 Proceedings of Engineering and Technology Innovation, vol. 13, 2019, pp. 10-19 Impact of DFIG in Wind Energy Conversion System for Grid Disturbances Sasmita Behera * , Sudeep Kumar Behera, Bibhuti Bhusan Pati Department of Electrical & Electronics Engineering, Veer Surendra Sai University of Technology, Burla, Odisha, India Received 09 April 2018; received in revised form 11 December 2018; accepted 25 January 2019 Abstract In this work, a grid-connected DoublyFed Induction Generator (DFIG) is studied for the transient and steady response. The vector control technique controls the Pulse Width Modulation (PWM) of both the back-to-back converters interfacing rotor to the grid. Reactive power supply and DC bus voltage are managed by the grid-side inverter. Active power and rotor angular speed are adjusted by the machine side inverter facilitating power generation for varying wind. The effect of voltage and frequency deviation from the grid on the control is observed. The controllers are found to work satisfactorily except for large frequency variation. The current harmonics are also within the allowed limit. The proposed controllers are expected to satisfy the revised grid code for wind energy. Keywords: vector control, PWM, voltage sag, voltage swells 1. Introduction Increasing share of wind power in grid connection draws attention to the study of its effect on power system stability. In the Wind Energy Conversion System (WECS), different generators are in use. The majority of variable speed WECS have Doubly Fed Induction Generator (DFIG) [1]. This is due to the efficient energy conversion, low cost, less mechanical stress etc. [2]. The increased proportion of wind power has forced the grid codes to be modified from its isolation by crowbar [1] to ride through the control during the grid disturbance [3]. Active and reactive power control for changing wind speed is accomplished by a stator flux oriented vector control with its real axis aligned to the stator flux vector rotating at the synchronous speed in [4]. The performance of such a system for various balanced and unbalanced fault conditions is explored in [5 -6]. There are many possible ways to control the outputs of the generator with different orientation of the real axis of the reference frame [7]. Minor but frequent disturbances in the grid such as voltage dip, sag and swell in the grid lead to poor of power quality, reliability,and equipment failure [8]. Such disturbances associated with voltage can be supplemented by reactive power balance. Different external reactive power support devices have also been in research, but with the additional cost [9-11]. In [12], upholding of DFIG by internal control for low voltage in stator voltage oriented reference frame is studied. In another study, the swell case of voltage is observed in [13]. Xiao et al. [14] have tentatively defined a maximum limit beyond which the crowbar protection should be activated. Selectively harmonics have been eliminated by proper control of DFIG in [15]. To minimize rotor current that activates crowbar during low voltage, additional feed-forward control has been proposed in [16]. Instead of voltage oriented vector control, a direct current control [17] has been proposed for faster control. Mohseni et al. [18-20] have proposed switching to hysteresis control during transient rather than utilizing Proportional-Integral (PI) current control for steady-state in order to satisfy grid code for the ride-through requirement. But, harmonics with hysteresis control is beyond the standard. PI with resonance control has been proposed in [21-22], to reduce torque pulsation and unbalanced voltage [23]. In addition, to reduce the transient time, torque oscillation for voltage swells virtual impedance control strategy has been proposed in [24]. *Corresponding author. E-mail address:sasmitabehera2000m@gmail.com Proceedings of Engineering and Technology Innovation, vol. 13, 2019, pp. 10-19 11 The Model Predictive Control (MPC) for switching converter instead of PWM has been reviewed in [25]. Though it was faster than PWM, the THD and steady-state performance were inferior. In a comparative study, Tremblay et al. [26] have shown robustness to noise and better control of the DFIG by PI control in vector control in comparison to Direct Power Control (DPC) and Direct Torque Control (DTC) approach. In recent literature, a demagnetizing current control based on vector control has been proposed for faster control for low voltage [27]. The studies are also limited to sag, swell or harmonic reduction. Having reviewed the literature for improvement of control to meet the latest grid codes for WECS, the vector control with PI controller is still unsurpassed though many other control techniques, which have been developed [28-32]. Herein, the vector control technique controls the PWM of both the back to back converters interfacing rotor to the grid to maintain reliable power. The sections 2-5 embody the equations governing the different parts of the model. Response to grid disturbances is presented in section 6 concluding the impact of DFIG in this condition. 