Volt-time curve and how it is plotted

Volt-time curve

A volt-time (V–t) curve of an insulation or a gap is the graph of the breakdown (flashover) voltage against the time to breakdown for impulse voltages of a given wave shape (normally 1.2/50 μs). Insulation does not break down instantly; for very fast impulses it withstands a higher voltage, so the curve rises steeply at short times and flattens at longer times.

In insulation coordination the V–t curve of the protective device (arrester or rod gap) must lie below the V–t curve of the protected equipment over the whole time range, with a safety margin of about 20–25%. Then the protective device always flashes over first.

 V |
   | \  equipment (transformer)
   |  \___
   |      \______________
   |  \                   margin
   |   \__  arrester
   |      \______________
   +-------------------------- t
     2 us              20 us

Step-wise plotting of the V–t curve

1. Set up the test: Connect the test object (gap, insulator or equipment) to an impulse generator. Measure voltage with a CRO and a voltage divider.
2. Fix the wave shape: Use a standard impulse, e.g. 1.2/50 μs, and keep the same shape for all shots; only the peak value is changed.
3. Start low: Apply an impulse below the expected flashover level. No breakdown occurs.
4. Increase the peak: Raise the peak in steps until flashover occurs on the tail of the wave (just above the critical flashover voltage). Record the time from the start of the wave to flashover.
5. Plot the point correctly:
   • If flashover occurs on the tail, plot the peak voltage of the applied wave against the time of flashover.
   • If flashover occurs on the front, plot the actual voltage at the instant of flashover against that time.
6. Repeat at higher voltages: Increase the applied peak further. Flashover occurs earlier, moving onto the front of the wave. Record voltage and time for each shot.
7. Repeat shots: Because breakdown is statistical, apply several impulses at each level and use mean values.
8. Join the points: Draw a smooth curve through the plotted points. This is the V–t characteristic.

 V |      x  flashover on front
   |     /  .   (plot V at instant)
   |    /     .
   |   /   o    .   o = peak, flashover
   |  /         ..   on tail (plot peak
   | /    applied     at time of FO)
   |/     waves
   +------------------------- t

Use in insulation coordination

• It shows whether the arrester or gap protects the equipment for both steep (fast) and slow surges.
• It is used to select the arrester and the BIL of equipment with proper margins.
• Two curves that cross mean protection fails over part of the time range, so devices are chosen whose curves do not cross.