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While conducting equipment inspections at a substation, the maintenance staff heard abnormal noises coming from the transformer; there was a slight discharge sound, and the intensity of these noises varied from time to time. Infrared testing revealed no abnormalities, so maintenance technicians were promptly sent to the site to investigate and determine the cause. Preliminarily, it is determined that the discharge sound originates from the installation point of the end screen of the C-phase bushing on the high-voltage side of the main transformer. An immediate report was made to the duty dispatcher, and the main transformer was taken out of service for maintenance so that further inspections and repairs could be carried out. Upon inspection, it was found that the high-voltage bushing of the main transformer was manufactured by a factory in Xi’an. The tightening screws at the ends of the grounding end shield and the grounding cover showed no signs of loosening. However, upon removing the grounding cover for further inspection, it was discovered that the tightening screws on the sealing flange were loose. There were no lockwashers on these screws; they became loose due to vibrations, resulting in an increase in the grounding resistance. This led to a relatively high floating potential, which in turn caused discharge. Lockwashers were then added to these flange screws and they were retightened. During the inspection of the end screens of the A- and B-phase bushings as well as the neutral point bushing, looseness at varying degrees was found. All issues were rectified, and oil samples taken from the bushings tested satisfactory results. After all work is completed, restore the main transformer to operation, and then check that there are no abnormal noises. 1. Various situations of abnormal noises during transformer operation. During operation, alternating magnetic fluxes exist within the transformer core, generating forces between the silicon steel laminations of the core, which results in a \"humming\" sound. “The magnitude of the “hum” is proportional to the voltage and current applied to the transformer. Under normal operation, the sound of the transformer core should be even. If the sound of the transformer becomes louder, sharper, or irregular, it indicates an abnormal condition. When a transformer operates under abnormal conditions, various unusual sounds may occur. These include a significantly louder but still uniform sound than usual; “clinking” or “hissing” noises during operation; crackling discharge sounds; or a bubbling sound similar to water boiling. During routine inspections, it is necessary to promptly judge any potential abnormalities based on sounds, determine whether they affect the safe operation of the transformer, and if required, take it out of service for maintenance. 2. Analysis of the causes of abnormal sounds during transformer operation: For different types of abnormal sounds in transformers, it is necessary to analyze the causes based on the actual site conditions and take appropriate measures. 2.1 When the noise emitted by the transformer increases compared to normal levels yet remains uniform, there may be two possible situations regarding the transformer. Overvoltage occurred in the power grid. When a single-phase ground fault occurs in the power grid or electromagnetic resonance overvoltage is generated, it will cause the transformer to make more noise. Transformer is overloaded. When a transformer is under heavy load, operating at full load, or running under overload conditions, the vibration of the silicon steel sheets in its core increases, resulting in a louder noise. In both of the above cases, a comprehensive judgment can be made by considering the readings of the voltage meters at the substation. 2.2 Occurrence of “clinking or rustling” noises during transformer operation When there are “clinking” sounds inside the transformer, it may be caused by loose clamping bolts on the core or loose individual components within the transformer. At this time, all indicator gauges are normal; the oil color, oil level, and oil temperature are also normal. Sounds caused by the collision of the transformer enclosure with other objects. This is because vibrations of the iron core inside the transformer cause other components to vibrate as well, resulting in collisions between contact points; this may produce a “rustling” friction sound or a regular knocking noise. For example, the hose carrying control wires on the transformer may collide with the casing or heat sinks, or there may be mechanical friction sounds resulting from wear in the bearings of the cooling fan; at such times, no abnormalities are observed in other parts of the transformer. The cause can be determined by locating the sound source, pressing it with the hand or a stick on the side where the sound is loudest, and then listening to see how the sound changes. 2.3 Occurrence of a \"cracking\" discharge sound during transformer operation: When a \"cracking\" discharge sound is heard during transformer operation, there may be internal or external faults in the transformer. When there are squeaking or crackling sounds inside a transformer, it may be due to an internal discharge fault or poor connections. Such as poor grounding of the core, poor contact of the tap changer, insufficient distance between the leads and the tank shell resulting in discharge, etc. When there are “squeaking” or “popping” sounds coming from outside the transformer, it may be due to severe dirt on the surface of the porcelain bushings, or to electrical corona discharge caused by weather conditions such as heavy fog or rain. At night, blue-colored corona or sparks can be observed. It is also possible that the tightening screws at the grounding end of the porcelain sleeve’s final screen, or the screws of the grounding cover, are loose; such looseness leads to an increase in the grounding resistance of the final screen, resulting in a higher floating potential and subsequent discharge. By locating the sound source, inspections and maintenance can be carried out after the device loses power, thereby determining the cause of the abnormality. 