In the search for green energy to combat climatechange, a shift from conventional energy sources such as coal,oil, and nuclear towards Renewable Energy Sources (RES) isneeded. This shift poses a threat to the stability of the powergrids as RES do not contribute with rotating mass in the system.A lack of rotating mass, or in other words inertia, jeopardizesthe ability of power systems to counteract large disturbances.Frequency Containment Reserves (FCR) units are responsiblefor controlling the frequency in power systems by regulatingthe balance between the generated and consumed power. If thefrequency deviates outside of the defined range from the nominalvalue, it can lead to system separation, blackouts, and systemequipment damage. The frequency deviations are faster in lowinertia systems, making it more difficult for FCR to keep thefrequency within accepted ranges. Hydro turbines are often usedas FCR units, but additional means of support could be neededfor low inertia systems. Viable support could be battery systems.This project investigates the change towards low inertia and thepossible implementation of a battery system as fast step-wisepower support with a frequency trigger. The investigation is donethrough case studies of simulated system models in Matlab andSimulink. / I jakten på grön energi för att bekämpaklimatförändringarna behövs en övergång från konventionellaenergikällor som kol, olja och kärnkraft mot förnyelsebaraenergikällor. Denna övergång utgör ett hot mot kraftnätensstabilitet då förnyelsebara energikällor inte bidrar med roterandemassa. Brist på roterande massa eller med andra ord tröghetäventyrar kraftsystemens förmåga att motverka stora störningar.Frequency Containment Reserves (FCR) är system som aktivt arbetarmed att styra frekvensen i kraftsystemet genom att reglerabalansen mellan den producerade och konsumerade effekten.Om detta misslyckas och frekvensen avviker för mycket frånden nominella frekvensen kan detta leda till systemseparation,strömavbrott eller skada hos systemkomponenter. I ett systemmed låg tröghet blir frekvensavvikelserna snabbare. Detta gördet svårare att använda sig av FCR för att hålla frekvenseninom accepterade intervall. Vattenkraftverk används ofta somFCR enheter, men för system med låg tröghet kan ytterliggarestöd behövas. Ett möjligt effektstöd kan vara batterisystem.Detta projekt undersöker förändringen till lägre tröghet i ettkraftsystem och möjlig implementering av ett batterisystemmed ett snabbt stegsvar för effektstöd, vilket aktiveras vid enförbestämd frekvens. Undersökningen görs genom studier avspecifika fall med en linjäriserad modell av ett kraftsystemet,simulerade i Matlab och Simulink. / Kandidatexjobb i elektroteknik 2021, KTH, Stockholm

The plans to reduce the energy-related greenhouse gas emissions stimulate the deployment of electronically interfaced renewable resources. The increased penetration of such intermittent sources together with phasing conventional power plants out and the installation of High Voltage Direct Current (HVDC) links for long-distance more efficient transmission, reduces the stored inertia in any electrical grid. This leads to a more vulnerable power system and increases the significance of studying the corresponding stability aspects. Decreasing the inertial response of a power system deteriorates the quality of both frequency and rotor-angle stability which are the dynamics of interest in this study. The thesis explores the role of the location of low inertia on varying the power system’s dynamics. This is to be conducted in isolation of all other factors that could affect the study outcomes, such as dealing with the same system’s inertia value upon lowering the inertia in different locations. To accomplish this objective, it is essential to analyze the inertia distribution of the examined power system following the alterations of inertia reduction location. Accordingly, an inherently previous work methodology, that estimates the relative distance of the system’s components to Center Of Inertia (COI), is utilized throughout this study. Both frequency response and small-signal stability are analyzed in light of the inertia distribution results. The thesis examines two different power systems, a small two-area model and a bigger more realistic power system. The former model, known as Kundur model, helps in building a conceptual process to apply the methodology and to benchmark the dynamics of interest. While the latter is a reduced model of the Swedish transmission grid, known as Nordic 32 model. Different scenarios of low inertia are considered to capture the current trend of integrating more Renewable Energy Sources (RES) and phasing out more conventional plants. DIgSILENT Powerfactory is the weapon of choice in this study. It is utilized to assess both the frequency stability by performing electromechanical transients’ simulations, and small-signal stability following modal analysis simulations.  Results show that the alterations of low inertia location are associated with variations in Instantaneous Frequency Deviation (IFD), Rate Of Change Of Frequency (ROCOF) and the damping ratio of the most critical inter-area oscillation mode. These variations have different levels of significance. Variations of the latter two metrics have the most considerable effects from the stability’s perspective. They can be utilized to prioritize the phasing out process of the conventional power plants, and to choose one of the scenarios of a specific low inertia location over the others. This helps in fulfilling proper long-term planning and short-term operation from the system operator’s perspective.