[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 3.3.5 Reactance-type LTC, inside view showing vacuum interrupter assembly.
Today the greater part of arcing tap switches is produced for voltage regulators, whereas the vacuumtype
LTC is going to be the state of the art in the field of power transformers.
Figure 3.3.5 shows a three-phase vacuum-type LTC with full insulation between phases and to ground
(nominal voltage level 69 kV). It consists of an oil compartment containing tap selector and reversing/
coarse change-over selector, vacuum interrupters, and bypass switches.
A typical winding layout and the operating sequence of the said LTC is shown in Figure 3.3.6. The
operating sequence is divided into three major functions:
1. Current transfer from the tap selector part preselecting the next tap to the part remaining in
position by means of the vacuum interrupter in conjunction with the associated bypass switch
(Figure 3.3.6b, positions A–C).
2. Selection of the next tap position by the tap selector in proper sequence, with the reclosing of the
vacuum interrupter and bypass switch (Figure 3.3.6b, positions C–F). Contrary to resistance-type
LTCs, the bridging position—in which the moving selector contacts p1 and p4 are on neighboring
fixed selector contacts (in Figure 3.3.6b, contacts 4 and 5, position F)—is a service position, and
therefore the preventive autotransformer/reactor (normally produced by the transformer manufacturer)
is designed for continuous loading; i.e., the number of tap positions is twice the number
of steps of the tap winding. In other words, the preventive autotransformer works as a voltage
divider for step voltage of the tap winding in the bridging position. In comparison with the
resistance-type LTC, the reactance-type LTC requires only half the number of taps of the tap
winding for the equivalent number of service tap positions.
3. Operation of reversing or coarse changeover selector in order to double the number of positions;
for this operation, the moving selector contacts p1 and p4 have to be on the fixed selector contact M
(Figure 3.3.6a).
For more detailed information about switching duty and phasor diagrams, see Annex B (IEEE, 1995
and IEC, 2003).
3.3.1.3 Tap Position Indication
There are no general rules for defining the numerals on the tap-position indicator dial. This is a question
of the user’s specifications or national standards. Some users are accustomed to designations such as 1
through 33 (or 0 through 32), while other have traditionally known 16L (lower), 15L, 14L, … N (neutral);
1R (raise), 2R … 16R. An additional point of confusion comes about with the selection of the placement
of the tap changer on the primary or secondary winding of the transformer. A tap changer on the primary
FIGURE 3.3.6 Reactance-type LTC. Top) Typical winding layout, LTC in position 16L; bottom) Switching sequence,
position 16L to 15L.
is sometimes designated such as 1 through 33, but position #1 may indicate the greatest degree of voltage
boost or buck, depending upon the transformer designer.
3.3.2 Applications of Load Tap Changers
3.3.2.1 Basic Arrangements of Regulating Transformers
The following basic arrangements of tap windings are used (Figure 3.3.7).
Linear arrang ement (Figure 3.3.7a), is generally used on power transformers with moderate regulating
ranges up to a maximum of 20%.
With a reversing chang eover selector (Figure 3.3.7b), the tap winding is added to or subtracted from
the main winding so that the regulating range can be doubled or the number of taps reduced. During
this operation the tap winding is disconnected from the main winding (for a discussion on problems
arising from this disconnection, see Section 3.3.5.1 entitled “Voltage Connection of Tap Winding during
Changeover Operation”). The greatest copper losses occur, however, in the position with the minimum
number of effective turns. This reversing operation is realized with the help of a changeover selector,
which is part of the tap selector or of the arcing tap switch. The double reversing chang eover selector
(Figure 3.3.7c) avoids the disconnection of tap winding during the changeover operation. In phaseshifting
transformers (PST), this apparatus is called an advance-retard switch (ARS).
By means of a coarse chang eover selector (Figure 3.3.7d), the tap winding is either connected to the
plus or minus tapping of the coarse winding. Also during coarse-selector operation, the tap winding is
disconnected from the main winding. (Special winding arrangements can cause the same disconnection
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC