Analysis, Design, and Implementation of Impulse-Injection-Based Online Grid Impedance Identification With Grid-Tied Converters

Grid impedance information occupies a fundamental and crucial position for the operation of grid-tied converters. Consequently, accurate and fast extraction of grid impedance is vital. The impulse injection method applied under the control loop of a grid-tied converter, which is commonly used to extract linear grid impedance online, is relatively reasonable in terms of device cost, measurement speed, simplicity, and design flexibility. However, the selection of impulse and design of impulse parameters mostly depend on rules of thumb, without theoretical guidance, leading to insufficient measurement accuracy in target frequency section and greatly burdening the system. To address these issues, a systematic guidance is proposed for analysis, design, and implementation of impulse-injection-based grid impedance identification. By comparing the spectra of several basic impulses with unipolar and bipolar forms and analyzing the impact when impulse parameters are altered, a guidance for impulse selection for different measurement scenarios is presented. Based on this guidance, an asymmetric bipolar sawtooth wave with a defined asymmetry ratio of
$\boldsymbol{\rho } = 0.5$
is chosen in this article for broadband measurement of grid impedance. Furthermore, a related parameter design procedure with optimal injection position and instant is also proposed to reduce system burden and eliminate overmodulation when injecting the impulse. Finally, theoretical results are validated by experiments.