constraints
The battery must enable continued operation over a time that is not:
- too short with respect to the backup time required by the application, in order not to underuse the battery and risk starting the Diesel Generator too often
- too close to the total battery backup time, in order to leave enough time for the Diesel Generator to start and ramp up before the end of battery power
UPS batteries have standard backup times of five to ten minutes, at a very reasonable cost in terms of installation and operation.
Diesel Generator Constraints
A Diesel Generator must operate a minimum amount of time (at least 15 minutes) to avoid excessive wear, and must be started only when necessary. A normal Diesel Generator requires approximately two minutes to start and ramp up to the point where the temperature is stable.
This process includes the following:
• the diesel engine driving the generator must start and reach its rated speed (eight to ten seconds).
• The motor and the generator must ramp up to full power (12 to 15 seconds).
• A Diesel Generator may not start on the first try and may stall during the ramp-up period.
A Diesel Generator combined with a UPS must not start until approximately three minutes after utility power has been lost:
• to provide the distribution-system automatic devices (reclosers, etc.) with the time required to restore power, if possible
• to avoid frequent and unnecessary starting of the Diesel Generator.
Our Conclusion and Suggestion
A backup power protection system providing ten minutes is well suited to Diesel Generators. It corresponds to a cost-effective, standard solution that ensures optimum use of the battery itself and the Diesel Generator.
A ten-minute backup time provides:
- three minutes for the distribution-system automatic devices to work, after which it is clearly necessary to start the Diesel Generator.
- two minutes for Diesel Generator start, ramp up and temperature stabilisation
- a margin to allow for a second Diesel Generator start, if necessary
- a second margin to take into account the real battery backup time (battery ageing, non-ideal battery conditions, etc.).
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