As electrical grids integrate increasing shares of renewable energy, the role of power electronic converters, particularly grid-forming (GFM) inverters, becomes increasingly critical. However, GFM inverters introduce new challenges in self-protection due to their synchronization mechanisms being tightly coupled with power dynamics. This paper proposes a layered self-protection scheme for GFM inverters, inspired by classical grid-following (GFL) strategies. The scheme comprises three hierarchical layers: (1) setpoint saturation, (2) control / software limitation, and (3) hardware limitation. Special emphasis is placed on adapting the first layer to the GFM context. The third layer remains largely applicable without significant changes. Additionally, hardware-imposed operational boundaries due to voltage and current constraints are examined and validated by simulation in order to define feasible inverter operation under fault conditions or setpoint misconfigurations.

The increased grid configuration variability and, among others, the massive integration of renewable energies, have created a challenging scenario for power converters. Due to this fact, active impedance estimation is an interesting characteristic that will allow power converter online control tuning and/or stability analysis. Pseudo Random Binary Sequence (PRBS) injection is one the most interesting and proposed methods in the literature to obtain this impedance estimation. Many papers use impedance estimation on dedicated power converters, or they often overlook the digital and power hardware constraints that power converters must meet. This is especially important in high-power converters, where low switching and control frequencies are used. This paper defines and proposes an implementation and design method for PRBS injection considering the abovementioned constraints. The proposed method is aimed to be implemented in an already operating converter, thus adding the impedance estimation as one additional functionality of the converter. Therefore, no dedicated power converter to estimate the grid frequency is required. Multi-harmonic and multi-sequence impedance estimation is also analyzed, thus demonstrating the ability of the proposed method to operate under highly variable and demanding grid conditions. The analysis is validated via simulation results in different case scenarios.

A more renewable energy generation model entails the replacement of synchronous generator-based power plants by converter-interfaced renewable sources, resulting in more susceptible power systems in terms of frequency and voltage oscillations under sudden power perturbations. In addition, dc-based power systems are becoming very popular thanks to the advantages they offer compared to classical ac systems. This has created a need for developing new time-domain simulation tools to represent the dynamic behaviour of converter-dominated, hybrid ac/dc power systems. This paper presents a Simulink® based simulation tool, named DFPF, that can be used to study the dynamic performance of ac, dc and hybrid power system scenarios. The DFPF tool consists of combining the dynamical models of grid-connected elements with the iterative solving of a static power flow algorithm of each part of the power system. The Simulink interface simplifies the implementation of element models and power system test scenarios, and since fast electromagnetic transients are not considered, medium-term simulations can be carried out rapidly for relatively complex scenarios. To test the tool, a hybrid scenario comprised by four ac grids interconnected by a multi-terminal dc grid is implemented and simulated. The results demonstrate that the DFPF tool can be employed to evaluate the transient response of interconnected ac and dc systems under different grid conditions.

This paper presents a pulse width modulation variation in order to overcome any drawback in the sampled reference signal due to its discretization. This discretization can be a problem in low switching frequency power converters; for instance, in Medium Voltage applications. This effect is amplified with interleaved power converter structures. The proposal is based on the polynomial linearization of the reference sampled point, instead of maintaining the reference constant in the whole switching update period. The graphical analysis is applied into three-phase, three-level power converter case study, but the conclusions can be extrapolated to any number of levels or any type of converter structure. The performance in one single and two interleaved power converters scenario scenarios is simulated in order to prove its superiority compared to the existing literature.

This paper presents an analysis of the reactive power injection limits of Medium Voltage STATCOM with harmonic active filtering capability. In order to obtain those limits, voltage and current requirements for the active filter application are estimated for a simplified case. In the same way, evolving the same analysis, some hardware and system issues, such as transformers de-rating and the impact of the phase angle of the compensated harmonic, are also addressed. Those limits are evaluated for a real full-scale power converter.

Focused on variable speed wind turbines with full control on the injected active and reactive power to the grid. This paper presents a control method oriented to use the kinetic energy stored in the wind turbine rotating masses for the primary frequency control. This way, the wind turbine can help to the frequency stability of the system increasing the output power at frequency dips. The control method is evaluated considering a small insular grid.