Improving Fault Ride-Through Capability of DFIG-Based Wind Turbine ...

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013

5601204

Improving Fault Ride-Through Capability of DFIG-Based Wind Turbine Using Superconducting Fault Current Limiter Mariam E. Elshiekh, Diaa-Eldin A. Mansour, Member, IEEE, and Ahmed M. Azmy Abstract—With increased penetration of wind energy as a renewable energy source, there is a need to keep wind turbines connected to the grid during different disturbances such as grid faults. In this paper, the use of superconducting fault current limiter (SFCL) is proposed to reduce fault current level at the stator side and improve the fault ride-through (FRT) capability of the system. To highlight the proposed technique, a doubly fed induction generator (DFIG) is considered as a wind-turbine generator, where the whole system is simulated using PSCAD/EMTDC software. Detailed simulation results are obtained with and without SFCL considering stator and rotor currents. In addition, the voltage profile at the generator terminals is analyzed. The effect of limiting resistance value is also investigated. The obtained results ensure that the SFCL is effective in decreasing the fault current. Moreover, both the voltage dip at the generator terminals and the reactive power consumption from the grid are decreased during the fault. The voltage dip characteristics are discussed in accordance with international grid codes for wind turbines. Index Terms—Doubly fed induction generator (DFIG), fault ride-through (FRT) capability, superconducting fault current limiter (SFCL), wind turbine.

I. I NTRODUCTION

T

HE last few years showed a great increase in electrical power demand. This was coincided with increasing penetration of renewable energy sources in order to reduce global warming and promote carbon free technologies. Among different renewable energy sources, wind energy has the major share due to their relative inferior cost. In addition they have low maintenance requirements and clean operation. Therefore, wind energy can be built on a large scale with prospective economical benefits. Recently, doubly-fed induction generator (DFIG), as a variable speed generator, has attracted a wide interest for application with wind energy. Using DFIG could achieve many advantages such as operation over a wide range of rotor speeds and decreasing the amount of power carried by the converter with substantial reduction in converter cost. However, DFIG suffers from high sensitivity to grid disturbances, especially grid faults [1]. When fault occurs into the grid, stator current increases and a voltage dip will appear at the generator terminals. In addition, Manuscript received October 7, 2012; accepted December 13, 2012. Date of publication December 20, 2012; date of current version January 20, 2013. The authors are with the Department of Electrical Power and Machines Engineering, Faculty of Engineering, Tanta University, Seperbay, Tanta 31521, Egypt (e-mail: [email protected]; [email protected]. edu.eg; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2012.2235132

excessive rotor current will flow due to the magnetic coupling between stator and rotor [2]. This current can cause failure of the rotor side converter (RSC). As a result, RSC will be blocked and wind turbine will be tripped. This problem becomes more severe with large penetration of wind energy and will cause a negative impact on the overall stability of the system. So, there is a need to improve the ability of wind turbines to remain connected to the grid during faults, which is termed as fault ride-through (FRT) capability. Several studies have been carried out to improve FRT capability of DFIG-based wind turbine [3]–[5]. The most wellknown method that is being used is the crowbar system. Crowbar system comprises a set of resistors connected with the rotor side through power electronic devices in order to bypass the rotor side converter. By the crowbar system, rotor currents could be successfully reduced. However, when the rotor side converter is isolated by the crowbar, the DFIG behaves as a conventional induction generator [6]. Thus, it consumes reactive power from the grid leading to further decrease of grid voltage [7]. As a result, several control techniques and strategies have been proposed to overcome these drawbacks [8]–[10]. However, most of these methods are too complicated for practical applications and need proper tuning of control parameters. In addition, these methods are not failure safe since they are usually based on power electronic devices. In this paper, the application of superconducting fault current limiter (SFCL) with DFIG-based wind turbine is proposed to improve FRT capability. SFCL is considered as a self healing method since it does not need any control action to be changed from superconducting to non-superconducting states. To ensure the validity of the proposed technique, the whole system is built using PSCAD/EMTDC software. Then, the effect of integrating SFCL on the stator and rotor currents during fault is studied. Also, the change in the voltage dip at the generator terminals is obtained with different values of limiting resistance as a conventional alternative. II. I NVESTIGATED S YSTEM A. System Description Fig. 1 illustrates a DFIG-based wind turbine, where the stator is directly connected to the grid, while the rotor is connected to a controlled back-to-back converter. The back-to-back converter consists of the rotor-side converter and the grid-side converter. The rotor-side converter controls the torque and the speed of the DFIG and the grid-side converter keeps the dc link voltage constant between the two converters. The DFIG is controlled by vector control strategy of the power converter.

1051-8223/$31.00 © 2012 IEEE

5601204


terms of temperature and current density as described by the following equation [12]: RSF CL ⎧ ⎪ ⎨ 0 n (J < Jc , T < Tc ) Superconducting state = f JJc Flux flow state (J > Jc , T < Tc ) ⎪ ⎩ f (T ) (T > Tc ) Normal state

Fig. 1. Schematic diagram of DFIG-based wind turbine with SFCL.

Under normal operating conditions, the active power is generated based on wind speed and wind turbine characteristics, while the reactive power is set at zero. The grid is represented as an infinite bus with frequency of 50 Hz and voltage of 20 kV. The DFIG is connected to the grid through a 0.69/20 kV step-up transformer and the SFCL is introduced after the transformer. The system has been simulated and analyzed using PSCAD/EMTDC software. B. DFIG-Based Wind Turbine Model Detailed modeling of DFIG-based wind turbine is explained in the literature. Here, only the important relations will be highlighted [7], [11]. The mechanical power extracted from the wind turbine is given by the following equation: Pw = 0.5CP AρV 3

(1)

where Cp is the power coefficient, A is the swept area of rotor, ρ is the air density and V is the wind speed. The voltage equations of the stator and rotor circuits of the generator are expressed in the d − q reference frame as follows: dλds dt dλqs = Rs iqs + ωs λds + dt

vds = Rs ids − ωs λqs +

(2)

vqs

(3)

dλdr dt dλqr = Rr iqr + (ωs − ωr )λdr + dt

vdr = Rr idr − (ωs − ωr )λqr +

(4)

vqr

(5)

where λ is the flux linkage, ω is the angular frequency and R is the resistance per phase. The subscripts d and q denote the direct and quadrature axes, respectively, while the subscripts s and r denote the stator and rotor quantities, respectively. C. SFCL Model In this study, the resistive type SFCL is considered due to its compact size and simple principle of operation. Resistive type SFCL is based on the transition from superconducting to normal state, which is recalled quenching process. With the second generation Yttrium barium copper oxide (YBCO) coated conductors; it became possible to build SFCL of high current density as well as fast transition and recovery. The current limiting behavior of the resistive type SFCL can be characterized by the resistance transition of YBCO tapes in

(6)

where J and T are the current density and temperature, respectively, while Jc and Tc are their critical values and n represents the exponent of E − J power law relation. The n value of E − J power law has a large value for YBCO coated conductors, normally higher than 20 [12]. Therefore, the flux flow resistance will increase in a step form within the first half cycle of the fault current [13]. From the dynamic point of view for DFIG, the flux flow resistance will play an important role in the present study, where after the first cycle of the fault current, the control action becomes active and the current drops rapidly. Therefore, the developed model of SFCL on PSCAD/EMTDC is represented as a constant resistance. The rms current is used to determine the critical point at which the SFCL will be converted to normal conducting state. The critical current of the developed model is set to 1 kA. III. R ESULTS AND D ISCUSSION A. Current Limitation Results In order to evaluate the effectiveness of SFCL for current limitation of DFIG-based wind turbine, a symmetrical fault was considered at the integration point with the grid as shown in Fig. 1. The fault is applied at 1.32 s and is cleared by normal protection devices after 0.1 s. For the results in this paper, the wind turbine operates at a wind speed of 11.5 m/s. Fig. 2 shows the current limitation capability of the SFCL through illustrating the stator current behavior regarding the mentioned fault. A 60 Ω current limiting resistance was considered in this study. Without connecting SFCL as shown in Fig. 2(a), the first peak of the current signal reaches about 4.4 kA for phase a, 3.1 kA for phase b and 5.6 kA for phase c. After fault clearance, the current exhibits an inverse peak, where fault clearance normally results in transient components similar to that at fault initiation but with less severity. After inserting the SFCL as represented in Fig. 2(b), the fault peak current was limited effectively to reach 3.3 kA for phase a, 2.4 kA for phase b and 3.5 kA for phase c. The difference of peak currents and corresponding limiting behavior between phases is attributed to the different fault starting angles. It is important to note that the inverse peak after fault clearance has also been decreased for all phases after connecting SFCL. So, the overall dynamic performance of DFIG has been improved. B. Voltage Dip Characteristics The evaluation of rms voltage in pu at the terminals of the wind turbine generator is shown in Fig. 3. Without SFCL, it is shown that the voltage is decreased to 0.12 pu during fault. After adding the SFCL, the voltage dip is decreased, where the voltage reaches 0.18 pu during fault. In accordance with international grid codes for wind turbines [14], this enhancement led to compliance with most of these codes. For instance, the grid

