Shahed Mortazavian

Medium-voltage distributed generation (DG) units can be subjected to a high penetration level of dynamic loads, such as induction motor (IM) loads. The highly nonlinear IM dynamics that couple active power, reactive power, voltage, and supply frequency dynamics can affect the stability of MV grid-connected converter (GCC)-based DG units. However, detailed dynamic analysis and, more importantly, stabilization approaches of GCC-based DG units with IM loads when subjected to the grid faults, are not reported in the current literature. In addition, the literature lacks a thorough study on the effect of the grid strength on the low-voltage ride-through (LVRT) performance of such practical systems. To fill in this gap, this paper presents comprehensive integrated modeling, stability analysis, and LVRT performance improvement methods for GCC-based DG units in the presence of an IM load considering different grid strengths. A detailed multi-stage small-signal model of the complete system is obtained, and the eigenvalue analysis is conducted considering both static and dynamic load modeling. Furthermore, a sensitivity analysis is performed to investigate the effect of the length of the power line between the DG unit and the IM on the stability and LVRT performance of the entire system. Finally, the LVRT performance of the DG unit under an unbalanced grid fault is investigated using three different reference current generation strategies to determine the best strategy to provide a stable and efficient LVRT performance under strong and weak grid conditions. The time-domain simulation and experimental results are also presented to validate the effectiveness of the proposed methods.

Supporting the host grid during voltage dips has become a major connection requirement for large distributed generation units. Because most of the grid faults are unsymmetrical, the recently developed grid codes suggest the injection of a flexible positive- and negative-sequence reactive current components proportional to the magnitude of the voltage dip at the point of common coupling. However, detailed dynamic analysis of the augmented grid-connected converter with the flexible positive- and negative-sequence current injection function and the characterization of the impact of the grid strength, converter control parameters, and proportionality constants used in the reference current generation block are not reported in the literature. To fill in this gap, first, a multi-stage linear model of the augmented nonlinear system dynamics is developed, and the small-signal stability analysis is performed on the system dynamic behavior before, during, and after the fault. The effects of different system and control parameters are studied and characterized. Second, a new and effective model-based controller design method is proposed to maintain the system stability during and after the fault with the consideration of the mutual interaction among different system controllers. Finally, the time-domain simulations and laboratory experiments validate the accuracy and effectiveness of the proposed control method.

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 re-designed 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.