Standby Generator With Step-up Xfmr

[cabletray]

When a 480 V standby generator is used to feed a 12.47 kV service entrance bus through a step-up xfmr, it seems difficult to attain the IEEE Red Book Ansi Range A 117 V on a 120 V base. With an approx. 5% voltage drop in the xfmr, the generator output (120 V on 120 V base) minus 5% (not counting feeder cable voltage drop) is only 114 V at the service bus and the standard requires a minimum 117 V. What is an effective solution? Increasing the generator full load voltage output by changing the exciter potentiometer setting? Using a lower xfmr tap setting?

 

[dpc]

You can't do anything about the transformer voltage drop, so if you want more voltage, you will have to either:

Increase generator voltage

Change transformer tap

Revise voltage regulator sensing such that it is looking at the 12.47 kV voltage instead of the 48O V voltage. If you do this, the voltage regulator will increase the excitation automatically to account for the transformer drop. You will need some overvoltage protection or limits on the 480V side to make sure you don't raise the voltage too high.

 

[mc5w]

I would also be careful to not have an amount of excitation that will burn up the exciter windings. The voltage regulator should have some means for programming or adjusting the maximum field current. Usually, a single pole motor overload relay with the contacts to trip the generator off line will do this as a backup device in case the current limit goes bad.