Buchholz relay is a gas actuated relay. It is generally used on all oil immersed transformers having a rating more than 500 KVA. It is installed between the conservator and main tank. Therefore, such relay can only be installed in the transformers equipped with conservator tanks.
The construction of a Buchholz relay is shown in Figure. It consists of two hinged floats in a metallic chamber. One of the floats actuates the mercury switch connected to the external alarm circuit and the other float actuates the mercury switch connected to the tripping circuit.
Whenever a fault occurs inside the transformer, the oil of the tank gets overheated and gases are generated. The generation of gases may be slow or violent according to nature of the fault.
When a predetermined amount of gases accumulate in the top of the chamber of the relay, the mercury type switch attached to the float is tilted, closes the alarm circuit and rings the bell.
When a severe fault occurs, a large volume of gas is generated, the lower float is tilted and the trip coil is energized. This opens the circuit breaker and supply to the transformer is switched off.
Buchholz relay is a very simple device used for transformer protection. Moreover, it detects the developing faults at a much earlier stage and enables us to protect transformer before serious damage occurs.
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