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International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 Voltage Source Converter Based HVDC Overhead Transmission System Mar Mar Win, Thet Tin Abstract— With the development of VSC technology and pulse width modulation (PWM), the VSC-HVDC can provide a number of potential advantages as compared with classic HVDC, such as short circuit current reduction, rapid and independent control of active and reactive power, etc. With those advantages VSC-HVDC are widely used in transmission and distribution systems. It is an efficient and flexible method to transmit large amounts of electric power over long distances by overhead, underground and underwater or submarine transmission lines. One of the advantages of applying a voltage-source converter (VSC) based HVDC system is its potential to be connected to very weak AC systems where the conventional line-commutated converter (LCC) based HVDC system has difficulties. In this paper, the typical transmission system consisting of VSC-HVDC connecting two AC grids is studied. In order to analyze the steady state and dynamic behavior of the developed VSC-based HVDC transmission system, various operation conditions are created. Thyristor) or the IGBT (Insulated Gate Bipolar Transistor). Overhead lines or underground / submarine cables can be used as transmission path. Based on the functions and the locations of the converter station, four main HVDC system configurations are used in power system transmissions. These four HVDC system configurations can be used for both voltage source converter (VSC) and current source converter (CSC) topologies. Monopolar HVDC system Bipolar HVDC system Homopolar HVDC System Back-to-back HVDC system Multiterminal HVDC system Keywords— Active and reactive power control, HVDC system, LCC, PWM converter, VSC. I. INTRODUCTION Today high voltage direct current (HVDC) transmissions are competitive to conventional HVAC transmission and the advanced flexible. But, DC power at low voltage could not be transmitted over long distances, thus giving rise to high voltage alternating current (AC) electrical systems. However, high voltage AC transmission links have disadvantages and engineers were engaged in the development of technology for DC transmission as a supplement to the AC transmissions. For the long transmission, DC is still more favorable than AC. The invention of mercury arc rectifiers and the thyristors valves, made the design and development of linecommutated current sourced converters possible. The first commercial HVDC line built in 1954 was a 98km submarine cable with ground return between the island of Gotland and the Sweden mainland. The first capacitor commutated converter (CCC) build in 1998. An improved in the thyristor-based commutation, the CCC concept is characterized by the use of commutation capacitors inserted in series between the converter transformers and the thyristors valves. The valves of these converters are built up with semiconductors with the ability not only to turn-on but also to turn-off. They are known as VSC (Voltage Source Converters). Two types of semiconductors are normally used in the voltage source converters: the GTO (Gate Turn-Off Figure-1 (a) Back-to-back (b) Monopolar (c) Bipolar (d) Parallel Multi-terminal and (e) Series Multi-terminal II. COMPONENTS OF VSC - HVDC Manuscript received Oct 15, 2011. First Author name, Electrical Power Engineering Department, Mandalay Technological University, ., (e-mail: [email protected]). Mandalay, Myanmar, 09-401569606, A typical VSC-HVDC system, shown in Fig 2, consists of AC filters, transformers, converters, phase reactors, DC capacitors and DC cables. 1 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 power balance during transients and reduce the voltage ripple on the DC side. F. DC Cable The cable used in the VSC-HVDC applications is a new developed type where the insulation is made of an extruded polymer that is Figure-2 . VSC based HVDC system particularly resistant to DC voltage. Polymeric cables are the preferred choice for HVDC mainly because of their mechanical strength, flexibility and low weight. A. Converter The converters are VSCs employing IGBT power semiconductors, one operating as a rectifier and the other as an inverter. The two converters are connected either back-to-back or through a DC cable, depending on the application. Figure-3 (a) Two-level VSC (b) Three-level VSC B. Transformer Normally, the converters are connected to the AC system via transformers. The most important function of the transformers is to transform the voltage of the AC system to a level suitable for the converter. C. Phase Reactor The phase reactors are used for controlling both the active and the reactive power flow by regulating currents through them. The reactors also function as AC filters to reduce the high frequency harmonic contents of the AC currents which are caused by the switching operation of the VSCs. Figure -4 (a) Single-core cable with lead sheath and wire armour (b) Three-core cable with optic fibers, lead sheath and wire armour III. TRANSMISSION SYSTEM HVDC transmission systems have three basic parts as shown in Figure-5: 1) converter station to convert AC to DC 2) transmission line 3) second converter station to convert back to AC. D. AC Filter The AC voltage output contains harmonic components, caused by the switching of the IGBTs. The harmonics emitted into the AC system have to be limited to prevent them from causing malfunction of AC system equipment or radio and telecommunication disturbances. High-pass filter branches are installed to mitigate these high order harmonics. With VSC converters there is no need to compensate any reactive power and the current harmonics on the AC side are related directly to the PWM frequency. Therefore, the amount of filters in this type of converters is reduced as compared with line commutated converters. Figure-5 HVDC Transmission System from source to consumers IV. SIMULATIONS AND RESULTS Figure-4 Filters used in HVDC transmission;(a) Single-tuned;(b) Double-tuned;(c) High pass (d) C type high-pass E. DC Capacitor On the DC side there are two capacitor stacks with the same size. The size of these capacitors depends on the required DC voltage. The objective of the DC capacitors is to provide an energy buffer to keep the A. Simulink Model Construction for VSC-HVDC For the simulation of voltage source converter used in HVDC system, HVDC transmission system is constructed as shown in Figure-7. The HVDC transmission system consists of four main portions as the AC grid 1, the AC grid 2, the HVDC link between the AC systems and the converter stations. The HVDC overhead line is connected to transmit the power from AC grid 1 to AC grid 2. Thus, rectifier station 2 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 is set up at AC grid 1 side and inverter station is at AC grid 2 side. The VSCs are located at both stations and the rectifier or inverter mode operation is executed by the firing angle control. The data for HVDC transmission system under construction is shown in Table 1. Rectifier Station HVDC Transmission frequency harmonics, shunt filtering is therefore relatively small compared to the converter rating. The 40 Mvar shunt AC filters are 27th and 54th high-pass tuned around the two dominating harmonics. Inverter Station Wind Power Plant AC Grid VSC Controller VSC Controller Figure-7 HVDC Transmission System for VSC Simulation Table 1. Data for System Model Name Specification AC Grid 1 AC Grid 2 Rectifier Station Inverter Station HVDC line 25kV,200MW,50 Hz 132kV,50Hz Three Level Bridge IGBT/Diode Three Level Bridge IGBT/Diode ±150kV,100km Bipolar HVDC Line Remark Three Phase AC Source Three Phase AC Source VSC Control VSC Control π Section Lines B. System Descriptions A 200 MW (± 150 kV) forced-commutated voltage-sourced converter (VSC) interconnection is used to transmit DC power from a 132 kV, 2000 MVA, and 50 Hz system to another identical AC system. The AC systems (1 and 2) are modeled by damped L-R equivalents with an angle of 80 degrees at fundamental frequency and at the third harmonic. The rectifier and the inverter are constructed with three-level Neutral Point Clamped (NPC) VSC converters using close IGBT/Diodes. The rectifier and the inverter are interconnected through a 100 km cable (i.e. 2 pi sections) and two 8 mH smoothing reactors. The sinusoidal pulse width modulation (SPWM) switching uses a single-phase triangular carrier wave with a frequency of 27 time’s fundamental frequency (1350 Hz).Along with converter, the station includes on the AC side: step down Y-Δ transformer, AC filters, converter reactor; the system on DC side has: the capacitors and DC filters. A converter transformer (Wye grounded /Delta) is used to permit the optimal voltage transformation. The present winding arrangement blocks triples harmonics produced by the converter. The 0.15 pu phase reactor with the 0.15 p.u transformer leakage reactance permits the VSC output voltage to shift in phase and amplitude with respect to the AC system Point of Common Coupling (PCC) and allows control of converter active and reactive power output. The tap position is rather at a fixed position determined by a multiplication factor applied to the primary nominal voltage of the converter transformers. The multiplication factors are chosen to have a modulation index around 0.85 (transformer ratios of 0.915 on the rectifier side and 1.015 on the inverter side). To meet AC system harmonic specifications, AC filters form an essential part of the scheme. They can be connected as shunt elements