The integration of renewable energy sources (RESs) is a key objective for energy sector decision-makers worldwide, aiming to establish renewable-rich future power grids. However, transitioning from conventional systems based on synchronous generators (SGs) or systems with a low RESs share presents challenges, particularly when accompanied by decommissioning large central generation units. This is because the reduction in inertia and system strength, traditionally provided by SGs, can lead to a loss of essential system support functions like voltage and frequency. While current converter technologies attempt to compensate for the grid support provided by SGs by enhancing converter capabilities, they still heavily rely on the presence of SGs to function effectively. These converters, known as grid-following (GFL) converters, depend on the grid to operate in a stable and secure manner. As the penetration of RESs increases, the efficacy of GFL converters diminishes, posing stability challenges in low inertia systems and limiting the integration of RESs. Therefore, it is crucial to reassess the existing GFL converter technologies, control mechanisms, and grid codes to understand their status and future requirements. This will shed light on the advancements and limitations of GFL converters, enabling greater RESs integration and grid support independent of SGs. This paper aims to provide an up-to-date reference for researchers and system operators, addressing the issues and challenges related to GFL converter technologies, control systems, and applications in low inertia systems. It serves as a valuable resource for facilitating the transition towards future systems with 100% RESs penetration scenarios.

Grid-forming inverters are anticipated to be integrated more into future smart microgrids commencing the function of traditional power generators. The grid-forming inverter can generate a reference frequency and voltage itself without assistance from the main grid. This paper comprehensively investigates grid-forming inverter modelling and control methodology. A decentralized method employing an active power versus frequency P − f droop and a reactive power versus voltage Q − V droop is exploited to drive the operation of the grid-forming inverter. This decentralized method ensures balancing the supply and demand beside the power-sharing task between two or more inverters. The performance of the grid-forming inverter is examined by monitoring the frequency and RMS voltage of the inverter bus for three different periods of a varying PQ load. In addition, the performance of the resultant droop is compared with the assumed droop to validate the effectiveness of the proposed method. Finally, two grid-forming inverters equipped with the same droop characteristics are connected to a single load to observe the power-sharing concept among them. All simulations are implemented and executed using Matlab/Simulink version R2014b.

The rapid integration of renewable energy sources (RESs) into the power grid has resulted in the high utilization of power-electronics devices and operating power systems where inverter-based resources (IBRs) are dominated. Such a transition has led to a reduction of inertia and system strength. In recent research, grid-forming converters (GFM) are introduced and developed to alleviate the grid-following converter (GFL) functionalities and to address the limitations concerning grid support capability, stability, and synchronization issues. However, the efficacy of GFM technology is still under investigation, and the level at which GFM converters can replace traditional GFLs is still under question. This paper aims to bridge the gap in literature by revisiting the up-to-date research on the capabilities and the limitations of the proposed GFM converters compared to the traditionally utilized GFL converters, allowing a better understanding of the current status and future requirements. This includes the GFM converter's topologies and their performance for small- and largesignal stability issues, the GFM converters' ability to enhance grid synchronization, and transient stability performances. Furthermore, the challenges and limitations of the dynamic behavior of GFM converters from the point of view of fault ride-through (FRT) capability, including grid codes and FRT requirements, FRT methods of GFM converters, post-fault behavior, and open research directions, are also comprehensively reviewed. Finally, the paper has been concluded by highlighting the main findings, considerations, and future recommendations.

Interfacing a weak grid imposes challenges on distributed generators (DGs). These challenges include transient frequency and voltage dynamics that can destabilize the system. Accordingly, this paper investigates the grid stiffness based on different scenarios and the dynamics of a grid feeding DG connected to a weak grid. Moreover, the dynamic effects of the physical and control parameters on the system’s stability are deeply analyzed and evaluated. Specifically, complete mathematical models and graphical representation are carried out to precisely examine the impact of the system parameters on the stability and performance of the DG. Therefore, stable deployment of renewable energy resources into power networks can be achieved as well as an efficient and robust performance of DGs can be ensured when connected to weak grids. The obtained results show the importance of the performed study in optimal sizing and designing the output filter of the inverter and the impact of tuning the control parameters on the system dynamics. As a result, a proper design of system physical and control parameters can be accurately achieved considering all factors affect the system performance. The paper also conducts detailed and elaborated analyses and simulations to evaluate the performance of a PI-controlled RC damped inverter connected to a weak grid. The proposed filter design of the interfacing inverter can significantly extend the integration of DGs into microgrids without requiring complex control schemes or oversized components.

The increasing integration of Power Electronics (PE)-based renewable energy sources into the electric power system has significantly affected the traditional levels and characteristics of fault currents compared to the ones observed in power systems dominated by synchronous generating units. The secure operation of a renewable rich power system requires the proper estimation of fault currents with wide range of scenarios of the high share of renewables. Although the utilization of detailed and complex time-domain dynamic simulations allows for calculating the fault currents, the resulting modeling complexity and computational burden might not be adequate from the operational perspective. Thus, it is necessary to develop alternative quicker data-driven fault current estimation approaches to support the system operator. For this purpose, this paper utilizes an Artificial Neural Network (ANN)-based tool to estimate the characteristics of short circuit currents in power systems with high penetration of power electronics-based renewables. The short circuits against different penetration of renewables are produced offline using the DIgSILENT PowerFactory considering the control requirements for renewables (e.g., fault ride through requirement). The resulting dataset is utilized to train the ANN to provide the mapping between the penetration level and the characteristics of the short circuit currents. The application of the approach using the modified IEEE 9-bus test system demonstrates its effectiveness to estimate the components of short circuit currents (sub-transient current, transient current, and peak current) with high accuracy based only on the penetration of power electronics-based renewables.

The sensitivity analysis becomes particularly critical for voltage stability analysis due to the fluctuation in power outputs of renewable energy resources. Besides, impacts of different load modeling and the operation mode of Distributed Generations (DGs) are not addressed in the well-known sensitivity analysis methods. Therefore, this work presents a new sensitivity analysis approach to find the relation between the Voltage Stability Margin (VSM) and the control variables of power systems, considering the voltage dependent characteristics of loads and DGs. The sensitivity analysis is performed on VSM, defined from equivalent nodal analysis, via its differential equation. To include the voltage dependent characteristics, loads are modeled as polynomial function (ZIP model) and DGs are considered to be operated with constant current and constant power modes. Based on this analysis, the sensitivity of VSM can be directly obtained by taking the derivatives of nodal voltages with respect to control variables. The validity of the developed approach is demonstrated on the IEEE 118 bus system.