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Buchholz relays have also been used in electric trains that operate on AC Electrified lines, including locomotives and multiple units. These trains take a high voltage supply from overhead wires, at up to 25,000 volts AC, which is then fed to an oil cooled transformer that 'steps down' the supply to voltage suitable for use in the trains traction motors - usually between 750 and 1000 volts. Depending on the age and design of the equipment, this voltage may then be rectified to Direct Current. A Buchholz Relay is installed in the transformer oil circuit, and should the relay detect 'gassing' of the oil, will trip out the trains main circuit breaker, so isolating the equipment from the supply and therefore protecting the equipment from overheating or short circuiting. Once tripped, the equipment can only be reset by maintenance staff.
Operating principle of buchholz relay
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electric power distribution
Electric power distribution is the final stage in the Power delivery, delivery of electricity. Electricity is carried from the Electric power transmission, transmission system to individual consumers. Distribution Electrical substation, substatio ...
and transmission, a Buchholz relay is a safety device mounted on some oil-filled power transformer
In electrical engineering, a transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces ...
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 Insulator (electricity), electrical insulator that can be Polarisability, polarised by an applied electric field. When a dielectric material is placed in an electric field, electric ...
failure inside the equipment. A generic designation for this type of device is "gas detector relay".
The relay was first developed by Max Buchholz (1875–1956) in 1921.
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.
Buchholz relays have also been used in electric trains that operate on AC Electrified lines, including locomotives and multiple units. These trains take a high voltage supply from overhead wires, at up to 25,000 volts AC, which is then fed to an oil cooled transformer that 'steps down' the supply to voltage suitable for use in the trains traction motors - usually between 750 and 1000 volts. Depending on the age and design of the equipment, this voltage may then be rectified to Direct Current. A Buchholz Relay is installed in the transformer oil circuit, and should the relay detect 'gassing' of the oil, will trip out the trains main circuit breaker, so isolating the equipment from the supply and therefore protecting the equipment from overheating or short circuiting. Once tripped, the equipment can only be reset by maintenance staff.
Operation
Depending on the model, therelay
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 switc ...
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 accumulates in the top of the relay and forces the oil level down. A float 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, which may produce visible light. An a ...
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 network, electrical circuit from damage caused by current in excess of that which the equipment can safely carry (overcurrent). Its basic function is to interr ...
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 current through a normally nonconductive medium such as air produces a plasma, which may produce visible light. An ar ...
, 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./ref>
See also
*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 switc ...
References