New contributions to frequency control based on virtual synchronous generators: application to power systems with high renewable energy sources integration

[SPA] Esta tesis doctoral se presenta bajo la modalidad de compendio de publicaciones. Tradicionalmente, servicios como la regulación y mantenimiento de la frecuencia de los sistemas eléctricos, cobertura de la demanda eléctrica o la existencia de las reservas rodantes (spinning reserves) han sido suministrados y asegurados por las fuentes de generación de energía eléctrica tradicionales. Sin embargo, los sistemas eléctricos han sufrido una serie de cambios en los últimos años que están afectando de manera directa al propio funcionamiento de los mismos. Por un lado, el aumento constante del consumo de energía y de la intensidad del propio uso energético, unido al aumento de las restricciones legislativas medioambientales, y por otro el concepto de la energía eléctrica como un producto comercial junto con la liberalización de los mercados energéticos, hacen que se tambaleen algunas de las premisas hasta ahora asumidas. En este sentido, y en un entorno de promoción de recursos renovables, hace que los servicios hasta ahora proporcionados sólo por la generación clásica deben también ser compartidos por todos los puntos de generación. No obstante, la alta penetración de este tipo de fuentes renovables en el sector eléctrico acarrea una seria de cuestiones derivadas de sus características y peculiaridades que es necesario abordar antes de proceder de manera masiva a su integración y, por tanto, a la independencia de la generación convencional. Adicionalmente, y debido a la naturaleza variable de la generación renovable (principalmente el viento y el sol) recobra mayor importancia el asegurar por parte de los organismos reguladores una reserva energética que permita actuar de manera eficiente y fiel en casos de desequilibrio de potencias. En este nuevo escenario, en el que el director de tesis ha trabajado a lo largo de la última década, se hace necesario contar con el desarrollo y adaptación de nuevas herramientas y soluciones que faciliten la integración de fuentes renovables sin que ello suponga una merma en las capacidades del sistema eléctrico en términos de estabilidad y de respuesta ante contingencias. Así pues, el objetivo principal de esta tesis consiste en el estudio, implementación y evaluación de sistemas eléctricos con alta penetración de recurso eólico y fotovoltaico con el fin de evaluar posibles soluciones para emular inercias virtuales y respuestas similares a las que se obtendrían con generación clásica, integrando así de manera efectiva el recurso renovable al control de la frecuencia del sistema eléctrico. En este escenario, resultaría crucial poder aliviar en parte las necesidades de almacenamiento de energía a los puntos de generación mediante la implementación de estrategias alternativas de control de respuesta ante excursiones de frecuencia en las unidades renovables, aportando éstas el apoyo necesario para mantener la frecuencia de red dentro de los límites establecidos. Por tanto, la solución aquí estudiada favorecería la integración masiva de recursos renovables, dentro de un escenario de estabilidad del sistema eléctrico apoyado por estas instalaciones, y donde la eliminación paulatina de elementos rotativos directamente conectados a la red debe sustituirse y/o emularse de manera que el sistema eléctrico ofrezca la misma fiabilidad que se percibe ante la presencia de generación convencional. Sólo así se conseguirá fomentar de manera argumentada las posibilidades tangibles de integración a gran escala de recursos renovables, adelantándonos a las necesidades que surgirán de manera inevitable como consecuencia de la disminución inicial de inercia del sistema (entendida de una manera clásica como elementos rotativos directamente conectados a red) y como consecuencia de la entrada de fuentes que poseen una variabilidad en sus niveles de generación. Destacar igualmente la importancia cada vez mayor del control de la frecuencia del sistema eléctrico, debido a la sensibilidad y dependencia que poseen de este parámetro la mayoría de las cargas y equipos con algún tipo de etapa de electrónica de potencia.[ENG] This doctoral dissertation has been presented in the form of thesis by publication. Over the last decades, most countries have been suffering an electrical energy transition, changing from a model based on non-renewable sources (mainly based on fossil fuels), to a new framework characterised by the integration of renewable energy resources (RES). These important changes have been mainly supported by the development of power electronics, environmental protection policies, and the need to reduce energy dependence on third countries. Moreover, the electrical sector stands out because of the diversity and heterogeneity of sources that can generate electricity. As a result, the current electrical scenario includes a high interest in the integration of variable renewable energy sources (vRES) shifting towards a new generation mix. In fact, these vRES (mainly photovoltaic and wind power installations) already play a relevant role, as some European countries have experienced generation levels over 50% during some time-periods of last years. As aforementioned, the two most mature renewable resources integrated into power systems are solar photovoltaic (PV) and wind power (especially variable speed wind turbines, VSWTs). Together with the integration of these two sources, and in contrast to traditional grids based on conventional power plants (i.e., hydro-power, thermal, and nuclear power plants), several important issues have emerged, needing to be analysed, assessed, and resolved.Los artículos que constituyen la tesis son los siguientes: 1. Fernández-Guillamón, Ana & Gómez-Lázaro, Emilio & Muljadi, Eduard & Molina-García, Ángel, 2019. "Power systems with high renewable energy sources: A review of inertia and frequency control strategies over time," Renewable and Sustainable Energy Reviews, Elsevier, vol. 115(C). 2. Ana Fernández-Guillamón & Jorge Villena-Lapaz & Antonio Vigueras-Rodríguez & Tania García-Sánchez & Ángel Molina-García, 2018. "An Adaptive Frequency Strategy for Variable Speed Wind Turbines: Application to High Wind Integration Into Power Systems,"Energies, MDPI, Open Access Journal, vol. 11(6), pages 1-21, June. 3. Fernández-Guillamón, A.; Vigueras-Rodríguez, A.; Gómez-Lázaro, E.; Molina-García, Á. Fast Power Reserve Emulation Strategy for VSWT Supporting Frequency Control in Multi-Area Power Systems. Energies 2018, 11, 2775. https://doi.org/10.3390/en11102775. 4. Fernández-Guillamón, Ana & Sarasúa, José & Chazarra, Manuel & Vigueras-Rodríguez, Antonio & Fernández-Muñoz, Daniel & Molina-Garcia, Ángel. (2020). Frequency control analysis based on unit commitment schemes with high wind power integration: A Spanish isolated power system case study. International Journal of Electrical Power & Energy Systems. 121. 106044. 10.1016/j.ijepes.2020.106044. 5. Fernández‐Guillamón, A., Vigueras‐Rodríguez, A. and Molina‐García, Á. (2019), Analysis of power system inertia estimation in high wind power plant integration scenarios. IET Renewable Power Generation, 13: 2807-2816. https://doi.org/10.1049/iet-rpg.2019.0220. 6. Fernández Guillamón, Ana; Martínez de Lucas, Guillermo; Molina García, Ángel y Sarasúa Moreno, José Ignacio (2020). An Adaptive Control Scheme for Variable Speed Wind Turbines Providing Frequency Regulation in Isolated Power Systems with Thermal Generation."Energies", v. 13 (n. 13); p. 3369. ISSN 1996-1073. https://doi.org/10.3390/en13133369. 7. Fernández-Guillamón, A.; Martínez-Lucas, G.; Molina-García, Á.; Sarasua, J.-I. Hybrid Wind–PV Frequency Control Strategy under Variable Weather Conditions in Isolated Power Systems. Sustainability 2020, 12, 7750. https://doi.org/10.3390/su12187750. 8. Fernández-Guillamón, Ana & Gomez-Lazaro, Emilio & Molina-Garcia, Ángel. (2020). Extensive frequency response and inertia analysis under high renewable energy source integration scenarios: application to the European interconnected power system.Escuela Internacional de Doctorado de la Universidad Politécnica de CartagenaUniversidad Politécnica de CartagenaPrograma de Doctorado en Energías Renovables y Eficiencia Energétic

