Introduction
Transformers play a pivotal role in modern power systems, responsible for the transmission, distribution, and voltage conversion of electrical energy, ensuring the normal operation of various electrical devices. However, with the increase in usage time and the influence of complex working conditions, the performance of transformers gradually deteriorates. When certain specific signals emerge, it implies that replacement may be necessary to avoid major safety accidents and power supply interruptions. A deep understanding of these signals is of great significance for ensuring the stable operation of power systems and equipment safety.
Abnormal Sound Signals
During normal operation, a transformer emits a uniform and stable “buzzing” electromagnetic sound due to the interaction of the iron core and windings in the alternating magnetic field, which can be regarded as the “normal breathing sound” of the transformer. However, when abnormal sounds occur, it demands immediate attention.
A “clanking” metallic impact sound may indicate that some internal components, such as loose bolts and nuts, are colliding with other parts under the action of electromagnetic force. This looseness not only intensifies the vibration of the equipment but may also lead to poor internal wiring contact, causing local overheating or even short – circuit faults. A “cracking” discharge sound is highly likely to suggest internal partial discharge in the transformer. This can be caused by aging insulation, impurities, or bubbles in the oil, resulting in uneven electric field distribution and subsequent discharge.Partial discharge will further damage the insulation materials, reducing the insulation performance of the transformer. In severe cases, it can lead to insulation breakdown and transformer damage. Additionally, a “gurgling” sound similar to boiling water may mean that the internal windings of the transformer are severely overheated, causing the insulating oil to decompose and generate gas. These gases rise in the oil to form bubbles, producing the boiling – like sound. This indicates a relatively serious overheating fault inside the transformer. If not dealt with promptly, it will lead to rapid aging of the insulation materials and may even trigger a fire.
Abnormal Temperature Signals
When a transformer is in operation, heat is generated due to winding resistance losses, iron core hysteresis, and eddy – current losses, causing the temperature of the transformer to rise. Under normal operating conditions, however, its temperature remains within a relatively stable range. Generally, for oil – immersed transformers, the top oil temperature should not exceed 85°C (and may not exceed 95°C for a short time under special circumstances). For dry – type transformers, the allowable maximum temperature of the windings varies according to different insulation classes. For example, for dry – type transformers with F – class insulation, the maximum winding temperature limit is 155°C.
When the temperature of the transformer continues to rise and exceeds the normal operating temperature range, and remains high even after reducing the load, it is a dangerous signal. There are various reasons for abnormal temperature increases. It may be due to faults in the cooling system, such as non – functioning cooling fans, damaged oil pumps, or blocked radiators, preventing the heat generated by the transformer from being dissipated in a timely manner. It could also be caused by internal winding or iron core faults, such as winding short – circuits or multiple – point grounding of the iron core, resulting in increased losses and excessive heat generation. Prolonged abnormal temperature increases will accelerate the aging of insulation materials, reduce insulation performance, significantly shorten the service life of the transformer, and may even trigger serious accidents such as insulation breakdown.
Abnormal Oil Level and Oil Quality Signals
For oil – immersed transformers, insulating oil not only provides insulation but also cools the transformer by circulating to carry away the heat generated during operation. Under normal circumstances, the oil level of the transformer fluctuates within a certain range with changes in oil temperature, and the insulating oil should be clear, transparent, and light yellow.
A sudden drop in oil level may indicate an oil leakage problem in the transformer. Leakage points may occur at tank welds, valves, bushing roots, and other locations. An excessively low oil level will expose the internal windings and iron core of the transformer to the air, depriving them of the insulation and cooling effects of the insulating oil, increasing the risk of insulation and overheating faults. Meanwhile, when the color of the insulating oil becomes turbid, black, or shows the presence of sludge and precipitates, it indicates that the insulating oil has deteriorated. The main reasons for the deterioration of insulating oil include long – term high – temperature operation, moisture ingress, oxidation, and arcs generated by internal faults. Deteriorated insulating oil will significantly reduce its insulation and heat – dissipation performance, failing to effectively protect the transformer. At this point, the transformer is in an unsafe operating state, and replacement should be considered.
Abnormal Electrical Performance Signals
Regular electrical tests on transformers can promptly detect potential internal faults and performance changes. When there is a significant deviation between the DC resistance test results of the transformer and the factory values or previous test values (generally, the three – phase unbalance rate should not exceed 2%), it may indicate problems such as inter – turn short – circuits in the internal windings or poor lead contact. Inter – turn short – circuits will reduce the effective number of turns of the windings, change the turns ratio of the transformer, and increase winding losses, causing local overheating.
In the insulation resistance test, a significant decrease in the insulation resistance value indicates a deterioration in the insulation performance of the transformer. This may be due to insulation moisture ingress, aging, or the presence of through – defects inside. In addition, a significant increase in the no – load loss and load loss of the transformer also reflects internal abnormalities. An increase in no – load loss may be due to iron core defects, such as damaged insulation between silicon steel sheets or loose lamination of the iron core. An increase in load loss may be related to factors such as increased winding resistance and poor contact between the windings and tap changers. These abnormal changes in electrical performance all indicate problems with the health status of the transformer, and further evaluation of whether replacement is necessary is required.
Protection Device Operation Signals
Transformers are usually equipped with various protection devices, such as gas protection, differential protection, over – current protection, etc., which act as the “guardians” of transformer safe operation. When the gas protection issues a light gas signal, it indicates that there may be a minor fault inside the transformer, generating a small amount of gas, causing the oil level in the gas relay to drop and the float to act and send out a signal. If the light gas signal occurs frequently, timely analysis and handling are required to determine the severity of the fault.
When the gas protection issues a heavy gas signal, or other protection devices such as differential protection and over – current protection trip, it indicates that a relatively serious fault has occurred inside the transformer, such as winding short – circuits or iron core faults. At this time, the transformer can no longer operate normally and must be stopped immediately for a comprehensive inspection and evaluation. In many cases, after inspection and analysis following the tripping of the protection device, if it is confirmed that the internal faults of the transformer are severe and cannot be restored to normal operation through maintenance, transformer replacement should be considered.
Abnormal Vibration Signals
During normal operation, the vibration of a transformer is relatively stable and regular. However, abnormal vibrations may imply internal problems. A sudden increase in vibration amplitude and a change in vibration frequency may be due to loose silicon steel sheets in the iron core, resulting in greater vibration under the action of electromagnetic force. It could also be caused by deformation or displacement of the windings, leading to an imbalance in the electromagnetic force between the windings and the iron core, thereby causing abnormal vibrations. Long – term abnormal vibrations will loosen the internal connecting components of the transformer, accelerate insulation aging, and may even trigger winding short – circuit faults, threatening the safe operation of the transformer. When such abnormal vibration conditions are difficult to repair, transformer replacement should be considered.
Odor Change Signals
Under normal conditions, transformers have almost no special odor. Once a pungent burnt smell is detected, it is highly likely that there is an overheating phenomenon inside the transformer, causing the decomposition of insulation materials and insulating oil, and generating peculiar smells. If the burnt smell is strong, it may mean that there has been severe overheating or even local combustion inside, which will cause irreversible damage to the insulation structure of the transformer and seriously affect its safety performance. When such a significant odor change occurs and the fault is difficult to eliminate, the transformer should be replaced in a timely manner to eliminate potential hazards.
Abnormal Oil Chromatographic Analysis Signals
Oil chromatographic analysis is an important means of monitoring internal faults of oil – immersed transformers. By analyzing the composition and content of dissolved gases in the insulating oil, it is possible to determine whether there are faults inside the transformer. For a normally operating transformer, the content of dissolved gases in the insulating oil is low and the composition is stable. When abnormal increases in the content of gases such as hydrogen (H₂), methane (CH₄), ethane (C₂H₆), ethylene (C₂H₄), and acetylene (C₂H₂) are detected, it requires high attention. For example, a significant increase in acetylene content often indicates the presence of arc discharge faults inside the transformer, while an increase in the content of hydrogen and hydrocarbon gases may suggest overheating faults. When the results of oil chromatographic analysis are continuously abnormal and it is determined that the internal faults are difficult to repair, replacing the transformer is a necessary measure to ensure the safety of the power system.
Abnormal Voltage and Current Fluctuation Signals
Under normal operating conditions, the input and output voltage and current of a transformer should fluctuate within a reasonable range. If the voltage is unstable, fluctuates significantly, or deviates from the rated value, and after excluding system – side causes, it may be due to problems with the internal windings of the transformer, such as inter – turn short – circuits or tap – changer faults, resulting in a change in the turns ratio and affecting voltage output. Abnormal current fluctuations, such as sudden increases or irregular fluctuations, may be caused by local short – circuits inside the transformer, load – side faults, or a decline in the performance of the transformer itself. When abnormal voltage and current fluctuations occur frequently and cannot be resolved through conventional maintenance, it means that the performance of the transformer can no longer meet the normal operating requirements, and replacement should be considered.
Abnormal Infrared Thermal Imaging Signals of Local Overheating
Infrared thermal imaging technology can be used to visually detect the temperature distribution on the surface of the transformer. For a normally operating transformer, the surface temperature distribution is relatively uniform with a small temperature difference. If a local area on the infrared thermal image has a significantly higher temperature than other parts, forming a “hot spot,” it indicates an overheating problem in this area. Hot spots may be caused by internal poor contact, local winding short – circuits, local overheating of the iron core, and other reasons. If the local overheating problems detected by infrared thermal imaging cannot be simply resolved and the overheating trend intensifies, it may cause serious damage to the insulation and other components of the transformer. In such cases, replacing the transformer can effectively prevent the further expansion of the fault.
Summary
The signals indicating the need for transformer replacement are often not single but rather a combination of multiple abnormal situations that are interrelated and interact with each other. In daily operation and maintenance, power system workers should pay close attention to the sound, temperature, oil level, electrical performance, protection device operation, vibration, odor, oil chromatographic analysis, voltage and current fluctuations, and infrared thermal imaging of transformers, promptly detect abnormal signals, and accurately determine the health status of the transformer through professional analysis and diagnosis. Once it is confirmed that the transformer has serious faults and cannot meet the requirements for safe and stable operation, decisive replacement measures should be taken to ensure the safe and reliable operation of the power system.
