Q5Power System - I
Question
Q.5. What is the importance of volt-time curves?
Answer
Volt-time curves show the relationship between the peak voltage a piece of insulation can withstand and the time duration for which that voltage is applied before breakdown occurs, and are essential for correctly coordinating the insulation strength of equipment with the characteristics of protective surge arresters.
A volt-time curve plots the breakdown (withstand) voltage of an insulation system against the time duration of the applied voltage impulse, showing that insulation can withstand a higher voltage for a very short duration than it can for a sustained or slowly rising voltage. This characteristic is critical for insulation coordination: it allows engineers to compare the withstand characteristic of equipment insulation against the protective (sparkover/discharge) characteristic of a surge arrester or protective gap over the same range of surge durations, ensuring that the arrester's protective level remains safely below the equipment's withstand curve across the entire relevant time range (not just at a single standard impulse test point), which is essential for reliably protecting equipment against the full variety of lightning and switching surge waveshapes that can occur in service.