Rapid dq-Frame Impedance Measurement for Three-Phase Grid Based on Interphase Current Injection and Fictitious Disturbance Excitation

Abstract

The accurate <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -frame impedance of three-phase grid is essential in the stability analysis of grid-connected VSC system. However, the conventional measurement methods relying on two separate independent injections extremely slow down the impedance measurement speed, lacking adaptability for the actual dynamic industrial grid. To address this shortcoming, this paper proposes a rapid <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -frame impedance measurement approach for three-phase grid. In this approach, only one injection of wideband harmonics is needed so that the injection times can be reduced to half of the conventional methods and then the measurement time is shortened by half, dramatically enhancing the real-time performance of measurement result. Firstly, an interphase QAM multitone current injection method is proposed, which can excite the first group of desired wideband responses through a single injection. Then, a fictitious disturbance excitation method is introduced to substitute for the second injection of actual disturbance, which can construct the second group of wideband responses by rotation transformation. Afterward, the wideband <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -frame impedance can be calculated with these two groups of response data. The feasibility and effectiveness of the proposed approach are validated by both simulation and experiment results.