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Buchholz Relay in Transformer Protection: Working & Applications

Illustration of a power transformer with conservator tank showing Buchholz relay, floats, and alarm/trip mechanism for internal fault protection
Buchholz Relay in Transformer – Key Components and Working Explained

Transformers are one of the most vital and expensive pieces of equipment in a power system. Any fault inside a transformer can lead to severe damage, long outages, and even fire hazards. To detect internal faults at an early stage, Buchholz Relay is widely used in oil-immersed transformers.

It is considered the most common and effective protective device for transformers above 500 kVA. This article explains the construction, working principle, operation, advantages, limitations, and applications of the Buchholz Relay.

What is a Buchholz Relay?

The Buchholz Relay is a gas-actuated protection device used in oil-filled transformers. It is installed in the pipe between the transformer’s main tank and the conservator.

It operates whenever an internal fault occurs, such as:

  • Insulation breakdown
  • Winding short circuit
  • Core heating
  • Oil leakage or decomposition

The fault causes heating and gas bubbles formation inside the transformer oil. These gases flow into the relay chamber, activating the relay contacts and tripping the circuit breaker.

Construction of Buchholz Relay

The Buchholz Relay consists of the following parts:

  • Metal Chamber – A container mounted in the pipeline between the main tank and conservator.
  • Float Mechanism (Upper Float) – Used for alarm signaling when a small amount of gas accumulates.
  • Mercury Switch – Provides electrical contacts for alarm and trip circuits.
  • Lower Float with Flap – Operates during severe faults by detecting oil surge.
  • Vent Pipe / Gas Collecting Device – Collects generated gases for testing.

Working Principle of Buchholz Relay

The Buchholz Relay works on the principle of gas formation and oil movement inside the transformer.

1. Minor Faults (Slow Gas Formation)

  • Insulation deterioration or small winding faults generate gas bubbles.
  • Gas rises to the conservator through the relay chamber.
  • Upper float tilts → mercury switch closes → alarm circuit activated.

2. Major Faults (Sudden Oil Surge)

  • Severe internal short circuit or winding failure produces a large gas volume.
  • Oil rushes towards the conservator, pushing the flap and lower float.
  • Lower mercury switch operates → trips the circuit breaker → transformer disconnected from supply.

Thus, the Buchholz relay provides two levels of protection:

  • Alarm (early warning)
  • Trip (circuit isolation)

Operation Stages

1. Gas Accumulation Test

  • A sample of collected gas is tested.
  • If gas burns → indicates internal fault.
  • If gas does not burn → may indicate air leakage.

2. Relay Action

  • Small faults → alarm signal.
  • Severe faults → circuit breaker trips.

Advantages of Buchholz Relay

  • Detects incipient (early-stage) faults before they become severe.
  • Provides dual protection – alarm + trip.
  • Simple and reliable device.
  • Offers a chance to analyze fault gases for diagnosing the type of fault.
  • Protects the transformer from both minor insulation failure and major winding faults.

Limitations of Buchholz Relay

  • Applicable only for oil-immersed transformers with conservators.
  • Not suitable for small distribution transformers (usually below 500 kVA).
  • Cannot detect external faults (line faults, overloads).
  • May not work effectively if oil level is too low or if there is air leakage.
  • Slight time delay in operation compared to fast relays like differential protection relay.

Applications of Buchholz Relay

  • Installed in power transformers above 500 kVA rating.
  • Widely used in generating stations, substations, and transmission systems.
  • Acts as a primary internal fault protection device in EHV transformers.
  • Also useful for fault analysis, as the type of gas can indicate the fault origin.

Difference Between Alarm and Trip Action in Buchholz Relay

Condition Operation Result
Minor Fault (slow gas formation) Upper float operates Alarm only
Major Fault (severe oil surge) Lower float + flap operates Trips the circuit breaker

Conclusion

The Buchholz Relay is a highly effective protection device for transformers. By detecting gas formation and oil surges, it safeguards transformers from insulation failure, short circuits, and overheating. Its ability to provide early warning (alarm) and circuit isolation (trip) makes it one of the most important protective relays in power systems.

For large oil-immersed transformers, the Buchholz Relay is indispensable and remains a trusted protection scheme despite the advent of modern numerical relays.

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