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low cost interconnection is needed. In this paper, a flexible ac power transmission link technology has been proposed for linking two asynchronous independent ...
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Flexible Asynchronous Ac Link for Power System Network Interconnection Imdadullah, M. Irshad, M. S. Jamil Asghar, member IEEE and S. Javed Arif

Abstract-- The primary reason for interconnections among different power grids is to reduce the overall economic costs as well as increase reliability and security of supplying electricity services. Thus, for electrical power flow from one power system or grid to another power system or grid a simple, reliable and low cost interconnection is needed. In this paper, a flexible ac power transmission link technology has been proposed for linking two asynchronous independent power systems. The proposed flexible asynchronous ac link (FASAL) system essentially consists of a rotating transformer which is put in the ac tie line between two separate power systems or grids. It controls the power transmission between these power systems which are asynchronous under some or all operating conditions. The direction and the magnitude of power flow is controlled by controlling voltage and/or frequency. Simulink model of proposed FASAL system has been developed for the analytical study and the result verifies the power transfer capability of the proposed system. Index Terms-- Flexible asynchronous ac link, power system interconnection, transfer of power, freely rotating asynchronous machine.

I. INTRODUCTION

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LECTRIC power supply systems are widely interconnectted for economic and reliable power delivery and to pool power plants and load centers in order to minimize the total power generation capacity and fuel cost . These transmission interconnections enable taking advantage of diversity of loads, availability of sources, and fuel price in order to supply electricity to the loads at minimum cost with a required reliability [1]. In fact, improved electric power systems reliability will increase the quality of service and reduction in power interruptions and average regional manufacturing costs in the commercial and industrial sectors that leads to an increase in the national gross domestic product (GDP) [2]. The AC link and DC link are two options available for large power system interconnections. Since, AC is the dominant mode of generation, transmission and distribution in power system, therefore, the AC link is the “natural” way of Imdadullah is with the Electrical Engineering Section, University Polytechnic, A.M.U., Aligarh, India (e-mail: [email protected]). Mohammad Irshad is with the Department of Electrical Engineering, A.M.U., Aligarh, India (e-mail: [email protected]) Mohammad Syed Jamil Asghar is with the Department of Electrical Engineering, A.M.U., Aligarh, India (e-mail:[email protected]). S. J. Arif is with the Department of Electronics Engineering, Aligarh Muslim University, Aligarh 202002, India (email: [email protected]).

978-1-4673-1835-8/12/$31.00 ©2012 IEEE

interconnecting existing AC power systems. The interconnections have been mostly realized by AC link since this option is technically feasible and economically justified [3]. Thus, for electrical power flow from one power system network to another power system network a simple, reliable and low cost interconnection is needed. Therefore, a flexible ac link is desirable to link one power system network to another power system network reliably such that either side is least affected by the disturbances in them (for example, due to fault condition or switching transients). When an alternator or an asynchronous power system is directly connected to the grid, or one asynchronous power system is connected to another power system, many problems may arise. A severe transient inrush current flows in the system at the instant of switching-in. Thus, to avoid this condition, asynchronous interconnection between power systems is achieved by high voltage direct current (hvdc) link. But, hvdc conversion is complicated due to the need of closely coordinate harmonic filtering, controls, and reactive compensation. Moreover, hvdc has performance limits when the ac power system on either side has low capacity compared to the hvdc power rating. Further, hvdc systems need conversion plants at both sides of the tie line which increases cost and undesirably require significant space, due to the large number of high voltage switches and filter banks [4]-[5]. Sometimes, an induction generator is also used to supply power (often low power generated by renewable energy sources e.g. wind, hydro etc) to the grid. It does not require any conventional synchronizing process. But, the induction generators supplying power to the grid are often repeatedly disconnected due to lower input power (speed below synchronous speed) or due to a system fault. Thereafter, switching as well as re-switching cause inrush current i.e. it draws large current from grid or power system. The condition become severe if the grid is weak. The magnitude of the inrush current depends on the phase angle of the voltage wave at the instant of switching, when the generator or incoming network gets reconnected to the grid. Moreover, the transient electromechanical torque may be negative and large enough to give the machine and the tower of the wind turbine a severe jolt. The actual amplitude and the peak of the transient torque are closely dependent on the rotor speed, switching instant of the voltage wave and duration of the interruption. In the worst case, the first peak may reach about 15 times the rated fullload torque. Frequent faults of this nature can cause shaft

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breakage due to fatigue loading, particularly at the coupling with the wind turbine [6]. An arrangement with a tap-changing transformer and a phase-shifting transformer is used to connect two different power systems and to control power flow along a transmission line [7]-[9]. It has the drawbacks of stepwise operation and slow about stability problem on the grid, and components wear out due to repetitive action. When the phase-shifting transformers are used with power electronics, some of these drawbacks are eliminated. However, they have their own limitations including problems of harmonics, risk of torsional interactions, risk of rapid bypass for grid disturbance, and short overload capability due to low thermal time constant [10]. Moreover, the classical arrangement of tap-changing transformers lacks the ability of reactive compensators to supply or absorb reactive power and thus this burden is left to the power system to handle [1]. Recently, an asynchronous ac link, an alternative of HVDC link, has been developed for two power systems. The requirement of HVDC link, phase shifting transformers and the problem of re-switching are eliminated by putting a variable frequency transformer (VFT) in between two asynchronous power systems[11]-[15]. In fact, VFT is rotated at a particular torque and a particular speed. In this scheme, a separate dc motor is used to control torque, speed and direction of rotation of VFT which in turn controls the power transmission from one power system to another power system at a constant frequency (e.g. 60 Hz). However, this VFT based asynchronous link suffers from a serious drawback that it requires a forced rotation of VFT. Moreover, a constant torque is required even at zero speed, when the frequencies of both grids are same. The system also requires frequent shutdown and maintenance due to replacement of carbon brushes of the high rating dc motor which is a part of the VFT system. Moreover, when there is a fault in power system, VFT requires very large torque to compensate [16]. This leads to requirement of very high rating dc drive as VFT has to handle bulk power. As an alternative of VFT system, here, a flexible asynchronous ac link (FASAL) system is proposed. The VFT is replaced by static ac to ac converter, rotating transformer and voltage regulator without the use of any high power dc drive system [17]. The electrical power transmission takes place between the two networks via magnetic coupling through the air gap. II. FASAL SYSTEM Figure 1 illustrates a conceptual system diagram of the FASAL system. The two separate electrical networks are connected to the stator and rotor, respectively. Electrical power is exchanged between the two networks by the magnetic coupling through the air gap. An adjustable frequency ac to ac converter and a voltage controller are used to control the direction and the magnitude of the power flow through the FASAL system.

Fig. 1. System diagram of the FASAL

In the proposed FASAL system, a doubly fed wound rotor induction machine operates as a rotating transformer (RT) at free-running condition which is used to link two power systems. In this system, the magnitude and direction of power flow is controlled independently by controlling the voltage and frequency. Moreover, during the fault condition, the speed of RT adjusts itself to compensate. Even in case of severe triple line (LLL) or triple line to ground (LLLG) fault, only a limited effect is slowly transferred to the other power system side. In fact, at this condition it behaves as shorted rotor circuit of a wound rotor induction motor (WRIM) and slowly picks up speed. Thus, no significant electrical power flow takes place through RT from the healthy side of the power system which is only magnetically coupled and electrically isolated. Only small power is needed for the acceleration of freerunning, unloaded and mechanically free (uncoupled) rotor of RT. It demands slowly a small amount of additional power (mechanical) for acceleration from the healthy power system (grid). Therefore, it gives sufficient cution period for protective gears to act and the effect of fault is thus delayed and localized. Therefore, it does not severely affect the stability of healthy side of power system network. III.

SIMULATION MODEL

A three-phase voltage source block is used as Grid # 1 and another three phase voltage source block is used as Grid # 2. An asynchronous machine SI unit block is used for induction machine model as shown in Fig. 2. Voltage and current measurement blocks are used as voltmeter and ammeter respectively. Scope and display blocks are used for graph plotting and data recording. Figures 3, 4 and 5 show the simulated results of real power flow from Grid # 1 to Grid # 2 at different frequencies (i.e. frequency of Grid # 1 side is kept higher than the Grid # 2 side) and voltage on stator/rotor side is varied which causes power transmission. In all three cases, the amount of power transfer is different depending upon the magnitude of frequency and voltages on stator/rotor side. Along with the power transfer, currents and voltages on both side is recorded and shown in the simulation graphs.

