Transformer Gas Relay

• Standard name: Gas Relay (Gas Relay, commonly known internationally as Buchholz Relay.

• core functionality: As the main protection specific to internal faults in oil-immersed power transformers, it is used to detect gases generated by the decomposition of insulation materials due to electrical discharges or localized overheating, as well as the impact of oil flow triggered by severe short-circuit faults.

• -protection range: Covers a wide range of faults within the transformer tank, and is particularly sensitive to initial latent faults such as winding turn-to-turn short circuits, core multi-point grounding, and poor tap changer contact.

• Working Principle: Mechanical relays based on physical phenomena. Utilizes the principle that the gas generated by a fault is less dense than the oil and rises up to collect (light gas signal), and the kinetic energy principle of the impact of the oil flow in case of a severe fault (heavy gas trip).

• mounting position: Horizontally mounted in the connecting line between the transformer tank body and the oil conservator, ensuring that all gases generated from the tank must pass through this relay during the ascent.

• applied valueIt is the simplest, most reliable and indispensable component of the transformer protection system, capable of giving early warning signals at the very early stage of a fault, or quickly removing the transformer in the case of a serious fault, effectively preventing the fault from expanding or even triggering an explosion in the tank.

I. Working principle and structure

A gas relay is a non-electrical quantity protective relay whose action does not depend on electrical parameters such as current and voltage, but responds to the physical phenomena generated by faults within the transformer. Its internal structure usually contains two core action elements, corresponding to two different protection logics.

1. Light Gas Alarm

• Mechanism of action: When there are minor or initial faults within the transformer, such as minor short circuits between turns of the windings, excessive eddy currents due to insulation damage between the silicon steel sheets of the iron core, or electrical sparks due to poor contact with the tap changer, these points of fault will cause the surrounding insulating oil and insulating paper to thermally decompose, producing a variety of gases, such as hydrogen, methane, ethylene, and so on. The density of these gases is much smaller than the absolute - edge oil, will be in the form of bubbles from the point of failure to escape and rise. Since the gas relay is installed in the path of the oil flow, these gases will eventually collect in the upper space of the relay.

• Structure & Movement:: The upper part of the relay is equipped with a cup-shaped float or hollow float with a downward opening, to which a reed switch or mercury contact is connected by a lever mechanism. As gas collects in the upper part, the oil level in the area drops and the float sinks as it loses buoyancy, thus driving the contact closed and signaling a "light gas" alarm.

• significance: The light gas signal is awarning signalIt suggests to O&M personnel that there may be latent faults within the transformer, and that tests such as dissolved gas analysis (DGA) of the oil need to be arranged as soon as possible to diagnose the nature of the faults and deal with them in a timely manner in order to avoid further development of the faults.

2. Heavy Gas Trip

• Mechanism of action: When a serious short-circuit fault occurs inside the transformer, such as a phase-to-phase short-circuit or a serious turn-to-turn short-circuit, the instantaneous generation of a huge arc energy will cause the surrounding insulating oil to vaporize dramatically, forming a powerful pressure wave and high-speed oil flow, surging from the tank body to the oil storage cabinet.

• Structure & Movement: The oil flow path inside the relay contains a baffle or stall plate that is connected to another independent reed switch or mercury contact. Under normal conditions, the oil flow rate is low enough to push the plate. However, in the event of a serious internal fault, the high velocity oil impulse will push the plate and deflect it enough to actuate the contact closure and issue a "heavy gas" trip command. This command acts directly on the circuit breakers on each side of the transformer, instantaneously removing the transformer from the grid.

• significance:: Heavy gas protection isMain protection tripping actionIt is designed to isolate the transformer as quickly as possible after a fault occurs, to prevent continuous input of fault energy, to minimize equipment damage, and to prevent the accident from expanding (e.g., tank fire, explosion).

II. Installation and operation requirements

• Installation Requirements:

  • It must be mounted horizontally on the connecting line between the tank top and the reservoir cabinet.

  • The line needs to have an upward inclination of 2%-4% to ensure that the gas flows smoothly toward the reservoir and into the relay.

  • The direction of the arrow on the relay must be pointing towards the oil reservoir.

  • The inside of the relay should be filled with insulating oil and no residual air should be present.

• Precautions during operation:

  • Newly commissioned or overhauled transformers may initially have air trapped in the insulation material escaping, which may cause a false alarm for light gas.

  • Operations such as refueling, oil filtering or cooler switching on the transformer may cause fluctuations in the oil flow, and it is usually necessary to temporarily switch the heavy gas trip circuit to the signal position or exit it for a short period of time in order to prevent the heavy gas protection from mis-activating.

  • Earthquakes or strong mechanical vibrations can also cause relays to malfunction, so modern relays are designed with a certain level of seismic performance.

III. Advantages and disadvantages of gas relay protection

Pros.

• high sensitivity: Sensitivity to various types of internal faults, especially initial faults such as turn-to-turn short circuits, is much higher than that of electrical quantity protection.

• high reliability: Simple structure, based on purely physical principles of action, with little influence from external electrical system operation, short-circuit current levels, CT saturation, and other factors.

• -The principle is clear.: Light gas alarms and heavy gas trips correspond to faults of different severity, making it easy to determine faults.

Disadvantages.

• non-selective: It is not possible to distinguish whether it is a winding fault or a core fault, and needs to be further analyzed in conjunction with other diagnostic tools (e.g., DGA).

• Dead zone exists: For faults in the lower part of the tank, where the resulting oil shock may have decayed before reaching the relay, and for faults inside the lead casing, the gas relay may not be able to protect effectively.

• Possible misbehavior: There is a possibility of false activation under non-fault conditions such as refueling, earthquakes, and oil level fluctuations due to traversing faults.

Frequently Asked Questions (FAQ)

1. How do you determine what kind of gas is present when a gas relay is activated?
When a light gas alarm occurs, O&M personnel should immediately collect the gas through the withdrawal valve on top of the relay. The gas collected can be determined to be flammable by a simple field test (open flames are strictly prohibited and specialized instruments should be used). Colorless, odorless and non-flammable gases are usually air; flammable gases are indicative of fault gases from decomposition of insulation materials. Accurate analysis of the composition needs to be sent to the laboratory for chromatographic analysis.

2. Why is the operating value of heavy gas protection set according to the oil flow rate?
Because the nature of a severe internal failure is characterized by a rapid release of energy, this inevitably leads to rapid expansion and flow of oil. The oil flow velocity is directly related to the magnitude of the fault energy. Therefore, setting the action threshold of heavy gas protection to a specific oil flow velocity (e.g., 0.6m/s, 1.0m/s, etc.) can effectively differentiate between normal oil circulation and dangerous faulty oil impacts, ensuring that the protection trips instantaneously only in the event of a truly severe internal fault.

3. How do "gas protection" and "differential protection" work together to protect against internal transformer faults?
They complement each other and constitute the two main protections that are at the core of the transformer's internal faults.

• gas protectionFor turn-to-turn short circuits, core failures, overheating and gas production, etc.No significant change in electrical quantitiesThe initial or non-metallic faults are particularly sensitive.

• differential protectionmetallicphase-to-phase short circuitThe fastest and most reliable response to high current faults such as.
  Either of the two actions is sufficient to remove the transformer. This redundant configuration greatly improves the overall reliability of the transformer protection.