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.