Dynamic Modeling and Simulation of Doubly-fed VSCF Wind Generator. Based on PSCAD/EMTDC

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1 Advanced Materials Research Online: ISSN: , Vols , pp doi:1.428/ 213 Trans Tech Publications, Switzerland Dynamic Modeling and Simulation of Doubly-fed VSCF Wind Generator Based on PSCAD/EMTDC Kangyi LI 1,a, Chao LU 1,b, Xiaozhen ZHANG 2,c, Miao YU 1,d 1 Department of Electrical and Engineering, Tsinghua University, Beijing 184,China 2 School of Electrical and Electronic Engineering, North China Electric Power University, Beijing 1226,China a lkangy@126.com, b luchao@tsinghua.edu.cn, c xiaozhennedu@126.com, d olivermiaoer@163.com Keywords: VSWT; DFIG; PI; MPPT; PSCAD/EMTDC Abstract. In order to build an aggregated wind farm model to research on the effect large scale wind power has on the transient performance of the power system, firstly, it is necessary to build an effective WTGs model. Based on the electro-magnetic transient simulation platform PSCAD/EMTDC, this paper built models of all the parts of Doubly-fed type VSCF wind generator, including wind model, wind turbine model, rotation system model, detailed converters model, and emphatically expounded the implementation process of the control strategy based on the stator flux vector oriented principle. Then through the change of wind speed and setting three-phase short-circuit fault on the net side, the model is verified on the decoupling control of active and reactive power for maximum point power tracking (MPPT) and the ability of low voltage ride-through. Introduction Doubly-fed asynchronous induction generators adopts flexible ac excitation control to regulate speed, which can have large scale changes and has fast response so that it can fast track wind speed change and run in the best operation, thus has a higher efficiency of wind power capturing in a great wind speed range; At the same time, its own has flexible reactive power control ability, and the converter's capacity can be only about 3% of the rated capacity. Therefore, compared with cage type wind generator and permanent magnet direct driving wind generator, doubly-fed induction generators have advantages in flexible power control, and lower-cost [1], and have become the main wind power type. A lot of research work on modeling and simulation of DFIGs has been done both at home and abroad, literature [2] established overall dynamic mathematical models of wind, wind turbine, doubly-fed asynchronous induction motor, converter and electrical control part of VSCF WTGs, just as shown in figure 1, but considering that because of high order of generator model, when conducting simulation with large wind farms models, the problem of dimension disaster may occur, literature [3] deduced the detailed reduced-order generator model, and discussed its application value in power market. VSCF wind power generation control unit focuses mainly on generator rotor field voltage control and wind turbines blade angle control, the goal is to make mechanical parts of the wind turbine cooperate with electrical parts of the generator, thus enhance the efficiency of converting wind energy to electricity and improve the power quality. Before the rated wind, state control units are realized by rotor voltage regulation, that is, control of the converter. And after the rated wind, state control units are mainly realized by blade adjustment. Literature [4] adopted the All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (#698896, Pennsylvania State University, University Park, USA-18/9/16,5:25:39)

