What is Buchholz Relay
Buchholz relay description
Power transformers are everywhere in the electrical system. Without power transformer the electrical system can't be possible. Transformer is an expensive machine in electrical system. There are certain safety systems which are used to protect transformer from external and internal faults. Buchholz Relay is a protection system for power transformer which protects the transformer from internal faults. Buchholz Relay is basically a gas activated relay. This relay functions on the formation of gases inside the transformer body or tank. Buchholz relay is connected between the conservator and the main transformer tank. The buchholz relay is an expensive equipment so it is used with transformers rating more than 500KVA. With smaller size transformers Buchholz relay is not connected due to cost consideration. Buchholz relay is a great protection relay.
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| Typical Buchholz Relay for transformer protection |
How Buchholz Relay function
When any type of electrical fault occurs inside a transformer, the transformer oil temperature increases. Oil converts in gas by evaporation process. The gas formation depend upon the intensity of the fault. The bigger the fault, more the gas generator. Smaller the fault, less gas would be generated. There are several reasons for faults inside transformers. Those are insulation breakdown for transformer windings or loose connections between transformer windings and connections.
When a fault occurs inside a transformer, a corona or arc produce inside the transformer which increases the transformer oil temperature. When the oil temperature increases, the oil evaporates and the oil converts in gas. The transformer system is kept sealed. Such gases operates the Buchholz relay and first an alarm is generated which tells the electrical system that abnormality had happened and it will lead to further failure. If still the defect present the Buchholz relay will detect it and it will send a tripping signal to the circuit breaker which will trip the transformer. Also it will generate an alarm to the electrical people to make maintenance on the system.
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| Buchholz Relay function |
Buchholz Relay construction
Buchholz Relay is located between transformer tank and oil conservator. Buchhose relay housing is fixed in between the pipe from oil tank to the oil conservator at a transformer. There are two sets of hanged contacts which are placed in the metal enclosure of Buchholz relay.
One of the hinged is located at the upper area of Buchholz Relay metal body and a mercury switch is connected with the hinge. The connection if that mercury switch are wired to the alarm system of electrical system. The second hinge is there in the underneath area of the first float and that hinge also contains mercury switch. The mercury switch contacts are wired to the circuit breaker of transformer for tripping.
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| Buchholz Relay construction and function diagram |
Buchholz Relay operation
When fault occurs in the transformer, corona or arc is produced inside the transformer. The produced corona or arc is hot and it increases the transformer oil temperature. The temperature increase leads to oil evaporation and gases production starts in the transformer tank. As the relay is located on the top of transformer, the gases are collected in the top area of the Buchholz Relay which activates the upper hinge of float which activates the alarming system of electrical to inform the operator about the defect.
IF the fault condition is still present and still the gas formation is in progress, the oil level in the conservator tank further reduces and gas accumulation is increased in the Buchholz relay which operates the lower float or hinge and the mercury switch connected with that hinge is activating the tripping circuit of the transformer circuit breaker.
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