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]

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

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

Wind Power Integration into Power Systems: Stability and Control Aspects

Power network operators are rapidly incorporating wind power generation into their power grids to meet the widely accepted carbon neutrality targets and facilitate the transition from conventional fossil-fuel energy sources to clean and low-carbon renewable energy sources. Complex stability issues, such as frequency, voltage, and oscillatory instability, are frequently reported in the power grids of many countries and regions (e.g., Germany, Denmark, Ireland, and South Australia) due to the substantially increased wind power generation. Control techniques, such as virtual/emulated inertia and damping controls, could be developed to address these stability issues, and additional devices, such as energy storage systems, can also be deployed to mitigate the adverse impact of high wind power generation on various system stability problems. Moreover, other wind power integration aspects, such as capacity planning and the short- and long-term forecasting of wind power generation, also require careful attention to ensure grid security and reliability. This book includes fourteen novel research articles published in this Energies Special Issue on Wind Power Integration into Power Systems: Stability and Control Aspects, with topics ranging from stability and control to system capacity planning and forecasting

Fast Assessment of Dynamic Behavior Analysis with Evaluation of Minimum Synchronous Inertia to Improve Dynamic Security in Islanded Power Systems

Over the last decades, renewable energy sources (RES) participation into the electricity supply mix has been constantly increasing not only in interconnected power systems but also in isolated power systems. This power supply transition seeks to accomplish renewable-based electricity generation targets and policies as well as the de-carbonisation of societies for a more sustainable energy future. Such transition should be achieved through investments and consequent substantial installation of wind and solar farms in power systems, not only because of their obvious environmental benefits but also because of their technological maturity and consequent steady cost declining.Despite renewable energy penetration growth in several power grids, there are some technical challenges to deal with when we are in presence of isolated power systems with variable renewable energy sources. Those technical issues are identical to the ones faced by larger and interconnected systems but they could intensify in these less robust type of systems. Therefore, issues like frequency control and spinning reserve management become even more important to guarantee acceptable levels of stability and security of the system. Moreover, the increasing participation of wind and photovoltaics in the generation mix, unlike conventional generators, leads to a significant reduction in the amount of synchronous inertia present in the system, which is essential to avoid a rapid rate of change of frequency (RoCoF) and large frequency deviations after a contingency. Thus, higher RoCoF and frequency deviations will be observed, which might trigger the protection devices resulting in a cascading outage and a blackout.This thesis presents a preventive control tool capable of evaluate power system stability and identify the minimum synchronous inertia required to maintain system stable for a certain operation scenario (characterized by its dispatch and demand) and, if necessary, to support the decision maker to perform a new power dispatch or consider the activation of synchronous condensers.For that purpose, a small and isolated power system with a significant participation of RES in the generation mix was considered. By performing a power dispatch, several operation scenarios were created and used as input in a MATLAB/SIMULINK model, which was used to study the dynamic response of the system when a disturbance occur. In this work two different disturbances were considered: active power output loss by the biggest thermal unit in the system and the loss of 50\% in both wind and PV active power output. Therefore, a dataset was created to train two different neural networks (one for each contingency) so that they could emulate the dynamic response of the system. By applying a sensitivity analysis by the neural networks it is possible to identify the minimum synchronous inertia required by the system to keep it secure and stable