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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
KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Electric Power and Energy Systems.ORCID iD: 0000-0001-6541-7892
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Abstract [sv]

Systemoperatörer av överföringssystem hanterar en allt större andel förnybara energikällor som ett resultat av pågående energiomställning. Förnybara energikällor, så som vind- och solkraft, ansluts till elsystemet genom kraftelektronik vilket innebär att de inte bidrar med tröghet till elsystemet. Minskning av tröghet i elsystemet försämrar frekvenshållningen, speciellt då stora obalanser inträffar. Frekvenshållningsreserverna (FCRs – frequency containment reserves) är utformade för att stabilisera frekvensen inom några sekunder efter att en obalans inträffat. I scenarier med låg tröghet i elsystemet och då en stor obalans inträffar kan frekvenshållningsreserverna vara otillräckliga för att stabilisera och hålla frekvensen inom acceptabla gränser. Detta innebär en risk för ofrivillig automatisk förbrukningsbortkoppling samt risk för nätsammanbrott och därmed allvarliga störningar i elförsörjningen. Syftet med denna avhandling är att undersöka snabb frekvensreserv i form av nödeffekt (EPC) från högspänd likströmsförbindelser (HVDC – high voltage direct current) som ett komplement till frekvenshållningsreserverna.

Tidigare forskning har inte tagit hänsyn till tekniska krav på frekvenshållningsreserverna och deras betydelse för utvärderingen av nödeffekt vilket denna avhandling avser att behandla. Dessutom har tidigare litteratur som undersöker nödeffekt inte utvecklat en effektiv och tillförlitlig lösning för ett elsystem där olika HVDC-länkar ingår.

I denna avhandling utvärderas olika designer av nödeffekt med syftet att minska frekvensavvikelsen efter att en stor obalans har inträffat samt syftet att undvika negativa stabilitetsinteraktioner. Dynamiska simuleringar genomförs och olika linjära reglertekniska metoder/teorier används i analyserna. Ett flertal olika elsystemmodeller används, från en enkel en-maskinekvivalenten till mer detaljerade fler-maskinmodeller. Syftet med att använda olika modeller är att kunna förklara och verifiera resultat från teoretiska beräkningar samt att fånga relevanta stabilitetsinteraktioner. Särskilt fokus är på spänningsberoende dynamik vilken ofta bortses vid utvärdering av frekvensresponsen. Avhandlingen undersöker även inverkan på småsignalstabilitet vid hög förstärkning i nödeffektregleringen. 

Nödeffekt som regleras proportionellt mot frekvensavvikelsen och som använder lokala mätningar utgör en säker och robust lösning för att förbättra frekvensresponsen i det nordiska kraftsystemet, både i nu och framöver. Avhandlingen visar dessutom stabilitets- och kostnadsfördelar vid noga avvägd distribuering av nödeffekt mellan olika HVDC-förbindelser och samordning med frekvenshållningsreserverna. Den enkla och robusta nödeffektdesignen gör det möjligt att analysera olika dynamiska interaktioner och varierande strategier för att undvika negativa stabilitetsinteraktioner. Slutligen visar avhandlingen den övergripande positiva påverkan och framtida rollen av föreslagen nödeffektdesign.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2024. , p. xviii, 129
Series
TRITA-EECS-AVL ; 2024:25
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-344534ISBN: 978-91-8040-861-5 (print)OAI: oai:DiVA.org:kth-344534DiVA, id: diva2:1845365
Public defence
2024-04-18, F3, Lindstedtsvägen 26 & 28, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20240320

Available from: 2024-03-20 Created: 2024-03-18 Last updated: 2024-03-20Bibliographically approved
List of papers
1. Supplementary Power Control of an HVDC System and Its Impact on Electromechanical Dynamics
Open this publication in new window or tab >>Supplementary Power Control of an HVDC System and Its Impact on Electromechanical Dynamics
2021 (English)In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 36, no 5, p. 4599-4610Article in journal (Refereed) Published
Abstract [en]

This paper presents a comprehensive analysis of the impact that supplementary power control of an HVDC link has on the electromechanical dynamics of power systems. The presented work addresses an interesting phenomenon that may occur when an HVDC power controller is installed to support frequency stability. In specific cases, a high gain HVDC frequency controller could deteriorate system damping. The given analytical study is the first of its kind that addresses this issue by including both: (i) the important higher-order generator dynamics that affect small signal stability simultaneously with an HVDC control as well as (ii) the available local angle/frequency input signals of the controller. The methodological approach here analytically formulates the impact an HVDC control has on the single generator dynamics. Furthermore, the relationship between the damping/synchronizing coefficients and the HVDC gain is explicitly derived when a frequency proportional HVDC controller is installed. The derived expressions confirm that, indeed, there is an optimal HVDC gain with respect to the damping coefficient and a typically positive impact of the HVDC controller on the synchronizing component. Finally, the developed theoretical foundation is demonstrated by the tools of linear and nonlinear analysis in a one-machine system case study.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2021
Keywords
HVDC transmission, Power system stability, Power system dynamics, Frequency control, Power control, Generators, Damping, Active power control, frequency stability support, high voltage direct current (HVDC) system, small signal stability assessment
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-301989 (URN)10.1109/TPWRS.2021.3056763 (DOI)000686891700069 ()2-s2.0-85101430510 (Scopus ID)
Note

