DC Float Voltage in UPS Systems - why 540v?

[shmojoe]

I have few building sized UPS (200kw - 500kw) and dozens of small rack-mount DC UPS's (1kw - 8kw) for telco closets and such.

One question that keeps bugging me is the float voltage that the batteries are kept. On the building UPS's, strings of (40) 12v batteries (in series) are kept on a boost charge of 13.5v - for a total string voltage of 540vdc. This then feeds the inverter (when needed) to supply the UPS 480vac output. I have always believed that the battery strings were kept at 540vdc to accommodate the load inrush (a french phrase I can't recall) and how this interacted with batteries. Is that the sole reason, or does it have something to do with inverter?

The same situation applies to the small rack-mount DC UPS - 54vdc float charge when the load is supplied 48vdc (only different here being that there is no inverter... just DC to DC load).


Many thanks...

ShmoJoe

 

[GeorgeB]

I'm a battery novice compared to some here, so you may (probably will) get better answers ...

From a gross voltage issue, the battery needs to be about 2.8* the RMS AC voltage, so 540 would allow about 190VAC out as a sine wave. With some distortion allowed, they probably get 208-3-60.

From a why 540 vs. 480 ...

A really brief battery charging lesson here. A well designed charger will charge to a CELL voltage of about 2.35V (varies somewhat with battery chemistry) then, with no load, drop to a (float) cell voltage of about 2.25V. With your 240 cell system, 540 would be the point it dropped to, and would try to hold. As it fell, and I don't have a good number for to where, the charger would kick back in and, at least periodically, take it up to the 2.35V level (564V for the string).

Does this help?

There is a very good website which maintains a battery FAQ ... imagine that, www.batteryfaq.org ... which offers almost certainly more than you want to know.

 

[shmojoe]

With you on the charging setup, but I don't follow your first line:

"From a gross voltage issue, the battery needs to be about 2.8* the RMS AC voltage, so 540 would allow about 190VAC out as a sine wave. With some distortion allowed, they probably get 208-3-60."

The UPS output is 277vac per phase. Could you explain why DC voltage needs to be 2.8* the RMS AC voltage? I've have never heard that one.

Thanks,

ShmoJoe

 

[Besoeker]

I have few building sized UPS (200kw - 500kw) and dozens of small rack-mount DC UPS's (1kw - 8kw) for telco closets and such.

One question that keeps bugging me is the float voltage that the batteries are kept. On the building UPS's, strings of (40) 12v batteries (in series) are kept on a boost charge of 13.5v - for a total string voltage of 540vdc. This then feeds the inverter (when needed) to supply the UPS 480vac output. I have always believed that the battery strings were kept at 540vdc to accommodate the load inrush (a french phrase I can't recall) and how this interacted with batteries. Is that the sole reason, or does it have something to do with inverter?

Assuming they are lead acid batteries the usual charging voltage is around 2.25V per cell but may vary some if the charger has temperature compensation.
This is a chart I made maybe a dozen or more years ago for our own chargers.

 

[GeorgeB]

The UPS output is 277vac per phase. Could you explain why DC voltage needs to be 2.8* the RMS AC voltage? I've have never heard that one.

Thanks,

ShmoJoe

I'm probably incorrect. The peak-to-peak of a sine is 2*sqrt(2) of the rms and I was thinking along the lines of needing that range. I suspect, from your question and thinking of others I've seen that they are able to "create" positive and negative so it would be 1.4 instead.

 

[__dan]

Float voltage has everything to do with cell lifetime. The tolerance is very tight. The manufacturer will specify cell float voltage +- maybe 3 or 4 hundredths of a volt. Over or undercharging will quickly shorten cell lifetime. Float voltage is not a variable, it is exact, and cells are added in series to achieve bank output voltage.

The large UPS has a design rating, something like 10 minutes at 100% load on battery. The inverter will have a low voltage cutout as bank DC voltage drops to the limit, maybe ~ 490 volts on a 540 volt string.

Battery maintenance is high. The smaller units, batteries are disposable pretty quickly, regularly. I would start looking at the specs, what are the ratings and the cell float voltages supposed to be and compare to the actual. If the bank draws current in float, that may be a bad sign. Are the units including the batteries on a maintenance contract? It's a specialty.

 

[shmojoe]

I get the float tolerances and temp compensation for charging batteries. And yes, the UPS dropout is a little above 400vdc. My curiosity was really just around the figure (540vdc). If the inverter is putting out 277vac on all three phases; to me, it would seem that something like 280 to 290vdc would be fed from the battery bank.

Thanks for the input.

 

[jdsmith]

As a side comment, DC bus voltage is one of the specifications that one should look at when evaluating UPS's. Many of the more robust units use a lower bus voltage and a multistage output inverter. Two of the most popular "industrial UPS" manufacturers quoted me 225 kVA units, 3 phase 480 input, 3 phase 480 output, with a 360VDC bus. Why 360V? I have no idea, but those manufacturers have determined that the best balance of reliability and cost for petrochemical and nuclear plants happens to use a 360 VDC bus. The typical commercial and low-end datacenter units are very cost competitive, have designs that are difficult to maintain, and incidentally have higher DC bus voltages.

Added thought: The best 3 phase UPS's are typically designed with Y connected electronics, so a fault on one phase will not affect the other two phases. Perhaps the folks using 540V busses are using delta connected inverters, which will cause L-G faults to affect multiple phases, but the inverter can be produced at a lower cost?

 

[__dan]

Small UPS's could be anything from something nice to something like two squirrels running on a wire wheel.

The large UPS, the output section after the inverter may have a delta to Y transformer and the IGBT's are closing line to line into the 480 ? v delta primary. Really, the line to line delta primary voltage could be anything with the turns ratio chosen to yield 480 / 277 Y from the DC bus voltage. One of the reasons for this would be to give the IGBT's a known calibrated impedance to close into. Things would get really squirrely if the large UPS IGBT's were closing directly into an open circuit at the end of a long run of wiring (excess capacitance). It would trip off. The UPS output would be the transformer Y secondary, probably seperately derived with system bonding jumper and GEC.

In utility sized equipment there are "voltage restorer" inverters that I believe will work from a lower variable input voltage to maintain the inverter output voltage. I have no idea what they're doing internally, maybe really big squirrels.

 

[shmojoe]

Thanks for the input. I had not considered the connection to the inverter (delta v Y).

jdsmith - Have not seen any 360vdc busses as of yet. Would like to learn more about the difference between the two. I have a PM coming up on one of the Pillar 500KVA units - I will pick the guy's brain and see what he has to say on the topic.

ShmoJoe

 

[jdsmith]

The two UPS manufacturers that come up most often in refineries and other petrochemical applications are Ametek (formerly known as Solidstate Controls or SCI) and Cyberex. You may want to look at their literature or have a technical conversation with them about bus voltages and inverter topologies. They both have similar thoughts that are different from many of the other manufacturers. I don't know your situation and I have no idea if these folks would even have a product that makes sense for you, but they both have good application engineers that might be good for you to talk to.

 

[T.M.Haja Sahib]

My curiosity was really just around the figure (540vdc). If the inverter is putting out 277vac on all three phases; to me, it would seem that something like 280 to 290vdc would be fed from the battery bank.

For a given load on the output side of the UPS,higher is the battery bank voltage,lower will be the D.C current and longer will be the life of the battery bank.However,increasing voltage causes increasing cost.So 540v dc might have been chosen by the manufacturer as optimum value.