[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 3.3.17 Methods of voltage connection (reversing changeover selector in mid-position): (a) fixed tie-in
resistor Rp; (b) with voltage switch Sp and tie-in resistor Rp.
FIGURE 3.3.16 Phase-shifting transformer, circuit (as shown in Figure 3.3.12): top) winding arrangement with
two windings and screens; bottom) recovery voltages (U r+ , U r– ) for tap windings 1 and 2 (phasor diagram).
• Voltages across the windings and electrical position of the windings within the winding arrangement
of the transformer that is adjacent to the tap winding
• Capacity between tap winding and adjacent windings or winding parts
• Capacity between tap winding and ground or, if existing, grounded adjacent windings
• Surge stress across half of tap winding
• Service and test power-frequency voltages across half of the tap winding
3.3.5.2 Effects of the Leakage Impedance of Tap Winding/Coarse Winding during the
Operation of the Arcing Switch When Passing the Mid-Position of the Resistance-
Type LTC
During the operation of the arcing switch from the end of the tap winding to the end of the coarse
winding and vice versa (passing mid-position, see Figure 3.3.19a), all turns of the whole tap winding and
coarse winding are inserted in the circuit.
This results in a leakage-impedance value that is substantially higher than during operation within
the tap winding, where only negligible leakage impedance of one step is relevant (Figure 3.3.19b). The
higher impedance value in series with the transition resistors has an effect on the circulating current,
FIGURE 3.3.18
Phase-shifting transformer — changeover operation by means of an advance–retard switch (ARS).
which is flowing in the opposite direction through coarse winding and tap winding during the arcing
switch operation.
Consequently, a phase shift between switched current and recovery voltage takes place at the transition
contacts of the arcing switch and may result in an extended arcing time.
In order to ensure optimum selection and adaptation of the LTC to these operating conditions, it is
necessary to specify the leakage impedance of coarse winding and tap winding connected in series.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC