Behavior of DFIG Wind Turbines with Crowbar Protection under Short Circuit

Transcription

1 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol: 1 No: 3 3 Behaior of DFIG Wind Turbines with Crowbar Protection under Short Circuit Omar Noureldeen a,b a Electrical Engineering Department, Faculty of Engineering, Islamic Uniersity, Madinah, King Saudi Arabia b Electrical Engineering Department, Faculty of Engineering, South Valley Uniersity, Qena, Egypt omar_noureldeen@su.edu.eg Abstract- Doubly Fed Induction Generators DFIG are nowadays widely used in ariable speed wind power plants. The dynamic interaction between ariable speed DFIG wind turbines and the power system which is subjected to disturbances, such as short circuit faults, is an important issue. The ability of the wind power plant to stay connected to the grid during disturbances is important to aoid a cascading effect due to lack of power. This paper inestigates the impact of fault ride through on the stability of DFIG wind turbine using crowbar resistance. Simulation test using MATLAB-Simulink toolbox is implemented on a 9 MW wind farm exports its power to 1 KV grid. The simulation is performed using different crowbar resistances. The ariations of rotor current, rotor speed, DC-link oltage, actie power and reactie power of the wind farm are inestigated. Index Terms- DFIG, Crowbar Resistance, Rotor Current, Actie Power, Reactie Power, Short Circuit. I. INTRODUCTION Due to the increasing of CO emissions, renewable energy systems, especially wind energy generation, hae attracted great interest in recent years. Nowadays, the most widely used wind turbines in wind farm are based on DFIG due to some adantages such as reduction of inerter cost, the potential to control torque and a slight increase in efficiency of wind energy extraction. Howeer, wind turbines based on the DFIG are ery sensitie to grid disturbance especially to oltage dips [1]. DFIG power conerter, which has a restricted oer-current limit, needs special attention especially during faults in the grid. When faults occur and cause oltage dips, subsequently the current flowing through the power conerter may be ery high oer-current. During this situation, it is common to block the conerter to aoid any risk of damage, and then to disconnect the generator from the grid. The most demanding requisite for wind farms especially with DFIG is the Fault Ride Through FRT capability. The wind farm must stay connected to the grid during system disturbances and support the network oltage and frequency. This implies some requirements for the safe operation of the rotor side inerter of the DFIG, since the rotor current will become ery large during these grid failures. Therefore, DFIG requires a protection system called actie crowbar disconnects the conerter in order to protect it turning the generator into a squirrel cage induction machine [-3]. The crowbar may comprise of a set of thyristors that will shortcircuit the rotor windings when triggered and thereby limit the rotor oltage and proide an additional path for the rotor current. Different alues of the crowbar resistors result in a different behaior. Using this technology, the DFIG can stay connected to the grid and resume operation as soon as possible. Seeral research works hae dealt with different crowbar strategies for Low Voltage Ride Through LVRT improement [-9]. The alue of crowbar resistance should be chosen carefully. There are two requirements that gie an upper and a lower limit to the crowbar resistance. It should be high enough to limit the short circuit rotor current and it should be low enough to aoid too high oltage in the rotor circuit. Motiated by the reason aboe, this paper proides a study of the dynamics of the grid connected with DFIG wind turbine. This paper inestigates a model of 9 MW wind farm exports its power to a 1 KV grid. The simulation is inestigated using SimPowerSystem toolbox. The impact of different crowbar resistance on the behaior of DFIG wind turbine during fault is inestigated. The crowbar resistances are simulated as a symmetric three phase Y-connection. II. DFIG MODEL In order to model the DFIG, a standard wound-rotor induction machine component from the SimPowerSystem toolbox has been used. The synchronous d q reference frame has been selected to perform all the simulations [1-11]: ω ψ = R i ω ψ qs + () = R i ω ψ dr + (3) = R i ω ψ qr + () ds ds = Rsi ds s qs + (1) qs s qs s ds dr r dr r qr qr r qr r dr where are oltages (V), i are currents(a), R are resistances (Ω), ψ are flux linkages (V s). Indices d and q indicate direct and quadrature axis components respectiely while s and r indicate stator and rotor quantities respectiely. All quantities are referred to the stator IJECS-IJENS June 1 IJENS

2 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol: 1 No: 3 33 III. DFIG CONTROL Figure 1 shows the schematic diagram of a DFIG connected to grid. It consists of a wound rotor induction generator with back-to-back oltage source conerters linking the rotor to the grid. The back-to-back conerter consists of a Rotor Side Conerter (RSC) and a Grid Side Conerter (GSC) connected to the grid by a line filter to reduce the harmonics caused by the conerter. The RSC is used to control the generator speed and reactie power, while the GSC is used to control DC-link oltage and reactie power exchange with the grid [1]. A. Rotor Side Conerter Control System The RSC controls independently the actie and reactie power injected by the DFIG into the grid in a stator flux dqreference frame. Figure shows the control scheme of the RSC. The q-axis current component I qr is used to control the actie power using a maximum power tracking strategy to calculate the actie power reference [1]. The actual speed of the turbine ω r is measured and the corresponding mechanical power of the tracking characteristic is used as the reference power for the power control loop. The reference alue for the actie power P r is compared with its actual alue P and the error is sent to a PI controller which generates the reference alue for the q-axis current I qr_erf. This signal is compared to its actual alue I qr and the error is passed through a second PI controller determining the reference oltage for the q-axis component V qr. The d-axis is used to control the reactie power exchanged with the grid, which in normal operation is set to zero in order to operate with unity power factor. In case of disturbance, if the induced current in the rotor circuit is not high enough to trigger the oer-current protection, the RSC is set to inject reactie power into the grid in order to support the oltage restoration. In such case, the actual oltage V at the collection bus is compared to its reference alue V ref and the error is passed through a PI controller to generate the reference signal for the reactie power of the DFIG. Similar to the control strategy of the q component, the error between the reactie power reference and its actual alue is passed through a PI controller to determine the reference alue for the d-axis current I dr_erf. This signal is compared to the d-axis current alue I dr and the error is sent to a third PI controller which determines the reference oltage for the d-axis component V dr. Figure Schematic diagram of rotor side conerter control system. B. Grid Side Conerter Control System Figure 3 shows the control system of the GSC which is used to regulate the DC link oltage between both conerters. In normal operation, the RSC already controls the unity power factor operation and therefore the reference alue for the exchanged reactie power between the GSC and the grid is set to zero. In case of disturbance, the GSC is set to inject reactie power into the grid whether the RSC is blocked or is kept in operation. The control of the GSC is performed using the dqreference frame. The actual oltage V dc at the DC link is compared with its reference alue V dc_ref and the error between both signals is passed through a PI controller which determines the reference signal for the d-axis current I d_gsc_ref. This latter signal is subtracted with its current alue I d_gsc and the error is sent to another PI controller to obtain the reference oltage for the d-axis component. As for the q-axis current, its reference alue depends whether the system operates in normal operation or during disturbance. Figure 3 Schematic diagram of grid side conerter control. Figure 1 Doubly fed induction generator controller In case of disturbance, the actual AC-side oltage of the GSC is compared with its reference alue and the error is passed through a PI controller which generates the reference IJECS-IJENS June 1 IJENS

3 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol: 1 No: 3 3 signal for the q-axis current. This reference signal is compared to its current alue and the error is sent to a second PI controller which establishes the reference oltage for the q axis component. The injection of actie and reactie power by the GSC is limited by its nominal capacity represented by the following equation in per unit base: I = I + I (5) con ( q ) ( d ) 1 During normal operation, the strategy does not present limitations with the control of the DC link oltage since the q- axis current is set to zero and therefore the conerter capacity is only used to control the DC link oltage. IV. DYNAMIC SIMULATION To demonstrate the effect of a grid fault on a DFIG wind farm connected grid, a detailed time domain model is inestigated. Figure shows a single line diagram of the studied system. It consists of six 1.5 MW wind turbines connected to a 5-kV distribution system exports power to a 1-kV grid through a 3 km transmission line. The stator windings are connected directly to the 6 Hz grid while the rotor is fed at ariable frequency through the RSC conerter. The GSC is connected to the grid by a line filter to reduce the harmonics caused by the conerter. The wind turbine has a protection system monitoring oltage, current and machine speed. The DC link oltage of the DFIG is also monitored. In this paper, the simulated disturbance is a three-phase to ground fault occurs at the wind farm terminals for 15 ms duration, where the protection system detects it after 1 ms from its occurrence. When the fault is detected by the protection system, the thyristors of the crowbar are switched on and the RSC is kept open. At the same time the rotor current is flowing through the crowbar resistor. When the crowbar is actiated the RSC pulses are disabled and the machine behaes like a squirrel cage induction machine directly coupled to the grid. After fault clearance, the crowbar resistance is still connected for 1 ms and then the RSC is reconnected. The main data of DFIG wind farm and system parameters are illustrated in Appendix A. The simulation scenario is performed for different crowbar resistance alues as shown in Table 1. Table 1: Relations between crowbar resistance and rotor resistance Case 1 without crowbar resistance Case crowbar resistance = 1 rotor resistance Case 3 crowbar resistance = 5 rotor resistance Case crowbar resistance = 1 rotor resistance Figure 5 shows the wind turbine power characteristics at different wind speed alues when the pitch angle is zero. For a wind speed of 1 m/s, the maximum turbine output is.55 pu of its rated power. T u rb in e output pow er (pu of n om inal m ech an ical pow er) Figure Single line diagram of the studied system 9 m/s 11 m/s 1 m/s Turbine speed (pu of nominal generator speed) Figure 5 Turbine power characteristic with zero pitch angle. V. SIMULATION RESULTS The simulation results show the ariations of rotor current, rotor speed, actie power, reactie power, and DC link oltage with different crowbar resistors during grid faults. As shown in Fig. 6, the rotor current is decreased by increasing the crowbar resistance alue. Also, the post fault current is affected by the crowbar resistance alue. As shown in Fig. 6 (a), the post fault current is aried between 1.6 pu and 1. pu when the crowbar resistance is not used. In case of using the crowbar resistance equals to 1 times of rotor resistance, the post fault current is aried between.88 pu and.91 pu as shown in Fig. 6 (b). It is IJECS-IJENS June 1 IJENS

4 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol: 1 No: 3 35 noticed that when the crowbar resistance is increased to 5 times or 1 times of rotor resistance, the post fault current has the same trend and aried between.73 pu and.85 pu. - - rotor current without crowbar resistance Without crowbar resistance - - (a) Crowbar resistance = 1 rotor resistance - - (b) Crowbar resistance = 5 rotor resistance - - (c) Crowbar resistance = 1 rotor resistance (d) Figure 7 shows the ariations of rotor speed during fault with different crowbar resistances. After fault occurrence and before the crowbar is actiated, the rotor speed is decreased from 1.9 pu to pu. The rotor speed is increased rapidly after actiating of crowbar resistance. It is noticed that the peak alue of rotor resistance is increased by increasing the crowbar resistance alue. The rotor speed is aried between pu and 1.97 pu according the crowbar resistance alue. R o t o r S p e e d ( p u ) without crowbar resistance 1.9 crowbar resistance = 1 rotor resistance 1.89 crowbar resistance = 5 rotor resistance crowbar resistance = 1 rotor resistance 1.88 Figure 7 Rotor speed ariations during fault with different crowbar resistances. Figure 8 shows the ariations of the generated actie power during fault in case of different crowbar resistances. Before fault occurrence, the total generated actie power is.7 MW where the wind farm