[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 3.5.4 Interconnected star-grounding transformer: (a) current distribution in the coils for a line-to-ground
fault and (b) normal operating voltages in the coils.
FIGURE 3.5.3 Current flow in double-wound transformer and autotransformer.
A disadvantage of the autotransformer is that the short-circuit current and forces are increased because
of the reduced leakage reactance. In addition, most three-phase autotransformers are wye–wye connected.
This form of connection has the same limitations as for the wye–wye double-wound transformers.
Furthermore, there is no electrical isolation between the primary and secondary circuits with an
autotransformer connection.
An autotransformer often has a delta-connected tertiary winding to reduce third-harmonic voltages,
to permit the transformation of unbalanced three-phase loads, and to enable the use of supply-station
auxiliary load or power-factor improvement equipment. The tertiary winding must be designed to accept
all of these external loads as well as the severe short-circuit currents and forces associated with threephase
faults on its own terminals or single line-to-ground faults on either the primary or secondary
terminals. If no external loading is required, the tertiary winding terminals should not be brought out
except for one terminal to ground one corner of the delta during service operation. This eliminates the
possibility of a three-phase external fault on the winding.
The problem of transformer insulation stresses and system transient protection is more complicated
for autotransformers, particularly when tapping windings are also required. Transient voltages can also
be more easily transferred between the power systems with the autotransformer connection.
3.5.4.4 Interconnected-Wye and Grounding Transformers
The interconnected-wye-wye connections have the advantages of the star–delta connections with the
additional advantage of the neutral. The interconnected-wye or zigzag connection allows unbalancedphase
load currents without creating severe neutral voltages. This connection also provides a path for
third-harmonic currents created by the nonlinearity of the magnetic core material. As a result, interconnected-wye
neutral voltages are essentially eliminated. However, the zero-sequence impedance of interconnected-wye
windings is often so low that high third-harmonic and zero-sequence currents will result
when the neutral is directly grounded. These currents can be limited to an acceptable level by connecting
a reactor between the neutral and ground. The interconnected-wye-wye connection has the disadvantage
that it requires 8% additional internal kVA capacity. This and the additional complexity of the leads make
this type of transformer connection more costly than the other common types discussed above.
The stable neutral inherent in the interconnected-wye or zigzag connection has made its use possible
as a grounding transformer for systems that would be isolated otherwise. This is shown in Figure 3.5.4.
The connections to the second set of windings can be reversed to produce the winding angular displacements
shown in Figure 3.5.2.
For a line-to-neutral load or a line-to-ground fault on the system, the current is limited by the leakage
reactance between the two coils on each phase of the grounding transformer.
3.5.4.5 Phase-Shifting Transformers
The development of large, high-voltage power grids has increased the reliability and efficiency of electric
power systems. However, a) the difference of voltages, impedance, loads, and phase angles between paralleled
power lines causes unbalanced line-loading. The phase-shifting transformer is used to provide a phase
shift between two systems to control the power flow. A phase-shifting transformer (PST) is a transformer
that advances or retards the voltage phase-angle relationship of one circuit with respect to another circuit.
In some cases, phase-shifting transformers can also control the reactive power flow by varying the voltages
between the two circuits.
There are numerous different circuits and transformer designs used for this application. The two
main type of PSTs used are shown in Figure 3.5.5a and Figure 3.5.5b. The single-core design shown in
Figure 3.5.5a is most commonly used. With this design, it is generally accepted practice to provide two
sets of three single-pole tap changers: one set on the source terminals and one set on the load terminals.
This permits symmetrical voltage conditions while varying the phase angle from maximum advance
to maximum retard tap positions. If only one set of three single-pole tap changers is used, the load
voltage varies as the tap-changer phase-shift position is varied. The single-core design is less complicated,
has less internal b) kVA, and is less costly than the other designs used. However, it has the following
disadvantages:
• The LTC and tap windings are at the line ends of the power systems and are directly exposed to
system transient voltages.
• The impedance of the PST varies directly with the square of the number of tap positions away
from the mid-tap position. The impedance of this type of PST at the mid-tap or zero-phase-shift
position is negligible. As a result, the short-circuit current at or near the mid-tap position is limited
only by the system impedance.
• The maximum capacity of this type of PST is generally limited by the maximum voltage or current
limitation of the tap changers. As a result, the maximum switching capacity of the tap changer
cannot be fully utilized. The space required by these tap changers cause shipping restrictions in
large-capacity PSTs.
• The transient voltage on the tap-changer reversing switch when switching through the mid-point
position is very high. Usually, additional components are required in the PST or tap changer to
limit these transfer voltages to an acceptable level.
• The cost of single-pole tap changers is substantially higher than three-pole tap changers used with
some of the other PST designs.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC