--- title: "Electrical considerations in conductor selection" subject: "Transmission and Distribution System Design" chapter: 4 key: "semester_8/transmission_and_distribution_system_design:conductor-electrical-considerations" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/conductor-electrical-considerations site: BCT Engineering --- # Electrical considerations in conductor selection Transmission and Distribution System Design · Chapter 4: Conductor and Support Selection - Asked in: 2071 Magh, 8 marks; 2068 Bhadra (old course), 6 marks; 2067 Mangsir (old course), 8 marks (asked 3 times) - Video: [Choice of Conductor Materials (L7 PTC)](https://www.youtube.com/watch?v=yJlD6CMP1lE) — Dr. Chakradhar Reddy (Dr. C C Reddy) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/conductor-electrical-considerations - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-4 ## Question Explain the electrical considerations required to be made while selecting a conductor for a high voltage transmission line. ## Short answer Key considerations: sufficient ampacity under maximum load and ambient conditions without exceeding temperature limit; low I²R losses for efficiency; voltage regulation within allowable drop; minimum diameter or bundling to prevent corona; stability limit for long lines; ac resistance from skin effect; short-circuit withstand; and radio/audible noise compliance. ## Answer A conductor for an HV line must carry the required power within safe temperature, loss, voltage-drop and corona limits. The electrical considerations are: 1. **Current-carrying capacity (thermal limit / ampacity):** the conductor must carry the maximum load current, including contingency and overload, without exceeding its maximum allowable temperature (about 75–85 °C for ACSR). Above that temperature the conductor anneals and loses strength, and its sag increases. Ampacity depends on ambient temperature, wind speed, solar radiation and emissivity: $I^2R = $ heat lost by convection and radiation minus solar heat gained. 2. **Line losses and efficiency:** $I^2R$ loss falls as the conductor area increases. Efficiency should usually be at least 94–95%. The economic choice balances the cost of losses against the capital cost (Kelvin's law and present-worth comparison). 3. **Voltage regulation:** the drop $IR\cos\phi + IX\sin\phi$ must keep regulation within limits (about 10–12% for transmission). $R$ falls with a larger area, while $X$ falls only slightly with diameter and more with bundling. 4. **Corona:** the conductor surface gradient must stay below the corona inception gradient in fair weather. Corona causes loss, radio and TV interference and audible noise. It sets a minimum conductor diameter (roughly 1.5 cm at 132 kV and 2–2.5 cm at 220 kV, depending on spacing and altitude), or bundled conductors at 400 kV and above. Check using Peek's formula $V_d = 21.1\, m\,\delta\, r \ln(D/r)$ kV. 5. **Stability and power-transfer limit:** a lower reactance (larger or bundled conductor) raises $P_{max} = V_sV_r/X$ and the SIL, which matters for long lines. 6. **Skin and proximity effect:** AC resistance is higher than DC resistance. ACSR places the aluminium outside the steel core, which suits the skin effect. 7. **Short-circuit rating:** the conductor must withstand the fault current for the fault duration without damage. 8. **Radio interference and audible noise limits:** these set the minimum diameter and the bundle design at EHV. 9. **Resistivity of material:** aluminium (ACSR or AAAC) is chosen over copper for lower cost and weight at equal conductance. The final conductor is the cheapest one that satisfies all these electrical checks together with the mechanical checks (tension, sag, vibration). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why must the conductor temperature be kept below its maximum allowable value (about 75-85 °C for ACSR)? A. Above it the conductor anneals, loses strength and sags more B. Above it the corona inception voltage becomes zero C. Above it the skin effect disappears D. Above it the resistivity of aluminium becomes negative Answer: A. Overheating anneals the metal, reduces tensile strength and increases sag, so ampacity is limited by temperature. (Key point: Thermal limit) 2. How is the economic conductor area chosen with respect to line losses? A. The largest available area is always chosen B. The smallest area that carries the current is always chosen C. Cost of I^2R losses is balanced against the capital cost (Kelvin's law, present worth) D. The area is chosen so that losses are exactly zero Answer: C. Larger area lowers I^2R loss but raises capital cost, so the two are balanced. (Key point: Economic size vs losses) 3. Which conductor choice is used to meet the corona limit at 400 kV and above? A. A smaller diameter single conductor B. Bundled conductors C. Copper instead of aluminium D. A lower spacing between phases Answer: B. Bundling gives a large equivalent radius, lowering the surface gradient below the inception gradient. (Key point: Corona and diameter) 4. How does a larger or bundled conductor help the power-transfer limit of a long line? A. It raises resistance and so damps swings B. It lowers reactance X, raising Pmax = VsVr/X and the SIL C. It increases the line length D. It removes the need for earth wires Answer: B. Lower series reactance raises the stability-limited power Pmax. 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