Abstract: Transformer gas protection features fast action, high sensitivity, and simple structure. It can detect various types of faults inside the transformer tank, especially when the number of short-circuited turns in the winding is small, the fault circulating current is large, which may cause severe overheating, but the external current change is small, making it difficult for various current-reflecting protections to operate. Gas protection has a special advantage in this type of fault. This paper elaborates on 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, and proposes anti-accident measures. Keywords: Gas protection; Transformer tank interior; Gas relay 1 Working Principle of Transformer Gas Protection Transformer gas protection is the main protective element for internal faults, and it can sensitively operate on faults such as inter-turn and inter-layer short circuits, core faults, bushing internal faults, winding internal open circuits, insulation deterioration, and oil level drop. 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 with the severity of the fault. Gas protection utilizes a gas relay (also known as a gas-response relay) 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 surge is generated, causing a sudden increase in pressure within the oil tank. This creates a large flow of oil impacting the oil conservator. The flow impacts the baffle plate, which overcomes the spring resistance, moving the magnet towards the reed contact and closing the mercury contacts. This activates the trip circuit, causing the circuit breaker to trip – this is known as "heavy gas." Heavy gas activation 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 closure of the signal circuit contact of the upper open cup inside the relay due to various reasons, causing the indicator light to illuminate. 2. Protection Scope of Gas Protection Gas protection is one of the main protections for transformers, and it can detect all faults within the oil tank. These include: multi-phase short circuits within the oil tank, inter-turn short circuits in the windings, short circuits between the windings and the core or the 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 oil 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. 3. 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: 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. Before installing a new gas relay, check for a valid inspection certificate, correct pipe 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. The gas relay should be installed horizontally, with the arrow on the cover pointing towards the oil tank. In practice, the relay's piping axis may be slightly higher towards the oil tank, but the inclination from the horizontal should not exceed 4%. Fill the gas relay with oil by opening the disc valve, and release the gas through the vent valve after it is full. If the oil tank has a bladder, pay attention to the filling and venting methods to minimize or avoid gas entering the oil tank. When wiring for protection, prevent incorrect connections and short circuits, avoid live operation, and prevent rotation of the conductive rod and oil leakage from the small ceramic head. Before commissioning, insulation resistance and transmission tests should be performed. 4. Test Items 4.1 Inspection of Gas Relays 4.1.1 Inspection of Internal and Mechanical Parts of Gas Relays The glass window, vent valve, probe, and lead terminals should be intact and free of oil leakage. When disconnecting or connecting leads, prevent the lead terminals from rotating with the relay. The float should be free of cracks and dents; the glass window should be intact, clear, and bright without crack lines. The mercury surface of the mercury contacts should be clean and shiny; the mercury should flow freely when shaken to open or close, and should not stick to the glass wall or electrodes. The electrodes and tips should be shiny and silvery-white. The mercury contact post should be firmly welded, and the base should be reinforced with a soft plastic tube; the contact lead should be of appropriate length, bendable freely, and free of broken strands; the ceramic bead should be intact, smooth, and free of burrs. The mercury contacts should be firmly fixed. When the contact distance is normal, the length of one pole immersed in mercury should be no less than 4mm. For other types of relays, the two poles should be disconnected when the contacts are normal. The polarity of the contact leads should be such that the contact that is frequently immersed in mercury is connected to the positive pole. If the contacts are not frequently immersed in mercury and connected to the positive terminal, long-term operation may damage the contacts due to the conductor of mercury vapor, potentially even triggering the gas relay protection. Moving parts should operate flexibly and reliably, with no jamming upon return. For float-type gas relays, the mercury contacts should be adjusted so that when the float is up, the distance between the contacts is not less than 4mm. When the float sinks and the mercury contacts just close, the float should continue to deflect approximately 5° in the downward direction to ensure reliable contact closure. 4.1.2 Gas Relay Sealing Test: Before the sealing test, check for any circuits caused by mercury vapor at the mercury contacts. The insulation resistance between the contacts can be measured with a 2500V megohmmeter; it should not be less than 50Ω, and there should be no momentary flashes or discharge luminescence. When using an AC 1000V withstand voltage test, good contacts will not emit light, while poor contacts will emit a strong purple light. Place the float, mercury, and reed switch contacts into a test apparatus filled with transformer oil. Apply 1.5 atmospheres of pressure and maintain the oil temperature at 90-95°C, conducting continuous sealing tests. The float weight should not differ by more than 0.1g before and after the test. Observe and judge the integrity of the contacts. Gently shake the mercury contacts; if a thin film appears on the mercury surface or the mercury breaks into small beads, it indicates oil leakage at the contacts. 