Enhancing LVRT Capability of FSIG Wind Turbine Using Current Source UPQC Based on Resistive SFCL

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1 Enhancing LVRT Capability of FSIG Wind Turbine Using Current Source UPQC Based on Resistive SFCL Amir Moghadasi, Student Member, IEEE Department of Electrical and Computer Engineering Florida International University Miami, Florida Arif Islam, Member, IEEE Department of Electrical and Computer Engineering Florida International University Miami, Florida Abstract This paper presents a current-source converter based unified power quality conditioner (UPQC) is used for a fixedspeed induction generator (FSIG) wind turbines in order to enhance the low voltage ride through (LVRT) capability. When voltage sag occurs due to grid side fault, the series compensator of the UPQC injects required voltage to prevent disconnecting of FSIG wind turbine. On the other hand, faults produced by wind turbine generator systems will impact not only the wind farms but also the interconnected system including the grid if proper protection is not ensured. Therefore, the resistive SFCL incorporated in series with the dc-link inductance of UPQC is proposed to limit excessive current in the event of the generator side fault. Detailed simulation studies implemented in PSCAD/EMTDC confirm that both of resistive SFCL and large dc-link inductance are effective in decreasing the fault current thereby reducing the voltage dip at the generator terminal. Index Terms-- Current-source converter, fixed-speed induction generator, low voltage ride-through (LVRT), superconducting fault current limiter. I. INTRODUCTION Wind turbines with grid-connected mode of operation play the significant role toward in sustainable energy development in the future. Some power quality (PQ) events such as voltage sag and voltage interruption corresponding voltage dips can lead to cut off a large wind farm and consequently serious effects on the power system operation. Therefore, a new set of grid codes have been defined recently, which includes the low voltage ride through (LVRT) requirements for wind turbine generator systems during the network disturbances. Fig. 1 shows a practical example of the LVRT curve defined by the Spanish grid operator. According to this requirement, the connection point voltage must reach the 80% of its nominal value within 0.5 s and the 95% within 15 s, after the clearing of the fault [1]. As more sensitive loads have come into wide use, power quality is a big issue of customers and utilities. In today's complex electronics environment many problems can occur due to poor quality of power. In this case, the active power The authors are with the Electrical Engineering Department, Center of Energy, Power & Sustainability (EPS), Florida International University (FIU), Miami 33174, USA ( amogh004@fiu.edu, arislam@fiu.edu). Figure 1. Proposed voltage-time curve set by Spanish grid operator [1]. filters (APFs) such as distribution STATCOM and Dynamic Voltage Restorer (DVR) have widely been used for compensation of many power quality problems in distribution system [2]-[4]. Also, the inverters play a pivotal role in DC charging of Electric Vehicles (EVs) [5]. The unified power quality conditioner (UPQC) which is integrated of series and shunt APFs have been commonly studied by many researchers as an ultimate device to improve voltage sag, voltage unbalance, voltage flicker, harmonics, dynamic active and reactive power regulation [6]-[8]. Academically, UPQC can be recognized by either a voltage-source converter (VSC) or a current-source converter (CSC) [9]. However, CSC topology proposes a number of significant advantages over VSC topology such as directly controlling the output current of converter, inherent short-circuit protection and high converter reliability [9], [10]. Due to large dc-link inductance and current control in the rectifier, the inherent protection from over-current, the power circuit of the CSC is simpler and more robust than that of the VSC. Furthermore, high penetration of the wind farms existing distribution networks leads to the increasing fault levels thereby more reducing terminal voltage in the wind turbine. Superconducting fault current limiters (SFCLs) are an appliance installed in electric power system to limit excessive current in the event of fault. A superconducting fault current limiter (SFCL) has many advantageous functions, such as automatic excessive current detecting, automatic recovering, and faster excessive current limiting operations [8], [12] /14/$ IEEE

2 This paper investigates the enhancement of the fault right through capability and power quality for the fixed speed wind generator using current source UPQC based on resistive superconducting fault current limiter (SFCL) system. During the normal operation, shunt APF suppresses the harmonic currents and compensates reactive power simultaneously. When voltage sag occurs due to grid side fault, the series APF injects a voltage equal to the difference of the grid voltage and the ideal terminal voltage to prevent disconnecting of the FSIG wind turbine. Similarly, in case of symmetrical fault on generator side, resistive SFCL along with dc-link inductance can perform as self-healing method by inserting the equivalent impedance to the network in order to limit fault current and to improve LVRT capability of wind turbine. To guarantee the validity of the proposed technique, the whole system is built using PSCAD/EMTDC software. T e T m 1/sT 1/J m Figure 2. Modeling of wind turbine system drive-train. P g P * g ω m ω * m Max B K β ω m II. FIXED SPEED WIND TURBINE MODEL A significant number of the wind farms in operation are equipped with fixed-speed squirrel-cage induction generators and the capacitor banks providing the generator reactive power requirements. The nonlinear model of the wind turbine commonly simplified to a static model of the aerodynamic rotor, a one-mass model of the drive train and a third-order model of induction generator. A. Aerodynamic Model The mechanical power extracted by a wind turbine from the wind is expressed by the well-known equation: 3 P 0.5 AV CP, (1) w w where ρ is the air density, A is the area of the rotor disk, V w is the wind speed and C p is the power coefficient. The power coefficient characterizes the rotor aerodynamics as a function of both tip speed ratio and the blade pitch angle. The tip speed ratio is defined as the relationship between rotor blade speed and the wind speed [12]. B. Drive Train System The drive train of the wind turbine generator is specified by a one-mass model based on deriving the state equation for the rotor angular speed at the wind turbine, represented by dm 1 Tm-Te-B m (2) dt Jm where J m is the moment of inertia for blades, hub and generator, Tm is the mechanical torque, T e is the electric torque and B is rotational damping. The applied block diagram for modeling of wind turbine system drive-train is shown in Fig. 2. C. Generation System Model In power system stability studies, the third-order model of FSIG can be achieved by neglecting the stator flux transients in the voltage relations and eliminating the rotor currents. Figure 3. Pitch angle Control used in FSIG-based wind turbine. However, the equations used for squirrel cage induction generator modeling are described in [14]. Also, Fig. 3 shows the control scheme of pitch angle regulator of the wind turbine. The inputs to the model are the mechanical speed of the machine ω m and the power output of the machine Pg. III. THE PROPOSED UPQC SYSTEM In this effort, the UPQC topology is composed of the integration of series-active and shunt-active power filters (APFs) connected back to back to a common dc link which is recognized as the most sophisticated power quality improvement. The main purpose of the series APF is to compensate voltage flicker/imbalance, voltage regulation and harmonic at the utility-consumer at point of common coupling (PCC). While, the shunt-apf absorbs current harmonics, compensates for reactive power and negative-sequence current injected by the load. Furthermore, it controls the current of the DC link inductor to a desired value. The passive filters are also used for neutralization of the switching frequency effects of the APFs and high frequencies. The current-source unified power quality conditioner (CS-UPQC) incorporated with the SFCL located in dc link is shown in Fig. 4 In current-source UPQC, DC current of the energy storage inductance (I dc ) must not be too small because converter cannot perform as perfect compensator. On the other hand, If I dc is set on the too high value it will lead to excessive loss in DC inductance (L dc ) and consequently passive filters cannot cancel the switching frequency. In practice, also L dc must be chosen large enough to limit the current ripple but the maximum amount of L dc is limited by some major factors such as the volume, weight, and cost. A. Series Active Filter Control The function of series active filter is to compensate the voltage disturbance in the grid side, which is due to the fault in the distribution system. The series APF injects a voltage equal to the difference of the grid voltage and the ideal load voltage. The state equation of the series active filter is given by

