12 Technical guide VUC | 1ZSC000562-AAX en, Rev. 1
Tap-changer principles of operation
Switching sequence, VUCG
By using an auxiliary contact system (MC, RC) in combination
with the vacuum interrupters (MVI, RVI) only two vacuum
interrupters are required per phase.
The switching sequence is that for an assymetrical pennant
cycle always going with the main contact first (called diverter
switch with one transition resistor #1 in IEC 60214-1,
2014-05). Fig. 8 shows the current path during normal
operation, from x to the neutral point (could also be to the
next phase). When commuting the load from x to v, the first
part of the operation sequence is to open the main vacuum
interrupter (MVI) and hence let the current flow through the
transition resistor(TR), Fig. 9. The main contact (MC) is then
rotated (Figs. 10 and11) in order to connect to v. The main
vacuum interrupter then closes, leading to an associated
circulating current driven by the difference in voltage potential,
Fig. 12. The load current is now via the normal path from v
to the neutral point. The voltage is now changed one step. In
Fig.13, the transition resistor is disconnected when opening
the resistor vacuum interrupters (RVI). The resistor contact
(RC) is then rotated and put in position according to Fig. 14.
Finally, the sequence is completed and next service position is
fully reached when the resistor vacuum interrupter is closed,
see Fig. 15.
The sequence below shows one pole only.
The single phase diagrams are available in the appendices,
pages 45-63.
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