4.1.3 Gas relay contact evaporation test: Fill a glass beaker with an appropriate amount of transformer oil. Place the mercury contacts in the upper part of the oil, with the contacts in the open position and the flexible wire exposed above the oil surface. Heat with an electric furnace to maintain the oil temperature at ~80°C for 2 consecutive hours. There should be no mercury evaporation. If mercury beads adhere to the electrodes and the glass tube wall, or mercury frost forms on the negative electrode, it indicates evaporation at the contacts. 4.1.4 Gas relay setting test: Light gas setting: When the gas accumulates in the casing to 250-300 cm³... When exposed to air, it should operate reliably. Heavy gas setting: For naturally oil-cooled transformers, it should operate reliably at an oil flow rate of 0.8–1.0 m/s; for transformers with forced oil circulation cooling, it should operate reliably at an oil flow rate of 1.0–1.2 m/s. 4.1.5 Gas relay contact reliability test: After 10 consecutive opening and closing cycles of rated voltage and current, there should be no arcing or burn marks. The contact resistance should not exceed 1 Ω. 4.2 Gas protection installation test: The transformer top cover should have a 1–15% slope along the direction of the gas relay. The oil pipe from the transformer body to the oil tank should have a 2–4% slope along the direction of the gas relay. However, the gas relay should remain horizontal. The transformer's explosion-proof pipe should be connected to the oil tank. The gas relay installation should ensure that the oil flow arrow points towards the oil tank side. The oil pipes on both sides of the gas relay should be of the same inner diameter. At the flange connection to the oil pipe, if an oil-resistant rubber gasket is used, its orifice diameter should be slightly larger than the inner diameter of the oil pipe to avoid obstructing oil flow. The valve from the gas relay to the oil tank should be open, and the valve should be a flat type, not a ball valve. The gas relay's lead terminals and vent valve should be oil-free. The gas relay's installation location should ensure an electrical safety distance for easy observation of the oil level and gas sampling. The gas relay's lead wires should be oil-resistant, passing through an intermediate terminal box before connecting to the cable. The lead wires should have several bends to prevent oil from flowing from the lead wires to the cable via capillary action. The intermediate terminal box should have a well-sealed rainproof design, and the cable should be armored; the buried portion should be protected by an iron conduit. Insulation testing of the gas protection system is required. ① The insulation resistance of the mercury contact should be no less than 50MΩ using a 1000V megohmmeter, and the insulation resistance of the secondary circuit should be no less than 1MΩ. ② The mercury contact and secondary circuit should be tested with an AC 1000V voltage withstand test for 1 minute or with a megohmmeter; there should be no flashing or discharge phenomena. Gas protection system inspection: ① Test the gas relay's activation signal using the air pump method. ② Test the correct and good wiring of the heavy gas protection circuit using an activation probe or short-circuit contact method; the circuit should trip. ③ For transformers with forced oil circulation cooling, the circulation pump should be started and stopped; the gas relay should not trip. 4.3 Gas protection inspection cycle: All gas protection inspections should be conducted in conjunction with transformer overhauls. A 1-2 inspection, insulation test, and system inspection should be performed annually. Every 2-3 years, the internal and mechanical parts of the gas relays of operating transformers should be inspected. 5. 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: The valve on the gas relay connection pipe should be in the open position. The transformer's breather should be in normal working condition. The gas protection connection piece should be correctly engaged. The oil level in the oil tank should be at the appropriate position, and the relay should be filled with oil. The waterproof cover of the gas relay must be secure. 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. 6. Operation When the transformer is operating normally, the gas relay should operate without any abnormalities. Regarding the operating status of the gas relay, the Procedures stipulate the following: When the transformer is running, the gas protection should be connected to the signal and trip settings; the gas protection of the on-load tap changer should be connected to the trip setting. The heavy gas protection signal should be switched during the following operations of a transformer: ① When one circuit breaker controls two transformers, and one transformer is switched to standby, the standby transformer's heavy gas protection signal should be switched. 