Volt-time curve significance and surge arrester

Significance of the volt–time characteristic in insulation coordination

The volt–time (V–t) curve of an insulation or a protective device plots the peak voltage at breakdown (flashover) against the time to breakdown, for impulses of a standard shape but increasing amplitude. With a steeper or higher impulse, breakdown happens sooner and at a higher voltage, so the curve rises at short times.

 V |\
   | \  equipment insulation (upper)
   |  \___________________
   |\
   | \__ protective device (lower)
   |    \_________________
   +-------------------------> t (us)

Significance:

1. Basis of coordination. The V–t curve of the protective device (arrester, rod gap) must lie below that of the protected insulation at all times, with a margin of about 15–25%. Then the device always operates first.
2. Short-time behaviour. For very steep fronts, a device with a rising V–t curve (such as a rod gap) may operate too late. The curves show whether it gives protection for steep waves.
3. Selection of BIL and arrester rating. Comparing the curves fixes the transformer BIL, the arrester protective level and the protective ratio.
4. Coordination of different insulations in a substation (bushings, line insulators, breakers) so that flashover, if any, happens at a harmless place.

General working principle of a surge arrester

A surge arrester (lightning arrester) is a non-linear device connected between line and earth near the equipment.

• Normal voltage: the arrester has a very high resistance (series gap open, or a metal-oxide block at low current). Only a tiny leakage current flows.
• Surge arrives: when the voltage exceeds the spark-over (or reference) level, the gap sparks over or the ZnO blocks become highly conductive. The surge current is diverted to earth.
• Clamping: because of the non-linear characteristic $V = kI^{\beta}$ (with $\beta \approx 0.02$–$0.05$ for ZnO), the voltage across the arrester (the residual voltage) stays almost constant even for large currents. The equipment sees only this clamped voltage.
• Resealing: after the surge, the voltage returns to normal. The resistance becomes high again and the power-follow current is interrupted (by the gap in gapped types), so the arrester is ready for the next surge.

 line ----+---- to equipment
          |
        [ZnO]  non-linear
          |
        earth