Running a project without compiling first is perfectly fine. If a Run command is issued, PSCAD will perform the Build Modified procedure before the run starts. See Running a Simulation for more details.
Build refers to the creation of the Fortran, data, and map files required for the building of a project executable file. You may choose to build the entire project (Build), or only modules that have been modified since a previous build (Build Modified). The latter choice comes in handy for very large projects, which may take minutes to compile in their entirety.
To build the entire project from scratch, select the Build button in the Home tab of the ribbon control bar:
To build only modules and transmission segments that have been modified since the last compile, select the Build Modified button in the Home tab of the ribbon control bar:
