Buchholz Relay Protection of Transformer: Complete Guide for Engineers

What Is Buchholz Relay Protection of Transformer?

Buchholz relay transformer protection relies on a mechanical non-electrical protection device, the Buchholz relay, which is mounted on the connecting pipeline between the transformer main tank and conservator tank.

Buchholz relay protection of transformer monitors two key abnormal conditions inside oil-immersed transformers:

1. Accumulation of decomposition gas
2. Sudden oil flow velocity and pressure fluctuation

This protection is only applicable to oil immersed transformers equipped with an oil conservator. Together with electrical power protection relays.

This protection is only applicable to oil immersed transformers equipped with an oil conservator. It provides early warning alarm and main trip protection for internal transformer faults that electrical transformer protection relays may fail to detect.

Why Is Buchholz Relay Important for Transformer Protection?

Internal transformer faults usually develop gradually. Minor faults such as inter-turn short circuits and local insulation degradation generate little fault current, making them difficult to be captured by differential or overcurrent protection.

Common Internal Transformer Faults Detected by Buchholz Protection

• Winding inter-turn short circuit
• Core overheating
• Partial discharge
• Internal arcing
• Insulation ageing and oil decomposition

All these failures break down transformer oil and produce gas or rapid oil surges. Buchholz relay detects these signatures at the early stage and prevents catastrophic transformer damage.

Working Principle of Buchholz Relay Protection

Buchholz Relay Operating Mechanism

1. Normal operation Transformer oil circulates stably. No gas gathers inside the relay; float contacts remain open. No alarm or trip signal is issued.
2. Minor developing faults (Gas alarm function) Slow insulation deterioration, loose connections or mild overheating decompose oil and generate gas. Gas accumulates at the top chamber of the relay and lowers the upper float, activating the alarm contact to send remote warning signals.
3. Severe internal faults (Oil surge trip function) Internal short circuits or arcing create massive gas instantly. Sharp oil flow surges push the lower float. The trip contact closes, triggering the circuit breaker to isolate the faulty transformer.

Construction and Main Components of Buchholz Relay

Component	Core Function
Upper float	Detect accumulated gas for alarm command
Lower float / baffle	Detect rapid oil surge for trip command
Magnetic / mercury contact switch	Transmit alarm or trip electrical signals
Relay housing	Sealed oil chamber, mechanical protection
Test valve	Manual function testing during commissioning

Buchholz Relay Protection Functions

Gas Accumulation Alarm Protection

Targets slowly evolving internal abnormalities: insulation ageing, local overheating, loose winding joints, slow oil leakage. It alerts maintenance crews to arrange inspections before faults escalate.

Oil Surge Trip Protection

Responds to sudden severe internal faults: winding short-circuit, internal flashover. It acts as critical non-electrical main protection to trip the transformer.

Fault Diagnosis Function

Engineers judge fault severity and type by sampling gas inside the relay, analysing gas volume, colour and composition. It delivers clear clues for post-fault troubleshooting.

Installation Requirements of Buchholz Relay on Transformer

Installation Position

• Install on the pipeline linking transformer tank and conservator
• Maintain specified inclination angle to guarantee gas collection
• Flow arrow on relay housing must point toward the conservator

Installation Considerations

• Avoid trapped air pockets inside pipelines
• Strictly prevent flange oil leakage
• Separate wiring for alarm circuit and trip circuit
• Install isolating valves for relay maintenance without draining transformer oil

Buchholz Relay Testing and Commissioning Procedure

This standard workflow is widely adopted by EPC teams during substation site acceptance.

1. Visual inspection Check mechanical integrity, sealing leakage and control wiring condition.
2. Alarm function test Inject compressed air into relay chamber and verify alarm contact actuation.
3. Trip function test Simulate oil surge movement to confirm reliable trip signal transmission to protection relay panels.
4. Contact resistance and insulation test Measure contact resistance and control circuit insulation resistance.

Common Buchholz Relay Problems and Troubleshooting

False Alarm Causes

• Air trapped inside transformer tank during oil filling
• Slow external oil leakage
• Incorrect installation inclination angle
• Continuous mechanical vibration on site

Unplanned Trip Without Transformer Internal Fault

• Excessive oil fluctuation caused by external load change
• Relay mechanical jamming
• Improper setting of oil surge threshold
• Short circuit on trip control wiring

No response during actual internal faults

• Broken internal contacts
• Mechanical blockage of floats
• Wrong wiring of secondary circuit

Difference Between Buchholz Relay and Other Transformer Protection Devices

Protection Device	Detection Target	Typical Application
Buchholz Relay	Internal gas & oil surge non-electrical faults	Oil immersed transformer with conservator
Differential Protection Relay	Electrical internal winding faults	All medium & large power transformers
Pressure Relief Device	Sudden tank pressure rise	Heavy-duty power transformers
Winding Temperature Relay	Overheating of windings and oil	All oil immersed transformers

Buchholz Relay Application in Power Transformer Systems

Distribution Transformers

Medium-capacity oil immersed distribution transformers for industrial parks and local power networks.

Substation Main Power Transformers

33kV, 66kV, 110kV and 220kV main transformers in transmission and distribution substations.

Industrial & Renewable Energy Transformers

Transformers deployed in mining facilities, thermal power plants, solar and wind farm collector substations.

How to Select a Reliable Buchholz Relay Supplier?

For EPC contractors and substation design teams, the following criteria reduce on-site failure risks during long-term operation:

Key Selection Criteria

• Product compliance with IEC relevant standards
• High-quality cast housing and corrosion-resistant internal components
• Stable mechanical action, low risk of contact sticking
• Complete FAT (Factory Acceptance Test) reports before shipment
• Manufacturer export experience and global after-sales technical support

Standard Project Execution Workflow

FAQ About Buchholz Relay Protection of Transformer

Q1: What is the purpose of Buchholz relay in transformer?

A: It is the primary non-electrical protection for oil immersed transformers. It detects internal faults by monitoring gas accumulation and oil surge, issuing early warning alarms or sending trip commands to avoid transformer explosion or severe burnout.

Q2: Which transformers require Buchholz relay installation?

A: Only oil immersed transformers fitted with an oil conservator. Dry-type transformers and transformers without conservators cannot adopt Buchholz relay protection.

Q3: Can Buchholz relay detect all transformer internal faults?

A: No. It cannot identify faults that do not generate gas or rapid oil movement. It must be deployed together with differential protection, overcurrent protection and temperature monitoring to achieve full-range transformer protection.

Q4: How often should Buchholz relay be tested?

A: Follow local grid specifications and manufacturer recommendations. Routine function inspection is required during annual transformer maintenance. Full commissioning testing must be conducted after new installation or transformer overhaul.

Q5: What is the difference between Buchholz relay alarm and trip?

A: Gas accumulation alarm responds to slow-developing minor faults for maintenance notification. Oil surge trip acts on violent short-circuit faults to cut off power supply and limit fault expansion.

Conclusion

Buchholz relay protection of transformer remains irreplaceable non-electrical protection equipment for oil immersed transformers. Correct installation, standard commissioning and periodic testing greatly improve transformer operational safety.

When combined with differential protection, backup overcurrent protection and temperature monitoring systems, it forms a complete protection scheme for power transformers in substations and industrial power projects.

Reference Technical Documents & Data Sources

1. IEC 60076 Power Transformers Series Standard
2. IEEE C57.104 Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers
3. IEC 61850 Substation Communication Standard
4. IEC 60255 Measuring Relays and Protection Equipment
5. International practice guide for transformer protection, Annual Conference for Protective Relay Engineers (CPRE)
6. Manufacturer standard testing specification for Buchholz relays