Gas protection is a critical safety mechanism in power transformers, designed to detect internal faults that may lead to serious damage. When a fault occurs within the transformer tank—such as a short circuit or a ground fault caused by dielectric breakdown—the high current and arc generated at the fault point cause the transformer oil (typically 25# insulating oil) and other insulating materials to decompose due to localized heating. This decomposition produces gas, which is lighter than oil and rises toward the top of the oil conservator.
In severe cases, the rapid expansion of oil creates a large volume of gas, which mixes with the oil flow and moves upward. This movement triggers the gas relay, causing its contacts to act and either send an alarm signal or automatically disconnect the transformer from the system. The device responsible for this function is known as gas protection.
The gas relay is the core component of gas protection systems. Models like FJ3-80 and QJ series are typically installed on the connecting pipe between the transformer tank and the oil conservator. To ensure optimal performance, the gas produced during a fault must pass smoothly through the relay. Therefore, during installation, the transformer tank should be inclined at 1–1.5% toward the oil conservator, and the connecting pipe should have a slope of 2–4% in the same direction.
Older gas relays, such as float-type mercury contact relays, were less reliable due to their poor vibration resistance. They could malfunction due to mechanical vibrations, leading to false trips. Modern gas relays, such as the open-cup baffle type, offer better stability and reliability, making them widely used in China’s power systems today.
Under normal operation, the gas relay is completely filled with transformer oil. When a minor fault occurs, small amounts of gas rise and accumulate in the upper chamber of the relay, causing the oil level to drop and triggering the upper magnetic switch to close a pair of dry spring contacts. This results in a light gas alarm signal.
In the case of a major fault, a large amount of gas is produced along with increased oil temperature, leading to rapid expansion and pressure buildup inside the tank. This creates an oil flow that strikes the lower baffle of the relay, bringing the lower magnetic switch close to another set of reed contacts. This action triggers a trip, isolating the transformer from the power system.
The setting of the gas relay can be adjusted using differential pressure flow tests or oil pump-based oil velocity testing equipment. The threshold for heavy gas is generally set between 0.5–1.5 m/s, depending on whether the transformer uses natural oil circulation (1–1.5 m/s) or forced oil circulation (0.6–1 m/s). For light gas, the threshold is based on the gas volume in the relay, typically ranging from 250–300 cm³, which triggers an alarm signal.
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