Analysis of Burnout Causes and Countermeasures for Dry-Type Transformers in a Substation

In the power system, dry-type transformers in substations are key equipment to ensure the stability of electricity supply for production and daily life. The dry-type transformers in the substations along the Shuohuang Railway are mainly responsible for providing electricity for on-site daily power supply, production lighting, equipment heating, and temperature-humidity monitoring. However, two consecutive burnout incidents of distribution dry-type transformers occurred in this substation within one year, which not only affected the normal power supply order but also exposed many problems in equipment operation and maintenance. Based on the content of the document, this article will comprehensively analyze this technical issue from three aspects: the core causes of dry-type transformer heating, the details of accident investigation, and targeted countermeasures, providing references for the safe operation of similar equipment.

1. Core Causes of Heating in Dry-Type Transformers

The heating problem of dry-type transformers is not caused by a single factor, but by the combined effects of abnormalities in multiple links such as insulation, iron core, winding, heat dissipation system, and load balance. These factors interact with each other, and if not handled in a timely manner, they can easily lead to equipment burnout accidents.

(1) Insulation Resistance Decline: Failure of the "First Line of Defense" for Equipment Safety

Insulation resistance is a key indicator to prevent current leakage and ensure the insulation performance of transformers. Its decline will directly lead to internal discharge and short circuit, which is an important cause of heating. Specifically, it can be divided into two situations: "moisture absorption" and "environmental pollution".

1. Insulation Deterioration Caused by Moisture Absorption: At present, all substations along the Shuohuang Railway have fully adopted resin-cast dry-type transformers, and the windings are sealed with resin to improve insulation performance. However, during actual operation, if the resin on the winding surface is damaged due to aging, vibration, or other factors, or if some insulating materials are in a humid environment for a long time, moisture will penetrate into the equipment. Corrosive substances in water cause the enameled wires to mildew and deteriorate, and the insulating paint to peel off, which in turn leads to discharge between turns (adjacent coils within the same winding) and between layers (different layers of the winding). This discharge initially manifests as local heating, and as the insulation is further damaged, abnormal odors and smoke will gradually appear. Therefore, when an abnormal odor is detected from the dry-type transformer, it is necessary to immediately cut off the power supply for inspection to prevent the expansion of the short-circuit range .
2. Insulation Decline Caused by Environmental Pollution: The substation is located in a special geographical position, adjacent to coal mines and highways, resulting in severe daily coal dust pollution; in spring and summer, a large number of catkins and poplar catkins fall, which easily enter and accumulate inside the equipment compartments. However, the existing equipment cleaning cycle is much longer than the pollutant accumulation cycle. These impurities will adhere to the surfaces of windings and insulation layers, reducing the insulation strength of the equipment and causing "surface discharge" (a discharge phenomenon formed by current leakage along the insulation surface). Over time, the insulation performance deteriorates continuously, laying hidden dangers for heating and burnout .

(2) Iron Core Heating: The "Invisible Killer" of Magnetic Circuit Abnormalities

The iron core is the core component for magnetic energy conversion in dry-type transformers, made by stacking silicon steel sheets. Its performance and installation quality directly affect the heating state of the equipment, with main problems focusing on "quality defects" and "multi-point grounding".

1. Iron Core Quality and Internal Insulation Defects: If the silicon steel sheets themselves are of substandard quality and have insufficient magnetic density, they will easily enter a magnetic saturation state during operation, leading to increased iron core loss, temperature rise, and directly affecting the load capacity of the transformer; in addition, if the construction process is improper during the gluing and assembly of silicon steel sheets (such as uneven glue application, excessive stacking gaps), local gaps will be formed inside the iron core, causing "partial discharge" — the heat generated by this discharge will continuously heat the iron core, forming a vicious cycle of "heating - insulation deterioration - more severe heating" .
2. Iron Core Multi-Point Grounding Fault: Under normal circumstances, the transformer iron core should be grounded at only one point (to avoid forming a circulating current). If multi-point grounding occurs, a closed loop will be formed inside the iron core, generating a "circulating current". This circulating current is equivalent to a small-range short circuit, and since the iron core has resistance, heat will be continuously generated when the circulating current flows through it. The causes of multi-point grounding are diverse: first, the insulation distance between the iron core and the metal shell is insufficient; second, the wrong installation direction of the fixing bolts (such as the bolt head being close to the iron core) leads to insufficient insulation distance; third, the insulation layer of the iron core is damp or damaged; fourth, foreign objects (such as metal debris, wires) are lapped on different parts of the iron core. If the multi-point grounding problem is not handled for a long time, the abnormal heating of the iron core will damage the internal insulation, and eventually may lead to the burnout of the iron core, or even cause an overall accident of the transformer .

