- for the very-fast-front overvoltage: the characteristics for this class of overvoltage
are specified by the relevant apparatus committees.
- for the slow-front phase-to-phase overvoltage: a standard combined switching
impulse with peak value equal to the peak value of the assumed maximum of the
slow-front phase-to-phase overvoltages.
NOTE 2 - An useful characteristic is the actual ratio, a, in service of the peak value of the negative
component, U-, to the peak value, U+ + U-, of the assumed maximum phase-to-phase overvoltage:
a = U-/( U+ + U-).
- for the slow-front [or fast-front] longitudinal overvoltage: a combined voltage
consisting of a standard switching [or lightning] impulse and of a power-frequency
voltage, each with peak value equal to the two relevant assumed maximum peak
values, and with the instant of impulse peak coinciding with the peak of the
power-frequency of opposite polarity.
4.3 Determination of the co-ordination withstand voltages (Ucw)
The determination of the co-ordination withstand voltages consists of determining the
lowest values of the withstand voltages of the insulation meeting the performance criterion
when subjected to the representative overvoltages under service conditions.
The co-ordination withstand voltages of the insulation have the shape of the
representative overvoltages of the relevant class and their values are obtained by
multiplying the values of the representative overvoltages by a co-ordination factor. The
value of the co-ordination factor depends on the accuracy of the evaluation of the
representative overvoltages and on an empirical, or on a statistical, appraisal of the
distribution of the overvoltages and of the insulation characteristics.
The co-ordination withstand voltages can be determined as either conventional assumed
withstand voltages or statistical withstand voltages. This affects the determination
procedure and the values of the co-ordination factor.
Simulations of overvoltage events combined with the simultaneous evaluationo f the risk of
failure, using the relevant insulation characteristics, permit the direct determination of the
statistical co-ordination withstand voltages without the intermediate step of determining
the representative overvoltages.
4.4 Determination of the required withstand voltages (Urw)
The determination of the required withstand voltages of the insulation consists of
converting the co-ordination withstand voltages to appropriate standard test conditions.
This is accomplished by multiplying the coordination withstand voltages by factors which
compensate for the differences between the actual in-service conditions of the insulation
and those in the standard withstand tests.
The factors to be applied shall compensate for:
- the differences in the equipment assembly;
- the dispersion in the product quality;
- the quality of installation;
- the ageing of the insulation during the expected lifetime:
- other unknown influences.