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ISSN 1859 1531 TẠP CHÍ KHOA HỌC VÀ CÔNG NGHỆ ĐẠI HỌC ĐÀ NẴNG, SỐ 11(96) 2015, QUYỂN 2 219 USING A STATCOM TO ENHANCE STABILITY OF A GRID CONNECTED WIND POWER SYSTEM SỬ DỤNG STATCOM ĐỂ NÂNG CAO ỔN ĐỊNH[.] SỬ DỤNG STATCOM ĐỂ NÂNG CAO ỔN ĐỊNH TRONG HỆ THỐNG ĐIỆN GIÓ KẾT HỢP VỚI LƯỚI ĐIỆN
ISSN 1859-1531 - TẠP CHÍ KHOA HỌC VÀ CƠNG NGHỆ ĐẠI HỌC ĐÀ NẴNG, SỐ 11(96).2015, QUYỂN 219 USING A STATCOM TO ENHANCE STABILITY OF A GRID CONNECTED WIND POWER SYSTEM SỬ DỤNG STATCOM ĐỂ NÂNG CAO ỔN ĐỊNH TRONG HỆ THỐNG ĐIỆN GIÓ KẾT HỢP VỚI LƯỚI ĐIỆN Nguyen Huu Vinh1, Nguyen Hung2, Le Kim Hung3 Hochiminh City Power Company, Vietnam; nguyenhuuvinhdlhcm@gmail.com Hochiminh City University of Technology, Vietnam; n.hung@hutech.edu.vn Danang University of Technology, Vietnam; lekimhung@dut.udn.vn Abstract - This paper presents the simulation results of using a static synchronous compensator (STATCOM) to improve damping of a grid connected wind power system (WPS) The operating performance of the studied WPS is simulated by an equivalent doubly fed induction generator (DFIG) driven by an equivalent wind turbine (WT) PID Damping controller of the proposed STATCOM is designed to reduce the fluctuation of the system under various operating conditions A time-domain scheme based on a nonlinear system model subject to a three-phase short-circuit fault at the power grid is utilized to examine the effectiveness of the proposed control schemes It can be concluded from the comparative simulation results that the proposed STATCOM joined with the designed controller is shown to be better for improving the stability of the system subject to a severe disturbance Tóm tắt - Bài báo trình bày kết mơ việc sử dụng thiết bị bù đồng tĩnh (STATCOM) để nâng cao ổn định cho hệ thống điện gió (WPS) kết nối với lưới điện Quá trình hoạt động hệ thống điện gió mơ tả mạch tương đương máy phát điện nguồn đôi (DFIG) truyền động tuabine gió (WT) Bộ điều khiển giảm dao động PID thiết kế cho STATCOM để giảm dao động cho hệ thống điều kiện làm việc khác Kết mô miền thời gian dựa vào mơ hình hệ thống phi tuyến có cố ngắn mạch pha tạo để kiểm tra tính hiệu điều khiển đề xuất Có thể kết luận dựa vào kết mô STATCOM kết hợp với điều khiển PID cho kết tốt việc nâng cao độ ổn định hệ thống có cố nghiêm trọng xãy Key words - wind power system (WPS), static synchronous compensator (STATCOM), stability improvement Từ khóa - hệ thống điện gió (WPS), thiết bị bù đồng tĩnh (STATCOM), nâng cao độ ổn định Introduction Wind energy is rapidly increasing all over the world According to The U.S Department of Energy, wind energy will make up 20% of U.S electrical capacity by 2030 [1] Relating to wind generators, doubly fed induction generator (DFIG) is the most employed generator due to its many merits such as high efficiency compared to direct drive wind power system with a full-scale power converter, capability of decoupled control active and reactive power for better grid integration [2] However, by connecting stator windings directly to the power grid, it is extremely sensitive to grid faults To reduce the influence of the power grid on DFIG-based wind farm, many papers have been published Particularly, a variable frequency transformer (VFT) has been used to smooth the fluctuating active power of the OWF injected into the power grid and improve the damping of the OWF [3] Nowadays, many flexible AC transmission systems (FACTS) devices have been proposed and implemented in power systems For instance, a static synchronous compensator (STATCOM) has been used to enhance the power stability using fuzzy controller in the same system [4] The applications of STATCOM to power-system stability improvement, using STATCOMs and the damping controller design of STATCOMs were presented in [5] A variable-blade pitch of a WT and design of an output feedback linear quadratic controller for a STATCOM to perform mechanical power control and voltage control under different operating conditions were studied in [6] Controller design and system modeling for quick load voltage regulation and suppression of voltage flicker using a STATCOM were explored in [7] The application of a STATCOM to damp torsional oscillations of a seriescapacitor compensated AC system was shown in [8] System Configuration Figure shows the configuration of the studied system containing a DFIG-based WPS with a 5-MVAR STATCOM The 20-MW WPS is represented by a large equivalent DFIG driven by an equivalent aggregated variable-speed WT The WPS and the STATCOM are connected to a point of common coupling (PCC) The PCC is connected to a power grid through a transmission line [3] The employed mathematical models of the studied system are described below 220 Nguyen Huu Vinh, Nguyen Hung, Le Kim Hung Figure Configuration of the studied system 2.1 Wind Turbine and DFIG Model The captured mechanical power by a WT can be written by Pw Ar Vw3 C p ( , ) (1) where is the air density (kg/m3), Ar is the blade impact area (m2), Vw is the wind speed (m/s), and Cp is the dimensionless power coefficient of the WT The power coefficient of the WT Cp is given by c c C p ( k , ) c1 ( c3 β c4 c5 c6 ) exp (2) k k c 1 39 in which (3) k c8 R b b (4) Vw where Rm is the pu equivalent resistance considering the equivalent electrical losses of the STATCOM, and iqs and ids are the pu q- and d-axis currents flowing into the terminals of the STATCOM, respectively The fundamental control block diagram of the employed STATCOM including a PID damping controller is shown in Figure The pu DC voltage Vdcs is controlled by the phase angle while the voltage vs is varied by changing the modulation index km where b is the blade angular speed (rad/s), Rb is the blade radius (m), λ is the tip speed ratio, is blade pitch angle (degrees), and c1-c9 are the constant coefficients for power coefficient Cp [3] The mathematical model of DFIG-based WT is presented in d-q axis and selected as a reference frame More detail can be referred to the model that is mentioned in [2] 2.2 STATCOM Model The pu q- and d-axis output voltages of the proposed STATCOM shown in Figure can be written by, respectively [5, 7] Y = CX + DU (10) where X is the state vector, Y is the output vector, U is the external or compensated input vector while A, B, C, and D are all constant matrices of appropriate dimensions The wind speed Vw = 12 m/s is properly selected as the nominal operating points for designing the PID damping controller The eigenvalues of the studied system without and with the STATCOM are listed in Table The following points can be found by examining the system eigenvalues: - All modes of the system are almost fixed on the complex plane without change regardless of the addition of the STATCOM - Two modes 16 and 17 are related to mechanical mode of WPS and they can be improved since the damping ratio is only 0.63113 The control block diagram of the STATCOM including the PID damping controller was shown in Figure In this paper the parametters of the PID controller were found by try and error method (5) vds = Vdcskmsin(pcc + ) (6) where vqs and vds are the pu q- and d-axis voltages at the output terminals of the STATCOM, respectively; km and are the modulation index and phase angle of the STATCOM, respectively; pcc is the voltage phase angle of the PCC, and Vdcs is the pu DC voltage of the DC capacitor Cm max Table Eigenvalues of the studied system Eigenvalues 0 Wind km0 Figure Control scheme of STATCOM The pu DC voltage-current equation of the DC capacitor Cm can be described by (Cm)( ̇ dcs) = b[Idcs (Vdcs/Rm)] (7) in which the DC current can be calculated as Idcs = iqskmcos(θpcc+ ) + idskmsin(θpcc+ ) (8) Subsystem vqs = Vdcskmcos(pcc + ) Frequency Domain Analysis The nonlinear system equations developed in the previous section are linearized around a selected nominal operating point to acquire a set of linearized system equations in matrix form as follows [3] ̇ = AX + BU (9) System without STATCOM System with STATCOM 1,2 3,4 5,6 -4.7032 ± j596,660 -4.504 ± j 697,260 -233.4 ± j 10,415 -4.504 ± j 697,260 -4.7032 ± j 596,660 -232.42 ± j 10,435 7,8 9,10 11 12 13 14 15 16,17 18 19 -218.51 ± j 5,540.2 -11.494 ± j 80.134 -102.57 -97.455 -100.64 -99.525 -16.012 -4.4316 ± j 5.6121 -8.0408 -0.40485 -225.2 ± j 5,484.2 -11.493 ± j 80.132 -100.12 -139.02 -100.64 -99.531 -14.522 -4.427 ± j 5.4409 -7.8805 -0.43053 WPS ISSN 1859-1531 - TẠP CHÍ KHOA HỌC VÀ CÔNG NGHỆ ĐẠI HỌC ĐÀ NẴNG, SỐ 11(96).2015, QUYỂN 20,21 Network 22,23 -61.647 ± j 976.01 -0.31783 ± j 374.95 -68.689 ± j 862.2 -0.32338 ± j 374.95 24,25 STATCOM 26,27 - -23.604 ± j 2423.1 -101.12 ± j 19.039 Time Domain Analysis This section uses the nonlinear system model developed in Section to compare the damping characteristics created by the proposed STATCOM joined with the designed PID damping controller on stability improvement of the studied system under a three-phase short-circuit fault at the power grid It is assumed that the WPS operates under a base wind speed of 12 m/s 1.2 PWind (p.u.) 0.8 0.6 0.4 0.2 t (s) 10 12 14 16 12 14 16 (a) Active power of WPS 0.3 QWind (p.u.) 0.2 0.1 -0.1 -0.2 t (s) 10 1.06 Wind (p.u.) 1.04 1.02 0.98 0.96 t (s) 10 12 14 16 (c) Rotor speed of WPS 1.2 VPCC (p.u.) 0.8 0.6 0.4 0.2 t (s) 10 of the studied system with and without the proposed STATCOM joined with the designed damping controller subject to a three-phase short-circuit fault at the power grid The three-phase short-circuit fault is suddenly applied to the grid at t = s and is cleared after cycles It is clearly observed from the comparative transient simulation results shown in Figure that proposed STATCOM with the designed PID damping controller can offer better damping to the studied system in which the active power, reactive power, rotor speed and voltage at PCC of aggregated WPS are shown It can be observed from this figure that when STATCOM is out of service the oscillations are damped out around 10s and when STATCOM joins with the designed PID controller the damping of the system is increased, so the system is stable in turn at 5s It shows that the proposed STATCOM with the designed PID damping controller can supply proper reactive power to the system and offer better damping characteristics to quickly damp out the inherent oscillations of the studied system than the studied system without the STATCOM Conclusion This paper has presented the stability improvement of an integrated WPS to power grid The STATCOM is proposed and is connected to the PCC of the integrated system To supply adequate reactive power to the system, a PID damping controller for the STATCOM has been designed It can be concluded from the simulation results that the proposed STATCOM joined with the designed damping controller has the best damping characteristics to improve the performance of the system under different operating conditions REFERENCES (b) Reactive power of WPS 1.08 0.94 221 12 14 16 (d) Voltage at PCC Figure Comparative transient responses of the studied system under a three-phase short circuit fault at infinite bus Simulation results of the proposed system with MATLAB/SIMULINK toolbox have been presented in Figure This figure plots the comparative transient responses [1] E Muljadi, T B Nguyen, and M A Pai, “Impact of wind power plants on voltage and transient stability of power systems,” in Proc IEEE Energy 2030 Conf., pp 1-7, 2008 [2] R Pena, J C Clare, and G M Asher, “Doubly fed induction generator using back-to-back PWM converters and its application to variable speed wind-energy generation,” in Proc Inst Elect Eng., Elect Power Appl., vol 143, no 3, pp 231-241, May 1996 [3] L Wang and L.-Y Chen, "Reduction of power fluctuations of a large-scale grid-connected offshore wind farm using a variable frequency transformer." IEEE Trans on Sustainable Energy, vol 2, no 3, pp 226-234, Apr 2011 [4] L O Mak, Y X Ni, and C M Shen, “STATCOM with fuzzy controllers for interconnected power systems,” Electric Power Systems Research, vol 55, no 2, pp 87-95, Aug 2000 [5] H Chong, A Q Huang, M E Baran, S Bhattacharya, W Litzenberger, L Anderson, A L Johnson, and A A Edris, “STATCOM impact study on the integration of a large wind farm into a weak loop power system,” IEEE Trans Energy Conversion, vol 23, no 1, pp 226-233, 2008 [6] H Gaztanaga, I Etxeberria-Otadui, D Ocnasu, and S Bacha, “Realtime analysis of the transient response improvement of fixed-speed wind farms by using a reduced-scale STATCOM prototype,” IEEE Trans Power Systems, vol 22, no 2, pp 658-666, 2007 [7] A Jain, K Joshi, A Behal, and N Mohan, “Voltage regulation with STATCOMs: Modeling, control and results,” IEEE Trans Power Delivery, vol 21, no 2, pp 726-735, 2006 [8] K V Patil, J Senthil, J Jiang, and R M Mathur, “Application of STATCOM for damping torsinal oscillations in series compensated AC system,” IEEE Trans Energy Conversion, vol 13, no 3, pp 237-243, 1998 222 Nguyen Huu Vinh, Nguyen Hung, Le Kim Hung (The Board of Editors received the paper on 08/15/2015, its review was completed on 10/01/2015) ... losses of the STATCOM, and iqs and ids are the pu q- and d-axis currents flowing into the terminals of the STATCOM, respectively The fundamental control block diagram of the employed STATCOM including... detail can be referred to the model that is mentioned in [2] 2.2 STATCOM Model The pu q- and d-axis output voltages of the proposed STATCOM shown in Figure can be written by, respectively [5, 7]... addition of the STATCOM - Two modes 16 and 17 are related to mechanical mode of WPS and they can be improved since the damping ratio is only 0.63113 The control block diagram of the STATCOM including