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Use load shedding relays. Check the "Model library" manual for the relay model LDSHxx. As an example: dyr entry to drop a fraction of total system load for a 60 Hz system, three load shedding stages of 5%, 10%, 10% load shedding relative to initial load:

//[UFLS Models]
// I, 'LDSHxx', LID,  f1  , t1,   frac1,  f2,   t2,   frac2,   f3,   t3,   frac3,   Tb 
   0, 'LDSHAL',   * , 59.3, 0.15, 0.05,   58.9, 0.1,  0.1,     58.5, 0.0,  0.1,     0.08 /

Using a single stage run, a fraction value would represent a fixed percentage of total load to drop whenever the trigger point (set by f1 and t1) is reached. Repeat the run for multiple values of the fraction of load to be shed. Here a python script to automate the study will be very helpful.