(3) Winding Heating: The "Weak Point" in the Current Transmission Link

The winding is the core of the transformer for voltage transformation, made by winding copper wires. Its heating is mainly caused by omissions in the design, manufacturing, maintenance, and monitoring links.

1. Manufacturing Link: Unreasonable Selection of Copper Wire Cross-Sectional Area: If the cross-sectional area of the copper wire is not selected according to the rated load of the transformer and the demand for inrush current (such as the instantaneous large current during motor startup and equipment closing) in actual operation, after commissioning, when encountering inrush current or impact load, the current density of the copper wire will be too large, and the heat generation will far exceed the design expectation, leading to a sharp rise in the winding temperature .
2. Mismatch Between Design and Environment: If the transformer is not fully designed to consider the space constraints of the operating environment — such as installing a high-load transformer in a narrow and closed substation room — the heat dissipation space will be insufficient; in addition, if the load of the equipment is at a high level for a long time during operation, the heat generated by the winding cannot be dissipated in a timely manner, forming a "heat accumulation effect" .
3. Maintenance Link: Human Operation Errors: During the annual periodic maintenance and testing, if the quality of maintenance personnel is insufficient, foreign objects such as short-circuit wires, fuses, and metal tools may be left inside the equipment. These foreign objects will cause short circuits in the windings, and the current at the short-circuit part will increase sharply, generating a large amount of heat instantaneously and causing local burnout of the windings .
4. Failure of Monitoring Components: If the temperature monitoring components (such as platinum resistors, thermocouples) installed outside the windings have poor contact, a "short-circuit loop" may be formed. This abnormal loop will consume additional electrical energy and convert it into heat, indirectly causing the winding temperature to rise .

(4) Poor Ventilation and Heat Dissipation System: "Blocked Channel" for Heat Discharge

Dry-type transformers do not have an oil cooling system and rely entirely on air circulation and environmental cooling for heat dissipation. Their heat dissipation capacity directly determines the upper temperature limit of the equipment.

• Defects in Heat Dissipation Structure: Dry-type transformers are exposed to the air and mainly rely on "natural convection + industrial air conditioning in the substation room" to adjust the temperature. However, the existing power distribution cabinets do not have special cabinet heat dissipation devices, only equipped with closed observation windows — this structure cannot achieve effective heat exchange. The copper loss (loss of current flowing through the windings) and iron loss (hysteresis and eddy current loss of the iron core) generated during equipment operation will be continuously converted into heat. If the heat cannot be discharged in a timely manner, it will accumulate inside the cabinet .
• Lack of Active Heat Dissipation: At present, the power distribution cabinets along the line are only equipped with automatic heating systems (used for anti-condensation in low-temperature environments) and no active heat dissipation devices (such as exhaust fans, cooling fans). When the transformer load increases and the ambient temperature rises (such as in summer), the heat dissipation capacity is far lower than the heat generation, and the winding temperature will continue to rise, damaging the insulation performance and accelerating the aging of insulating parts .

(5) Three-Phase Load Imbalance: "Imbalance Hidden Danger" in System Operation

Transformers are designed to operate in three-phase balance. If the three-phase load is unevenly distributed, a series of chain reactions will be triggered, intensifying equipment heating.

1. Increased Transformer Loss: Transformer losses are divided into iron loss (constant, independent of load) and copper loss (proportional to the square of the load current). When the three-phase load is unbalanced, the current of one (or two) phases will far exceed the rated value, leading to a sharp increase in the copper loss of the winding of that phase, a rise in the winding temperature, accelerated insulation aging, and shortened service life of the transformer; at the same time, the unbalanced load will generate "zero-sequence magnetic flux", which cannot be closed through the iron core and can only flow through components such as the transformer's metal shell and oil tank — due to the poor magnetic conductivity of steel, the zero-sequence magnetic flux will generate hysteresis loss and eddy current loss in these components, causing the metal components to heat up and further intensifying the overall temperature rise of the transformer .
2. Increased Line Loss: According to specifications, the three-phase load unbalance degree of the transformer should not exceed 15%, and the neutral line current of a three-phase transformer with only a small amount of single-phase load should not exceed 25% of the rated current. If the three-phase load is unbalanced, the zero-sequence magnetic flux will be converted into "zero-sequence current" in the neutral line, resulting in additional loss of the neutral line (the neutral line has no current during normal balanced operation). For the three-phase four-wire power supply system used in the substation, the line power loss is the smallest under balanced conditions; however, in the unbalanced state, the "additional loss" generated by the unbalanced current will cause the line to heat up, which not only increases electrical energy waste but also impacts the line insulation .
3. Shortened Load Service Life: The unbalanced three-phase load will lead to unequal output currents of each phase, which in turn causes unequal voltage drops of each phase, triggering "three-phase voltage imbalance". The phase with excessively high voltage will cause the user equipment (such as lighting and heating devices) connected to that phase to burn out due to overvoltage; the phase with excessively low voltage will make the equipment unable to start or operate normally (such as reduced motor speed and sharply decreased heating efficiency). At the same time, the continuous impact of the high-load phase will accelerate the aging of equipment components, increase maintenance costs, and seriously affect the safety and quality of power consumption .

2. Accident Investigation and Analysis: Seeing the Superposition Effect of Problems from Actual Cases

To accurately identify the causes of burnout, the article conducts a detailed investigation into the two dry-type transformer burnout accidents, and clarifies the process of the accident caused by the superposition of multiple factors through "sorting out related events + verifying test data + logical reasoning".

(1) Accident Overview: Repeated Failures of Similar Equipment

The interval between the two accidents was 9 months, and the burned transformers were of the same model — all SCB10-80/10 epoxy-cast dry-type transformers with a rated capacity of 80 kV·A. Among them, Transformer No. 1 (1# SB) suffered winding burnout due to a single-phase grounding fault, while Transformer No. 2 (2# SB) suffered winding burnout due to excessive operating temperature. Although the fault manifestations were different, there were commonalities in the root causes .

(2) Related Events: Dual Mistakes of Human and Environmental Factors

1. Water Inflow into Cable Trenches and Inadequate Drying: The substation was a construction and renovation site. During the excavation of the outdoor site, due to inadequate on-site management and control, no rainproof and seepage-proof measures were taken for the cable trenches. During the thunderstorm season, rainwater directly poured into the cable trenches. Although drainage treatment was carried out afterwards, the cable trenches and surrounding equipment were not effectively dried — the moisture continued to evaporate, leading to an increase in the air humidity in the substation room and dampening of the equipment insulation layer, laying hidden dangers for subsequent insulation decline .
2. Severe Imbalance in Load Connection: Due to the long distance between the substation and the station area, the original heating equipment could not meet the daily heating needs of employees, so the logistics department added graphene wall-mounted heating devices on its own. However, before construction, no communication was conducted with the professional person in charge of the production department, and the three-phase load balance was not considered, so all the load of the heating devices was connected to Phase B. Actual measurements showed that:

• When the wall-mounted heating devices were disconnected, the three-phase currents were 7 A (Phase A), 6 A (Phase B), and 9 A (Phase C), respectively, and the three-phase load unbalance degree had reached 23% (exceeding the specification limit of 15%), at which time the operating temperature of the transformer iron core was 37℃ (within the normal range);
• When the wall-mounted heating devices were connected, the three-phase currents changed to 7 A (Phase A), 37 A (Phase B), and 9.2 A (Phase C), respectively, the unbalance degree soared to 117%, and the iron core temperature surged to 67℃ — far exceeding the normal operating temperature, leading to a sharp increase in the heating of windings and iron core .

(3) Test Data: Verification of Performance Deterioration After Failure

Through insulation resistance, DC resistance, and winding transformation ratio tests on the transformers before and after burnout, the differences in data intuitively reflect the degree of equipment damage.

1. Test Data of 1# SB (Single-Phase Grounding Fault):

• Insulation Resistance: Before burnout, the insulation resistance between high voltage and low voltage (and ground) was 2500 GΩ, between low voltage and high voltage (and ground) was 365 GΩ, and the overall insulation resistance to ground was 389 GΩ (all meeting the specification requirements); after burnout, the corresponding values dropped sharply to 40 MΩ, 270 MΩ, and 70 MΩ, and the insulation performance almost completely failed .
• DC Resistance: Before burnout, the resistance of Phase BC was 18.38 Ω, with a deviation of only 0.326%; after burnout, it dropped to 16.9 Ω, and the deviation expanded to 16.08%, indicating that there was local burnout or short circuit in the winding, resulting in abnormal changes in resistance value .
• Winding Transformation Ratio: Before burnout, the transformation ratio of AB/ab was 25.01, with a deviation of -0.01% (close to the design value); after burnout, it dropped to 24.79, and the deviation expanded to 7.19%. The deviations of the transformation ratios of BC/bc and CA/ca also reached -1.75% and -5.36%, respectively, indicating that the number of winding turns or the insulation structure had been damaged, and normal voltage transformation could not be achieved .

1. Test Data of 2# SB (Excessive Temperature Fault): There was no significant change in DC resistance and winding transformation ratio (indicating no obvious short circuit or turn damage in the winding), but due to severe burnout, the insulation resistance could only be measured for a single phase to ground, and the insulation resistance of each phase was only a few thousand ohms, far below the specification requirements, which was completely unqualified insulation .

(4) Accident Inference: The "Fatal Combination" of Multi-Factor Superposition

Combining related events and test data, three core inferences can be drawn:

1. Environmental Moisture + Insufficient Heat Dissipation: "Dual Blow" to Insulation: Water inflow into the cable trenches caused the equipment to be damp, and the insulation resistance decreased; at the same time, the ventilation and heat dissipation capacity of the substation room was insufficient, and heat could not be discharged. When operating under high load, the winding heating intensified, further damaging the insulation — the superposition of the two accelerated the deterioration of the equipment insulation performance, becoming the "basic inducement" of the accident .
2. Severe Three-Phase Imbalance: The "Direct Driver" of Heating: The unreasonable connection of wall-mounted heating devices led to a three-phase load unbalance degree of up to 117%, the zero-sequence current increased sharply, and the neutral line and windings heated severely. Although the transformer adopted Class F insulating material (maximum operating temperature of 155℃, maximum temperature rise limit of 100℃), under normal circumstances, the maximum operating temperature was not high enough to burn out the equipment. However, the insulation performance decreased due to moisture, and the heat resistance was greatly reduced. Finally, burnout occurred under the superposition of "high temperature + low insulation" .
3. Hidden Equipment Quality Hazards: "Potential Risk" in Long-Term Operation: The transformer had only been in operation for 3 years, and the daily load was relatively small, but burnout still occurred. It cannot be ruled out that there were defects in the manufacturing link — such as the internal insulation layer not being fully dried before subsequent processing, and there was slight discharge inside the insulation layer during operation. After long-term accumulation, the discharge range expanded, and finally, it interacted with environmental and load factors to cause the accident .

3. Countermeasures: Building a Protection System from Multiple Dimensions of Management, Technology, and Monitoring

In view of the core causes of dry-type transformer heating and accident lessons, the article proposes a set of systematic countermeasures, covering five dimensions: operation management, operation specifications, temperature control, technological empowerment, and load balance, aiming to prevent faults from the source and improve the safety level of equipment.

