Master Thesis Proposal: Real-time and off-line simulation of DC Grids

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1 Master Thesis Proposal: Real-time and off-line simulation of DC Grids Background The revival of direct current (DC) for long-distance power transmission began in 1954 when ABB linked the island of Gotland to mainland Sweden with high-voltage DC (HVDC) lines. Now, more than 145 projects using HVDC are operational worldwide [1]. Glancing at the European DC Super Grid, the European Union is vigorously promoting the European electricity transmission system development, which is mainly based on DC, designed to facilitate large-scale sustainable power generation in remote areas for transmission to centers of consumption [2]. DC power transmission allows for transporting large amounts of electricity across long distances, exceeding the transport capacity of AC lines. Especially when considering the ultimate goal of smart grids which enables sustainable energy system, these lines will be crucial to bring power from renewable sources like wind farms and solar to main consumption center. In the last 20 years the importance of HVDC long distance interconnections and back to back couplings has increased significantly all over the world. Classic, thyristor based HVDC systems have been built up to ± 800 kv DC voltage [3]. Recently, a new HVDC technology based on voltage source converters (VSC) has emerged. In particular, VSC-HVDC can address not only conventional network issues such as bulk power transmission, asynchronous network interconnections, back-to-back ac system linking, and voltage/stability support to mention a few, but also niche markets such as the integration of large-scale renewable energy sources with the grid and most recently large onshore/offshore wind farms [4]. This project will focus on the VSC-based HVDC transmission system. Even though the application of point-to-point HVDC links is well known, there are plentiful open questions of the operation of HVDC links within a mesh grid. Unfortunately very few detailed DC grid models, especially large DC grid models, using realistic data are available for research due to commercial and trade secret reasons. In order to alter this situation and facilitate DC grid research and development, CIGRE 1 promoted collaboration with experts in DC grid and tried to improve DC grid modeling by developing a generic model and corresponding DC grid test system. The Smart Transmission Systems Laboratory (SmarTS Lab) at KTH collaborated with CIGRE for DC grid model simulation and validation. On the other hand, to avoid modeling ambiguity is of crucial need for model exchange among different software not only for this project, but also is an important subject for electric power system modeling. Different scales of DC grid models in the software EMTP-RV, Simulink and Hypersim will be developed and simulated (both off-line and real-time) in this project and we are also trying to look into the model exchange challenge and outline our possible solution as well. Project Outline 1 The International Council on Large Electric Systems (in French: Conseil international des grands réseaux électriques or CIGRÉ) is a world organization in the field of high voltage electricity. KTH Background 1

2 This project is one part of EIT 2 project for DC grid development. Two students will be recruited for this project and carry out their work at both the SmarTS Lab (at KTH) and RTE 3 (in Paris, France) (more detailed information will be negotiated and provided after finishing the recruitment process). This project will start with the study and improvement of higher level control for the VSC in Simulink. Then different scales of DC test system need to be developed, including pint-to-point DC link, threepoint DC grid, a three-point DC grid with large AC grid, and the CIGRE DC Grid Test System (9 Converters). All the systems will be implemented in Simulink and Hypersim, and comparisons of simulation results need to be carried out with available models in EMTP-RV 4 for validation. In addition, the Simulink model will be applied into RT-LAB 5 for real-time simulation with the Opal-RT real-time simulators at SmarTS Lab. The payment for this master thesis will be provided by Electric Power System Department (after successful completion of the project and defense of the MSc thesis) and RTE (more detailed information will be negotiated and provided after finishing the recruitment process). The application for this master thesis should be sent to the main contact Wei Li with the personal CV and transcripts of Bachelor and Master attached. The deadline for the application will be the end of January Objectives 1. Perform literature review on the subject, especially considering DC grid modeling and simulation. The newly developed generic control model based on voltage source converters (VSC) need to be covered as well. 2. Study and improve the higher level control for VSC in Simulink and try to transfer it into Dynamic Linked Library (DLL) by using C compiler. In the case of Hypersim C code needs to be generated using the Real Time Workshop (MATLAB/Simulink). 3. Develop the point-to-point DC link test system, three-point DC grid test, a three-point DC grid with large AC grid, and the CIGRE DC Grid Test System (9 Converters)and Hypersim, respectively. 4. Off-line simulations for above three cases are carried out in Simulink and Hypersim. 5. Familiarize with the real time simulator in the lab. 6. Transfer the Simulink model into RT-LAB and carry out real-time simulation for above three cases. 2 The EIT is a body of the European Union established in March Their mission is to increase European sustainable growth and competitiveness by reinforcing the innovation capacity of the EU. 3 RTE is the French Transmission System Operator. 4 EMTP-RV is professional software for simulation and analysis of transients in power systems. 5 RT-LAB is the open Real-Time Simulation software environment that has revolutionized the way Model-based Design is performed. It enables engineers to conduct Real-Time Simulation of Simulink models with Hardwarein-the-Loop, in a very short time, at a low cost. KTH Objectives 2