QC 20210916

Available from: 2021-09-16 Created: 2021-09-16 Last updated: 2024-03-18Bibliographically approved
2. Frequency Dynamics of the Northern European AC/DC Power System: A Look-Ahead Study
Open this publication in new window or tab >>Frequency Dynamics of the Northern European AC/DC Power System: A Look-Ahead Study
Show others...
2022 (English)In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 37, no 6, p. 4661-4672Article in journal (Refereed) Published
Abstract [en]

In many power systems, the increased penetration of inverter-based renewable generation will cause a decrease in kinetic energy storage, leading to higher frequency excursions after a power disturbance. This is the case of the future Nordic Power System (NPS). The look-ahead study reported in this paper shows that the chosen units participating in Frequency Containment Reserves (FCR) cannot keep the frequency above the prescribed threshold following the outage of the largest plant. This analysis relies on a detailed model of the Northern European grid. The latter is compared to the classical single-mass equivalent, and the impact of voltage-dependent loads is assessed in some detail. Next, the paper focuses on emergency power control of the HVDC links that connect the NPS to the rest of the European grid, which can supplement or even replace part of the FCR. The proper tuning of that control is discussed. Finally, the analysis is extended to the HVDC links connecting the future North Sea Wind Power Hub under two configurations, namely low and zero inertia. The impact of outages in the latter sub-system is also assessed. The material to simulate the system with industrial software is made publicly available.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2022
Keywords
HVDC transmission, Load modeling, Europe, Voltage control, Power systems, Kinetic energy, Power system stability, Frequency containment reserves, hvdc frequency support, nordic power system, north sea wind power hub
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-321030 (URN)10.1109/TPWRS.2022.3154720 (DOI)000871076600045 ()2-s2.0-85125298720 (Scopus ID)
Note

QC 20221104

Available from: 2022-11-04 Created: 2022-11-04 Last updated: 2024-03-18Bibliographically approved
3. Linear System Study of the Voltage Dynamics Impact on Frequency Control Performance
Open this publication in new window or tab >>Linear System Study of the Voltage Dynamics Impact on Frequency Control Performance
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The progressive retirement of conventional power plants is decreasing the total system inertia and making the frequency control more delicate. In this context, voltage-sensitive loads will have a more pronounced impact on frequency. Therefore, there is an interest in understanding and quantifying how voltage dynamics impact the frequency response. For this purpose, a linearization of the system and the analysis of the obtained transfer functions are in focus. The paper aims to complement the traditional eigenvalue analysis, capture a system’s response to large disturbances, and estimate the frequency overshoots. The study also shows that controllers affecting the load voltage, such as Power System Stabilizers (PSSs), can impact frequency response during a large disturbance. The impact of PSS on system responses and its transfer functions is carried out by applying a general control configuration. Both conventional PSS1A and multi-band PSS designs are analyzed and compared. Thereby, the paper explicitly interlinks a PSS design, and its parameters with properties of the system’s frequency response reflected in the system transfer function. Illustrating enhancements facilitated by the presented approach, the explicitly derived PSS designs demonstrate the potential to reduce frequency overshoot. As an example, this reduction is achieved by improving a common mode damping or changing the position of the dominant zero of the closed-loop system.

Keywords
Frequency containment reserves, general control configuration, low inertia, power system stabilizers, voltage-dependent load.
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-344533 (URN)
Note

Submitted to journal

QC 20240320

Available from: 2024-03-18 Created: 2024-03-18 Last updated: 2024-03-20Bibliographically approved
4. Assessment and Design of Frequency Containment Reserves with HVDC Interconnections
Open this publication in new window or tab >>Assessment and Design of Frequency Containment Reserves with HVDC Interconnections
2018 (English)In: 2018 2018 North American Power Symposium (NAPS), Institute of Electrical and Electronics Engineers (IEEE), 2018, article id 8600593Conference paper, Published paper (Refereed)
Abstract [en]

