inrush current of pdu transfomers and inst. setting of ups breaker

[sahelrahman]

Hi,
I am currently doing a coordination study and I would like to know how to set the UPS instantaneous setting. I know I have to calculate and add the inrush current of all the pdu transformers and set the instantaneous setting of the static bypass switch circuit breaker to a level where it can withstand the inrush current of all PDU transformers. Does the same apply to the instantaneous setting of the UPS circuit circuit breakers too?

Any help would be appreciated..

Thanks

 

[sibeen]

What do you mean by 'the UPS circuit breakers'?

Are these circuit breakers internal to the UPS or the circuit breakers on the switchboard feeding the UPS?

Most UPS systems don't use circuit breakers as their internal switching elements. Normally just a basic switch is used for operation and fuses will be used to safeguard the rectifier input, inverter output and bypass (static switch) lines.

 

[sahelrahman]

By UPS breakers, I mean both the breakers on the switchboard and the internal breakers. These ups has internal breakers with long time and instantaneous settings. So I wonder, what should be the inst. settings of both the breakers (internal breakers and the one on the swbd feeding ups) ?

Thanks

 

[sibeen]

Hopefully the UPS manufacturer has done his homework and the settings of the circuit breakers internal to the UPS are set for correct fault operation.

As for circuit breakers external and Upstream of the UPS it depends how they are configured.

Is there a single circuit breaker which supplies the UPS or is there individual circuit breakers that supply the rectifier input and the Reserve (bypass) input?

During the inrush to the PDU transformers the UPS will transfer to bypass and wait for the current to subside before transferring back to the inverter supply. Your upstream circuit breaker, supplying the bypass line, must be able to handle this short term over current.

With a separate circuit breaker feeding the rectifier input, this circuit breaker does not see the inrush current as the UPS has transferred to the bypass line.

 

[prashious12]

@sahelrahman : Its very good point.

About setting the internal CB instantaneous setting, I believe, you have to make sure that internal breaker also should not trip on inrush current though the UPS may transferred to Bypass because of inrush current as rightly mentioned by Sibeen. The internal Breaker is provided as safeguard to the inverter rectifier Unit of UPS when the load is fed through UPS where as the Main is provided for Protection specifically when its on bypass and back-up when its on UPS.
Set the UPS internal breaker to take care the inrush-current but set it to trip on actual fault and the Bypass on little above (I am assuming the bypass is main Breaker which supplies UPS in normal run and Bypass on abnormal run, correct me if i am wrong by giving the either single line diag or configuration).

prashious