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 offline, 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 2.18.
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 of the transformer. This option is specified on about 60% of new medium and large power transformers. Figure 2.19 illustrates the basic operation by providing a sample schematic and connection chart for a transformer supplied with an LTC on the low-voltage (secondar y) side. It should be recognized that there would be slight differences in this schematic based on the specific LTC being used. Figure 2.19 also shows a sample schematic where an auxiliary transformer is used between the main windings and the LTC to limit the current through the LTC mechanism.
It is also possible for a transformer to have dual voltage ratings, as is popular in spare and mobile transformers. While there is no physical limit to the ratio between the dual ratings, even ratios (for example 24.94 _ 12.47 kV or 138 _ 69 kV) are easier for manufacturers to accommodate.
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 2.18.
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 of the transformer. This option is specified on about 60% of new medium and large power transformers. Figure 2.19 illustrates the basic operation by providing a sample schematic and connection chart for a transformer supplied with an LTC on the low-voltage (secondar y) side. It should be recognized that there would be slight differences in this schematic based on the specific LTC being used. Figure 2.19 also shows a sample schematic where an auxiliary transformer is used between the main windings and the LTC to limit the current through the LTC mechanism.
It is also possible for a transformer to have dual voltage ratings, as is popular in spare and mobile transformers. While there is no physical limit to the ratio between the dual ratings, even ratios (for example 24.94 _ 12.47 kV or 138 _ 69 kV) are easier for manufacturers to accommodate.
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