LOW FREQUENCY TRANSMISSION FOR REMOTE POWER GENERATING SYSTEMS

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LOW FREQUENCY TRANSMISSION FOR REMOTE POWER GENERATING SYSTEMS A Thesis Presented to The Academic Faculty by Anupama Keeli In Partial Fulfillment of the Requirements for the Degree Master of Science in
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LOW FREQUENCY TRANSMISSION FOR REMOTE POWER GENERATING SYSTEMS A Thesis Presented to The Academic Faculty by Anupama Keeli In Partial Fulfillment of the Requirements for the Degree Master of Science in the School of Electrical and Computer Engineering Georgia Institute of Technology AUGUST 2011 COPYRIGHT 2011 BY ANUPAMA KEELI LOW FREQUENCY TRANSMISSION FOR REMOTE POWER GENERATING SYSTEMS Approved by: Dr. A.P. Sakis Meliopoulos, Advisor School of Electrical and Computer Engineering Georgia Institute of Technology Dr. David Taylor School of Electrical and Computer Engineering Georgia Institute of Technology Dr. Deepakraj M Divan School of Electrical and Computer Engineering Georgia Institute of Technology Date Approved: June 28, 2011 ACKNOWLEDGEMENTS The MS thesis has been funded by the Power System Engineering Research Center project S-42, Low Frequency transmission for wind farm power. I am very thankful for the support. I would like to thank my advisor, Dr. A.P.Meliopoulos for giving me the opportunity to work on this project and his guidance through my research and time at Georgia Tech. Dr. Dionysios Aliprantis and Chen Hao from Iowa State University were also a part of the project. I am very grateful for their support and time in discussing the ideas and progress of the project. I am also grateful for the MS Thesis reading committee members, Dr. David Taylor and Dr. Deepak Divan and their willingness to support and help improve the thesis. During the course of my Masters, I have had the opportunity to interact and exchange ideas with many of my fellow students and this has helped me in my research. For this, I thank Yong Hee Lee, Ravishankar Nilakantan, Rohit Ajitkumar, Xuebei Yu, Matthew Reno, Satya Sridevi, and Umar Tariq. I would like to thank my parents for their love and support and continued encouragement which has been a source for all my success. iii TABLE OF CONTENTS Page ACKNOWLEDGEMENTS LIST OF TABLES LIST OF FIGURES LIST OF SYMBOLS AND ABBREVIATIONS SUMMARY iii vii viii xi xiii CHAPTER 1 INTRODUCTION Problem Statement Research Objectives 1 2 LITERATURE REVIEW Introduction Technologies for Wind Farm Power Transmission Wind Farm Connections and Cost Analysis 4 3 TOPOLOGIES FOR LOW FREQUENCY TRANSMISSION Introduction Wind farm topologies Wind system configuration 1: AC wind farm, Nominal iv frequency, Network connection Wind system configuration 2: AC wind farm, AC/DC transmission, Nominal frequency, Network connection Wind system configuration 3: Series DC wind farm, Nominal frequency, Network connection Wind system configuration 4: Parallel DC wind farm, Nominal frequency, Network connection Wind system configuration 5: Series DC wind farm, Low frequency Radial AC transmission Wind system configuration 6: Parallel DC wind farm, Low frequency Radial AC transmission Wind system configuration 7: Series DC wind farm, Low frequency transmission Network Wind system configuration 8: Parallel DC wind farm, Low frequency transmission Network 16 4 VOLTAGE LEVEL SELECTION Wind farm configuration 1: Series DC wind farm, radial transmission Calculation of transmission loss (up to the Main Dc Bus) ($/yr) Calculation of cost of the cable and converter equipment in ($/yr) Wind farm configuration 2: Series DC wind farm network Calculation of transmission loss (up to the Main Dc Bus) ($/yr) 24 v 4.2.2 Calculation of cost of the cable and converter equipment in ($/yr) Wind farm configuration 3: Parallel DC wind farm, radial transmission Calculation of transmission loss (up to the Main Dc Bus) ($/yr) Calculation of cost of the cable and converter equipment in ($/yr) Wind farm configuration 4: Parallel DC wind farm network Calculation of transmission loss (up to the Main Dc Bus) ($/yr) Calculation of cost of the cable and converter equipment in ($/yr) 32 5 WinIGS-F MODEL EXAMPLE Wind Farm Modeling Wind system configuration 1_ model Wind system configuration 4_ model Wind system configuration 4_ model CONCLUSIONS AND FUTURE WORK 40 APPENDIX A: DEVICE MODELS AND LINE CONFIGURATIONS 42 REFERENCES 64 vi LIST OF TABLES Table 5.1: Transmission power loss for Wind system configuration 1 (60 Hz transmission) 35 Table 5.2: Transmission power loss for wind system configuration 4 (60 Hz transmission) 38 Table 5.3: Transmission power loss for wind system configuration 8 (20 Hz transmission) 39 Page vii LIST OF FIGURES Figure 3.2.1: Wind system configuration 1: AC wind farm, Nominal frequency, Network connection 7 Figure 3.2.2: Wind system configuration 2: AC wind farm, AC/DC transmission, Network connection 9 Page Figure 3.2.3: Wind system configuration 2: Series DC wind farm, Nominal frequency, Network connection 10 Figure 3.2.4: Wind system configuration 4: Parallel