Lightning protection schemes

Lightning protection is the set of devices that intercept a lightning stroke or divert the resulting surge safely to earth so that system insulation, equipment and people are not damaged.

Need for lightning protection

• A lightning stroke carries 10–200 kA with very steep front (1–10 μs), producing surges of several MV on lines, far above insulation levels (BIL).
• Without protection it causes insulator flashover, puncture of transformer and machine windings, damage to substation equipment, prolonged outages, fires and danger to life.
• Nepal's hilly terrain has high thunderstorm activity, so lines and substations are frequently hit.

Types of lightning protection schemes

1. Earthing screen (for substations): a network of earthed copper/steel wires stretched over the substation, connected to the earth mat. Direct strokes hit the screen and go to earth. It does not protect against surges coming in along the lines.
2. Overhead ground (shield) wires (for lines): earthed wires above phase conductors, bonded at each tower. They shield conductors against direct strokes within a 30°–45° protective angle; low tower-footing resistance (below about 10 Ω) avoids back flashover.
3. Lightning conductors/masts: vertical rods or masts with good earthing protect buildings and outdoor yards within a protective cone.
4. Lightning arresters (surge diverters): protect equipment from travelling waves that reach terminals. Types:
   • Rod gap: two rods set at a gap; cheap but no follow-current control.
   • Horn gap: arc rises on horn-shaped electrodes and extinguishes itself; with series resistor.
   • Multi-gap: series of small gaps with resistors.
   • Expulsion type: fibre tube; gases blow out the arc; used on distribution lines.
   • Valve type (SiC): series gaps with non-linear silicon carbide resistor blocks.
   • Metal oxide (ZnO) arrester: gapless, highly non-linear; now the standard at all voltages.
5. Surge absorbers: reduce steepness of the incoming wave before it reaches transformer windings.

   Shield wire
  ===========  (earthed at tower)
  |  a  b  c |
  |  |  |  | |
  Tower    Line --+--> Substation
                  |
               [Arrester] --- earth mat

A complete scheme uses shield wires and earthing screens against direct strokes, and arresters at line entrances and at transformer terminals against travelling waves, all connected to a low-resistance earth.