We have recently replaced 11KV XLPE cable of Alternator (from alternator to 11KV GCB) of one of our Gas Turbine (Fr 5, GE make, 20 MW).
After replacement of the HT cable when we first started the GT, we wanted to raise the voltage manually step by step so that voltage would build up gradually and hence put the DAVR (Brush make) in manual mode.
But the moment we switched ON the excitation, the voltage rose automatically to 11 KV, which was not desired. Kindly guide us where might the problem be for rectification purpose.
Based on the information provided, it's most likely the problem was the assumption that by putting the excitation in Manual that it would start at some minimum (very low) voltage and could be raised using the Manual RAISE VOLTAGE button (or RAISE CURRENT).
Most excitation systems have on off-line voltage setpoint (or off-line current setpoint) they will go to when the unit is running at rated speed and the excitation system is switched on.
It's most likely that the manual exciter wasn't lowered to its minimum (which usually isn't all that low, for protection purposes) before it was switched on.
Someone at site needs to read and study the Brush excitation manual to understand how it works and is configured to determine if what you want to accomplish can even be done, without some kind of special or unusual circuitry or configuration.