3 Utility 380 V, 50 Hz Zgrid Grid Bus Igrid LF UPQC System Based Current Source Converter CF IF Shunt APF IPCC Ldc Idc RSFCL Series APF VF CF VC PCC Non-Linear Load 50 kva IL Cbank 660 kw, 380 V FSIG Figure 4. Structure of the proposed system: FSIG-based wind turbine with current source UPQC and SFCL connected to dc link. VC * K VPCC * Vgrid V F VF * K. where V * C reference load voltage and V F is the series APF voltage. The reference voltage is compared with V F in a hysteresis controller band to generate the switching signals for IGBTs. The control block diagram series APF is shown in Fig. 5. V * PCC V grid V F V F V * F V * C K Hysteresis Module Figure 5. Generation of the switching signals for series APF. Switching Signal B. Shunt Active Filter Control Shunt active power filter were developed for suppressing the harmonic currents and compensating reactive power, simultaneously. The shunt active filter controller was designed using the instantaneous power method based on -0 [5]. The reference value of compensating current, which is including V PCC (a,b,c) I PCC (a,b,c) I α I β V * dc V β V α Power calculation using (8) V dc q P Switching Signal HPF P loss PWM Module p I * F I * α (7) Current calculation using (9) Figure 6. Generation of the switching signals for shunt APF. the part harmonic and reactive, is shown in the block diagram of Fig. 6. p v vi. q v v i i 1 v v p p loss. 2 2 i v v v v q V α V β I * β (8) (9) IV. THEORETICAL MODEL OF THE RESISTIVE SFCL Superconducting Fault Current Limiters (SFCLs) can limit fault currents without adding any impedance to the system during normal operations. Among different SFCL devices, the resistive type SFCL is considered due to its compact size and simple principle of operation [15]. However, during the excessive current, SFCLs switch into a high-impedance state automatically. Moreover, after the fault current is suppressed successfully, the SFCL can be recovered to the normal state without external assistance. The need for SFCLs is driven by the rising fault current levels in power grids as clean energy sources, such as wind and solar plant. The resistive type of SFCL is based on superconducting bars or double non-inductive spirals [16] for effective elimination of inductance. YBCO coated conductors are generally used for resistive type of SFCLs. The current limiting behavior of the resistive type SFCL can be practically subdivided into superconducting state, flux flow state and normal conducting state in which is assumed superconducting state at a temperature and a current below the critical values; flux flow state at a current above the critical value but temperature below the critical values; and (c) normal conductive state at a temperature above the critical value. Detailed formulation can be found in [12]. When the fault occurs in generator side, fault current increases in amplitude and reaches critical current and SFCL will be converted to normal conducting state. Under this condition, the certain value of a nonlinear resistance and the dc-link inductance are inserted to the network through transformer of series APF in order to limit fault current. The equivalent impedance of the current source UPQC during the generator fault is given by following 2 2 Zeq Zt n ZL n RSFCL (10) where Z t is impedance of the isolation transformer, Z L is impedance of dc link and R SFCL is the superconductor resistance. According to (10), injected impedance also depends isolation transformer turns ratio, n. V. SIMULATION RESULTS In this section, simulation studies using PSCAD/EMTDC were carried out to verify the effectiveness of the proposed structure shown in Fig. 4 by enhancing the LVRT capability of the FSIG-based wind and damping the fault current occurred in wind turbine terminal. Here, one fixed-speed squirrel-cage induction generator (660 kw) is directly connected to the PCC. A capacitor bank, C bank, has been considered to be connected to the terminal of induction generator for reactive power compensation at steady state. The value of capacitor is chosen so that power factor of the wind generator during the rated operation becomes unity. The nonlinear and sensitive load of a three-phase diode bridge (L1, 50 kva) supplied by grid was considered to prove ability of UPQC in mitigating current harmonics. For limiting purpose of fault current, the resistive SFCL incorporated in series with the dc-link inductance of UPQC is introduced and modeled in this work. The characteristics of the system and the current source UPQC parameters are given in Table I.

4 TABLE I CHARACTERISTICS OF THE UPQC AND THE SYSTEM Symbol Quantity Value V S Supply voltage 380 V R S Resistance of the system 0.01 Ω L S Inductance of the system 0.01mH L Non-Linear load 50 kva f SW Switching frequency 10 khz THD L1 Total Harmonic Distortion L 30% I dc-ref Inductance reference current 100 A L dc Dc-link Inductance 100 mh L f Inductance of the passive filter 0.4mH C f Capacitor of the passive filter 50μF Trans Rating Turn ratio 70 kva 2:1 (c) Figure 7. Current harmonic compensation. Load current. Shut APF current. (c) Grid Current. Fig. 7 shows the current waveform of: 1) the load and the grid and 2) shunt active filter, in which the load current can be compensated by the shunt active filter current, keeping the grid current sinusoidal. In order to evaluate the operating characteristic of the current source UPQC and limiting behaviors of the resistive SFCL, simulations based on the sample parameters were carried out by considering the following cases: Case 1: Three-phase Fault on the grid side A symmetrical three phase fault occurring at 0.2 s and clearing after 0.1 s is assumed at the grid bus of the power system. The simulation results are shown in Fig. 8. Under fault condition, the amplitude of wind turbine terminal drops to about 80% of its nominal voltage without UPQC support [see Fig. 8]. In Fig. 8, UPQC is controlled to compensate the voltage disturbance in the grid side. (c) Figure 8. Voltage sag compensation. generator side voltage. Series APF voltage. (c) Grid Current. Consequently, low voltage ride-through capability will be improved. Fig. 8(c) illustrates the terminal voltage of the wind generator in which is maintained at 1 pu. Case 2: Three-phase Fault on the wind turbine side As earlier aforementioned, one of the features of current source UPQC is the inherent ability in limiting the fault current by the dc-link inductance (L dc ). Therefore, in the following discussion, we assume that an error has occurred near the wind turbine terminal at 0.2 s and during of 0.1 s. In this case, the current with amplitude of an 1800 A is injected into the grid without using of UPQC. In case with UPQC, L dc prevents from extreme changes and reduce the amplitude of the fault current to 900 A. Fig. 9 shows the fault current without and with current source UPQC support. Obviously, the larger L dc (i.e. greater impedance) leads to further decrease of the fault current. But, by increasing the L dc, the installation cost of the UPQC and losses increase significantly. Thus, in order to utilize the current source UPQC effectively as fault current limiter like compensating operation, implementation of the resistive SFCL in series with L dc is proposed. The feasibility of the model for limiting the fault current and resistance of SFCL are shown in Fig. 10. Without connecting SFCL, the peak of the dc current reaches about 250 A. With SFCL, dc current was limited effectively to reach about 150 A in duration of the fault [see Fig. 10]. On the other hand, according to Fig. 10, the limiting resistance of the SFCL went up to 3.7 ohm after five cycles of the fault to limit fault current to approximately 400 A as shown in Fig. 11.

