[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 2.1.14 Layer windings (single layer with two strands wound in parallel).
FIGURE 2.1.16 Basic disc winding layout
FIGURE 2.1.15 Helical winding during assembly.
2.1.5.3 Taps-Turns Ratio Adjustment
The ability to adjust the turns ratio of a transformer is often desirable to compensate for variations in
voltage that occur due to the regulation of the transformer and loading cycles. This task can be accomplished
by several means. There is a significant difference between a transformer that is capable of
changing the ratio while the unit is on-line (a load tap changing [LTC] transformer) and one that must
be taken off-line, or de-energized, to perform a tap change.
Most transformers are provided with a means of changing the number of turns in the high-voltage circuit,
whereby a part of the winding is tapped out of the circuit. In many transformers, this is done using one of
the main windings and tapping out a section or sections, as illustrated by the schematic in Figure 2.1.18.
FIGURE 2.1.17 Disc winding inner and outer crossovers.
With larger units, a dedicated tap winding may be necessary to avoid the ampere-turn voids that occur
along the length of the winding. Use and placement of tap windings vary with the application and among
manufacturers. A manually operated switching mechanism, a DETC (de-energized tap changer), is normally
provided for convenient access external to the transformer to change the tap position. When LTC capabilities
are desired, additional windings and equipment are required, which significantly increase the size and cost
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC