[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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