Analysis and Handling Experience of Common Transformer Fault Cases
As a core equipment in power systems, the operational reliability of transformers directly determines the stability of power supply. Once a fault occurs, it may lead to regional power outages, causing serious impacts on industrial production and residents' lives. This article systematically analyzes common transformer fault types, causes, and handling technologies based on practical cases, providing references for operation and maintenance work.
Ⅰ. Winding Faults
1. Turn-to-turn Short Circuit
A main transformer in a 110kV substation showed abnormal temperature rise during routine inspection, accompanied by the operation of the gas relay. Oil chromatographic analysis revealed a significant exceedance in total hydrocarbon content, with acetylene concentration reaching 12μL/L (far exceeding the 5μL/L warning value). Winding DC resistance testing showed that the resistance of the low-voltage side B-phase was 8% lower than that of the other two phases, exceeding the 2% allowable range specified in GB/T 1094.1-2013. Inspection after lifting the cover confirmed a turn-to-turn short circuit in the low-voltage B-phase winding, with the insulation layer at the fault point showing carbonization marks and local overheating deformation of the conductor.
Fault Mechanism: Insulation defects remaining during winding manufacturing gradually expand under long-term electrodynamic and thermal cycles, leading to insulation breakdown and the formation of a short-circuit path. Handling requires removing the damaged winding and rewinding with wires of the same specification, strictly controlling insulation thickness and winding tension during the rewinding process. After repair, insulation resistance, dielectric loss, and partial discharge tests must be conducted, and the transformer can be put into operation only when all indicators are qualified. After this repair, the operating temperature of the transformer returned to 65℃ (previously reaching a maximum of 82℃), and the oil chromatographic data remained normal.
2. Winding Open Circuit
After a sudden power outage in a 10kV distribution transformer of a factory, there was no output on the low-voltage side when power was restored. Insulation resistance testing showed normal insulation on the high-voltage side A-phase, but DC resistance testing showed infinity. Disassembly inspection found that the welding point between the high-voltage winding lead-out wire and the terminal had a cold solder defect, which melted under the impact of short-circuit current.
Fault Analysis: Poor welding techniques resulted in excessive contact resistance. The Joule heat generated during long-term operation gradually oxidized the welding point, which eventually broke under current impact. Handling requires re-welding using silver-copper brazing techniques, and a joint temperature rise test must be conducted after welding (passing rated current for 30 minutes, with a temperature rise not exceeding 60K). After this repair, there were no abnormalities during 6 months of continuous operation.
Ⅱ. Core Faults
1. Multiple Grounding of Core
During a preventive test on a main transformer in a 35kV substation, the measured core grounding current reached 1.2A (standard value ≤0.1A). Segmented inspection found that the insulation pad between the core and the clamp had a 0.5mm penetrating crack due to mechanical damage, forming a multiple grounding loop.
Hazard Mechanism: Multiple grounding causes circulation in the core, leading to local overheating and, in severe cases, burning of core laminations. Handling requires replacing the 3mm thick epoxy glass cloth insulation pad, ensuring that the insulation resistance between the core and the clamp is ≥1000MΩ during installation. After repair, the grounding current dropped to 0.03A, meeting the operating standards.
2. Short Circuit of Core Silicon Steel Sheets
The no-load loss of a 220kV transformer increased by 15% compared with the factory value, accompanied by an 8K rise in top oil temperature. Core insulation resistance testing showed that the insulation resistance between silicon steel sheets dropped to 500MΩ (standard ≥1000MΩ). Inspection after lifting the cover found that about 3% of the insulation coating on the surface of the silicon steel sheets had worn off due to electromagnetic vibration, forming eddy current channels.
Handling Measures: Insulation restoration is performed on the insulation-damaged areas - after removing the surface oxide layer, apply Class F insulating paint, with the paint layer thickness controlled between 0.05-0.08mm. For severely worn silicon steel sheets, overall replacement is carried out to ensure the lamination factor is ≥0.93. After repair, the no-load loss returned to the factory level, and the temperature rise curve returned to normal.
Ⅲ. Tap Changer Faults
1. Poor Contact of On-Load Tap Changer
A 220kV transformer experienced voltage regulation failure after voltage regulation operation, with abnormal noise from the tap changer itself. Disassembly inspection found 8 ablation points on the changeover switch contacts, with a maximum ablation depth of 0.3mm, and the contact resistance increased to 500μΩ (standard ≤50μΩ). Further inspection showed that wear of the transmission mechanism bearing led to insufficient contact pressure of the contacts, resulting in arc ablation.
Handling Plan: Repair the contact surface using precision grinding technology, replace worn bearings and spring components, and adjust the contact pressure to 25-30N. After assembly, 100 mechanical operation tests are conducted to ensure the switching time deviation is ≤2ms. After commissioning, 30 consecutive voltage regulation operations were normal, with the contact resistance stable at 35μΩ.
2. Incorrect Position of Off-Load Tap Changer
A 10kV distribution transformer showed a 15% output voltage deviation after maintenance. Inspection found that the actual position of the tap changer did not match the gear indication, resulting in a wrong transformation ratio. Such faults are mostly caused by the failure to implement the gear checking process after maintenance.
Handling Specification: Adjust the tap changer to the corresponding gear according to the grid voltage level (for example, when the 10kV system voltage is 10.5kV, it should be placed in the -5% gear). After adjustment, measure the transformation ratio error (should be ≤±0.5%). After this adjustment, the output voltage deviation was controlled within ±2%.
Ⅳ. Bushing Faults
1. Bushing Flashover Discharge
The A-phase bushing of a 110kV transformer had a surface flashover after a thunderstorm, with the insulation resistance dropping sharply from 2500MΩ to 800MΩ. Inspection found that the salt density of the contamination layer on the bushing surface reached 0.25mg/cm² (Grade III pollution area standard ≤0.1mg/cm²), forming a conductive channel after being infiltrated by rainwater.
During handling, first, use live water washing to remove surface contamination, then spray RTV anti-pollution flashover coating to ensure the dry film thickness is not less than 0.3mm. At the same time, verify the arrester characteristics to ensure the residual voltage at 10kA does not exceed 260kV. After handling, a 184kV, 1-minute power frequency withstand voltage test showed no abnormalities, and the pollution flashover monitoring for the subsequent 6 months was also qualified.
2. Bushing Oil Leakage
Oil leakage occurred at the top seal of the high-voltage bushing of a 220kV transformer, with the oil level dropping by about 2mm per day. Disassembly inspection confirmed that the nitrile rubber seal gasket lost elasticity due to aging, with a compression amount of only 0.5mm, failing to meet the standard requirement of ≥1.2mm.
During repair, replace with a fluororubber seal gasket (temperature resistance range -20℃~200℃). During installation, control the parallelism deviation of the flange surface to not exceed 0.1mm/m, and uniformly tighten the bolts according to the design torque of 450N·m. A 30-minute seal test under 0.05MPa air pressure was then conducted to confirm no leakage. After 3 months of operation, the oil level remained stable, and no more leakage occurred.
Conclusion
The key to handling transformer faults lies in accurate diagnosis, scientific schemes, and standardized processes. In actual operation and maintenance, strengthen condition monitoring, and detect potential defects as early as possible through technical means such as oil chromatographic analysis and partial discharge detection. Usually, pay attention to accumulating fault cases and summarizing the common laws of similar faults. This can effectively improve handling efficiency and better ensure the safe operation of equipment throughout its life cycle.