DC Wind farm, Nominal frequency, Network connection 11 Figure 3.2.5: Wind system configuration 5: Series DC wind farm, Low frequency Radial AC transmission 13 Figure 3.2.6: Wind system configuration 6: Parallel DC Wind Farm, Low frequency, Radial transmission 14 Figure 3.2.7: Wind system configuration 7: Series DC wind Farm, Low frequency AC transmission Network 15 Figure 3.2.8: Wind system configuration 8: Parallel DC wind Farm, Low frequency AC transmission Network 16 Figure 4.1: Wind farm configuration 1: Series DC wind farm, radial connection 19 Figure 4.2: Plot of voltage at the main DC bus vs. total cost for m i = 10, Pt = 3 MW 22 Figure 4.3: Wind system configuration 2: Series DC wind farm, nominal frequency, network connection 24 Figure 4.4: Plot of voltage at the main DC bus vs. total cost for m i = 5, n f = 5, Pt = 2 MW 26 Figure 4.5: Wind farm configuration 6: Parallel DC wind farm, low frequency, radial connection 28 Figure 4.6: Plot of voltage at the main DC bus vs. total cost for m i m i = 10, n f =1, Pt = 3 MW 30 Figure 4.7: Wind farm configuration 4: DC parallel wind farm, radial connection 32 viii Figure 4.8: Plot of voltage at the main DC bus vs. total cost for m i = 10, n f =2, Pt = 3 MW 33 Figure 5.1: Wind system configuration 1 (60 Hz transmission) 35 Figure 5.2: Wind system configuration 1 (20 Hz transmission) 36 Figure 5.3: Wind system configuration 4 (60 Hz transmission) 37 Figure 5.4: Wind system configuration 8 (20Hz transmission) 38 Figure A.1: Generator parameters for WinIGS-F model 1 42 Figure A.2: Parameters of three phase step up transformer at the wind turbine system 43 Figure A.3: Parameters of three phase transformer at the collector substation 44 Figure A.4: Cable model for WinIGS-F model 1 45 Figure A.5: Cable parameters 45 Figure A.6: Parameters of three phase overhead transmission line (length = 54 mi, operating voltage = 69 kv) 46 Figure A.7: Parameters of three phase overhead transmission line (length = 54 mi, operating voltage = 115 kv) 47 Figure A.8: Parameters of three phase overhead transmission line (length = 54 mi, operating voltage = 138 kv) 48 Figure A.9: Parameters of three phase overhead transmission line (length = 100 mi, operating voltage = 69 kv) 49 Figure A.10: Parameters of three phase overhead transmission line (length = 100 mi, operating voltage = 115 kv) 50 Figure A.11: Parameters of three phase overhead transmission line (length = 100 mi, operating voltage = 138 kv) 51 Figure A.12: Parameters of Constant Power three-phase Electric load 52 Figure A.13: Parameters of the slack generator 53 Figure A.14: Parameters of the ground impedance model 54 Figure A.15: Parameters of the 20 Hz transmission line operating at 138 kv 55 ix Figure A.16: Three phase equivalent circuit of the 20 Hz transmission 56 Figure A.17: 2-Terminal connector model 56 Figure A.18: Model for the Two-Primary-Bus Connector Model 57 Figure A.19: Parameters of the Series R model 58 Figure A.20: Conductor library 58 Figure A.21: Tower library 59 Figure A.22: Power flow solution report for wind system configuration 1 (60 Hz transmission) 60 Figure A.23: Power flow solution report for wind system configuration 1 (20 Hz transmission) 61 Figure A.24: Power flow solution report for wind system configuration 4 (60 Hz transmission) 62 Figure A.25: Power flow solution report for wind system configuration 8 (20 Hz transmission) 63 x LIST OF SYMBOLS AND ABBREVIATIONS A AC ACost CCost Cost conv Cost inv CCost m d i DC ft HVDC Hz i I DC kv l L LFAC Loss m m m i Annual investment for cable and converter Alternating Current Acquisition cost of the cable per feet Acquisition cost of cable for entire wind farm Cost of converter Cost of inverter Cost of m parallel cables Distance between adjacent wind turbines in feeder i Direct Current feet High Voltage Direct Current Hertz Interest rate DC current in the DC circuit Kilovolt Distance from the last wind turbine to the collection point Total length of cable Low frequency AC transmission Loss for m parallel cables Number of parallel cables Number of wind turbines in the radial feeder i xi MW n N n f P P L P t r R V gn V xn WinIGS WTS X Mega Watt Life time Number of wind turbines Number of radial feeders Acquisition cost of the cable and converter Transmission power loss Power rating of the wind turbine Resistance of the cable per unit length Positive sequence resistance of medium voltage cable per unit length Nominal generator voltage Nominal high side transformer voltage Windows Integrated Grounding System Wind Turbine System Positive sequence reactance of medium voltage cable per unit length xii SUMMARY The goal of this Masters Thesis research is to evaluate alternative transmission systems from remote wind farms to the main grid using low-frequency AC technology. Low frequency means a frequency lower than nominal frequency (60/50Hz). The lowfrequency AC network can be connected to the power grid at major substations via cycloconverters that provide a low-cost interconnection and synchronization with the main grid. Cyclo-converter technology is utilized