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The variables 'isload' and 'bus' are assumed to be integers,if not, use the function int(x) on them. The code below worked fine on the savnw.sav case for load at bus=3008, with change= 120.0
# to scale load:
if int(isload):
psspy.bsys(1,0,[0.0,0.0],0,[],1,[bus],0,[],0,[])
psspy.scal_2(1,0,1,[0,0,0,0,0],[0.0,0.0,0.0,0.0,0.0,0.0,0.0])
psspy.scal_2(1,0,2,[0,1,0,1,0],[change,0.0,0,-.0,0.0,-.0,0])