Comments on "Digital Current Control in a Rotating Reference Frame-Part I: System Modeling and the Discrete Time-Domain Current Controller With Improved Decoupling Capabilities"

A recent paper by Hoffmann et al. presents a discrete-time model in a rotating dq reference frame of an R-L filter and its current control. The purpose of this note is, first, to show that the discrete model presented in the paper behaves differently to the sampled continuous-time model of the plant, formulated in the stationary alpha beta reference frame; second, to find the proper discretization of the plant in dq coordinates; and third, to verify that there is cross coupling between axes d and q in the closed-loop system if the original model is used, and that this coupling is not present when using the model found in this note. In the note, it is verified that having a precise model of the plant allows us to fulfill the control objective of obtaining the complete decoupling between axes.