Frequency control is one of the main actions in power system operation, since large frequency deviation from the nominal value can lead to automatic frequency protection triggering to avoid equipment damaging. The three main factors which affect the dynamical response of the frequency include the amount of power imbalance due to a disturbance, available reserves and total inertia of the system. Due to increased integration of renewable energy sources, the total inertia of the system decreases and makes the speed of the response more sensitive to power balance disturbances. This paper assesses the dynamical performance of generators involved in the Frequency Containment Reserves and correlates them with additional Emergency Power Control from High Voltage Direct Current (HVDC) interconnections. The currently used constant power ramp control and a new proposed frequency droop control of HVDC interconnections are investigated for different amounts of inertia in a test system representing the Nordic Power System. The performance of each HVDC control is evaluated with respect to the maximum Instantaneous Frequency Deviation and the amount of power required for provided frequency control actions.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2018
Series
North American Power Symposium, ISSN 2163-4939
Keywords
Emergency Power Control, Frequency Containment Reserves, HVDC interconnections, Nordic power system
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-245093 (URN)10.1109/NAPS.2018.8600593 (DOI)000458721400055 ()2-s2.0-85061837116 (Scopus ID)978-1-5386-7138-2 (ISBN)
Conference
2018 North American Power Symposium, NAPS 2018; Fargo; United States; 9 September 2018 through 11 September 2018
Note

QC 20190308

Available from: 2019-03-08 Created: 2019-03-08 Last updated: 2024-03-18Bibliographically approved
5. Assessment of HVDC Frequency Control Methods in the Nordic Test System
Open this publication in new window or tab >>Assessment of HVDC Frequency Control Methods in the Nordic Test System
Show others...
2020 (English)In: Proc. CIGRE conference, CIGRE , 2020Conference paper, Published paper (Refereed)
Abstract [en]

The Frequency Containment Reserve (FCR) is one of the balancing actions to keep the frequency within acceptable limits. The objective of the FCR (also known as primary frequency control) is to stabilize the system frequency within a short time interval after a disturbance. Related to that, maximum steady-state frequency deviation and maximum Instantaneous Frequency Deviation (IFD) are defined. With higher integration of renewable energy sources, power systems will reduce its impact on pollution, but face much more often with low inertia scenarios. With low inertia values, the system decreases its inherent property to react to large power disturbances. In these cases, IFD is profoundly affected, and there is a need for fast and cost-effective solutions.  

High-Voltage Direct-Current (HVDC) links, with appropriate control strategies, could offer a solution to this problem. According to current system requirements, HVDC links must be capable of providing frequency support. Several studies have focused on control methods that adjust the power output of HVDC converters in response to frequency deviations; however, which method performs best in terms of reliability, robustness, and cost-effectiveness has still not been proved. 

The aim of this work is to apply and compare two control methods for HVDC frequency support in a test system representing the Nordic Power System (NPS), where this control mode is referred to as Emergency Power Control (EPC). The first method, the current paradigm in the NPS, is based on ramp power injections and frequency triggering activations. The second method, here proposed as the method for future EPC operation in the NPS, is a droop frequency-based EPC. The performance of these two methods is tested for two different disturbances, with the same EPC  power capacities. The main objective of the EPC is to meet the frequency requirements and avoid any negative interactions. The results show how the new proposed method outperforms compared to the current one, highlighting the benefits of a change of paradigm. The Nordic test system has been designed by authors to capture the frequency response of the NPS. Additionally, a single machine model is used to study the performance of the proposed EPC for the low inertia scenarios.

Place, publisher, year, edition, pages
CIGRE, 2020
Keywords
Droop Frequency Control, Emergency Power Control, Frequency Containment Reserves, HVDC Frequency Support, Low Inertia System, Nordic Power System.
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-344523 (URN)
Conference
CIGRE 2020 Session, Paris, France, 24 Aug - 03 Sep 2020
Note

The paper is presented in both CIGRE 2020 Symposium and CIGRE 2021 Symposium.

QC 20240320

Available from: 2024-03-18 Created: 2024-03-18 Last updated: 2024-03-20Bibliographically approved
6. Distributed HVDC Emergency Power Control: Case study Nordic Power System
Open this publication in new window or tab >>Distributed HVDC Emergency Power Control: Case study Nordic Power System
2022 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Frequency Containment Reserves might be insufficient to provide an appropriate response in the presence of large disturbances and low inertia scenarios. As a solution, this work assesses the supplementary droop frequency-based Emergency Power Control (EPC) from HVDC interconnections, applied in the detailed Nordic Power System model. EPC distribution and factors that determine the EPC performance of an HVDC link are the focus of interest. The main criteria are the maximum Instantaneous Frequency Deviation and used EPC power. The presented methodology is motivated based on the theoretical observation concerning linearized system representation. However, the assessed and proposed properties of interest, such as provided EPC active and reactive power, their ratio, and energy of total loads and losses in the system due to the EPC, concern highly nonlinear system behavior. Finally, based on the obtained study, remarks on the pragmatical importance of the EPC distribution to the frequency nadir limitation are provided.

Keywords
Control distribution, Frequency Containment Reserves, Emergency Power Control, HVDC power control, Nordic Power System
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-344530 (URN)
Conference
11th Bulk Power Systems Dynamics and Control Symposium (IREP'2022), July 25-30, 2022, Banff, Canada
Note

QC 20240320

Available from: 2024-03-18 Created: 2024-03-18 Last updated: 2024-03-20Bibliographically approved

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PhD thesis - Danilo Obradovic - KTH(7781 kB)114 downloads
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