Comparison of control schemes of STATIC Compensator for Grid Connected Wind System

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1 Vol. 1, Issue. 4, Dec. 214, ISSN omparison of control schemes of STATI ompensator for Grid onnected Wind System Abstract The from wind and solar energy sources varies due to environmental conditions. Due to the fluctuation nature of the wind, the wind injection into an electric grid affects the quality. The influence of the wind sources in the grid system concerns the quality such as the active, reactive, variation of voltage, and harmonics. In this paper quality issues such as active, reactive and harmonics have been considered when wind turbine is installed to grid side. A Static ompensator (STATOM) is connected at a point of common coupling with a battery energy storage system (ESS) to improve the above quality issues. The battery energy storage is used to maintain constant real from variable wind. Here two control schemes such as angang current controller and Instantaneous real and reactive theory have been proposed to STATOM and compared the two schemes for effectiveness of the controllers. The operation of the STATOM with two control schemes for maintaining the quality of the grid connected wind energy system is investigated using MATLA/SIMULINK. Keywords: STATOM, quality, wind generating system, attery Energy Storage System (ESS), ang ang current controller, and Instantaneous real reactive theory I.INTRODUTION Injecting wind into the system grid effects quality problems such as reactive compensation, voltage regulation, hormones produced in the grid. We know that induction generators reactive problems may come due to non liner loads balanced and unbalanced loads some kind of electronic devices such as arc lamps welding machines etc. this all are switching actions harmonics will present in the system so that complete grid effects and also it effects on source side. oth electric utilities and end users of electric are increasingly concerned about the quality of. Power quality can be defined as any problem manifested in voltage, current and frequency those results in failure or mal operation of the customer equipment [1]. Injection of the wind into an electric grid affects the quality [2]. The group of devices used for mitigation of quality problems is known by the name of ustom Power Devices (PDs). The family of compensating devices mainly has the following members: Static Synchronous ompensator (STATOM), Dynamic Voltage Restorer (DVR) and Unified Power Quality onditioner (UPQ). The work analyses the Performance of STATI OMPENSATOR (STATOM) with a battery energy storage system (ESS) connected at the point of common coupling of wind energy generating system and the existing system to mitigate the quality issues [1].During the normal operation, wind turbine produces a continuous variable output. The main quality issues are voltage sag, swell, flickers, harmonics etc [3]. One of the simple methods of running a wind generating system is to use the induction generator connected directly to the grid. The induction generator has inherent advantages of cost effectiveness and robustness. However, induction generators require reactive for magnetization. II. TOPOLOGY FOR POWER QUALITY IMPROVEMENT The STATOM is a three phase voltage source inverter Having the capacitance on its D link and connected at the point of common coupling. The STATOM injects a ompensating current of variable magnitude and frequency component at the bus of common coupling [1].The wind energy system and STATOM with ESS is connected to the grid. The current controlled voltage source inverter based STATOM injects the current into the grid in such a way that the source current (grid current) are harmonic free and they are in phaseangle with respect to source voltage. The injected current will cancel out the reactive part and harmonic part of the induction generator current and load current, thus it improves the quality [4]. A. Wind Energy Generating System. In this configuration, wind energy generation is based on constant speed topologies with pitch control turbine. Available 5

2 Vol. 1, Issue. 4, Dec. 214, ISSN ontrol Scheme: The first control scheme approach is based on Injecting the currents into the grid using bangbang ontroller [1].The controller uses a hysteresis current ontrolled technique as shown in Fig 2.. The current controller block receives reference current and actual current as inputs and are subtracted so as to activate the operation of STATOM in current control mode [5].The second control scheme is Instantaneous real and reactive theory Grid Synchronisation :In the threephase balance system, the RMS source Voltage amplitude is calculated from the source phase Voltages ( V, V, V ) and is expressed as sample template (sampled peak voltage),v Fig (1) System operational scheme in grid system..1. The induction generator is used in the proposed scheme because of its simplicity, it does not require a separate field circuit, it can accept constant and variable loads, and has natural protection against short circuit. The available of wind energy system is presented as: P = ρav (1) Where ρ = air density (kg/m3), A = area swept out by Turbine blade (m ), V = wind speed( m s) It is not possible to extract all kinetic energy of wind. Thus extracts a fraction of the called coefficient p of the wind turbine, and is given by P = P (2) The mechanical produced by wind turbine is given y P = πr c (3) Where, R = Radius of the blade (m). The battery energy storage system (ESS) is used as an energy storage element for the purpose of voltage regulation [1].The ESS will naturally maintain dc capacitor voltage constant and is best suited in STATOM since it rapidly injects or absorbs reactive to stabilize the grid system. System Operation:The shunt connected STATOM with battery energy storage is connected at the interface of the induction generator and nonlinear load at the P [4]. The Fig.1 represents the system working scheme in grid system. The STATOM output is different according to the control strategy, so as to maintain the quality norms in the grid system. The current control strategies for STATOM are the angang controller and Instantaneous real and reactive theory.. V = (V V V ) (4) The inphase unit vectors are obtained from source voltage in each phases and the RMS value of unit vector is shown below. U = V V U = U = (5) The inphase reference currents generated are derived using inphase unit voltage template as shown below. i = I U, i = I U, i = I U (6) Where I is proportional to magnitude of filtered source voltage for respective phases. This ensures that the source current is controlled to be sinusoidal [6].. angang urrent ontroller: It is implemented in the current control scheme. The reference current is generated as in equation (6).The actual current are detected by current sensors and are subtracted for obtaining a current error for a hysteresis based bangbang controller. Thus the ON/OFF switching signals for IGTs of STATOM are derived from hysteresis controller [1].The switching function SA for phase a is expressed as: (i sa i sa ) < H = S A = 1 (7) (i i ) > H = S = This is same for phases b and c. Fig 2 ontrol scheme Available 6

3 Vol. 1, Issue. 4, Dec. 214, ISSN D. INSTANTANEOUS REAL AND REATIVE POWER THEORY (IRP Theory): Instantaneous PQ Theory was initially proposed by Akagi. This theory is based on the transformation of three phase quantities to two phase quantities in αβ frame and the Instantaneous active and reactive is calculated in this frame. Sensed inputs ( V, V, and V ) & i, i, i are fed to the controller and these quantities are processed to generate reference commands (i, i, i ) which are fed to a hysteresis based PWM current controller to generate switching pulses(g 1,g 2 and g 3 ) for STATOM.The system terminal voltages are given as V = V sin(ωt) (8) V = V sin ωt π (9) V = V sin ωt π (1) The respective load current are given as i = I sin{n(ωt) θ } (11) i = I sin n ωt π θ (12) i = I sin n ωt π θ (13) In a, b and c coordinates a, b and c axes are fixed on the same plane apart from each other by 2π/3. Reference urrent control strategy: The control scheme of the STATOM must calculate the current reference signals from each phase of the inverter using instantaneous realreactive compensator. The block diagram as shown in Fig.3 that control scheme generates the reference current required to compensate the source current harmonics and reactive of induction generator. V V = V 2 3 V 3 (14) 2 2 V i 1 i = i 2 3 i 3 (15) 2 2 i Where α and β axes are the orthogonal coordinates. onventional instantaneous for three phase circuit can be defined as P = V I V I (16) Where p is equal to conventional equation p = V I V I V I (17) Similarly, the instantaneous reactive is defined as q = V I V I (18) Therefore in matrix form, instantaneous real and reactive are given as p q = V V V i V iβ (19) The αβ currents can be obtained as v v = v v v v p q (2) Where = v v Instantaneous active and reactive s p and q can be decomposed into an average (dc) and oscillatory component p = p p (21) q = q q (22) Where p and q are the average dc part and p and q are the oscillatory (ac) part of these real and reactive instantaneous. Reference currents are calculated to compensate the instantaneous oscillatory component of the instantaneous active and reactive v i i = v v p (23) q Figure 3: Reference current generator using Instantaneous real and reactive theory. These phases can be transformed into αβ coordinates using larke s transformation as follows. The oscillatory part of real p and reactive q is obtained by using 4th order low pass utterworth filter of cutoff frequency 25 Hz. These currents can be transformed in abc quantities to find reference currents in abc coordinates using reverse larke s transformation. 1 i i i = i i (24) Available 7

4 A a b c T.Suneel, R.Dileep Kumar and G.Subba Reddy / International Journal of New Technologies in Science and Engineering Vol. 1, Issue. 4, Dec. 214, ISSN III.SYSTEM PERFORMANE: The proposed control scheme is simulated using MATLA/SIMULINK in system block set. The simulation parameters used for the system is given Table:1 System parameters S.L No Parameters Ratings 1. Grid voltage 3 phase,415v,5hz 2. Induction Motor/Generat or 3. Line series Inductance 4. Inverter Parameters 3.35 kva,415v,5hz, Speed=15rpm,Rs=2Ω Rr=2Ω,Ls=.6H,Lr=.6H.5mH D link voltage=8v D link capacitance=6μf Switching frequency=2khz 5. Load parameter Nonlinear load 6.5KW 6. Wind turbine ase wind speed=11 m/s A. Voltage Source Inverter Operation The IGT based three phase inverter is connected to grid. The generation of switching signals from reference current is simulated within hysteresis band of.8 for angang current controller [1].. The real transfer from the batteries is also supported by the controller of this inverter. The three phase inverter injected current are shown in fig 5. STATOM Performance Under Load Variations The wind energy generating system is connected to the grid having the nonlinear load. The bangbang current controller and instantaneous real and reactive theory for STATOM is implemented in MATLA/SIMULINK. The main SIMULINK diagram of the control schemes with STATOM is shown in fig 4 Vac Vac1 Vac2 Measuremntes L L1 L2 v V3 Scope2 Scope3 Aa b c VI v V2 Stat Goto1 Repeating S equence co m L5 L4 Di screte, Ts = 5 e6 s Fig 4. Matlab/Simulink Model of STATOM circuit. Aa b c VI3 p owerg ui ThreePhase reaker L3 g A Universal ridge1 Aa b c VI1 dc D Voltage Source v V1 A aa b c VI2 Nonlinear load V dc1 Goto2 1 onstant [Vdc] Goto Scope1 L6 Tm A ontrol i kcuit m induction generator Gate 1 Fig.5. Three phase injected inverter urrent source current load current inverter current time induction generator current time Fig.6. (a) Load urrent (b) Source urrent. (c) Wind generator (Induction generator) current. (d) Inverter Injected urrent. The performance of the system is measured by switing OFF the STATOM at time T=.2s to.3s.the load current and source currents are shown in Fig.6 (a) and Fig.8 (b) respectively. While the injected current from STATOM is shown in Fig.6 (d) and the generated current from wind generator at P is shown in Fig.6(c). Table:2 %THD of source current with and without STATOM by using ang ang current controller. S.No urrent THD(%) 1. Load urrent Wind Generator urrent Injected inverter current.1 4. Source urrent(without STATOM) Source urrent (with STATOM) 3.7 The Fourier analysis of waveforms with and without using angang controller is performed and the THD obtained for the source current at P without Available 8

5 Vol. 1, Issue. 4, Dec. 214, ISSN STATOM is 27.6% and the THD of source current (grid current) is only 3.7% as shown in Table 3. The injected currents also have harmonics and it cancel out reactive and harmonic part produced by the induction generator and the nonlinear load. Thus it improves the quality. The real and reactive of the wind energy system, STATOM, grid and in load indicates the performance of STATOM for different conditions of the system as shown in Fig7. When the output of the wind generating system is reducing from.1s to.45s, the real and reactive for load is provided by STATOM and the source real and reactive is increased for The period of.1s to.2s and.3s to.6s in providing the necessary real and reactive for load.this is a burden for source. 2 x 14 1 source x load compensator induction generator time Fig7. Real and reactive for (a) load (b)source (c)statom (d) wind energy generating system. Fig8. FFT analysis of source current by using bang bang current controller. Implementation Using Instantaneous real and reactive theory: The model of this system is also developed in MATLA/SIMULINK. To investigate the performance of the system a load variation is inserted at time T=.1 s and the STATOM is ON from time T=.1s. Fig9. shows the load current, inverter injected current, wind generator current and source current. Whatever changes occurs in the load or induction generator occurs it can t be seen in the source current and it is free from harmonics by the suitable operation of STATOM by using instantaneous real and reactive theory. s ou rc e c urrent (A ) non linear load c u rre nt (A ) inv erte r c u rre nt(a ) ind uc tio n gen era tor c u rre nt(a ) time(s) Fig.9. (a) Load urrent (b) Source urrent. (c) Wind generator (Induction generator) current. (d) Inverter Injected urrent Analysis of the system by reducing the output of the wind generating system output is also carried out. Output is reducing at time T=.1s to.45s and restoring at time T=.45s, STATOM is ON from T=.s and OFF at time T=.7 s and again ON at time T=.8s. From Fig1, it can be observed that the induction generator current remains constant by the operation of STATOM even though the output of wind generating system is varied. Fig9. shows the load current, source current, wind generator current, and inverter current when the output of wind generating system is reducing from.1 s to.45s and the STATOM is OFF from.2 s to.3s. Table: 3 %THD of source current with and without STATOM by using Instantaneous real reactive theory. S.NO URRENT %THD 1. Load current Injected inverter urrent 3. Induction generator current Source current (without) Source current.53 Available 9