(1) Formulating a Sound Operation Management System: Consolidating the Safety Foundation

1. Daily Monitoring and Environmental Control: Incorporate transformer load monitoring into daily inspections, and use an infrared thermometer to detect the temperature of the operating transformer once a day, focusing on monitoring the temperature of windings, iron cores, and outlet terminals to detect abnormal heating points in a timely manner; at the same time, strengthen the waterproof and moisture-proof management of the substation room, strictly prevent rainwater from pouring in (such as adding rainproof covers to cable trenches and setting drainage slopes), and if the equipment is damp, immediately use drying equipment (such as hot air drying, vacuum drying) for treatment; regularly open the fire door of the substation room for ventilation and heat exchange every week to ensure that the indoor humidity is controlled within a reasonable range (generally not exceeding 70%) .
2. Power Supply Rotation Mechanism: Design the two-way AC power supply as a hot standby state, and implement the "alternate operation in odd and even months" system — switch the power supply on the 1st day of each month, use Power Supply No. 1 in odd months, and use Power Supply No. 2 in even months. This rotation can not only avoid fatigue loss caused by long-term operation of a single power supply but also quickly switch when one power supply fails, ensuring the continuity of power supply .

(2) Strengthening Standardized Operation Management: Eliminating Human Errors

1. Maintenance Quality Control: Strictly implement the standardized process for maintenance operations, and focus on doing a good job in "post-maintenance re-inspection" — form a multi-level review team composed of team leaders, work leaders, team management personnel, and substation attendants to conduct a comprehensive inspection of the equipment after maintenance, ensuring that no foreign objects such as short-circuit wires, metal tools, and fuses are left, and avoiding winding short circuits caused by foreign objects; at the same time, standard parts (such as fixing bolts) must be used during maintenance, and the length of bolts must be strictly controlled to prevent insufficient insulation distance between the iron core and the shell due to excessively long bolts, causing multi-point grounding faults .
2. Insulation Fault Location: When insufficient insulation resistance is found during the test, do not disassemble the equipment blindly, but carry out insulation testing step by step — measure the insulation resistance to ground of individual windings, clamps, through bolts, insulation layers, and other components in sequence, and accurately locate the part with insufficient insulation through data comparison, avoiding equipment damage caused by large-scale disassembly and improving the efficiency of fault handling .

(3) Strengthening Winding Temperature Control: Blocking the Vicious Cycle of Heating

1. Temperature Monitoring and Emergency Handling: Regularly record the winding operating temperature using an infrared thermometer. If an abnormal temperature rise is found (such as exceeding the temperature rise limit of 100℃ for Class F insulation), immediately count the current operating load to determine whether the heating is caused by excessive load — if so, adjust the power load in a timely manner (such as transferring single-phase load, reducing the electrical equipment of the high-load phase); if the load is normal, immediately switch the AC power supply system and check whether there is a power supply side fault (such as abnormal voltage, excessive harmonics) .
2. Environmental Cooling and Dust Cleaning: Install a temperature-humidity meter in the substation room, monitor the indoor temperature in real time during high-temperature periods in summer, put the industrial air conditioning in the substation room into use in a timely manner, and regularly open the substation room door for ventilation (to avoid closed heat accumulation); during each maintenance, thoroughly clean the dust on the transformer windings, iron cores, and insulation layers (using compressed air blowing, soft brush cleaning, etc.) to eliminate the obstruction of dust to heat dissipation and improve heat dissipation efficiency .

