What is the difference between buchloz relay and sudden pressure relay and pressure relief in transformer protection ?

Dear their purpose is totally differant one is electromechanical other is pure mechanical

bucholz relay is connected to main differantial and purpose is to isolate transformer from network incase of sudden gas formation , and they suppose to act most quickly to isolate the transformer ,where is PRD's are fixed on some location ,now adays on multiple location as well as at cable boxes to avoid any rupture / explossion as the arch formation is THE MOST UNPREDICTED PHENOMENON IN side transformer winding coils it can happen any where and at any moment it's like lightening of the nature read cigre2010 three papers on transforemr explossion issue AND PLEASE  REFER NEC-450 for furtehr transformer safety

Buchholz relay is a kind of oil gas activated security relay. 

The purpose is to give the protection to the transformer from the different faults happening in the transformer like

~ Short circuit

~ Inter-turn

~ Core 

~ Incipient

This relay will sense these faults and shut the alarm circuit