Transformer Protection Relays (Buchholz Principle) - EMB Gmbh

3. Function
The Buchholz relay is installed in the pipe between the tank of the device to be protected (transformer,
reactor) and the conservator. During normal operation it is filled completely with insulating
liquid.
Due to buoyancy the float of the single-float relay and both floats of the double-float relay are at
their top position.
The upper and lower switching systems form a functional unit in the single-float Buchholz relay so
that in the event of a fault the transformer is immediately disconnected from the power system.
In the following the function of a Buchholz relay is explained using the example of a double-float
Buchholz relay. If a fault occurs inside the transformer, the Buchholz relay responds as follows:
3.1. Gas accumulation
Fault: Free gas is available in the insulating liquid.
Response: The gas in the liquid moves upwards, accumulates in the Buchholz relay and displaces
the insulating liquid level.
The moving float actuates a switch contact (magnet contact tube). An alarm signal is tripped.
The lower float is not affected as from a certain gas volume the gas flows through a piping to the
conservator.
Oil Öl
Gas
Figure 8 - Gas accumulation
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