Transformer-based operation, maintenance and fault handling solutions

Power transformers play an important role in power plants. However, due to their structure, process, operation and maintenance, transformer failures frequently occur in power plants, greatly affecting the normal production of power plants. Therefore, strengthening the regular maintenance of transformers and taking effective measures to prevent transformer failures are of great significance to ensure the safe and stable operation of transformers.
1 Classification and causes of common transformer failure phenomena
(1) Problems that exist when the transformer leaves the factory. Such as loose ends, loose pads, poor welding, poor core insulation, insufficient short-circuit resistance, etc.

(2) Line interference. Line interference is the most important factor causing transformer accidents. It mainly includes: overvoltage generated when closing the switch, voltage peaks that occur during low-load stages, line faults, flashovers and other abnormal phenomena. This type of failure accounts for a large proportion of transformer failures. Therefore, it is necessary to conduct impulse protection tests on transformers regularly to detect the strength of transformers to resist excitation surge current.

(3) Accelerated aging of transformer insulation due to improper use. The average life of a general transformer is only 17.8 years, which is much lower than the expected life of 35 to 40 years.

(4) Overvoltage caused by lightning strike.

(5) Overload. Overload refers to the transformer operating at a power level exceeding the rated power for a long time. Overload often occurs when the power plant continues to increase the load slowly, the cooling device does not operate normally, the transformer has internal faults, etc., which eventually causes the transformer to operate at overload. The resulting high temperature will cause premature aging of the insulation. When the transformer's insulation paperboard ages, the paper strength decreases. Therefore, the impact of external faults may cause insulation damage and then failure.

(6) Moisture: such as flooding, pipeline leakage, top cover leakage, moisture intrusion into the oil tank along the bushing or accessories, and moisture in the insulating oil.

(7) Failure to perform proper maintenance.

2 Analysis and treatment measures for common faults in transformer operation
(1) Main insulation and turn-to-turn insulation faults of the winding. The main insulation and turn-to-turn insulation of the transformer winding are prone to failure. The main reasons are: due to long-term overload operation, poor heat dissipation conditions, or long service life, the transformer winding insulation ages and cracks, and the dielectric strength is greatly reduced; the transformer is subjected to short-circuit shocks many times, causing the winding to deform under stress, hiding insulation defects, and once encountering voltage fluctuations, the insulation may be broken down; water in the transformer oil greatly reduces the insulation strength and cannot withstand the allowable voltage, causing insulation breakdown; in the high-voltage winding reinforcement section or low-voltage winding part, due to insulation expansion, the oil channel is blocked, affecting heat dissipation, causing the winding insulation to age due to overheating, and a breakdown short circuit occurs; due to imperfect lightning protection facilities, insulation breakdown occurs under the action of atmospheric overvoltage.

(2) Transformer bushing failure. Mainly bushing flashover and explosion. The high-voltage side of the transformer generally uses capacitor bushings. Due to poor bushing porcelain or sand holes and cracks, the bushing is not tightly sealed and there is oil leakage; too much scale on the bushing may cause flashover and explosion.

(3) Core insulation failure. The transformer core is made of stacked silicon steel sheets with insulating paint films between them. If the silicon steel sheets are not properly fastened, the paint films will be damaged, generating eddy currents and causing local overheating. Similarly, if the insulation of the through screws and pressure irons that clamp the core is damaged, overheating will also occur. In addition, if there are iron filings or welding slags left in the transformer, the core will be grounded at two or more points, which will cause core failure.

(4) Tap changer failure. The transformer tap changer is one of the common transformer failures. Since the switch relies on pressure contact for a long time, the spring pressure will be insufficient, reducing the effective contact area of ​​the switch connection part, and the silver plating of the contact part will wear and fall off, causing the tap changer to heat up and be damaged during operation. The tap changer has poor contact and cannot withstand the impact of the short-circuit current, causing the tap changer to burn out and fail; in transformers with on-load voltage regulation, the oil tank of the tap changer and the transformer oil tank are generally not connected to each other. If the tap changer tank is seriously short of oil, a short circuit fault may occur during the switching of the tap changer, causing the tap changer to burn out.

(5) Gas protection fault. Gas protection is the main protection of the transformer. Light gas acts on the signal and heavy gas acts on the trip. The following is an analysis of the reasons and treatment methods for the gas protection action: First, the light gas protection action sends a signal. The reasons are: there is a minor fault inside the transformer; there is air inside the transformer; there is a secondary circuit fault, etc. The operator should check immediately. If no abnormal phenomenon is found, gas sampling and analysis should be carried out. Second, when the gas protection action trips, there may be a serious fault inside the transformer, causing the oil to decompose into a large amount of gas, or there may be a secondary circuit fault, etc. If the gas protection action trips, the standby transformer should be put into operation first, and then an external inspection should be carried out. Check the oil pillow explosion-proof door, whether the welding seams are cracked, and whether the transformer shell is deformed; finally, check the flammability of the gas.