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. ③ When working on the gas protection system and its secondary circuit. ④ When opening venting, draining, and inlet valves in all locations except for oil sampling and the venting valve on the upper part of the gas relay. ⑤ When the oil level gauge shows an abnormal rise or there are abnormalities in the suction system, requiring the venting or draining valves to be opened. During earthquake prediction, the operation mode of the heavy gas protection should be determined based on the specific conditions of the transformer and the seismic performance of the gas relay. For transformers that are shut down due to heavy gas protection activation caused by an earthquake, the transformer and gas protection system should be inspected and tested before being put back into operation. Only after confirming that there are no abnormalities can the transformer be put back into operation. 7. Main Reasons for Gas Protection Signal Activation If the gas protection activates and the transformer switch trips, the accident process is generally over, but the consequences are quite serious. Therefore, it is essential to carefully inspect, analyze, and correctly judge the situation when the gas signal activates, and take immediate measures. Transformer gas relays come in different models, such as float type, baffle type, and open cup type. Their signal circuit is connected to the upper open cup, and the trip circuit is connected to the lower baffle. Gas protection signal activation refers to the closure of the signal circuit contact of the upper open cup inside the relay due to various reasons, resulting in a light gas activation. The reasons for contact closure are generally as follows: Air enters the transformer and gradually accumulates in the upper part of the gas relay, forcing the oil level inside the relay to drop, causing the reed contacts to close and sending a light gas signal. There are three ways air can enter a transformer in operation: ① When changing or adding oil, if the oil to be changed or added is not thoroughly vacuum degassed or strictly follows the vacuum oil injection process, air in the oil, air adhering to the core, windings, and accessory surfaces, and air in the pores of organic solid insulation materials will gradually accumulate and rise into the gas relay after the transformer is put into operation through oil convection circulation and the expansion and contraction of the transformer core's magnetic circuit, triggering a signal activation. ② After replacing the absorbent (such as silica gel) in the transformer's thermosiphon, if the oil immersion and settling time is short, air may not be completely expelled, entering the main circulation from the thermosiphon and then entering the gas relay, triggering a signal activation. ③ In transformers with forced oil circulation, poor sealing of the submersible oil pump can cause air to enter the transformer's main circulation due to the slight negative pressure generated during pump operation, accumulating in the gas relay and triggering a gas signal activation. ④ A sudden drop in ambient temperature can cause the transformer oil to cool and contract rapidly, resulting in a decrease in the oil level, or severe oil leakage in the transformer can also cause a decrease in the oil level, i.e., oil flow triggering a gas relay signal activation. Faults in the secondary signal circuit of the gas relay, including short circuits due to damaged signal cable insulation, short circuits at terminal block contacts, and individual signals connected in the signal circuit, can cause the reed contacts to close, triggering the gas signal. Internal discharge or overheating faults in the transformer can cause the solid insulation material and transformer oil to decompose, producing hydrogen, carbon monoxide, carbon dioxide, and low-molecular-weight hydrocarbon gases. These gases gradually turn into bubbles with the oil convection circulation and rise to accumulate on the upper part of the gas relay, forcing the oil level inside the relay to drop and triggering the gas signal. A through-fault fault in the transformer can also cause problems. Under the influence 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 windings to heat up. When the fault current multiple is very large, the winding temperature rises rapidly, causing the oil volume to expand and resulting in a malfunction of the gas relay. All of the above factors can cause the gas protection signal to activate. 8. Handling Strategies for Gas Protection Device Operation 8.1 Analysis and Diagnosis Procedures (1) Check for gas accumulation in the relay; (2) Perform ignition test and chromatographic analysis. 8.2 Basic Principles and Handling Strategies for Analysis and Diagnosis After the gas signal is activated, whether there is gas accumulation in the relay is the most basic principle for distinguishing between the causes of signal activation, such as oil level drop, secondary circuit failure, air entering the transformer, and internal transformer failure. Due to secondary circuit failure, the gas signal activation caused by oil level drop cannot generate gas. Therefore, when there is no gas accumulation in the relay, a step-by-step judgment should be made. First, inspect the transformer for serious oil leaks. If so, report to the superior dispatcher and supervisor immediately and take measures to plug the leaks. If not, determine whether the oil level drop is caused by a sudden drop in ambient temperature. It is necessary to observe whether the oil level indicator position of the transformer oil tank is normal and whether the oil passage is blocked. If not, take corresponding measures. If it is not caused by the above reasons, the secondary signal circuit failure is more likely. It is necessary to check and eliminate the secondary circuit defects. Is the gas accumulated in the relay air or flammable gas? If the gas inside the relay is air, the following should be determined in sequence: whether the air was not completely purged after the oil change or ~lN,rh; whether the air was not completely purged due to a short settling time during the replacement of the transformer thermosiphon adsorbent; if so, vent air from the