Recently while trying to synch to grid, turbine was spinning at 3600 with field closed in. Turbine tripped on vibration and excitation never opened up. As the turbine starts rolling down to gear, excitation will keep adding field voltage and current to bring terminal voltage to operating limits (One would assume).

What damage could occur if field was left on when on gear? Damage to the Alterrex and main transformer?

 

It is very dangerous situation. Generator rotor coils will be damaged permanently. As a result, you have to replace the coils. While doing the commissioning itself, they need to check these kind of safety interlocks like if the GCB closed with excitation off, it should give a trip to turbine. At any condition, if turbine trips, it should trip excitation immediately.