Re: 600V Nominal
Sorry, I didn't give the whole picture. Slightly oversimplified, this facility will have two 13.2kV 500A services in a '2N' configuration (completely redundant), and (10) 2MW 13.2kV diesels in a '2N' configuration as standby power. The expected load is approximately 8MW (10MW with some cushion).
We are trying to utilize the switchgear and conductors to their maximum capacity.
Since the substation transformers are multitap, but not automatic multitap, they are set initially to provide appropriate voltage when the load reaches steady state (approximately 3 months from when the door opens). The client doesn't want to mess with the transformer taps when the building is "live". The transformer taps will be selected so that the voltage would provide 600V at the secondary of the multiple substation transformers at steady state. Since the substation transformers will have approximate;y 6% impedance, the secondary voltage for the first 3 months (before steady state) will be approximately 3 to 5% high, so to account for the voltage loss/drop across the transformer when they are eventually loaded at steady state. So, before loading @ steady state, the transformer secondary voltage may be in excess of 600V, like 620V or so.
Hence my concern. Normally, in other designs/facilities, when we run high with a 480V secondary, the equipment is still well below its rating of 600V and not a problem. Since now we will be running @ nominal 600V, the actual rating of the equipment and conductors, when the voltage is high, we will be exceeding the voltage rating of the equipment and conductors.
For example, take any industrial facility that is in full operation and measure the voltage at the service entrance transformer. Then turn off all the loads in the facility and measure the service entrance voltage. You will see the voltage is higher when the transformer does not have to contend with the full extent of its own impedance.
