Buchholz Relay – Working Principle, Construction and Operation

In large oil-immersed transformers, we can find a cylindrical object installed in the pipe connecting the conservator to the main tank. This cylindrical object, that looks like a solenoid valve is called Buchholz relay. It is a type of oil activated relays. It can be found in all oil-immersed transformers rated higher than 500kva. This article can help you understand the operation, construction and operation of a Buchholz relay in detail.

What is the purpose of using a Buchholz relay in a transformer?

Before learning the operating principle and design features, it is necessary to understand “why Buchholz relay is used in a transformer?”. Buchholz relays are used to sense the faults occurring in the transformer winding and core such as inter-turn faults, 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.  

Basically, a Buchholz relay is an oil actuated relay. It gives an alarm or trips the input supply 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.

The principle of operation of a Buchholz relay

Faults that occur inside an oil-filled transformer generates heat, much enough to decompose the insulating oil. Decomposition of oil produces gases such as hydrogen, carbon monoxide, methane etc. These gases gradually move towards the conservator through the connecting pipe, but a part of which 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 towards the oil conservator. This oil surge pushes down the flap attached to the transformer side outlet of the Buchholz relay. This part can be understood only after reading the following sections.

Construction and design features of a Buchholz relay

Construction of Buchholz Relay
Cross Section of a Buchholz Relay

The external casing and terminal box of a Buchholz relay are made up of aluminium 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 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 oil level inside the relay.

A gas release cock 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 aluminium alloy terminal box fitted at the top of the relay. A drain cock is provided at the bottom of the casing. The electrical circuitry is Modern relays come with provision for analogue and digital signals option for continuous gas accumulation and flow speed monitoring.

Mounting of Buchholz relay

A Buchholz relay is mounted 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.

Mounting of buchholz relay
Image Credit: EMB

Alarm and Trip circuit in a Buchholz Relay

The upper reed switch (attached to the upper float switch) is connected to an alarm circuit and 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.

Fault diagnosis

Samples of gases trapped in the Buchholz relay are collected on the occurrence of a fault. The colour 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 in a Buchholz relay.

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

In case of slight faults gas passing from the main tank to the conservator get 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 Buchholz relay, connecting pipe and the 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, avoiding severe faults.

Smart Buchholz relay

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

Advantages and disadvantages of Buchholz relays

Buchholz relay can detect the 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.

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