2. Model of WT-DFIG System The power of the wind drives the Wind Turbine (WT) coupled to DFIG. With the gearbox in between, the rotational speed is stepped up suitably and the DFIG generates electricity from the mechanical input. Fig. 1 shows a DFIG vector control strategy. For the accurate design of the dynamic model in the d-q frame, the transient reactance of the generator is considered. The equivalent circuits of DFIG and induction machine are analogous [7, 33]. Fig. 1 Overall structure of the DFIG model Definingthe different electrical circuit parameters in the d-q frame as in Eqs. (1-5), it yields   qr rr sss d L L E )( '  (1)   dr rr ms q L L E )( '  (2) LX ssss  (3)                  L L LX rr m ssss 2 '  (4) R L T r rr '0 (5) Proceedings of Engineering and Technology Innovation, vol. 13, 2019, pp. 10-19 12 the DFIG is modelled as in Eqs. (6) to (10) :     iXE T v L L EsiXX T Rv dt diX qssq s dr rr m ddsss s sds s dss '' ' 0 '' ' 0 ' 1 1 1         (6)     iXE T v L L EsiXX T Rv dt diX dssq s qr rr m qqsss s sqs s qss        '' ' 0 '' ' 0 ' 1 1 1  (7)   iXXE T v L L E dt dE qssdqr rr m sqs d s '' ' 0 ' ' 1  (8)   iXXE T v L L E dt dE dssqdr rr m sds q s '' ' 0 ' ' 1  (9) dr rr mdr dsi iL L  (10) i iL Lqr rr qr m qs  (11) where , L , R , X , v , E , i are flux linkage, inductance, reactance, voltage, emf, and current respectively. The first subscript d, q stands for the directors and the quadrature component respectively, and the second subscript r and s represent as rotor and stator respectively. The repetition of subscript s and r for L stand for the corresponding self-inductance, whereas single m stands for mutual inductance. t is the synchronous speed, s is the slip, 0 ' T is the electrical time constant of the rotor,the superscript ′ represents a transient condition. Equating the LHS of Eqs. (3) and (4), and neglecting rotor resistance dsv .Then substituting for dr and qr , it reduces to ( ) ( ) dr s r rr qr s r m qs V L i L i        (12) ( ) ( ) qr s r rr dr s r m ds V L i L i       (13) 3. The Drive Train To model drive train, the two-mass model is simple and it amply represents the dynamics of WT suitable for transient state analysis [34]. The two masses are the generator-gearbox and the hub with blades. Mathematically, it is expressed by TT dt d H sw s t   2 (14)   )1( s dt d strt t  (15) TT dt ds H seg 2 (16) dt d DKT t stss    (17) where tH and gH represent the inertia of the hub and the generator, respectively, t is the turbine angular speed, r is the generator rotor angular speed, t is the shaft torsion angle, sD and sK are the is the stiffness and damping coefficients of shaft respectively, sT is the shaft torque, wT is the effective torque input to the WT, eT is the electromagnetic torque. wT and eT are given by Proceedings of Engineering and Technology Innovation, vol. 13, 2019, pp. 10-19 13 V CR T w t pa w 3 25.0    (18) / e s s T P  (19)    0068.0)54.0 116 (5176.0 21   eC i i p (20)  3 1 035.0 08.0 11     i (21) t w R V    (22) where aP is the air density, R is the radius swept by WT blade, wV is the velocity of wind,  is the pitch angle,  is the blade tip speed ratio, sP is the stator active power, pC is the power coefficient. By controlling t , maximum pC and maximum power from wind can be achieved. 4. Converter Modeling From Fig. 2(b), the active power equation is: DCr gP P P  (23) where r P is the rotor power that charges the DC bus capacitor with DC P , and delivers g P to the grid. These are given by r dr dr qr qrv i v iP   (24) g dg dg qg qgv i v iP   (25) DC DC DC DC DC C dt dv v i vP    (26) where v stands for the voltage and i the current with the second subscript r and g represent for rotor and grid respectively. DC v , DC i and C are the voltage, current,and capacitance of the capacitor in DC link. Now, rewriting Eq. (18), the power balance in terms of voltages and currents gives: ( )DC VDC dg dg qg qg dr dr qr qr dv C v i v i v i v i dt     (27) 5. Controller Modeling Fixing d-axis of the d q reference frame with dsv , qsv nullifies and dsv equals the grid voltage. gP and reactive power g Q can be regulated separately by dg i and qg i respectively in a decoupled manner by vector control resolving three phases to d q components [35]. The stator active power and voltage can also be adjusted separately by qr v and dr v as described below. 