What is Buchholz relay? -Construction, Diagram, Working

A transformer is the main device of the power system hence protection must be provided to it. Buchholz relay is a mechanically operated relay that is used to protect the transformer from internal faults that occur inside the transformer tank like an inter-turn fault. Buchholz relay is an oil and gas-operated device.

It is used to give an alarm in case of an incipient fault in the transformer and to disconnect the transformer from the main power supply in the event of serious internal faults. Buchholz relay purpose is to disconnect the transformer from the supply in a faulty condition and give an alarm indicating fault type. An installation location is shown in the below diagram.

Buchholz relay construction installation diagram
Buchholz relay installation diagram

The Buchholz relay is installed in the pipe between the transformer tank and the conservator. The pipe is normally tilted at an angle of 5 degrees. During the normal operation of the transformer, it is filled completely with transformer oil.

Due to an internal fault in the transformer, the temperature of the transformer oil rise and this generate gases into the tank. The generation of gases may be slow or fast depending upon fault type major or minor. The function of the relay depends on the mechanical phenomenon.

Working principle of Buchholz relay :

Buchholz relay working principle and operation is base on the gases generated in the transformer tank during the internal fault, whenever fault generated in the tank, Bucholz relay operates and disconnects the transformer from supply or generates an alarm in case incipient fault.

When gas is generated in the transformer tank because of fault, it rises towards the conservator tank and collects in the upper chamber of the relay. Hence the oil level in the relay drops and the top float triggers an alarm switch. Gas shall not freely pass from the relay body and escape into the pipework before the alarm contact has operated.

For a serious fault, large volumes of gases are generated in the main tank. which causes violent flow of transformers oil, this oil in the main tank rushes towards the conservator tank through Buchholz relay connected in the pipe, which connects the transformer tank and conservator. Due to the turbulence flow of oil, the lower float tilts to close the contact of the mercury switch. This completes the tripping circuit to open circuit breaker and transformers get disconnected from the supply.

Check after actuation

Alarm signal

When the alarm signal is given, the colour of the gas should be observed through the inspection-windows.

The gas may be released or gas samples can be taken for analysis.

Colour of gas due to the operation and fault type:

Whitish gas:  It is caused by electric arcing in contact with paper, cotton, and silk. 

Yellowish gas: it is caused by wood and Cardboard. 

Greyish gas: it is caused by a breakdown of the magnetic circuit. 

Black gas: it is caused by free arcing in the oil

Trip signal

If the relay disconnects the transformer, similar checks on the gas should be made to determine the color and the quantity of gas collected. It is always good practice to make a gas analysis. In any case, the transformer should not be immediately re-energized, as this would increase the seriousness of the fault. 

Buchholz relay construction:

Buchholz relay diagram and construction are shown below.
Buchholz-relay-working-diagram
Buchholz relay working diagram

Buchholz relay consists of two hinged floats, one at the upper side and another one at the bottom side in the chamber. An upper side mercury switch is connected to an alarm circuit which used to give an alarm during an incipient fault in the transformer tank. And bottom mercury switch is connected to the circuit breaker which isolates the transformer from the supply in case of a series fault in the transformer.

Testing Procedures of Buchholz Relay


There are many testing procedures some of them are

1)Measurement of the volume of gas necessary to trip the alarm.
2)Pressure Withstand Test 2.5 bar for 2 minutes with oil at 100°C.
3)Vacuum Withstand Test of 2500 Pa for 24 hrs.
4)Rate of oil flow test to operate trip contacts
5)Non-stationary vibration tests

Advantages of Buchholz relay


1. This relay detects the internal faults of the transformer and it helps to prevent the major faults.
2. Intensity and type of fault can be determined without even opening or disturbing the transformer.
3. If a serious fault occurs, the transformer can be isolated automatically with the help of the Buchholz relay.these are advantages. 

Disadvantages of Buchholz relay

The disadvantages of the Buchholz relay are listed below.

1) This relay can only be implemented in oil-immersed transformers.
2) Its operation is based on the oil level in the transformer tank.
3) The response time of the Buchholz relay is too high.

Applications

The applications of Buccholz relay include the following.

1) It can use at the Entry of air bubbles in the oil
2) Insulation failure of the transformer Core bolt
3) It can use where the reduction of oil level will be low due to leakage
4) This relay can be used in loose and bad electrical contacts
5) transformer Winding short circuit

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