How to Identify Internal Faults in a Transformer?

Measure DC resistance: Use a bridge to measure the DC resistance of each high- and low-voltage winding. Check whether the resistance values among phases are balanced and consistent with the manufacturer’s original data. If phase resistance cannot be measured directly, line resistance may be measured instead. The DC resistance values can indicate whether the windings are intact, whether there are short circuits or open circuits, and whether the contact resistance of the tap changer is normal. If the DC resistance changes significantly after switching tap positions, the issue likely lies in the tap contact points rather than in the windings themselves. This test also verifies the quality of connections between bushing studs and leads, and between leads and windings.

Measure insulation resistance: Measure the insulation resistance between windings and between each winding and ground, as well as the polarization index (R60/R15). Based on these measured values, it is possible to determine whether the insulation of any winding has become damp, or whether there is a risk of breakdown or flashover between windings or to ground.

Measure dielectric loss factor (tan δ): Use a GY-type Schering bridge to measure the dielectric loss factor (tan δ) between windings and between windings and ground. The test results can indicate whether the winding insulation is damp or has undergone overall degradation.

Take an insulating oil sample for simplified testing: Use a flash point tester to check whether the flash point of the insulating oil has decreased. Inspect the oil for carbon particles, paper fibers, and note whether it has a burnt odor. If a gas chromatography analyzer is available, the gas content in the oil can be measured. These methods help identify the type and nature of internal faults.

No-load test: Perform a no-load test on the transformer to measure the three-phase no-load current and no-load power loss. These values help determine whether there are faults between the silicon steel laminations in the core, short circuits in the magnetic circuit, or short circuits within the windings.