ELSHIEKH et al.: IMPROVING FAULT RIDE-THROUGH OF DFIG-BASED WIND TURBINE USING SFCL

Fig. 2. Stator current behavior with and without SFCL. (a) Without SFCL. (b) With SFCL.

5601204

Fig. 4. Active and reactive power responses: (a) Active power response. (b) Reactive power response.

C. Active and Reactive Power Behavior Fig. 4 shows the active and reactive power responses with and without connecting SFCL. As shown in Fig. 4(a), the active power drops to approximately zero after fault without connecting SFCL. So, the electromagnetic torque of DFIG is reduced to zero and the rotor accelerates. After connecting SFCL, the drop in the active power decreased, where it is attained about 0.15 kW. In addition, the reactive power characteristics have been improved as shown in Fig. 4(b), after connecting SFCL. SFCL limits the deviation of reactive power at the fault instant, and also limits the reactive power drawn from the grid at fault clearance. This would enhance the stability of the overall system. D. Rotor Current Behavior

Fig. 3.

Voltage-dip characteristics at the generator terminals.

code requirements issued by USA, Ireland and Canada AESO (Alberta Electric System Operator) are depicted in Fig. 3 as a solid line. It is recognized that the required minimum voltage level of 0.15 pu was achieved after using SFCL. The highest minimum voltage level required worldwide is 0.25 pu [14] that is not attained by the current limiting resistance considered here (60 Ω). But it is expected to achieve various grid codes with increasing the current limiting resistance as will be described later in this paper.

Increased current in the rotor circuit is very harmful to the rotor side converter. It may exceed the rating of the converter leading to failure of its components. In addition, the protection setting on rotor circuit is governed by current. This setting is responsible of blocking the rotor circuit and tripping the wind turbine from the grid. Fig. 5 shows the change of rotor current under fault with and without SFCL. After connecting SFCL at the stator circuit, the rotor current is limited to about 0.9 kA by the effect of magnetic coupling between rotor and stator, while this current was about 1.7 kA without SFCL. With adjusting the current limiting resistance to a proper value, the rotor currents can be limited within their safety margins. This would prevent the rotor side converter from disconnection from the generator during faults and keep the wind turbine connected to the grid. Consequently, fault ride-through capability will be improved. Moreover, with

5601204


voltage level required worldwide [14]. Accordingly, the wind turbine integrated with a current limiting resistance of 100 Ω became complied with various international grid codes. IV. C ONCLUSION

Fig. 5. Change of rotor current under fault with and without SFCL.

The utilization of superconducting fault current limiter (SFCL) to improve fault ride-through capability of DFIG has been proposed. The DFIG model integrated with SFCL has been built using PSCAD/EMTDC software. With using SFCL, the stator currents have been limited effectively and the minimum voltage level at the generator terminals has been increased leading to compliance with international grid codes. The reduction of stator current has been reflected on the rotor currents due to the magnetic coupling. Rotor currents have been limited to about 50% of its value without SFCL. In addition, the overall dynamics of DFIG, represented by active and reactive power, has been improved. With increasing the current limiting resistance, further improvements in the obtained characteristics have been achieved. The obtained results pointed out the effectiveness of using SFCL with DFIG-based wind turbine. R EFERENCES

Fig. 6. Effect of SFCL resistance on stator currents.

