Chapter 2. Power Transformers - 2 Power Transformers 2.1...

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2 Power Transformers H. Jin Sim Waukesha Electric Systems Scott H. Digby City of Wilson, NC 2.1 Introduction. ........................................................................ 2 -1 2.2 Rating and Classifications. .................................................. 2 -2 Rating . Insulation Classes . Cooling Classes 2.3 Short-Circuit Duty. ............................................................. 2 -5 2.4 Efficiency, Losses, and Regulation . .................................... 2 -6 Efficiency . Losses . Economic Evaluation of Losses . Regulation 2.5 Construction. ....................................................................... 2 -8 Core . Windings . Taps-Turns Ratio Adjustment 2.6 Accessory Equipment. ....................................................... 2 -17 Accessories . Liquid-Preser vation Systems 2.7 Inrush Current. .................................................................. 2 -20 2.8 Transformers Connected Directly to Generators . .......... 2 -20 2.9 Modern and Future Developments. ................................. 2 -22 High-Voltage Generator . Hig h-Temperature Superconducting (HTS) Transformer 2.1 Introduction ANSI = IEEE defines a transformer as a static electrical device, involving no continuously moving parts, used in electric power systems to transfer power between circuits through the use of electromagnetic induction. The term power transformer is used to refer to those transformers used between the generator and the distribution circuits, and these are usually rated at 500 kVA and above. Power systems typically consist of a large number of generation locations, distribution points, and interconnections within the system or with nearby systems, such as a neighboring utility. The complexity of the system leads to a variety of transmission and distribution voltages. Power transformers must be used at each of these points where there is a transition between voltage levels. Power transformers are selected based on the application, with the emphasis toward custom design being more apparent the larger the unit. Power transformers are available for step-up operation, primarily used at the generator and referred to as generator step-up (GSU) transformers, and for step-down operation, mainly used to feed distribution circuits. Power transformers are available as single-phase or three-phase apparatus. The construction of a transformer depends upon the application. Transformers intended for indoor use are primarily of the dry type but can also be liquid immersed. For outdoor use, transformers are usually liquid immersed. This section focuses on the outdoor, liquid-immersed transformers, such as those shown in Figure 2.1 . ß 2006 by Taylor & Francis Group, LLC.
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2.2 Rating and Classifications 2.2.1 Rating In the U.S., transformers are rated based on the power output they are capable of delivering continu- ously at a specified rated voltage and frequency under ‘‘usual’’ operating conditions without exceeding prescribed internal temperature limitations. Insulation is known to deteriorate with increases in temperature, so the insulation chosen for use in transformers is based on how long it can be expected to last by limiting the operating temperature. The temperature that insulation is allowed to reach under operating conditions essentially determines the output rating of the transformer, called the kVA rating.
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This note was uploaded on 10/19/2010 for the course ENGINEERIN ELEC121 taught by Professor Tang during the Spring '10 term at University of Liverpool.

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Chapter 2. Power Transformers - 2 Power Transformers 2.1...

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