On Load Tap Changer

BZZZT! Oh, you mean with proper personal protective equipment?

Since it has no load, why don't you turn it off and do some preventive maintenance/inspection on the gear? Sounds like a good opportunity.

Manually operating it will change the secondary voltage in the same way it changes for automatic operation, except that the change is initiated by you without it sensing a need to change. That's what the manual control is for. It won't hurt the tap changer - you can even do it on load. It could put the voltage outside of the desired range, though.

Thanks for the reply, the OLTC to our transformer was recently faulted during commissioning and I was wondering whether it had everything to do with the fact that it was operated on no load conditions

Try it, but go down in voltage rather than up and don't go too far. A while ago I had a problem with VFD's failing at the weekend. We tracked the problem down to one of the 20MVA transformers OLTC's drive motor. It had tripped out on overload so at the week end the voltage would rise and destroy the VFD's.