Buchholz relay is a protection device which is used to detect the fault occurs inside the oil filled transformers . Such faults are termed as incipient faults. Insulation breakdown between the winding turns, impulse breakdown of the insulating oil or winding to core fault because of weakening of insulation are examples of such faults. Incipient faults are slow developing faults. These faults are not significant at the start but slowly develop into serious hazardous faults. Buchholz relay used in oil immersed transformers has a rating of above 750 KVA.
Buchholz relay is a gas actuated relay. It is installed in the pipeline that runs from the main tank to the conservator tank. It consist of an oil filled metallic chamber with two internal elements. The upper element consists of float. One mercury switch is connected to the float. The lower element in the chamber consists of a vane connected with another set of mercury switch placed at the bottom of chamber. Mercury switch present at the top of chamber (operated by float) is used for generating an alarm. The other mercury switch present at the bottom of chamber (operated by vane) is used for tripping the circuit breaker.
Operation of Buchholz Relay :
Whenever an incipient fault occurs inside the transformer, oil present in the transformer tank gets heated which causes the decomposition of transformer oil. Gases produced due to the decomposition of oil in the main tank move towards the conservator and during that process gets collected at the relay chamber. Buchholz relay detects the nature of fault from the amount of gas accumulated in the chamber. In case of minor fault, Buchhlolz relay only generates an alarm. When severe fault occurs inside the transformer, Buchholz trip circuit operates the circuit breaker and isolates the transformer from the supply.
Buchholz Relay Working :
When internal (incipient) fault occurs inside the transformer, transformer oil will decomposed into different gases. The amount of gases produced depend on the intensity of fault occurs inside the transformer. Gases produced in the main tank will gradually move towards the conservator through the connecting pipe and gets entrapped in the relay chamber. In case of minor faults, these gases will accumulate in the upper part of relay chamber. When specific amount of gases accumulated in the upper part of relay chamber, it causes the oil level to fall. Decrease of oil level causes the float present at the top of chamber to sink and operates the mercury switch by closing its contacts. Since the mercury switch on the upper part of chamber is connected with an alarm circuit, hence the contacts of mercury switch will generate an alarm.
If major fault occurs inside the transformer, it causes the severe heating of oil present in the transformer tank. Since the generated heat is high, it produces large amount of gases. Gases produced due to the decomposition liberated from the oil that heats up to around 350°C. High build up of oil pressure causes the oil in the main tank to rush towards the conservator. A vane is placed in the path of high pressure (surge) oil at the bottom of chamber (see the above diag.). Since the amount of gas produced in cases of major fault is large, it causes the vane to tilt. When the vane tilts, it causes the mercury switch present at the bottom to close its contacts. Once the contacts of mercury switch is closed, it causes the immediate tripping of circuit breaker which isolates the transformer from the supply.
Limitations of Buchholz Relay:
Buchholz relay can only be used in large oil filled transformers.
The response time of Buchholz relay is high. The reason for keeping the response time high is so that it cannot operate during the time of earth quakes, mechanical shocks etc.(since these are not considered as faults)
Buchholz relay is activated by the amount of gas produced during the faults. In case of minor faults, the amount of gas produced accumulated in the upper part of relay chamber and causes the oil level to fall. In that case, Buchholz trip circuit will only generate an alarm. When major fault occurs inside the transformer, large amount of gases produced flows rapidly from the main tank to the conservator tank. High pressure oil causes the vane present in the path of surge oil to tilt which operates the Buchholz trip circuit in order to isolates the transformer from the supply.
That’s all. Hope this will helps you.