Why do we convert generators and loads??

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In a regular power flow calculation, the generators and loads are represented as sources or sinks of active and reactive power. Take a look at any textbook to see how the equations are structured. In a dynamic simulation, generators are modelled as voltage-controlled current sources. So, you need to convert the generators to current sources before you transition from power flow calculation to dynamic simulation. The load in a regular PSSE power flow calculation has no dependency on terminal voltage or frequency. In a dynamic simulation, you want to consider these dependencies, or at least consider the voltage dependency. To do this, you need to convert the loads to impedances and/or current sources. More information about these processes can be found in the PSSE manuals.