to minimize costs which result in systems of 20/16.66 Hz (for 60/50Hz systems respectively). Low frequency transmission has the potential to provide an attractive solution in terms of economics and technical merits. The optimal voltage level selection for transmission within the wind farm and up to the interconnection with the power grid is investigated. The proposed system is expected to have costs substantially lower than HVDC and conventional HVAC systems. The cost savings will come from the fact that cyclo-converters are used which are much lower in cost than HVDC. Other savings can come from optimizing the topology of the wind farms. Another advantage of the proposed topologies is that existing transformers designed for 60 Hz can be used for the proposed topologies (for example a 345kV/69 kv, 60Hz transformer can be used for a 115 kv/23kv, 20 Hz system). The results from this research indicate that the use of LFAC technology for transmission reduces the transmission power losses and the cost of the transmission system. The alternate topologies suitable for various geographical locations and optimal voltage for transmission within the wind farms are presented in the thesis. xiii CHAPTER 1 INTRODUCTION 1.1 Problem Statement Renewable sources of energy are widely available and proper utilization of these resources leads to decreased dependence on the fossil fuels. Wind is one such renewable source available in nature and could supply at least a part of the electric power. In many remote locations the potential for wind energy is high. Making use of the available wind resources greatly reduces the dependence on the conventional fuels and lowers the emission rates. There are a few problems associated with the wind which makes the wind energy more expensive than other forms of electric power generation. The two main issues are: (a) Large wind farms are located in remote locations which make the cost of transmission of wind power costly, and (b) the intermittent supply of power due to the unpredictability of the wind that results in lower capacity credits for the operation of the integrated power system. These issues are addressed by designing alternative topologies and transmission systems operating at low frequency for the purpose of decreasing the cost of transmission and making the wind farm a more reliable power source. The use of DC transmission within the wind farm enables the output of wind generators to be rectified via a standard transformer/rectifier arrangement to DC of appropriate kv level. 1.2 Research Objectives The following tasks have been carried out for the development of alternate topologies with low frequency transmission technology. 1 Literature study of previous research on low frequency AC transmission and wind farm topologies. Design of alternate topologies. Calculation of optimal transmission voltage levels for different topologies. Modeling the system using WinIGS-F software. Optimization of the system is done depending on the area covered by the wind farm. Storage can be provided at the DC buses. The storage is not unique to lowfrequency transmission. An inverter is used to transform DC to AC of low frequency, preferably of 1/3 of the normal power frequency. The low frequency AC voltage is transformed to higher voltages for efficient transmission. The above-described basic approach produces a number of alternative topologies for specific geographical arrangements. In addition, it will allow the development of other forms of storage systems, such as hydro or pumped hydro that may be located in remote areas. 2 CHAPTER 2 BACKGROUND AND LITERATURE REVIEW 2.1 Introduction In recent years the amount of electricity produced from wind has grown rapidly. Large wind farms located remotely or offshore are used to meet the increasing power demand. This requires transmission of power over long distances. In this study, low frequency AC (LFAC) transmission technology is proposed for various designs of the wind farms. 2.2 Technologies for Wind Farm Power Transmission The possible solutions for transmitting power from wind farms are HVAC, Line commutated HVDC and voltage source based HVDC (VSC-HVDC). Low frequency AC transmission (LFAC) is particularly beneficial in terms of cost savings and reduction of line losses [4] in cases where the distance from the power generating stations to the main power grid is large. The use of fractional frequency transmission system (FFTS) for offshore wind power is discussed in [6]. The author proposes LFAC as an alternative to HVAC and HVDC technologies for a short and intermediate transmission distances. HVAC is more economical for short transmission distances. For longer distances, HVAC has disadvantages like increase in the cable cost, terminal cost and charging. HVDC transmission systems and wind farm topologies are discussed in [12]. HVDC being a matured technology is used for longer distances. Compared to HVDC, the LFAC system reduces the usage of an electronic converter terminal which reduces the investment cost. HVDC technology is used only for point-to-point transmission [11], and LFAC can be used for similar networks as AC transmission. 