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POWER QUALITY IMPROVEMENT IN GRID CONNECTED SYSTEM USING STATCOM T.Rajithavani M.tech student scholar Department of Electrical&Electronics Engineering Shri Vishnu Engineering College for Women Bhimavaram Abstract: - The main objective of this paper is to improve the power quality and transient stability of grid connected wind farm using a Static synchronous compensator(STATCOM).when the grid is affected by voltage fluctuations, faults and harmonics which are occurred at point of common coupling(PCC) are reduced by the static synchronous compensator (STATCOM) connected to grid system. Here a novel control strategy is applied on the STATCOM to improve the reliability and to run the STATCOM in safe limit. The Applied control strategy is proportional resonant control strategy (PR) which reduces the steady state error between reference and measured signals and the generated signals from PR controller are given to STATCOM to effectively suppress the STATCOM over current. Here PLL is used for grid synchronization.PR controllers are working in stationary reference frame in which transformations are reduced and simpler. Index terms power quality, transient stability, grid system, point of common coupling (PCC) wind, STATCOM, phase Locked loop (PLL) [6].PR controller, stationary reference frame 1. INTRODUCTION An increasing demand for more electric power coupled with depleting natural resources has led to an increased need for energy production from renewable energy sources such as wind and solar. Now a day the usage of renewable energy sources is increased widely to improve the short comings of conventional energy resources. The latest technological advancements in wind energy conversion and an increased support from governmental and private institutions have led to increased wind power generation in recent years Wind power is the fastest growing renewable source of electrical energy.By using wind fed to the wind turbine generators mechanical energy fed to the Induction Generator through the transmission line connected to the grid. Fluctuations in wind energy [4], voltage fluctuations and harmonic compensation due to nonlinear loads S.Dileeep Kumar Varma Associate professor Department of Electrical & Electronics Engineering Shri Vishnu Engineering College for Women Bhimavaram are reduced by using a static synchronous compensator. In the past decade, researchers have explored use of proportional-resonant (PR) regulators for control ac-dc converters using the stationary α-β frame. PR regulator can offer fast dynamic response and zero steady state error .PR controllers are carried out in stationary reference frames with phase locked loop .in stationary reference frames three phase two phase transformations are required .these three phase equations are converted into two phase by Clarke transformation. PCC Grid system RL load, non linear load, step load, faulty load Control output Inverter control Switchi -ng control inputs STATCOM Wind energy system Fig.1: Basic block diagram When variable loads, non linear loads and faulty loads are connected to the PCC they cause voltage drop, harmonics, and transient instability in the grid system. To overcome this faults STATCOM is a FACTS device to control reactive power dynamically, utilizing a voltage source converter connected in shunt to the power system which main purpose is to perform reactive power compensation and voltage control at the PCC. A proper control scheme in wind energy generation system is required to mitigate these power quality problems. Efficient control scheme of STATCOM based on PR (proportional resonant) control strategy with PLL control strategy is presented to control the grid currents. Here the Application of a PR control strategy on STATCOM is for controlling the STATCOM over current and PCC voltage. It is also concluded that the proposed control strategy can effectively reduce the effect of abnormal conditions. II PROPOSED CONTROL SYSTEM Wind System Line inductance Utility distribution grid Non linear load R1 RL Load R2Rrrr L1 R2r PR current control strategy L2 L2 R3L3 L3 STATCOM Step load Fig.2: System configuration Fig 2 shows the grid connected wind energy system with and without STATCOM is analyzed under RL load, nonlinear load, step change in load and faulty loads which are connected at PCC. When RL load is connected voltage is dropped at PCC and effect the grid voltage. When a nonlinear load is connected to the PCC the harmonics in the load will affect the grid. When step change in load voltage effect the PCC voltage. By the STATCOM performance with PR controller plays predominant role and THD also reduced effectively + + +………. X100 (1) V1 STATCOM is connected in shunt between the source and load via a coupling inductance Lc, so that it provides the necessary reactive current for the respective functions of the STATCOM such as power factor correction, harmonic elimination, etc. The controller of the STATCOM in this work is based on proportional resonant control theory. %THD = III.PROPOSED CONTROL STRATEGY Here PR controller is achieved in stationary reference frame. The transformation is done from ABC to αβ reference frame. Here three phase sinusoidal variables are controlled instead of constant variables. Three phase sinusoidal quantities produces two phase quantities in a fixed reference frame vary sinusoidal with time. Where as in DQ rotating frame they are stay constant. Fig 3 shows the Grid system converter control using PR controller which is carried out in stationary reference frame. Fig.3 Block diagram of STATCOM control From the above block diagram three phase voltage signals from the grid are synchronized with system using PLL .As well as three phase current signals from grid are converted to two phase stationary signals by Clarke transformation .Then this signals are compared with the two phase stationary signals which are generated from the PLL. These signals are compared error signal is given to PR controller. The generated two phase signals are converted to three phase by inverse Clarke transformation those are given to PWM then pulse from PWM given to STATCOM to operating in safe manner. HPR(s) = KP+KI S/ (2) Transfer function from the block diagram shows proportional plus resonant gains. Here the compensation terms are no need because they are not cross coupling current terms. Here the transformation is implemented in stationary (Clarke) transformation.[2]The transfer function of single and three phase PR controllers and filters are derived using internal model control, modified state transformation [13] Single phase PR controller in stationary reference frame and their equivalence in synchronous reference frame. Fig.5.Equivalent representations of PR and synchronous PI controllers In single phase PI control, PI control is approached by closest equivalence is developed to multiply the feedback error (t), by sine and cosine functions. The functions are usually synchronized with grid voltage using a phase-locked loop (PLL).This achieves the same effect of transforming the component at the chosen frequency to DC ,leaving all other components as AC quantities. In case of non-linear loads connected to the grid thus the harmonic component is taken as the error signal. PR CONTROLLER e(t)=E1 COS(ωt+θ1)+E3cos(3ωt+θ3) The objective of current controller s is to have zero phase and zero magnitude error .The principle is to find an equivalence ac compensation network with same frequency response characteristic to the synchronous frame controller. Instead of transforming the feedback error to the equivalent synchronous frame for processing, an alternative approach of transforming the controller GDC(s) from the synchronous to the stationary frame is also possible. GAC(S)= GDC(s-jω)+ GDC(s+jω) (3) (4) Where GAC(S) represents the equivalent stationary frame transfer function. Therefore, for ideal and non ideal integrators. The ideal and non ideal integrators are Fig.4: Block diagram for PR controller = and = (1+( )) (5) ω represents controller gain and ωc is cut off frequency where ki and ωc <<ω).the derived ac integrators GAC(S) are expressed = (6) = = cut (7) off frequency ω=controller gain = this control structure is used according to the internal model principle; in introduce a mathematical model that can generate the required sinusoidal reference along the open loop control path. And therefore can ensure overall zero steady state error [12] connecting nonlinear load, and increasing step change in the load voltage sag is created in PCC voltage, harmonics in source voltage and current. when step change in load at PCC then STATCOM absorb the injected voltage. V.SIMULINK RESULTS AND DISCUSSIONS The control scheme has been MATLAB/ SIMULINK model. simulated on A. voltage regulation in the grid under variable load By connecting the RL load at PCC the voltage dropped from the rms voltage 240v to 224v. For three phase systems the transfer function is follows (8) Fig7. VOLTAGE SAG WITHOUT STATCOM The above Equation have no cross coupling nondiagonal terms, implying that each of α and β stationary