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