Voltage Sag Impact on Wind Turbine Tower Vibration - IEEE Xplore

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Voltage Sag Impact on Wind Turbine Tower Vibration Roohollah Fadaeinedjad, Student Member, Gerry Moschopoulos, Member, IEEE, and Mehrdad Moallem, Member, IEEE

Abstract — In order to study the Voltage Sag impact on mechanical vibration of wind turbine structure a detailed model that considers all three electrical, mechanical and aerodynamic aspects of the wind turbine must be considered. A drawback of many works in the area of wind turbine simulation is that either a very simple mechanical model is used with a detailed electrical model or vice versa. Hence the effects of interactions between electrical and mechanical components are not accurately taken into account. In this paper, three simulation programs - TurbSim, FAST, and Simulink - are used to model the wind, mechanical and electrical parts of a wind turbine, and its controllers in detail. Simulation results obtained from the model are used to observe the interaction of all three factors affecting the operation of a wind turbine system. Especially, the voltage sag impact on tower vibration is investigated, considering different power system characteristics (i.e. Short Circuit Level (SCL) and X/R ratio), and wind turbine operating conditions. Index eT rms — Voltage Sag, Tower Vibration, Doubly Fed Induction Generator (DFIG), Simulink, FAST, TurbSim.

I. I NTRODUCTION

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IND energy has been used for thousands of years by humans. Ancient Persians used wind energy to pump water before the birth of Christ [1]. The past decade has seen a growing interest in wind energy generation as environmental concerns are on the rise. In spite of this growth, more technology advances are needed to make wind energy competitive with many other energy supply methods. Due to an increase in the number and size of wind turbine installations, many utilities have revised their view and now request that wind turbines support the grid following disturbances. With this regard, voltage sag ride through capability is one of the most important requirements that dictate the connection of the wind turbines to power system. Considerable research has been done on ride-through capability of wind turbines [2], [3], [4], [5], [6], [7]. Most of this research, however, has been done using relatively simple mechanical and aerodynamic models of wind turbines that neglect a number of significant characteristics from mechanical point of view. It is very important to study both the electrical and mechanical aspects of wind turbines simultaneously, especially since an electrical disturbance such as a voltage sag can affect mechanical performance (i.e. can cause vibrations), and output power quality can be affected by aerodynamic and mechanical The authors are with the Department of Electrical and Computer Engineering, University of Western Ontario, London, Ontario N6A 3K7 Canada (e-mails: [email protected]; [email protected]; [email protected])

1-4244-1298-6/07/$25.00 ©2007 IEEE.

behavior. A detailed model that combines both electrical and mechanical aspects of a wind turbine system is therefore needed. To study the vibration in wind turbine structure it is important to calculate the blade aerodynamic forces since they can cause vibration of the mechanical structure and influence the generated torque (power). In this research, the mechanical torque, the deflection, and velocities of the mechanical structure are calculated based on the aerodynamic forces for each part of the blades. The effects of the blade deflection and velocities on blade aerodynamic forces are accounted for. These subjects have not been considered in previous works, which have mainly studied the problems from electrical point of view [2], [3], [4], [5], [6], [7], [8], [9] In this paper, it will be shown that a combination of three different simulation packages, namely (TurbSim, FAST, and Simulink), can be used to model the electrical and mechanical aspects of a wind turbine in detail. The simulation environment is used to study the voltage sag impact on wind turbine tower vibration under different conditions. The results obtained from the model are discussed and conclusions are made about the interaction of all three aspects of a wind turbine system in response to a voltage sag. II. S YSTEM C ONFIGURATION AND S IMULATION T OOLS The choice of network configuration, to study the voltage sag ride-through, is dependent largely on the utility in question and may be either meshed or radial, whereas the interconnection may be at a strong or weak point in the system either at the distribution or transmission level. Often they are connected at weak parts in the network, which may serve small remote loads. Therefore, the connection is often of the radial type to systems with a low SCL. Fig. 1 shows a typical connection for a wind turbine, which can be used to consider the effect of a voltage sag on the tower vibration. It consists of a wind turbine, serving a remote load, connected to a distribution system through a radial line. As can be seen, this system consists of a wind turbine with a variable-speed constant frequency doubly-fed induction generator. This means that the stator is directly connected to the system while the rotor winding is connected via slip rings to an inverter. The inverter is designed so that the induction generator can operate in a limited variable-speed range. The gear-box ratio is set so that the nominal speed of the induction generator corresponds to the middle value of the rotor-speed range of the wind turbine. This is done in order to minimize

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Fig. 1. Simulation structure of the variable speed wind turbine with DoublyFed Induction Generator (DFIG) and grid connection.

the size of the inverter, which will vary with the rotor-speed range. In this work, the three dimensional turbulent wind has been simulated by TurbSim [10]. FAST has been used to simulate the mechanical parts of wind turbine, whereby AeroDyn [11] has been used to model the aerodynamic forces [12]. The controllers, DFIG, and power system have been modelled by Simulink blocks.

Fig. 2.

Aerodynamics of airfoil [13].

Fig. 3.

Variation of CD

III. A ERODYNAMIC S IMULATION U SING T URB S IM AND A ER DYN Since a key element in wind turbine simulation is the calculation of the blade aerodynamic forces, some sort of wind simulator is needed. TurbSim is a stochastic, full-field, turbulent-wind simulator used to provide simulated inflow turbulence environments that incorporate many of the important fluid dynamic features known to adversely affect turbine aeroelastic response and loading, such as tower shadow and wind shear [10]. To study the vibration in wind turbine structure it is important to calculate the blade aerodynamic forces since they can cause vibration of the mechanical structure and influence the generated torque (power). In this research, the mechanical torque, the deflection, and velocities of the mechanical structure are calculated based on the aerodynamic forces for each part of the blades. The effects of the blade deflection and velocities on blade aerodynamic forces are accounted for. In order to study the forces and moments acting on a wind turbine structure, it is useful to understand the elements of blade aerodynamics. Thus Blade Element Momentum (BEM) theory is briefly presented in this section [13]. The air flow over a stationary blade causes a lift force FL , and a drag force FD . As can be seen in Fig. 2, the lift force is perpendicular to the direction of air flow and the drag force is in the direction of air flow. If the airfoil blade moves in the direction of the lift force, it sees the relative velocity W , where the wind velocity is V . The angle α made by the relative wind velocity with the blade chord line is called the Angle Of Attack (AOA). The angle made by the chord line with the plane of rotation is the summation of the twist angle φ and the pitch angle β. The lift and drag forces on the blade can be written as FL = 0.5ρLCL W 2 and FD = 0.5ρLCD W 2 , where ρ is density of air, L is the chord length of airfoil section, CL is the lift coefficient, and CD is the drag coefficient.

and CL

with AOA

A subroutine package named AeroDyn has been developed at the University of Utah [11]. Using BEM theory, the aerodynamic forces are calculated in AeroDyn and passed back to the dynamic subroutines in FAST [12], [11]. To use this subroutine, some input data files should be prepared including wind profile, airfoil lift and drag coefficients and some definable parameters. In this work, each blade has been divided into fifteen parts and for each part, certain airfoil type, pitch twist angle, and chord length have been defined. The variation of the lift and drag coefficients with AOA is shown in Fig. 3. IV. S IMULATION OF M ECHANICAL PARTS OF W IND T URBINE U SING FAST Different methods may be used to study the mechanical dynamics of a wind turbine. If the tower, for example, is modelled by a single mass-spring model, one cannot calculate the bending and the mechanical loads on certain parts of the tower. Moreover, this model is not suitable for a flexible structure, which has more than one vibration mode.

