I am trying to have the bus loads in PSS/E to change by using python program. So I am trying to write a script in python where I could change loads to different values between two buses in PSS/E.
You can use API routine called "LOAD_CHNG_4" (search for this routin in API.pdf documentation). This routine belongs to the set of load data specification functions. It can be used to modify the data of an existing load in the working case.
Python and PSS/E