If I understand you correctly you have a 300KVA generator, and a 300KVA UPS. In my opinion your generator is way undersized.
Here are a few points:
1: A UPS of this type is probable a dual conversion type unit, meaning rectifier-to-dc, dc-to-inverter. Each stage at best will have 80 to 90% efficiency. So work the math backwards. 300 KVA out the UPS output to the dc stage through the inverter is 375 KVA. 375 KVA from the dc stage back to source through rectifier is 468 KVA at full load. Since you mentioned it is an older UPS the efficiency could be as low as 60% if they use SCR rather than switch mode rectifiers.
2: Another factor is the battery recharge current. Not only do the rectifiers have to supply the inverter power, but they must also have the capacity to recharge the batteries at the same time. This is usually an additionally 20 %. So now lets back up to the DC stage and see what the rectifiers must actually be able to produce and work backwards. 375 KVA from the last step plus 20% recharge current equals 450 KVA at the rectifier output. Assuming the rectifiers efficiency is 80% we get 562.5 KVA input power requirement at the UPS during recharge.
3. Now lets talk generators. Just because a generator is rated at 300 KVA doesn’t mean it can deliver 300 KVA.
Taking all this into consideration a 300KVA UPS would need a minimum 500 KVA generator depending on the efficiency of the UPS, actual load, and battery recharge demand. I design large scale UPS with generator back up and typically we double the generator output over the UPS depending on the above items. Sounds to me who ever designed it overlooked some things.
Good Luck
Dereck
[This message has been edited by DBC1 (edited 03-15-2003).]
