Factors affecting choice of BIL

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.