Power systems with high renewable energy sources: A review of inertia and frequency control strategies over time

Traditionally, inertia in power systems has been determined by considering all the rotating masses directly connected to the grid. During the last decade, the integration of renewable energy sources, mainly photovoltaic installations and wind power plants, has led to a significant dynamic characteristic change in power systems. This change is mainly due to the fact that most renewables have power electronics at the grid interface. The overall impact on stability and reliability analysis of power systems is very significant. The power systems become more dynamic and require a new set of strategies modifying traditional generation control algorithms. Indeed, renewable generation units are decoupled from the grid by electronic converters, decreasing the overall inertia of the grid. ‘Hidden inertia’, ‘synthetic inertia’ or ‘virtual inertia’ are terms currently used to represent artificial inertia created by converter control of the renewable sources. Alternative spinning reserves are then needed in the new power system with high penetration renewables, where the lack of rotating masses directly connected to the grid must be emulated to maintain an acceptable power system reliability. This paper reviews the inertia concept in terms of values and their evolution in the last decades, as well as the damping factor values. A comparison of the rotational grid inertia for traditional and current averaged generation mix scenarios is also carried out. In addition, an extensive discussion on wind and photovoltaic power plants and their contributions to inertia in terms of frequency control strategies is included in the paper.This work was supported by the Spanish Education, Culture and Sports Ministry [FPU16/04282]

Frequency control studies: A review of power system, conventional and renewable generation unit modeling

Over the last decades, renewable energy sources have increased considerably their generation share in power systems. As a consequence, in terms of frequency deviations, both grid reliability and stability have raised interest. By considering the absence of a consensual set of models for frequency control analysis, both for the different generation units (conventional and renewables) and the power system itself, this paper provides extensive and significant information focused on the models and parameters for studies about frequency control and grid stability. An extensive analysis of supply-side and power system modeling for frequency stability studies over the last decade is presented and reviewed. Parameters commonly used and assumed in the specific literature for such simulations are also given and compared. Modeling of generation units are described as well, including both conventional and renewable power plants.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

Vertical wind profile characterization and identification of patterns based on a shape clustering algorithm

Wind power plants are becoming a generally accepted resource in the generation mix of many utilities. At the same time, the size and the power rating of individual wind turbines have increased considerably. Under these circumstances, the sector is increasingly demanding an accurate characterization of vertical wind speed profiles to estimate properly the incoming wind speed at the rotor swept area and, consequently, assess the potential for a wind power plant site. The present paper describes a shape-based clustering characterization and visualization of real vertical wind speed data. The proposed solution allows us to identify the most likely vertical wind speed patterns for a specific location based on real wind speed measurements. Moreover, this clustering approach also provides characterization and classification of such vertical wind profiles. This solution is highly suitable for a large amount of data collected by remote sensing equipment, where wind speed values at different heights within the rotor swept area are available for subsequent analysis. The methodology is based on z-normalization, shape-based distance metric solution and the Ward-hierarchical clustering method. Real vertical wind speed profile data corresponding to a Spanish wind power plant and collected by using a commercialWindcube equipment during several months are used to assess the proposed characterization and clustering process, involving more than 100000 wind speed data values. All analyses have been implemented using open-source R-software. From the results, at least four different vertical wind speed patterns are identified to characterize properly over 90% of the collected wind speed data along the day. Therefore, alternative analytical function criteria should be subsequently proposed for vertical wind speed characterization purposes.The authors are grateful for the financial support from the Spanish Ministry of the Economy and Competitiveness and the European Union —ENE2016-78214-C2-2-R—and the Spanish Education, Culture and Sport Ministry —FPU16/042

Offshore wind power integration into future power systems: Overview and trends

Nowadays, wind is considered as a remarkable renewable energy source to be implemented in power systems. Most wind power plant experiences have been based on onshore installations, as they are considered as a mature technological solution by the electricity sector. However, future power scenarios and roadmaps promote offshore power plants as an alternative and additional power generation source, especially in some regions such as the North and Baltic seas. According to this framework, the present paper discusses and reviews trends and perspectives of offshore wind power plants for massive offshore wind power integration into future power systems. Different offshore trends, including turbine capacity, wind power plant capacity as well as water depth and distance from the shore, are discussed. In addition, electrical transmission high voltage alternating current (HVAC) and high voltage direct current (HVDC) solutions are described by considering the advantages and technical limitations of these alternatives. Several future advancements focused on increasing the offshore wind energy capacity currently under analysis are also included in the paper.This work was supported by ‘Ministerio de Educación, Cultura y Deporte’ of Spain (grant numbers FPU16/04282 and EST18/00738). This research received no external funding

An adaptive control scheme for variable speed wind turbines providing frequency regulation in isolated power systems with thermal generation

The lack of synchronous inertia, associated with the relevant penetration of variable speed wind turbines (VSWTs) into isolated power systems, has increased their vulnerability to strong frequency deviations. In fact, the activation of load shedding schemes is a common practice when an incident occurs, i.e., the outage of a conventional unit. Under this framework, wind power plants should actively contribute to frequency stability and grid reliability. However, the contribution of VSWTs to frequency regulation involves several drawbacks related to their efficiency and equipment wear due to electrical power requirements, rotational speed changes, and subsequently, shaft torque oscillations. As a result, wind energy producers are not usually willing to offer such frequency regulation. In this paper, a new control technique is proposed to optimize the frequency response of wind power plants after a power imbalanced situation. The proposed frequency controller depends on different power system parameters through a linear regression to determine the contribution of wind power plants for each imbalance condition. As a consequence, VSWTs frequency contribution is estimated to minimize their mechanical and electrical efforts, thus reducing their equipment wear. A group of sixty supply-side and imbalance scenarios are simulated and analyzed. Results of the case study are compared to previous proposals. The proposed adaptive control reduces the máximum torque and rotational speed variations while at the same time maintaining similar values of the load shedding program. Extensive results and discussion are included in the paper.This work was partially supported by ‘Ministerio de Educación, Cultura y Deporte’ of Spain (ref. FPU16/04282) and by ‘Ministerio de Economía y Competitividad’, under the project “Value of pumped-hydro energy storage in isolated power systems with high wind power penetration” of the National Plan for Scientific and Technical Research and Innovation 2013–2016, grant number ENE2016-77951-R