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The aim of this paper is to compare three different categories of grid-forming converter control strategies based on their structural differences, and ability to fulfill the requirements that have been recently identified by the European Network of Transmission and System Operators for Electricity (ENTSOE). Among these requirements, the ones associated with the frequency characteristics of the converter system, such as its ability to prevent the risk for adverse control interactions, and act as a sink for harmonics are in focus. These properties are assessed through the frequency-dependent input-admittance models of the converter system. In addition, robust performance of the converter system at various grid strength is paramount, which is investigated both analytically and through detailed time-domain simulations in PSCAD.

With the increasing penetration of renewable and decentralized energy sources into the power grid, an extended use of DC voltages is expected on both distribution and transmission levels. Generation of DC voltages by means of voltage source converters is associated with a wide spectrum of harmonic distortions at converter terminals, both on the ac and on the dc sides. This can lead to partial discharges in power cables, which deteriorate insulation material thus weakening its performance and reducing cable life-time. In the previously published paper, the effect of harmonic distortion on appearance of partial discharges in cable insulation was evaluated. Here, the study related to the PD behavior has been made focusing on the influence of multilevel converter parameters such as the number of submodules and the switching frequency chosen to generate different voltage stresses. In addition, effects of using a function generator for producing desired waveform instead of the dSPACE board as in the previous work, are highlighted. In particular, it is shown implementation of the former made it possible utilizing the Pry-Cam synchronization and thus the realization of the PD pattern.

In this paper a tool is presented for the automation of horizon scanning. Horizon scanning is the systematic process of examining the scientific literature to identify opportunities, risks, threats and emerging issues. This tool has been named the Autonomous information Comprehension Engine (AiCE). AiCE has the ability to ingest large quantities of unstructured text based data from a variety of sources, extract a wide variety of features and analyse trends across the corpus of collected source material. AiCE is used to analyse science and technology literature and identify developments which may be of strategic relevance to a specific domain. In this paper AiCE is introduced and its components are outlined. The benefit of the proposed system is demonstrated through a time expenditure study comparing this new system to the traditional manual process of horizon scanning. The proposed system is also demonstrated by analysing literature from the computer vision community to extract relevant research to the natural language processing community.

The aim of this paper is to investigate the impact of steady-state deviations in the grid-frequency on the performance of grid-forming converter control strategies. In particular, the virtual synchronous machine (VSM) control structure has been the focus of study in this paper. Two alternative solutions to address the issue with the conventional VSM structure are suggested, and their impact on the active power response and the power dissipation properties of the converter are compared. For the latter, the ac-side input admittance of the grid-connected converter is derived for the different control structures under consideration. Finally, the dynamic performance of the different control structures is verified through the time-domain simulations.

The aim of this paper is to evaluate the low-frequency passivity properties of grid-forming converters. Through the analysis of the frequency-dependent input admittance of the converter, the impact of various control loops is investigated. A simplified analytical model that is valid in the low-frequency range is also derived in order to identify possible control modifications to enhance the passivity of the system.

Lately, the research interest for modular battery systems has increased due to the possibility of a better utilization of individual battery packs/cells and the steadily reducing costs of low voltage power electronics. This paper deals with the output voltage synthesis of a modular battery system based on a seven level Cascaded H-bridge (CHB) inverter topology used in a small passenger vehicle. Two methods are considered, Multilevel Pulse Width Modulation (MPWM) and Fundamental Selective Harmonic Elimination (FSHE). Using simulations, the inverter and battery losses, as well as the current THD, are used to assess the effectiveness of both techniques for the broad operating range of a vehicle’s drivetrain. It has been shown that FSHE cannot be applied at a modulation index below 0.25, because of the high current THD (> > 5%). Exceeding a modulation index of 0.25, FSHE reduces the battery and inverter losses in comparison to MPWM, while maintaining an acceptable current THD. Operating at higher speeds, FSHE achieves an even better current THD than MPWM. Consequently, it seems reasonable to use a hybrid modulation technique, using MPWM at low and FSHE at higher speeds, respectively. The exact boundary between MPWM and FSHE can vary in accordance with the individual optimization weightings of current THD and drivetrain efficiency.

