| 1. |
- Beza, Mebtu Bihonegn, 1981, et al.
(författare)
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Analytical Derivation of the AC-Side Input Admittance of a Modular Multilevel Converter with Open- and Closed-Loop Control Strategies
- 2018
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Ingår i: IEEE Transactions on Power Delivery. - 0885-8977 .- 1937-4208. ; 33:1, s. 248-256
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Tidskriftsartikel (refereegranskat)abstract
- This paper deals with the derivation of the ac-side input admittance of a grid-connected modular multilevel converter (MMC) under both open- and closed-loop ac-side current control strategies. The contribution of the internal dynamics of the MMC on the input admittance depends on the type of ac-side controllers implemented. For this reason, the input admittance is first derived with the MMC operating in open-loop on the ac side. The results are then modified to include various acside controllers in order to analyze the impact of the internal dynamics of the MMC on the total input admittance. Based on the findings, recommendations are given on modeling the input admittance of the MMC. Finally, the derived analytical models are verified using a detailed switching model of the MMC in the frequency domain.
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| 2. |
- Cutululis, Nicolaos Antonio, et al.
(författare)
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Challenges Towards the Deployment of Offshore Grids: the OffshoreDC Project
- 2014
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Ingår i: 13th International Workshop on Large-Scale Integration of Wind Power into Power Systems as well as on Transmission Networks for Offshore Wind Plants, Berlin, Germany, 11-13 Nov. 2014. - 9783981387094
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Konferensbidrag (refereegranskat)abstract
- This paper summarizes challenges towards the deployment of offshore grids which are dealt with in the Nordic OffshoreDC project. The OffshoreDC project studies the techno-economic challenges related to assessment of the value and use of optimization in the design of offshore grids. The project also studies the technical challenges related to control, protection and provision of ancillary services from HVDC grids with connected wind power plants. Finally, the transients in the DC grids are studied in the project.
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| 3. |
- Song, Yujiao, 1986, et al.
(författare)
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Analytical investigation of poorly damped conditions in VSC-HVDC systems
- 2016
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Ingår i: 2016 IEEE 55th Conference on Decision and Control, CDC 2016. - 9781509018376 ; , s. Art no 7798762, Pages 3279-3285
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Konferensbidrag (refereegranskat)abstract
- In this paper, strong AC-grid connected VSC-HVDC systems are studied. Under specific conditions, such systems can suffer from both stability and poor damping related issues, which warrants a stability study. Analytical eigenvalue expressions can directly demonstrate the impact of physical or control parameters on the system stability. However, especially in case of high-order systems, such expressions are challenging to obtain. This paper suggests a method to symbolically represent approximative eigenvalues of two-terminal VSC-HVDC systems, which could also be used to analyze the system dynamics. In addition, by applying symbolic-isolation method, the order of a multi-terminal VSC-HVDC system canbe reduced to an equivalent two-terminal VSC-HVDC system,which enables the proposed method to provide symbolic pole expressions. Numerical studies based on Matlab simulations are presented, showing the accuracy of the analytical eigenvalue expressions and providing useful hints on the impact of physical or control parameters.
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| 4. |
- Stamatiou, Georgios, 1984, et al.
(författare)
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A novel decentralized control strategy for MultiTerminal HVDC transmission grids
- 2015
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Ingår i: 7th Annual IEEE Energy Conversion Congress and Exposition, ECCE 2015, Montreal, Canada, 20-24 September 2015. - 2329-3721. - 9781467371506 ; , s. 5794-5801
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Konferensbidrag (refereegranskat)abstract
- The concept of Voltage Source Converter based MultiTerminal High Voltage Direct Current (VSC-MTDC) grids, represents both challenges and opportunities for the future of large power transfer and integration of renewable energy sources. For this kind of grids, the control aspect is of great importance, with voltage-droop based methods considered as one of the most attractive solutions. All existing strategies are designed to maintain the level of voltage in the MTDC grid constant during unexpected events, thus sacrificing the power flow. The aim of this paper is to propose a new droop controller structure that maintains the dc-grid voltage close to the nominal values and at the same time tries to preserve the power flow in the dc grid, following events such as faults or disconnection of stations. The control scheme is presented and simulations are carried out in a four-terminal MTDC grid, proving the validity of the concept.
