Buchholz Relays

The following protective devices are used so that, upon a fault development inside a Transformer, an alarm is set off or the Transformer is disconnected from the circuit. In the event of a fault, oil or insulations decomposes by heat, producing gas or developing an impulse oil flow.
To detect these phenomena, a Buchholz relay is installed.

Buchholz Relay
The Buchholz relay is installed at the middle of the connection pipe between the Transformer tank and the conservator.
There are a 1st stage contact and a 2nd stage contact as shown in Fig. (28). the 1st stage contact is used to detect minor faults.
When gas produced in the tank due to a minor fault surfaces to accumulate in the relay chamber within a certain amount (0.3Q-0.35Q) or above, the float lowers and closes the contact, thereby actuating the alarm device.
 

Fig. (28). Buchholz Relay

The 2nd stage contact is used to detect major faults. In the event of a major fault, abrupt gas production causes pressure in the tank to flow oil into the conservator. In this case, the float is lowered to close the contact, thereby causing the Circuit Breaker to trip or actuating the alarm device.