Fig. 7. Effect of SFCL resistance on voltage dip at the generator terminal.

using SFCL, the rotor current after fault clearance returns to its steady state value without any further oscillations. E. Effect of Current Limiting Resistance Several cases have been studied with varying the value of the SFCL resistance in order to evaluate the effect of current limiting resistance. Only the stator currents and the voltage dip characteristics will be presented, since they are the main motivations for other characteristics of DFIG. For stator currents, with increasing SFCL resistance further enhancement in current limitation will be achieved as shown in Fig. 6. Three SFCL resistance values were considered and compared with zero limiting resistance. The difference among phases is attributed to the different fault starting angles. Regarding voltage dip characteristics at the generator terminals, it was also improved for higher SFCL resistance values. The minimum voltage level under fault attained 0.25 pu at a current limiting resistance of 100 Ω as shown in Fig. 7. This achieved the highest minimum

[1] J. Lopez, E. Gubia, E. Olea, J. Ruiz, and L. Marroyo, “Ride through of wind turbines with doubly fed induction generator under symmetrical voltage dips,” EEE Trans. Ind. Electron., vol. 56, pp. 4246–4254, Oct. 2009. [2] G. Pannell, D. J. Atkinson, and B. Zahawi, “Analytical study of grid-fault response of wind turbine doubly fed induction generator,” IEEE Trans. Energy Convers., vol. 25, pp. 1081–1091, Dec. 2010. [3] J. Morren and S. W. H. de Haan, “Ridethrough of wind turbines with doubly-fed induction generator during a voltage dip,” IEEE Trans. Energy Convers., vol. 20, pp. 435–441, Jun. 2005. [4] V. F. Mendes, C. V. Sousa, S. R. Silva, B. C. Rabelo, and W. Hofmann, “Modeling and ride-through control of doubly fed induction generators during symmetrical voltage sags,” IEEE Trans. Energy Convers., vol. 26, pp. 1161–1171, Dec. 2011. [5] A. M. S. Yunus, M. A. S. Masoum, and A. Abu-Siada, “Application of SMES to enhance the dynamic performance of DFIG during voltage sag and swell,” IEEE Trans. Appl. Supercond., vol. 22, pp. 5702009, Aug. 2012. [6] L. G. Meegahapola, T. Littler, and D. Flynn, “Decoupled-DFIG fault ridethrough strategy for enhanced stability performance during grid faults,” IEEE Trans. Sustain. Energy, vol. 1, pp. 152–162, Oct. 2010. [7] L. Yang, Z. Xu, J. Ostergaard, Z. Y. Dong, and K. P. Wong, “Advanced control strategy of DFIG wind turbines for power system fault ride through,” IEEE Trans. Power Syst., vol. 27, pp. 713–722, May 2012. [8] D. Santos-Martin, J. L. Rodriguez-Amenedo, and S. Arnaltes, “Providing ride-through capability to a doubly fed induction generator under unbalanced voltage dips,” IEEE Trans. Power Electron., vol. 24, pp. 1747– 1757, Jul. 2009. [9] S. Foster, L. Xu, and B. Fox, “Coordinated reactive power control for facilitating fault ride through of doubly fed induction generator- and fixed speed induction generator-based wind farms,” IET Renew. Power Gener., vol. 4, pp. 128–138, Mar. 2010. [10] K. E. Okedu, S. M. Muyeen, R. Takahashi, and J. Tamura, “Wind farms fault ride through using DFIG with new protection scheme,” IEEE Trans. Sustain. Energy, vol. 3, pp. 242–254, Apr. 2012. [11] B. Fox, D. Flynn, L. Bryans, N. Jenkins, D. Milborrow, M. O’Malley, R. Watson, and O. Anaya-Lara, Wind Power Integration: Connection and System Operational Aspects. Stevenage, U.K.: Inst. Eng. Technol., 2007. [12] K. Nam, C. Lee, D. K. Park, T. K. Ko, and B. Y. Seok, “Thermal and electrical analysis of coated conductor under AC over-current,” IEEE Trans. Appl. Supercond., vol. 17, pp. 1923–1926, Jun. 2007. [13] S. M. Blair, C. D. Booth, and G. M. Burt, “Current-time characteristics of resistive superconducting fault current limiters,” IEEE Trans. Appl. Supercond., vol. 22, p. 5600205, Apr. 2012. [14] M. Tsili and S. Papathanassiou, “A review of grid code technical requirements for wind farms,” IET Renew. Power Gener., vol. 3, pp. 308–332, Sep. 2009.