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40220140505005
40220140505005
40220140505005
40220140505005
40220140505005
40220140505005
40220140505005
40220140505005
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40220140505005

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  • 1. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 36-43 © IAEME 36 A REVIEW: LOW VOLTAGE RIDE THROUGH (LVRT) IN WIND FARM Pravin S. Phutane PG student, Department of Electrical & Electronics Engineering, RKDF College of Engineering, Bhopal, M.P. India A.K. Jhala Associate Professor, Department of Electrical & Electronics Engineering, RKDF College of Engineering, Bhopal, M.P. India ABSTRACT In concept of green energy, wind energy is one of the promising energy sources of electrical power. Renewable wind power is fastest growing generation globally. Wind generation system has the potential for grid support. However, when the wind power is connected to an electric grid affects the power quality and poses problems like stability, grid voltage disturbances. It become very important to address these challenges and problems arise due to wind generation as Induction generators are very sensitive to the grid voltage disturbances and need retrofit solution to enhance their low voltage ride through (LVRT) capability. This paper analyses the extent to which the low voltage ride through (LVRT) capability of wind farms using induction generators can be enhance by the use of FACTS controllers such as STATCOMs or MERs (Magnetic Energy Recovery Switch),compared to the thyristors controlled Static Var Compensators(SVSs). The transient stability margin is proposed as indicator of LVRT capability. To know the effect of STATCOMs or MERs on transient stability margins a torque slip characteristic approach is considered. The method for estimating the required rating of different compensation devices to ensure stability after fault is suggested based on same approach. Index Terms: Wind Farm, Grid Code, STATCOM, Low Voltage Ride Through (LVRT), MERS. INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) ISSN 0976 – 6545(Print) ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 36-43 © IAEME: www.iaeme.com/ijeet.asp Journal Impact Factor (2014): 6.8310 (Calculated by GISI) www.jifactor.com IJEET © I A E M E
  • 2. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 36-43 © IAEME 37 I. INTRODUCTION Research of grid connected wind turbines has gained great interest in the recent years. Due to this, new guidelines and regulations has been set up for connection of large wind farms including guidelines for fault tide through or low voltage ride through (LVRT) capability [1] As the wind power plants increase in size and command a larger share of supply portfolio, they are required to stay operational and not disconnect from the grid supporting the grid with reactive power during and after voltage sags [2]. Such requirements are known as Fault Ride-Through (FRT) or Low Voltage Ride-Through (LVRT) capability. Low voltage ride-through is a condition required to the wind generators when the voltage in the grid is temporarily reduced due to a fault or large load change in the grid. The required low voltage ride-through (LVRT) behavior is defined in grid codes. In this paper, a survey on recent LVRT solutions for different generator topology is discussed. LVRT capability must be provide to wind turbines as penetration of wind power continues. Crowbars are commonly used to protect the power converters during voltage dips and their main drawback is that the DFIG absorbs reactive power from the grid during grid faults. we know that there are emergency grid code requirement, as per this requirement if any fault which may be external or any voltage failure condition occurs in that case wind generator should continue its generation and should remain connected to grid and continue own reactive power supply. II. TYPES OF WIND GENERATORS Following are the types of generator used for wind generation a) Squirrel Cage Induction Generators (SCIG) b) Doubly Fed Induction Generator (DFIG) c) Permanent Magnet Synchronous Generators (PMSG) Figure 1: Schematic diagram of the electric system of a fixed speed wind turbine Figure 2: WECS with squirrel cage induction generator (SCIG)
  • 3. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 36-43 © IAEME 38 Figure 3: WECS with Doubly-Fed Induction Generator (DFIG) Figure 4: WECS with Permanent Magnet Synchronous Generator (PMSG) Low voltage ride through (LVRT) of squirrel cage induction generators in wind farms is challenging due to the large required reactive power after low voltage period [3].Using power electronic converters with reduced capacity in Doubly-Fed Induction Generator (DFIG) based wind turbines makes them vulnerable to over-current during grid disturbances[4]. Solutions on LVRT such as blade pitch angle control, control of fully rated converters, and capacitor sizing. Other solutions on LVRT include active crowbar rotor circuit and the DC bus energy storage circuit are illustrated for permanent magnet synchronous generators (PMSG) in [1]. III. WIND TURBINE TECHNOLOGY The kinetic energy of moving air molecules are converted into rotational energy by the rotor of wind turbine. This rotational energy in turn is converted into electrical energy by wind electric generator. The amount of power, which the wind transfers to the rotor, depends on the density of air, the rotor area, and the wind speed. The power contained by wind is given by, P =0.5*(air mass flow rate)*(wind velocity)2 = 0.5* (ρ*A*V) * (V)2 =0.5ρAV3 where, P = power contained in the wind (W) ρ = air density (kg/m3) A = rotor area (m2) V=wind velocity before rotor interference (m/s)
  • 4. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 36-43 © IAEME 39 The power coefficient (Cp) describes the efficiency of a turbine that converts the energy in the wind to rotational power. Hence power output of the turbine is given by Pο = 0.5ρAV3Cp ----------------1 The tip speed ratio of the wind turbine is given as λ =ωR/V ------------ 2 Where R = radius of the swept area in meters ω= angular speed in radians per second [6] IV. COMPARISON OF LVRT TECHNOLOGIES In the past, wind generators were regarded as devices for distributed generation and thus were made to trip following even minor disturbances. However, as the penetration of wind power penetration increases, their impact on the grid can no longer be ignored. Today, wind turbines are required to operate like conventional generators in many utilities; consequently, many new grid codes for wind generators have been established [8], governing their LVRT capability, reactive power capability, and real power control. Of these requirements, LVRT capability represents the greatest challenge to wind turbine manufacturers. LVRT requires that wind generators must connect to the grid and stable during most grid faults, so that they contribute to the grid stability [7]. To enhancement of wind farms the LVRT capability, several approaches have been proposed, which can be divided into three categories: installation of additional hardware, improvement of the control of the power converter, and determination of the optimal point of common coupling (PCC). A commonly used solution for satisfying the LVRT requirement by the additional hardware installation method involves the use of an active crowbar circuit [10], which provides a low-impedance path for the rotor current during the fault. This method is relatively simple and cost-effective. However, upon activation of the crowbar, control over the machine is lost, and the DFIG behaves as a squirrel cage induction machine; therefore, the issue of reactive power consumption during grid faults must be addressed. Another method that involves the installation of hardware is to insert an energy storage system (ESS) [11], such as super capacitors or batteries , at the dc bus inside the rotor-side converter, where the energy produced during the fault is momentarily stored and is later exported to the grid following fault clearing. The ESS can also be applied under steady-state conditions to attenuate power fluctuation caused by wind variations. However, larger ESS systems require more space; accordingly, a dc chopper can be added inside the dc-link [12]. This dc chopper dissipates the excess power through its resistor and maintains the dc-bus voltage in a safe range during the critical period. V. WIND FARMS AND GRID CODES For any public electricity network, technical requirement must comply and agreed by its connected consumers and generators. It is well known that to provide many control functions electric networks are depends on generators, this led to increase complexity in technical requirements for generators. These technical requirements are termed ‘Grid Codes’ [13]. Above stated complex technical requirement which governs relationship between system operator and generators, should be defined clearly. This process makes more complicated with the introduction of renewable generation. Generators used in renewable generation have different characteristics than conventional synchronous generators. Hence in some countries like Germany a specific grid code has been
  • 5. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 36-43 © IAEME 40 developed for wind farms, and in others countries the aim has been to define these grid codes which is nothing but the technical requirement in which renewable generators technical requirements is independent of conventional generators. The technical requirements within grid codes and related documents vary between electricity systems. The typical requirements for generators can be classified as follows for simplicity: • Tolerance - the range of conditions on the electricity system for which wind farms must continue to operate; • Control of reactive power - often this includes requirements to contribute to voltage control on the network; • Control of active power - often this includes requirements to contribute to frequency control on the network; • Protective devices; and • Power quality. Basic Requirement of the Grid Code on LVRT Wind turbine low voltage ride through (LVRT) means that, when the voltage at point-of- common coupling of wind farm drops, wind turbine should maintain its grid-connected state and even provide a certain amount of reactive power to the grid to support the grid recovery until its normal state is reached. Thus low-voltage time/regional is crossed Figure 5: LVRT Requirements of Wind Farm
  • 6. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 36-43 © IAEME 41 Figure 5 is a Chinese standard [9] on LVRT for wind farms, given as follows: a) For different types of power system faults, if the voltage at point-of-common coupling of wind farm stands above the contour line in Figure 6, wind turbine must ensure not escaping from the grid and maintain continuous operation, otherwise, should allow wind turbine to cut out. b) For the wind farm not cutting out the grid during power system fault, after the fault is cleared, its active power should restore at least 10% of rated power per second until to the value of pre-failure. c) For the wind farm group whose total installed capacity is beyond one million kW, when three- phase short-circuit fault of power system causes its voltage dropping, each wind farm should have dynamic reactive supporting ability during LVRT process. VI. METHODOLOGY USED Basically different techniques are used to know the transient behavior of cage induction generators if the grid voltage is going to be changed. Following are the techniques: A. SVC Static Var Compensator B. STATCOM C. MERS type series FACTS controller A. SVC Static Var Compensator Traditionally, the use of synchronous condensers, SCs, and/or static var compensators, SVCs, have been the most commonly employed techniques for the stability improvement of ac systems. The disadvantages of the synchronous condensers systems are known as slow response and high losses. Apart from being costly solutions, the use of SVCs has the disadvantage that it becomes less effective when the ac voltage level is reduced, i.e. during faults and large disturbances. The SVC consists of a thyristor controlled reactor, and thyristor or mechanically switched capacitors. For the purpose of this investigation, the SVC can be considered as shunt impedance determined by the parallel connection of the capacitor and the effective inductance of the thyristor controlled reactor [14]. B. STATCOM The STATCOM is a power electronics device based on the voltage source converter principle. The technology typically in use is, depending on voltage level and total rating, a two- or three-level voltage source converter, controlled by digital techniques and connected to the power system in shunt through a filter and possibly a coupling transformer [14]. C. MERS type series FACTS controller Reactive power compensation with shunt compensation can improve the LVRT capability, but requires a large rating. The alternative use of series compensation is studied. Series compensation is based on increasing the reactive power transfer from the grid, and can by this reduce the required rating of the compensator compared to shunt compensation. A series compensator called magnetic energy recovery switch (MERS) is found suitable due to large control range and good over-current capability.
  • 7. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 36-43 © IAEME 42 VII. ANALYSIS The DFIG is an induction machine which requires reactive power compensation during grid side disturbances. STATCOM is a feasible option to provide the necessary reactive power compensation when connected to a weak grid [15]. STATCOM can support the fixed speed wind power plant in order to fulfilment the required voltage-dip ride-through capability. An 85% Low Voltage Ride Through (LVRT) for 150 ms on the grid side is studied based on E.ON grid code [16].MERS can improve the LVRT capability by injecting a series voltage during and/or after the low voltage period [3]. VIII. CONCLUSION Finally, evaluation of series compensation suggests cost reductions compared to shunt compensation; however, the influence on power system stability of increased reactive power transfer from the grid after low voltage period should be investigated in future works. REFERENCES [1] Ibrahim, R.A. ; Hamad, M.S. ; Dessouky, Y.G. ; Williams, B.W. “A review on recent low voltage ride-through solutions for PMSG wind turbine” Power Electronics, Electrical Drives, Automation and Motion (SPEEDAM), 2012 International Symposium. [2] F.K.A. Lima, A. Luna, P. Rodriguez, E.H. Watanabe and F. Blaabjerg, “Rotor Voltage Dynamics in the Doubly-Fed Induction Generator during Grid Faults”, IEEE Transactions on Power Electronics, vol. 25, nº. 1, pp. 118-130, 2010. [3] Wiik, J.A. ; Fonstelien, O.J. ; Shimada, R.”A MERS type series FACTS controller for low voltage ride through of induction generators in wind farms” Power Electronics and Applications, 2009. EPE ‘09.13th European Conference. [4] Mohaghegh Montazeri, M. ; Xu, David ; Bo Yuwen “Improved Low Voltage Ride Thorough capability of wind farm using STATCOM” Electric Machines & Drives Conference (IEMDC), 2011 IEEE International 2011. [5] R.P.S. Leão, J.B. Almada, P.A. Souza, R.J. Cardoso, R.F. Sampaio, F.K.A. Lima1, J.G. Silveira and L.E.P. Formiga “The Implementation of the Low Voltage Ride-Through Curve on the Protection System of a Wind Power Plant”. [6] K. Sree Latha, Dr. M.Vijaya Kumar “Enhancement of Voltage Stability in Grid Connected Wind Farms Using SVC’ International Journal of Emerging Technology and Advanced Engineering Volume 3, Issue 4, April 2013. [7] Wen-Tsan Liu, Yuan-Kang Wu, Ching-Yin Lee, and Chao-Rong Chen “Effect of Low- Voltage-Ride-Through Technologies on the First Taiwan Offshore Wind Farm Planning”, IEEE Transactions On Sustainable Energy, Vol. 2, No. 1, January 2011. [8] Federal Energy Regulatory Commission (FERC), Interconnection for wind energy Docket No RM05-4-000, Order No. 661, Jun. 2005, Issue 2. [9] Xiaoqiang Yang, Yong Qu, Jianqiang Chen, Lei Chen “Low-voltage Ride Through Practice of Double-fed Wind Turbine System” 2012 China International Conference on Electricity Distribution (CICED 2012) Shanghai, 5-6 Sep. 2012. [10] G. Joos, “Wind turbine generator low voltage ride through requirements and solutions,” in Proc. Power and Energy Society General Meeting—Conversion and Delivery of Electrical Energy in the 21st Century, 2008, pp.1–7.
  • 8. International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 36-43 © IAEME 43 [12] M. Z. C. Wanik and I. Erlich, “Simulation of microturbine generation system performance during grid faults under new grid code requirements,” in Proc. 2009 IEEE Power Tech. Conf., Bucharest, 2009, pp. 1–8. [13] María Paz Comech, Miguel García-Gracia, Susana Martín Arroyo and Miguel Ángel Martínez Guillén “Wind Farms and Grid Codes”. [14] Marta Molinas, Member, IEEE, Jon Are Suul, and Tore Undeland, Fellow, IEEE “Low Voltage Ride Through of Wind Farms With Cage Generators: STATCOM versus SVC”. [15] Aditya P. Jayam, Nikhil K. Ardeshna, Badrul H. Chowdhury “Application of STATCOM for Improved Reliability of Power Grid Containing a Wind Turbine”. [16] Omar Noureldeen, “Low Voltage Ride through Strategies for SCIG Wind Turbines Interconnected Grid”, International Journal of Electrical & Computer Sciences IJECS-IJENS Vol: 11 No: 02. [17] Chandrasekaran, S. ; Dept. of Electr. Eng., Univ. of Bologna, Bologna, Italy ; Rossi, C. ; Casadei, D. ; Tani, A., “Improved Control Strategy of Wind Turbine With DFIG For Low Voltage Ride Through Capability”, Power Electronics, Electrical Drives, Automation and Motion (SPEEDAM), 2012 International Symposium , Print ISBN:978-1-4673-1299-8. [18] Nadiya G. Mohammed, “Application of Crowbar Protection on DFIG-Based Wind Turbine Connected to Grid”, International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 2, 2013, pp. 81 - 92, ISSN Print : 0976-6545, ISSN Online: 0976-6553. [19] Ameer H. Abd and D.S.Chavan, “Impact of Wind Farm of Double-Fed Induction Generator (DFIG) on Voltage Quality”, International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 1, 2012, pp. 235 - 246, ISSN Print : 0976-6545, ISSN Online: 0976-6553. [20] S.Dileep Kumar Varma and Divya Dandu, “Modelling and Simulation of Hybrid Renewable Energy Sources Connected to Utility Grid”, International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 5, 2013, pp. 155 - 164, ISSN Print: 0976-6545, ISSN Online: 0976-6553. [21] Mustafa Jawad Kadhim and Prof. D.S.Chavan, “Overview LVRT Capability of DFIG Techniques”, International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 3, 2013, pp. 75 - 81, ISSN Print : 0976-6545, ISSN Online: 0976-6553.

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