operates at wind speed of 1 m/s. During fault, the generated actie power is decreased to 1.68 MW when crowbar resistance is not used while it decreases to.7 MW when the crowbar resistance is actiated for different alues. After fault clearance and reconnection of the RSC conerter, the generated actie power is increased to 7 MW for all studied cases and then the system returns to the steady state operation. A c t i e P o w e r ( M W ) 1 5 without crowbar crowbar resistance=1 rotor resistance crowbar resistance=5 rotor resistance crowbar resistance=1 rotor resistance Figure 6 Rotor current ersus time for different crowbar resistances a) without crowbar resistance. b) crowbar resistance = 1 times of rotor resistance. c) crowbar resistance = 5 times of rotor resistance. d) crowbar resistance = 1times of rotor resistance. -5 Figure 8 Actie power ariations of DFIG wind farm during fault with different crowbar resistances IJECS-IJENS June 1 IJENS

5 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol: 1 No: 3 36 Figure 9 shows the ariations of the reactie power during fault in case of different crowbar resistances. Before fault occurrence, the reactie power is nearly zero. After fault occurrence, in case of crowbar resistance is not actiated, the RSC conerter acts as STATCOM and injects 5.95 MVAR reactie power to the grid. During fault period, when there is no crowbar resistance, the reactie power is aried between.98 MVAR as absorbed reactie power and. MVAR as injected reactie power. In the case of crowbar resistance is actiated, the wind farm generators absorb reactie power of. MVAR for different alues of crowbar resistance. After fault clearance, the reactie power is exchanged between the wind farm and the grid before the system returns to the steady state operation. After fault clearance and reconnection of RSC, the injected reactie power is increased to 3.5 MVAR when the crowbar resistance equals 1 times of rotor resistance. Also, it is increased to 3.95 MVAR when the crowbar resistance equals 5 and 1 times of rotor resistance, where it was.9 MVAR when the crowbar protection is not used. On the other hand, the absorbed reactie power is increased to 3.5 MVAR when there is no crowbar resistance and it equals.5 MVAR when the crowbar protection is used for all cases. R e a c t i e P o w e r ( M V A R ) without crowbar crowbar resistance=1 rotor resistance crowbar resistance=5 rotor resistance crowbar resistance=1 rotor resistance - Figure 9 Reactie power ariations of DFIG wind farm during fault with different crowbar resistances. Figure 1 shows the ariations of the DC-link oltage during fault with different crowbar resistances. The DC-link capacitance equals 6 mf with nominal oltage of 1 V. During fault occurrence, the grid oltage falls and the GSC is not able to transfer the power from the RSC to the grid. Therefore, the additional energy goes into charging the DClink capacitor and thus its oltage rises rapidly. When the system operates without crowbar protection, the DC-link oltage is increased to 9 V during fault period. When the crowbar resistance is triggered at the instant of 1.1 s, the DClink oltage alue is 1716 V and starts to decrease for the case of crowbar resistance equals 1 times of rotor resistances. But for other two cases, it will continue in increasing until it reaches 181 V and then starts in decreasing. D C V o lt a g e ( o lt ) without crowbar crowbar resistance=1 rotor resistance crowbar resistance=5 rotor resistance crowbar resistance=1 rotor resistance 5 Figure 1 DC-link oltage ariations during fault with different crowbar resistances. VI. CONCLUSIONS This paper inestigates the behaior of DFIG wind farm during terminal fault in presence of crowbar protection system. Dynamic simulation model of 9 MW wind farm connected grid is inestigated. The DFIG rotor current, rotor speed, actie power, reactie power and DC-link oltage are monitored in steady state and fault state conditions. The simulation scenario is performed for different crowbar resistances alues. The simulated disturbance is a three-phase to ground fault occurs at the wind farm terminals for 15 ms duration. The crowbar protection resistance is actiated after fault occurrence with delay time of 1 ms. Also, it will deactiated after fault clearance with delay time of 1 ms. It shows that absence of crowbar resistance leads to high rotor current, high DC-link oltage and more reactie power fluctuations during and post fault periods. When the crowbar protection is used, rotor current is decreased while rotor speed is increased. When the crowbar protection is not used, the generated actie power alue during fault is more than that in the cases of using crowbar resistances. After fault clearance and reconnection of RSC, the DFIG can proide reactie power support to the grid and thus help in stabilizing of the grid oltage. During post fault period, the absorbed reactie power by the wind farm generators is decreased when the crowbar protection is used. Finally, the rotor current, rotor speed, DC-link oltage, actie power and reactie power are affected by a certain alue of the crowbar resistance. Therefore, the crowbar resistance alue should be chosen carefully IJECS-IJENS June 1 IJENS

