[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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FIGURE 3.3.2 Switching sequence of tap selector — arcing switch.<br />
positions a–c). Then the arcing switch transfers the load current from the tap in operation to the<br />
preselected tap (Figure 3.3.2, positions c–g). The LTC is operated by means of a drive mechanism. The<br />
tap selector is operated by a gearing directly from the drive mechanism. At the same time, a spring energy<br />
accumulator is tensioned. This operates the arcing switch — after releasing in a very short time —<br />
independently of the motion of the drive mechanism. The gearing ensures that this arcing switch<br />
operation always takes place after the tap preselection operation has been finished. With today’s designs,<br />
the switching time of an arcing switch lies between 40 and 60 ms.<br />
During the arcing switch operation, transition resistors are inserted (Figure 3.3.2, positions d–f), which<br />
are loaded for 20 to 30 ms, i.e., the resistors can be designed for short-term loading. The amount of<br />
resistor material required is therefore relatively small. The total operation time of an LTC is between<br />
3 and 10 sec, depending on the respective design.<br />
An arcing tap switch (Figure 3.3.3) carries out the tap change in one step from the tap in service to<br />
the adjacent tap (Figure 3.3.4). The spring energy accumulator, wound up by the drive mechanism<br />
actuates the arcing tap switch sharply after releasing. For switching time and resistor loading (Figure 3.3.4,<br />
positions b–d), the above statements are valid.<br />
The details of switching duty, including phasor diagrams, are described by IEEE (Annex A [IEEE,<br />
1995]) and IEC (Annex A [IEC, 2003]).<br />
3.3.1.2 Reactance-Type Load Tap Changer<br />
For reactance-type LTCs, the following types of switching are used (Annex B of [IEEE, 1995 and IEC,<br />
2003]):<br />
• Arcing tap switch<br />
• Arcing switch with tap selector<br />
• Vacuum interrupter with tap selector<br />
FIGURE 3.3.3 Design principle — arcing tap switch.<br />
FIGURE 3.3.4 Switching sequence of arcing tap switch.<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC