Đang tải... (xem toàn văn)
IMPROVING STABILITY FOR INDEPENDENT POWER CONTROL OF DFIG WITH SFOC AND DPC DURING GRID UNBALANCE
20 Nguyen Thanh Hai, Vo Viet Cuong IMPROVING STABILITY FOR INDEPENDENT POWER CONTROL OF DFIG WITH SFOC AND DPC DURING GRID UNBALANCE Nguyen Thanh Hai2, Vo Viet Cuong1 HCMC University of Technology and Education, Vietnam; hai_nguyenthanh2012@yahoo.com.vn Le Hong Phong High School for The Gifted, Ho Chi Minh City, Vietnam; hoc_vien@yahoo.com.vn Abstract - This paper presents modified Stator Fed Oriented Control (SFOC)for Doubly Fed Induction Generator (DFIG) in wind turbines during grid unbalance,and improves stability by using Notch filter to eliminate second order harmonic components.The proposed schemes apply multiple PI controllers with Fuzzy and anti-windup (PI-F) to obtain commanded rotor currents and also introduce extra commanded values for rotor currents Comparison of the proposed controller with Direct Power Control (DPC) using Notch filters for improvement during grid voltage unbalance is also included The modifications are applied to rotor side converter (RSC) for active and reactive power controls of wind turbine The turbine, generator and control units are also described on MATLAB/SIMULINK Simulation results show improved stability of active and reactive powers stator Cp (): the aerodynamic efficiency which depends on the tip speed ratio λ, and blade pitch angle β According to Betz’s efficiency, the maximum theoretical efficiency is 59.3% [10] The tip speed ratio λ is defined as the speed at which the outer tip of the blade is moving divided by the wind speed = turb R (2) vw Key words - DFIG; Unbalanced Voltage Dip; PI controller; Anti-windup; SFOC; Notch Filters; Fuzzy Introduction Doubly fed induction generators have been the popular choice in wind power generation due to the low rating of power electronic circuit connected to the rotor side of the generator and the grid [1] The active and reactive powers delivered by DFIG can be controlled independently by Stator Flux oriented Control and Direct Power Control which are designed for operation with balanced grid voltage [2] However, most of the grids experience the problems of voltage unbalance, which raises the winding temperature and causes pulsation of torque and power [3] This paper will investigate the stabilities of active and reactive powers during transient unbalance of grid voltage for traditional and modified stator flux oriented control and direct power control of DFIG The modifications are hybrid PI-Fuzzy controller and Sequence Component controller The grid unbalance is modeled with a reduction of 25% of voltage in one phase Wind speed varies randomly during the process Mathematical Model Of Wind Turbine The model of wind turbine and its formula of shaft torque, turbine torque, power transferred to generator and related parameters are presented in this session Figure 1illustrates the mechanical system of wind turbine which is often used in large wind turbine systems The power extracted from the wind is: Pturb = Avw3 C p ( , ) Figure Model of the mechanical part of Wind Turbine [9] Direct Power Control and Stator Flux Oriented Control Of DFIG Structure of control method with DPC for DFIG is shown in Figure and The proposed control structure with SFOC is shown in Figure Appropriate voltage vectors for rotor side converter are selected to control generated active and reactive power in DPC Converters on rotor side of DFIG are controlled by stator flux oriented control to achieve the independent control of active and reactive powers Modification of the control system by using hybrid PI-Fuzzy controller has provided better performance of the generated powers [5] However, this is only verified with balanced grid voltage To improve stability of the powers during voltage unbalance, inclusion of Notch filter has been suggested by [6, 11] and presented in Figure and to eliminate second order harmonic components VDC * Vrabc αβ ωr Isabc abc αβ Vsαβ abc Where:ρ = 31.22 (kg/m3) air density Vsabc Encoder Isαβ abc * Vrαβ DFIG Vsabc (1) A=R2 (m2) the cross-sectional area through which the wind passes R(m): length of turbine’s blades vw (m/s):the wind speed normal to the cross-session area A PWM dt Ps , Qs Ps Qs Vsαβ Calculation -jθs e θr Vsdq αβ PLL θs d/dt ωs j(θs-θr) e * Vrdq Required Rotor voltage calculation Psref Qsref Balanced control scheme Figure DPC for grid-connected DFIG-based wind generator without Notch filter [11] ISSN 1859-1531 - THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO 12(97).2015, VOL Qs = Figure The typical configuration of a grid-connected DFIGDPC with Notch Filter 3 Lm Vs (vqs ids − vds iqs ) = Vs − iqr 2 Ls s Lm 21 (10) PI-Fuzzy controllers as shown in Figure are used to control the errors between the required and actual values of both the active power and reactive power delivered to the grid by the generator The parameters of the PI-Fuzzy are adjusted by the fuzzy rules to obtain the best output to drive the errors to zero The variable parameters of the controllers, which are fixed in traditional PI controllers, will help to achieve the best performance of the system The outputs of these controllers are commanded values of d-q components of rotor current in the stator flux oriented reference frame These commanded values of currents are used to regulate the RSC for provision of the rotor phase voltage to DFIG The fuzzy rules for parameters of PI-FUZZY controllers are presented in Table and Table The rules are developed by trial and error method LN, SN, ZE, SP, and LP represents large negative, small negative, zero, small positive and large positive respectively S, M, H stand for small, medium and high respectively Table rule base of Kp [5] Kp e LN SN ZE SP LP LN H H M M M de/dt SN ZE SP H H H M M M S S S M M M H H H LP H H M H H Table rule base of Ti [5] Ti e LN SN ZE SP LP LN H H H H H SN H M M M H de/dt ZE SP H H M M S M M M H H LP H H H H H Figure The proposed control scheme for the RSC of a DFIG using PI+F controller and Notch filters In both control scheme in Figure 3, Notch filters are used to eliminate second order harmonic components in positive and negative sequences of stator voltage In Figure 4, Notch filters are used with positive sequences of stator voltage and rotor current Figure shows the spatial relationships between the stationary (α,β)s reference frame, the rotor (α,β)r reference frame rotating at the speed of ωr, and the dq+ and dq− reference frames rotating at the angular speed of ωs and −ωs, respectively As shown, the d+-axis of the dq+ reference frame is fixed to the positive sequence stator voltage V+sd+ According to Figure 5, the transformations between (α,β)s, (α,β)r and dq+ and dq− reference frames are given by − j slip t + − = I ( )r e (3) I dq = I (−dq )−r e− j 2 slip t r j slip t − − = I ( )r e I dq = I (+dq )−r e j 2 slip t r (4) Figure PI-Fuzzy controller I (t ) = I + (t ) + I − (t ) (5) The triangular membership functions of inputs and outputs of PI-Fuzzy controller are shown in Figure 7, 8: d dq+ s + j s dq+ s dt I d+ = I d+ + + I d+ − = I d+ + + I d− − e − j 2 t [6; 7; 8] I q+ = I q+ + + I q+ − = I q+ + + I q− − e− j 2 t [6; 7; 8] Active and reactive power of stator: 3 Lm Ps = (vds ids + vqs iqs ) = − Vs iqr 2 Ls Vdq+ s = Rs I dq+ s + slip r r r r r slip r Figure Relationships between (α,β)s, (α,β)rand dq+ and dq− reference frames [6] r r r r (6) (7) (8) (9) Figure Membership functions of two inputs of fuzzy bloc 22 Nguyen Thanh Hai, Vo Viet Cuong Inertia Inertia of Rotor Figure Membership functions of two outputs of fuzzy bloc Simulation and Results Simulation implementation of proposed control method for 2.3 MW DFIG is carried out, Table The grid voltage unbalance happens after 35 seconds, the commanded values of reactive power and active power change at 50s and 60s respectively Comparisons of average values of active and reactive powers in steady state with different controllers are presented in Table and Both actual values and percentage of references are shown Average electromagnetic torque of the generator is shown in Table 6.The randomly variable wind speed is shown in Figure and Figure 10 is grid unbalance at 35s Figure Random variation of wind speed 800 600 Value 159.2 (μH) 159.2 (μH) 5.096 (mH) (mΩ) (mΩ) 100π (rad/s) 400 Vabcs [V] Symbol LS Lr Lm RS Rr p ωS IWTR 93.22 (kg.m2) 17.106(kg.m2) The simulation results with different controllers are shown in figures 11 to 16 for active and reactive output power respectively These figures demonstrate the power responses when voltage unbalance happens and when the commanded values of powers change under voltage unbalance Torque response of the generator is shown in Figure 17 Table Parameters of DFIG 2.3MW Parameter Stator inductance Rotor inductance Magnetic inductance Stator resistance Rotor resistance Number of pole pairs Frequencyof the electric system Igen 200 -200 -400 -600 -800 29.95 29.96 29.97 29.98 29.99 30 30.01 Time [s] 30.02 30.03 30.04 30.05 Figure 10 The grid voltage unbalance happens after 35 seconds Table Average value of Ps[MW]in steady state for controllers Grid Voltage Psef =2 DPC WITHOUT NOTCH FILTER Mean Max Min Balanced (11-19s) 2.002 0.1% 2.086 4.3% Unbanced (31-49s) 2.001 0.05% 2.113 5.7% Deviation (%) = DPC WITH NOTCH FILTER SFOC WITH PI-F & NOTCH FILTER Mean Max Min Mean Max Min 1.920 -4% 2.002 0.1% 2.08 4.2% 1.919 -4% 2.001 0.1% 2.138 6.9% 1.908 -4.6% 1.904 -4.8% 2.001 0% 2.1 5% 1.915 -4.2% 2.02 1% 2.225 11.3% 1.867 -6.7% P − Psref (%) Psref During the unbalanced voltage, best performances of active power are observed for DPC with Notch Filter, then the traditional DPC without Filter In detail, the lowest value of PMax for DPC with Notch filters is 5.0% of the commanded The highest value of PMin for DPC with Notch Filter is -4.2% of the commanded value Table Average value of Qs [MVAR] in steady state for controller Grid Voltage Qsref =1 Balanced (11-19s) Unbanced (31-49s) DPC WITHOUT NOTCH FILTER Mean Max Min 1.007 1.073 0.928 0.1% 7.3% -7.2% 1.051 1.09 0.879 5.1% 9% -12.1% Deviation (%) = DPC WITH NOTCH FILTER Mean 1.00 0% 1.00 0% Max 1.073 7.3% 1.057 5.7% Min 0.928 -7.2% 0.891 -10.9% SFOC WITH PI-F & NOTCH FILTER Mean Max Min 1.00 1.114 0.889 0% 11.4% -11.1% 0.997 1.117 0.881 0.3% 11.7% -11.9% Q − Qsref (%) Qsref During the unbalanced voltage, best performances of active power are observed for DPC with Notch Filter, then the traditional DPC without Filter In detail, the lowest value of QMax for DPC with Notch filters is 5.7% of the commanded The highest value of QMin for DPC with Notch Filter is -10.9% of the commanded value ISSN 1859-1531 - THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO 12(97).2015, VOL 23 Table Average value of generator’s torque in steady state for the controllers Discussion DPC has shown good steady state of active power responses during the voltage balance and unbalance as shown in Table The deviation of the mean value of active power from the reference value is almost zero percent with the inclusion of Notch filter SFOC also gives good performance with small deviation (about 1%) The fluctuation of active power is smallest for DPC with Notch filter during the unbalance Steady state responses of reactive power are also very good when Notch filters are included The deviations are 0% and 0.3% respectively for DPC and SFOC The deviation is much higher without Notch filter during the voltage unbalance as shown in Table There is no significant difference observed between the responses during the voltage balance, with or without Notch filters The fluctuation is observed to be smallest for DPC with Notch filter SFOC however gives smallest torque variation during voltage unbalance as shown in Table The results obtained in Table are further demonstrated in Figure 11 SFOC’s active power response when voltage unbalance happens has higher ripples while the responses obtained with the two DPC schemes are not significantly distorted The responses to change in the commanded values during the unbalance are good for the three control scheme as shown in Figure 12 DPC schemes give faster responses as shown in Figure 13 DPC WITH NOTCH FILTER DPC WITHOUT NOTCH FILTER 2.3 2.2 2.2 2.2 2 Ps [MW] 2.3 Min 12065 8500 1.8 20 40 Time [s] 30 1.8 20 40 Time [s] 2 1.7 1.7 1.7 1.4 1.4 1.4 1.1 1.1 1.1 0.8 49.5 0.8 49.5 50 50.5 30 40 Time [s] Ps [MW] DPC WITHOUT NOTCH FILTER SFOC WIT H PI+F& NOT CH FILT ER 2.3 2.3 2.3 2 1.7 1.7 1.7 1.4 1.4 1.4 1.1 1.1 0.8 20 40 Time [s] 60 0.8 20 Time [s] 60 0.8 20 50.5 0.8 49.5 50 50.5 Time [s] DPC WITH NOTCH FILTER DPC WITHOUT NOTCH FILTER SFOC WITH PI+F&NOTCH FILTER 1.2 1.2 1.2 1.1 1.1 1.1 1 0.9 0.9 0.9 0.8 20 0.8 20 30 40 Time [s] 0.8 20 30 40 Time [s] 30 40 Time [s] Figure 14 Reactive power of DFIG when voltage unbalances happen DPC WITH NOTCH FILTER DPC WITHOUT NOTCH FILTER SFOC WITH PI+F&NOTCH FILTER 2.2 2.2 2.2 1.9 1.9 1.9 1.6 1.6 1.6 1.3 1.3 1.2 1 20 40 Time [s] 60 0.7 20 40 Time [s] 60 07 20 40 60 Time [s] Figure 15 Reactive power of DFIG during transient states DPC WITH NOTCH FILTER DPC WITHOUT NOTCH FILTER SFOC WITH PI+F&NOTCH FILTER 2.2 2.2 2.2 1.9 1.9 1.9 1.6 1.6 1.6 1.3 1.3 1.2 1 49.5 50 Time [s] 50.5 0.7 49.5 50 Time [s] 50.5 07 49.5 50 50.5 Time [s] DPC WITHOUT NOTCH FILTER FOC WITH PI+F& NOTCH FILTER 20 20 20 18 18 18 15 15 15 12 12 12 9 6 3 20 20 40 60 Time [s] 80 40 60 Time [s] 80 20 40 60 Time [s] 80 Figure 17 Torque of DFIG 1.1 40 50 Time [s] Figure 16 Dynamic responses of DFIG’s reactive power during the change of commanded value Figure 11 Active output power of DFIG when voltage unbalances happen DPC WITH NOTCH FILTER SFOC WITH PI+F& NOTCH FILTER 2.3 Figure 13 Dynamic responses of DFIG’s active output power during the change of commanded value Te [ KN.m] 30 DPC WITHOUT NOTCH FILTER 2.3 DPC WITH NOTCH FILTER 1.8 20 SFOC WITH PI-F & NOTCH FILTER Mean Max Min 12603 13982 11342 12522 15899 10195 2.3 Time [s] SFOC WITH PI+F& NOTCH FILTER 2.3 Max 13158 17046 DPC WITH NOTCH FILTER Ps [MW] During the unbalanced voltage, best performances of active power are observed for DPC with Notch Filter, then the traditional DPC without Filter In detail, the lowest value of TMax for DPC with Notch filters is 15899 (N.m) of the commanded The highest value of TMin for DPC with Notch Filter is 10195 (N.m) of the commanded value Mean 12606 12571 Qs [MVAR] Balanced (11-19s) Unbanced (31-49s) DPC WITH NOTCH FILTER Qs [MVAR] DPC WITHOUT NOTCH FILTER Mean Max Min 12605 13168 12056 12568 17327 8273.7 Qs [MVAR] Grid Voltage 40 60 Time [s] Figure 12 Active output power of DFIG during the transient states Higher ripples are also observed in reactive power responses of SFOC when voltage unbalance occurs as shown in Figure 14 The observation is consistent with statistics presented in Table Reactive powers in the three control scheme follow the commanded values under the 24 Nguyen Thanh Hai, Vo Viet Cuong condition of voltage unbalance as shown in Figure 15 Time responses of reactive power in DPC control schemes are also less than those of SFOC as shown in Figure 16 Torque responses observed in Figure 17 are also consistent with the statistics shown in Table 6 Conclusion The proposed SFOC scheme for DFIG with the inclusion of PI-Fuzzy controllers and Notch filters has improved the stability of independent control of active and reactive power during grid voltage unbalance The responses of active and reactive power are compared with a traditional DPC and modified DPC using Notch filters to increase the stability The observations are made during the occurrence of voltage dip in one phase, transient states as well steady states of the powers under unbalanced condition In all the observations, the independent control of the powers is maintained for the proposed scheme However, high fluctuations in active and reactive powers are present in the responses obtained with the proposed scheme although lower ripples are observed for generator’s torque Experimental verification of the new control scheme should be carried out to validate the results obtained with simulation REFERENCES [1] Ackermann, T., Wind power in power systems, John Wiley and Sons, USA, 2003 [2] Leonhard, W., Control of electric drives, Springer-Verlag, 3rd [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] edition, USA, 2001 Muljadi, E., Yildirim, D., Batan, T., and Butterfield, C.P., “Understand the unbalanced-voltage problem in wind turbine generation”, Proceeding of IEEE Industry Application Conference, Phoenix, USA, 1999, pp.1359-1365 Baggu, M M.; “Advanced control techniques for doubly fed induction generator – based wind turbine converters to improve low voltage ride- throught during system imbalances”, PhD Thesis, Missouri University of Science and Technology, 2009 Pham-Dinh, T., Pham-Trung, H., Le-Thanh, H., “PI-Fuzzy Controller for Doubly Fed Induction Generator Wind Turbine”, Proceedings of ASEAN Symposium on Automatic Control ASAC 2011, Vietnam, 2011, pp.79 – 81 Phan, V T., Lee, H H., Chun, T W.; “An Effective rotor current controller for unbalanced stand – alone DFIG systems in the rotor reference frame”, Journal of Power electrionics, Vol.10, No.6, 2010, pp 194-202 L Xu, Y Wang, “Dynamic modeling and control of DFIG based wind turbines under unbalanced network conditions”, IEEE Trans Power Syst 22 (1) (2007) 314–323 A Peterson, L Harnefors, T Thiringer, “Comparison between stator-flux and grid flux oriented rotor current control of doubly-fed induction generators”, The 35th Annual IEEE Power Electronics Specialist Conference, vol 1, 20–25 June,2004, pp 482–486 Sorensen, P.; Hansen, D.A.; Christensen, P.; Mieritz, M.; Bech, J.; Bak-Jensen, B.; Nielsen, H.; “Simulation and Verification of Transient Events in Large Wind Power Installation”, Project Report, Risø National Laboratory, Roskilde, Norway; 2003 Masters, M G Renewable and Efficient Electric Power Systems, John Wiley and Sons, Inc., Publication; 2004 Jia-bing HU, Yi-kang HE; “Modeling and enhanced control of DFIG under unbalanced grid voltage conditions”, Electric Power Systems Research 79(2009); pp 273-281 Hai Nguyen-Thanh; “Improved Control of DFIG Systems under Unbalanced Voltage Dip for Torque Stability Using PI-Fuzzy Controller”; International Journal of Electrical Energy, Vol 2, No 4, December 2014; pp 300-307, USA (The Board of Editors received the paper on 15/05/2015, its review was completed on 05/07/2015) ... proposed SFOC scheme for DFIG with the inclusion of PI-Fuzzy controllers and Notch filters has improved the stability of independent control of active and reactive power during grid voltage unbalance. .. Dynamic responses of DFIG? ??s reactive power during the change of commanded value Figure 11 Active output power of DFIG when voltage unbalances happen DPC WITH NOTCH FILTER SFOC WITH PI+F& NOTCH... Psref During the unbalanced voltage, best performances of active power are observed for DPC with Notch Filter, then the traditional DPC without Filter In detail, the lowest value of PMax for DPC with