(6) Treatment of automatic transformer tripping. When the circuit breakers on either side of the transformer trip automatically, first operate the control switch of the tripped circuit breaker to the tripped position, and quickly put the standby transformer into operation, adjust the operation mode and load distribution, and maintain the operating system and equipment in normal condition. Then check the protection action and conduct external inspection. If it is not an internal fault but an external fault (through fault) or a human error, it can be put into operation without internal inspection. If the main protection action is differential, heavy gas, quick-break, etc., all equipment within the protection range should be checked. Before the cause is found, it is forbidden to put the transformer into operation.

(7) Transformer fire is also a dangerous accident. Due to the damage or flashover of the transformer bushing, the oil flows out under the action of the oil pressure of the oil pillow and burns on the transformer top cover; the internal fault of the transformer causes the oil to burn and the outer shell to rupture. Because there are many flammable substances in the transformer, if it is not handled in time, it may cause an explosion or expand the fire. When such an accident occurs, the transformer protection should be activated to disconnect the circuit breaker. If the circuit breaker is not disconnected for some reason, you should manually disconnect the circuit breaker immediately, pull open the isolating switch that may lead to the transformer power supply, and quickly put the standby transformer into service, restore power supply, stop the operation of the cooling equipment, and extinguish the fire. When extinguishing a transformer fire, it is best to use a foam fire extinguisher or a dry powder fire extinguisher. If necessary, sand can be used to extinguish the fire.

3 Inspection of the normal operation of the transformer
In order to understand the operating status of the transformer, the operation of the transformer should be checked regularly so that abnormal conditions can be discovered and handled in time. The inspection items of the transformer are: First, check whether the sound of the transformer is normal. Second, check whether the oil level and oil color of the oil pillow and the oil-filled bushing are normal, and whether there is oil leakage in each part. Third, whether the upper oil temperature of the transformer is normal. The upper oil temperature of the transformer is different due to different cooling methods, but the upper oil temperature should not exceed the specified value. When the operating personnel inspect, in addition to paying attention to the upper oil temperature not exceeding the specified value, they should also compare with previous data based on the current load conditions, ambient temperature and cooling device investment to determine the cause of the temperature increase. Fourth, check whether the transformer bushing is clean, and whether there are any damage, cracks and discharge marks. Fifth, check whether the lead joints are in good contact. Each lead joint should not change color, overheat, redden, etc. The temperature indicator at the contact of the joint should not melt. Tested with an infrared thermometer, the temperature of the contact point shall not exceed 70 ℃. Sixth, check whether the respirator is normal and intact, whether the silicone has discoloration, and if the silicone fails, it should be replaced in time. Seventh, the explosion-proof diaphragm should be intact and not broken. Eighth, the cooler of the transformer should operate normally. Whether the number of coolers put into use is correct, whether the oil pump and fan operate normally, whether there is any abnormal sound, and whether the oil flow indicator indicates in the "flow position". Ninth, check the gas relay. Tenth, check the transformer core grounding wire and the shell grounding wire, the grounding should be good and there should be no broken wire. Eleventh, check whether the voltage tap position is correct. Twelfth, when the weather changes, special inspections should be carried out. In strong winds, check whether the lead wire swings violently, and there should be no debris at the transformer top cover and bushing lead wire; on snowy days, the contacts of each part should not melt immediately or have discharge after the snow falls; on foggy days, check whether there is spark discharge in each part, etc.

4 Daily maintenance of transformers
Daily maintenance of transformers includes: First, check the cleanliness of the bushing and magnetic skirt and clean them in time, keep the magnetic bushing and insulator clean to prevent flashover. Second, when the cooling device is running, check that the butterfly valves of the cooler inlet and outlet oil pipes are in the open position; the radiator air inlet is unobstructed, and the inlet is clean and free of debris; check that the submersible pump is turning correctly, there is no abnormal sound or obvious vibration during operation; the fan is running normally; the branch power automatic switch in the cooler control box is closed well, there is no vibration or abnormal sound; there is no oil leakage in the cooler. Third, ensure that the electrical connection is tight and reliable. Fourth, regularly check the tap changer and check the tightness, burns, scars, rotation flexibility and contact positioning of the contacts. Fifth, the coil, bushing and lightning arrester of the transformer should be inspected every 3 years. Sixth, check the reliability of the lightning arrester grounding every year. The lightning arrester grounding must be reliable, and the lead should be as short as possible. In the dry season, the grounding resistance should be tested and its value should not exceed 5 Ω. Seventh, replace the desiccant and oil bath oil of the respirator. Eighth, test the fire-fighting facilities regularly.

