VACUUM OIL FILLING PROCEDURE TUTORIALS

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.

POWER TRANSFORMER WINDING CONNECTION DIAGRAM

The actual winding connections are shown in a diagram with each winding and its taps labeled. A set of tables then specifies the voltage ratings, ampere ratings, and the connections for all the available taps. For transformers with load tap changing equipment, the connection diagrams and the accompanying tables are quite extensive.

The connection diagram usually also gives the general physical layout of the transformer, showing the placement of the bushings and the locations of current transformers (CTs) and a schematic representation of the load tap changing equipment, including the preventative autotransformer, moving contacts, arcing contacts, transfer switch, and reversing switch.

A portion of an actual nameplate that shows the winding connection diagram is illustrated in Figure 7.2. The nameplate depicted is rather interesting. The transformer has a load tap changer.

From the connection diagram we see that the buried tertiary is also a tapped winding that supplies a buck/boost voltage to the secondary windings through auxiliary transformers connected between the tertiary and the secondary.

Therefore, the tertiary simultaneously provides four important functions:

1. It provides a path for third harmonic currents.

2. It helps stabilize voltages in the Y-Y primary-secondary connection.

3. It provides a grounding bank action by providing a path for zero sequence currents.

4. It provides the necessary voltage taps for regulating the low-side voltage.

The only function that the buried tertiary cannot perform is to supply an external load. The voltage rating of the buried tertiary is not given because it cannot be connected to a system voltage, but one corner of the Δ connection is grounded internally.

This grounding is done so the winding potential voltage does not ‘‘float’’ because of capacitive coupling to the other windings. Without this ground connection, capacitively induced voltages are indeterminate and could be large enough to cause insulation damage.

The voltage taps for the primary and secondary are shown on the connection diagram and on the winding rating tables in Figure 7.2. These also specify which terminal numbers and letters are connected for each tap.

This transformer has a total of 14 current transformers that are used for metering, protective relaying, and other purposes. Note the CTs marked ‘‘LDC’’ and ‘‘WDG. TEMP.’’ The term LDC stands for line drop compensation. The LDC CT supplies metered line current to a compensating device in the voltage regulator controls.

The compensating device effectively moves the voltage control point into the system connected to the secondary winding. The CT labeled WDG.TEMP supplies current to the winding temperature gauges.

These gauges use a heating element surrounding a temperature probe mounted in the top oil in order to mimic the winding temperature. The ratios of these CTs would be shown on an actual nameplate, but this information is not shown in Figure 7.2.

Just below the connection diagram is a layout sketch showing the physical locations of the bushings, the load tap changing compartment and the operating handle for the tap changer at deenergized conditions. The load tap changer is represented schematically in the connection diagram.

Note the terminals labeled P1, P2, and P3. These terminals correspond to the connections to the preventative autotransformer. The two series arcing contacts per phase that are in series with the movable contacts are shown as well.

FIGURE 7.2 Part of a transformer’s nameplate showing the voltage ratings, MVA ratings, percent impedances, connection diagram, physical layout, vector diagram, tap connections, CT connections, and BIL ratings.