Chapter 12. Electrical Bushings

Chapter 12. Electrical Bushings - 12 Electrical Bushings...

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12 Electrical Bushings Loren B. Wagenaar WagenTr ans Consulting, LLC 12.1 Purpose of Electrical Bushings . ...................................... 12 -1 12.2 Ty pes of Bushings . ........................................................... 12 -2 According to Insulating Media on Ends . According to Construction . According to Insulation Inside Bushing 12.3 Bushing Standards. .......................................................... 12 -7 12.4 Important Design Parameters . ....................................... 12 -8 Conductor Size and Material . Insulators . Flange . Oil Reservoir . Clamping System . Temperature Limits 12.5 Other Features of Bushings . ......................................... 12 -12 Voltage Taps . Bushing Current Transformer Pockets . Lower Support = Lower Terminal . Lower-End Shield 12.6 Bushings for Special Applications. ............................... 12 -14 Hig h-Altitude Applications . Highly Contaminated Env ironments . High-Current Bushings within Isolated-Phase Bus Ducts 12.7 Accessories Commonly Used with Bushings . ............. 12 -16 Bushing Potential Dev ice . Upper Test Terminals . Draw-Lead Conductors 12.8 Tests on Bushings . ......................................................... 12 -18 Categories of Tests . Dielectric Tests . Mechanical Tests . Thermal Tests 12.9 Maintenance and Troubleshooting . ............................. 12 -22 Oil Level . Power-Factor = Capacitance Measurements . Damage or Contamination of Air-End Insulator . Improper Installation of Terminals . Misaligned or Broken Voltage Tap Connections . Dissolved-Gas-in-Oil Analysis 12.1 Purpose of Electrical Bushings ANSI = IEEE Std. C57.19.00 [1] defines an electrical bushing as ‘‘an insulating structure, including a through conductor or providing a central passage for such a conductor, with provision for mounting a barrier, conducting or otherwise, for the purpose of insulating the conductor from the barrier and conducting current from one side of the barrier to the other.’’ As a less formal explanation, the purpose of an electrical bushing is simply to transmit electrical power in or out of enclosures, i.e., barriers, of an electrical apparatus such as transformers, circuit breakers, shunt reactors, and power capacitors. The bushing conductor may take the form of a conductor built directly as a part of the bushing or, alternatively, as a separate conductor that is drawn through, usually through the center of, the bushing. Since electrical power is the product of voltage and current, insulation in a bushing must be capable of withstanding the voltage at which it is applied, and its current-carrying conductor must be capable of carrying rated current without overheating the adjacent insulation. For practical reasons, bushings are not rated by the power transmitted through them; rather, they are rated by the maximum voltage and current for which they are designed. ß 2006 by Taylor & Francis Group, LLC.
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12.2 Types of Bushings There are many methods to classify the ty pes of bushings. These classifications are based on practical reasons, which w ill become apparent in the follow ing discussion in three broad areas. Bushings can be classified: 1. According to insulating media on ends 2. According to construction 3. According to insulation inside bushing 12.2.1 According to Insulating Media on Ends One method is to designate the t y pes of insulating media at the ends of the bushing . This classification depends primarily on the final application of the bushing .
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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 12. Electrical Bushings - 12 Electrical Bushings...

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