Renewable Energy Sources are posing critical issues related to the mining of power systems stability, essentially due to a decrement of the inertial response during system's contingencies. As a consequence, to preserve the security and the reliability of the system, it is necessary to adopt new frequency adjustments mechanisms. This issue becomes particularly critical in isolated power systems, like those of small islands not supplied by the main grid, in the case of high shares of production from unpredictable renewabies such as photovoitaic and wind sources. In this framework, the present work deals with the analysis of the variation of the inertia time constant of a small island in various scenarios characterized by a different share of renewable energy. The study represents the first step of a more extended feasibility study for the conversion of a traditional power system based on fossil fuel towards a renewable energy-based power system.

The reduction of the inertial response of the generation system has been one of the main obstacles to the penetration of high sharing of renewable energy sources in islanded systems. This issue can be solved thanks to innovative control that makes wind turbines able to provide the so-named synthetic inertia. The paper investigates some scenarios with different levels of wind power penetration in a small island whose grid is not supplied by the main grid, by the use of the software NEPLAN 360, simulating the wind power plants accordingly to the standard IEC 61400-27. The aim of the paper is to assess the maximum level of penetration in the island of wind generation, with traditional control systems, that does not threaten the stability of the system.

The properties of complex space-vector models for asymmetric three-phase systems are investigated in this article. Most importantly, three alternative methods for the stability analysis of the asymmetric closed-loop systems are presented. The end results avoid the usage of matrix manipulations. It is shown how the theory can be applied to modeling and stability analysis of a grid-connected voltage-source converter (VSC). The methods are compared using numerical examples.

Nesting of microgrids enhances supply reliability, operational efficiency. and ancillary service support by optimizing distributed energy resource utilization. The availability of local reserve capacity within the nested microgrids (NMGs) enhances the local black start capability and can provide extra ancillary services to the main grid. However, NMG requires complex control and multiple communication layers, which makes it more vulnerable to control instability and cyber-attacks. Coordination of multivendor MGs is another challenge. This paper reviews the structure of a NMG, its associated benefits, challenges, and measures to assess its potential for large-scale application

This paper attempts to provide another view of the application of sequence domain analysis to study the stability of ac-grid connected converters. The approach is based on the definition of positive and negative sequence electrical quantities and on the use of the generalized Nyquist stability criterion in its basic form. Thus, a simple expression to perform the stability analysis is obtained, which allows to identify the contribution of each subsystem on the overall system stability.

The transient stability of a wind-farm connected to an ac grid depends on the control settings of the involved converters, the grid resonances and the system's operating conditions. This paper focuses on investigating the impact of converter control algorithms on possible high-frequency resonance interactions that could endanger the overall stability of these kinds of systems. Mainly, two control strategies namely the conventional cascade control structure, consisting of an inner current-control loop and various outer-loop controls, and the immediate voltage control structure, which does not include a current-control loop, are compared. The impact of control parameters for both methods will be investigated as well. For this purpose, the ac-side input admittance of a wind-turbine converter is derived for the two control strategies and passivity-based frequency-domain analysis is used to assess their impact on system stability. Finally, the analytical findings are verified through detailed time-domain simulations and recommendations to increase system stability are given accordingly.

A controller for a hybrid converter for STATCOM applications, based on a cascaded H-bridge star in series with a generic converter connected at the star point, is proposed. The controller utilizes the generic converter in an optimized way to maximize the operating range of the converter, with focus on unbalanced operating conditions. So for each operating condition, the generic converter reference voltage minimizes the voltage rating of the star part. Theoretical investigations of the hybrid converter using the proposed control approach show that the hybrid converter allows to effectively extend the operating range of the converter under unbalanced conditions of the system, given that the generic converter is large enough to circulate the needed active power between the converter phase legs. This indicates that, depending on the requirements from the operator in terms of negative-sequence injection capability, the hybrid converter can be a valuable solution to overcome the limitations of the CHB star.

Double fed induction generators (DFIG) for wind turbines are very sensitive to grid disturbances especially voltage dips. Understanding and improving fault-ride-through (FRT) capacity of wind turbine generators (WTG) demands accurate assessment of the impact of voltage dips. In many FRT requirements, only voltage dip magnitude and duration are considered. However, additional characteristics point-on-wave (POW) and phase-angle-jump (PAJ) have great impact on DFIG. This paper aims to study the behavior of DFIG-based WTGs during various types of voltage dips. PAJ and POW are specifically taken into consideration; intensive simulation tests show that their impact is significant and should be included in FRT studies. Theorical analysis are also provided to explain the mechanism behind the observed phenomena. And conclusions of paper could be used to provide useful information for FRT related works and other applications.