From GridLAB-D Wiki
The #set macro is used to set a global variable in GridLAB-D.
Set only works if the variable is already defined and will not create a new variable if it does not already exist unless the strictnames global variable is set to 1 or TRUE.
GLM
#set variable=value
Note: the macro is not followed by a semicolon unless the semicolon is part of the value.