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A comprehensive analysis of a flexible structure can be done by software based on finite element methods or similar approaches. In this regards, a program called FAST has been developed and refined [12]. This software is specifically intended for simulating both two- and three-bladed wind turbines. The code uses a modal approach in combination with Kane Dynamics to develop the equations of motion. The code also runs significantly faster than a large comprehensive code such as ADAMS because of the use of the modal approach with fewer degrees of freedoms (DOFs) to describe the most important parts of turbine dynamics. The FAST code can model a three-bladed Horizontal Axis Wind Turbine (HAWT) with 24 DOFs. In this research, FAST has been used to model an upwind, three-bladed rotor HAWT with a rigid hub and foundation, considering 16 of 24 DOFs including first and second flapwise blade mode, edgewise blade mode, drivetrain rotationalflexibility, yaw angle, first and second tower fore-aft bendingmode, first and second tower side-to-side bending-mode, and generator azimuth angle. The DOFs for rotor-furl, tail-furl, and platform have been neglected. Because, rotor-furl and tail-furl are usually used for small wind turbines and it is assumed that the platform is installed on the ground not on the sea. Using a linear modal representation, FAST models flexible elements, such as the tower and blades. The reliability of this representation depends on the generation of accurate vibration mode shapes, which are input into FAST. A program called Modes [14] can be used to generate these mode shapes. As mentioned before, FAST allows us to specify four different mode shapes, the two fore-aft modes are defined separately from the two side-to-side modes, for the tower. In this work, however, same mode shapes for fore-aft and side-side bending modes have been used. The mode shapes take the form of a sixth-order polynomial with the zeroth and first terms always being zero. This is because the mode shapes are cantilevered at the base so they must have zero deflection and slope there. At the top of the tower, where the normalized height is 1, the deflection must have a normalized value of 1. This means the sum of the polynomial coefficients must add to 1. The tower vibration mode shapes, used in this research, have been shown in Fig. 4. FAST reads mechanical system parameters from some input files and creates some output files. Moreover, it can be used to exchange data with Simulink [12]. The controller and the electrical parts of a wind turbine can be implemented by Simulink blocks where the mechanical parts of wind turbine is simulated by FAST. Some information about wind turbine can be found in Table I. V. T IME DOMAIN SIMULATION OF THE DFIG CONNECTED TO THE GRID

The time domain simulation of a DFIG connected to a grid will be explained in this section. The simulation consists of a bidirectional PWM converter, the power system, and an induction machine. In order to link a DFIG model to FAST in a Simulink environment, the model must be in time domain. There is a model for wind turbines with DFIG in Simulink,

Fig. 4.

The tower vibration mode shapes.

but it is not used in this work because it is phasor based rather than a time-domain model [15]. Some information with regard to induction generator and grid connection have been given in Table II. A. Power system connection The study has been conducted on the system shown in Fig.1, which represents a typical situation. It consists of a Wind Turbine Generator (WTG) rated at 1.5M W and 575V . The WTG unit is electrically attached to a pad mounted transformer that steps up the voltage to a medium voltage level rated at 34.5KV , where a local load 1.9M W , is supplied. The power system is represented by a constant voltage source connected in series with its Thevenin equivalent impedance. In other words, the grid connection has been considered based on SCL at Point of Common Coupling (PCC). All power system impedances, which are referred to the 575V voltage level, are modelled by Simulink blocks using lumped, linear, RLC components. To create the voltage sag a 12-cycle, three-phase to ground fault has been applied to 34.5KV bus. B. Bidirectional converter and grid connection Six controlled voltage and current sources with their limiters have been used to model the bidirectional PWM converter. The converter dynamics including switch dynamics have not been considered in this work. C. Induction machine The electrical part of the induction machine is represented by a fourth-order state-space model and its mechanical part by a second-order system. All electrical variables and parameters are referred to the stator. All stator and rotor quantities are in the arbitrary two-axis reference frame (d − q frame). The relation between three phase quantities and dq components has been defined by Park’s transformation [16]. The model of the induction machine in Simulink is based on Park’s model equations [16]. To solve them, they have to be rearranged in

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state space form. The induction machine modelling equations in state space form can be written as  Rs
LM d (1) ϕqr − ϕqs ϕqs = vqs− ωe ϕds + dt σLs Lr  Rs
LM d ϕ d r − ϕd s (2) ϕds = vds + ωe ϕqs+ dt σLs Lr  Rr
LM d ϕqs− ϕqr (3) ϕqr = vqr − (ωe − ωr )ϕdr + dt σLr Ls  Rr
LM d ϕd s − ϕd r (4) ϕdr = vdr + (ωe − ωr )ϕqr + dt σLr Ls LM 1 1 ϕqs− ϕqr,ids = ϕd s − σLs σLr Ls σLs 1 LM 1 iqr = ϕqr − ϕqs,idr = ϕd r − σLs σLr Ls σLs 1.5pLM Te = (ϕqsϕdr − ϕds ϕqr) σLs Lr For the mechanical part we have iqs =

p d d ωr = (Te − Tm ), θr = ωr dt J dt

LM ϕd r σLr Ls LM ϕds σLr Ls

(5)

