Checklist Before Energizing a New or Replaced OIP Bushing at Pre-Commissioning Stage and at Recharging Stage.

Transformer bushings are crucial components that ensure the safe transfer of electrical energy between transformer winding leads and transmission line. Before energizing a new or replaced Oil-Impregnated Paper (OIP) bushing, whether during pre-commissioning or recharging, it is essential to inspect its condition to prevent premature failure and ensure long-term reliability.

The following aspects provide an overview of the recommended checks:

Settling time and orientation

Proper oil redistribution within the bushing is important. If the bushing has been stored (including during transport) for up to one month, it should be positioned vertically or in its projected service position for at least 12 hours before energization. For storage periods longer than one month, a minimum of 48 hours is recommended. This practice helps stabilize the internal dielectric and prevents air entrapment.

Visual oil level check

Inspect the oil indicator on the bushing to ensure that the oil is visible and at the normal level corresponding to room temperature level. Low oil level may indicate presence of oil leakage in bushing & this needs to be attended.

Mounting and earthing

Securely mount the bushing flange to the transformer turret/tank cover. Connect the flange to the earth potential using an adequately sized earthing strip/flexible conductor.

Visual inspection for absence of leakage

Carefully examine the bushing for absence of oil leaks at critical locations such as mounting flange to transformer tank joint, air release plug, HV terminal cover, conservator, and HV air-end seals. Leakage at any of these points may cause oil leak and moisture ingress which adversely affects insulation quality and must be addressed before energization.

Test tap earthing & cap sealing

Test Tap is provided for measurement of capacitance and Tan Delta of bushing. After the measurements, the test tap stem is required to be connected to earth. For this earthing, test tap cap is required to be screwed on, which ensures that test tap stem is connected through cap to mounting flange. The test tap cap is also required to be fixed and tightened to ensure that O-ring/ gasket sealing is foolproof.

Electrical testing

Perform diagnostic measurements to assess insulation health: Capacitance (C1) and Tan Delta at 10 kV to evaluate the dielectric health at pre-commissioning stage and the test results are required to be near about same as those mentioned on rating plate.

Reference manufacturer guidelines

Always review the OEM's instruction manual for model-specific procedures and recommendations to supplement these general checks.

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