3 Further, VSC-HVDC replaces the thyristors with IGBTs and is considered to be the most feasible solution for long distance transmission. However, addition of converter stations on both sides of the transmission line increase the investment cost of the VSC- HVDC system [7] compared to LFAC. Hence, due to the limitations of the HVAC and HVDC the proposed LFAC is used in the design of transmission systems. The use of LFAC can be extended to long transmission distances. Cyclo converter technology is used for converting the AC of nominal frequency to AC of one third frequency i.e Hz/20 Hz for a 50 Hz/ 60 Hz transmission system`. Several advantages of the LFAC are identified. The transmission system used for conventional AC system can be used for LFAC without any modifications and the LFAC system increases the transmission capacity. 2.3 Wind Farm Connections and Cost Analysis Thomas et al., [2] proposed that a series connection of the wind farm leads to the elimination of the offshore platform and the turbines output would be DC. As a result the desired high transmission voltage can be obtained directly without a large centralized DC/DC converter. Various wind farm designs are proposed based on this idea with low frequency and nominal frequency transmissions. Lundberg [7, 8] presents various wind farms and energy production costs for these wind farms. The different electrical configurations for the wind farm discussed are large AC, AC/DC, small DC, large DC and series DC system. A result of the study is that a series DC wind farm has promising energy production cost, if the transmission distance is above 20 km. Wind power is an intermittent source of energy. The author in [1] presents a computationally efficient recursive algorithm to calculate the wind farm power output distribution. The model focuses on generation and transmission system reliability. In the 4 present study various wind farm configurations are designed to determine the total cost of the energy production and transmission system reliability issues need to be addressed. Increased penetrations of wind energy cause operational problems to the utility grid [3]. An AC/DC/AC interface resolves the operational problems caused due to the increased wind energy penetration by introducing protection and coordinated control of the wind farm output. The goal of this thesis is to evaluate alternative transmission systems from remote wind farms to the main grid using LFAC. The wind farm design, cost and efficiency of the transmission system influence the economics of the overall system [12]. The main focus of this thesis is to develop comprehensive methodology for determining the optimal topology and optimal transmission voltage for a LFAC system. 5 CHAPTER 3 TOPOLOGIES FOR LOW FREQUENCY TRANSMISSION 3.1 Introduction The literature survey indicates that many topologies and systems have been proposed for transmitting power from wind farms to the main power grid. In this section a number of topologies and systems are presented that are further evaluated for their costs and operating voltages. 3.2 Wind farm topologies A wind farm is a group of interconnected wind turbines located in the same geographical area. Individual wind turbines are interconnected by medium voltage power collection system. In order to connect the local wind turbine grid to the transmission system the voltage is increased. A wind farm contains the following elements: wind turbines, local wind turbine grid, collecting point, transmission system and wind farm interface to the point of common connection (PCC). The power from the wind turbine units is collected at the collecting point where the voltage is increased to a level suitable for transmission. The power is then transmitted to the wind farm grid interface over the transmission system. The following are the different wind farm topologies: Wind system configuration 1: AC wind farm, Nominal frequency, Network connection Wind farms that are built today have an AC electrical system from the wind turbines to the PCC. Two different types of AC wind farms referred in this thesis are radial and network connections. Radial wind farms are suitable for small wind farms with a short transmission distance. In a small AC wind farm, the local wind farm grid is used both for connecting all wind turbines in a radial fashion and transmitting the generated 6 power to the wind farm grid interface. Network connected wind farms are usually large AC wind farms where the local wind farm grid has a lower voltage level than the transmission system. Figure 3.2.1: Wind system configuration 1: AC wind farm, Nominal frequency, Network connection The wind system configuration 1 shown in figure has network connection of wind turbines and AC power collection system. The configuration is described with the following parameters: n f : Number of radial feeders m i : Number of wind turbines in radial feeder i d i : Distance between adjacent wind
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