2 Advanced Materials Research Vols air-gap field oriented control strategy, and realized decoupling control of the stator's active and reactive power, however, in the actual system, it is not easy to accurately orient air-gap field, and it increased the complexity of control systems. Literature [5] used direct power control, and the results showed good robustness, however, because of using of hysteresis comparator, the adjustment of excitation controller had large impact on generator and its system, literature [6] used sliding mode control for direct voltage control in micro nets, the effect is good but it is still not verified in the power system. At present vector control strategy is commonly used, which can realize two-way flow of converter power, thus realize decoupling control of the reactive and active power, literature [7] first applied nonlinear vector control strategy to VSCF system, and completed a preliminary design of conversion control system, literature [8] put forward two different types of converter models, and based on stator flux vector oriented principle made basic control design of the rotor side converter and net side converter, literature [9] made detailed mathematical reasoning of the structure design of the rotor side converter and verified the rationality of the structure. According to the not flexible enough structure, literature [1] put forward a kind of novel structure, reactive power by changing the control strategy, but the structure of the robust stability remains to be theory to prove it, literature [11] used indirect matrix converter for the design of the rotor current control, and did not need special strictly mathematical derivation and robustness. This paper adopts the stator flux oriented principle, and establishes dynamic model of the control strategy of both rotor side and net side converters, with organization structure as follows: the first part introduces each part of models of WTGs, the second part details control strategies; the third part, through the simulation results validity the model; The fourth part, as an end part of this paper, summarizes this paper's main work and points out the direction for the next step. Modeling of DFIGs The basic configuration of DFIGs model is as Figure 1 shown, the wind turbine connects with the induction motor by mechanical rotation system, which includes the high-speed shaft and low speed shaft and a gear box between them. The stator of wound rotor induction motor is directly connected to the grid and the rotor connected to the grid through the VFC. In order to make the rotor transfer power whose voltage and frequency can be constant to the grid, when the rotor runs at sub-synchronous or super-synchronous speed, it is necessary to control the amplitude and phase of current between the rotor side and the net side. Therefore, VFC,which is called back-to-back model, consists of two four-quadrant IGBT pulse width modulation (PWM) converters, the RSC (rotor side converter) and the GSC (generator-side converter), and the DC capacitor, which connects the two converters. Fig.1 Configuration of DFIGs

3 75 Progress in Renewable and Sustainable Energy 1.1 Aerodynamic model of wind turbines Aerodynamic models can be depicted with the well-known CP -λ-β curve, wind energy utilization and a function of tip speed ratio and pitch angle. CP is the coefficient of ω t λ = R (1) v w n n i j CP ( β, λ) = αijβ λ i= j= (2) The letter R is the radius of the blade, ω t is the rotational speed, v w is the wind speed. The Cp -λ-β curve depends on the design of the blade and will be provided by the wind turbine manufacturer. Once you knowc, the mechanical power of wind turbine can be obtained by equation (3): P 1 2 p 3 = ρ A v C ( λ, β ) (3) m r w P The symbol ρ is the air density, A r is the swept area of the blade. For a fixed wind speed, there will be only one v w corresponds to C (maximum coefficient of wind energy utilization), and pm thus we can get the maximum output power. If the wind speed is below the rated, wind turbines run at the mode of variable speed, through controlling the active power, the wind turbine's speed can be adjusted so that thec maintains at C p pm.in this mode, the control system of the Pitch angle is not involved in the adjustment, the symbol β is a given value. However, if the wind speed exceeds the rated value, the pitch angle control system will increase the pitch angle to limit the wind turbine s mechanical output. For the sake of simplicity, in this article, Cp is assumed to be a constant value of The generator model The structure of doubly-fed machine is similar to the wound rotor induction motor, therefore, this article uses a wound rotor induction motor in the PSCAD software library, and the model is established according to the state equation of the motor. A,B,C represent for the stator winding's side and a,b,c represent for the rotor winding's side; W is the input of rotational speed control ; T L is the input of the torque control, when T L is a negative value,it means the motor is in the generation state,otherwise, motor state; S is the input of the mode selection control, is for the torque control mode and 1 is for speed control mode, this paper sets initial value of S as 1, after the motor run at the steady state for.5 seconds, switches speed control mode to torque control model, that is, switch the value of Sfrom1 to.

