Whenever a fault in the transformer develops slowly, heat is produced locally, which begins to decompose solid of liquid-insulated materials and thus to produce inflammable gas and oil flow. This phenomenon has been used in the gas protection relay or popularly known as Buchholz relay. This relay is applicable only to the so-called conservator type transformer in which the transformer tank is completely filled with oil, and a pipe connects the transformer tank to an auxiliary tank or ” Conservator” which acts as an expansion chamber. The figure is shown as Buchholz relay connected into the pipe leading to the conservator tank and arrange to detect gas produced in the transformer tank. As the gas accumulates for a minor fault the oil level falls and, with it a float ‘F’ which operates a mercury switch sounding an alarm. When a more serious fault occurs within the transformer during which intense heating takes place, an intense liberation of gases results. These gases rush towards the conservator and create a rise in pressure in the transformer tank due to which the oil is forced through the connecting pipe to the conservator. The oil flow develops a force on the lower float shown as “V” in the figure and overtrips it causing it contacts to complete the trip circuit of the transformer breaker. Operation of the upper float indicates an incipient fault and that of the lower float a serious fault.
Figure: Buchholz Relay
Buchholz relay Operation : Certain Precautions:
The Buchholz relay may become operative not only during faults within the transformer. For instance, when oil is added to a transformer, air may get in together with oil, accumulate under the relay cover and thus cause false operation of the gas relay. For this reason when the ‘Gas’ alarm signal is energized the operators must take a sample of the gas from the relay, for which purpose a special clock is provided. Gases due to faults always have color and an odor and are inflammable.
The lower float may also falsely operate if the oil velocity in the connection pipe through not due to internal faults, is sufficient to trip over the float. This can occur in the event of an external short circuit when overcurrents flowing through the windings over-heat the copper and the oil and cause the oil to expand. If mal-operation of Buchholz relay due to overloads or external short circuits is experienced it may be necessary that the lower float is adjusted for operation for still higher velocities.
In installing these relays the following requirements should be fulfilled.
a) The conductor connecting the contacts to the terminals on the cover must have paper insulation, as rubber insulation may be damaged by the oil.
b) The floats must be tested for air tightness by for example, submerging them in hot oil to create a surplus pressure in them.
c) The relay cover and the connection pipe should have a slope of 1.5 to 3 percent and not have any protruding surface to ensure unrestricted passage of the gases into the conservator.