Dual UPS replacement - one at a time

[ankledeep]

I have a pair of 21 year old 50 KVA UPSs that are going to be replaced with a pair of new 150 KVA units. The input is 480 3 phase, the output is 120/208 to a pair of 42 position panels (one panel per UPS, 2/0 cable 100 feet away. The load on each UPS is 16 KVA (UPS-1: A phase=48amps, B phase=23amps, C phase=53amps, neutral=27amps UPS-2: A phase=40amps, B phase=38amps, C phase=38amps, neutral=28amps). 20 years ago I put a 3 phase 100 amp tie breaker in each panel to allow for powering(back feeding if you will) the opisite panel in the event of a UPS failure or UPS off line for maintenance. (I have never used the breakers. Never had a failure and never have done any off line maintenance :-0 .....)

Well, it's replacement time(one UPS at a time)!!! One new STS/PDU is in place. The UPS portion will soon be ready to feed the STS/PDU. It's time to wire the new PDU to feed the panel's main input breaker and neutral (while keeping the panel HOT via the 100 amp tie breakers)to supply the loads (the meaning of the U in UPS!!!!) I have closed one of the 100 amp tie breakers, measured across the second open tie breaker, phase A to A = 5 volts, phase B to B = 4.5 volts, phase c to c = 3.4 volts neutral to neutral = 1.8 volts.

It all looks good to go to me!! I will put BOTH UPSs in "Maintenance Bypass" (remeasure all of the voltages and neutrals prior to connecting the two neutrals together and closing the second tie breaker), Close the second tie breaker and open the panel's Main input breaker, Since I will need to remove the phase feeds from the one panel's main input breaker as well as the neutral in order to rewire the panel feed breaker and neutral to the new STS/PDU.

I don't see any potential (pardon the pun) problems in doing this....

I have consulted three of our Electrical Engineers (Telecom, Substation & Relay protection) and they say it should be just fine.....

Does anyone out there see a problem with doing this??
Has anyone done anything like this, as far as a live UPS replacement goes?????

Any comments???? Thanks!!!!

PS I just hope that the new UPS/STS/PDU's 480 to 120/208 transformer's unloaded output voltage will be close for going back to the new UPS system!!

 

[davidbeach]

I've seen similar done before with no problems. Once you have the main breaker open on the one panel you can go out of maintenance bypass until you need to transfer the panel onto the new UPS.

 

[ykee]

I was aware that a UPS could be used as a transformer, but was not aware that it could do so in bypass mode. I guess it would have to in order to be useful. Presumably there is an xformer right at the input to the UPS & this does the reduces the voltage level, not the UPS's inverter?

 

[ankledeep]

ykee,

Actually, the transformer is at the output of the inverter and static switch. The inverter runs on a 540 volt battery plant. The bypass breaker bypasses the static switch from the 480 volt input bus around the inverter and static switch to feed the transformer directly. The transformer is a 480 to 120/208.