8.8 Nonelectrical protection
8.8.1 Pressure devices
A very sensitive form of transformer protection is provided by relays based upon a mechanical
principle of operation. When a fault occurs inside an oil-filled transformer tank, the fault arc
produces gases, which create pressure waves inside the oil. In the ‘conservator’ type of tank
construction,2 which is more common in Europe, the pressure wave created in the oil is detected
by a pressure vane in the pipe which connects the transformer tank with the conservator. The
movement of the vane is detected by a microswitch, which can be used to sound an alarm, or trip
the transformer. This type of a relay is known as a Buchholz relay, named after its inventor.
In the USA, the more common transformer construction is of the tank type with a gas cushion
at the top of the tank.5 In such a transformer, the pressure wave is detected by a SPR mounted on
the side of the transformer. The mechanical sensor consists of a bellows and a pressure equalizer,
which together is insensitive to slow changes of pressure, for example those caused by thermal or