where d, q : direct and quadrature axis, r, s : rotor and stator variables, Rr , Rs : rotor, stator resistance, Llr , Lls : rotor, stator leakage inductances, Lr , Ls , LM : rotor, stator and magnetizing inductances, vqs, vds , iqs, ids : q and d-axis of stator voltage and currents, vqr, vdr , iqr, idr : q and d-axis of rotor voltage and currents, ϕqs, ϕds : q and d-axis of stator flux, ϕqr, ϕdr : q and d-axis of rotor flux, Te , Tm : electrical and mechanical torque, θr , wr : rotor angular electrical position and speed, θm , wm : rotor angular position and speed, we : stator angular electrical frequency, σ : leakage cooefficient, J : generator and shaft inertia, p : number of pole pairs, Assuming a large inertia for the mechanical shaft, ωr can be considered as a constant in equations (3) and (4). With this assumption, the mechanical dynamics of generator (equations (5)) including the mechanical parts of wind turbine (gear box, low speed shaft, and high speed shaft) have been modelled by FAST in longer time step. The d-q model requires that all the three-phase variables be transformed to the two-phase rotating frame. Consequently, the induction machine model will have the blocks transforming the three-phase voltages to d-q frame and the d-q currents back to the three-phase. The induction machine is controlled in a synchronously rotating dq axis frame with the d-axis aligned along the stator-flux position. This permits decoupled control of electromagnetic torque and rotor excitation currents. The control

method for DFIG with back-to-back PWM scheme is similar to previous work [17], however, in this work a power controller loop is used instead of the speed controller. All wind turbine control scheme has been implemented by Simulink control blocks.

VI. S IMULATION S TUDY A voltage sag can cause oscillations in the mechanical parts of wind turbine, which can be sustainable or make the system unstable. Moreover, these mechanical vibrations, depending on the type of the wind turbine, can cause oscillations in system voltage, which can be enough to violate transient undervoltage criteria and sometimes cause the wind plants to trip out due to overvoltage [18]. The duration and severity of the mechanical vibrations depend on the power system characteristics, wind turbine specifications and operating conditions. It is important to have detailed models of both mechanical and electrical parts of a wind turbine to study certain key issues such as voltage sag causing tower vibration. The effects of a voltage sag on mechanical oscillations, especially lateral tower vibration, is studied in the following sections. At first, the effect of voltage sag on wind turbine systems is shown. Then, the impacts of different power system characteristics (i.e. SCL and X/R ratio) on tower vibration are explained. Moreover, it is shown how a voltage sag causes more sustainable mechanical oscillations with higher speed wind or lower tower damping.

A. The Impact of Voltage Sag on Wind Turbine systems The effects of a voltage sag on wind turbine systems are studied in this section. In this study, the voltage sag occurs while the wind turbine is generating rated power at unity power factor. The SCL and X/R ratio of the power system connection have been considered 50M V A and 5 respectively. The voltage and currents of the induction generator are shown in Fig. 5. The stator voltage sags to 80% at t = 15 seconds, because of a three-phase fault at 34.5KV side and recover to normal voltage level at t = 15.2 seconds, when the fault is cleared. As can be seen in Fig. 5, the voltage sag creates some transients on the stator and rotor currents and the rotor voltage. However, the situation after this voltage sag is not serious enough to trigger the rotor protection devices. The control schemes of the DFIG operate as normal and try to restore the wind turbine’s normal operation after the disturbance is cleared. The voltage and currents transients significantly change the generator active and reactive powers, which are shown in Fig. 6. The lateral tower acceleration at top, generator torque, and the low speed shaft twist angle are shown in Fig. 7. Fig. 7 also depicts how an electrical disturbance causes side-to-side vibration on the tower. In addition, the shaft twist angle shows how the disturbance may create problems for the shaft. As can be seen in Fig. 8, the voltage sag reduces the generator torque and thus the rotation speed will be increased, where the pitch controller try to keep that constant.

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Fig. 5.

The voltage and currents of stator and rotor (phase A).

Fig. 6. The effect of voltage sag on stator active and reactive powers, rotor active power, and total output power.

Fig. 8. The effect of voltage sag on pitch controller, rotation speed, and generator torque.

Fig. 9. The effect of the voltage sag on the tower acceleration with different network SCL (MVA): 200(thick), 100(thin) , 75(- -), 50(-.), 25(...).

B. The Impact of Network SCL on Tower Vibration As mentioned before, a voltage sag can cause tower vibration. The magnitude of these oscillations is influenced by power system strength (SCL), which is investigated in this section. In order to study the role that SCL has on the tower vibration, a three-phase fault identical to section VI-A is applied to power system with same wind turbine operating conditions, while the SCL is varied from 200M V A to 25M V A. As can be seen in Fig. 9, the tower vibration is significantly increased by decreasing the SCL. In other words, the voltage sag cause severer tower vibration, when wind turbine is connected to a weaker power system. C. The Impact of Network X/R Ratio on Tower Vibration Fig. 7. The effect of voltage sag on lateral tower acceleration, generator torque, and low speed shaft twist angle.

The effect of network X/R ratio on tower vibration is studied in this section. When a remote wind farm connects with the distribution network, in general, the transmission impedance

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Fig. 10. The effect of the voltage sag on the tower acceleration with different network S/R ratio: 10(thin) , 8(- -), 6(-.), 4(...), 2(thick).

Fig. 11. The effect of the voltage sag on the tower acceleration with two different tower dampings (ξ): 1.435% (thick) and 0.796% (thin).

ratio X/R is in the area from 2 to 10. For example, in offshore wind farm, the submarine cable ratio is close to 2 [3]. In order to study the effect of X/R ratio, the disturbance explained in section VI-A is applied to system while the X/R ratio is changed from 2 to 10 by step 2. As can be seen in Fig. 10, the tower vibration is slightly increased by increasing the X/R ratio. D. The Impact of Tower Damping Ratio on Tower Vibration One of the most important parameters in tower vibration studies is tower mechanical damping. The effects of the voltage sag on wind turbine with two different tower damping ratio, ξ, are investigated in this section. The damping ratio is found from the logarithmic damping decrement, δ , by the following relation [19]: δ (6) 2π Damping of tower vibrations is due to both aerodynamic and structural factors. The damping decrement suggested by Germanischer Lloyd (1993) is 0.1 for reinforced concrete and between 0.05 − 0.15 for steel. It means that the damping ratio should be from 0.796% to 2.387% for steel. An electrical disturbance identical to that of section VI-A is applied to the network. In this case, however, the dampings for all tower vibration modes are decreased from 1.435% to 0.796%. The tower accelerations for two tower damping ratios are shown in Fig. 11. As can be seen, the magnitude of tower acceleration will increased by decreasing the tower damping ratio, however, the remarkable effect of tower damping is seen in Fig. 12, depicting the tower acceleration in longer time duration. As can be seen, the voltage sag causes sustainable oscillations in case of lower damping. Moreover, Fig. 12 shows the actual pitch angle of the blades and the rotation speed for two different damping ratios; the wind speed at nacelle height is shown in Fig. 13 (thin line). As can be seen, the electrical disturbance causes oscillations on the tower in two different modes and on the rotation speed, ξ=

Fig. 12. The interaction of the lateral tower acceleration with rotation speed and pitch actuation in two different tower dampings (ξ): 1.435% (thick) and 0.796% (thin).

which interact with the pitch control system. As expected, the more sustainable tower oscillations are shown in the case of lower tower damping. E. The Impact of Wind Speed on Tower Vibration The voltage sag can cause severe tower vibrations when wind turbine is working in higher speed and more turbulent wind. This subject is investigated in this section. An electrical disturbance identical to that of section VI-A is applied to the network. In this case, however, the wind turbine is subjected to a lower speed and smoother wind distribution. The wind speeds at nacelle height are shown in Fig. 13. Initially, the wind turbine operates below rated wind speed and generates 740KW at unity power factor. The system is then subjected to the voltage sag at t = 15 seconds. The tower accelerations for two different wind distributions are shown in Fig. 14 and Fig. 15. It can be concluded from these figures that lower speed and smoother wind results in more damped and

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Fig. 13.

The high speed (thin) and low speed (thick) wind distributions.

Fig. 14. The effect of the voltage sag at low speed (thick) and high speed (thin) winds on the tower lateral acceleration.

lower magnitude vibrations on tower. As mentioned in section III, lower speed wind produces lower mechanical forces. These forces, themselves, can cause vibration on mechanical structure, especially blades and tower [19]. VII. C ONCLUSION It was shown that TurbSim, FAST, and Simulink can be used to model the electrical, mechanical, and aerodynamic aspects of a wind turbine with DFIG in detail. The model, illustrating the relationship between a voltage sag and tower vibrations, was used to study several cases. As a result, some conclusions can be made, that are explained in this section. An important to consider point is that a power system disturbance (i.e. a voltage sag) can affect not only the electrical dynamics of wind turbine but also the mechanical performance of wind turbine, especially, it can cause vibration on tower, which have not been studied in previous works [4], [5], [6], [7].

Fig. 15. The effect of the voltage sag at low speed (thick) and high speed (thin) winds on the tower lateral acceleration.

The voltage sag cause severer tower vibration, when wind turbine is connected to a weaker power system. In other words, the tower vibration is significantly increased by decreasing the network SCL. Whereas, these oscillations are slightly increased by increasing the network X/R ratio. The more sustainable tower oscillations are shown in the case of lower tower damping. In general, the tower oscillations can affect rotation speed and interact with pitch control action. Then, to study mechanical oscillations, caused by voltage sag, in variable speed wind turbines, not only the generator control action should be studied, but also the pitch controller action should be considered. Moreover, it is necessary to have accurate parameters, i.e. tower damping ratio ξ, in this study. The possibility of tower oscillations, caused by voltage sag, during turbulent and high speed wind is greater than that during smooth and low speed wind because turbulent wind creates higher mechanical forces. Development of new control strategies to tackle problems such as vibration and ride-through, and the mechanical phenomena that affect electrical quantities such as output power and voltage are subjects of ongoing and future work. A PPENDIX Numerical data for induction generator, grid, and wind turbine parameters are given in Tables I and II. R EFERENCES [1] National Resources Canada, The history of wind energy, www.canren.gc.ca, July 2000. [2] A. Petersson, L. Harnefors, and T. Thiringer, “Evaluation of current control methods for wind turbine using doubly fed induction machines,” IEEE Trans. on Power Electronics, vol. 20, no. 1, pp. 227–235, January 2005. [3] F. Zhou, G. Jos, and C. Abbey, “Voltage stability in weak connection wind farms,” in Power Engineering Society General Meeting, vol. 2. June: IEEE, San Francisco, California, USA 2005, pp. 1483–1488. [4] J. Morren and S. W. H. de Haan, “Ride-through of wind turbines with doubly-fed induction generator during a voltage dip,” IEEE TRANSACTIONS ON ENERGY CONVERSION, vol. 20, no. 2, pp. 435–441, JUNE 2005.

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TABLE I P RINCIPLE DATA OF WIND TURBINE Parameter Rotor diameter Cut-in wind speed Cut-out wind speed Rated wind speed Rated rotor speed Tower hight Hub radius Tilt angle Blade pitch angle range Cone angle Blade type Hub mass less blades Blade mass Nacelle mass Blade number Hub inertia about rotor axis (includes blades and LSS) Drivetrain torsional damper Drivetrain torsional spring Gearbox ratio

value 70m 4 m/s 25 m/s 12 m/s 19.7rpm 82.39m 1.75m 5.0◦ 4◦ − 90◦ 0.0◦ S818, S825, S826 15148kg 3912kg 51170kg 3 34.6 × 103 kg.m2 1.0 × 107 N m/s 5.6 × 109 N m/rad 75.7098

TABLE II I NDUCTION GENERATOR AND GRID Parameter Rated generator apparent power Rated generator power Rated generator voltage(line to line) Frequency Stator/rotor turns ratio Stator resistance Stator leakage inductance Rotor resistance Rotor leakage inductance Magnetizing inductance Generator inertia about HSS Pole pairs Transformer rated apparent power Transformer impedance Load power at 34.5KV Bus Load power factor

PARAMETERS

value 1.5/0.9M V A 1.5M W 0.575kv 60Hz 1 0.0014Ω 8.998 × 10−5 H 9.9187 × 10−4 Ω 8.2088 × 10−5 H 1.526 × 10−3 H 53.036kg.m2 3 1.75M V A 0.0077 + j0.0579pu 1.9M W 0.8

[5] C. Chompoo-inwai, C. Yingvivatanapong, K. Methaprayoon, and W.-J. Lee, “Reactive compensation techniques to improve the ride-through capability of wind turbine during disturbance,” IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, vol. 41, no. 3, pp. 666–672, MAY/JUNE 2005. [6] A. Petersson, S. Lundberg, and T. Thiringer, “A GIFD wind turbine ride-through system. influence on the energy production,” WIND ENERGY, no. 8, pp. 251–263, July 2005. [7] Y. Lei, A. Mullane, G. Lightbody, and R. Yacamini, “Modeling of the wind turbine with a doubly fed induction generator for grid integration studies,” IEEE TRANSACTIONS ON ENERGY CONVERSION, no. 99, pp. 1–8, 2005. [8] R. Pena, J. C. Clare, and G. M. Asher, “A doubly fed induction generator using back to back pwm converters supplying an isolated load from a variable speed wind-energy generation,” IEE Proc. Electronic Power Applicaion, vol. 143, no. 3, pp. 231–241, May 1996. [9] A. Tapia, G. Tapia, J. X. Ostolaza, and J. R. Saenz, “Modeling and control of a wind turbine driven doubly fed induction generator,” IEEE Trans. on Energy Conversion, vol. 18, no. 2, pp. 194–204, June 2003. [10] B. J. Jonkman and M. L. J. Buhl, “Turbsim user’s guide, www.nrel.gov,” Nov. 2005. [11] P. J. Moriarty and A. C. Hansen, “Aerodyn theory manual, www.nrel.gov,” December 2005. [12] J. M. Jonkman and M. L. J. Buhl, “Fast user’s guide, www.nrel.gov,” Aug. 2005.

[13] T. Burton, D. Sharpe, N. Jenkins, , and E. Bossanyi, Wind Energy Handbook. John Wiley and Sons Ltd, 2001. [14] M. L. J. Buhl, “A simple mode-shape generator for both towers and rotating blades. CTWN design codes (modes), www.nrel.gov,” April 2002. [15] Power System Blockset for use with Simulink(User’s guide), The Mathworks Inc., 3 Apple Hill Drive, Natick, MA 01760-2098 USA, 2004. [16] B. K. Bose, Modern Power Electronics and AC Drives. pp. 56-74: Prentice Hall PTR, 2002. [17] R. Fadaeinedjad, G. Moschopoulos, and M. Moallem, “Simulation of a wind turbine with doubly-fed induction machine using FAST and Simulink,” in Prceeding of International Symposium on Industrial Electronics ISIE06. Montreal,Quebec,Canada: IEEE, July 2006, pp. 2648–2653. [18] D. Jacobson, J. Laninga, and R. Brandt, “Wind interconnection in manitoba,” in CIGRE Conference on Power Systems, paper 4. Montreal, Quebec, Canada: CIGRE, October 2006. [19] J. Manwell, J. McGowan, and A. Rogers, Wind Energy Explained Theory Design and Application. John Wiley and Sons Ltd, 2002.

Roohollah Fadaeinedjad (S’06) received the B.S. and M.Sc degrees in Electrical Engineering from Shiraz University,Shiraz, and Isfahan University of Technology, Isfahan, Iran, in 1992 and 1995 respectively. From 1996 to 1999, he was with Shahid Bahonar University of Kerman as a lecturer. From 1999 to 2004, he worked in Protection and Control Departments of Kerman Regional Electric Company (KREC) and Rasanir Company as a senior protection engineer and project manager. He is currently pursuing the Ph.D. degree with the Department of Electrical and Computer Engineering at the University of Western Ontario (UWO). His research interests include wind energy, control systems, power electronics, and power system protection.

Gerry Moschopoulos (S’89-M’98) received the B.Eng., M.A.Sc., and Ph.D. degrees from Concordia University, Montreal, QC, Canada, in 1989, 1992, and 1997, respectively, all in electrical engineering. From 1996 to 1998, he was a Design Engineer in the Advanced Power Systems Division, Nortel Networks, Lachine, QC. From 1998 to 2000, he was a Postdoctoral Fellow at Concordia University where he performed research in the area of power electronics for telecommunications applications. He is presently an Assistant Professor at the University of Western Ontario, London, ON, Canada. Dr. Moschopoulos is a Registered Professional Engineer in the province of Ontario.

Mehrdad Moallem (M’95) received the B.Sc. and M.Sc. degrees in electrical and electronic engineering from Shiraz University, Shiraz, and Sharif University of Technology, Tehran, Iran, in 1986 and 1988, respectively, and also the Ph.D. degree in electrical and computer engineering from Concordia University, Montreal, Canada, in 1997. From 1997 to 1999, he held postdoctoral and research positions at Concordia University, and Duke University, Durham, NC, respectively. He is currently with the Department of Electrical and Computer Engineering at the University of Western Ontario (UWO), Canada, and an Associate Scientist at the Canadian Surgical Technologies and Advanced Robotics (CSTAR) research group at UWO. Dr. Moallem is a Registered Professional Engineer in the province of Ontario.