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| 5. |
- Stamatiou, Georgios, 1984
(författare)
-
Analysis of VSC-based HVDC systems
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main objective of this thesis is to perform stability and control studies in the area of VSC-HVDC systems. A major part of the investigation focuses on the development of procedures, whose aim is to understand, explain and avoid poorly-damped conditions or instability that may appear due to dc-side resonances, which stem from the interaction of converters and passive elements in such systems.An analytical approach is initially considered, where the eigenvalues of VSC-HVDC systems are approximated by analytical closed-form expressions. The Similarity Matrix Transformation (SMT) method is introduced and applied to the reduced 4th order state-space model of a two-terminal VSC-HVDC system. The results show that the SMT offers improved accuracy in approximating the actual eigenvalues of the system, compared to the already established LR method. Nevertheless, the two analytical methods are not free of limitations. The increase in modeling accuracy of a system renders the analytical approach impractical or impossible to use. A frequency-domain approach proves ideal in performing a stability analysis in such cases, and is therefore considered and applied to a detailed two-terminal, two-level converter-based VSC-HVDC system. The latter is modeled as a Single-Input Single-Output (SISO) feedback system, where the VSC-system and dc-grid transfer functions are defined and derived. The passivity analysis and the net-damping criterion are separately utilized and assessed on their potential to be adequate analysis tool in VSC-HVDC stability studies.In contrast with the typical Two-Level Converter (2LC), the Modular Multilevel Converter (MMC) has a fundamentally different structure that introduces internal dynamics and requires additional control for the converter to operate properly. The dc-side input admittance of the MMC is analytically derived, allowing the dynamic impact of MMCs in two-terminal VSC-HVDC systems to be analyzed from a frequency-domain perspective. The contribution of the MMC's circulating-current control to the closed-loop system stability is investigated and the differences of the MMC and the 2LC in terms of their passivity characteristics are highlighted.Finally, studies are performed in VSC-based Multiterminal grids, with the objective of proposing advanced control strategies that can offer robust performance during steady-state and transient conditions, with improved power flow and direct-voltage handling capabilities. The properties of the proposed controllers are assessed through simulations of four- and five-terminal grids, where their benefits compared to those of their conventional counterparts are shown.
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| 6. |
- Stamatiou, Georgios, 1984, et al.
(författare)
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Analytical derivation of poorly-damped eigenvalues in two-Terminal VSCHVDC systems
- 2017
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Ingår i: 2017 19th European Conference on Power Electronics and Applications, EPE 2017 ECCE Europe. - 9789075815276 ; 2017-January
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Konferensbidrag (refereegranskat)abstract
- Under specific conditions, Voltage Source Converter based High Voltage Direct Current (VSC-HVDC) systems can experience stability, or poor-damping related issues. Analytical investigation of the system eigenvalues can demonstrate the impact of physical or control parameters on the system stability. However, especially in case of high-order systems, such expressions are challenging to obtain and so extensive in form that they are impractical to use and cannot provide useful hints on the impact of single parameters on the system's stability. Therefore, current methods in power system analysis rather rely on a purely numerical approach to describe eigenvalues and are deprived of the benefits of the symbolic representation. This paper suggests a method to analytically describe a two-Terminal VSC-HVDC link with respect to its dominant eigenvalues, providing a useful tool for the investigation of poorly-damped performance and system stability, by deriving compact analytical eigenvalue expressions.
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| 7. |
- Stamatiou, Georgios, 1984, et al.
(författare)
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Analytical Derivation of the DC-Side Input Admittance of the Direct-Voltage Controlled Modular Multilevel Converter
- 2017
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Ingår i: IET Generation, Transmission and Distribution. - : Institution of Engineering and Technology (IET). - 1751-8687 .- 1751-8695. ; 11:16, s. 4018-4030
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Tidskriftsartikel (refereegranskat)abstract
- The input admittance of a converter, connected to other dynamic subsystems, is a useful tool to investigate whether poorly damped oscillations or even unstable conditions might occur at certain frequencies. This is of interest in applications employing the modular multilevel converter (MMC), where the internal dynamics of the converter and the increased number of control loops greatly affect the MMC’s dynamic behaviour, compared to other types of converters. In this paper, the dc-side input admittance of the direct-voltage controlled MMC is derived analytically and verified via small-signal perturbation in a detailed nonlinear time-domain simulation model. The MMC’s input admittance is parametrically studied and compared to the dc-side input admittance of an equivalent two-level converter.
