Generation of Impulse Voltages
- Transient overvoltages due to lightning and switching surges cause steep build-up of voltage on transmission lines and other electrical apparatus .
- It is therefore necessary to generate these kind of transient overvoltages in laboratories so as to check our protective systems. These transient overvoltages are of impulse type.
- Impulse voltage is a large voltage generated within a very small time period.
- A typical impulse voltage waveform is shown in Figure.
- 10 % to 90 % time period is called as front time or rise time. The wavefront time of an impulse wave is the time taken by the wave to reach its maximum value starting from zero value.
- T2 is called as the fall time or tail time.
- The tolerance allowed in the rise and tail time are + -30 % and +/- 20 % respectively.
- The impulse wave is represented mathematically as
V = Vo[exp(-αt) – exp ( –βt)] …(1)
- It is shown that lighting overvoltages can also be represented by this expression.
- According to Indian Standard specification the impulse voltage is defined as 1.2 / 50 µsec and 1000 kV.
- Thus 1.2 / 50 µs , 1000 kV wave represents an impulse voltage wave with a front time of 1.2 µs, fall time to 50 % peak value of 50 µs and peak value of 1000 kV. The wave front time is specified as 1.25 times (t2 – t1)
where t1 – Time to reach 10 % of peak, value.
t2 – Time for the wave to reach to its 90 % of peak value.
The tolerance allowed in the peak value is +/- 3 % .