A buchholz relay is a protection device for monitoring the gas and oil movements in oil immersed transformers. It is used on practically all power transformers with the exception of small distribution sizes.
In practice it has proved to be the only protective device that can clear certain types of faults. The buchholz relay relies on the principle that during fault conditions, gas is generated inside the transformer tank from the insulating oil. An example of a buchholz relay device is shown in figure (44).
The buchholz relay is sited in the pipe work between the transformer and its conservator as illustrated in figure (45), and is filled with oil during normal transformer operation.
The internal mechanism of a buchholz relay mainly comprises two floats and is illustrated in figure (46). During normal operation, the relay is completely filled with oil keeping the floats in their top limit or rest position.
The contact mechanisms in the relays respond to:
- Slight faults causing a slow evolution of gas in the transformer (e.g. overheating)
- Serious faults creating an immediate surge of oil (e.g. shortcircuits etc.)
- Oil leakage
Buchholz Relay Operation
When a slight fault occurs in the transformer, the small bubbles of gas which pass upwards towards the oil conservator tank are trapped in the relay housing this causing its oil level to fall. As a result, the upper float drops and activates the external alarm switch. If gas continues to be generated then the second float operates the second switch that is normally used to isolate (trip) the transformer.