Abstract: This paper details the basic working principle, protection range, installation method, daily inspection items, operating status, and reasons for the signal action of the gas protection device for oil-immersed power transformers, as well as the basic principles and handling methods for accident analysis and diagnosis. Anti-accident measures are also proposed. Keywords: Oil-immersed power transformer, gas protection, handling methods, anti-accident measures 1 Introduction Currently, most of the power transformers used in our company are still oil-immersed transformers. Since starting my work, I have frequently participated in the design, installation, commissioning, and maintenance of transformers, accumulating a wealth of knowledge about transformers. This paper will now provide a detailed introduction to the gas protection of transformers. 2 Working Principle Gas protection is the main protective element for internal transformer faults. It can sensitively operate against faults such as inter-turn and inter-layer short circuits, core faults, bushing internal faults, winding internal open circuits, insulation deterioration, and oil level drops. When an internal fault occurs in an oil-immersed transformer, the electric arc will decompose the insulating material and generate a large amount of gas, the intensity of which varies depending on the severity of the fault. Gas protection utilizes a gas relay (also known as a gas-response relay) that reacts to gaseous conditions to protect against internal transformer faults. Inside the gas protection relay, there is a sealed float at the top and a metal baffle at the bottom, both equipped with sealed mercury contacts. The float and baffle can rotate around their respective axes. During normal operation, the relay is filled with oil, the float is immersed in the oil and in the floating position, and the mercury contacts are open; the baffle, due to its own weight, sags, and its mercury contacts are also open. When a minor fault occurs inside the transformer, the gas is generated slowly. As the gas rises to the oil conservator, it first accumulates in the upper space of the gas relay, causing the oil level to drop. The float then descends, closing the mercury contacts and triggering a delayed signal – this is known as "light gas." When a serious fault occurs inside the transformer, a strong gas flow is generated, causing a sudden increase in pressure within the oil tank. This creates a large oil flow that impacts the oil conservator. The oil flow impacts the baffle, which overcomes the spring resistance, moving the magnet towards the reed contact, closing the mercury contacts, and triggering the trip circuit, causing the circuit breaker to trip – this is known as "heavy gas." Heavy gas tripping immediately cuts off all power to the transformer, preventing the accident from escalating and protecting the transformer. Gas relays come in different models, including float type, baffle type, and open-cup type. Currently, the QJ-80 type relay is most commonly used, with its signal circuit connected to the upper open-cup and the trip circuit connected to the lower baffle. The so-called gas protection signal action refers to the closing of the signal circuit contact of the upper open cup inside the relay due to various reasons, causing the indicator light to illuminate. 3. Protection Scope Gas protection is the main protection for transformers, and it can detect all faults within the tank. These include: multi-phase short circuits within the tank, inter-turn short circuits in the windings, short circuits between the windings and the core or casing, core faults, oil level drops or leaks, poor contact of the tap changer, or poor welding of the wires, etc. Gas protection is fast, sensitive, reliable, and has a simple structure. However, it cannot detect faults in the external circuits of the tank (such as those on the lead-out lines), so it cannot be the sole protection device for internal transformer faults. Furthermore, gas protection is prone to malfunction under interference from external factors (such as earthquakes), and corresponding measures must be taken to address this. 4. Installation Method The gas relay is installed on the connecting pipeline from the transformer to the oil conservator. During installation, the following should be noted: 4.1 First, ensure the butterfly valve on the gas relay pipeline is tightly closed. If the butterfly valve cannot be tightly closed or other issues arise, it is necessary to drain the oil from the oil tank to prevent excessive oil overflow during operation. 4.2 Before installing a new gas relay, check for a valid inspection certificate, correct diameter and flow rate, and any damage to internal or external components. Remove any temporary bindings inside. Finally, check the reliability of the float, baffle, signal, and trip contacts, and close the vent valve. 