Resonance impact on inter-area oscillations of the Nordic power system

Maintaining the robustness and stability of the Nordic power systemis an essential task nowadays. The Nordic electricity network has the characteristics of bearing heavy generation in the northern region while supplying large consumption in the southern region through longtransmission lines. Therefore, its operation mightbecomemore complex which infact can lead to operational issues that did not exist in the pastand might lead to major problemsofthe network.The resonance interaction caused by forcing oscillations occurring at frequencies near the inter-area mode frequenciesis such an issue and is studied thoroughly in this thesis project.Small signal stability analysis is the basis for the research of this phenomenon, as the inter-area modes need to be calculated. For verifying this phenomenon, simulations were implemented in DigSilent PowerFactory software. The Nordic 32 test model is utilized, as it reflectsthe original Nordic system. Initially, the modes of the system are calculated,and the inter-area modes of interest are identifiedbased on their frequency and damping ratio value. The observability, controllability and participation factorsof the modes areconsidered for picking the modes of interestfor further analysis. Then, forced oscillations in the formof load variations are implemented to study the resonance impact withthe inter-area modes. As a mean to quantify the resonance impact, the active power flow of tie lines in the system is used.A sensitivity study was performed to study how various factors of the forced oscillations influence the resonance impact. The sensitivity study considered the following aspects of the forced oscillations: frequency, amplitude, location, and type. The amplification of the resulting oscillations seems to be affected by the value of controllability of the mode whose frequency couples with the inter-area frequency. This connection is evaluated in the simulations of this project and interesting results occur for further analysi

Statistical Evaluation of New Estimators used in Forced Oscillation Source Localization

Forced oscillations occur in power systems when a piece of equipment injects a periodic disturbance into the system. Under certain conditions, these oscillations can become large and widespread, impacting the system\u27s reliability. Several methods have been proposed to identify the oscillation\u27s source so that it can be addressed. One of the most promising is based on Dissipating Energy Flow (DEF). In this paper, a new expression for the DEF is developed under a set of commonly observed conditions. Based on this new expression, three new DEF estimators are proposed. The performance and sensitivities of these estimators, along with a previously published method, are evaluated using statistical methods. Results from simulated and measured power system data validate the DEF estimators and highlight the advantages and disadvantages of each

