The gas- and oil-actuated (Buchholz) relay is including two lower and upper elements. The gas-operated relay can only be fitted to transformers having conservator vessels, and is installed in the pipeline between the transformer and its conservator tank. The relay comprises an oil-tight container fitted with two internal elements which operate mercury switches connected to external alarm and tripping circuits.
Normally, the device is full of oil and the elements, due to their buoyancy, rotate on their supports until they engage their respective stops. An incipient fault within the transformer generates small bubbles of gas which, in passing upwards towards the conservator, become trapped in the housing of the relay, thereby causing the oil level to fall. The upper element rotates as the oil level within the relay falls, and when sufficient oil has been displaced the mercury switch contacts close, thus completing the external alarm circuit. In the event of a serious fault within the transformer, the gas generation is more violent and the oil displaced by the gas bubbles flows through the connecting pipe to the conservator. This abnormal flow of oil causes the lower element to be deflected, thus actuating the contacts of the second mercury switch and completing the tripping circuit of the transformer circuit breaker, so disconnecting the transformer from the supply. Typical values of the oil velocity required to operate the lower element under oil surge conditions and the volume of gas required to operate the upper alarm element are given in Table below.
Reference:The J & P Transformer Book