Introduction
The Gas Relay (also known as a Buchholz Relay) is a critical protective device in oil-immersed transformers, designed to detect faults by monitoring gases generated inside the transformer. When internal failures-such as overheating, arcing, or insulation degradation-occur, insulating oil decomposes, producing combustible gases (e.g., hydrogen, methane, acetylene). The gas relay detects either the accumulation of these gases or sudden oil flow surges, triggering alarms or trip signals to prevent catastrophic failures.
Construction
1.plug 2.weight 3.cup(float) 4.signal magnet 5.connection point 6.magent 7.baffle plate 8.connnection point 9.connection point 10.adjusting screw 11.Spring 12.Relay upper cover 13.conducting rod 14.Probe 15.Stop screw
Working Principle
The gas relay is typically installed in the pipe connecting the transformer tank and the conservator. It operates based on two mechanisms:
(1) Minor Fault Detection (Gas Accumulation Alarm)
- Slow-developing faults (e.g., localized overheating) cause insulating oil to decompose, releasing gases that rise and accumulate in the relay’s upper chamber.
- When gas volume reaches a preset threshold (usually 250–300 mL), a float drops, activating an alarm contact (“light gas” warning).
(2) Major Fault Detection (Oil Flow Trip)
- Severe faults (e.g., short circuits, arcing) rapidly decompose oil, generating high-pressure gas bubbles and oil turbulence.
- The resulting oil flow displaces a flap inside the relay, triggering a trip contact to disconnect the transformer within milliseconds.
Advantages and Disadvantages
Advantages
- High Sensitivity: Detects incipient faults (e.g., insulation aging) before they escalate.
- Rapid Response: The “heavy gas” trip operates in milliseconds, minimizing damage.
- No External Power: Mechanical design ensures reliability even during power outages.
- Fault Diagnosis: Gas analysis (e.g., DGA-Dissolved Gas Analysis) identifies fault types (overheating, arcing, etc.).
Disadvantages
- Oil-Immersed Transformers Only: Not applicable to dry-type transformers.
- Environmental Sensitivity: Vibration or improper installation may cause false trips.
- Maintenance-Intensive: Requires periodic gas venting and seal inspections.
Analysis of Common Transformer Faults Detectable by Gas Relay
Detectable Fault Types and Gas Characteristics |  |
5.1.1 Partial Discharge
- Fault Characteristics: Localized electric field concentration causing weak discharges in insulation
- Gas Generation:
Primary gas: Hydrogen (H₂, 60-70%)
Secondary gas: Methane (CH₄)
Key indicator: Very low Acetylene (C₂H₂, <5ppm)
- Relay Action: Typically triggers only a light gas alarm
- Risk Level: Moderate (may escalate if untreated)
5.1.2 Thermal Faults
Low-Temperature Overheating (150-300°C)
- Common Causes: Core grounding issues, poor connections
- Gas Generation:
Dominated by Methane (CH₄)
Emerging Ethylene (C₂H₄)
Low Hydrogen (H₂)
High-Temperature Overheating (>700°C)
- Common Causes: Winding overheating, blocked oil ducts
- Gas Generation:
Significant Ethylene (C₂H₄)
Increased Ethane (C₂H₆)
Trace Acetylene (C₂H₂)
- Relay Action: Prolonged overheating may trigger heavy gas trip
5.1.3 Arcing (High-Energy Discharge)
- Fault Characteristics: Winding short circuits, tap changer failures
- Gas Generation:
High Acetylene (C₂H₂, typically >50ppm)
Elevated Hydrogen (H₂)
Possible carbon particles in oil
- Relay Action: Always causes heavy gas trip
- Risk Level: Critical (requires immediate shutdown)
5.1.4 Moisture Ingress
- Fault Characteristics: Excessive water content in oil
- Gas Generation:
Predominantly Hydrogen (H₂, >80%)
Minor Methane (CH₄)
- Relay Action: Frequent light gas alarms
- Diagnosis: Requires confirmation via oil moisture testing
Fault Diagnosis Methods |  |
5.2.1 Gas Ratio Methods (Rogers Ratio)
Key gas ratios for fault identification:
| Ratio | Range | Fault Type |
| CH₄/H₂ | <0.1 | Partial Discharge |
| C₂H₄/C₂H₆ | >3 | High-Temperature Overheating |
| C₂H₂/C₂H₄ | >0.5 | Arcing Fault |
5.2.2 Duval Triangle (IEC 60599 Standard)
Advanced three-ratio diagnostic method for precise fault classification.
5.2.3 Gas Generation Rate Analysis
Attention Threshold: Total hydrocarbon generation rate > 0.5 mL/h
Warning Threshold: Total hydrocarbon generation rate > 1 mL/h
Case Studies |  |
Case 1: Faulty Tap Changer Contacts
- Symptoms: Frequent light gas alarms
- Gas Analysis:
CH₄: 45%
C₂H₄: 30%
C₂H₂: <1ppm
- Diagnosis: Medium-temperature overheating (200-400°C)
- Action: Inspect and clean tap changer contacts
Case 2: Inter-Turn Winding Short Circuit
- Symptoms: Heavy gas trip
- Gas Analysis:
H₂: 55%
C₂H₂: 35%
Carbon particles in oil
- Diagnosis: High-energy arcing fault
- Action: Perform internal winding inspection
Maintenance Recommendations |  |
- Regular Gas Sampling: Conduct Dissolved Gas Analysis (DGA) at least every 6 months.
- Event Logging: Record relay activations with corresponding electrical parameters.
- Relay Calibration: Annually verify float and flap mechanisms.
- Supplementary Monitoring: Integrate with online monitoring systems for enhanced reliability.
