Dynamic Analysis and Improved LVRT Performance of Multiple DG Units Equipped With Grid-Support Functions Under Unbalanced Faults and Weak Grid Conditions

Due to the increased integration of multiple distributed generation (DG) units into the distribution network, riding through short-term faults and supporting the host grid have been requested by the new grid codes in many countries. However, the literature lacks the detailed dynamic analysis and control coordination of multiple grid-connected converter-based DG units, equipped with advanced controllers in the synchronous reference frame. To fill this gap, this paper initially presents a detailed small-signal modeling framework for typical medium-voltage multi-bus power distribution systems with multiple DG units equipped with grid-support functions to operate under the unbalanced conditions. This model encompasses the positive and negative sequences of the current and voltage and is developed for three stages of the fault (i.e., before, during, and after the fault) to cover a wide range of system operating points. In addition, to precisely study the interactions among DG units, four different control modes in DG units are considered to study the system dynamics under low-voltage and unbalanced conditions and at different grid strengths. Using the proposed detailed state-space models and based on the small-signal stability analyses, different control parameters are redesigned using the eigenvalue analysis on the complete multi-DG system. As a second contribution, sensitivity analyses are performed to study the effects of different system parameters, such as line characteristics, loading levels, and unbalanced fault characteristics, on the stability of the multi-DG system under unbalanced faults. Comparative simulation and experimental results are also reported to show the accuracy and effectiveness of the theoretical analyses.