3 7. Compare the simulation results in Simulink and Hypersim with the one in EMTP-RV, including both off-line simulation and real-time simulation. 8. Conclude the DC grid modeling and simulation experience, and propose the future directions in order to develop a library for DC grid modeling and simulation in the different software platforms. Deliverables The students will carry out their work in SmarTS Lab at the Electric Power Systems Department and RTE. The reports on the progress of the work need to be presented to their supervisor during regular team meetings on one week basis. These reports will serve as a basis for discussion and feedback on the work in progress. At the end of the project a written report should be produced and a final presentation held. The development of models should be clearly documented and any resulting models and software in any of the simulation platforms shall be provided to SmarTS Lab and RTE for further development. Additionally, the students need to be part of the writing process of a paper on the subject with the results obtained and are required to enroll the course EG2011 Publication of Mater s Project within Electric Power Systems simultaneously. Time Plan The master thesis is planned to start as soon as possible and will be carried out during 28 weeks (not necessary, depending on the workload). Weeks Tasks Deliverables 1-4 Literature review about DC grid modeling and simulation, generic control model based on List of references. Reports for the materials review. voltage source converters. 5-7 Study and improve the higher level control for Higher level control model. VSC in Simulink and try to transfer it into DLL Develop and simulate the point-to-point DC grid test system in HyperSim. Compare with the results in Simulink model and EMTP-RV model Develop and simulate the three-point DC link test system in Simulink and HyperSim. Compare with the results in EMTP-RV Develop and simulate the three-point DC grid with large AC grid test system in Simulink and HyperSim Study how to do real-time simulation in RT-LAB by using the real time simulator in the lab Writing the master thesis with analysis of the results. The pint-to-point DC grid test system in HyperSim. The three-point DC link test system in Simulink and HyperSim. The three-point DC grid with large AC grid test system in Simulink and HyperSim. Real-time simulation results for all the models. Final report and presentation KTH Deliverables 3

4 Supervisor and Examiner Contact Information Supervisors: Wei Li (Main contact and supervisor in KTH) Ph.D. candidate School of Electrical Engineering, Electric Power System, KTH Phone: Mobile: +46 (0) wei3@kth.se Sébastien Dennetière (Supervisor in RTE) RTE - National Center For Grid Expertise Paris - La Défense - France Tel : +33 (0) Sebastien.dennetiere@rte-france.com Examiner: Dr. Luigi Vanfretti Docent and Assistant Professor School of Electrical Engineering, Electric Power System, KTH Phone: luigi.vanfretti@ee.kth.se Homepage: Bibliography [1] M. Kanellos, "Direct Current Gets Another Look," The New York Times, 17 November [2] L. Weimers, "A European DC Super Grid--A Technology Providers View," ABB, Brussel, [3] Siemens Smart Grid Division, "AC/DC grids: Concepts and designs for mixed AC/DC grids," Siemens, Deutschland, [4] N. Flourentzou, V. G. Agelidis and G. D. Demetriades, "VSC-Based HVDC Power Transmission Systems: An Overview," IEEE TRANSACTIONS ON POWER ELECTRONICS, vol. 24, no. 3, pp , KTH Supervisor and Examiner Contact Information 4

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