4 Advanced Materials Research Vols Converter model In this paper, a detailed model of converters is adopted, the converter is composed by two four-quadrant IGBT pulse width modulation (PWM) converters and through the DC capacitor to link them, which is called back to back model, and is imposed a crowbar protection circuit to avoid the rotor excitation current out-of-limit. The rotor side adopts current-driven controlling circuit and the net side adopts voltage-driven controlling circuit, the model can fully represent the dynamic characteristics of DFIGs when power grid is under transient state. Design of the converter control system 2.1 Design of RSC control system The RSC control block diagram consists of two cascaded control loops. Inner current control loop controls the components of the d-axis and q-axis rotor current idr and i qr, stator flux oriented synchronous rotating coordinate system is generally used at present, and the outside power control loop is used to adjust the stator s active power (or generator rotor speed) and reactive power so that the decoupling of active and reactive power can be realized. The theoretical basis and implementation process of RSC decoupling control can refer to literature reference [12]. The overall vector control system block diagram is shown in Figure 2: v dc i sabc vsabc irabc θ s ρ s ω r ω r P s P s qr i qr v v qr qr1 v qr2 v dr Q s Q s dr idr v dr1 v dr 2 Fig.2. Overall vector control scheme of the RSC 2.2 Design of GSC control system The GSC control system also consists of two cascaded control loops. The inner current control loop can separately control d- axis and q-axis components of GSC s ac side current, the outer control loop can adjust the voltage of dc capacitor and reactive power flow between the GSC and the grid. Therefore we need to separately design the internal current control loop, the dc capacitor voltage controller and reactive control part, the specific theoretical derivation can refer to literature reference [13], and the overall GSC vector control block diagram is shown in Figure 3. vsabc igabc θ g v dc dg i dg v v dg dg1 v qg v dc v dg 2 Q g Q g qg iqg v qg1 v qg 2 Fig.3. Overall vector control scheme of GSC

5 752 Progress in Renewable and Sustainable Energy 2.3 Design of pitch angle control system The pitch angle control system works only when wind speed was high, in this case, the wind turbine speed will no longer increase so as to control the speed within the operating limits, otherwise, generator or converter will be overloaded. Therefore, the pitch angle control system is used to limit the wind turbine mechanical output and to prevent the wind turbine run at an excessive speed. The pitch angle control system s structure is shown in Figure 4. dβ ( dt ) max ω r P e ω r β β 1 T dβ ( dt ) min 1 S β P e Fig.4.Wind turbine pitch angle controller Simulation Analysis Through constructing each part of the models and strategy adopted, combined with the existing models in the PSCAD, the whole WTGs model is built. Through changing wind speed and imposing three-phase short-circuit fault on the grid side, transient characteristics of the model is simulated and analyzed. Wind turbine firstly runs at rotational speed control mode, then switches to torque control mode when it run in the stable state, then the wind speed switches from 5m / s to 12m / s in 5 seconds; three-phase short-circuit fault is imposed on the grid side in 1 seconds, setting the failure time for.5s, the simulation results are shown in Figure w ref.5 T e 1.1 w pu. T m kv/kv Fig.5. Rotate speed Fig.6. Torque i rd 3 Q g 4 25 Q g_ref KA 2 kvar Fig.7. Reactive power Fig.8. D-axis rotor current i dr

6 Advanced Materials Research Vols i rq 4 p g kw 2 KA Fig.9. Active power Fig.1.D-axis rotor current i dr 1.2 v g_rms 1 E cap pu Fig.11.Terminal voltage of WTGs Fig.12.Voltage of dc capacitance Figure 5 shows the curve of the rotational speed, through regulating rotational speed of Wind turbine,it can well track the reference speed,thus achieve maximum wind energy; Figure 6 shows the curve of wind turbine torque, when the wind speed switches from 5m / s to 12m / s in 5 seconds, the mechanical torque increased rapidly but the electromagnetic torque has not changed temporarily due to the inertia of the rotating system, the imbalance of the torque leads to the rotational speed increases, so that the mechanical torque decreases, at this time, the electromagnetic torque began to increase, finally runs in the new equilibrium point; In figures 7-8, through the comparison of the two curves, it can be easily seen that reactive power that the generator output can be controlled by the rotor current direct-axis component; From figure 9-1, you can see the active power that the wind turbine output can be controlled by the rotor current quadrature-axis component.the above four figures show this model realizes the decoupling control of active and reactive power; Figure 11 shows the curve of the terminal voltage of WTGs, when three-phase short-circuit fault occurs in 1 seconds, the voltage has dropped to be about.1, but thanks to the powerful means of control, the motor does not trip and quickly restores to normal value when the failure is not yet removed, this shows that it has low-voltage ride through capability; Figure 12 shows the curve of dc capacitor s voltage, when a short-circuit fault occurs, the voltage can reach a new stable voltage value. Thus, figures 5-12, show a good validation of WTGs model this paper has built on largest wind energy capturing, active and reactive power decoupling control and low voltage ride through capability. Conclusion In this paper, doubly-fed VSCF wind turbine model is built through constructing each part of the models of WTGs based on PSCAD simulation platform, then through changing wind speed and imposing three-phase short-circuit fault on the grid side,the model is verified. However, in the