(4) Using Technological Means: Improving Monitoring and Protection Capabilities

1. Installing Arc Protection Devices: Install arc protection devices inside the transformer cabinet. These devices have the function of automatic fault location — when an arc short circuit (such as winding discharge, foreign object short circuit) occurs inside the equipment, they can quickly locate the fault point (with an accuracy of up to the centimeter level), and automatically conduct self-inspection on the system, issue an audible and visual alarm signal, and notify the attendant to exit the faulty system in a timely manner to prevent the expansion of the fault range .
2. Adding High-Precision Temperature Monitoring Units: The existing temperature-humidity meters are only installed on the upper cabinet of the power distribution cabinet and cannot monitor the temperature of the core parts of the transformer (such as the inside of the winding). Add temperature monitoring units to the rear cabinet, using a combined scheme of "sensing probe + fluorescent optical fiber + temperature measuring device" — the diameter of the sensing probe is less than 3 mm, which can penetrate into the winding gaps, with a measurement range covering -20~150℃ and an accuracy of ±1℃, and has anti-electromagnetic interference capability (adapting to the complex electromagnetic environment of the substation room); the monitoring data is connected to the substation integrated automation system to realize real-time temperature collection and abnormal alarm (such as automatically triggering SMS notifications when the temperature exceeds the limit), achieving "early detection and early handling" of temperature abnormalities .
3. Transforming the Rear Cabinet Observation Window: Balancing Observation and Heat Dissipation: Design and manufacture a "visual rear cabinet exhaust device", using industrial exhaust fans as the core of heat dissipation, taking power from the AC power supply of the front cabinet, and linking with the existing temperature-humidity controller — when the indoor temperature is lower than the set value (such as 5℃), the controller starts the heater to prevent condensation; when the temperature is higher than the set value (such as 35℃), it automatically turns on the exhaust circuit, and the exhaust fan starts to dissipate heat; at the same time, the visualization function of the observation window is retained, and the attendant can directly check the equipment operation status through the observation window, realizing the dual functions of "ventilation and heat dissipation + status monitoring" .
4. Increasing Partial Discharge Tests: Incorporate the partial discharge test of air-insulated switchgear into the periodic maintenance project (recommended once every six months), and use a dedicated partial discharge tester to detect whether there is partial discharge inside the equipment (such as discharge inside the insulation layer and in the iron core gaps). This test can detect potential hidden dangers that are not easy to be found in conventional maintenance (such as insulation resistance testing), and take targeted measures in a timely manner (such as replacing insulating parts, handling iron core gaps) to prevent the expansion of partial discharge and cause equipment damage .

(5) Strengthening Load Management: Realizing Three-Phase Balanced Operation

1. Combining Monitoring and Assessment: Incorporate the three-phase load monitoring of the transformer into the daily management regulations of the substation, and establish an assessment mechanism — the attendant records the three-phase current and neutral line current every day, calculates the unbalance degree, and if the unbalance degree exceeds 15% or the neutral line current exceeds 25% of the rated current, it is necessary to report and rectify immediately; assess the responsible persons who fail to detect or handle the load imbalance in a timely manner to strengthen management responsibilities .
2. Standardizing the Power Access Process: Formulate a management system for the access of daily power consumption (such as heating and lighting equipment) in the substation, and clarify that all newly added electrical equipment must be reviewed by professional personnel of the production department — the review content includes equipment power and access phase to ensure the reasonable distribution of three-phase loads; private access to high-power equipment is strictly prohibited. If it is necessary to increase the load (such as winter heating), the three-phase balance must be recalculated, and if necessary, a three-phase power supply method (instead of single-phase access) should be adopted .
3. Controlling Zero-Sequence Current and Neutral Line: During peak load periods (such as winter heating and summer cooling), focus on monitoring the change of zero-sequence current using electricity meters or current monitoring devices. Once the unbalance degree exceeds 25%, adjust the load immediately (such as transferring the equipment of the high-load phase to the low-load phase); when installing the transformer, select the neutral line cross-sectional area reasonably according to the maximum neutral line current (generally not less than 50% of the phase line cross-sectional area; if the single-phase load is large, the same cross-sectional area as the phase line can be selected) to avoid the neutral line being burned out due to overcurrent heating .

4. Conclusion

As the core equipment of the substation, the safe operation of dry-type transformers is directly related to the stability of electricity supply for production and daily life. Through the analysis of two transformer burnout accidents in the substation along the Shuohuang Railway, this article reveals that insulation decline, iron core faults, winding defects, insufficient heat dissipation, and three-phase load imbalance are the core causes of equipment heating and burnout, and proposes countermeasures from multiple dimensions of management, technology, and monitoring. These measures can not only scientifically plan the low-voltage load distribution of the substation and standardize the collection and arrangement of basic data but also improve the substation operation and management system in a "point-to-face" manner, providing references for the safe operation of dry-type transformers in similar substations. In the future, with the improvement of the intelligence level of the power system, technologies such as AI monitoring (such as fault early warning models based on big data) and remote control can be further introduced to continuously improve the operation and maintenance efficiency and safety guarantee capabilities of dry-type transformers.

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