Two-Float
Buchholz Gas Relay
Однопоплавкове
Газове реле Бухгольца
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High-Voltage Equipment
Types of Transformer Protection Relays (Buchholz Principle)
The product presented here is the standard version. Modifications are possible and can be easily requested from us. We will be happy to provide consultation and work with you to find the optimal solution.
Description
The gas relay was invented in 1921 by Max Buchholz, senior advisor of the Prussian Electrotechnical Joint Stock Company in Kassel. Since then, it has become an important protection and monitoring device for transformers with a liquid dielectric, equipped with an expansion tank and arc-quenching coils, as well as for the separate monitoring of oil-filled bushings or cable connection boxes.
The relay is installed in the cooling circuit of the protected device and responds to disturbances such as gas formation, losses, or accelerated flow of insulating liquid.
In transformers with a hermetically sealed connection using a hydrocompensator (“rubber bag”) in the expansion tank, Buchholz gas relays can also be used as monitoring devices (“bag rupture indicators”) for this hydrocompensator.
Gas relays can be used both in outdoor installations and indoor environments.
The variety of gas relay types is determined by regulations and standards as well as the specific requirements of customers. The rated power and design of the protected device determine the type of gas relay to be used. Our product range ensures optimal compatibility with these parameters.
The Buchholz gas relays supplied by AFC comply with EN 50216-2 and IEC 60076-22-1 standards. They are characterized by ease of maintenance, high reliability, and a long service life. The operating characteristics of the gas relays are guaranteed when using all standard insulating liquids. Compatibility and functionality with new insulating liquids are verified in close cooperation with manufacturers.
Housing
The housing is made of aluminum alloy resistant to weather conditions and features a paint coating according to the specifications. Depending on the design, it can have either a flanged connection (Fig. 1 / No. 1) or a threaded connection (Fig. 2 / No. 2). Housing variants are available for both single-float and double-float gas relays. Other versions are available upon request.
For monitoring the switching system’s functions, the housing is equipped with inspection windows (Fig. 2 / No. 3). The scale printed on the glass allows the volume of accumulated gas to be determined.
The devices can be fitted with hinged covers (Fig. 1 / No. 4) positioned in front of the inspection windows.
The control relay responds to excessively intense oil flow in the direction of the expansion tank and sends a signal that can be used to disconnect the tap changer and the transformer.
Control relays for tap changers are suitable for both outdoor installations and indoor environments.
Cover
The cover is made of aluminum alloy resistant to weather conditions and features a paint coating according to the specifications. At the top of the cover is a terminal box (Fig. 3 / No. 1). In front of the terminal box are an air valve (Fig. 3 / No. 2) and a test button covered with a cap nut (Fig. 3 / No. 3), as well as a nameplate (Fig. 3 / No. 4) with instructions for operating the test button.
In addition to the grounding terminal (Fig. 3 / No. 5), the box contains electrical terminals (Fig. 4 / No. 6) installed at the bottom of the cover. The number of these terminals determines the configuration of the switching systems, including the type and quantity of reed contacts.
The terminal box is protected by a protective cover (Fig. 3 / No. 7) that prevents accidental contact and contamination. On the inner side of the cover, the symbol markings and terminal layout diagram (Fig. 3 / No. 8) are provided.
The connection cable is inserted through a threaded cable gland (Fig. 4 / No. 9), ensuring a secure and sealed connection. The design of the cover allows it to be removed for maintenance or replacement of individual components without compromising the tightness of the entire system.
All metal parts have anti-corrosion coating, ensuring durability and resistance to external factors. Upon customer request, additional options are available, such as different types of electrical terminals, a modified air valve design, or alternative housing materials.
The Buchholz gas relay is designed for the protection and monitoring of transformers filled with insulating liquid, equipped with an expansion tank and a grounding arc suppression reactor, as well as for the separate monitoring of oil-filled bushings or cable connection boxes.
The relay is installed in the cooling circuit of the protected device and responds to faults such as gas formation, insulating liquid leakage, or increased liquid flow velocity, issuing a warning or trip signal. This prevents severe transformer damage, up to and including complete destruction.
The gas relay is installed in the connecting pipe between the tank of the protected device (transformer, grounding reactor) and the expansion tank. Under normal operating conditions, it is completely filled with insulating liquid.
The float of a single-float relay and both floats of a double-float relay rise upward due to buoyancy and remain in their highest position.
The upper and lower switching systems of a single-float relay function as a single unit, ensuring immediate disconnection of the transformer from the network in the event of a fault.
The operating principle is described below using a double-float gas relay as an example. In the event of internal transformer faults, the gas relay responds as follows:
Gas Emission
Fault: Free gas is present in the insulating liquid.
Response: The gas in the insulating liquid rises upward, accumulates in the gas relay, and displaces the insulating liquid. As the liquid level drops, the upper float descends.
As the float moves, a switching contact (magnetically operated reed switch) is activated, triggering a warning signal. The lower float is not affected, since once a certain gas volume is reached, it moves through the pipeline into the expansion tank.
Insulating Liquid Leakage
Fault: Leakage of insulating liquid due to loss of tightness.
Response: As the level of insulating liquid decreases, the expansion tank, pipeline, and gas relay empty. The upper float descends first, triggering the alarm signal. With further liquid loss, the lower float descends, activating the switching contact, which results in the disconnection of the transformer.
Accelerated Flow of Insulating Liquid
Fault: As a result of a spontaneous event, a shock wave occurs and moves in the direction of the expansion tank.
Response: The liquid flow acts on a valve flap located within the flow. If the flow velocity exceeds the activation threshold of the valve flap, it begins to move in the direction of the flow. As a result of this movement, a switching contact is triggered, leading to the disconnection of the transformer. After the shock wave subsides, the lower switching system returns to its initial position.
In the Buchholz gas relays supplied by AFC, the valve flap is held in place by a permanent magnet.
| Parameter | Value / Data | Remarks |
|---|---|---|
| Voltage | AC 5 V – max. 250 V DC 5 V – max. 250 V |
|
| Current | AC 0.01 A – max. 6 A DC 0.01 A – max. 6 A |
Cos φ > 0.5 L/R < 40 ms |
| Connected Power | AC max. 1500 VA DC max. 1250 W |
|
| Dielectric Strength | AC 2500 V AC 2000 V (make, break) AC 1000 V (switch) |
Between circuit and ground Between open contacts |
| Temperature Range: Ambient Temperature |
-40 °C to +55 °C -40 °F to +131 °F |
Climatic test according to DIN EN 60068-2-78: 2002-09 Others on request |
| Temperature Range: Operating Range Insulating Liquid Temperature |
-40 °C to +115 °C -40 °F to +239 °F Up to +135 °C – optional version 21 |
|
| Temperature Range: Operating Range Insulating Liquid Viscosity |
1 mm²/s to 1100 mm²/s | |
| Insulating Liquid | Mineral oil | Others on request |
| Vibration Resistance | Vibration: 2–200 Hz, 2 g Shock: 25 g, 6 ms |
According to class 4M6 per DIN EN 60721-3-4 |
| Compressive Strength Limit | 0.25 MPa | |
| Vacuum Strength | < 2.5 kPa | |
| Magnetic Field Immunity | 25 mT | Static magnetic field in any direction and polarity |
| Switching System | Magnetically operated reed contact, held by magnet < 0.1 s |
|
| Switching System at: | 200 cm³ to 300 cm³ min. 0.65 to max. 3.00 m/s ± 15% |
Others on request Ordering information in the "Type Indicator" |
| Cable Gland | M20x1.5; M25x1.5 | Others on request |
| Nominal Installation Position | 0° to 5° | With upward slope towards the expansion tank |
| Protection Class | IP 56 | Others on request |
| Housing Coating | Two-component structural paint | Polyurethane-based |
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