Master's thesis: Inverter-Driven Oscillations in the Nordic power system: existing and future challenges

Background Environmentally friendly ways to provide reliable, safe electrification are essential for sustainable societies. Achieving this goal depends on the effective large-scale integration of Renewable Energy Sources (RES) and Battery Energy Storage Systems (BESS). Despite providing numerous benefits, such as a low carbon footprint and running costs, RES also introduces various technical challenges that impact power system stability. Unlike conventional power plants that rely on synchronous machines, RES and BESS are connected to the system via power electronic inverters and are commonly referred to as Inverter-Based Resources (IBRs). Other devices supporting the energy transition, such as High-Voltage Direct Current (HVDC), Flexible AC Transmission Systems (FACTS), and electrolyzers, are also interfaced to the system through inverters. Therefore, the large-scale integration of RES and the increasing electrification of energy systems must be followed by careful stability and control assessment of inverter-dominated power systems. Substantial expansion of IBRs has introduced new types of oscillations across a wide range of time scales. These oscillations arise from cross-couplings among IBR dynamics, synchronous machine electromechanical dynamics and network electromagnetic properties. If not properly managed, these oscillations may result in Inverter-Driven Instabilities (IDI) that manifest in critical electrical quantities such as power, frequency, and voltage. Moreover, they can propagate through the network and eventually cause system-wide disruptions, such as blackouts and equipment damage. Description RISE Electric Power Systems Group has several ongoing projects that relate to the above-described domain. Specifically, RISE is coordinating the European project Navigating Uncertainties: Advanced Control Solutions for Inverter-Dominated Power Systems (NU-ACTIS) and leading WP5, Benchmarking and synthesis of the project’s technical solutions. The project is supported by Hitachi Energy Sweden, whose HVDC and RnD departments provide crucial expertise in inverter technology from an industry perspective. An integral part of NU-ACTIS project and WP5 is the use of power system simulation tools and models to provide reliable, close-to-reality analysis and evaluation of phenomena related to IDI. The focus is on the Nordic power system, following the documented ambitious goals of the Nordic Transmission System Operators (TSOs) related to fossil-free power systems and maintaining high standards of reliability and resilience. The master thesis’s work should contribute to WP5 by building up on the knowledge of power system modelling, dynamics, stability, and control under high penetration of IBRs. Main Tasks The proposed Master theses would support the work of NU-ACTIS by: - Evaluating the dynamics of the Nordic power system test model under various (inverter-rich) operating and control scenarios - Identification of the appropriate test disturbances - Assess the properties of IDIs appearing under these changing scenarios - Identify some of the key properties leading to the unwanted oscillations through sensitivity studies - Propose and demonstrate a control mitigation strategy by controlling the inverters in the affected system that would mitigate the oscillations under a variety of system conditions Status and relation to previous work: - State-of-the-art Nordic RMS test system model is available in PowerFactory with a scenario exemplifying the IDI - Initial Python scripts for running the test system and automating the process of sensitivity analysis, multiple contingencies, operating points, etc. - Literature supporting the work - Access to the NU-ACTIS project meetings and discussions with academic and industry partners as a valuable source of feedback and exposure The developed model, including the script and user description, shall be delivered and presented to RISE and Hitachi Energy R&D before finalizing the Master thesis project. Qualifications • The ongoing MSc studies in Sweden • Fluent in English (both written and spoken) • Power system analysis and stability-related courses • Knowledge of dynamical systems • Knowledge of power electronics • Experience with control engineering • Basic experience with power system modelling • Programming knowledge; preferably Python Conditions Location: The work is foreseen to be performed mainly at the RISE office at Drottning Kristinas väg 61, Stockholm, and/or Vasagatan 12, Västerås. Working conditions: on-site or hybrid Estimated start: January 2027 Estimated duration: 20 weeks Credits: 30 ECTS Compensation: After project completion and approval = 39,990 SEK in total RISE supervisors: Danilo Obradović (danilo.obradovic@ri.se), Stefan Stanković (stefan.stankovic@ri.se) Hitachi Energy Research supervisor: Rasool Heydari (rasool.heydari@hitachienergy.com) Welcome with your application! The application must contain: - 1or 2-page CV with clear details on the candidate's qualifications with respect to the described project - Full transcript of records from both BSc and MSc studies All documents must be written in English. Contact person: Danilo Obradović (danilo.obradovic@ri.se) Deadline: September 15, 2026