Gas Relay: Complete Technical Guide for Oil‑Immersed Transformers

What is Gas Relay

For most international power standards, gas relays are generally mandatory accessories for oil‑immersed transformers above 800 kVA. It acts as early warning protection before electrical differential relays detect abnormal signals, capturing incipient faults which cannot be easily identified by conventional electrical protection equipment. A built‑in sight glass and gas sampling cock allow field technicians to collect fault gas samples for dissolved gas analysis (DGA), supporting further fault diagnosis.

Core Structure & Working Principle

1. Light‑gas alarm action: Minor faults produce gas which gathers in the top chamber. Accumulated gas displaces insulating oil inside relay housing, lowering local oil level and making upper float sink. Magnetic reed switch closes and sends remote alarm signal to control room, reminding operators to perform inspection without immediate power cut. Typical trigger threshold for light‑gas alarm is 250‑300 mL collected gas volume.
2. Heavy‑gas trip action: Severe internal faults generate huge gas volume instantly, pushing high‑speed oil flow rushing toward conservator. This oil surge strikes the lower baffle plate inside gas relay. Baffle deflection drives heavy‑gas contact closure, activating circuit‑breaker trip loop to isolate transformer from power grid. Typical operating oil‑flow velocity ranges 0.6‑1.5 m/s according to nominal pipe diameter.

Important installation note: Gas relay must be mounted horizontally, following flow‑direction marking printed on housing. The connecting pipe shall maintain a small upward slope toward conservator, ensuring all fault gas can flow smoothly into relay chamber without dead gas pockets.

Main Types of Gas Relay

Key Technical Specification Reference Table

Table 1: Common Nominal Diameter & Operating Parameters for Gas Relay

Practical remark: Operating velocity setting can be adjusted within allowable range during factory calibration. Improper setting value will cause false tripping or refusal to operate under real fault conditions.

Table 2: Gas Relay Trigger Event Classification

Critical Selection Guidelines for Gas Relay

1. Transformer capacity and pipe nominal diameter

   Match gas relay DN size with the connecting pipe diameter between main tank and conservator. Mismatched diameter will alter actual oil‑flow velocity passing through relay, resulting in deviation of heavy‑gas operating threshold. Small‑medium distribution transformers mostly adopt DN50; large power transformers use DN80 or DN100 gas relay.
2. Contact function requirement

   Ordinary distribution transformers require standard dual‑function contacts: light‑gas for alarm only, heavy‑gas for trip. For unattended substations, select gas relay with extra auxiliary status contacts for remote monitoring system access. For on‑load tap‑changer device, special tap‑changer dedicated gas relay shall be adopted.
3. Environmental adaptability

   Standard gas relay works within operating temperature range of ‑40 °C ~ +85 °C. For cold‑climate regions, confirm low‑temperature resistance of internal sealing gaskets to prevent gasket hardening and oil leakage. Outdoor‑installed units require junction box with good waterproof performance to avoid water ingress and secondary‑circuit short‑circuit.
4. Anti‑misoperation performance

   External short‑circuit current may create transient oil surge and trigger false heavy‑gas tripping. For large‑capacity transformers, select gas relay with optimized anti‑surge baffle structure, or configure reasonable time‑delay logic in protection control system according to project specification.

Installation & Commissioning Best Practices

• Keep horizontal installation strictly; follow arrow mark for oil flow direction. Do not install in tilted status.
• Ensure connecting pipeline maintains upward slope toward conservator, preventing gas accumulation before relay inlet.
• After oil filling and transformer commissioning, repeatedly release trapped air through gas sampling cock, to eliminate false light‑gas alarm caused by residual air.
• Perform contact function test before putting into service; verify alarm and trip loop conduction.
• Keep junction‑box sealing intact; avoid water and moisture entering secondary‑circuit terminals.

• Do not perform welding work nearby installed gas relay; high temperature may damage internal float and magnetic components.

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Common Faults and Troubleshooting of Gas Relay

1. Frequent false light‑gas alarm without real internal fault

   Root causes: residual air trapped during oil injection, poor sealing leading to air suction, water ingress in terminal box. Solution: exhaust trapped gas, inspect flange and gasket sealing, repair waterproof performance of junction box.
2. Gas relay heavy‑gas false tripping

   Root causes: improper oil‑flow velocity setting, transient oil surge caused by external short‑circuit or cooling pump start‑stop. Solution: recalibrate operating flow‑velocity parameter, evaluate anti‑misoperation configuration for protection logic.
3. Contacts fail to act under real fault condition

   Root causes: float or baffle mechanical jamming, magnetic reed‑switch aging, contact oxidation. Solution: perform periodic maintenance test, check flexible movement of internal moving parts.
4. Oil leakage at flange or housing

   Root causes: aging or uneven compression of sealing gaskets. Solution: replace oil‑resistant rubber gasket, apply uniform bolt tightening torque during reassembly.

Summary