Start-up of virtual synchronous machine: methods and experimental comparison

A modern grid is smarter mainly in the advance in information and communication technologies, while the power processing mechanism does not make a big difference. To make a modern grid smarter, the grid control should be improved to process the power in a smarter way. Therefore, it is easily foreseen that virtual synchronous machines, which emulates the synchronous machines based on power converters, may have big potentials in a future energy internet. This paper uses the Synchronous Power Controller with emulated and improved synchronous machine characteristics for renewable generation systems and proposes two start-up strategies. The proposed strategies are explained in detail, verified and compared by experimental results.Peer ReviewedPostprint (published version

Control of Voltage-Source Converters Considering Virtual Inertia Dynamics

Controlling power-electronic converters in power systems has significantly gained more attention due to the rapid penetration of alternative energy sources. This growth in the depth of penetration also poses a threat to the frequency stability of modern power systems. Photovoltaic and wind power systems utilizing power-electronic converters without physical rotating masses, unlike traditional power generations, provide low inertia, resulting in frequency instability. Different research has developed the control aspects of power-electronic converters, offering many control strategies for different operation modes and enhancing the inertia of converter-based systems. The precise control algorithm that can improve the inertial response of converter-based systems in the power grid is called virtual inertia. This thesis employs a control methodology that mimics synchronous generators characteristics based on the swing equation of rotor dynamics to create virtual inertia. The models are also built under different cases, including grid-connected and islanded situations, using the swing equation with inner current and voltage outer loops. Analysis of the simulation results in MATLAB/Simulink demonstrates that active and reactive power are independently controlled under the grid-imposed mode, voltage and frequency are controlled under the islanded mode, and frequency stability of the system is enhanced by the virtual inertia emulation using swing equation. On this basis, it is recommended that the swing equation-based approach is incorporated with the current and voltage control loops to achieve better protection under over-current conditions. Further works are required to discover other factors that could improve the effectiveness of the models

Control of distributed renewable energy generation systems in converter-dominated microgrid applications

Mención Internacional en el título de doctorThere is a growing interest in the use of renewable Distributed Energy Resources (DERs) that increase the efficiency of the transmission system and reduce the ecological impact of renewable energy infrastructures. At the same time, they reduce the associated capital requirements, thus increasing the potential installation of renewable energy. Microgrids have been proposed as a solution to improve the integration of renewable DERs. By the use of advanced control techniques, they provide a reliable frame for DERs to support the power system operation. As such, Microgrids can be a promising solution to increase renewable energy penetration. However, since renewable DERs are usually interfaced by Power Electronic Converters (PECs), they do not provide the common stabilization characteristics of traditional generation interfaced by Synchronous Generators (SGs). Therefore, there are concerns about the stability of converter-dominated Microgrids. This Thesis focus on the specific requirements of PEC-interfaced renewable DERs operating in Microgrids. An overview of available solutions show that, for PECs to support the Microgrid operation in both grid-connected and islanded modes, they require a synchronizing mechanism that does not rely on the measurement of an external frequency. A promising alternative is to emulate the behavior of traditional SGs in the PEC control system with the so-called Virtual Synchronous Machine (VSM) solutions. The synchronization system underlying to these proposals is analyzed. A comparison with the use of traditional frequency measurement systems, namely Phase-Locked Loops (PLLs), in the support of the Microgrid power balance is addressed, showing that the PEC synchronization system has a direct effect on the Microgrid stability. The Thesis includes a new proposal to ensure synchronous operation based on the use reactive power, instead of active power as in VSMs, that does not require frequency measurements. A dynamic model of a grid-connected PEC is used to demonstrate that reactive power can be used to ensure synchronism. This Reactive Power Synchronization system is used to propose a solution for the black-start of Wind Energy Conversion Systems (WECSs), so that they can contribute to the restoration of the power system following a blackout. The proposed control systems are validated with experimental results of a grid connected PEC and an isolated WECS.Programa Oficial de Doctorado en Ingeniería Eléctrica, Electrónica y Automática por la Universidad Carlos III de MadridPresidente: Luis Rouco Rodríguez.- Secretario: Emilio José Bueno Peña.- Vocal: Roberto Alves Baraciart

Synchronous power control of grid-connected power converters under asymmetrical grid fault

Control of grid-connected power converters is continuously developing to meet the grid codes, according to which the generation units should keep connected to the grid and further provide ancillary services, such as voltage and frequency support, negative sequence current injection, inertia emulation, etc. A virtual admittance controller is proposed in this paper for the objective of voltage support under asymmetrical grid faults. By using independent and selective admittances for positive and negative sequence current injection, the unbalanced voltage can be significantly compensated during asymmetrical faults. The controller is based on the generic control framework of the synchronous power controller (SPC), which is able to control a power converter with emulated and improved synchronous generator characteristics. Simulation and experimental results based on two paralleled 100 kW grid-connected power converters demonstrate the controller to be effective in supporting unbalanced voltage sags.Peer ReviewedPostprint (published version

Offshore Wind Farm Black Start Service Integration:Review and Outlook of Ongoing Research

A review of the ongoing research on black start (BS) service integrated with offshore wind farms (OWFs) is presented in this paper. The overall goal is to firstly gain a better understanding of the BS capabilities required by modern power systems. Subsequently, the challenges faced by OWFs as novel BS service providers as well as an outlook on the ongoing research which may provide solutions to these are presented. OWFs have the potential to be a fast and environmentally friendly technology to provide BS services for power system restoration and, therefore, to ensure resiliency after blackouts. As a power electronic-based system, OWFs can be equipped with a self-starter in the system in order to perform BS. The self-start unit could be a synchronous generator (SG) or a power electronic unit such as a grid-forming (GFM) converter. Preliminary BS studies performed in PSCAD/EMTDC are presented in a simplified OWF system via an SG as the self-start unit. Consequently, technical challenges during the BS procedure in an OWF benchmark system are outlined via theoretical discussions and simulations results. This is useful to understand the threats to power electronics during BS. Finally, the most relevant GFM strategies in the state-of-the-art literature are presented and their application to OWF BS is discussed

Issues and Challenges of Grid-Following Converters Interfacing Renewable Energy Sources in Low Inertia Systems : A Review

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.© 2024 The Authors. This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/fi=vertaisarvioitu|en=peerReviewed