[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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