1. Remove all oil from the transformer.

2. Test new, unprocessed oil for dielectric strength using ASTM Method D877. The oil must have a minimum breakdown voltage of 30 kV.

3. After assembly, pressurize the transformer to 2 psig by adding drynitrogen. Check the transformer for leaks.

4. For transformers rated 115 kV and above, after waiting for a 24 h period, make a dew-point check to determine the dryness of the transformer insulation. For new transformers and transformers in warranty, refer to the manufacturer’s instructions for acceptable dew-point readings.

For transformers not in warranty, refer to Table 8.1.5. If it does not pass dew-point test, a hot oil dryout is required. After dryout, repeat step 4.

5. Draw a vacuum of 2 mmHg or less. Hold this vacuum for a period of time specified in the manufacturer’s instruction book, if the transformer is new or in warranty. If the transformer is not in warranty, use Table 8.1.6.

6. Maintaining a vacuum of 2 mmHg or less, admit oil into the top of the transformer connection. Once oil filling is started, it must not be interrupted. Oil degassing equipment is required for transformers rated 115 kV and above.

7. If the transformer is a conservator type, stop filling when oil reaches a level 2 in. below the transformer cover. If the transformer is equipped with a nitrogen bottle, stop filling when the oil level gauge is slightly over the 25°C level. This is to compensate for the transformer expanding when vacuum is broken and for oil cooling.

8. Break the vacuum with dry nitrogen. If the transformer has a conservator with air bag, or air separation membrane, add the remaining oil to the expansion tanks in accordance with the manufacturer’s recommendations.

9. Bleed the air from transformer oil pump vents. Turn on all pumps and leave them running while the oil cools.

10. Allow the transformer to stand before energizing (with oil pumps running) according to the timetable shown in Table 8.1.7. Run one half of the pumps for half the time and the other half of the pumps for the second half of the time.

11. Prior to energizing the transformer, check oil levels in all compartments. Pump oil into the top, if necessary, to raise the oil level to the 25°C mark.

12. Prior to energizing the transformer, shut off all oil pumps and place controls on automatic so that no pumps are running prior to energizing. This is important to eliminate static electrification of the oil, which could cause an internal failure. This hold time must be a minimum of 12 h.

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