The LTC design that is normally applied to larger powers and
higher voltages comprises an arcing switch and a tap selector. For lower
ratings, LTC designs are used where the functions of the arcing switch and the
tap selector are combined in a so-called arcing tap switch.
With an LTC comprising an arcing switch and a tap selector
(Figure 3.3.1), the tap change takes place in two steps (Figure 3.3.2). First,
the next tap is preselected by the tap selector at no load (Figure 3.3.2, positions
a–c). Then the arcing switch transfers the load current from the tap in
operation to the preselected tap (Figure 3.3.2, positions c–g).
The LTC is operated by means of a drive mechanism. The tap
selector is operated by a gearing directly from the drive mechanism. At the
same time, a spring energy accumulator is tensioned.
This operates the arcing switch — after releasing in a very
short time — independently of the motion of the drive mechanism. The gearing
ensures that this arcing switch operation always takes place after the tap
preselection operation has been finished.
With today’s designs, the switching time of an arcing switch
lies between 40 and 60 ms. During the arcing switch operation, transition
resistors are inserted (Figure 3.3.2, positions d–f), which are loaded for 20
to 30 ms, i.e., the resistors can be designed for short-term loading.
The amount of resistor material required is therefore
relatively small. The total operation time of an LTC is between 3 and 10 sec,
depending on the respective design.
An arcing tap switch (Figure 3.3.3) carries out the tap
change in one step from the tap in service to the adjacent tap (Figure 3.3.4).
The spring energy accumulator, wound up by the drive mechanism actuates the
arcing tap switch sharply after releasing. For switching time and resistor
loading (Figure 3.3.4, positions b–d), the above statements are valid.
The details of switching duty, including phasor diagrams,
are described by IEEE (Annex A [IEEE, 1995]) and IEC (Annex A [IEC, 2003]).
