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]

Inertia emulation control of VSC-HVDC transmission system

The increasing penetration of power electronics interfaced renewable generation (e.g. offshore wind) has been leading to a reduction in conventional synchronous-machine based generation. Most converter-interfaced energy sources do not contribute to the overall power system inertia; and therefore cannot support the system during system transients and disturbances. It is therefore desirable that voltage-source-converter (VSC) based high voltage direct current (HVDC) interfaces, which play an important role in delivery of renewable power to AC systems, could contribute a virtual inertia and provide AC grid frequency support. In this paper, an inertia emulation control (IEC) system is proposed that allows VSC-HVDC system to perform an inertial response in a similar fashion to synchronous machines (SM), by exercising the electro-static energy stored in DC shunt capacitors of the HVDC system. The proposed IEC scheme has been implemented in simulations and its performance is evaluated using Matlab/Simulink

Inertia emulation control strategy for VSC-HVDC transmission systems

There is concern that the levels of inertia in power systems may decrease in the future, due to increased levels of energy being provided from renewable sources, which typically have little or no inertia. Voltage source converters (VSC) used in high voltage direct current (HVDC) transmission applications are often deliberately controlled in order to de-couple transients to prevent propagation of instability between interconnected systems. However, this can deny much needed support during transients that would otherwise be available from system inertia provided by rotating plant

Provision of Frequency Response from Wind Farms: A Review

Renewable sources of energy play a key role in the process of decarbonizing modern electric power systems. However, some renewable sources of energy operate in an intermittent, non-dispatchable way, which may affect the balance of the electrical grid. In this scenario, wind turbine generators must participate in the system frequency control to avoid jeopardizing the transmission and distribution systems. For that reason, additional control strategies are needed to ensure the frequency response of variable-speed wind turbines. This review article analyzes diverse control strategies at different levels which are aimed at contributing to power balancing and system frequency control, including energy storage systems.This research was funded by the Basque Government, through the project EKOHEGAZ (ELKARTEK KK-2021/00092), Diputación Foral de Álava (DFA) through the project CONAVANTER, and UPV/EHU through the project GIU20/063

Frequency support characteristics of grid-interactive power converters based on the synchronous power controller

Grid-interactive converters with primary frequency control and inertia emulation have emerged and are promising for future renewable generation plants because of the contribution in power system stabilization. This paper gives a synchronous active power control solution for gridinteractive converters , as a way to emulate synchronous generators for inerita characteristics and load sharing. As design considerations, the virtual angle stability and transient response are both analyzed, and the detailed implementation structure is also given without entailing any difficulty in practice. The analytical and experimental validation of frequency support characteristics differentiates the work from other publications on generator emulation control. The 10 kW simulation and experimental frequency sweep tests on a regenerative source test bed present good performance of the proposed control in showing inertia and droop characteristics, as well as the controllable transient response.Peer ReviewedPostprint (author's final draft

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

Modified PSO-Based Virtual Inertia Controller for Optimal Frequency Regulation of Micro-Grid

Owing to the growing need to address the energy crisis by the traditional sources (e.g. Thermal power plants), as well as the associated environmental concerns posed, the power system witnessed increased penetration of power electronics-based power sources like solar, wind, and energy storage in terms of battery technologies. Consequently, modern compared with traditional power systems have become more susceptible to large frequency fluctuations due to the emergence of stability issues. Prominent among these include the reduction of system properties such as damping and inertia which are significant characteristics of system stability. Insufficient inertia drives the grid frequency outside the acceptable range under severe disturbances and this may lead to an outage of generators and tripping, unscheduled shedding of load, system collapse, and in the severe scenario, an entire power blackout, this threatens the system dynamic security. To preserve the system's dynamic security, this paper proposes an alternative approach to frequency regulation built upon a PID-based Virtual Inertia Control (VIC) which imitates the inertia property. The proposed virtual inertia uses the frequency derivative to emulate virtual inertia. The optimality search capability of the Particle Swarm Optimization (PSO) technique is used to design the proposed controller. Evaluation of the robustness of the proposed controller is demonstrated through Time Domain Analysis, considering different system operating ranges for improving frequency stability and resilience. Improved performance of the proposed controller when paralleled with the traditional virtual inertia controller shows a 69.2% reduction in frequency nadir under the condition of reduced system inertia, 70% without RESs integration. Also, 50.7% and 44.4% improvement in the reduction of frequency nadir and maximum overshoot respectively were observed under the situation of nominal system inertia, 100%, and Renewable Energy Systems (RESs) penetration