5.1.Machine-side control The controller on the rotor-side retains the terminal voltage to set point and controls the DFIG output active power varying with the WT torque [27]. The control of active power and voltage are achieved by qr v and drv , respectively. The control structure is given in Fig. 2(a), while the control is expressed as: Proceedings of Engineering and Technology Innovation, vol. 13, 2019, pp. 10-19 14  2 1 1 21 2p p i i m rrqr qr r s ds r s qrpv x i x s i s iK K K K L L        (28)  2 3 3 23 4p p i i rrdr dr r s drvv x i x s iK K K K L      (29) where the gain of the PI controller is xy K . The first subscript x with notation p and i represent proportional and integral gains respectively. The second subscript 1, 2, 3 are for the power controller, machine-side current controller, and grid voltage controller, respectively. dg ef i r and dg ef i r are the references for the respectivecomponents of the current on the rotor, s ref v  is the reference for stator terminal, ref P is the reference of active powerfound from r P B t ef tB P    (30) where tB is the WT angular speed base, BP is the maximum output active power at tB . 5.2.Grid-side control The grid-side converter controller (Fig. 2(b)), controls DCv by dgi and the reactive power by dgi . The change in intermediary states 1 x , 2x and 3x , shown in Fig. 2(b) are expressed as: 1 DC ref DC DC dx dt v v v    (31) 2 1pdg Idgdg ref dg DC dg dx dt K Ki i v x i      (32) 3 qg ref qg dx dt i i   (33) 2 2pg Igdg dt dx K Kv x  (34) 3 3Igqg pg dt dx K Kv x  (35) (a) Machine-side converter control (b) Grid-side converter control Fig. 2 The control of converters [12] where pdg K and Idg K are the proportional and integral gains of the DC bus voltage controller, respectively. Its output dg ef i r is the set point for dg i ; pg K and Ig K are the proportional and integral gains of the grid-side converter current regulator, respectively, DC ef v r is the DC bus voltage set point and dq efi r is the reference for dqi [7]. DC link voltage drop due to increased active power demand due to or wind speed variation in sub-synchronous speed operation is controlled by a change in dg v (cf. Eq. (22)). Reactive power is controlled by controlling dgv . PWM gating pulses are generated in a similar manner from above two grid voltages as shown in Fig. 2(a). Proceedings of Engineering and Technology Innovation, vol. 13, 2019, pp. 10-19 15 6. Result Discussion The WECS considered is integrated to a 120 kV, 3 phase 2500 MVA grid with transformers in between. The model is built in MATLAB/SIMULINK with system specifications: w V :15 m/s,DFIG: 9kW, 575 V, 6 pole, 1440 rpm, 60 Hz; DC bus voltage 1150 V with a capacitor of 60 mF; frequency of the PWM carriers 1.62 kHz and 2.7 kHz for machine-side and grid-side converters respectively.To reduce the injection of harmonics from DFIG via the grid side converter to the grid, the switching frequency is kept high. 6.1. Balanced grid The stator currents and voltages of DFIG under steady-state condition (the wind speed constant at 15m/s) are depicted in Fig. 3(a). The active power (blue) produced by DFIG is 0.9 PU which is excluding electromechanical losses from the mechanical input, which is similar to the findings in [4]. The reactive power (green) reference is 0 kVAR. The DC bus voltage remains stable at 1150 V. The rotor angular speed is 1.2 PU, the rated value. (a) No disturbance (b) With voltage sag Fig. 3 Simulation results of performance parameter variation with times 6.2. Voltage sag on the grid Grid voltage sag occurs due to generation outage, line outage and switching of a heavy fluctuating load. When voltage sag of 0.15 PU [16, 21] is uniformly applied in all phases at 0.05 s for 9 cycles, the effects are depicted in Fig. 3(b). Normall y the deviation is limited to ±10% as per IEEE 1159. Abnormal violations of grid voltage, which sag beyond ±10%, are taken to verify the effectiveness of the control system. On abrupt deviation of the grid voltage from its rated, the stator current rises and the active power P fluctuates and the active power control