Buchholz Relay

• The Buchholz relay is a safety device which protects the transformer from internal incipient faults(slow developing faults such as insulation failure of winding, core heating, fall of oil level due to leaky joints etc..).
•  It is the gas actuated relay installed in oil immersed transformers for protection against all kinds of faults.
• A buchholz relay can be installed on transformer having the rating higher than 750 KVA.
• Buchholz relay is placed between the main tank and the conservator.

Construction:

Buchholz relay is placed between the main tank and the conservator. Buchholz relay consists of an oil filled chamber. It consists of two element, the upper element, and the lower element.Where the upper element includes a mercury type switch linked to a float and The mercury switch on the upper float is connected to an alarm circuit during incipient faults Similarly, the lower element comprises of a mercury switch increased on a hinged kind flap situated on the straight line of oil flow and that on the lower float is connected to an external trip breaker in case of severe internal faults.

Working :

1. In case of incipient (minor slow developing) faults within the transformer, the heat due to fault causes the decomposition of transformer oil in the main tank and bi product of decomposition contains more than 70 % of hydrogen gas and other gases. Hydrogen gas bubbles are light in weight tries to go into the conservator through buchholz relay. These gas bubbles flow in upward direction and get collected in the buchholz relay. The collected gas displaces the oil in buchholz relay and the displacement is equivalent to the volume of gas collected. The displacement of oil causes the upper float to close the upper mercury switch which is connected to an alarm circuit. Hence, when minor fault occurs, the connected alarm gets activated.
2. If serious faults occurs in the transformer, an enormous amount of gas is generated in the main tank. the oil in the main tank rushes towards the conservator through the buchholz relay and in doing so tilts the flap to close the contact of mercury switch. This completes the trip circuit to open the circuit breaker controlling the transformer.

Advantages of Buchholz relay.

• Simplest form of transformer protection.
• It detects the incipient faults at a stage much earlier than as possible.

Disadvantages of Buchholz relay.

• The relay is used only in oil immersed transformer.
• It can only detect the fault below oil level.
• The response time of the relay is high.The minimum operating time of the relay is 0.1 seconds.

Related terms:

Fuses & Circuit breakers

Protective Relays