A surge can be represented by a current source, modeling as a triangular ramp function or one of the surge models of EMTP. A general grounded or ungrounded source in series with your system is the component used to represent lightnings. What problem you're currently facing?
I am using ATP/EMTP software.
Double exponential or Heidler model are available in the tool .
Usually 30 kA 1,2/50 us for shielding failure and 200 kA 4/250 us for Backflashover.
OHL overhead are modelled with J-marti, cable with fixed frequency and 100 kHz tune frequency. For 4/250 us I noticed 25 kHz is better. Transformer models need windings capacitance to be considered. Tower foot resistance, Tower propagation and caratcteristic impedance (i.e. Zcharacteristic =130 Ohm). Insulation coordination standards has reference for the max overvoltage values. 60071-1 and -2. Let me know if you need something else.
Andrea
As Andrea said in ATP/EMTP software you can use Double-exponential model or Heidler function with standard surge wave shape with 10-20 or 30kA and 1,2/50 us. For swithching transients times are slower.
Srete
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