electric power distribution Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmissi ...

and transmission, a Buchholz relay is a safety device mounted on some oil-filled power

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

s and reactors, equipped with an external overhead oil reservoir called a "conservator". The Buchholz relay is used as a protective device sensitive to the effects of

dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...

failure inside the equipment. A generic designation for this type of device is "gas detector relay".

Application

Buchholz relays have been applied on oil-filled power and distribution transformers at least since the 1940s. The relay is connected to the oil piping between the overhead conservator tank and the main oil tank of a transformer. The piping between the main tank and conservator is arranged so that any gas evolved in the main tank tends to flow upward toward the conservator and gas detector relay. Vermogentransformator 1

Operation

Depending on the model, the

relay A relay Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off A relay is an electrically operated switch ...

has multiple methods to detect a failing transformer. On a slow accumulation of gas, due perhaps to slight overload, gas produced by decomposition of

insulating oil Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers (wet transformers), some types of high-voltage capacitors, fluorescent lamp El ...

accumulates in the top of the relay and forces the oil level down. A

float switch A float switch is a type of level sensor, a device used to detect the level of liquid within a tank. The switch may be used to control a pump, as an indicator, an alarm, or to control other devices. One type of float switch uses a mercury switch ...

in the relay is used to initiate an alarm signal. Depending on design, a second float may also serve to detect slow oil leaks. If an

electrical arc An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. ...

forms, gas accumulation is rapid, and oil flows rapidly into the conservator. This flow of oil operates a switch attached to a vane located in the path of the moving oil. This switch normally will operate a

circuit breaker A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk ...

to isolate the apparatus before the fault causes additional damage. Buchholz relays have a test port to allow the accumulated gas to be withdrawn for testing. Flammable gas found in the relay indicates some internal fault such as overheating or

arcing An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma (p ...

, whereas air found in the relay may only indicate low oil level or a leak. Through a connected gas sampling device the control can also be made from the ground. Depending on the requirements, the Buchholz relay has a flange or threaded connection. The classic Buchholz relay has to comply with the requirements of the DIN EN 50216-2 standard. Depending on the requirements, it is equipped with up to four (2 per float) switches or change-over switches, which can either send a light signal or switch off the transformer. The relay was first developed by Max Buchholz (1875–1956) in 1921.

References

{{Electric transformers Safety switches Electric transformers Relays German inventions

Application

Operation

See also

References