I would like to ask why the input of the PI controller is divided by Vdc_ref?(Vdc_ref is the reference value of the DC bus voltage). If we divide the input by Vdc_Ref the controller will handle smaller values of the voltage error. Does it make the performance of the controller better or there is another explanation?
That’s just a normalization that expresses the error as a fraction of the reference voltage. It has no bearing on the design of the controller, and makes it applicable to any regulator with different reference voltage.
Input and output variables of a controller are always application dependent. Normalizing these in some understandable way can lead to a better insight into the controller operation.