Buchholz relay in transformers -Construction, Working and Diagram:
General : This relay was invented by Max Buchholz in 1931. it is used in those oil filled transformer having conservators.
look and construction: How a buchholz relay looks like from outside, see picture below.
This relay is connected between transformer tank and conservator.
Now what is inside the buchholz relay .. see the below picture.
What you have seen in above picture. you have seen two arrow which indicates oil flow from transformer tank to conservator tank as relay is connected between conservator and transformer tank.
Except these two arrows, what you see, there are two switches with two floats( ball shaped floats, you all have seen in your home overhead tank). so there are two floats but in the picture above, you see four floats .. what is that .. in normal condition, the relay has filled with oil completely and upper & bottom float both remains up in tilted position, during abnormal condition, oil level inside the relay falls, so floats comes down. so both normal and abnormal position of floats are shown in the diagram.
During normal condition, oil level inside relay is above top float. during a internal fault inside transformer, the oil heats up because of heat and gas is produced by decomposition of this oil .
this gas from the transformer tank comes in the relay as the relay is connected in the pipe between conservator and top of the transformer tank.
this gas displaces the oil and oil level inside the relay drops. when it drops below a certain level. than upper float becomes down as show in above picture. when this floats falls below, it closes the switch and activates the alarm circuit means everybody is aware that there is something wrong with transformer i.e. alarming condition.
During an heavy fault, more gas is produced and all the conservator oil goes to transformer tank and oil level inside relay becomes very very low.. negligible ,.. we can say no oil in relay .. all are gas inside relay .. this causes lower float of the relay to fall below.. causing switch to act and as this is serious condition. so switch activates the tripping circuit and transformer is tripped and taken out of service.
You also seen a paddle switch inside the relay.normally it remains in vertical position. during a severe fault, suddenly oil rushes from conservator to transformer tank . this rush activates this paddle switch and float magnetic switch and trips the transformer.