Dionysios Moutevelis

Modern electrical energy systems face the dual challenge of decarbonization while maintaining reliability and cost efficiency. Efforts to address these challenges focus on several key areas. First, renewable energy sources like photovoltaic and wind, which rely on power electronics for grid connection, are becoming prominent. Their variable nature requires flexibility to balance generation and demand from equipment such as energy storage units and flexible loads. Second, during the integration of these diverse units, developing additional high-voltage dc (HVdc) transmission systems and flexible ac transmission systems is essential to maintain electrical variables within the design limits. Finally, the electrification of demand is growing, particularly in sectors like mobility, driven by the rise of electric vehicles and their charging infrastructure.

Due to the fundamental transition to a power electronic dominated power system, the increasing diversity of dynamic elements underscores the need to assess their similarity to mature electrical engineering models. This article addresses the concept of the Dynamic Similarity Index (DSI) for its use in power electronics-dominated networks. The DSI is a multi-purpose tool developed to be used by different stakeholders (e.g., converter manufacturers and system operators). This index is calculated per frequency, which serves to anticipate potential differences in particular frequency ranges of interest between the model under study and a reference model. Within the scope of this study, the dynamic similarity of inverter-based generators and clusters of network components to an ideal voltage source behind an impedance is assessed, due to the relevance of this fundamental circuit in the representation of generation units in power system studies. The article presents two potential applications based on this mathematical framework. First, for manufacturers to evaluate control performance compared to a reference model and second, for operators to diagnose buses with voltage vulnerability based on a user-defined reference Short-Circuit Ratio (SCR) value. The DSI results for these two case studies are validated using Matlab Simulink simulations.

Virtual synchronous machine (VSM) control is commonly used to control power converters in islanded microgrids (MGs). Frequently, the dynamic model of the MG is unknown and, therefore, the estimation-based, impedance criterion is preferred to design the control parameters. However, the original impedance criterion does not consider the fundamental frequency dynamics and its application may result in technical challenges, such as instabilities, during the connection of new VSMs to already operating MGs. This is relevant for cases where frequency dynamics are critical, as in the case of islanded MGs. In this paper, a methodology to design the parameters of a VSM connected to a MG is presented, based on an extension of the impedance criterion that accounts for the frequency dynamics. It is shown that the proposed method facilitates the understanding of the impact of the VSM parameters on the electromechanical and electromagnetic timescales and on the overall system stability, establishing an intuitive approach for their design. Furthermore, it is shown that the MG frequency dynamics have a relevant effect on the dynamic performance of the VSM, which would not be captured using the conventional impedance criterion. Theoretical developments are verified using simulations and laboratory tests of a representative islanded MG.

This paper proposes a taxonomy of power converter control schemes based on the recently proposed concept of complex frequency . This quantity captures local frequency variations due to the change of both the phase angle and amplitude of bus voltages and current injections. The paper derives the analytical expressions of the link between complex power variations and complex frequency of each converter controller as well as the identification of critical control parameters. The main contribution of this work is to provide a general framework that allows classifying converters synchronization mechanisms and controllers. This framework also allows comparing converters with synchronous machines. To validate the theoretical results, extensive simulations are performed using a modified version of the WSCC 9-bus system. Examples of how the theoretical formulations of the paper can be used to improve power converter control in power system applications are showcased.

This paper proposes a recursive, virtual admittance based, secondary controller for DG units that improves the voltage profile in distribution networks. First, the adaptation of the virtual admittance concept for the goal of voltage regulation is explained. Then, a recursive secondary controller is developed to periodically update the virtual admittance gains. The controller is formulated as an optimization problem with current and stability limitations as constraints. Measurements across the grid, transmitted through low-bandwidth communications, are used to simplify the calculations, resulting in a recursive algorithm. Weight vectors are included in the objective function to allow participation flexibility of each converter. Results show that the primary virtual admittance controller is able to mitigate over-and undervoltages in steady state and under transient conditions. Subsequently, the secondary controller is shown to further improve the voltage profiles across the grid. Experimental results obtained from a laboratory environment, comprising three DG units and a grid emulator, validate the functionality of the complete control structure.

The aim of this article is to analyze the parameter stability and power transfer limits of active distribution networks with power converters and load tap changers by using bifurcation theory.First, the nonlinear model of a representative distribution grid with mixed loads and power converters is derived. Frequently neglected effects of converter capacity limits and transformers with load tap changers are considered, and for this purpose, static and dynamic saturations are modeled by using approximations based on smooth functions. In addition, a solution based on an antiwindup scheme is provided to model load tap changers without numerical convergence problems. These methods allow their integration to the model in a comprehensive way. Results show the impact of converter parameters and its limits on system stability as well as the impact of weak grid conditions. Moreover, they show that these effects can destabilize the system under, seemingly, safe operating conditions. The results indicate that reactive power support from converters can improve stability margins and the performed analysis can help in the design of the converter control parameters. Experimental results obtained from the laboratory environment comprising two distributed generators and a grid emulator are used for the validation of the contributions of this article.