axes can be treated as a single-phase system. The transfer function of pr controller and filter is shown in figure [5].It is derived from the closed loop control which having a proportional plus resonant gains. From fig.3 voltage sag in grid is reduced after t=0.15” because the STATCOM come into action By comparing the reference and measured signals error sign IV. SIMULINK MODEL Fig.8. Voltage sag with STATCOM After the transition time voltage is in phase with the current Fig.6.Simulink model with PR control strategy The control circuit is achieved by the basic block diagram and after going all conversions at PCC by connecting RL load voltage is dropped, and by Fig9. Power factor B. Suppressing harmonics in the grid under nonlinear load currents. When a nonlinear load is connected to the PCC harmonics in the load will affect the grid. From fig.10 the harmonics in grid is reduced after t=0.05” because the STATCOM come into action along with PR controller. Fig12.Powerfacotr of source voltage and current D. Transient stability under step change in load When the load is step change at PCC will effect the grid voltage. Change in load voltage is controlled by STATCOM. After transition time 0.05” there is no change in the source voltage. The increased amount is carried out by STATCOM. Fig 10.Three phase source currents After transition time STATCOM is come into contact with system then the harminc currents are injected to STATCOM.SO after 0.05s STATCOM current have ripples. Fig 13.Step change in load voltage when load is step changed STATCOM absorb the load voltage fig 14.shows the STATCOM current after transition time. Fig 11.STATCOM current C. Power factor correction under nonlinear load. From Fig.12 the power factor becomes unity when STATCOM connected to PCC after t=0.05”. Simulation waveforms of the power factor are compared with and without STATCOM. Fig.14. STATCOM current E. Reduction of fault voltage When a faulty load is connected to PCC there is fault voltage or current in that phase cause voltage sag at PCC effects the load.fig 15 shows the phase A fault voltage. Fig.15: Phase a fault voltage By connecting STATCOM at PCC after 0.1s STATCOM is come into contact with system then fault in that load is minimized then voltage is raised after 0.15s. Fig.16: Phase A Voltage wave with STATCOM When non linear load is connected at the point of common coupling (PCC) THD with and without STACOM is compared. the PR current controller for harmonic reduction place a predominant role. Fig18. THD with STATCOM VI CONCLUSION This paper provides the grid model based PR control strategy when the grid is effected by non linear load, RL load and step load. The effect of these loads on the grid is reduced by specially designed stationary reference frame with PLL control strategy. Control strategy proposed has lesser computational burdens and has less steady state error between actual and measured signals. Simulation results shown that the PCC voltage has been effectively regulated and THDi indicate that PR control improves it effectively.PR control is a good alternative to implement an inverter system control with reduced harmonic content injected into the grid. Simulation results have been compared with and without STATCOM. The STATCOM based PR current controller with PLL control strategy guarantees fast transient response and high static performance via an internal current control loops. REFFERENCES [1]. Synchronization methods for three phase distributed power generation systems. An Overview and evaluation.Author: F. Bleiberg A. Tim bus, R. Teodorescu and M. Leisure.Journal: Power Electronics Specialists Conference, 2005. PESC ’05. IEEE Vol.36pp:2474–2481, 16–1. 6 June 2005. Fig17. THD without STATCOM When STATCOM is connected to the system the harmonics in the source current is suppressed when [2]. Zmood, D.N., and Holmes, D.G.: ‘Stationary frame current Regulation of PWM inverters with zero steady-state error’, IEEETrans. Power Electron. 2003, 18, pp. 814–822 [3]. Yuan, X.,Merk,W., Stemmler, H., and Allmeling, J.: ‘Stationary frame generalized integrators for current control of active power filters with zero steady-state error for current harmonics of concern under unbalanced and distorted operating conditions’, IEEE Trans. Ind.Appl., 2002, 38, pp. 523–532 [4]. Mattavelli, P.: ‘A closed-loop selective harmonic compensation foractive filters’, IEEE Trans. Ind. Appl., 2001, 37, pp. 81–89 [5]. LCL Filter design for grid-connected NPC inverters in offshore wind turbines. 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