Project Server Workflows need to run under the context of a user. However, they do not run under context of the user that started the project, instead, the workflows are run under the Workflow Proxy Account. This means that the user account which you specify as the workflow proxy account must have the proper rights to execute all of the commands a project server workflow will need to do. All workflow activities should be happening under the workflow proxy account. By default all PSI-calls from built-in activities are running under this proxy account. However, it is possible to perform PSI-calls from custom workflow activities under the Application Pool account.