tailieunhanh - power systems analysis phần 5

Khóa học này là sự tiếp nối của lươn 3216, cung cấp cho sinh viên với một kiến thức làm việc của các vấn đề hệ thống điện và kỹ thuật máy tính được sử dụng để giải quyết một số vấn đề. Các công thức trên trang web này có thể được sử dụng trong các hệ thống điện thiết kế điều chỉnh yếu tố và các ngân hàng lọc hài hòa. | 104 4. TRANSMISSION LINE PARAMETERS The selection of an economical voltage level for the transmission line IS based on the amount of power and the distance of transmission. The voltage choice together with the selection of conductor size is mainly a process of weighing RR2 losses audible noise and radio interference level against fixed charges on the investment. Standard transmission voltages are established m the United States by the American National Standards Institute ANSI . Transmission voltage lines operating at more than 60 kV are standardized at 69 kV 115 kV 138 kV 161 kV 230 kV 345 kv 500 kV 765 kv line-to-line. Transmission voltages above 230 kV are usually referred to as extra-high voltage EHV and those at 765 kV and above are referred to as ultra-high voltage UHV . The most commonly used conductor materials for high voltage transmission lines are ACSR aluminum conductor steel-reinforced AAC all-aluminum conductor AAAC all-alummum alloy conductor and ACAR aluminum conductor alloy-reinforced . The reason for their popularity IS their low relative cost and high strength-to-weight ratio as compared to copper conductors. Also aluminum IS in abundant supply while copper IS limned m quantity. A table of the most commonly used ACSR conductors IS stored in lie Characteristics of other conductors can be found in conductor handbooks or manufacturer s literature. The conductors are stranded to have flexibility. The ACSR conductor consists of a center core of Steel strands surrounded by layers of aluminum as shown in Figure . Each layer of strands IS spiraled in the opposite direction of its adjacent layer. This spiraling holds the strands in place. FIGURE Cross-sectional view of a 24 7 ACSR conductor. Conductor manufacturers provide the characteristics of the standard conductors with conductor sizes expressed m Circular mils cmil . One mil equals inch and for a solid round conductor the area m circular mils IS defined as the square of diameter m