Control of power converter in modern power systems

A la portada consta el nom del programa interuniversitari: Joint Doctoral Programme in Electric Energy Systems [by the] Universidad de Málaga, Universidad de Sevilla, Universidad del País Vasco/Euskal Erriko Unibertsitatea i Universitat Politècnica de CatalunyaPower system is undergoing an unpreceded paradigm shift: from centralized to distributed generation. As the renewable-based generations and battery storage systems are increasingly displacing conventional generations, it becomes more and. more difficult to maintain the stability and reliability of the grid by using only conventional generations. The main reason for the degradation of grid stability is the rapid penetration of nonconventional sources. These new generations interface with the grids through power electronics converters which are conventionally designed to maximize conversion efficiency and resource utilization. Indeed, these power converters only focus on their internal operation despite the grid conditions, which often worsens the grid operation. To overcome such a drawback, the grid-forming concept has been proposed for power converters, aiming to redesign the control of the power converters to enforce more grid-friendly behaviours such as inertia response and power oscillation damping to name a few. Despite the rich literature, actual adaptation of grid-forming controller in real-world applications is still rare because incentives for renewable power plants to provide services based on such advanced grid-forming functions were at best scarce. In the last years, however, several system operators have imposed new requirements and markets for grid-supporting services. In addition, the existing grid-forming controllers require modification to low-level control firmware of a power converter, which is often unrealistic due to the control hardware limitations as well as necessary testing and certifications. To ensure a stable operation of a grid-forming converter under adverse operating conditions, a robust voltage sensorless current controller is developed in this PhD thesis. The proposed controller is able to handle most of the possible abnormal conditions of the grid such as impedance variations, unbalanced voltage; harmonics distortion. These abnormalities of the grid are mathematically represented using equivalent linear models such that they can be used for calculating the controller gains. Linear matrix inequality techniques are also used to facilitate parameter tuning. In fact, the performance and stability of the current control loop can be determined through only two tuning parameters instead of eight parameters for a controller of a similar structure. The existing grid-forming implementations are designed considering that the control firmware of the power converter can be upgraded at will. However, modifications of the control firmware are not straightforward and cost-effective at mass scale. To overcome such a limitation, an external synchronous controller is presented in this PhD thesis. The external synchronous controller uses measurements, which are either provided by the power converter or a dedicated measurement unit, to calculate the actual active and reactive power that should be injected by the power converters in a way that the power plant acts as an aggregated grid­forming converter. As a result, any conventional power converters can be utilized for providing grid-supporting services with minimal modification to the existing infrastructure. Power converters can provide even better performance than a synchronous generator if a proper control scheme is used. In this regard, the final chapter of this PhD thesis presents the multi-rotor virtual machine implementation for grid-forming converter to boost their damping performance to power oscillations. The multi-rotor virtual machine-controller implements several virtual rotors instead of only one rotor as in typical grid-forming strategies. Since each of the virtual rotors is tuned to target a specific critical mode, the damping participation to such a mode can be increased and adjusted individually. The controllers presented in this PhD thesis are validated through simulators and experiments in the framework of the H2020 FlexiTranstore project. The results are throughout analysed to assess the control performance as well as to highlight possible implications.A medida que las generaciones basadas en energías renovables y los sistemas de almacenamiento de baterías desplazan la generación convencional, se vuelve cada vez más difícil mantener la estabilidad y confiabilidad de la red. Estas nuevas generaciones interactúan con las redes a través de convertidores de electrónica de potencia que están diseñados tradicionalmente para maximizar la eficiencia de conversión y la utilización de recursos. Estos convertidores centran su funcionamiento interno independientemente de las condiciones de la red, lo que a menudo empeora el funcionamiento de la red. Para esto, se ha propuesto el concepto de convertidores de potencia formadores de red (grid-forming), con el objetivo de rediseñar el control de los convertidores de potencia para imponer comportamientos más favorables a la red, por ejemplo, la respuesta inercial y la amortiguación de oscilaciones de potencia. No en tanto, la adaptación real del controlador grid-forming en aplicaciones del mundo real todavía es escasa debido a los pocos incentivos para que las plantas de energía renovable proporcionen servicios basados en funciones de formación de red tan avanzadas. Aunque en los últimos años, operadores de sistemas han impuesto nuevos requisitos y mercados para servicios auxiliares, los controladores grid-forming existentes requieren cambios en el firmware de control de bajo nivel de un convertidor de potencia, algo poco realista debido a las limitaciones del hardware de control, así como a las pruebas y certificaciones necesarias. En esta tesis se desarrolla un controlador de corriente robusto, sin sensor de tensión, para garantizar el funcionamiento estable de un convertidor grid-forming en condiciones de operación adversas. Este controlador es capaz de manejar la mayoría de las condiciones anormales de red, como variaciones de impedancia, tensión desequilibrada y distorsión de armónicos. Estas anomalías de la red se representan matemáticamente mediante modelos lineales equivalentes, utilizados para calcular las ganancias del controlador. También, usando técnicas de desigualdad matricial lineal para facilitar el ajuste de parámetros. De hecho, el rendimiento y la estabilidad del bucle de control de la corriente pueden determinarse mediante sólo dos parámetros de sintonización. Las implementaciones de formación de red existentes están diseñadas considerando que el firmware de control del convertidor de potencia puede actualizarse a voluntad. Sin embargo, las modificaciones del firmware de control no son sencillas ni rentables a gran escala. Por tanto, esta tesis presenta un controlador síncrono externo que utiliza las mediciones proporcionadas por el convertidor de potencia o por una unidad de medición dedicada para calcular la potencia activa y reactiva real que deben inyectar los convertidores de potencia, de forma que la central eléctrica actúe como un convertidor grid-forming agregado. Como resultado, cualquier convertidor de potencia convencional puede utilizarse para proporcionar servicios de apoyo a la red con una modificación mínima de la infraestructura existente. Los convertidores de potencia pueden ofrecer mejor rendimiento que un generador síncrono utilizando un esquema de control adecuado. El último capítulo de esta tesis presenta la implementación de una máquina virtual multirrotor para que los convertidores de red aumenten su rendimiento de amortiguación de las oscilaciones de potencia. El controlador de la máquina virtual multirrotor implementa varios rotores virtuales en lugar de un solo rotor como en las estrategias típicas de grid-forming. Dado que cada uno de los rotores virtuales está sintonizado para dirigirse a un modo crítico específico, la participación de la amortiguación a dicho modo puede aumentarse y ajustarse individualmente. Los controladores presentados en esta tesis doctoral han sido validados mediante simulaciones y experimentos en el marco del proyecto H2020 FlexiTranstore.Postprint (published version

Internal dynamics of a medium-sized subarctic lake: field measurements and numerical modeling

