UPS Bypass Transformer Grounding

[tranny]

I have a UPS system in which the essential load is normally fed from the ups. I event of an overload or failure of the ups, a static switch will transfer the load to bypass. Bypass consists of a 480 to 120 v single phase constant voltage transformer. The essential bus that is fed from this transformer and the inverter consists of a 120 volt panel where there are two pole breakers being used. The line and neutral wires can be switched with the breaker. Is this normal??
Also, should the secondary of the constant voltage transformer be grounded anywhere? Right now, X1 and X2= 120V, X2 and X3= 120 volts and X1-X3= 240v. the only wires hooked up is x1 and x2. X3 is floating. if there was a fault on one of the secondaries, nothing would trip, right? Unless the other leg faulted, then there would be smoke, correct? Thank you

 

[peebee]

No, switching the neutral is not normal.

See IEEE Std. 142, Chapter 1, Figure 10 (Configuration 2) for the recommended connection. Chapter 1 is available for free download from IEEE at

http://standards.ieee.org/colorbooks/sampler/Greenbook.pdf

.

 

[dydt]

hi..

on your 120V panel, you should not run your neutral wire via the breaker...only the hot wire..my answer then is that it is NOT normal..

it seems that you are only using one winding of the secondary side of the CVT which is X1 and X2..you are not using the full capacity of the CVT and this may create an assumption that you are fully loaded on an undersized winding..you should be able to connect the other 120v winding in parallel with this..just watch out for the polarity..as far as i know the primary of the CVT is electrically isolated with the secondary winding and having said this ..you should be able to ground this leg and feed your neutral on the panel..

dydt

 

[tranny]

that is the way it is shown to hook it up in the book. The CVT is a General Electric "Stabiltron" the now one that was installed is a SOLA.

 

[DanDel]

What exactly is the load being served? Is it designed to be an isolated system?

 

[tranny]

Yes, that is the purpose. The loads are SCADA, Telephone, DCS System, Power System Stabilizers, RTU's, Fire Alarm, some lighting and receptacles.

 

[DanDel]

Disconnection of the neutral for an isolated system may not be unusual, it depends on the design. Usually this type of system has monitoring equipment for ground faults.
I would pose some questions to the manufacturer.

 

[nukeman]

I have a transfer switch on a ungrounded system that switches both the hot and neutral.

AS DYDT stated, the secondary of a CVT is isolated from the primary so you should be able to ground its neutral.

You are correct in stating that if there were a fault on the secondary nothing on the primary would trip. In fact, you could put a bolted short across the output of the CVT and the input should not trip. The CVT is a current limiting device.

 

[dydt]

i agree with nukeman..

that's the beauty of the CVT


dydt