4.3 The gas relay should be installed horizontally, with the arrow on the top cover pointing towards the oil tank. In practice, the relay's pipeline axis may be slightly higher towards the oil tank, but the inclination from the horizontal should not exceed 4%. 4.4 Open the disc valve to fill the gas relay with oil. After filling, release the gas through the vent valve. If the oil tank has a bladder, pay attention to the filling and venting methods to minimize or avoid gas entering the oil tank. 4.5 When wiring the protection circuit, prevent incorrect connections and short circuits, avoid live operation, and prevent the conductive rod from rotating and the small ceramic head from leaking oil. 4.6 Before commissioning, insulation resistance and transmission tests should be performed. 5 Test Items The following inspection and test items should be performed on the gas relay before installation and use: 5.1 General Inspection Items: The glass window, vent valve, control pin, and lead terminals should be intact and free of oil leakage; the float, open cup, and glass window should be intact and free of cracks. 5.2 Test Items 5.2.1 Sealing Test: Apply oil pressure (20 MPa, duration 1 hour) to test for leaks; there should be no leakage. 5.2.2 Terminal Insulation Strength Test: The outgoing terminals and the area between them should withstand a power frequency voltage of 2000V for 1 minute. Alternatively, a 2500V megohmmeter can be used to measure the insulation resistance for 1 minute instead of the power frequency withstand voltage. The insulation resistance should be above 300 MΩ. 5.2.3 Light Gas Operation Volume Test: When 250-300 cm³ of air accumulates inside the casing, the light gas relay should operate reliably. 5.2.4 Heavy Gas Operation Flow Rate Test. 6. Routine Inspection Items The Operating Procedures for Power Transformers DL/T572-95 (hereinafter referred to as the "Procedures") stipulates that the first item in the routine inspection of transformers should be checking for gas in the gas relay. The following points should be noted during the gas inspection: 6.1 The valve on the gas relay connection pipe should be in the open position. 6.2 The transformer's breather should be in normal working condition. 6.3 The gas protection connection piece should be correctly engaged. 6.4 The oil level in the oil tank should be at the appropriate position, and the relay should be filled with oil. 6.5 The waterproof cover of the gas relay must be secure. 6.6 There should be no oil leakage at the relay terminals, and it should be able to prevent the intrusion of rain, snow, and dust. The power supply and its secondary circuits should have waterproof, oil-proof, and antifreeze measures, and waterproof, oil-proof, and antifreeze checks should be carried out in spring and autumn. 7. Operation When the transformer is operating normally, the gas relay will work without any abnormalities. Regarding the operating status of gas relays, the regulations stipulate the following: 7.1 When a transformer is running, the gas protection should be connected to both signal and trip settings; the gas protection of an on-load tap changer should be connected to trip settings. 7.2 When the following operations are performed on a transformer during operation, the heavy gas protection should be switched to signal settings: 7.2.1 When one circuit breaker controls two transformers, and one of them is switched to standby, the standby transformer's heavy gas protection should be switched to signal settings. 7.2.2 When filtering oil, replenishing oil, replacing the submersible pump or replacing the adsorbent in the oil purifier, and opening/closing valves on the gas relay connection pipe. 7.2.3 When working on the gas protection and its secondary circuits. 7.2.4 Except for oil sampling and venting at the vent valve on the upper part of the gas relay, when venting, draining, and inlet valves are opened in all other locations. 7.2.5 When the oil level gauge shows an abnormal rise or there are abnormal phenomena in the suction system, requiring the venting or draining valves to be opened. 7.3 During earthquake prediction, the operation mode of heavy gas protection should be determined based on the specific conditions of the transformer and the seismic performance of the gas relay. For transformers whose heavy gas protection is shut down due to an earthquake, the transformer and gas protection should be inspected and tested before being put back into operation. Only after confirming there are no abnormalities can the transformer be put back into operation. 