Utilities do not want their trfs to be tripped for minor seismic tremors and for trfs to be installed in seismic areas utilities specify special Buchholtz relay and manufacturers do provide them.
Some cases Buchholtz relay trip can be slightly desensitised by adjusting the trip contacts with out adversely affecting the relay operation in case of a major fault.This will be required in case of maloperation of relay contact with the starting of oil cooling pumps.Other case is the operation of relay for an external line fault,the case explained by Peter.Many years ago , a hydro station was feeding power to 220 KV grid.Employees agitating for higher wages thought of pressing their demand by toppling a 220 KV tower near to station. Buchholtz relays in all generator trfs in the station tripped as the fault current caused the movement of windings inside trf which in turn cause a surge of oil to conservator tank causing the relay to trip.This will happen esp in case of water cooled trfs where oil qty will be less.Problem was solved by slightly desensitising the relays.But such desensitisation will not be helpful in case of frequent minor tremors,which are common in seismic regions.
