Buchholz relay – Working Principle, Construction and Operation

A Buchholz relay in a transformer is an oil-activated relay, used to sense abnormalities such as short circuits, overheating, and oil leakages inside a transformer. It is a cylindrical object found in the pipe connecting the conservator to the main tank, in large oil-immersed transformers. It is very common in all oil-immersed transformers rated higher than 500kva.

Working principle of Buchholz relay

Faults that occur inside an oil-filled transformer generate heat, much enough to decompose the insulating oil into gases such as hydrogen, carbon monoxide, methane, etc. These gases gradually move toward the conservator through the connecting pipe, but a part of them get trapped inside the Buchholz relay. The trapped gases displace oil inside the relay. Hence the level of oil in it falls, activating the float switches inside.  

In the case of large faults, a huge amount of gases are produced by the decomposition of oil. This results in an oil surge that moves toward the oil conservator. This oil surge pushes down the flap attached to the transformer side of its outlet. This part can be understood only after reading the following sections.

Construction and design features

The external casing and terminal box of a Buchholz relay are made up of aluminum alloy. It is a weatherproof and oil-tight design that holds inside two float switches, one at the top and the other at the bottom, for sensing oil level and flap to sense oil surge. Two reed switches/mercury switches, one for alarm and the other for circuit breaker trip, are attached to the float switch. These switches are connected to the terminals within the terminal box at the top of the relay. An inspection window is provided to monitor the oil level inside it. The viewing window is fitted with scaled sight glasses which enables the monitoring of the oil level inside the relay.

A gas-release cork is provided at its top to expel the accumulated gases. An electrical circuit contact test button and the terminals are enclosed in a weather-resistant cast aluminum alloy terminal box fitted at the top of the relay. A drain cork is provided at the bottom of the casing. The electrical circuitry is Modern relays that come with provision for analog and digital signals option for continuous gas accumulation and flow speed monitoring.

Why is the Buchholz relay used in the transformer?

It is used to detect the faults that occur inside the main tank of the transformer such as inter-turn faults, insulation breakdowns, short circuits, earth faults, overheating of the core, oil leakage, etc. It senses these faults and interrupts the transformer’s input supply, so that greater damages may be prevented. It also serves the purpose of fault diagnosis.

It triggers the alarm and/or trips the input supply to the transformer based on the level of oil inside it. It also responds quickly to the unusual oil flow from the transformer main tank to the oil conservator. It is a protection and monitoring equipment not only for transformers but also for oil-immersed chokes with an oil conservator.

Fault diagnosis

It is important to diagnose the reasons for the trip before reenergizing the transformer. The samples of gases trapped in the relay are collected on the occurrence of a fault. The trapped gases must be analyzed immediately. The color of the gases collected indicates the type of fault as follows:

• 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  

Protection features

A Buchholz relay protects the transformer from the following conditions:

• short-circuited core laminations
• short-circuit between phases/ earth
• overheating of transformer winding
• oil leakage from the main tank.
• air Inlet
• puncture of bushing insulators inside the tank
• Earth faults

In case of slight faults gas passing from the main tank to the conservator gets trapped in the relay. The oil level lowers slightly, so as the upper float. This operates the alarm contact. The same thing shall happen if free gases are present in the main tank.

In case of severe faults such as oil leakages, overheating of core/windings oil level in the relay and conservator falls continuously. Eventually, the relay, connecting pipe, and conservator are emptied and the lower float switch is operated. This activates the trip contacts and the transformer is disconnected from the supply.

In the case of short circuits, the gas generation is violent and causes oil to rush from the main tank to the conservator. This lowers the flap fitted to the lower float and activates the trip contacts, thereby isolating the input supply to the transformer, and avoiding severe faults.

Mounting arrangement

A Buchholz relay is located on the pipe connecting the main tank and the conservator of the transformer. The pipe is normally tilted at an angle of 5 degrees to the horizontal in order to allow easy oil flow.

Alarm and Trip circuits

The upper reed switch (attached to the upper float switch) is connected to an alarm circuit and the lower reed switch (attached to the lower float switch) is connected to the trip circuit of the circuit breaker at the incoming side of the transformer. During large faults, the lower reed switch activates the trip circuit of the circuit breaker and interrupts the power supply to the transformer, thereby avoiding large accidents.

Smart Buchholz relay

Smart Buchholz relays are equipped with sensors for sensing the oil temperature and moisture content of the transformer oil and the properties of gas accumulated in the relay during faults. These sensors send the sensed data to Programmable logic controllers or Substation automation systems. This enables the continuous monitoring of the relay from a remote location. It is also equipped with the standard features of conventional relays.

Advantages and disadvantages of Buchholz relays

They can detect faults at the early stage and is the simplest form of transformer protection. But the drawback is that it can be used only in the oil-filled transformer.