For a protection of transformer Buchholz relay in transformer is mounted on the connecting pipes between the main tank & the conservative tank. It consists of two element. The upper element consist of a float & the lower element consist of a baffle & mercury switch.
Buchholz relay is mainly used with the transformer having capacity of 500 KVA.
Buchholz relay is to detect internal fault of the transformer.
Construction of Buchholz Relay
Buchholz relay in transformer is an oil container mounted on the connecting pipes between the transformer & the conservator tank. It has mainly two elements. The upper element consists of a float. The float is attached to a hinge in such a way that it can move up & down depending upon the oil level in the Buchholz relay container. One mercury switch is fixed on the float. This mercury switch is used for activating the alarm.
The lower element consists of a baffle plate & the mercury switch on it will energize the trip circuit.
Buchholz Relay Working Principle
The Buchholz relay working principle & operation is very simple. When the fault occurs inside the transformer such as insulation fault between turns, core high temperature, breakdown core of the transformer, the transformer oil splits into the gaseous form, CO & CO2.
The upper part of the Buchholz relay will be used for measure minor fault of the transformer & the lower part of the Buchholz relay will be used for measure major fault in transformer tank.
Under minor fault, the transformer oil splits into the gaseous form & gets collected at the top of the Buchholz relay due to this the mercury switch on the float get operated & energize the alarm circuit.
Under major fault like short circuit condition a large amount of oil will run from the main tank to the conservator tank. This making the mercury switch on the baffle plate to operate therefore the trip circuit get energize & the contact of circuit breaker gets open. This is used to detect only internal fault of the transformer.