2.4 Presence of a boiling \"gurgling\" sound during transformer operation: When a gurgling sound similar to that of boiling water is heard inside the transformer, it may indicate a serious fault in the internal windings, such as an inter-turn short circuit or poor contact in the tap changer. Due to severe local heating at the fault site, the oil boils locally, producing a \"gurgling\" sound. This kind of sound requires special attention. 2.5 When the system fails, abnormal noises can also be heard from the operating transformer. When there is a phase break or single-phase ground fault in the system or inside the transformer, a high-pitched \"buzzing\" sound can be heard from within the transformer. If a short-circuit fault occurs in the system, the transformer experiences large amounts of non-periodic current, causing the core to become severely saturated. The magnetic flux then becomes distorted into a non-sinusoidal waveform, which results in the entire transformer casing vibrating due to strong electromagnetic forces. At this time, very loud \"buzzing\" sounds along with other vibrating noises are produced both inside and outside the transformer. When a fault occurs in the system, both the sound and the readings on the transformer indicator instruments (ammeter, voltmeter) change simultaneously, making it easy to identify the issue. 3. Methods for dealing with abnormal noises from transformers: During the operation of a transformer, when, under the same load conditions, the noise level is higher than normal, the electromagnetic vibration sounds are uneven, or any abnormalities are detected, it is necessary to increase monitoring. At the same time, other possible abnormalities should be checked, the causes of these abnormal noises should be analyzed, and the situation should be reported immediately to the dispatch team and relevant supervisors so that appropriate measures can be taken. If the abnormal noise from the transformer is caused by overload, it should be handled in accordance with the principles for dealing with overloads; the load can be reduced by adjusting the operating mode. During operation, the transformer only produces noise; there are no other abnormal phenomena. If it does not affect the operation of the transformer, no action need to be taken for now; record the situation, increase monitoring, inform the dispatching staff and relevant supervisors, and wait to inspect and address it during a power outage. If there is an abnormal increase in noise inside or outside the transformer, or if there are sounds of cracking, it indicates that the insulation inside or on the surface of the transformer has been damaged; the transformer should be shut down immediately for inspection and repair. When there is a gurgling boiling sound inside the transformer, along with sudden changes in temperature and an increase in oil level, it should be considered that there is a serious fault in the transformer’s windings, and the transformer should be shut down immediately for inspection and testing. When the system fails, the transformer generates a large amount of non-periodic components, resulting in abnormal noises from the transformer. Normal operation was restored after fault isolation. 4. Matters requiring attention in transformer maintenance and operation. Before putting a transformer back into service after maintenance, it is necessary to check whether the grounding of the high-voltage bushing’s terminal screen is proper; any abnormalities should be addressed promptly. When the terminal screen is in a grounded state, use a multimeter to measure whether the resistance value between the grounding pad and the transformer casing (ground) meets the requirements. When multiple ground faults are found in the transformer core during transformer maintenance, they should be addressed promptly. The transformer core must have, and can only have, one point of grounding; if there are two or more points of grounding, it is considered multi-point grounding. Operating a transformer core with multiple grounding points will cause faults within the core, posing a threat to the safe operation of the transformer. If heating is detected at the joint connections during transformer operation, the cause of the heating should be analyzed and appropriate actions taken promptly. Since the current-carrying connections of the lead wires are important components of the transformer itself, poor connections can cause overheating or even wire burnout, severely affecting the normal operation of the transformer. Any overheating in these connections must be addressed promptly. During transformer maintenance, the on-load switch requires a transition resistance test, during which it is necessary to measure the resistance value between the arc contact and the main contact. Also, pay attention to whether there are any bypass circuits between the arc contacts and the main contacts, in order to avoid measurement errors caused by such bypass circuits. During the transformer maintenance, it was found that the DC resistance value of the on-load switch was out of spec, which may be caused by the presence of an oil film or an oxide film. Before the test, 10 cycles should be operated first to address the oxide film issue on the load switch; if it is a oil film problem, the core of the switch must be removed and the main contacts as well as the moving contacts polished. When the DC resistance is not within the specified range, human factors and instrument-related issues must be ruled out. Strengthen the routine inspections of transformer operation; pay attention to any slight abnormal noises detected in the transformers. Accurately determine the possible location of the defects and address them promptly, so as to prevent accidents caused by failure to detect and fix these defects in time, and ensure the safe operation of the transformer equipment. Strengthen the inspection of transformer operation; when a light gas alarm is triggered in the transformer itself, the gas should be collected promptly, its composition analyzed, and gas release procedures carried out immediately. The cause of gas generation should also be analyzed and identified to prevent accidents resulting from untreated internal faults in the transformer, thus ensuring the safe operation of the transformer equipment.