competently brings it to its rating within the next 4 cycles. For voltage sag reactive power is supported by DFIG with an increased injection of vqg and adjusting vdr . It supplies reactive power to the grid at that time and settles to 0 kVAR by appropriate control action. For a variation of up to 25%, such rise of current due to sag has a negligible effect on DFIG and the controller works well. The DC bus voltage oscillates at the switching instants and comes down to its set value of 1150 V by the grid side converter. The rotor speed is constant (1.2 PU) as V w is fixed. Proceedings of Engineering and Technology Innovation, vol. 13, 2019, pp. 10-19 16 6.3. Voltage swells on the grid Under symmetrical rise of voltage by 0.2 PU at 0.05 s continuing for 9 cycles, the variations are depicted in Fig. 4(a).The grid voltage may swell under outage of a large load. When the grid voltage swells, the stator current falls to maintain powerconstant. The reactive power Q falls suddenly due to a decrease in power demand and then comes back to 0 kVAR within 4 cycles which is better than that in [8] and [13]. It sustains as per Australian grid code [20]. Finally, the system settles to its set value. 6.4. Fall of grid frequency When the grid frequency drops from 60 Hz rated to 58 Hz at 0.05 s, the proposed control strategy does not act upon it as seen from Fig. 4(b). Such condition of frequency fall may arise due to loss of large generation. Though the case is quite unusual, as the IEC standard limits grid frequency deviation to ±0.5 Hz, it is shown to check the prominent effect in the simulation. It is indicative of the modification of control required to counter to such large change in frequency. However, for the deviation within standard limits the controller performs to stabilize. Since the machine rated frequency is 60 Hz, it is not responding to 58 Hz, and the machine collapses with a gradual fall of rotor angular speed and DC bus voltage. Compared to the grid, the capacity of the generator is small, hence this DFIG can’t support frequency stabilization of the grid. (a) Voltage swell (b) With voltage sag Fig. 4 Simulation results of performance parameters variation with times 6.5. Current harmonics Fig. 5 THD of the stator current The harmonics in the stator current of the DFIG are also analyzed and given in Fig. 5. The dominant odd harmonics have suppressed considerably. The THD is 1.89 %, which is satisfied as per the standards. Proceedings of Engineering and Technology Innovation, vol. 13, 2019, pp. 10-19 17 7. Conclusions A DFIG-WT system with a back-to-back VSC is studied with its PWM controlled by PI controllers without a crowbar. The impact of DFIG is investigated under steady state, for a large deviation of ±20% in voltage and 3% in the frequency of the grid. The stator voltage, current, active and reactive power exchange between the grid and the DFIG are observed. In conclusion, the DFIG supports grid within stability limits for the cases being discussed except for variation in frequency with the implementation of the vector control strategy. In the grid-connected DFIG, the harmonic suppression measure has already been presented. Since the harmonicfilter at the input to the controller, it is expected that the control will work in the presence of harmonics. The stator current has 1.89% THD, well within the limit. Further study can be focused on the improvement of control performance in the larger rating of DFIG and unbalanced faults. Acknowledgement The facilities created by the project AICTE/8023/RID/RPS/19/11-12 Dt.15.5.12 of the department entitled “Transient stability analysis and control of power system with excitation control”, are gratefully acknowledged. Conflicts of Interest The authors declare no conflict of interest. References [1] J. López, E. Gubía, E. Olea, J. Ruiz, and L. Marroyo, “Ride through of wind turbines with doubly fed induction generator under symmetrical voltage dips,”IEEE Transactions on Industrial Electronics,vol. 56, no. 10, pp. 4246-4254, 2009. [2] R. Mittal, K.S. Sandhu, and D.K. Jain, “An overview of some important issues related to wind energy conversion system (WECS),” International Journal of Environmental Science and Development, vol. 1, no. 4, pp. 351-363, 2010. [3] A.K. Pathak, M.P. Sharma, and M. Bundele, “A critical review of voltage and reactive power management of wind farms,”Renewable and Sustainable Energy Reviews,vol. 51, pp. 460-471, 2015. [4] B. C. Babu and K.B. 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