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IV. EXPERIMENTAL RESULTS The proposed FASAL system shown in Fig.1 is realized with the help of three phase wound rotor induction machine, three phase static ac to ac converter and voltage controller. The Grid # 1 is connected to the stator through a three phase ac to ac converter and Grid # 2 is connected to the rotor through voltage controller of wound-rotor induction machine. The power transfer from Grid # 1 to Grid # 2 takes place when voltage on stator/rotor side and frequency on stator side is varied. The direction of power flow is controlled by controlling the frequency. Power flows from high frequency side to lower frequency side of wound-rotor induction machine. Figure 6 shows that real power flow from Grid # 1 to Grid # 2 as frequency of Grid # 1 side is increased keeping the voltage on both the side same. In this case the power transfer is about 95W. Figure 7 shows that real power flow from Grid # 1 to Grid # 2 as frequency of Grid # 1 side is increased and the voltage on both the side is same (i.e. 150V). In this case the power transfer is about 180W. Figure 8 shows that real power flow from Grid # 1 to Grid # 2 as frequency of Grid # 1 side is kept higher (58.4 Hz) than Grid # 2 side (50 Hz) and voltage on stator side is increased keeping rotor side voltage constant (i.e. 100V). In this case the power transfer is about 175W. Figure 9 shows that real power flow from Grid # 1 to Grid # 2 as frequency of Grid # 1 side is kept higher (58.4 Hz) than Grid # 2 side (50 Hz) and voltage on stator side increases, keeping rotor side voltage constant (i.e. 200V). In this case the power transfer is about 475W.

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COMPARISON OF EXPERIMENTAL AND SIMULATED RESULTS

VI. CONCLUSION

The experimental results have been compared with simulated results obtained from Simulink model for different values of stator voltage, rotor voltage and Grid # 1 side frequency, as shown in the Figures 10, 11 and 12. The simulated and experimental results verify the real power transfer from Grid # 1 to Grid # 2.

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This paper proposes a new flexible asynchronous ac link (FASAL) system for linking two asynchronous independent power systems. It controls the power transfer between these power systems which are asynchronous under some or all operating conditions. The successful implementation of the proposed FASAL system is elaborated by using simulation and experimental results. The proposed FASAL system can also be used in joining two renewable energy power systems making a micro grid with minimum infrastructure for reliable transfer of power. The proposed FASAL system is a viable alternative to back-to-back HVDC link, VFT system and replacement for phase shifting transformers based link for the interconnection of asynchronous power systems.

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VII. REFERENCES

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N.G. Hingorani and L. Gyugyi, “Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems,” IEEE press/Standard Publishers Distributors, Delhi, 2001. A report published by UN Department of Economic and Social Affairs, Division for Sustainable Development, “Multi dimensional issues in international electric power grid interconnections” United Nations, New York, 2006, available : http://www.un.org/esa/sustdev/publications/energy/interconnections.pdf J. Wu, J. Wen, H. Sun, and S. Cheng, “Feasibility study of segmenting large power system interconnections with ac link using energy storage technology,” IEEE Trans. Power Syst. on page (s): 1 - 8, March 2012. DOI- 10.1109/TPWRS.2012.2185255 (Early Access Articles). Einar Vaughn Larsen, Charlton, N.Y. “Power Flow Control and Power Recovery with Rotary Transformers,” U.S. Patent Number: 5,953,225; Date of Patent: Sep.14, 1999. H.Wang and M.A. Redfern, “The advantages and disadvantages of using HVDC to interconnect AC networks,” in Proc. IEEE 45th Int. UPEC, Cardiff, Wales, Aug. 31- Sept. 3, 2010, pp.1-5 M.R. Patel, “Wind and Solar Power Systems: Design, Analysis and Operation,” second edition, CRC Press, Boca Raton, Florida, 2006. J. Verboomen, D. V. Hertem, P.H. Schavemaker, W. L. Kling, R. Belmans, “Phase Shifting Transformers: Principles and Applications,” in proc. 2005 IEEE Int. Conf. Future Power Systems, page(s) 1-6. J. Verboomen, D. V. Hertem, P. H. Schavemaker, W. L. Kling and R. Belmans, “Analytical Approach to Grid Operation with Phase Shifting Transformers,” IEEE Trans. Power Syst., vol. 23, no.1, pp. 41-46 ,Feb. 2008. K. K. Lim and M. Ilie, “Control of Underload-Tap Changing Transformer,” in proc. 1991 IEEE American Control Conference, pp. 1021 - 1025 E. Larsen, “A classical approach to constructing a power flow controller,” in IEEE Power Eng. Soc. Summer Meet., vol. 2, pp. 1192– 1195, 1999. R. Gauthier. (2004, Nov.). A world-first VFT installation in Quebec. Transm. Distrib. World [Online]. Available: http://tdworld.com/mag/ power_worldfirst_vft_installation/ E. Larsen, R. Piwko, D. McLaren, D. McNabb, M. Granger, M. Dusseault, L. Rollin, and J. Primeau, “Variable Frequency Transformer – A New Alternative For Asynchronous Power Transfer,” presented at Canada Power, Toronto, Ontario, Canada, Sep. 28-30, 2004. P. Doyon, D. McLaren, M. White, Y. Li, P. Truman, E. Larsen, C. Wegner, E. Pratico, and R. Piwko, "Development of a 100MW Variable Frequency Transformer," presented at Canada Power, Toronto, Ontario, Canada, Sep. 28-30, 2004. M. Dusseault, D. Galibois, J. Gagnon, M. Granger, D. McNabb, D. Nadeau, J. Primeau, E. Larsen, S. Fiset, C. Wegner, and E. Pratico, “First VFT Application and Commissioning,” presented at Canada Power, Toronto, Ontario, Canada, Sep. 28-30, 2004. A. Merkhouf, P. Doyon, and S. Upadhyay, “Variable Frequency Transformer—Concept and Electromagnetic Design Evaluation,” IEEE

Trans. Energy Convers, vol. 23, no. 4, pp. 989-996, Dec. 2008. [16] D. Nadeau, “A 100-MW Variable Frequency Transformer (VFT) on the Hydro- Québec TransÉnergie Network –The Behavior during Disturbance,” IEEE Power Engineering Society General Meeting, 2007. On page(s): 1-5, ISSN: 1932-5517, ISBN: 1-4244-1296-X [17] M.S. Jamil Asghar and Imdadullah, “A flexible Asynchronous AC link (FASAL) system,” Patent Application #1116/DEL/2009A. Indian Official journal of the patent office, Issue No. 01/2010, p. 08, Jan.1, 2010.

VIII. BIOGRAPHIES Imdadullah was born in West Champaran, India in 1982. He received the B.Tech. degree in electrical engineering and the M.Tech. degree in power systems and drives from Zakir Husain College of Engg. and Tech. (ZHCET), Aligarh Muslim University (AMU), Aligarh, India in 2003 and 2006. He is currently an Assistant Professor in electrical engineering with the University Polytechnic, AMU, Aligarh, India. His areas of interests are renewable energy, power systems and drives and instrumentation and measurement. Mohammad Irshad received the B.Tech. degree in electrical engineering and the M.Tech. degree in power systems and drives from Zakir Husain College of Engg. and Tech. (ZHCET), Aligarh Muslim University (AMU), Aligarh, India in 2008 and 2010. His research interests include analysis, design, modeling and simulation of Power System, Power Electronics and Drives.

M. S. Jamil Asghar (M’94) was born in Patna, India. He received the B.Sc.

(Engg.) degree in elec-trical engineering, M.Sc. (Engg.) degree in power systems, and Ph.D. degree in power electronics from Aligarh Muslim University, Aligarh, India. He joined the Department of Electrical Engineering, AMU, in 1983, where he is currently a Professor. He established the Centre of Renewable Energy, Department of Electrical Engineering, AMU, from the funds of UGC (Government of India). He has written a text book Power Electronics (Prentice-Hall of India), and he is a chapter author of Power Electronics Handbook (Academic/Elsevier, CA, under a joint program of the University of West Florida, Pensacola, and University of Florida, Gainesville). He has successfully completed many government-funded research projects and has guided many research thesis and 17 M.Tech. thesis. He is the holder of several patents. He has published more than 60 papers in refereed journals and conference proceedings, including several singleauthored papers in IEEE TRANSACTIONS. His research and teaching interests include power electronics, renewable energy systems, and electrical machines. Dr. Asghar is a Fellow of Institution of Electronics and Telecommunication Engineers (IETE), India.

Syed Javed Arif received the B.Sc. (Engg.) degree in electrical engineering and the M.Sc. (Engg.) degree in instrumentation and control from Aligarh Muslim University (AMU), Aligarh, India. He was an Electronics Engineer with AMU from 1991 to 1997. Since 1997, he has been with the faculty of the AMU, where he is currently an Assistant Professor with the Department of Electronics Engineering. His area of interest is instrumentation and measurement.