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Determining Optimal Start Time Delays For Generators




August 8, 2013 - Data Centers

The standby-generator start-time delay programming adjustment is driven by a number of factors.

Opinions range from 0.5 seconds to 30 seconds. One concern is that the majority of utility power bumps last less than 3 seconds. Therefore you can have quite a few unnecessary engine starts with start programming set for less than 3 seconds.

The short power bumps typically result from utility re-closers that automatically open and close quickly in an attempt to "shake loose" a problem on the lines, such as tree branches, animals, etc. Re-closers are often programmed by the utility to open and re-close quickly a couple times, resulting in power bumps of 1 second or less, then to remain open up to about 3 seconds on the third and typically final attempt. The fourth time a re-closer opens it typically stays open for many minutes or longer (awaiting manual intervention).

Another concern is that many times the utility fails, then returns, and almost immediately fails again. A generator start timer (typically located in the ATS or generator switchgear) will typically reset when power returns. For UPS backup energy storage, the recharge time is typically 10 times longer than the discharge time, so rapid short utility bumps can cumulatively draw down short-term storage (a very real concern for flywheel UPS systems). Multiple 3 to 10 second utility power failures within a short duration can leave a UPS flywheel too depleted to provide ride-through time.

Most engineers recommend programming generator start time delay settings in the 3 to 5 second range. If utility power is down for more than several seconds it will probably be down for several minutes or hours, so you might as well start your engines. It's best not to challenge UPS batteries any longer than necessary and risk the chance of a UPS failure. The longer a data center runs with no cooling while waiting for generator power, the greater the risk of the data center overheating. However, starting the generator for every light flicker takes its toll on equipment, with impacts on reliability, maintenance, and environmental emissions.

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