6 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol: 1 No: 3 37 APPENDIX A. DFIG DATA AND SYSTEM PARAMETERS DFIG parameters Rated power (MW) 1.5 Rated oltage (V) 575 Rated frequency (Hz) 6 Stator resistance (pu).83 Rotor resistance (pu).377 Stator leakage inductance (pu).18 Rotor leakage inductance (pu).1791 Mutual inductance (pu) 6.77 Transmission line parameters Positie sequence resistance (ohm/km).1153 Zero sequence resistance (ohm/km).13 Positie sequence inductance (henries/km).15 Zero sequence inductance (henries/km).33 Positie sequence capacitance (farads/km) 11.33e-9 Zero sequence capacitance (farads/km) 5.1e-9 Transformer( T1) parameter Rated power (MVA) 1 Turns ratio 575V/5KV Impedance (pu).17+j.5 Transformer( T) parameter Rated power (MVA) 7 Turns ratio 5KV/1KV Impedance (pu).53+j.16 Grid impedance Impedance (pu) REFERENCES.+j. [1] Min Min Kyaw, V.K. Ramachandaramurthy, "Fault ride through and oltage regulation for grid connected wind turbine", ScienceDirect, Renewable Energy 36 (11) 6-15, doi:1.116/j.renene [] L. Shi, N. Chen and Q. Lu, "Dynamic Characteristic Analysis of Doublyfed Induction Generator Low Voltage Ride-through", ScienceDirect, Energy Procedia 16 (1) , doi:1.116/j.egypro [3] Christian Wessels, Fabian Gebhar and Friedrich W. Fuchs, "Dynamic Voltage Restorer to allow LVRT for a DFIG Wind Turbine", IEEE International Symposium on Industrial Electronics (ISIE), 1, doi: 1.119/ISIE [] J. Lopez, P. Sanchis, X. Roboam, L. Marroyo Dynamic behaior of doubly fed induction generator during three phase oltage dips IEEE Transactions on Energy Conersion, ol., no. 3, 7, pp [5] M. Garcia-Gracia, M. P. Comech, J. Sallan, and A. Llombart, Modelling wind farms for grid disturbance studies, Science direct, renewable energy, 33, 8, pp [6] M. Rahimi, M. Parniani, Grid fault ride through analysis and control of wind turbines with doubly fed induction generators, Science direct, Electrical Power System Research, 8, 1, pp [7] S. Chondrogiannis, M. Barnes, Specification of rotor side oltage source inerter of a doubly-fed induction generator for achieing ride-through capability, IET Renewable Power Generation, ol., no. 3, 8, pp [8] Anca D. Hansen, Gabriele Michalke, Fault ride-through capability of DFIG wind turbines ScienceDirect, Renewable Energy 3 (7) , doi:1.116/j.renene [9] Francois B., Yongdong Li, Improed Crowbar Control Strategy of DFIG Based Wind Turbines for Grid Fault Ride-Through Applied Power Electronics Conference and Exposition, 9, pp , doi /APEC [1] Mingyu Wang, Bin Zhao, Hui Li, Chao Yang, Renjie Ye, Z. Chen, Inestigation of Transient Models and Performances for a Doubly Fed Wind Turbine under a Grid Fault WSEAS TRANSACTIONS on CIRCUITS and SYSTEMS, Issue 11, Volume 1, Noember 11. [11] Omar Noureldeen, Mahmoud Rihan, Barakat Hasanin, "Stability improement of fixed speed induction generator wind farm using STATCOM during different fault locations and durations", ScienceDirect, Ain Shams Engineering Journal (11), 1 1, doi:1.116/j.asej [1] MATLAB/Simulink Documentation. Aailable: Omar Noureldeen is an assistant professor in the department of electrical engineering, Faculty of Engineering,Qena, South Valley Uniersity, Egypt. He receied his Ph. D. in electrical power and machines from Faculty of Engineering, Cairo Uniersity in. From to 6 he has been assistant professor at the department of electrical engineering, Higher Institute of Energy, South Valley Uniersity. Since 7 he is assistant professor at the department of electrical engineering, Faculty of Engineering - Qena, South Valley Uniersity. Since 11 till now he is assiatant professor at the department of electrical engineering, Faculty of Engineering, Islamic Uniersity, Madinah, King Saudi Arabia. His fields of interest are digital protection of power systems, power system stability, and renewable energy systems IJECS-IJENS June 1 IJENS