1. DC Resistance Test

Measure the DC resistance of each phase winding using a three-phase DC resistance tester and compare the three-phase imbalance. If the resistance deviation is excessive, the values fluctuate abruptly, or the three-phase difference exceeds the standard, it can be determined that the winding has defects such as inter-turn short circuits, loose connections, poor soldering, broken strands in the conductors, or internal breaks.

2. Transformation Ratio and Connection Group Test

Use a transformer ratio tester to detect the ratio error and wiring group of each tap position. Exceeding the ratio tolerance or incorrect polarity may indicate inter-turn short circuits, incorrect turns, winding deformation, or internal breaks, while also identifying issues such as poor contact in the tap changer.

3. Insulation Resistance and Absorption Ratio Test

Measure the insulation resistance between windings and ground, as well as between phases, using a megohmmeter, and calculate the absorption ratio and polarization index. A significant decrease in insulation resistance and a low absorption ratio indicate winding moisture, insulation aging, or severe contamination. A sudden drop in values often accompanies inter-turn or inter-layer insulation defects.

4. Winding deformation test (frequency response analysis FRA + low voltage impedance)

By detecting the frequency response curve of the winding through the frequency response method and comparing it with the three-phase curve or historical curve, significant shifts or distortions in the curve indicate deformation, displacement, twisting, swelling, or damage from short-circuit impacts in the winding. Combined with short-circuit impedance testing, the overall deformation degree of the winding can be determined.

5. Partial Discharge Detection

Offline or online partial discharge tests are conducted using a partial discharge meter to monitor the discharge amount and patterns. Exceeding the partial discharge limit or the occurrence of continuous pulse discharges often indicates early insulation defects such as weak匝间 insulation, corona discharge at sharp corners, insulation cracking, or internal gap discharge.

6. Chromatographic Analysis of Dissolved Gases in Oil

Gas chromatography analysis was conducted on transformer oil to monitor the content of acetylene, hydrogen, and total hydrocarbons. A significant increase in acetylene and hydrogen levels typically indicates internal faults such as arc discharge, inter-turn short circuits, localized overheating, or wire breakage with arcing in the windings.

7. No-load and Load Loss Tests

Measure no-load loss, load loss, and impedance voltage. An abnormal increase in no-load loss often corresponds to inter-turn short circuits or core faults; sudden changes in load loss and abnormal impedance voltage usually indicate winding deformation, short circuits, broken strands, or poor connections.