[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

150
V(%)
100
50
0
-50
40 80 T(s) 120
FIGURE 2.3.16 Lightning impulse response (bypassed PST).
of this design. On the other hand, the axial arrangement of the regulating winding in core-type transformers,
as shown in Figure 2.3.15b, offers the advantage of direct access and saves space. But because
the pattern of the stray field is more complicated, thorough field calculations have to be performed using
the appropriate computer programs.
It is possible that a client may operate the PST in a bypassed condition. In this case, the source and
load terminals are directly connected phasewise. In this state, a lightning impulse would penetrate the
series winding from both ends at the same time. Two traveling waves would meet in the middle of the
winding and would theoretically be reflected to double the amplitude (Figure 2.3.16). Therefore the series
winding must either be designed with a high internal capacitance or be protected by external or internal
surge arresters. A high internal capacitance can be obtained using any of the measures that are used to
improve the lightning-impulse voltage distribution along the winding, e.g., interleaving or shielding.
Again, the axial arrangement is more advantageous and easier to make self-protecting.
Figure 2.3.15c shows the arrangement of a two-core design with two coarse and one fine tap winding,
which is a variation of the circuit drawn in Figure 2.3.12 (see also Section 2.3.7, Details of On-Load Tap-
Changer Application). If a two-tank solution has to be used, the connection between the main and series
unit requires an additional set of six high-voltage bushings, which means that a total of nine high-voltage
bushings would have to be arranged on the series unit. Because a short circuit between the main and
series units could destroy the regulating winding, a direct and encapsulated connection between the two
tanks is preferred. This requires a high degree of accuracy in the mechanical dimensions and the need
for experienced field engineers to assemble both units on site. If required, special oil-tight insulation
systems allow the separation of the two tanks without the need to drain the oil in one or both units. In
the latter case, an extra oil-expansion system is needed for the connecting tubes. Figure 2.3.17 shows a
double-tank PST design at a testing site.
2.3.7 Details of On-Load Tap-Changer Application
OLTCs are subject to numerous limits. The most essential limit is, of course, the current-interrupting or
current-breaking capability. In addition to this limit, the voltage per step and the continuous current are
also limited. The product of these two limits is generally higher than the capability limit, so the maximum
voltage per step and the maximum current cannot be utilized at the same time. Table 2.3.1, Table 2.3.2,
and Table 2.3.3 show examples for design power, voltage per step, and system current as functions of
phase-shift angle, throughput power, system voltage, and number of voltage steps.
These limits (e.g., 4000 V/step, 2000 A) determine the type of regulation (the need for only a fine tap
winding or a combination of fine and coarse tap windings) and the number of parallel branches. But it
must be noted that the mutual induction resulting from the use of a coarse/fine-tap winding configuration
also has to be taken into account and may influence the decision.
FIGURE 2.3.17 Two-tank PST at a test site (coolers not assembled) (650 MVA, 60 Hz, 525/52520*1.2˚ kV).
TABLE 2.3.1 Design Power of PSTs as a Function of Throughput Power and Phase-Shift Angle, MVA
Throughput
Power, MVA
Design
Power, MVA
Phase-Shift Angle ˚
10 20 30 40 50
100 17.4 34.7 51.8 68.4 84.5
250 43.6 86.8 129.4 171.0 211.3
500 87.2 173.6 258.8 342.5 422.6
750 130.7 260.5 388.2 513.0 633.9
1000 174.3 347.3 517.6 684.0 845.2
TABLE 2.3.2 Step Voltage as a Function of System Voltage and Phase-Shift Angle for a 16-Step OLTC, V
System
Voltage, kV
Step
Voltage, V
Phase-Shift Angle ˚
Steps:16 10 20 30 40 50
69.0 434 865 1,289 1,703 2,104
115.0 723 1,441 2,148 2,839 3,507
138.0 868 1,729 2,578 3,406 4,209
161.0 1,013 2,018 3,007 3,974 4,910
230.0 1,447 2,882 4,296 5,677 7,015
345.0 2,170 4,324 6,444 8,516 10,522
500.0 3,145 6,266 9,339 12,342 15,250
765.0 4,812 9,587 14,289 18,883 23,332
Another problem that also is not specific to PSTs, but may be more significant, is the possible altering
of the potential of the regulating winding when the changeover selector is operated. In this moment, the
tap selector is positioned at tap “K” (see Figure 2.3.18), and the regulating winding is no longer fixed to
the potential of the excitation winding. The new potential is determined by the ratio of the capacitances
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC