[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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FIGURE 2.4.21 A 36-pulse system made up of 12 transformers with six types of six-pulse transformers for highcurrent<br />
electrochemical rectifier duty. Each transformer is 11,816-kVA OFWF with 426-V delta secondary windings.<br />
Each transformer has a 34,500-V primary with the shifts shown in Table 2.4.7 for a 36-pulse system. (Photo courtesy<br />
of Niagara <strong>Transformer</strong> Corp.)<br />
FIGURE 2.4.23 A 37,500-kVA OFAF regulating autotransformer with LTC and saturable core reactors and dc power<br />
supplies. The high voltage is dual voltage for 13,800-V wye and 23,000-V wye. The LTC and saturable-core reactors<br />
permit the secondary voltage to range from 14,850-V wye down to 7,150-V wye. The LTC makes coarse taps, while<br />
the saturable-core reactors provide infinite variability between taps. This transformer powers downstream to an<br />
electrochemical service diode rectifier. The transformer weighs over 170,000 lb, as it includes the main transformer,<br />
a series transformer, a preventive autotransformer, and saturable core reactors. The dc power supplies provide control<br />
to the saturable-core reactors. (Photo courtesy of Niagara <strong>Transformer</strong> Corp.)<br />
FIGURE 2.4.22 A 5000-kVA 18-pulse transformer ready for shipment. Motor drive duty, 13,800-V delta primary<br />
to secondaries of 1400-V delta, 1400-V delta +20, and 1400-V delta –20. (Photo courtesy of Niagara <strong>Transformer</strong><br />
Corp.)<br />
2.4.13 Electrostatic Ground Shield<br />
It is usually desirable to have an electrostatic ground shield between the primary and secondary windings.<br />
The electrostatic ground shield provides capacitive decoupling of the primary and secondary windings.<br />
Generally, the winding connected to the rectifier circuit is ungrounded. Without the presence of the<br />
electrostatic ground shield, transients on the primary side transfer to the secondary side of the transformer.<br />
These may be approximately 50% of the magnitude of the primary transient if there are no<br />
grounds in the system. This is high enough to fail secondary windings and core insulation or to cause<br />
rectifier-circuit failures. The other normally considered advantage to the system is the minimization of<br />
high-frequency disturbances to the primary system due to the rectifier.<br />
2.4.14 Load Conditions<br />
Load conditions generally are categorized by the service to which the transformer will be subjected.<br />
ANSI/IEEE C57.18.10 gives the limits of rectifier transformer winding temperatures for defined load<br />
cycles. These limits are the hottest-spot temperature limits of the applicable insulation systems. These<br />
are the same limits that one may have with a standard power transformer that is not subject to loads<br />
rich in harmonics. However, the harmonic losses are to be included in the calculation and test values<br />
used to determine thermal capability for rectifier transformers. The standard service rating classes are<br />
as follows:<br />
1. Electrochemical service<br />
2. Industrial service<br />
3. Light traction or mining service<br />
4. Heavy traction service<br />
5. Extra-heavy traction service<br />
6. User-defined service<br />
User-defined service is a catch-all for load patterns not defined in items 1 through 5.<br />
2.4.15 Interphase <strong>Transformer</strong>s<br />
Interphase transformers help to combine multiple rectifier outputs. They may be external or internal to<br />
the rectifier transformer. The interphase transformer supports ac voltage differences between the rectifier<br />
outputs. They cannot balance steady-state differences in dc voltage, since they only provide support to<br />
ac voltage differences. The interphase-transformer windings carry both ac and dc currents. The windings<br />
are in opposition so as to allow dc current to flow, but this causes opposing ampere-turns on the core.<br />
The core usually has to be gapped for the expected dc current unbalances and to be able to support the<br />
expected magnetizing current from the ripple voltage. Excellent sources on this are Shaefer (1965) and<br />
Paice (1996, 2001) listed in the references below.<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC