International society of academic and industrial research www.isair.org IJARAS International Journal of Academic Research in Applied Science (2): 2-28, 202 ijaras.isair.org Wind Farm Evaluation and Control Saswati Kumar Jadavpur University, India kumarsuswati@yahoo.com Abstract Wind power is the conversion of wind energy into a useful form of energy, such as using: wind turbines to make electricity, windmills for mechanical power, wind pumps for water pumping or drainage, or sails to propel ships. A large wind farm may consist of several hundred individual wind turbines which are connected to the electric power transmission network. Offshore wind farms can harness more frequent and powerful winds than are available to land-based installations and have less visual impact on the landscape but construction costs are considerably higher. Small onshore wind facilities are used to provide electricity to isolated locations and utility companies increasingly buy surplus electricity produced by small domestic wind turbines. Doubly fed induction generators (DFIG) are commonly used wind turbine generators (WTG) in electricity networks. In DFIG there are three control loops which should be implemented in order to appropriate performance of WTG. These control loops are pitch angle control, rotor speed control and voltage control. The results of wind power on the system performance are compared with a conventional thermal power plant. Keywords Wind Turbine Generators; Doubly Fed Induction Generators; Multi Machine Electric Power System
. Introduction Wind turbines generators (WTGs) are usually controlled to generate maximum electrical power from wind under normal wind conditions. However, because of the variations of the wind speed, the generated electrical power of a WTG is usually fluctuated. Currently, wind energy only provides about % 2% of the U.S. s electricity supply. At such a penetration level, it is not necessary to require WTGs to participate in automatic generation control, unit commitment, or frequency regulation. In order to meet power needs, taking into account economical and environmental factors, wind energy conversion is gradually gaining interest as a suitable source of renewable energy. The electromagnetic conversion is usually achieved by induction machines or synchronous and permanent magnet generators. Squirrel cage induction generators are widely used because of their lower cost, reliability, construction and simplicity of maintenance [, 2]. But when it is directly connected to a power network, which imposes the frequency, the speed must be set to a constant value by a mechanical device on the wind turbine. Then, for a high value of wind speed, the totality of the theoretical power cannot be extracted. To overcome this problem, a converter, which must be dimensioned for the totality of the power exchanged, can be placed between the stator and the network. In order to enable variable speed operations with a lower rated power converter, doubly-fed induction generator (DFIG) can be used as shown on Figure. The stator is directly connected to the grid and the rotor is fed to magnetize the machine. Control of electrical power exchanged between the stator of the DFIG and the power network by controlling independently the torque (consequently the active power) and the reactive power is an important issue in DFIG utilization [3]. Several investigations have been developed in this direction using converters and classical proportional-integral regulators [4-6]. In this paper a unified scheme is used to adjust DFIG controllers in the same time. The results of wind power on the system performance are compared with a conventional thermal power plant. Figure : Variable speed wind turbine with doubly fed induction generator 22
2. Mathematical model of the DFIG For a doubly fed induction machine, the Concordia and Park transformation's application to the traditional a,b,c model allows to write a dynamic model in a d-q reference frame as follows: 3. DFIG control In DFIG there are three control loops which should be implemented in order to appropriate performance of wind farm. These control loops are pitch angle control, rotor speed control and voltage control. These control schemes are thoroughly explained in Figures 2-4. Figure 2: Rotor speed control scheme 23
Figure 3: Voltage control scheme Figure 4: Pitch angle control scheme 4. Illustrative test case In order to evaluate the effect of WTG on the system performance, a multi machine electric power system which is IEEE 4 bus test system is considered as case study [7]. In the proposed test system two following cases are considered: Case : power generator at bus is a thermal power plant Case 2: power generator at bus is a wind turbine power plant Figure 5: IEEE 4-bus test system 24
Both the cases are simulated and compared under disturbance. Figure 5 shows the proposed test system with a WTG installed in bus. Also in this paper the wind speed is modeled as Weibull Distribution [7]. The simulation results are carried out on the proposed test system with both the cases. Figures 6-0 show the results following a 2 cycle three phase short circuit is assumed at bus 0. It is clearly seen that WTG has a great effect on the system stability and increasing damping of oscillations. With WTG, in all figures the oscillations are damped out successfully and the transient and dynamic stability margin of the system in increased. Also, voltage of bus 3 is demonstrated in figure 0. It is clearly seen that WTG not only has a great effect on system stability but also has a positive effect on the voltage of bus..0003.0002.000 Speed G (pu) 0.9999 0.9998 0.9997 0.9996 0.9995 0.9994 0.9993 0 2 3 4 5 6 7 8 9 0 Figure 6: Speed of generator Solid (wind power plant at bus ); Dashed (thermal power plant at bus ) 25
.0004.0002 0.9998 Speed G2 (pu) 0.9996 0.9994 0.9992 0.999 0.9988 0.9986 0.9984 0 2 3 4 5 6 7 8 9 0 Figure 7: Speed of generator 2 Solid (wind power plant at bus ); Dashed (thermal power plant at bus ).002.005.00 Speed G3 (pu).0005 0.9995 0.999 0.9985 0 2 3 4 5 6 7 8 9 0 Figure 8: Speed of generator 3 Solid (wind power plant at bus ); Dashed (thermal power plant at bus ) 26
.0025.002.005.00 Speed G4 (pu).0005 0.9995 0.999 0.9985 0.998 0 2 3 4 5 6 7 8 9 0 Figure 9: Speed of generator 4 Solid (wind power plant at bus ); Dashed (thermal power plant at bus ).04.03.02 Bus voltage (pu).0 0.99 0.98 0.97 0 2 3 4 5 6 7 8 9 0 Figure 0: voltage of bus 3 Solid (wind power plant at bus ); Dashed (thermal power plant at bus ) 5. Conclusion A unified tuning of DFIG controllers successfully carried out in this paper. The proposed DFIG was compared with conventional generators in thermal power plants. It showed that DFIG has a great effect on the system stability and performance. DFIG also successfully controlled the voltage. The paper showed that utilization of DFIG not only is suitable from 27
view of energy and environmental effects, but also is appropriate from view of system stability and performance. References [] A.G. Abo-Khalil, Synchronization of DFIG output voltage to utility grid in wind power system, Renewable Energy, 44 (202) 93-98. [2] V. Akhmatov, System stability of large wind power networks: A Danish study case, International Journal of Electrical Power & Energy Systems, 28 (2006) 48-57. [3] M. Alonso, H. Amaris, C. Alvarez-Ortega, A multiobjective approach for reactive power planning in networks with wind power generation, Renewable Energy, 37 (202) 80-9. [4] A.H.M.A. Rahim, I.O. Habiballah, DFIG rotor voltage control for system dynamic performance enhancement, Electric Power Systems Research, 8 (20) 503-509. [5] M. Rahimi, M. Parniani, Dynamic behavior analysis of doubly-fed induction generator wind turbines The influence of rotor and speed controller parameters, International Journal of Electrical Power & Energy Systems, 32 (200) 464-477. [6] M. Verij Kazemi, M. Moradi, R. Verij Kazemi, Minimization of powers ripple of direct power controlled DFIG by fuzzy controller and improved discrete space vector modulation, Electric Power Systems Research, 89 (202) 23-30. [7] F. Milano, An open source power system analysis toolbox, Power Systems, IEEE Transactions on, 20 (2005) 99-206. 28