Professional Development

Question 2

Determine theI2tcurrent rating of 80mm² ‘Grasshopper’ conductor 7/3.9mm AAAC

having a resistance at 20°C of 0.3406

O/km and a weight of 230 kg/km. The initial temperature is

15°C, the allowable maximum temperature is 200°C and the coefficient of electrical resistance with

temperature is 0.0036 per °C.

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Version ESchool of Engineering and Applied Science Continuing Professional Development Module 3 – Overhead Lines Course Co-ordinator Dr. M. SadeghzadehElectricity Distribution Module 3 Version E 1 INDEX1. Advantages and Disadvantages of Overhead Line versus Cable 2. Types of Support 3. Earthed and Unearthed Construction 4. Mechanical Design of Overhead Lines 5. Conductor Spacing – Wind Induced Oscillation 6. Conductor Vibration 7. Conductor Characteristics 8. Conductor Current Rating 9. Conductor Sag and Tension 9.1 Overview 9.2 Design Tension Limit 9.3 Conductor Sag 9.4 Equivalent or Ruling Span 9.5 Variation of Sag with Temperature 9.6 Sag/Tension/Temperature Table 9.7 Sag Templates 9.8 Conductor Clearance 9.9 Conductor Uplift 10. Line Survey 11. Route Selection 12. Profiling 13. Insulators 14. Lightning and Overhead Lines 14.1 Use of Arc (Horn) Gaps 14.2 Use of Non-Linear Resistors (Surge Arresters or Surge Diverters) 14.3 Effects of Lightning on Pole Transformers Appendix A – BS1990 Pole SizesElectricity Distribution Module 3 Version E 2 1. Advantages and Disadvantages of Overhead Line versus Cable Cost -In the UK, Engineering Recommendation P21/5 provides guidance on the comparative cost of overhead lines and cables at various system voltages and is reproduced below. Table 1 U/G Cable versus O/H line System Voltage Ratio 132kV Double circuit on steel lattice tower 8:1 132kV Single circuit on steel lattice tower 7:1 33kV Double circuit on wood poles 7.5:1 33kV Single circuit on wood poles 7.5:1 11kV Single circuit on wood poles 2.3:1 Low voltage on wood poles 1.7:1 These are generalised values which vary with individual projects, due to the following factors: 1. The underground cable route may be longer than the overhead line route because cables are laid in publicly owned roads and footpaths which usually do not follow the most direct route, whereas overhead lines follow a more or less direct route, crossing private land to do so.

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