[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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Source<br />
Bypass Switch<br />
Source<br />
Bypass Switch<br />
A<br />
A<br />
B<br />
C<br />
N<br />
Disconnect<br />
Series<br />
Lightning<br />
Arrester<br />
S<br />
L<br />
SL<br />
Shunt<br />
Lightning<br />
Arrester<br />
L<br />
S SL S<br />
L<br />
SL<br />
B<br />
C<br />
Disconnect<br />
Series<br />
Lightning<br />
Arrester<br />
S<br />
L<br />
SL<br />
Shunt<br />
Lightning<br />
Arresters<br />
S<br />
L<br />
SL<br />
FIGURE 2.7.5 Connection of three regulators regulating a three-phase, four-wire, multigrounded wye circuit.<br />
L<br />
S<br />
120°<br />
120°<br />
SL<br />
System<br />
Voltage<br />
120°<br />
S<br />
L<br />
0 ± 10%<br />
Regulation<br />
FIGURE 2.7.7 Connection of two voltage regulators regulating a three-phase, three-wire wye or delta circuit.<br />
A’<br />
A<br />
010%<br />
Output Voltage<br />
System Voltage<br />
B<br />
S<br />
L<br />
FIGURE 2.7.6 Phasor diagram of three regulators regulating a three-phase, four-wire, multigrounded wye circuit.<br />
on the expected unbalance in load. Either method allows a path for the unbalanced current to flow. The<br />
primary windings are connected in a wye configuration tying their neutral back to the neutral of the<br />
three regulators, while the secondary windings are connected in a delta configuration.<br />
Two single-phase regulators connected in an open-delta bank, as shown in Figure 2.7.7, allow for two<br />
of the phases to be regulated independent of each other, with the third phase tending to read the average<br />
of the other two. A 30 phase displacement between the regulator current and voltage, as shown in<br />
Figure 2.7.8, is a result of the open-delta connection. Depending on the phase rotation, one regulator<br />
has its current lagging the voltage, while the other has its current leading the voltage.<br />
When a three-phase, three-wire circuit incorporates three single-phase voltage regulators in a closeddelta-connected<br />
bank as shown in Figure 2.7.9, the overall range of regulation of each phase is dependent<br />
on the range of regulation of each regulator. This type of connection will give approximately 50% more<br />
regulation (15% vs. 10%) than is obtained with two regulators in an open-delta configuration. A 30<br />
phase displacement is also realized between the regulator current and voltage as a result of the delta<br />
connection. The phasor diagram in Figure 2.7.10 shows this. Depending on the phase rotation, the current<br />
will lag or lead the voltage, but the lag/lead relationship will be consistent for all three regulators.<br />
Contributing to the effect is that the phase angle increases as the individual range of regulation of<br />
each regulator increases. A 4 to 6% shift in the phase angle will be realized with the regulators set at the<br />
same extreme tap position. A voltage improvement of 10% in the phase obtained with the regulator<br />
operation leads to a 5% voltage improvement in the adjacent phase. If all three regulators operate to the<br />
same extreme position, the overall effect increases the range of regulation to 15%, as shown in the<br />
phasor diagram of Figure 2.7.10.<br />
C’<br />
010%<br />
Output Voltage<br />
FIGURE 2.7.8 Phasor diagram of two voltage regulators regulating a three-phase circuit.<br />
Source<br />
A<br />
B<br />
C<br />
Disconnect<br />
Series<br />
Lightning<br />
Arrester<br />
Bypass Switch<br />
S<br />
L<br />
SL<br />
Shunt<br />
Lightning<br />
Arrester<br />
FIGURE 2.7.9 Connection of three voltage regulators in a three-phase, three-wire delta circuit.<br />
C<br />
L<br />
S SL S<br />
Figure 2.7.11 reflects the power connection of a closed-delta bank of regulators. Current within the<br />
windings reaches a maximum of approximately minus or plus 5% of the line current as the regulators<br />
approach the full raise or lower positions, respectively. Because the operation of any regulator changes<br />
the voltage across the other two, it may be necessary for the other two to make additional tap changes<br />
to restore voltage balance. The 30º phase displacement between the regulator current and voltage at unity<br />
power-factor load for open- and closed-delta connections also affects the resulting arc energy and<br />
corresponding life of the tap-changer load-breaking contacts.<br />
L<br />
SL<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC