Overvoltage is any voltage, whose peak value exceeds the appropriate peak value of the highest operating voltage in the LV power system. Overvoltage is usually an accidental phenomenon, which differs in time history and the place of its occurrence. Its parameters are defined by its cause (lightning stroke, switching in heavy-current network and so on) and also by electrical character of the circuit (wave resistance, ending impedance, discharge ability and so on). In the past few years the range of current and voltage courses for different uses has been standardized. These courses enable implementation of testing on equipment and constructive elements under the same conditions.
In the following text the most important parameters of the most used standardized courses will be defined (according to EN 61 643-11, IEC 60-1 and CSN 34 5640).
Peak value (amplitude) Umax , Imax
– peak value is the maximal value of voltage or current, which is achieved by monitored impulse course.
Front of impulse
– a part of voltage or current impulse before the peak value.
Front time of current impulse T1
– 1,25multiple of the time interval between moments, when actual current value rise from 10% to 90% of the peak value.
Front time of voltage impulse T1
– 1,67multiple of the time interval between moments, when actual voltage value rise from 30% to 90% of the peak value.
Tail of impulse
– a part of voltage or current impulse after the peak value.
Time to half-value T2
– the time interval between virtual beginning of impulse and the moment, when observed course reduces to 50% of its peak value.
Note: The virtual beginning is an intersection of time axis and bisector, which goes through points, where actual value of the front of impulse at first time reaches partly given lower value and partly given higher value …in detail see the following two figures.
Standardized testing current impulse
– two basic types of testing current impulses are used during SPD tests:
a) testing impulse of lightning current Iimp(10/350) – it is used for simulation of lightning current (so-called test by lightning current)
b) testing current impulse Imax(8/20) – it is used for simulation of indirect effect of lightning and switching overvoltages Arrester must discharge cca 17,5x higher charge during test by the testing impulse of lightning current Iimp(10/350), than during testing by the current impulse Imax(8/20) with the same amplitude.
Also resulting in a different construction of the lightning current arresters tested by the lightning current impulse Iimp(10/350) and surge arresters tested by the current impulse Imax(8/20).