Enhancing Transient and Steady-State Performance in Grid-Connected VSCs: A Comprehensive Frequency Region Analysis of Decoupling Current Units

This paper introduces an innovative synthesis of inner loop control for grid-connected Voltage Source Converters (VSCs), comprising a Decoupling Current Unit (DCU) with a classic current tracking controller. The objective is to enhance both transient and steady-state performance by reducing the inherent coupling between the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i>- and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i>-axis current control loops. The proposed DCU is formulated through a generalized control design approach that can be configured according to the dynamics of practical filter types, including L, LC, and LCL structures. Decoupling effectiveness is assessed through a model-independent Relative Gain Array (RGA) analysis, while the robustness of the inner control loop under parameter uncertainties is evaluated to ensure reliable steady-state performance. Theoretical analysis confirms that the proposed DCU effectively suppresses inter-axis coupling, producing RGA profiles that closely align with ideal decoupling across different filter configurations. These findings are further validated through simulation studies under both linear and nonlinear load conditions. Compared to a classic non-decoupled control scheme, the proposed method achieves substantial improvements: approximately 55% reduction in overshoot, up to 70% suppression of cross-coupling effects following power step changes, more than 60% improvement in reactive power tracking accuracy.