Chapter 5. Rectifier Transformers

Chapter 5. Rectifier Transformers - 5 Rectier Transformers...

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5 Rectifier Transformers Sheldon P. Kennedy Niagara Transformer Corporation 5.1 Background and Historical Perspective. .......................... 5 -2 5.2 New Terminology and Definitions . ................................. 5 -3 Fundamental kVA . Harmonic Loss Factor 5.3 Rectifier Circuits. ............................................................... 5 -4 5.4 Commutating Impedance. ................................................ 5 -7 5.5 Secondary Coupling. ......................................................... 5 -7 5.6 Generation of Harmonics. .............................................. 5 -10 5.7 Harmonic Spectrum. ....................................................... 5 -11 5.8 Effects of Harmonic Currents on Transformers. .......... 5 -13 5.9 Thermal Tests. .................................................................. 5 -16 5.10 Harmonic Cancellation. .................................................. 5 -16 5.11 DC Current Content. ...................................................... 5 -18 5.12 Transformers Energized from a Converter = Inverter . ... 5 -19 5.13 Electrostatic Ground Shield. ........................................... 5 -20 5.14 Load Conditions. ............................................................. 5 -21 5.15 Interphase Transformers. ................................................ 5 -21 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 transform- ers, 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. ß 2006 by Taylor & Francis Group, LLC.
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5.1 Background and Historical Perspective Rectifier transformers can be liquid-immersed, dry-type, or cast-coil technology. Dry-type trans- formers were primarily used in distribution-voltage classes. Impregnation systems have improved with the development of vacuum pressure impregnation (VPI) technology. These types of trans- formers 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
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Chapter 5. Rectifier Transformers - 5 Rectier Transformers...

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