As the Arctic Circle opens up due to the effects of global warming, the surrounding regions, and in particular the subarctic zone, experience both climatic and societal changes, which makes it imperative to understand the existing ecosystems. Lake Lagarfljót, an Icelandic fjord lake with glacial inflow, is used to evaluate processes occurring in weakly stratified water bodies in the sub-Arctic. Field observations during the summers of 2010 and 2011 show that the lake experiences a short period of stratification, with overturning occurring when the heat fluxes become negative. The internal-wave regime evolves rapidly as the stratification strengthens, higher vertical modes and Kelvin waves are identified. Glacial inflow to Lake Lagarfljót delivers large volumes of fine sediment. It is expected that most of the sediments remain in suspension and this can alter the density of the water-column, resulting in inverse thermal gradients. Additionally, the glacial river is deflected to the eastern edge of the lake by Coriolis forces, causing the inflow to remain in shallow water before entering the center of the lake. Results from the Münnich model demonstrate that the natural oscillation of the lake basin is close to the oscillation period of the wind. The 3D hydrodynamic model Si3D was adapted to account for suspended sediment, and it successfully reproduced the motions observed in the lake, showing its utility for the study of weakly stratified lakes. Results suggest that wind is the main driver of internal waves in Lake Lagarfljót, but that the river influences stratification, meaning that it cannot be ignored when considering lake circulation. The model confirms the presence of Kelvin waves in the basin. Tracking the pathways of the inflowing glacial water, it appears that the initial intrusion depth is dependent on internal waves. During summertime stratification, the river penetrates the lake as interflows or underflows. Underflows can become trapped, causing them to oscillate and becoming flushed episodically when large glacial discharges enter the lake as underflow. Water entering the lake as interflow mixes more easily and it is flushed quasi-continuously out of the basin. Because the circulation of Lake Lagarfljót is sensitive to the strength of the stratification, the internal dynamics of the lake are expected to change significantly in a warming climate.Hlýnun jarðar hefur mikil áhrif á Norðurskautssvæðinu og þar eru að opnast nýir möguleikar til samgangna og annarra athafna. Hlýnuninni fylgja miklar veðurfars- og samfélagsbreytingar á aðliggjandi svæðum, einkanlega í kaldtempraða beltinu. Áríðandi er að auka skilning á ýmsum vistkerfum á þessu belti og viðbrögðum þeirra við hlýrra loftslagi. Í ritgerð þessari er gerð grein fyrir rannsókn á lagskiptingu í stöðuvatninu Leginum, sem er jökulmyndað vatn efst í Lagarfljóti á Héraði. Vettvangsrannsóknir sem gerðar voru á Leginum sumurin 2010 og 2011 sýna skammæja lagskiptingu sem hverfur þegar hitaflæði verður neikvætt. Innri bylgjukerfi í vatnsbolnum þróast hratt þegar lagskipting styrkist, lóðrétt streymi eykst og Kelvin bylgjur eru sjáanlegar. Jökulsá í Fljótsdal ber með sér mikið magn fíngerðra setagna í Löginn. Viðbúið er að mest af setögnunum haldist í sviflausn og getur það breytt eðlisþyngd vatnssúlunnar, sem veldur viðsnúnum hitastigli. Þar að auki sveigist straumur jökulvatns að eystri bakka Lagarins vegna Coriolis kraftsins og helst því lengur á grynningum en ella áður en það rennur út á mitt stöðuvatnið. Niðurstöður úr Münnich líkaninu sýna að náttúrulegar sveiflur í vatninu eru nálægt sveiflutíma vinds. Þrívíða straumfræðilega reiknilíkanið Si3D var aðlagað til að taka tillit til svifagna. Líkanið endurspeglar innri hreyfingar sem mælast í vatninu og sýnir rannsóknin því fram á nytsemi þessa líkans við greiningu veikrar lagskiptingar í stöðuvötnum. Niðurstöður benda til þess að vindur sé helsti drifkraftur innri bylgjuhreyfinga Lagarins. Áhrif innstreymis Jökulsár á hringrásina í vatninu og þar með á lagskiptinguna eru einnig veruleg. Líkanið staðfestir tilvist Kelvin bylgna í vatninu. Þegar fylgst er með hvernig Jökulsá í Fljótsdal blandast við vatnið virðist innstreymisdýptin í vatnsbolnum vera háð innri bylgjuhreyfingum hans. Við þá lagskiptingu sem ríkir á sumrin rennur áin inn í vatnið sem innra flæði um miðbik vatnsins og eftir botninum sem undirflæði. Vatn sem rennur eftir botninum getur lokast þar af og dýpi þess sveiflast til. Innikróað vatn skolast svo út stöku sinnum þegar mikið af nýju jökulvatni rennur eftir botni vatnsins. Jökulvatn sem berst í vatnið sem innra flæði um miðbik þess blandast betur og skolast nokkuð jafnt út úr því aftur. Þar sem vatnshringrásin í Leginum er háð styrkleika lagskiptingarinnar er viðbúið að innri hrif muni breytast verulega við hækkandi vatnshita.Energy Research Fund of the National Power Company (Landsvirkjun