6 Vol. 1, Issue. 4, Dec. 214, ISSN real and reactive s of source real and reactive s of non linear load real and reactive s of induction generator real and reactive s of inverter 2 x x x time(s) Fig.1. Real and reactive for (a) load (b)source (c)statom (d) wind energy generating system. Table: 4 omparison of two control schemes: S.No Type of ontroller %THD 1. angang current controller(%thd 3.7% in source current) 2. instantaneous real and reactive theory(%thd in source current).53% The above fig show the source current FFT analysis by using Instantaneous real reactive theory. The current waveform before and after the STATOM operation is analysed.without using instantaneous real and reactive theory for STATOM the source current THD is 27.41%, and using instantaneous real and reactive theory the source current (grid current) THD is reduced to.53% as shown in Table 3. It indicates that when we are using Instantaneous real and reactive theory the harmonics are reduced more as compared to ang ang controller. V. ONLUSION The STATOMbased control scheme for quality improvement in grid connected wind generating system with non linear loads has been presented effectively by reducing the %THD of source current. The STATOM has been designed by using the ang ang current control technique and instantaneous real and reactive theory. The % THD of source current has been reduced from to 3% by using ang ang current control technique and by using instantaneous real and reactive theory, the source current %THD is reduced from 27.41% to.53%. The THD analysis revealed that the instantaneous real and reactive theory is good as compared to bangbang controller. The instantaneous real and reactive theory is simpler and has faster response REFERENES [1] Sharad W. Mohod, Mohan V. Aware A STATOM control scheme for grid connected wind energy system for quality improvement IEEE SYSTEMS JOURNAL, VOL. 4, NO. 3, SEPTEMER 21 [2]. Han, A. Q. Huang, M. aran, S. hattacharya, and W. Litzenberger, STATOM impact study on the integration of a large wind farm into a weak loop system, IEEE Trans. Energy onv., vol. 23, no. 1,pp , Mar. 28. [3] M. I. Milands, E. R. adavai, and F.. Gonzalez, omparison of control strategies for shunt active filters in three phase four wire system, IEEE Trans. Power Electron., vol. 22, no. 1, pp , Jan. 27. [4] Sharad W. Mohod, Member, IEEE, and Mohan V. Aware Micro wind generator with battery storage IEEE SYSTEMS JOURNAL, VOL. 6, NO. 1, MARH 212 Fig.12. FFT analysis of source current [5] H. Akagi, E. H. Watanabe, and M. Aredes, Instantaneous [7] G.D. Marques, "A comparison of active filter Power Theory and Applications to Power onditioning, control methods in unbalanced and nonsinusoidal Hoboken, NJ: Wiley, 27. conditions, " Proc. 24lh IEEE Annual onf. IEON '98, [6] H. Akagi, Y. Kanazawa, and A. Nabae, "Instantaneous vol.l, pp , reactive compensator comprising switching devices without energy storage components," IEEE Trans. Ind. Appl., vol. IA 2, no. 3, pp ,Mar Available 1

7 Vol. 1, Issue. 4, Dec. 214, ISSN T.Suneel received his achelor degree in Electrical and Electronics Engineering from Gudlavalleru Engineering ollege, Gudlavalleru (INDIA) in 27 and M.Tech in Power Electronics and Drives from VIGNAN Engineering ollege, JNTU University Kakinada, (INDIA) in 29. He is currently working as an Assistant Professor in Electrical and Electronics Engineering Department at V.R.Siddhartha Engineering ollege Vijayawada, (INDIA). His research interests includes Power Electronics, Power Electronics Drives and Power Systems. R.Dileep Kumar received his achelor degree in Electrical and Electronics Engineering from sri sunflower college of Engineering & technology. Lankapalli (INDIA) in 212 and persuing M.Tech in Power Systems Engineering at V.R.Siddhartha Engineering ollege Vijayawada, (INDIA) G.Subba Reddy: received his achelor degree in Electrical and Electronics Engineering from Prasad V Potluri Siddhartha institute of technology (pvpsit), Vijayawada and M.Tech in Power Systems Engineering from RVR&J college of engineering, Guntur (INDIA) in28. He is currently working as an Assistant Professor in Electrical and Electronics Engineering Department at Audisankara ollege of Engineering & Technology Gudur(INDIA). His research interests includes systems, digital signal processing Available 11