Definition: The Buchholz relay protects the transformer from internal faults. It is the gas actuated relay. The Buchholz relay is placed between the main tank and the conservator. Such type of relay is used in the transformer having the rating higher than 500KVA. It is not used in small transformer because of economic consideration.
Working Principle of Buchholz Relay
When the fault occurs inside the transformer, the temperature of the oil increases. The oil evaporates in the form of the gas. The generation of the gas depends on the magnitude of the fault occurs inside the transformer. The internal failure occurs in the transformer either because of the insulation breakdown between the winding or the winding have the weak initial contact.
The fault induces the arc which increases the temperature of the gas. The oil becomes evaporated and moves upwards. The Buchholz relay detects the failure and gives the alarm to the personnel. The transformer is disconnected from the main supply for maintenance.
Construction of Buchholz Relay
The Buchholz relay has two hinged which is placed in the metallic chamber. This metallic chamber is connected through the pipe between the conservator and main tank.
The one of the hinged is placed in the upper portion of the metallic chamber along with the mercury switch. This mercury switch is used for activating the alarm. The other float is placed in the lower portion of the metallic chamber along with the mercury switch. The mercury switch is used for actuating the tripping circuit.
The internal fault of the transformer induces the arc inside the main tank. The oil of the transformer starts heating because of the thermal effect. The gas moves upwards, and few of their vapours are collected in the upwards area of the main tank. Because of the evaporation, the level of oil inside the transformer tank starts decreasing.
The mercury switch placed inside the metallic chamber becomes tripped, and the relay gives the alarm to the personnel. The supply of the transformer becomes close, and it is disconnected to the system for maintenance. The relay has test cock which is used for releasing the pressure of the chamber.
When the severe fault occurs inside the transformer, the lower mercury switch placed inside the metallic chamber becomes slightly tilted because of which the tripping circuit becomes closed. Thus, the transformer is disconnected from the main circuit.
Limitations of Buchholz Relay
The following are the disadvantages of Buchholz relay.
- The relay is used only in oil immersed transformer.
- It can only detect the fault below oil level.
- This relay does not protect the connecting cables. Hence separate protection is used for the cables.
- The response time of the relay is high.
The minimum operating time of the relay is 0.1 seconds.