relay vent and monitor the transformer operation; whether the signal action was caused by air entering the transformer body from the submersible oil pump; if so, use a step-by-step shutdown test to determine which pump the air entered from and request pump shutdown for maintenance; if the gas inside the relay is flammable gas, there is overheating inside the transformer, possibly a discharge fault, or a combination of overheating and discharge fault. In this case, gas and oil samples should be taken simultaneously from the relay (oil sample taken from the bottom of the transformer body) for chromatographic analysis. The nature, development trend, and severity of the fault should be determined according to the dissolved gas analysis and judgment guidelines for transformer oil. Based on the analysis conclusions, continued monitoring or shutdown for inspection should be implemented. The method to identify whether the gas inside the relay is air or flammable gas is to collect these gases and perform ignition tests and chromatographic analysis. 8.3 Identification of Gases in Relays: Ignition and Chromatographic Analysis of Methane Gas. The "Operating Regulations for Power Transformers" / T572-1995 stipulates that if there is gas inside the 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 ignition test involves igniting the gas collected with a syringe using a match through the vent. If the gas spontaneously combusts with a light blue flame, it is flammable, indicating a fault inside the transformer. If it does not spontaneously combust, it is air, indicating that the signal activation was caused by air intrusion. Chromatographic analysis refers to the qualitative and quantitative analysis of the collected gas, including hydrogen, oxygen, carbon monoxide, carbon dioxide, methane, ethane, ethylene, and acetylene, using a chromatograph. Based on the names and contents of the components, the nature, development trend, and severity of the fault can be accurately determined. The ignition test and chromatographic analysis are two different methods for determining whether there is a fault inside the transformer, but they have the same purpose. The ignition test was a method prescribed before the use of chromatographic analysis for qualitative and quantitative analysis of the gases contained in the power transformer; it is simpler and more rudimentary. The accuracy of this method depends on the skill and experience of the test personnel, but it cannot determine the nature of the fault. Since the adoption of chromatography, this method has not been removed from transformer operation procedures. The original intention was to quickly determine whether a transformer has a fault on-site, but limitations such as whether on-site personnel can correctly collect the gas, ignite it correctly, and make accurate judgments prevent it from achieving the desired effect of long-term training for short-term use. Whether to perform an ignition test or chromatographic analysis depends on the gas relay signal activation volume setting value. Theoretically, as long as the signal is activated, approximately 250-300 ml of gas can be collected. Two tubes can be collected using a 100 ml syringe. One tube can be used for the ignition test on-site, and the other for chromatographic analysis. Sometimes, internal transformer faults develop rapidly, and the generated gas rises and accumulates in the relay before reaching oil saturation. If the gas is not collected promptly after the signal activation, some gas will dissolve back into the oil and escape, resulting in less than 100 ml of collected gas. If a single 100 ml syringe is used, an ignition test should not be performed; chromatographic analysis should be conducted immediately. This conflicts with the regulations for transformer operation. The solution is to collect the gas using two small-capacity syringes (each tube no less than 10 ml). If the transformer is close to the chromatography laboratory, an ignition test is unnecessary. If the on-site operators are trained and capable of collecting and conducting ignition tests, they should be responsible for this task. If they lack this capability, it should be handled by relevant professionals. 9. Anti-Accident Measures for Gas Protection Gas protection activation can range from a minor signal alerting maintenance personnel to immediate transformer intervention, to a major tripping of the transformer switch, causing immediate transformer shutdown and compromising power supply reliability. To address this, anti-accident measures for gas protection are proposed: 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. To prevent short circuits due to water leakage, rainproof measures should be taken for its terminals and the cable bow terminal box. Oil-resistant wires should be used for the gas relay leads. The gas relay leads and cables should be connected separately to terminals inside the cable bow terminal box. 10. Conclusion After a transformer gas signal is activated, the operating personnel must inspect the transformer. They must follow the analysis and diagnostic procedures to determine the cause of the activation and immediately report to the superior dispatcher and supervisor. If a spare syringe is available on site, the on-duty operating personnel should collect two tubes based on the total gas volume inside the relay; one tube should be used for an ignition test, and the other should be given to a professional for chromatographic analysis. 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. Appropriate countermeasures should be taken based on relevant guidelines and on-site analysis conclusions to prevent accidents and ensure the safe and economical operation of the transformer. Click to download: Discussion on Transformer Gas Protection