5 Issues that need to be paid attention to during transformer operation
(1) Leakage treatment. Transformer leakage is divided into oil-side leakage and gas-side leakage. The so-called oil leakage is the leakage of oil in the oil tank (or bushing) into the atmosphere, which belongs to oil-side leakage; the leakage of atmosphere into the oil tank or bushing is gas-side leakage. All parts filled with oil may have oil-side leakage. The characteristic of oil-side leakage is the residual oil traces at the leakage point, which pollutes the appearance of the transformer. When the oil pressure at the leakage point is greater than the osmotic pressure of the oil solution (contaminated oil), moisture, gas and other impurities in the atmosphere cannot penetrate into the oil tank; when the oil pressure at the leakage point is less than the osmotic pressure of the oil solution, mutual penetration occurs between molecules, and moisture, gas and other impurities in the atmosphere may penetrate into the oil tank. Gas-side leakage occurs in the parts of the transformer where there is gas, such as the upper end of the copper tube for high-voltage bushing cable, the part above the oil surface of the safety airway, the air cavity left on the top of the oil-carrying transformer, the upper part of the oil chamber of the oil storage cabinet, the negative pressure area of ​​the strong oil circulation, and the upper part of the bushing oil storage cabinet. The characteristic of gas-side leakage is that due to the thermal expansion and contraction of the retained gas, it "breathes" at the leakage point. When the atmosphere is sucked in, the moisture, gas and other impurities in the atmosphere are sucked into the oil tank or the inside of the bushing. Especially in rainy and snowy weather, a lot of moisture may be sucked in at one time, causing the insulation to be damp locally, reducing the insulation's withstand voltage strength and leading to breakdown discharge. Since gas leakage does not leave traces like oil leakage and cannot be detected by the naked eye, the probability of causing accidents is very high and the consequences are particularly serious. The oil-paper insulation of the transformer or bushing is an insulation that is isolated from the atmosphere after being sealed. Since "its electrical strength has nothing to do with atmospheric conditions", it belongs to "internal insulation". When leakage occurs on the gas side or oil side, its electrical strength is related to atmospheric conditions. Therefore, leakage of the transformer or bushing is a kind of damage to the internal insulation. The decrease or even loss of insulation strength caused by leakage is a more serious damage to the internal insulation.

(2) Maintenance of the plastic bag sealed oil pillow. In order to slow down the oxidation of transformer oil, a diaphragm or capsule (also called a plastic bag) is placed on the oil surface of the oil pillow. The upper opening of the capsule is connected to the atmosphere, so that the oil surface of the oil pillow is completely isolated from the atmosphere. The volume of the capsule increases or decreases with the change of oil temperature. When refueling the oil pillow, care should be taken to exhaust the air between the outside of the capsule and the inner wall of the oil pillow as much as possible. Otherwise, it will cause false oil level and gas relay action, so refueling should be fully sealed. When refueling the oil pillow, attention should be paid to the appropriate amount of oil and oil inlet speed to prevent the oil speed from being too fast. When the amount of oil is too much, it may cause the explosion-proof pipe to spray oil, and the pressure release device to send a signal or spray oil.

(3) Operation and maintenance of the oil purifier. There is a flange interface at the upper and lower parts of the transformer box shell, and a metal barrel filled with silica gel or activated alumina is installed between the two flange interfaces. The maintenance work mainly includes: When the transformer is running, check that the upper and lower valves of the oil purifier are in the open position to keep the oil flowing smoothly between them. The silica gel in the oil purifier should be replaced with qualified silica gel after a long period of use. When the oil purifier is put into operation, first open the lower valve to fill the oil purifier with oil, and open the small exhaust valve on the upper part of the oil purifier to discharge the air inside. When the small valve overflows oil, the small valve can be closed, and then the upper valve of the oil purifier can be opened.

6 Conclusion
Whether the transformer can operate normally depends not only on the transformer structure design and manufacturing process, but also has a lot to do with daily operation, maintenance management and other aspects. Transformer failure is extremely harmful to the operation of the power grid system. In order to avoid accidents, daily operation inspection management and effective maintenance measures should be strengthened to ensure the safe and stable operation of the transformer.