Gas protection is a critical safety mechanism used in power transformers to detect internal faults. When an internal fault occurs—such as a short circuit or a ground fault caused by dielectric breakdown—the transformer oil, typically 25# insulating oil, along with other insulating materials, undergoes thermal decomposition due to the heat generated by the fault current and arc. This process produces gas, which is lighter than oil and tends to rise toward the top of the oil conservator. In severe cases, the rapid expansion of oil and the generation of large amounts of gas can create a strong gas flow that moves upward, triggering the gas relay. This relay then activates the control circuit, sending an alarm signal or initiating the automatic disconnection of the transformer.
The gas relay is the core component of the gas protection system. Models such as FJ3-80 and QJ series are commonly installed on the connecting pipe between the transformer tank and the oil conservator. To ensure the sensitivity and reliability of gas protection, it's essential that the gas produced during a fault flows smoothly through the relay. Therefore, during installation, the transformer tank should be inclined at 1 to 1.5% towards the oil conservator, and the connecting pipe should have a slope of 2 to 4% in the same direction.
Older gas relays, such as float-type mercury contact relays, were known for their instability and poor vibration resistance. They could malfunction due to external vibrations causing the mercury to move. Modern open-cup baffle gas relays, however, offer better stability and reliable operation, making them the preferred choice in China’s power systems today.
Under normal operation, the gas relay is fully 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 the upper magnet to approach a pair of dry spring contacts, triggering a light gas signal. In the case of a severe fault, a large volume of gas is produced alongside rising oil temperature and increased pressure inside the tank, creating a rapid oil flow that pushes the lower baffle of the relay. This action brings the lower magnet close to another set of reed contacts, resulting in a trip signal.
The setting of the gas relay can be adjusted using either a differential pressure flow test or an oil pump-based oil speed test equipment. The threshold for heavy gas is generally set between 0.5 to 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 setting is based on the gas volume within the relay, typically ranging from 250 to 300 cm³. These settings help ensure timely detection and response to internal transformer faults.
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