--- title: "Factors affecting choice of BIL" subject: "Transmission and Distribution System Design" chapter: 3 key: "semester_8/transmission_and_distribution_system_design:bil-choice-factors" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/bil-choice-factors site: BCT Engineering --- # Factors affecting choice of BIL Transmission and Distribution System Design · Chapter 3: Overhead Line Insulator Design - Asked in: 2073 Bhadra, 6 marks - 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/bil-choice-factors - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-3 ## Question Explain the factors affecting the choice of BIL of a transmission line for insulator discs selections. ## Short answer BIL depends on system voltage, lightning activity, shielding/footing resistance, switching surges (at EHV), arrester protective level and neutral earthing (effective earthing enables reduced BIL), altitude, and pollution. Higher BIL needs more discs, balancing reliability against cost. ## Answer The **basic insulation level (BIL)** is the crest value of the standard 1.2/50 µs lightning impulse voltage that the insulation must withstand without flashover. It sets the minimum impulse flashover voltage of the insulator string, and so it largely sets the number of discs. The main factors are: 1. **System voltage:** the BIL rises with the nominal and maximum system voltage. Standard values (IEC/IS) are, for example, 550 kV for 132 kV, 900–1050 kV for 220 kV and 1425–1550 kV for 400 kV. 2. **Lightning performance required (isokeraunic level):** in areas with many thunderstorm days, more lightning surges reach the line. A higher BIL, or better shielding, is chosen to keep tripouts within the target. 3. **Shielding and tower footing resistance:** the earth wire, its shielding angle and the footing resistance decide the back-flashover voltage across the string. With high footing resistance (rocky hills), a higher BIL is needed. 4. **Switching surges at EHV:** above about 300 kV, switching surges (SSR × peak phase voltage) may need more insulation than lightning does. The insulation is then coordinated for switching (BSL), and the BIL follows from it. 5. **Surge arresters and protective level:** arresters at line ends limit the surge. The BIL must keep a margin of about 15–25% above the arrester's protective level (insulation coordination). 6. **Earthing of the neutral:** effectively earthed systems have lower temporary overvoltage, so a reduced BIL can be used. Insulated or resonant-earthed systems need full BIL. 7. **Altitude and atmosphere:** low air density at high altitude (as in Nepal's hills) lowers flashover voltage. The required BIL is corrected by the air-density factor $\delta$. 8. **Pollution and rain:** contaminated or wet conditions lower the flashover voltage, so more discs or a higher margin are needed. 9. **Economics and reliability:** a higher BIL means more discs, longer strings, larger towers and more cost. It is balanced against the cost of outages. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is BIL? A. The crest value of the standard 1.2/50 us lightning impulse the insulation must withstand without flashover B. The rms power-frequency withstand for 1 minute C. The switching surge ratio D. The maximum current of the line Answer: A. BIL sets the minimum impulse flashover of the string and so largely the number of discs. (Key point: BIL definition) 2. How does high tower footing resistance (rocky hills) affect BIL choice? A. It has no effect B. It lowers the BIL required C. It replaces the need for earth wires D. It increases back-flashover risk, so a higher BIL is needed Answer: D. High footing resistance raises the voltage across the string during a stroke. (Key point: Footing resistance) 3. How is BIL related to the surge arrester? A. BIL is set below the protective level B. BIL keeps a margin of about 15-25% above the arrester protective level C. BIL equals the system voltage D. Arresters do not affect BIL Answer: B. Insulation coordination demands margin above the protective level. (Key point: Arrester margin) 4. Why is BIL corrected for altitude? A. Altitude affects only the tower weight B. High altitude raises flashover voltage C. Altitude changes conductor material D. Low air density reduces flashover voltage, so a correction by air-density factor is applied Answer: D. Thinner air at altitude lowers breakdown strength. 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