What is Buchholz Relay
The Buchholz relay is a gas-actuated device that is used to protect against faults in transformers. Buchholz, the inventor of this relay, had originally designed it for incipient faults. This device can be located in the pipe connecting the conservator to the main tank and will give an alarm if there are any faults developing inside of your transformer. Buchholz relays are used all over the world and have been a reliable product for decades. Practically all oil immersed transformers rating over 750 KVA use Buchholz relays.
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 the 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
Figure: Buchholz Relay
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
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.