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