power consumption of UPS with respect to battery charging

[VIK]

Hi..

Please help me understand below situation:-

For Industrial UPS, say Galaxy VS of 20KW, the data sheet says it has efficiency of ~96% at normal operation on 400V and the battery charging power is ~20%. Also, suppose my total connected load on output side is ~15KW.

Now does it mean:

a) maximum load that can be connected at output is 96% of 20KW i.e. 19.2KW ? Meaning we can still connect more load from 15KW to upto 19.2KW ?

b) battery charging power consumption is 20% of 19.2KW i.e. 3.84KW ?

c) input power to UPS is sum of its output to load and battery charging power ?

d) input power consumption of this 20KW UPS, to be given to feeder supplier of the Building Power System shall be 20KW ? or it shall be 23.04KW (19.2 KW as max load and 3.84KW as battery charging power) ?

Kindly reply item wise.

[dpierce4776 · Accepted solution]

Typically speaking no electric device can take more than 80% of its input breaker size. In your case APC calls for a 40 amp input breaker and a 32 amp bypass breaker. So you would take the lower of these for load as the extra 8 amps on the input is reserved for charging the batteries under normal operation.

On top of that you have to also consider that the UPS in question can only handle up to 20KVA. Which is not "sustained" meaning that while it can "handle 20KVA" it should not be the goal. With all electrical devices the 80% rule for steady load should be maintained. 

For a UPS 4 things must always be considered.

1. The running load should not exceed 80% of the total UPS rating.
2. All breakers (rule of thumb) will trip under sustained loads of 80% of there rating.
3. If input and bypass breakers differ in size all calculations for sustained loads should be made using the lower of the two.
4. Battery charging is on top of total load in amps. and is usually a percentage of the total, not the variable load.   

Also I am not seeing in the data sheet where it says "battery charging power is ~20%" But all things considered that would be 20% of the total load on the input breaker. Not 20% of whatever load you have. So for a 20KVA UPS you can for rule of thumb use 4amps (20% of 20KVA in a perfect world of a 1 to 1 power factor)

So your total load calculation should be roughly total load amps = load amps + 4 amps. Since the bypass breaker is 32 amps your total load should not exceed 80% of that or 25 amps for the breaker, as well as 80% of the UPS rating which is ~16KVA or 25 amps. KVA is not usually equal to KW unless in a 1 to 1 power factor ratio which almost never happens.

If you are running 15KW that with a 400 volt system that works out to be 21.7 amps. Add your 4 amps for charging and you have 25.7 amps. I would not add anymore to the UPS it is fine under normal operation as you input breaker of 40 amps can handle that. But if you go to bypass of any reason you will be playing with tripping the breaker and dropping the load.

[VIK]

Thanks a lot for such a detailed explanation 🙏 🙏, much appreciated !!