The Buchholz relay is a gas- and oil-actuated protective relay installed in the piping between the transformer tank and the conservator (oil reservoir). As the primary internal fault protection device for oil-immersed transformers, it detects the generation of fault gases and abnormal oil flow caused by incipient (slow-developing) faults and severe (arcing) faults, then issues an alarm or trip signal to safeguard the equipment.
The QJ4-80 is a conventional float-type Buchholz relay. It is suitable for transformers rated 750 kVA and above with a conservator design. With a robust hermetically sealed construction and reliable mercury-free contacts, it operates safely in outdoor environments and under extreme service conditions.
Working Principle
Under normal conditions, the relay housing is completely filled with transformer oil and the float is in its uppermost position, holding the alarm contact open.
Light fault (incipient): Minor internal faults (e.g. local overheating, partial discharge) decompose the oil and slowly release gas, which accumulates in the top of the relay chamber. As the oil level falls, the float drops and closes the alarm (signalling) contact at a preset gas volume, typically 250–300 cm³.
Severe fault (arcing): A major internal fault generates a large volume of gas instantaneously, creating a rapid oil surge through the relay. The resulting pressure wave deflects the lower oil-flow flap, which rotates and closes the trip contact to disconnect the transformer from the network without delay.
2. Model Designation & Construction
2.1 Model Designation
Code
Meaning
QJ
Buchholz Relay (gas relay)
4
Design series number
80
Nominal pipe / flange diameter, DN 80
2.2 Construction
The relay consists of a cast aluminium alloy or steel housing forming a sealed oil chamber. Inside are two independent operating elements:
Upper element — a hinged float carrying a mercury-free magnetic reed or micro-switch. It responces to gas accumulation (falling oil level) and operates the alarm contact.
Lower element — an oil-flow controlled flap (vane) with a separate contact. It responds to rapid oil surge and operates the trip contact.
The housing is fitted with a transparent inspection window for visual observation of the oil level and gas collection, plus a manual gas-release / test valve and mounting flanges for integration into the conservator piping.
3. Technical Data
Parameter
Specification
Model
QJ4-80
Application
Oil-immersed power & distribution transformers
Nominal diameter
DN 80
Gas (alarm) setting
Approx. 250–300 cm³ accumulated gas
Oil-surge (trip) setting
Adjustable, approx. 0.8–1.2 m/s flow velocity
Contact configuration
1 × alarm (SPDT) + 1 × trip (SPDT)
Contact rating
AC 250 V / 5 A (resistive), or as specified
Housing material
Cast aluminium alloy / welded steel
Mounting orientation
Horizontal, in tank-to-conservator pipe
Connection flange
DN 80, drilled to standard or customer drawing
Operating temperature
∐40 °C to +85 °C
Ingress protection
IP65 / IP66
Standards
IEC 60076-1, GB/T 6451
4. Key Features
<b>Two-stage protection:</b> independent gas-accumulation alarm and oil-surge trip elements provide complete coverage of both incipient and severe internal faults.
<b>Reliable contacts:</b> mercury-free reed / micro-switch contacts ensure stable signalling over a long service life with no risk of contact welding.
<b>Adjustable trip:</b> the oil-surge flap setpoint is field-adjustable to match transformer rating and piping geometry, preventing nuisance operation.
<b>Visual indication:</b> a transparent window allows on-site inspection of oil level, gas volume and relay status without opening the housing.
<b>Hermetic & robust:</b> sealed cast housing resists oil leakage, vibration and outdoor weathering for maintenance-free long-term operation.
<b>Test & release valve:</b> integrated manual valve permits periodic function testing and safe release of collected gas during routine maintenance.
5. Applications
Oil-immersed power transformers (750 kVA and above) with conservator design
Generator step-up transformers and auto-transformers
HV reactors, shunt reactors and large instrument transformer installations
Retrofit and OEM projects requiring conventional float-type Buchholz protection
6. Why Choose Us
<b>Full OEM/ODM support</b> — flanges, contacts and settings tailored to your transformer design.
<b>100% factory tested</b> — every relay undergoes gas-volume and oil-surge calibration before shipment.
<b>Proven field reliability</b> — robust construction and stable performance under harsh outdoor conditions.
<b>Fast response</b> — technical support and quotation within 24 hours.
7. Ordering Information
Please provide the following information when requesting a quotation:
No.
Required Data
1
Transformer rated power / voltage level
2
Nominal diameter and flange standard (DN 80, or customer drawing)
3
Contact type and rating (alarm / trip, SPDT, voltage & current)