8. Main Causes of Gas Protection Signal Activation 8.1 Causes of Light Gas Protection Activation: 8.1.1 Air enters the transformer due to inadequate oil filtration, refueling, or cooling systems. 8.1.2 The oil level drops below the gas relay's light gas float due to temperature drop or oil leakage. 8.1.3 A small amount of gas is generated due to a transformer fault. 8.1.4 A through-circuit fault occurs in the transformer. Under the action of the through-fault current, the oil flow velocity in the oil gap increases. When the pressure difference between the oil gap and the outside of the winding changes significantly, the gas relay may malfunction. The through-fault current causes the winding to heat up. When the fault current multiple is very large, the winding temperature rises rapidly, causing the oil volume to expand, resulting in a malfunction of the gas relay. 8.1.5 Gas relay or secondary circuit fault. All of the above factors may cause the gas protection signal to activate. 9. Handling Gas Protection Device Activation After the transformer gas protection device activates, it should be immediately and carefully inspected, analyzed, and correctly judged, and immediate corrective measures should be taken. 9.1 When the gas protection signal activates, the transformer should be immediately inspected to determine the cause of the activation, whether it is due to air accumulation, low oil level, secondary circuit fault, or internal transformer malfunction. If there is gas in the gas relay, the gas volume should be recorded, the gas color observed, and its flammability tested. Gas and oil samples should be taken for chromatographic analysis. The nature of the transformer fault can be determined according to relevant regulations and guidelines. Chromatographic analysis refers to the qualitative and quantitative analysis of the collected gas, including hydrogen, oxygen, carbon monoxide, carbon dioxide, methane, ethane, ethylene, acetylene, etc., using a chromatograph. Based on the names and contents of the components, the nature, development trend, and severity of the malfunction can be accurately determined. If the gas in the gas relay is colorless, odorless, and non-flammable, and the chromatographic analysis determines it to be air, the transformer can continue to operate, and the air ingress defect should be eliminated promptly. If the gas in the gas relay is flammable and the chromatographic analysis results of the dissolved gases in the oil are abnormal, a comprehensive judgment should be made to determine whether the transformer should be shut down. 9.2 When the gas relay trips, the transformer must not be put back into operation until the cause is identified and the fault is eliminated. To identify the cause, the following factors should be considered in making a comprehensive judgment: a. Whether there is obstruction in ventilation or incomplete venting; b. Whether the protection and DC secondary circuits are normal; c. Whether there are any abnormal phenomena on the transformer's appearance that clearly reflect the nature of the fault; d. Whether the gas accumulated in the gas relay is flammable; e. The chromatographic analysis results of the gas in the gas relay and the dissolved gases in the oil; f. The results of necessary electrical tests; g. The operation status of other relay protection devices on the transformer. 10. Accident Prevention Measures for Gas Protection Gas protection activation can range from minor incidents (sending a protection signal to alert maintenance personnel for immediate transformer intervention) to major incidents (tripping the transformer switch, causing immediate transformer shutdown and compromising power supply reliability). To address these issues, the following accident prevention measures for gas protection are proposed: 10.1 Replace the lower float of the gas relay with a baffle type and the contacts with a vertical type to improve the reliability of heavy gas operation. 10.2 To prevent short circuits caused by water leakage in the gas relay, rainproof measures should be taken for its terminals and the cable lead terminal box. 10.3 The gas relay leads should use oil-resistant wires. 10.4 The gas relay leads and cables should be connected separately to terminals inside the cable lead terminal box. 11. Conclusion After a transformer gas signal activation, operating personnel must inspect the transformer, determine the cause of the activation, and immediately report to the superior dispatcher and supervisor. The superior supervisor should immediately dispatch personnel to the site to collect gas samples, oil samples, and body oil samples from the relay for chromatographic analysis. Based on relevant guidelines and on-site analysis conclusions, take corresponding countermeasures to avoid accidents and ensure the safe and economical operation of transformers.