The procedure for insulation coordination consists of
a) Determination of voltage stresses
b) Selection of the insulation strength to achieve the desired probability of failure
The voltage stresses can be reduced by the application of surge-protective devices, switching device insertion resistors and controlled closing, shield wires, improved grounding, etc.
Determination of the system voltage stress
System transient analyses that include the selection and location of the overvoltage limiting devices are performed to determine the amplitude, waveshape, and duration of system voltage stresses.
The overvoltage stress may be characterized either by
— The maximum crest values, or
— A statistical distribution of crest values, or
— A statistical overvoltage value [this is an overvoltage generated by a specific event on the system (lightning discharge, line energization, reclosing, etc.), with a crest value that has a 2% probability of being exceeded].
The results of the transient analysis should provide voltage stresses for the following classes of overvoltage:
— Temporary overvoltage (phase-to-ground and phase-to-phase)
— Switching overvoltage (phase-to-ground and phase-to-phase)
— Lightning overvoltage (phase-to-ground and phase-to-phase)
— Longitudinal overvoltage (an instantaneous combination of switching or lightning surge and a power-frequency voltage)
Comparison of overvoltages with insulation strength
To compare the overvoltages with the insulation strength, the insulation strength must be modified because of the (1) nonstandard waveshape of overvoltages and (2) nonstandard atmospheric conditions. The dielectric strength of insulation for surges having nonstandard waveshapes is assessed by comparison to the dielectric strength as provided by standard chopped wave tests.
The rules for the atmospheric correction of withstand voltages for external insulations are specified in IEEE Std 4-1995. For insulation coordination purposes, wet conditions are assumed and only the relative air density corresponding to the altitude needs to be taken into account.
In addition, a safety margin may be necessary based on consideration of
— Statistical nature of the test results
— Factory or field assembly of equipment
— Aging of insulation
— Accuracy of analysis
— Other unknown factors
The overall protective margin is derived from experience and further described in IEEE P1313.2