As it is a relay so first understand, what is a relay; Relay is a device when it is placed in a circuit then it protects the circuit by faults. For this, it detects an abnormal change in the parameters of the circuit such as voltage, current, temperature, pressure, etc. And closes the circuit before any unforeseen happening. If it can not do it by self, then it gives a tripping signal to a circuit breaker. Tripping single is often in the form of electrical voltage, which could be 5 v or 12 v.
Now let's talk about Buchholz relay, it protects electrical transformers. However, transformers are a static device in it the frequency of fault is much less as compared to motors and generators. But fault produced in it can halt the entire power transmission or distribution system. Because transformers are the most important part of these systems.
The Buchholz relay was first used in 1921 by MaxBuchholz. It is a gas actuated relay used in the oil-immersed transformer that means for the transformers of a higher rating. Generally, it is not used in distribution transformers as fuse can protect them. It is mostly used to protect transformers having a rating above 500 kVA.
It protects the transformer from;
1) Core heating fault
2) Winding fault
3) Short circuit fault
4) Incipient fault
It, connect between the main tank and the conservator tank by using a pipe. an arrow over it signifies the gas is coming to it form the main tank and going up to the conservator tank i.e. this indication helps in the proper installation. It, consisting of two arms inside it, the first arm hinged at this point and the other at this point. Upper arm consists of a floating ball while the lower arm consisting of a flap in front of the main valve. each arm consists of a mercury switch. this switch is for alarm circuit and this switch is for the trip circuit. the chamber of Bucholz relay consists of release cock at the top and test cock at the bottom.
If the transformer does not have any fault then we will not find any large pockets of gas bubbles coming out from the transformer tank, when we do have faults within the main electrical transformer tank it could be between the winding is in the core, between the winding himself those small electrical discharges will heat up the insulating liquid and they would cause insulating liquid to break down, when insulating liquid breaks down it actually produces gases now the severity of the break down dictates which gases are created. if you got a high energy discharge this means a quit large fault in a transformer, and produce more complex gas such as acetylene or ethylene and also form some solid components who starts depositing at the bottom of the main tank, these gas bubbles rise up to go to the "conservator tank" but gets trap in the Buchholz relay and the level of oil starts to fall which lower down the upper arm, which turns ON the mercury switch and hence the alarm gets ON. Incipient faults can make this condition, which means that faults which produced gradually, this can also happen when, quality of oil decreases. it also gives a sign that the time has come to change the oil. To stop the alarm, the gas released from the release coke and the sample of the gas is also taken. So that by doing test in the lab, we can find what type of fault occurring inside the transformer and whether to change the oil or not. Which is a necessary process if the oil is not changed on time, this can cut the cooling action of the transformer in a way that it will damage the entire insulation.
In the faults other than "incipient faults" such as winding fault, core heating fault, winding fault and short circuit fault, the level of discharge increases in such a way that the insulation at that point get permanently damaged and the turns of the winding get short or it may cause electrical contact between winding and core. the discharge thus produced heats up the oil at very high temperature. the transformer oil is rapidly heated and starts boiling so that it expands rapidly and moves toward the "conservator tank". due to heavy flow of gas and oil, the flap of lower arm moves backward and this mercury switch gets ON which gives tripping signal to the circuit breakers. so that the transformer get removed from the main supply.
so in this way, Buchholz relay protects the electrical transformer from faults,
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