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| 8. |
- Stamatiou, Georgios, 1984
(författare)
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Converter interactions in VSC-based HVDC systems
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main objective of this thesis is to perform stability and control studies in the area of VSC-HVDC. A major part of the investigation focuses on the explanation of poorly-damped conditions and instability that are linked to dc-side resonances. Initially, a frequency domain approach is considered, applied to a two-terminal VSC-HVDC connection that is modeled as a Single-Input Single-Output (SISO) feedback system, where the VSC-system and dc-grid transfer functions are defined and derived. The passivity analysis and the net-damping criterion are separately applied, demonstrating the superiority of the latter as an analysis tool. Furthermore, it was discovered that the net-damping of a system and the damping factor of its poorly-damped dominant poles are correlated in an almost linear way.The occurrence of poorly-damped conditions is further analyzed from an analytical perspective, where the eigenvalues of a two-terminal VSC-HVDC system are approximated by closed-form expressions. This offers the benefit of a deeper understanding in the way selected parameters of the system can affect the frequency and damping characteristics of its eigenvalues. The Similarity Matrix Transformation (SMT) method is introduced in this thesis and applied to the reduced 4th order state-space model of a two-terminal VSC-HVDC system. The results show that the SMT offers improved accuracy in approximating the actual eigenvalues of the system, compared to the already established LR method.Finally, studies are performed in VSC-MTDC grids, with the main objective of proposing advanced control strategies that can offer robust performance during steady-state and transient conditions, with improved power flow and direct-voltage handling capabilities. The advantageous properties of the proposed controllers are proven through simulations of four- and five-terminal MTDC grids, in which their benefits compared to their conventional counterparts are shown.
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| 9. |
- Stamatiou, Georgios, 1984, et al.
(författare)
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Decentralized converter controller for multiterminal HVDC grids
- 2013
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Ingår i: 15th European Conference on Power Electronics and Applications (EPE), 2-6 Sept. 2013, Lille, France. - 9781479901166 ; , s. Article number 6634680-
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Konferensbidrag (refereegranskat)abstract
- Multiterminal High Voltage Direct Current (MTDC) grids based on Voltage Source Converters (VSC) are considered ideal for the future integration of large scale and highly distributed renewable power generation, as well as interconnection of remote power systems. The control of such grids is challenging as it is necessary to provide accurate power flow control and multiple voltage controlling stations. Existing control techniques either fail to fulfill those requirements or achieve them at a cost of maintaining a communication channel between the local station controllers and a master central control level. This paper suggests a decentralized controller for MTDC grids that ensures accurate power flow control, provides voltage control capabilities to all stations and requires no need for communication between the stations. The validity of the controller was tested in a five-terminal MTDC model proving its functionality.
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| 10. |
- Stamatiou, Georgios, 1984, et al.
(författare)
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Economics of DC wind collection grid as affected by cost of key components
- 2011
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Ingår i: World Renewable Energy Congress – Sweden, 8–13 May, 2011, Linköping, Sweden. - : Linköping University Electronic Press. - 1650-3740. - 9789173930703 ; 15:057, s. 8-
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Konferensbidrag (refereegranskat)abstract
- Using High Voltage Direct Current (HVDC) transmission lines to connect offshore wind parks to the onshore power grid has been proven technically advantageous to AC solutions and more cost-effective for relatively long transmission distances (>70km).The concept of applying DC technology can be expanded to the collection grid. DC collection grid offshore wind parks can be developed only if several key components currently nonexistent are available. The technical challenges involved can result in the unpredictability of their costs. This paper investigates the effect the uncertainty of the key components’ cost can have on the overall economic performance of DC collection grid offshore wind parks. Results for a wide cost range are presented and corresponding cost boundaries which secure the economic viability of such parks were determined.
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