As a core component of power systems, the safety and stability of an oil-immersed transformer directly impact the operational quality of the entire power network. For electrical engineers or enterprise operation and maintenance teams, conducting thorough inspections and preparations before officially loading an oil-immersed transformer is essential to avoid failures and extend equipment life.
Xinhong Electrical, a brand specializing in power transmission and distribution equipment, has summarized the following critical points to note before putting an oil-immersed transformer into service, based on years of field experience.
After the transformer is installed, a comprehensive visual inspection should be performed:
Check that the tank, radiators, high and low voltage bushings, oil conservator, and explosion-proof tube are intact.
Ensure bolts are tightened, with no loose parts or oil leakage.
Verify that the grounding device is secure and reliable, and that grounding resistance meets specification requirements.
Xinhong Electrical recommends paying special attention to signs of collision or displacement during transportation, and documenting or repairing any damage as necessary.
Before operation, use a megohmmeter to measure the insulation resistance between windings, between each winding and ground, and of the core. Compare the results with factory values. A significantly lower reading indicates moisture or insulation defects, requiring drying treatment.
Generally, the insulation resistance of an oil-immersed transformer should not be less than 70% of the factory value.
Oil serves as both an insulating and cooling medium, so its condition is critical:
Check that the oil level in the conservator is within the normal range.
Take oil samples for dielectric strength testing and moisture content analysis.
Observe whether the oil is clear and transparent, with no visible suspended matter or sediment.
Xinhong Electrical emphasizes that if oil quality deteriorates or moisture content exceeds limits, the oil must be filtered or replaced. Never put the transformer into operation under such conditions.
Oil-immersed transformers are typically equipped with off-load or on-load tap changers. Before energizing, adjust the tap changer to the appropriate position based on system voltage. Also:
Manually operate the tap changer to check for smooth and accurate switching.
Measure DC resistance at each tap position; the three-phase imbalance should comply with standards.
The transformer should be equipped with a Buchholz relay, thermometer, pressure relief valve, oil level gauge, and other protective components. Before operation:
Calibrate the accuracy of thermometers and pressure gauges.
Check that the Buchholz relay is filled with oil and properly vented, and that wiring is correct.
Test the operation of the pressure relief valve.
As a manufacturer dedicated to high-quality power equipment, Xinhong Electrical reminds users not to ignore the functional testing of protection relay circuits — this is the last line of defense against accident escalation.
After completing the above checks, it is recommended to perform a no-load inrush current test (energization test), typically as follows:
Apply full voltage from the high-voltage side for five inrush cycles.
Maintain the required interval between each energization (generally 5–10 minutes).
During each inrush, listen carefully for abnormal internal sounds and monitor oil level and protection device responses.
This step effectively verifies the internal insulation strength and mechanical stability of the transformer.
All inspection data, commissioning records, and test reports should be formally documented for future comparison during maintenance. A complete commissioning file is also an important guarantee for the long-term safe operation of the transformer.
Every pre-service check of an oil-immersed transformer represents a commitment to safety and responsibility. Whether for new substation construction or equipment replacement, following standard procedures and choosing reliable transformer brands significantly reduces operational risks.
