Transformers play a critical role in electrical power systems, facilitating the efficient transmission and distribution of electricity across vast networks. However, ensuring the reliable and safe operation of transformers requires diligent maintenance and inspection practices. This article presents a comprehensive overview of transformer maintenance, covering routine tasks, inspection procedures, and preventive measures to enhance reliability and safety.
From cleaning bushings to checking insulation resistance, each maintenance task is essential for mitigating risks such as flashovers and insulation failures. Additionally, regular inspections of components like tap changers and lightning arresters are crucial for detecting potential issues early and preventing costly downtime. Moreover, preventive measures such as monitoring insulation aging and oil quality are vital for prolonging transformer lifespan and minimizing failures.
By adhering to best practices outlined in this guide, electrical engineers and maintenance personnel can ensure transformers operate at peak performance, minimizing the risk of outages and enhancing the overall resilience of electrical grids.
(1) Check and clean bushings and magnetic skirts to keep them clean and prevent flashover.
(2) During the operation of cooling devices, check that the butterfly valves on the cooling system’s inlet and outlet oil pipes are open; ensure the radiator inlet is clean and unobstructed; check that the oil pump is rotating correctly, running without abnormal noise or significant vibration; ensure fans are operating normally; check that the individual power supply automatic switches in the cooler control box are closing properly without vibration or abnormal noise; and ensure the cooler has no oil leaks.
(3) Ensure electrical connections are securely fastened.
(4) Regularly inspect the tap changer, checking the tightness, scorching, scars, rotational flexibility, and contact positioning of the contacts.
(5) Inspect the coils, bushings, and lightning arresters every three years.
(6) Annually check the reliability of the lightning arrester grounding; the grounding must be reliable, and the lead should be as short as possible. In dry seasons, test the grounding resistance, ensuring it does not exceed 5Ω.
(7) Replace the desiccant in the breather and the oil in the oil bath.
(8) Regularly test fire protection equipment.
(1) Ensure all short-circuit grounding wires and other safety measures are removed; the transformer cover and surrounding area should be clean and free of debris.
(2) Verify that the tap changer position is correctly set.
(3) If phase-shifting work is possible during transformer operation, perform phase verification before paralleling.
(4) Check for oil leaks, normal oil levels, clear oil color, and fully opened oil valves on radiators, oil filters, and gas relays; ensure the explosion vent diaphragm is intact, gas relay oil level is normal, oil regeneration device and filter are functioning, and the desiccant in the breather is dry.
(5) Bushings should be clean, undamaged, free of cracks, and discharge marks.
(6) Verify that the gas relay and thermometer lead insulation is in good condition and free of corrosion.
(7) Ensure all bushing terminals have good contact and the transformer case grounding wire is securely connected and intact.
(8) Check that the transformer protection circuit is functioning well; perform open/close tests on the disconnector, and test the protection circuit trip functions.
(9) Use a 1000V (or 2500V) insulation resistance tester to measure the insulation resistance and absorption ratio (R60/R15 > 1.3) of the high-voltage coil to ground; use a 500V (or 1000V) tester to measure the low-voltage coil to ground and between high and low voltage coils. If the measured values are less than 1/3 to 1/5 of previous readings, investigate and resolve the cause.
(10) After repairs, perform three successful switching operations before putting the transformer into normal operation.
(11) Verify that the status displayed on the monitoring system and protection devices matches the actual conditions.
(1) Under rated conditions, transformers can operate at rated capacity year-round. For oil-immersed transformers, the top oil temperature should generally not exceed 85℃, with a maximum of 95℃, and the temperature rise should not exceed 60℃.
(2) For forced-air-cooled transformers, activate the cooling system when the load reaches 70% of the rated capacity or when the top oil temperature exceeds 55℃. Deactivate the fans when conditions are reversed. When the load exceeds 100% of the rated value, activate the cooling fans regardless of the oil temperature.
(3) Transformers can operate continuously at ±5% of the rated voltage, provided the current on both high and low voltage sides does not exceed the rated value. Depending on operational conditions (reserve capacity, load curve, and cooling temperature), transformers can handle normal overload and emergency overload related to the daily load curve. For each 10% decrease in load, the maximum load can exceed the rated value by 3%, permitted only when ambient temperature does not exceed 35℃.
Periodically use a megger to test the insulation resistance absorption ratio, and perform DC resistance testing to detect issues early based on the balance coefficient of the winding DC resistance. If necessary, measure the DC leakage current and compare it with previous or factory data. Perform AC voltage withstand tests for insulation strength while monitoring the top oil temperature to manage transformer load variations and avoid prolonged peak load operation.
Preventive measures for oil quality involve testing the dielectric loss, using gas chromatography to analyze latent faults. Gas chromatography is highly sensitive and is a crucial method for monitoring the operation of oil-filled equipment.
(1) Keep porcelain bushings and insulators clean; in the oil cooling system, check for leaks, rust, dirt buildup, and any mechanical damage restricting oil flow. Ensure electrical connections are securely fastened; regularly inspect the tap changer.
(2) Check the tightness, scorching, scars, rotational flexibility, and contact positioning of the tap changer contacts; inspect the coils, bushings, and lightning arresters every three years for dielectric loss; annually check the reliability of the lightning arrester grounding.
(3) Ensure reliable grounding with short leads. In dry seasons, test grounding resistance, ensuring it does not exceed 5Ω; consider using online monitoring systems for critical transformers.
Regular and preventive maintenance of transformers effectively extends their lifespan, reduces faults, and enhances the reliability and safety of the power system. This comprehensive guide on transformer daily maintenance, inspection procedures, and preventive maintenance provides valuable insights and practical guidance for electrical engineers.
Transformer Series Maintenance Testers:
