[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

2.3.8.1.2 Series Connection
• The throughput power of a single unit is equal to the total throughput power.
• The phase-shift angle of a single unit is equal to 1/n of the total phase-shift angle.
• The impedance of a single unit in ohms is 1/n of the total impedance.
• The impedance of a single unit in percent, referred to the throughput power, is 1/n of the total
impedance in percent.
• When one unit is lost or bypassed, the throughput power remains constant, and the total phaseshift
angle and the total impedance in ohms and in percent are reduced by (n – 1)/n.
2.3.8.2 Tests
In case of a two-tank design, the PST should be completely assembled with the two units connected
together, as in service.
Auxiliary bushings may be necessary to make inner windings accessible for measurements of resistances,
losses, and temperatures or to allow dielectric tests.
2.3.8.2.1 Special Dielectric Tests
When bypassing of the unit is a specified operating condition, a special lightning impulse and a special
switching impulse has to be performed with both terminals of the series winding connected.
2.3.8.2.2 No-Load Phase-Shift Angle
From Figure 2.3.5, the phase shift angle can be calculated as
(2.3.41)
with V S , V L , and V S–L being the absolute values of the voltages of corresponding source and load terminals
to ground and between them, respectively.
To differentiate between advanced and retard angle, the following criteria can be used:
advanced phase-shift angle:
retard phase-shift angle:
V S1 – V S2 > V S1 – V L2
V S1 – V S2 < V S1 – V L2
The tolerance depends on the accuracy of the voltages. ANSI/IEEE C57.135 recommends that this be
1%. This considers the worst case, when V S and V L are at the upper tolerance limit (1.005) and V S–L is
at the lower limit (0.995).
References
V V V
arccos
2 * V * V
2 2 2
S L S L
S L
ANSI/IEEE, Guide for the Application, Specification and Testing of Phase-Shifting Transformers, ANSI/
IEEE C57.135-2001, Institute of Electrical and Electronics Engineers, Piscataway, NJ, 2001.
Brown, F.B., Lundquist, T.G. et al., The First 525 kV Phase Shifting Transformer — Conception to Service,
presented at 64th Annual Conference of Doble Clients, 1997.
Krämer, A., On-Load Tap-Changers for Power Transformers, Maschinenfabrik Reinhausen, Regensburg,
Germany, 2000.
Seitlinger, W., Phase Shifting Transformers — Discussion of Specific Characteristics, CIGRE Paper 12-
306, CIGRE, Paris, 1998.
2.4 Rectifier Transformers
Sheldon P. Kennedy
Power electronic circuits can convert alternating current (ac) to direct current (dc). These are called
rectifier circuits. Power electronic circuits can also convert direct current to alternating current. These
are called inverter circuits. Both of these circuits are considered to be converters. A transformer that has
one of its windings connected to one of these circuits, as a dedicated transformer, is a converter transformer,
or rectifier transformer. IEC standards refer to these transformers as converter transformers,
while IEEE standards refer to these transformers as rectifier transformers. Because it is IEEE practice to
refer to these transformers as rectifier transformers, that same term is used throughout this discussion.
Transformers connected to circuits with a variety of loads, but which may contain some electronic
circuits that produce harmonics, are not considered to be rectifier transformers. However, they may have
harmonic heating effects similar to rectifier transformers. Those transformers are covered under IEEE
Recommended Practice for Establishing Transformer Capability when Supplying Non-Sinusoidal Load
Currents, ANSI/IEEE C57.110.
Electronic circuits provide many types of control today, and their use is proliferating. These circuits
are generally more efficient than previous types of control, and they are applied in many types of everyday
use. Rectifier circuits are used to provide high-current dc for electrochemical processes like chlorine
production as well as copper and aluminum production. They are also used in variable-speed-drive motor
controls, transit traction applications, mining applications, electric furnace applications, higher-voltage
laboratory-type experiments, high-voltage direct-current power transmission (HVDC), static precipitators,
and others. While HVDC transmission and static precipitators are not directly covered in this
chapter, much of the basic information still applies.
2.4.1 Background and Historical Perspective
Rectifier transformers can be liquid-immersed, dry-type, or cast-coil technology. Dry-type transformers
were primarily used in distribution-voltage classes. Impregnation systems have improved with the development
of vacuum pressure impregnation (VPI) technology. These types of transformers have been
developed to 34-kV and 46-kV classes, although basic-impulse-insulation levels (BIL) are often less than
in liquid-immersed transformers. Cast-coil technology has developed as a more rugged, nonliquid-filled
technology. Both of these types of transformers — dry type and cast coil — are limited by voltage and
kVA size. They have advantages over liquid-filled transformers for fire protection, since they have no
liquids to ignite. However, liquid-immersed transformers can be built to all voltage levels and current
levels. High-fire-point fluids can be used for fire-protection considerations. Auxiliary cooling can be
utilized to cool larger levels of power loss developed in higher-current applications.
The early rectifiers were pool-cathode mercury rectifiers. These had high levels of short-circuit failures
on transformers and suffered from arc-backs. When one phase faulted, all phases would dump through
the faulted phase. So on a six-phase transformer with 10% impedance, instead of 10 times rated current
during a fault, it could develop up to 60 times rated current. Usually the fault would not be this high,
but it could still be in the area of 40 times rated current. This is an extremely high fault current for a
transformer to withstand. Transformers had to be built very ruggedly and were extremely heavy compared
with most transformers, which greatly increased the cost of these systems. They also had the disadvantage
of the environmental problems associated with mercury. These transformers were built to comply with
ANSI/IEEE C57.18-1964, Pool Cathode Mercury-Arc Rectifier Transformer. The latest revision of this
standard was 1964, but it was reaffirmed later.
Semiconductor rectifiers advanced higher in voltage and current capability, and finally semiconductor
technology developed to the point that pool-cathode mercury-arc rectifiers were replaced. Semiconductor
rectifiers also brought the ability of control with thyristors in addition to diodes, without the use of
magnetic devices such as the saturable core reactors and amplistats that had been used for this purpose.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC