1PSCAD/EMTDC Based Dynamic Modeling and Analysis of a Variable Speed Wind Turbine Seul-Ki Kim, Eung-Sang Kim, Jae-Young Yoon and Ho-Yong Kim Abstract-- The paper presents dynamic modeling and simulation of a grid connected variable speed wind turbine (VSWT) using PSCAD/EMTDC, a widely used power system transient analysis tool. A variable speed wind generator with power electronics interface is modeled for dynamic simulation analysis. Component models and equations are addressed and their incorporations into the EMTDC are provided. Controllable power inverter strategies are intended for capturing the maximum power under variable speed operation and maintaining reactive power generation at a pre-determined level for constant power factor control or voltage regulation control. The component models and control schemes are constructed by user-define function provided in the simulation program. Simulation case studies provide the variable speed wind generator dynamic performance for changes in wind speed. This modeling work can be employed to evaluate impacts on power grid as well as the control scheme and output performance of a variable speed wind power system at the design state. base of built-in components available in the program but also user-defined models are used for assembling the wind turbine system. Overall control strategy and some major part of the system elements, i.e. wind turbine, are modeled by the userdefine functions. The study results demonstrate the modeling work provide a reliable and useful simulation tool for evaluating the dynamic performance of a variable speed wind turbine integrated into power system. VSWT Wind Rectifier VSI Transformer Power Grid SG DC link Fig. 1. Schematic representation of modeled VSWT II. EMTDC BASED MODELING Index Terms— Grid connection, Maximum power capture, Power electronics interface, Reactive power control, Variable speed, Wind turbine The variable speed wind turbine model consists of the following components. - I. INTRODUCTION V RIABLE speed operation yields 20 to 30 percent more energy than the fixed speed operation, providing benefits in reducing power fluctuations and improving var supply. Falling prices of the power electronics have made the variable speed technology more economical and common [1]. Such a wind turbine system as other types of dispersed generation is mostly connected to distribution feeders and the generation system cannot be easily connected to the electric power network without conducting comprehensive evaluations of control performance and grid impacts. This requires a reliable tool for simulating and assessing dynamics of a grid connected variable speed wind turbine. The purpose of the work is to provide the capability of simulating and analyzing the dynamic performance and grid impacts of a variable speed wind energy conversion system using a reliable power system transient analysis program, PSCAD/EMTDC. The modeled system includes a fixed-pitch type wind blades, a direct-drive synchronous generator without a gear-box, and a controllable power electronics system, which consists of a six-diode rectifier and a IGBT voltage source inverter. The entire schematic diagram of the modeled wind generation is shown in Fig. 1. Models of the elements and the system control scheme are proposed in the form of mathematical equations and graphical control blocks and implemented in PSCAC/EMTDC [2]. Not only the large Wind model Wind turbine Synchronous Machine Rectifier and voltage source inverter Power electronics control Fig.2 depicts the component blocks of a VSWT model. VBASE WE VGUST + + VWIND + WT SG TM POWER ELECTRONICS + WM VRAMP EF VDC IF PWT, QWT, VEX_REF VNOISE EXC CONTROL VWT, IWT Fig. 2 Components of a VSWT simulation model For modeling the shaft and synchronous generator, models provided by the EMTDC program are used, and models of the wind speed, the wind turbine, power electronics block and the control block are built into the program by user define function of coding in FORTRAN or assembling the built-in elements and modules. A. Wind Model A wind model selected for this study is a four-component 3 shows a userdefined wind turbine component and windows for entering data and parameters in this study. (3) and (4). of which frequency and magnitude are adjustable. which is based on generalized machine theory [2] and with this model both sub-transient and transient behavior can be examined.00184 (λ − 2) β (5) 13 − 0. B. . and can be described by equation (1).44 − 0. VBASE = base wind speed [m/s] VGUST = gust wind component [m/s] VRAMP = ramp wind component [m/s] VNOISE = noise wind component [m/s] Vwind The base component is a constant speed and wind gust component can be usually expressed as a sine or cosine wave function [4]. User defined component (wind turbine) C. a combination of different cosine functions is used for wind gust. The exciter plays a role of meeting the dc link voltage requirement. VDC ≥ 2 2 ⋅ V AC _ RMS DMAX (6) where VDC = dc link voltage of power electronics VAC_RMS = RMS value of the AC line to ground voltage of the inverter DMAX = maximum duty cycle Since the synchronous generator is a direct drive type with low speed and a high number of poles. In this simulation. It is considered that the synchronous generator is equipped with an exciter identical to IEEE type 1 model [6]. Therefore. as may be described by equation (6). Synchronous Machine The PSCAD/EMTDC provides a fully developed synchronous machine model. The shaft dynamics and the rotating mass can be represented by multi-mass torsional shaft model of PSCAD/EMTDC. and is driving the generator. shaft dynamics can be characterized by a swing equation on a single mass rotating shown in equation (7). Based on the four components. CP may be expressed as a function of the tip speed ratio (TSR) λ given by equation (2) [5]. The noise component of wind speed is defined in this study by a triangle wave function.3β where β is the blade pitch angle. Where. for the three-phase voltage source inverter to create voltage waveforms with a nominal value of magnitude. a wind speed model is constructed by integrating the built-in functions and logic circuits provided in the program. VWIND = VBASE + VGUST + VRAMP + VNOISE (1) value of β is set to a constant value. The ramp wind component can be represented by the built-in ramp function model of the program.2 model [3]. Wind Turbine The wind turbine is described by the following equation (2). 3. the wind turbine and the generator are rotating at the same mechanical speed via the same shaft. λ= ωM R VWIND 1 ρπR 2C PVWIND 3 2 3 1 ω = ρπR 5C P M3 2 λ PM TM = (2) PM = ωM (3) (4) where λ = tip speed ratio ωM = blade angular speed [mechanical rad/s] R = blade radius [m] VWIND = wind speed [m/s] PM = mechanical power from wind blades [kW] ρ = air density [kg/m3] CP = power coefficient TM = mechanical torque from wind blades [N⋅m] The mechanical torque obtained from equation (4) enters into the input torque to the synchronous generator. Since there is a triangle wave generator available in the commercial version.0167 β ) sin π (λ − 2 ) − 0. C P = (0. Fig. it is used as a noise generator. which can be easily interfaced with the synchronous machine model. For a fixed pitch type the WIND TURBINE w Tm Fig. Power Electronics Control Several types of power electronics interfaces have been investigated [7]. Rectifier and VSI model g enerato r Fig. Since the voltage remains at a level of the grid AC voltage and the voltage variation is very small compared to changes in the magnitude of IQ and ID. 5. 2 P= 3 Q = − VO I D 2 (11) where |VO| is the instantaneous voltage magnitude of the wind turbine system. The electrical base frequency of the machine in the built-in models must be set to a value corresponding to the rated mechanical speed of the wind turbine specified by a manufacturer or a designer. 4 shows a rectifier and VSI system model that has been implemented in PSCAD/EMTDC. a combined system of a sixdiode rectifier and a six-IGBT switch voltage source inverter. The desired current vector IABC_REF and the actual output current vector IABC_WT of the wind system are compared and the error signal vector IERR is compared with a triangle waveform vector to create the switching signals. ωB. The VSI is a voltage harmonic source in the point view of ac system and a harmonic filter need be placed appropriately to reduce the voltage harmonics it generates[8]. For the modeling study. Equation (8) and (9) give the value for the electrical base speed of the synchronous machine. The reference values PREF and QREF of the wind generation are specified by what VSI’s control strategies are taken for real and reactive power output. Current-controlled VSIs can generate an ac current which follows a desired reference waveform so can transfer the captured real power along with 3 VO I Q . 5 illustrates DQ control decouples real and reactive components and enables real power and reactive power to be separately controlled by specifying the respective reference values of PREF and QREF for the both power outputs and independently adjusting the magnitude of the d-axis current IQ and that of the q-axis current ID. P= 3 3 (V D I D + V Q I Q ) . Variables in the ABC three phase coordinates may be transformed into those in the d-q reference frame rotating at synchronous speed by the rotational d-q transformation matrix [2]. the instantaneous active and reactive power outputs. The six diodes rectifier converts ac power generated by the wind generator into dc power in an uncontrollable way and so control has to be implemented by the power electronics inverter.3 JM dω M = TM − TE − Dω M dt (7) where JM = a single rotating inertia [kg⋅m2] TE = electric torque produced by generator [N⋅m] D = damping [J⋅s/rad] In variable speed operation. IQ_UPPER PREF PI + WTUR PREF & QREF generator PWT QREF QWT OR VMAG IABC_REF PI + IA_REF ID_UPPER IQ_LOWER - IQ_REF ID_REF DQ to ABC IB_REF IC_REF ID_LOWER IERR firing signals + - IABC_WT SPWM generato r comparator SPWM Triangle signals Fig. A L-C harmonic filter consisting of a series interconnection inductor and a parallel capacitor is located at the VSI terminal. the rotating speed of the wind generator is not consistent with the electrical synchronous speed of the electric network and generally much slower than the speed. so the equation (10) may be contracted into simpler equation (11). P and Q. The firing signals are generated by the sine pulse width modulation (SPWM) technique. Fig. P and Q are mainly subject to the d-axis current and q-axis current respectively. VQ is identical to the magnitude of the instantaneous voltage at the wind generation system and VD is zero in the rotating d-q coordinates. has been modeled for AC-DC-AC conversion. which is less expensive than others and commonly put into industrial use. Current control scheme of a voltage source inverter Firing Signals . DQ control method that is widely used for VSI current control is employed. Q = (V D I Q − V Q I D ) (10) 2 2 where VD = d-axis voltage at the wind turbine VQ = q-axis voltage at the wind turbine ID = d-axis current at the wind turbine IQ = q-axis current at the wind turbine. P RPM TUR fB = ⋅ 2 60 ω B = 2πf B =π ⋅P⋅ RPM TUR 60 (8) controllable reactive power. of the wind turbine are described by equation (10). In the three-phase balanced system. 4. Here. In this study. (9) where fB = electrical base frequency of the generator [Hz] P = number of poles RPMTUR = mechanical rated speed of the turbine [rpm] D. Fig. Capturing the maximum power The maximum aerodynamic power available from wind energy can be described by equation (12) and it can be depicted by Fig. 6. Power vs turbine speed curve Fig. 7. [9]. SPWM switching control F. In constant power factor control (PFC) mode. Fig. 8.4 Fig. Current control block Fig. C MAX 1 3 PMMAX = πρR 5 P3 ω M λOPT 2 PREF = ηPMMAX (12) (13) Where CPMAX = the maximum power coefficient λOPT = value of λ where CPMAX = CP (λOPT) η = electrical loss in generator and inverter PM P M MAX re wo pl aic na hc eM 0 V1 V2 V3 Tu rb in e s p e e d [ ra d / s ] ωM Fig. Possible control modes include power factor. kvar. 9. 9. One way of enabling the maximum power capture is to specify the reference value of real power for the inverter control as the available maximum power multiplied by the inverter efficiency. current and voltage. as shown in equation (13). 6 shows the current control model for this modeling study and Fig. the reference . Reactive Power Control Various control modes can be used for determining the amount of reactive compensation to provide. options of control modes and some desired values. 7 depicts the window boxes of the user-defined component of the real and reactive power reference generator to enter the basic parameters of the wind turbine. User defined component of P and Q reference generator Fig. 8 presents the SPWM switching signal generator to give firing pulses into the IGBT switches of the VSI. Constant power factor mode and voltage regulation mode are implemented in this analysis. This simply means that the maximum power may be achieved by varying the turbine speed with varying wind speed such that at all times it is on the track of the maximum power curve [1]. E. 10 shows a VSWT model implemented in PSCAD/EMTDC. III. as shown in Fig. 16 presents magnitude of the voltage at the terminal of the wind turbine in constant power factor. The voltage waveforms at the primary busbar (0. 17 shows the dc link voltage. a sudden increase of reactive load by 600kVar at the second winding busbar (22. 10.2 kHz. The real and reactive power output of the wind turbine in power factor control with varying wind speed is shown in Fig. The wind speed curve used for this study is shown in Fig.005 pu. QREF = PREF ⋅ 1− PF 2 PF (14) Where PF is power factor and PREF is the reference value of real power output of the VSWT.44 that the turbine speed has been well controlled to capture the maximum energy with varying wind speed.5 Fig. In voltage regulation (VR) mode.69kV side) of the VSWT transformer are shown in Fig. 19. The phenomenon comes from the interaction between the mechanical torque applied on the wind turbine and the electrical torque produced by the power system. the real power output and MVA rating of the inverter respectively. Such a limitation is required to be considered in the modeling study. the cut-in and cut-out speeds are 6 m/s and 25 m/s respectively. In power factor control the set value is unity and in voltage regulation the desired voltage is set to 1. reactive power compensation is controlled in such a manner that the voltage magnitude of the VSWT-connected bus being kept constant at a specified level. The rating capacity is chosen to be 1MVA. In order to see the voltage control capability in VR mode. It can be demonstrated by observing the power coefficient reaching the maximum value of 0. Inertia smoothing effects are apparent in the real power curve. 12(a). both types of reactive compensation. It is assumed that the system operates in a balanced condition. the terminal . 2 2 QLIMITS = ± S INV − PINV (15) Where QLIMITS. Whether the mode controls constant power factor or voltage. Figs.35 m/s. Also. the reactive power capability of a VSWT is limited. The reactive capability limits of the wind turbine used in this study are determined by MVA rating of the inverter which may be described by equation (15). VSWT implemented in PSACD/EMTDC value of the reactive power of the wind turbine. The reference magnitude of the voltage to be regulated must be set as the nominal voltage of the AC grid where the wind turbine is considered as being interconnected. In such a case. The turbine angular speed variation respoding to varying wind speed is shown in Fig. as shown in Fig. 15. The current reference is well being tracked by the actual current. The rated speed of the rotor is chosen to be 26. constant power factor control and voltage regulation control. Fig. 13 presents the power coefficient profile corresponding to change in the turbine speed. A high pole modular synchronous generator which has 42 pole pairs is considered as the wind generator. It should be noted that the voltage is varying with power fluctuations and the power variations result from changes in wind speed. PINV and SINV are the reactive power limits. 11. 18. SIMULATION RESULTS The proposed model is implemented into PSCAD/EMTDC software and simulated for analyzing the dynamic behaviors of a wind turbine with varying wind conditions. The rated wind speed is 12. At the moment of adding the additional load. Fig. were simulated to compare the impacts on the bus voltage of the wind turbine. The speed swings still remained as subsychronous oscillations.8 rpm. QREF. Fig. whose frequency is approximately 20[Hz]. may be specified by equation (14).9kV side) of the VSWT transformer was applied. 20 and 21 show the results of PFC and VR operation respectively. The VSWT has been connected to the power grid at 0.5 [sec]. It is observed that at the instance when the wind turbine was integrated oscillations in the turbine speed occurred and gradually damped. Fig. 14 shows the mechanical torque into and electrical torque from the wind generator. 12(b). The switching frequency of the grid interface inverter is 7. Such oscillations can be damped below the appropriate level by employing damping factors. the terminal voltages and reactive power outputs in PFC and VR modes were compared. Fig. 18.200 0. Voltage waveforms at primary busbar of VSWT transformenr Terminal voltage (PFC mode) Torque Mechnical Torque Electrical Torque Vmag 1.80 2.0 .8k 0.0 20.0150 1.0010 4. .0060 Voltage Waveforms Cp 0.0 10. 25.50 2.0 15.30 15.0 25.00 0. 0.5k Va Vb -1.3k 0.0 30.150 1.00 . Case of PFC operation with reactive load connected at 1 [sec] . .6 Wind Speed 14.0 40.0 5.0050 Fig.0 40. Fig.0 10. (b) Subsynchronous oscillation (15-16 [sec] period of curve (a)) Fig.0 5.0150 12.0 30.000 -1.0 35. .60 15. 19.0 35.0 25.0 15.9850 0. 0.00 1.00 1.040 Fig.0030 0.0 .40 2.0 1.060 . rad/s] 2. 15. 20.50 2.0 Turbine Speed 30.2k -600 0.0 20.40 2.70 15.8k 0.240 2. 2. .0 35.00 [sec] 20.0 30.20 2.00 0. .450 4.0 30.100 [sec] 4.80 15.0 40. 12.280 Actual Current (phase A) 350 300 2.10 15.0 40.0 30. .0k [sec] Reactive Pow er of VSWT 5.0 5.0 25. Current reference and actual current POWER COEFFICIENT 0.0 10.0000 0.4k 0. 17.00 0.400 1.0080 -0.0 40.00 . Wind speed for case study [sec] 0.0 [sec] Vmag 1.0 Fig.0 8. 16.3k 1. Power coefficient CP [sec] 4. Fig.0 20.0k 0. Fig. .0 20. 2.50 15.0 .0 10.0 40.0050 voltage [pu] [PU] Vc -0.50 3.8k 0.80 2. . Turbine speed of wind turbine 0 -50 [sec] 4.0100 voltage [pu] m/s 11.0 35. DC link voltage (a) Turbine angular speed (0-40 sec) Turbine Speed Wtur Currents 2. -0.00 0.0 Fig.000 4.9900 7.9950 0. .0 -700 -0. .20 4.3k 0.0100 0.2k [sec] -0.260 150 100 2. . 11.0 .220 250 200 A [mech.0 15.0 25. 13.300 0.60 2.0 10. Real and reactive power of VSWT in PFC mode 30.0 5.00 1.80 1. rad/s] 25.0 4.0 2.50 1.0 15.40 0.0 20.0000 0. . DC link Voltage V_dc 2. .0 20.0000 4.5k 0.0k 0. 4. .9950 0.0050 1.0 .0 .3k 5.60 [kV] [mech. .20 1. . Mechanical and electrical torque [sec] 0.0 25. .50 1. .0 10.0 15.020 4.250 [sec] 4.4k -300 -400 -500 0.0 Reactive pow er into Grid 0 1.9850 5.9900 0.0 15.2k Real Pow er of VSWT Reactive Power of VSWT -100 -200 [kVar] kW / kVar 0. 14. .0 35.0 Terminal voltage (PFC mode) Wind Speed 13.00 .0040 0.200 [sec] Reference Current (phase A) 450 400 50 15.0 Fig.0070 0.0 V 0. .50 3.0020 4.0 .0200 0. .0 10.0 6.0 1.60 0.20 15. . Terminal bus voltage in PFC mode Wtur 2.40 15.0 .050 0.0 9.00 (b) Reactive generation of VSWT and reactive injection into grid Fig.00 15.0 15.0 10.0 35.350 0.0 40.00 (a) Terminal voltage magnitude Reactive Power (PFC mode) 100 VSWT OUTPUT 1. .90 16.0 1.0 35. .20 2.6k 0. 50 2. pp. Spooner.0000 0.00 2. Tennakoon.9950 0. and Ph. 37. B. “Computer Representation of Excitation Systems”. P. 5.S and Ph. [7] Z. he has been working in the research field of power system analysis including custom power systems. IEE Proc. PSCAD/EMTDC Power System Simulation Software User’s Manual. 2. “ Pitch-Controlled Variable-Speed Wind Turbine Generation”.. 21(a). M. grid interface of intermittent generation sources such as wind turbines has been a challenge that can cause lower power quality in power systems. December 1983.USA in 1982 and 1985 respectively. Williamson.00 1. Electric Power Applications. Component models of a VSWT and its control scheme have been built by using user define functions and built-in components provided in the software. PSCAD/EMTDC. Sept. 1996. A wind model was integrated into the modeling to see the wind impact. no. Bath. pp. . CRC Press.00 . Currently. Energy Conversions. Barton. Wind and Solar Power Systems. pp. Ho-yong Kim is with the Korea Electrotechnology Research Institute as a Principle Research Engineer since 1986. dispersed generating system integration and grid-connection of dispersed generations. UK. Since 1987. Javid.1999. S. IEEE Trans. Power Apparatus and systems. May 1983. Patel. UPEC’92. He is currently a Director of Power System Research Lab.00 . Butterfield. Since 2000. No.9900 0.. the work done in this study provides a reliable tool for evaluating the performance of variable speed wind turbines and their impacts on power networks in terms of dynamic behaviors as a preliminary analysis for their actual integrations and operations. He received his BSc. “Grid Power Quality with Variable Speed Wind Turbines”. Dynamic responses of the wind turbine to varying wind speeds and under different reactive control schemes were simulated and analyzed based on the modeled system. [5] A. ㅡ VI. Therefore. His research interests are gridconnection of wind turbines. as shown in Fig. pp. No. Vol. 1. Purdue University Report TR-EE 79-20. Korea in 1979 and MS. pp. IEEE Trans. C. on Power Apparatus and Systems. His research areas are power system modeling.13401347. PAS-102.00 1. R. 20(a). vol. 143. pp. (a) Terminal voltage magnitude Reactive Pow er (VR mode) 400 Reactive Pow er of VSWT Reactive pow er into Grid 300 200 [kVar] 100 0 -100 -200 -300 -400 [sec] 0.00 2. It’s just because the VSWT produced zero reactive generation as programmed to generate unity power factor and the power system supplied such amount of reactive power. He received BS degree from Seoul National University. 388-395. Case of VR operation with reactive load connected at 1 [sec] voltage made a sudden drop in Fig. "Control Design and Performance Analysis of a 6 MW wind Turbine Generator".0100 voltage [pu] 1.50 3. CONCLUSIONS A dynamic model of a variable speed wind generation with power electronic interface was proposed for computer simulation study and implemented in a widely used power system transient analysis program. Chen and E. So comprehensive impact studies are absolutely necessary before wind turbines being added to real networks. no. he has been working as a principal researcher in power system research group of Korea Electrotechnology Research Institute. from University of Texas at Austin . analysis and evaluation including system interconnection study.50 2. His research interests are power quality. Spooner.0150 1. . Sept. PAS-87. S. 81-82. 12. BIOGRAPHIES Seul-Ki Kim received B. (b) Reactive generation of VSWT and reactive injection into grid Fig. Murdoch.00 0.00 0. 5. Manitoba HVDC Research Center. Vol. R. June 1968. Also. 6. Vol. B. J. No. A. June 2001. 3791-3795.7 [2] Terminal voltage (VR mode) Vmag 1. Jae-young Yoon is the head of the Power System Research Group at the Korea Electrotechnology Research Institute .H. on Power Apparatus and Systems.D. 148-154. Chen and S. Version 3. on Industry Applications. Catto. On the other hand VR operation kept the voltage at the specified level. 21. Anderson and Anjan Bose. . His main research areas are distribution automation and AI applications to power systems and Power System Interconnection. korea in 1998 and in 2000 respectively. and G. 1992. 1998 release. V.0050 1. January/February 2001. 21(b) that the VSWT shared the added reactive demand by supplying about 300kVar to the power grid. as shown in Fig. IEEE Trans. “Harmonic filter considerations for voltage source inverter based advanced static Var compensator”. [4] Reynolds. Eung-Sang Kim received B.S and M. [9] Eduard Muljadi and C.50 3. 640-643. Winkelman. [8] Z. IEEE Trans. voltage stability analysis and power flow analysis.50 1. IEEE Trans. vol. . PAS-102. pp. . IV. [10] E. he has been working as a researcher in power system research group of Korea Electrotechnology Research Institute (KERI). Ph. In the view point of electric utilities. “Modular design of permanent-magnet generators for wind turbines”. USA. D degree in electrical engineering from Soong-sil University. Vol. No. REFERENCES [1] Mukund R. Michael G. [6] IEEE Committee Report.50 1. MSc. “Stability Simulation of Wind Turbine Systems”. .D degree in electrical engineering from Busan National University. 20(b). 16. “Stability of Wind Turbine Generators to Wind Gusts”. [3] P.S degree in electrical engineering from Korea University.9850 [sec] 0. users who intend to install wind turbines in networks must ensure their systems meet the requirements for grid connection. 240-246. It should be noted in Fig. and M.S degree in electrical engineering from Seoul National University of Technology.