[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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FIGURE 3.3.10 LTC with delta connected windings.<br />
FIGURE 3.3.12 Phase-shifting transformer — direct circuit arrangement.<br />
FIGURE 3.3.11 LTC in autotransformers.<br />
• For regulation of low voltage U 2 the circuits in Figure 3.3.11b,d,e,f are applicable. The arrangements<br />
in Figure 3.3.11e and 3.3.11f are two core solutions. Circuit Figure 3.3.11f is operating with<br />
variable flux in the series transformer, but it has the advantage that a neutral-end LTC can be<br />
used. In case of arrangement according to Figure 3.3.11e, main and regulating transformers are<br />
often placed in separate tanks to reduce transport weight. At the same time, this solution allows<br />
some degree of phase shifting by changing the excitation connections within the intermediate<br />
circuit.<br />
3.3.3 Phase-Shifting <strong>Transformer</strong>s (PST)<br />
In recent years, the importance of PSTs used to control the power flow on transmission lines in meshed<br />
networks has steadily been increasing (Krämer and Ruff, 1998). The fact that IEEE has prepared a “Guide<br />
for the Application, Specification and Testing of Phase-Shifting <strong>Transformer</strong>s” proves the demand for<br />
PSTs (IEEE C57.135-2001). These transformers often require regulating ranges that exceed those normally<br />
used. To reach such regulating ranges, special circuit arrangements are necessary. Two examples<br />
are given in Figs. 3.3.12 and 3.3.13. Figure 3.3.12 shows a circuit with direct line-end regulation (singlecore<br />
design). Figure 3.3.13 shows an intermediate circuit arrangement (two-core design).<br />
Figure 3.3.12 illustrates very clearly how the phase-angle between the voltages of the source- and loadsystem<br />
can be varied by the LTC position. Various other circuit arrangements have been realized.<br />
The number of LTC operations of PSTs is much higher than that of other regulating transformers in<br />
networks (10 to 15 times higher). In some cases, according to regulating ranges — especially for line-end<br />
FIGURE 3.3.13 Phase-shifting transformer — intermediate circuit arrangement.<br />
arrangements (Figure 3.3.12) — the transient overvoltage stresses over tapping ranges have to be limited<br />
by the application of non-linear resistors. Furthermore, the short-circuit-current ability of the LTC must<br />
be checked, as the short-circuit power of the network determines the said current. The remaining features<br />
of LTCs for such transformers can be selected according to the usual rules (see Section 3.3.5 entitled<br />
“Selection of Load Tap Changers”).<br />
Significant benefits resulting from the use of a PST are:<br />
• Reduction of overall system losses by elimination of circulating currents<br />
• Improvement of circuit capability by proper load management<br />
• Improvement of circuit power factor<br />
• Control of power flow to meet contractual requirements<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC