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WEEK 06: TRANSMISSION: OVERHEAD AC Sections: Voltage | Conductor | Environment | Line | Mechanical | Support | Accessories | Foundation Definition A transmission system include all land, conversion structures and equipment at a primary source of supply; lines, switching and conversion stations between a generating plant or receiving point and the entrance to a distribution center or wholesale point, all lines and equipment whose primary purpose is to augment, integrate or tie together sources of power supply. The fundamental purpose of the electric utility transmission system is to transmit from generating units to the distribution system which ultimately supplies the loads. This objective is served by transmission lines that connect the generators into the transmission network, interconnect various areas of the transmission network, interconnect one electric utility with another, or deliver the electrical power from various areas within the transmission network to the distribution substations. Transmission system design is the selection of the necessary lines and equipment which will deliver the required power and quality of service for the lowest overall average cost over the service life. The system must also be capable of expansion with minimum changes to existing facilities. Methods There are two general ways of transmitting electric current -- overhead and underground. Overhead transmission of electric power remains one of the most important elements in electric power system. Underground cable systems are used for electric-energy transport where overhead construction is impracticable, unsafe, costly, or environmentally unacceptable. The principal applications have been in urban areas where overhead rights-of way are unavailable or prohibitively costly, or where local ordinances mandate undergrounding. There are also many spot locations where undergrounding maybe practical for reasons of safety, security, reliability, or aesthetics [e.g., CCP complex]; these include airport approaches [e.g., NAIA], station and substation exits, long water crossings [e.g. Negros to Cebu], and areas of unusual scenic value or with extreme vulnerability to damage by natural forces or vandalism. Components There are six basic devices of power transmission: transformers, circuit breakers, circuit switchers, voltage regulators, power capacitors, and fuses. Towers and frames form part of transmission components, which also include conductors, insulations, etc. Design Considerations Electrical design of ac systems include (1) power flow requirements, (2) system stability and dynamic performance, (3) selection of voltage level, (4) voltage and reactive power flow control, (5) conductor selection, (6) losses, (7) corona-related performance (radio, audible, and television noise), (8) electromagnetic field effects, (9) insulation and overvoltage design, (10) switching arrangements, (11) circuit breaker duties, and (12) protective relaying. Mechanical design includes (1) sag and tension calculations, (2) conductor composition, (3) conductor spacing, (4) types of insulators, and (5) selection of conductor hardware. Structural design includes (1) selection of the type of structure to be used, (2) mechanical loading calculations, (3) foundations, and (4) guys and anchors. Miscellaneous features of transmission line design are (1) line location, (2) acquisition of right of way, (3) profiling, (4) locating structures, (5) inductive coordination (considers line location and electrical calculations), (6) means of communication, and (7) seismic factors Sections: Voltage | Conductor | Environment | Line | Mechanical | Support | Accessories | Foundation
Standard transmission voltages are established by the NPC. Usually, worldwide, there is no clear delineation between distribution, subtransmission, and transmission levels. The following shows the standard voltages listed in ANSI Standard C84 and C92.2. The nominal system voltages of 345, 500, and 765 kV are classed as extra-high voltages (EHV), these are extensively used in the U.S. In addition, 400-kV EHV transmission is used, principally in Europe. One voltage level above 800 kV, namely 1100 kV nominal (1200 kV maximum) is presently standardized. | ||||||||||||||||||||||||||||