5 Figure 9. Fault current drown form grid in the absence and presence of current source UPQC. Figure 10. Fault current drown form grid in the absence and presence of current source UPQC. Figure 11. Fault current drown form grid in the absence and presence of current source UPQC. VI. CONCLUSION This paper has designated the current source UPQC system to improve the power quality and low voltage ridethrough capability of the FSIG-based wind turbine. Despite, UPQC with CSC topology proposes a number of significant advantages such as directly controlling the output current of converter, inherent short-circuit protection and high converter reliability it takes advantages of series and shunt APFs for compensating the distortions of grid voltages and load currents. In this effort, it has been demonstrated that, during the normal operation, shunt APF of the UPQC maintains sinusoidal current at the grid bus and when voltage sag occurs due to grid side fault the series APF of the UPQC can inject appropriate deficit voltage to prevent disconnecting of the FSIG based wind turbine. In case of symmetrical fault on wind turbine side, resistive SFCL with alone dc-link inductance can perform as selfhealing method by inserting the equivalent impedance to the network in order to limit fault current and to improve LVRT capability of wind turbine. With using SFCL, the grid current has been limited effectively and the minimum voltage level at the generator terminal has been increased leading to compliance with international grid codes. VII. REFERENCES [1] Voltage sag response requirements of wind power facilities. Operation procedure. Spanish System Operator (REE), PO 12.3; [2] Shukai Xu; Qiang Song; Yongqiang Zhu; Wenhua Liu, "Development of a D-STATCOM prototype based on cascade inverter with isolation transformer for unbalanced load compensation," Industrial Technology, ICIT [3] Sensarma, P.S.; Padiyar, K. R.; Ramanarayanan, V., "Analysis and performance evaluation of a distribution STATCOM for compensating voltage fluctuations," Power Delivery, IEEE Transactions on, vol.16, no.2, pp.259, 264, Apr [4] Ghosh, A.; Ledwich, G., "Compensation of Distribution System Voltage Using Dynamic Voltage Restorer (DVR)," Power Engineering Review, IEEE, vol.22, no.8, pp.71, 71, Aug [5] M.H. Amini, B. Nabi, M. Parsa Moghaddam and S.A. Mortazavi, Evaluating the Effect of Demand Response Programs and Fuel Cost on PHEV Owners Behavior, A Mathematical Approach", Second Iranian Conference on Smart Grid, [6] H. Akagi, H. Fujita, A new power line conditional for harmonic compensation in power systems, IEEE Trans. Power Del, vol. 10, no. 3, pp , Jul [7] Moghadasi, A.; Torabi, S.M.; Salehifar, M., "Combined operation of the unified power quality conditioner with SFCL and SMES," Power Quality Conference (PQC), 2010 First, vol., no., pp.1,7, Sept. 2010Amir [8] Moghadasi; Heydari, H.; Salehifar, M., "Reduction in VA rating of the Unified Power Quality Conditioner with superconducting fault current limiters," Power Electronic & Drive Systems & Technologies Conference (PEDSTC), st, vol., no., pp.382, 387, Feb [9] Hingorani NG, Gyugyi L. Understanding FACTS: concepts and technology of flexible AC transmission systems. New York: IEEE Press; [10] Melin, P.E.; Espinoza, J.R.; Baier, C.R.; Guzman, J.I.; Espinosa, E.E., "Unified Power Quality Conditioner based on current source converters for harmonic mitigation using a decoupled control strategy," IECON th Annual Conference on IEEE Industrial Electronics Society, vol., no., pp.4152,4157, 7-10 Nov [11] Ajami Ali, Armaghan Mehdi. Modeling and state feedback controller of SSSC based current source converter In: Second international conference on computer and electrical engineering, ICCEE, vol. 1; December p [12] R. Sharifi, H. Heydari, Multiobjective optimization for HTS fault current limiters based on normalized simulated annealing, IEEE Trans. Appl. Superconduct, vol. 19, no. 4, pp , Aug [13] S. Heier, Grid Integration of Wind Energy Conversion Systems. Chicester: John Wiley & Sons, [14] E. S. Abdin and W. Xu, Control design and dynamic performance analysis of a wind turbine-induction generator unit, IEEE Trans. Energy Convers, vol. 15, no. 1, pp , Mar [15] H.heydari, A.H, Moghadasi, "Optimization Scheme in Combinatorial UPQC and SFCL Using Normalized Simulated Annealing," IEEE Trans, Power Delivery, vol. 26, NO. 3, [16] D. K. Park, M. C. Ahn, S. E. Yang, Y. S. Yoon, B. Y. Seok, CLee, H. M. Chang, Development of 220 V/300 A class non-inductive winding type fault current limiter using 2G HTS wire, IEEE Trans. Appl. Superconduct, vol. 17, no. 2, pt. 2, pp , Jun, 2007.