on the AC system side or the converter side of the converter transformer. Since there are only high Figure-8 VSC-Based HVDC Transmission Link 200 MVA (+/- 150kV) C. Design Procedure In the present work, the rectifier/inverter are three levels VSC that use the IGBT/diode module available in the MATLAB/Simulink/Simpower system. The case study is done for a VSC based HVDC transmission link rated 200 MVA (200MW, 0.95), ±150kV. The system on AC side has: step down Y-Δ transformer, AC filters, Converter reactor. The system on DC side has: Capacitors and DC filters. The design of the components on AC and DC side are shown below. DC voltage rating: ±150kV System frequency: 50Hz Source AC voltage: 230kV line voltage Rated DC current=Rated DC power/Rated DC voltage D. AC System Modelling AC system is modeled as a simple three phase AC source with internal resistance and inductance that is calculated from short circuit level MVA calculations. (MVA)B = 200MVA (KV)B = 230 kV (Phase to Phase rms), 50 Hz E. Transformer Design Y grounded /Δ Transformer is used to permit the optimal voltage transformation. It also blocks the triplen harmonics produced by the converter. The following data for the transformer is considered: Nominal Power =200MVA (total for three phases) Nominal frequency=50Hz. Winding1 specifications: Y connected, Nominal voltage = 230kV rms (Line to Line) X 0.915 (to simulate a fixed tap ratio) = 210.45kV Resistance = 0.0025pu, Leakage reactance = 0.0075pu Winding 2 specifications: Δ connected, nominal voltage = 150kV rms (Line to Line), Resistance = 0.0025pu, Leakage reactance = 0.075pu Magnetizing losses at nominal voltage in % of nominal current: Resistive 5 %( =500pu). F. AC Filter Nominal voltage: 150kV Nominal frequency: 50Hz 3 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 Nominal reactive power: 20% of real power (200MW) = 78.5Mvar Tuning frequency= 27*50 and 54*50. Quality factor= 15. G. Simulations Results Figure-16. Three phase Current at receiving end Figure-9. DC Voltage at receiving end Figure-17. DC voltage at sending end (X) Figure-10. DC Voltage in p.u at receiving end Figure-18. DC voltage in p.u at sending end (X) Figure-11. DC real power in p.u at receiving end (Y) Figure-19. DC real power in p.u at sending end (X) Figure-12. Receiving end (Y) AC voltage measured in p.u Figure-20. Sending end (X) AC voltage measured in p.u Figure-13. Active power in p.u at receiving end (Y) Figure-21. Active power in p.u at sending end (X) Figure-14. Reactive power in p.u at receiving end (Y) Figure-22. Reactive power in p.u at sending end (X) Figure-15.Three phase Voltage at receiving end 4 All Rights Reserved © 2012 IJSETR International Journal of Science, Engineering and Technology Research (IJSETR) Volume 1, Issue 1, July 2012 Figure-23. Three phase Voltage in p.u at sending end (X) Figure-24. Three phase Current in p.u at sending end (X) From the above simulation results it can be observed that by VSC HVDC transmission line, the reactive power at Y end can be minimized as well as the receiving end voltage can be maintained at 1pu without using any compensation. V. CONCLUSION This Paper presents the steady-state performance of AC Transmission System and VSC based HVDC transmission system. The modeling details of HVDC system with three levels VSC are discussed. From the simulation results, it is concluded that the system response is fast; high quality AC voltages and AC currents can be obtained; and that the active power and the reactive power can be controlled independently and are bi-directional. The proposed scheme also ensures that, the receiving end voltage is maintained at 1 pu without any compensation. ACKNOWLEDGMENT The author is deeply grateful to her supervisor, U Thet Tin, lecturer, Department of Electrical Power Engineering, Technological University (Mandalay), for help, permission and suggestions. The author would also like to thank to Dr. Khin Thuzar Soe, Associated Professor and Head of Department, Department of Electrical Power Engineering, Technological University (Mandalay), to her helpful suggestions, valuable advice and permission to carry out of this paper. The author appreciates and thanks all her teachers for their support, and guidance during theoretical study and thesis preparation. REFERENCES [1] [2] [3] [4] “Voltage Source Converter Based HVDC Transmission”, International Journal of Engineering Science and Innovative Technology (IJESIT) Volume 1, Issue 1, September 2012 Stan, A.I., 2010, “Control of VSC-Based HVDC Transmission System for Offshore Wind Power Plants”, Master Thesis, Aalborg University, Denmark Shire, T. W., 2009, “VSC-HVDC based Network Reinforcement”, M. Sc. Thesis, Delft University of Technology, U. S. A. The Math Works Inc, 2010a, MATLAB 7.0, http://www.mathworks.com. 5 All Rights Reserved © 2012 IJSETR