Replacement of voltage transformer (high voltage)

Voltage Transformers are instrument transformers that convert high primary voltage (usually 69 kV and above) to low secondary voltage (usually 115 V and 66.4 V)for utilization of other instruments like protective relays and meters. These instrument transformers are also known as potential transformers. The most common insulation of these instrument transformers is insulating oil.
If the insulating oil of these instrument transformers is below the recommended limit (breakdown voltage for oil is 26 kV minimum), then it is recommended that these equipments must be replaced. This is to prevent explosion of transformers due to insulation failure. We had replaced our Trench electric voltage transformer last Thursday because the insulation of its insulating oil is not good already. Bad insulating oil can also be ssen through the transformer oil sight glass. The color of the oil in the sight glass was dark-colored already. Our criteria in choosing the replacement are the following: it must have the same voltage rating, the secondary voltage must be the same, the number of cores must satisfy the existing used cores, check if both transformers have HF terminals. The HF terminal is present for capacitor voltage transformer so that the transformer can be utilized for power line communication. Other provisions like mounting, pedestal base and height are also taken into consideration. We devised a checklist to follow in the replacement of our unit. This served as our guide so that it can be done orderly and no important item will be missed.
1. Test the replacement voltage transformer (insulation test, power factor test, ratio test)
2. Remove the terminal pad of the voltage transformer for replacement.
3. Remove wires terminated to secondary terminals (including HF terminal). Label the wires. Take note of the PT ratio.
4. Remove the pipes for wires from the voltage transformer secondary box.
5. Remove the Voltage transformer grounding connected to its body.
6. Secure the voltage transformer with the truck to be used in lifting the equipment.
7. Next, remove the bolts that secures the equipment to its base.
8. Carefully lower down the voltage transformer from its pedestal.
9. Bore the new holes where the bolts of the replacement voltage transformer will be fastened.
10. Place the replacement voltage transformer in the pedestal.
11. Fasten the pedestal bolts to secure the equipment.
12. Reconnect the body grounding of the voltage transformer.
13. Replace the pipes used for wiring of secondary terminals.
14. Terminate back the secondary and HF terminals with the correct PT ratio and termination.
15. Reconnect the terminal pad.
16. Put silicon sealant on pipes to avoid entry of moisture and rain.
17. Re-test the installed voltage transformer (insulation test, power factor test, ratio test).
18. If all is normal, then the voltage transformer is ready for energization.

The Capacitor Voltage Transformer that was replaced

On-going replacement of the capacitor voltage transformer

Lifting of the capacitor voltage transformer from its pedestal