What is Buchholz Relay?
A relay that is used for the protection of the transformer from internal incipient fault is called the Buchholz relay. A Buchholz relay is placed in the pipe connecting the main and conservator tanks. The Buchholz relay is an oil and gas actuated relay means the relay operates when the gases are produced in the main tank due to an internal winding fault.
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| What is Buchholz Relay in Transformer? |
The Buchholz relay is used in transformers having ratting higher than 500Kva. The Buchholz relay is not suitable for smaller transformers because of economic considerations. The Bucholz relay is a safety device for the transformer.
When the incipient fault occurs in the transformer first Buchholz relay gives the alarm. If the internal fault becomes severe, the Buchholz relay disconnects the transformer from the supply.
Construction of Buchholz relay
The Buchholz relay consists of Three elements
1. Upper element - Alarm unit
2. Lower element - Tripping unit
3. Gas release pocket
4. Drain plug
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| Buchholz Relay |
1. Upper Element
The Buchholz relay consists of two elements, an upper element and a lower element. The upper element consists of a float made from plastic-type weightless material which is attached to the hinge in such a way that the float can move up and down. The mercury switch is attached to the hinge. The upper element works as an alarm unit. In normal operating conditions, the mercury switch is in open condition. when the fault occurs inside the transformer and gases are produced, float got tilt and mercury switch shorted and gives the command to the alarm unit.
2. Lower Element
The lower element also consists of float which is attached to the baffle plate. The baffle plate is also attached to the hinge in such a way that it can move up and down depending upon the oil pressure. The mercury switch is attached to the baffle plate. The baffle plate is provided in front of the inlet of the Buchholz relay of the transformer in such a way that when high-pressure oil enters the Buchholz relay from the inlet of the relay, the position of the baffle plate with mercury switch will change. The lower element works as a tripping unit.
Both the elements are enclosed in a domed vessel that is connected between the main tank and the conservator. The electrical wires from both mercury switches are taken out through the terminal block.
3. Gas release pocket
The Buchholz relay consists of a gas release pocket at the top of the doomed vessel. The gas release pocket is used to release the gases when a fault occurs in the transformer and gases are produced. It is called test cock.
4. Drain plug
The drain plug is provided at bottom of the Buchholz relay. The drain plug is used for manual testing of the Buchholz relay by draining the oil from Buchholz relay tank.
Working of Buchholz relay
When an internal fault occurs inside the transformer like insulation failure between the turns of winding, core failure, core heating, it generates the heat inside the oil due to the very high fault current and which is decompose the insulating oil different hydrocarbon gases like methane, Ethan and carbon monoxide and carbon dioxide.
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| Buchholz Relay |
The decomposition of transformer oil produces more than 70 % of hydrogen gas. Due to the thermal effect, insulating oil evaporates and gases are produced. So the oil level inside the main tank is reduced. The produced gases are lower in weight than oil. The produced gases are trapped in the main tank upper portion and Buchholz relay upper portion.
The produced gases are accumulated upper portion of the Buchholz relay and the oil level in the Buchholz relay is reduced. The upper element float is tilted due to the lowering of the oil level and the mercury switch gets shorted and gives the command to the alarm circuit.
When a major fault occurs inside the transformer, an enormous amount of hydrocarbon gases are generated inside the transformer. The oil from the main tank rushes towards the conservator tank through the Buchholz relay. The lower element float tilted and the mercury switch gets shorted. This gives the tripping commands to the tripping coil of the breaker.
The breaker gets tripped and disconnects the transformer from the supply. During inter trip, the Low voltage side breaker also tripped. So transformers are isolated from the supply from both sides. This way, the Buchholz relay saves the costly equipment from major damage.
The Buchholz relay also gives the alarm or tripping in case of a low oil level in the main tank.
Advnatages of the buchholz relay
1. The overcurrent or earth fault relay can not detect the small internal fault inside the transformer. The small internal fault can be detected by the Buchholz relay easily.
2. The construction of the Buchholz relay is very simple.
3. The Buchholz relay saves the costly transformer from major damage.
4. The severity of the internal fault inside the transformer can be determined without dismantling the transformer.
Disadvantages of the Buchholz Relay
1. The Buchholz relay is only used for oil-immersed transformers having a conservator tank.
2. The Buchholz relay can detect the fault inside the transformer tank. For the protection of the outer portions like cable leads, separate protection is required.
3. The response time for the Buchholz relay is very high. The response time for the Buchholz relay is 0.1 seconds.
4. The Buchholz relay can operate without any fault in the transformer. When the oil is added to the transformer main tank, the air may get with transformer oil and accumulated at the top of the relay and giving the false tripping.
5. The setting of the mercury switch is not kept very sensitive otherwise it is operated during vibrations, earthquakes or mechanical shocks.
6. The Buchholz relay only detects the fault below the oil level.
Applications of the Buchholz relay
1. The Buchholz relay can detect an inter-turn fault in the transformer.
2. The Buchholz relay is used to detect the insulation failure of the core bolt.
3. The Buchholz relay detects the short circuit of the winding inside the transformer.
4. The Buchholz relay is used to detect short circuits between the different stages.
5. The Buchholz relay can detect local overheating inside the transformer.
6. The Buchholz relay is used to detect the bad or loose contacts inside the transformer.
7. The Buchholz relay is used to detect the bushing puncture and winding earth fault.
8. The Buchholz relay is used to detect the loss of oil or reduction of oil level due to the leakage in the transformer main tank.
9. The Buchholz relay is used to detect the short-circuited lamination in the transformer core.
Important Points about Buchholz relay
When the transformer tripped in the Buchholz relay and gases are observed from the test cocks of the Buchholz relay Dissolved gas analysis tests are carried out.
1. If the presence of C2H2 and H2 gases shows the arcing in oil between the constructional parts inside the transformer.
2. If the presence of C2H2, CH4 and H2 gases shows the deterioration of paper insulation or tap changer fault.
3. If the presence of CH4, C2H4 and H2 gases also indicate the hot spot in core joints.
4. If the presence of C2H4, C3H6, H2 and CO2 gases shows a Hot spot in transformer windings.
Various coloured gases were produced during the fault in the transformer which is released from the Buchholz relay
1. Black gas: The black coloured gases are produced when the free arcing occurred in the transformer oil.
2. Whitish gas: The whitish gases are produced when the electric arcing is caused between the contact and cotton, paper and silk.
3. Yellowish gas: The yellowish gas is produced when the fault occurred in cardboard and wood.
4. Greyish gas: The greyish gas is produced when the breakdown of the magnetic circuit takes place.