Gas-Actuated Relay for Oil-Immersed Transformer Protection
Model: QJ12-80 | DN80 | Complies with IEC 60076-1
1. Product Introduction
The QJ12-80 Buchholz Relay (also known as a gas-actuated relay) is a critical protective device installed in the connecting pipe between the transformer tank and the conservator (oil storage tank). It continuously monitors the condition of the insulating oil and provides early warning of internal faults by detecting gas accumulation and sudden oil displacement.
The QJ12-80 is a dual-float type relay: two independent floats operate upper and lower alarm and trip functions respectively, each driving a sealed mercury-tilt switch. It contains NO oil-flow flap, relying instead on precise float displacement for highly sensitive two-stage protection.
Working Principle
Under normal operation, both floats are submerged in oil and both mercury switches remain in their rest position. As minor internal faults (e.g. local overheating, corona) generate dissolved gas, the gas rises and collects in the upper chamber of the relay.
Alarm Stage (Upper Float): As gas accumulates, the oil level drops and the upper float descends. Once the preset gas volume (approx. 250–300 cm³) is reached, the upper mercury switch tilts and closes the alarm contact, triggering a gas-accumulation alarm to the control room.
Trip Stage (Lower Float): Under a severe fault (e.g. winding short-circuit, core failure), a large volume of gas is generated rapidly. The lower float drops under the falling oil level and its mercury switch closes the trip contact, initiating immediate transformer disconnection to prevent damage.
Oil-Flow Trip (optional): In addition to gas accumulation, a sudden oil surge caused by an internal arcing fault can also drive the lower float, providing fast-acting overpressure / flow protection.
Because the relay responds to both slow gas accumulation (alarm) and rapid oil displacement (trip), it is regarded as the primary and most sensitive internal-fault protection for oil-immersed transformers.
2. Model Designation & Construction
2.1 Model Designation
Code
Meaning
QJ
Gas relay (Buchholz relay) code
12
Design series (dual-float type)
80
Nominal connecting-pipe diameter = DN80
2.2 Construction
Cast Aluminum Body: Rugged, oil-tight housing with viewing windows for visual inspection of the oil level and float positions.
Upper Float (Alarm): Detects gradual gas accumulation; actuates the alarm (mercury-tilt) switch at the preset gas volume.
Lower Float (Trip): Detects severe faults / rapid oil surge; actuates the trip (mercury-tilt) switch to disconnect the transformer.
Two Sealed Mercury Switches: One for alarm, one for trip; hermetically sealed, reliable and maintenance-free over a long service life.
Vent & Test Valve: Top vent plug for gas release and routine testing; oil-drain plug at the bottom.
Mounting Flanges: Standard DN80 flanges for direct installation in the tank-to-conservator pipe, with gasketed, leak-proof sealing.
3. Technical Data
Parameter
Value / Specification
Model
QJ12-80
Type
Dual-float Buchholz relay (mercury switch type, no flap)
Nominal Diameter
DN80
Alarm (Gas Accumulation)
Upper float, approx. 250–300 cm³ (adjustable)
Trip (Severe Fault / Surge)
Lower float, approx. 0.8–1.2 m/s oil surge equivalent
Contacts
2 × SPDT (Form C), one alarm + one trip
Switch Type
Mercury-tilt switch, hermetically sealed
Contact Rating
AC 250V / 5A (or per customer specification)
Body Material
Cast aluminum alloy
Mounting Position
In connecting pipe between tank and conservator, inclined ~2–4°
Ingress Protection
IP65 / IP66
Operating Temperature
-40 °C to +85 °C
Standards
IEC 60076-1, GB/T 6451
4. Key Features
Dual-Float, Two-Stage Protection — Independent upper (alarm) and lower (trip) floats provide clear separation between early warning and emergency trip, avoiding nuisance operations.
High Sensitivity — Float-based design responds to both slow gas build-up and rapid oil surges, detecting internal faults at the earliest possible stage.
Mercury-Tilt Switches — Sealed, contact-free switching ensures long service life, stable contact resistance and immunity to oil contamination.
No Oil-Flow Flap — Unlike single-float Buchholz relays, the dual-float design has no flap, making it more sensitive to gas accumulation and simpler to maintain.
Clear Visual Indication — Transparent viewing windows allow operators to observe oil level and float positions directly without disassembly.
Built-in Test & Vent Valve — Enables periodic gas-release testing and functional checks without removing the relay from the pipeline.
5. Applications
Power transformers (generally ≥750 kVA and above) and step-up / step-down transformers.
Oil-immersed reactors, shunt reactors and phase-shifting transformers.
HVDC converter transformers and other large oil-filled power equipment.
New installations as well as retrofit / upgrade projects for existing transformer fleets.
6. Why Choose Us
Customizable setpoints (alarm gas volume, trip oil-surge level) and contact configuration to match your protection relay scheme.
Every relay is individually calibrated and 100% tested for float actuation, switch operation and oil-tightness before shipment.
Proven performance under harsh outdoor conditions; corrosion-resistant cast-aluminum housing and hermetically sealed switches for long-term reliability.
Experienced OEM/ODM partner with fast lead times and responsive technical support (24h quotation).
7. Ordering Information
To receive an accurate quotation, please provide the following information when ordering:
No.
Required Information
1
Transformer rated power and voltage (kVA / kV)
2
Pipe connection diameter and flange standard (DN80, …)
3
Alarm / trip setpoints (gas volume & oil surge) if special
4
Contact type and quantity (SPDT × 2, mercury switch)
5
Order quantity and target delivery date
Send us your transformer specification or tank-to-conservator pipe drawing and we will provide a complete QJ12-80 Buchholz relay proposal — including setpoints and delivery schedule — within 24 hours.
