Symmetra UPS and Standby Generator

[Anonymous user]

I'm trying to figure out what size generator we need to get to work with a new Symmetra 16kW UPS we're installing. I'm being told 1.5 times, 3 times and 5 times the size, which equates to 25kw, 45kw and 70kw. This all seems so fuzzy and no one I talk to will commit to guarantee me that their recommendation will work except the guy telling me to get a 70kw unit. What I'm hearing is that the UPS doesn't see that the generator is providing power and so it switches back and forth between battery mode and utility line mode until the battery finally shuts down.

Anyone have any experience or history with this that actually knows exactly what to do?

Don't get me wrong, I appreciate feedback, but I don't need any more "Well I heard..." or "from what I've been told..." I need an "I have X size generator installed with an X size UPS and it works perfectly."

Thanks for any light that can be shed.

[upsguy_apc]

I would recommend reading these white papers to determine the proper thinking that goes along with adding a generator to your IT facility.

http://www.apcmedia.com/salestools/SADE-5TNRKS_R0_EN.pdf

http://www.apcmedia.com/salestools/SADE-5TNNEP_R4_EN.pdf

http://www.cumminspower.com/www/literature/technicalpapers/PT-7007-SizingGensets-en.pdf

[BillP]

Hi Rushweb,

A quick search for "Symmetra generator" in the kBase at www.apc.com/support returns the article "[Generator Sizing for Single-Phase Symmetra UPS Systems|http://nam-en.apc.com/cgi-bin/nam_en.cfg/php/enduser/std_adp.php?p_faqid=7300]."

Here is the full text of the article:

"A generator, Genset, should be sized to 1.5 times the size of the Symmetra's full capacity.

Many standard off-the-shelf Gensets are sized at 0.8 power factor for continuous duty operation ( > 30 mins of runtime ). When the Genset is sized < 1.5 times Symmetra frame size, the interaction of the Symmetra PFC (power factor corrected) circuitry with output impendence of the generator results in generator output waveform distortion. This distortion can lead to alarms on the Symmetra related to input voltage & frequency.


For these reasons, the recommendation is to size the generator at 1.5 times the Symmetra's full capacity and not exceed 75% of the generator's load capacity. (This load includes other devices that may be powered by the generator but not by the UPS such as AC, heating, and lighting)."

[Anonymous user · Accepted solution]

rushweb wrote:
I'm trying to figure out what size generator we need to get to work with a new Symmetra 16kW UPS we're installing. I'm being told 1.5 times, 3 times and 5 times the size, which equates to 25kw, 45kw and 70kw. This all seems so fuzzy and no one I talk to will commit to guarantee me that their recommendation will work except the guy telling me to get a 70kw unit. What I'm hearing is that the UPS doesn't see that the generator is providing power and so it switches back and forth between battery mode and utility line mode until the battery finally shuts down.

Anyone have any experience or history with this that actually knows exactly what to do?

Don't get me wrong, I appreciate feedback, but I don't need any more "Well I heard..." or "from what I've been told..." I need an "I have X size generator installed with an X size UPS and it works perfectly."

Thanks for any light that can be shed.

It is 2009. Technology is advancing faster than guidelines for load matching guideline. If the guideline was designed in 70s and 80s when computers were assumed to be straight power supplies (i.e. like home computers) with power factor of 0.5 to 0.7 and extremely high amount of third harmonics, then 3 -5 times oversizing is necessary. Generators do not like non-sinusoidal load(to put it in plain English, these loads are "pulsating load" that do not load the generator evenly).

If the Symmetra has the delta conversion with input side power factor correction (that is, UPS' dobule conversion circuit having power factor correction), then gross oversizing is unnecessary. Ask APC engineering if front end is power factor corrected. If so, tell the generator vendor that the "loads are near unity power factor". Ask about THD @ xx % of capacity @ unity power factor. Ask APC if it's within their acceptable limits. Double conversion UPS essentially regenerates power to correct power quality so it is more tolerant to poor power quality than standby UPS which only pass through power or derive power from battery. Standby unit would hunt back and forth between input and battery.

If you're going to also power HVAC on the same generator, it is not a big deal if the compressors starting causes brown out and frequency swing for a few seconds. UPS will cover it. You just need to make sure that the compressors in air conditioners can actually start without causing the generator to go into trouble mode and turn itself off.

If the HVAC system will be fed directly from the generator, they should have a five minute restart delay and be able to restart on its own without user intervention. If it doesn't have a delay, the compressor may not be able to turn over and cause the generator to go into overload trip. If it doesn't have a self-restart... well you know, it won't start. The room will get hot and things break. If the climate is mild, having outside air economizer is a pretty good idea to reduce AC dependency is a good idea, so the servers can continue to operate even if the AC won't fire back up or fails.

What will the generator power?