Buchholz relay

Buchholz relay is a gas actuated relay which used in the transformer to identify the fault inside it. It means that the formation of gas causes the relay to operate.

BUCHHOLZ RELAY

Construction:

Buchholz relay is the dome shaped equipment towards upper side of its structure. It is placed in the pipe which is connected between transformer and the conservator tank in the slanting position. It should be placed below the level of conservator tank.

It consist of two floats attached to the mercury switch. One is upper float or upper element and the another one is lower float or lower element.

Note :

1. The upper float will be actuated during any lower or insipient fault.

2. The lower float will be actuated only during the time of severe fault.

Tilt mercury Switch :

It is an electrical switch that opens and closes when a small amount of liquid mental mercury connects or disconnects the metal electrode by flowing above it.

Working:

Case I:

when insipient fault occurs in the transformer. It cause heating the core of the transformer. It will cause the decomposition of some transformer oil. Then It produce the product that contains 70% of hydrogen gas which is very light in nature. So it will try to move towards conservator tank (without disturbing the flap in the lower element). Then it is trapped in the upper part of the Buchholz relay. When a sufficient amount of gas accumulates, Then it exert pressure on the float and it tilt the float downside and closes the contacts of the metal electrode with the help of mercury flow above it. It completed the ararm circuit and gives an alarm.

Buchholz Relay - Alarm Circuit

Case II

During Short circuit, an enormous amount of gas will be generated in the main tank of the transformer. At that time, not only the gas but also oil present in the main tank will starts to move towards the conservator tank forcefully via Buchholz relay. It will hit the flap present in the front and tilt the mercury switch attached to it. It closes the trip circuit. 

Buchholz Relay - Trip Circuit

Lowering oil Level :

when oil of the transformer falls below the lower element, the float tilt downwards. It will also cause the trip circuit to close.

Advantages

1. It is the simpler form of the transformer protection

2. It detects the faults earlier than the other forms of protection.