7 754 Progress in Renewable and Sustainable Energy actual operation process of wind power system, the wind speed is always changing in large scale so that the model's performance will be affected, how to take the impact of various environmental factors into account on the wind turbine running status on the basis of the model structures, and then build more effective WTGs model, and then to build a detailed model of the entire wind farm is the important contents that will be studied next. Acknowledgements This work was supported by the Major State Basic Research Development Program of China (212CB21526) and the Major Program of National Natural Science Foundation of China (51372). References [1] Mitsutoshi Yamamoto,Osamu Motoyoshi.Active and Reactive Power Control for Doubly-fed Wound Rotor Induction Generator[J]. IEEE Trans.ON Power Electronics,1991,6( 4) : [2] LI Jing,SONG Jia-hua,WANG Wei-sheng.Modeling and Simulation of Large VSCF Wind Power Generators[J].Proceedings of the CSEE,24,24(6):1-15.(In Chinese) [3] Pulgar-Painemal H.A.,Negrete-Pincetic M..Wind turbine reduced-order model for power system analysis[c].environment and Electrical Engineering (EEEIC), 211 1th International Conference on,rome,211. [4] ZHANG Jun-feng, MAO Cheng-xiong,LU Ji-ming. Direct Power Control Strategy of Doubly-fed Induction Generator[J].Electric Power System Automation Equipment,26, 26( 4) : (In Chinese) [5] Zhang Qi-wei, Cai Xu.Maximum Wind Energy Tracking Wind Turbine System and Simulation. Electric Machines & Control Application,Vol34, pp:42-46,may 27(In Chinese). [6] Aghatehrani R.,Kavasseri R..Sliding Mode Control Approach for Voltage Regulation in Microgrids with DFIG Based WTGs[C]. Power and Energy Society General Meeting,211 IEEE, San Diego,CA,211 [7] Ben Alaya J.,Khedher A.,Mimouni M.F.Nonlinear Vector Control Strategy Applied to a Variable Speed DFIG Generation System [C].Systems,Signals and Devices(SSD),211 8th International Multi-Conference on,sousse,211. [8] Wei Qiao.Dynamic Modeling and Control of Doubly Fed Induction Generators Driven by Wind Turbines[C]. Power Systems Conference and Exposition,PSCE '9,IEEE/PES,Seattle, WA, 29. [9] Flores Mendes V.,De Sousa C.V.,Rocha Silva S.,Cezar Rabelo B., Hofmann W..Modeling and Ride-Through Control of Doubly Fed Induction Generators During Symmetrical Voltage Sags[J]. Energy Conversion,IEEE Transactions on,211,26(4): [1] Dongkyoung Chwa,Kyo-Beum Lee.Variable Structure Control of the Active and Reactive Powers for a DFIG in Wind Turbines[J].Industry Applications,IEEE Transactions on, 21,46(6): [11] Calvillo C.F.,Martell F.,Elizondo J.L.,Avila A.,Macias M.E, Rivera M.,Rodriguez J..Rotor Current Fuzzy Control of a DFIG with an Indirect Matrix Converter[C].IECON th Annual Conference on IEEE Industrial Electronics Society,Melbourne,VIC,211. [12] Masaud,T.M.;Sen,P.K.Modeling and Control of Doubly Fed Induction Generator for Wind Power[C].North American Power Symposium (NAPS), Boston,MA 211 [13] Wei Qiao. Dynamic Modeling and Control of Doubly Fed Induction Generators Driven by Wind Turbines [C].Power Systems Conference and Exposition,PSCE'9,IEEE/PES,Seattle, WA, 29.