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Coordinated Frequency Control Using DC Interconnections Between AC Systems

Utilizing Fast Frequency Support through HVDC Links and Evaluating the Newly Uncovered Dynamics in Low-Inertia Power Systems

Time: Thu 2024-04-18 10.00

Location: F3 (Flodis), Lindstedtsvägen 26 & 28, Stockholm

Language: English

Doctoral student: Danilo Obradovic , Elkraftteknik

Opponent: Associate Professor Damian Flynn, University College Dublin, Dublin, Ireland

Supervisor: Robert Eriksson, Elkraftteknik; Mehrdad Ghandhari, Elkraftteknik

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QC 20240320


Transmission system operators are increasingly adopting renewable energy sources in response to the escalating need to reduce environmental pollution. However, renewable energy sources, like wind and solar power, connect to the grid through power electronics, offering no inherent inertia. This reduction in inertia substantially deteriorates the frequency responses during large power disturbances. Frequency Containment Reserves (FCR) are designed to counteract these disturbances and stabilize frequency within a few seconds after an imbalance has occurred. However, in scenarios with low inertia and large power disturbances, relying solely on FCR may prove insufficient to maintain frequency within acceptable limits, risking power system blackouts and severe disruptions. This thesis, therefore, conducts a comprehensive evaluation of fast frequency support in the form of Emergency Power Control (EPC) from High Voltage Direct Current (HVDC) links as a complement to FCR.

Unlike prior research, which overlooked the consideration of technical requirements of FCR responses and their significance for EPC evaluation, this thesis fills these gaps. Additionally, previous literature examining EPC has not confirmed a reliable solution for a system of various HVDC links.

Various EPC designs are evaluated to reduce frequency deviations and avoid negative interactions. This thesis employs dynamic simulations and, where appropriate, various linear control theories. A spectrum of system models is used, from simplified single-machine equivalents to detailed multi-machine models, aiming to highlight common findings, explain disparities, and capture relevant stability interactions. Particular attention is given to voltage-dependent dynamics, which are often overlooked in frequency control assessments. Moreover, considering EPC's ability to apply large gains, the thesis explores its impact on small-signal stability.

The droop frequency-based EPC using local inputs emerges as a key and safe solution for controlling the frequency in the Nordic power system for present and future operations. It is shown that the proposed EPC reduces the frequency deviations when appropriate droop values are chosen. Even more, the research demonstrates stability and cost benefits when efficiently distributing EPC among different HVDC links and coordinating it with the FCR. The simple EPC design allowed for analyzing various dynamic interactions and derivations of strategies for avoiding the ones of a negative nature. Finally, the thesis confirms the overall positive and sustainable role of the proposed EPC.