Chapter4-1.pdf

Module 4 Chapter 4.1: Transmission Line Design Consideration All Rights Reserved 1 Transmission Line Design Consideration • Goals of transmission line design are: 1. 2. To develop a simple model for transmission lines To gain an intuitive feel for how the geometry of the transmission line affect...

Module 4 Chapter 4.1: Transmission Line Design Consideration All Rights Reserved 1 Transmission Line Design Consideration • Goals of transmission line design are: 1. 2. To develop a simple model for transmission lines To gain an intuitive feel for how the geometry of the transmission line affects the model parameters • The most common methods for transfer of electric power are • Overhead (AC or DC) • Underground (AC or DC) • An overhead transmission line consists of: • • • • Conductors Insulators support structures shield wires (most cases). 2 Common Conductor • Aluminum has replaced copper as the most common conductor metal for overhead transmission because:  lower cost  lighter weight  more supply of aluminum available • Common Conductor: • ACSR: Aluminum Conductor Steel-Reinforced (ACSR) • one of the most common conductor types • consists of layers of aluminum strands surrounding a central core of steel strands. • high strength-to-weight ratio • No insulating cover (for purposes of heat dissipation 3 Other Conductors • AAC - all-aluminum conductor • AAAC - all- aluminum-alloy conductor • ACAR - aluminum conductor alloy-reinforced • Alumoweld - aluminum-clad steel conductor Higher- temperature conductors capable of operation in excess of 150o C: • ACSS - aluminum conductor steel supported Composite materials: • ACFR - aluminum conductor carbon fiber reinforced (ACFR) • ACCR - aluminum fiber conductor composite reinforced 4 Insulators Insulators for transmission lines above 69 kV are typically suspension-type insulators, which consist of a string of discs constructed porcelain, toughened glass, or polymer. The standard disc has a 10-in. diameter, 53-in. spacing between centers of adjacent discs, and a mechanical strength of 7500 kg. 5 Support Structure Transmission lines employ a variety of support structures. • a self-supporting, lattice steel tower typically used for 500- and 765-kV lines. • Double-circuit 345-kV lines usually have self-supporting steel towers with the phases arranged either in a triangular configuration to reduce tower height or in a vertical configuration to reduce tower width • Wood frame configurations are commonly used for voltages of 345 kV and below 6 Shield Wires • Shield wires located above the phase conductors protect the phase conductors against lightning. They are usually high- or extra-high-strength steel, Alumoweld, or ACSR with much smaller cross section than the phase conductors. • The number and location of the shield wires are selected so that almost all lightning strokes terminate on the shield wires rather than on the phase conductors. • Figures have two shield wires. Shield wires are grounded to the tower. As such, when lightning strikes a shield wire, it flows harmlessly to ground, provided the tower impedance and tower footing resistance are small. 7 Electrical Factors • Type, size, and number of bundle conductors per phase • Number of insulator discs • Vertical or V-shaped string arrangement • Phase-to-phase clearance • Phase-to-tower clearance • Number, type, & location of shield wire • Conductor spacing, types, and sizes also determine the series impedance and shunt admittance 8 Other Factors • Mechanical Factors: o Strength of conductors, insulators and support structure • Environmental Factors: o Land usage o Visual impact o Biological effects • Economic Factors: o Lowest overall cost 9