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INTERNATIONAL Electrical Engineering and Technology (IJEET), ISSN 0976 – International Journal of JOURNAL OF ELECTRICAL ENGINEERING & 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME TECHNOLOGY (IJEET) ISSN 0976 – 6545(Print) ISSN 0976 – 6553(Online) Volume 3, Issue 1, January- June (2012), pp. 247-260 IJEET © IAEME: www.iaeme.com/ijeet.html Journal Impact Factor (2011): 0.9230 (Calculated by GISI) ©IAEME www.jifactor.com HARMONIC MITIGATION FOR NON-LINEAR LOADS USING THREE-PHASE FOUR WIRE UPQC CONTROL STRATEGY Mr. Laith O. Maheemed1, Prof. D.S. Bankar2 1 Bharatividyapeeth university .M.TECH Student, Electrical Department , COE, Pune, India Email: eng.laithpower@gmail.com 2 Bharatividyapeeth university, Associated professor ,Electrical Department, COE, Pune, India Email: bankardeepak@indiatimes.com ABSTRACT This paper presents three-phase four-wire active filter for power line conditioning (PLC) to improve power quality in the DFIG wind turbine grid network. In addition to the power-factor correction, load balancing and mitigation of voltage and current harmonics, it can regulate the load voltage against voltage sag/swell and voltage dip in a three-phase four-wire distribution system for different non-linear loads. The active power filter (APF) is implemented with PWM based current controlled voltage source inverter (VSI). This VSI switching signals are generated through proposed two-level hysteresis current controller (HCC) that achieves significant reduction in the magnitude and variation of the switching frequency; The synchronous reference frame (SRF) theory is used to get the reference signals for series and shunt active power filters (APFs). The reference signals for the shunt and series APF of UPQC are derived from the control algorithm and sensed signals are used in a hysteresis controller to generate switching signals for shunt and series APFs. The UPQC is realized using two voltage source inverters (VSI) connected back to back, to a common dc link capacitor. MATLAB/Simulink based simulations are obtained, which support the functionality of the UPQC. Keywords: DFIG, Active Power Line Conditioners (APLC), PI-Controller, Hysteresis Current Controller (HCC), Harmonics, Power quality. I . INTRODUCTION Wind energy is the fastest growing and most widely utilized emerging renewable energy technologies in electrical energy conversion systems at present [1]. This high penetration of wind energy in the power system has been closely related to the advancement of the wind turbine technology and the way of how to control. There are basically three types of generators that are commonly used with commercial wind turbines. They are (1) fixed-speed system with squirrel- 247 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME cage induction generator, (2) variable-speed system with Doubly-Fed Induction Generator (DFIG) (3) variable-speed system with a direct-drive synchronous generator. DFIG based variable speed wind energy conversion systems are currently the most admired one, due to its important advantages such as, high capacity with high energy efficiency, four-quadrant active and reactive power controls and the small converter size with a rating of only 20%–30% of the rated Wind turbine power [2]. The DFIG consists of a Wounded Rotor Induction Generator with the stator windings directly connected to the constant frequency three-phase grid and with the rotor winding connected to a bidirectional back-to-back PWM voltage Source Converter. The ever-growing proliferation of power-switching devices for source conditioning and motion control in single-phase and three-phase modern industrial applications has increased the occurrence of unacceptable current harmonics levels in three-phase distribution systems. The harmful and costly effects of harmonics have been discussed extensively in literature [3-6]. A major effect of harmonic voltages and currents in rotating machinery (DFIG included) is increased heating due to iron and copper losses at the harmonic frequencies. The harmonic components thus affect the machine efficiency [7]. For instance, the fifth and seventh harmonics can combine to produce a torsional stimulus on a generator rotor at the sixth harmonic frequency. If the frequency of a mechanical resonance exists close to the frequency of electrical stimulus, high-stress mechanical forces can be developed. Another generally greater concern is the flow of harmonic currents in the rotor. The flow of each current in the stator will produce a magneto- motive force in the air gap that will induce current flow in the rotor of the machine. Just as each characteristic harmonic can be defined as being a positive or negative sequence, the rotation of that harmonic will be either forward or backward with respect to rotor rotation. The fifth harmonic will rotate in a backward direction (negative sequence), so a harmonic current will be induced in the rotor with a frequency corresponding to the net rotational difference between the fundamental air gap frequency and the fifth, i.e., the fifth plus one, or the sixth harmonic. Since the seventh harmonic will rotate in a forward direction (positive sequence), a harmonic current will be induced in the rotor with a frequency corresponding to the net rotational difference between the seventh and the fundamental air gap frequency, i.e., the seventh minus one, or the sixth harmonic. Thus, from a rotor heating standpoint, the fifth and the seventh harmonics in the stator combine to produce a sixth harmonic current in the rotor. The 11th and the 13th harmonics act in the same manner to produce the 12th harmonic current in the rotor, and so on with higher order harmonic pairs. There are two major concerns with these rotor harmonics: (1) Resultant rotor heating; (2) Pulsating or reduced torques. To solve these problems, passive power filters have been widely used for a long time. Passive power filters consists of a combination of inductors and capacitors tuned to a certain frequency. Although they are simple in structure and have a relatively low investment cost, they can cause unwanted resonance and amplify harmonic currents. To overcome the disadvantage of passive power filters and restrictions on their performance, research in active power filters has been carried out actively. Active power filters can be classified as series or parallel by their system configuration. The combination of series and parallel active power filters is called the Unified Power Quality Conditioner (UPQC). Although its main drawback is its high cost and complexity of control, interest in UPQCs is growing due to its superior performance. UPQCs offer not only harmonics elimination but also compensation for reactive power, load current unbalance, source voltage sags, source voltage unbalance and power factor correction [2]. UPQC is mainly designed to inject compensating current and voltage into the system, in order to mitigate the system harmonics [3]. The UPQC have been studied and applied to regular three-phase power systems, operated in 50/60 Hz [4, 5], however, there are no many applications of UPQC in single-phase systems. This article is based on the steady state analysis of the behavior of UPQC with a 248 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME distorted source voltage and a nonlinear load condition. Aim is to maintain the load bus voltage sinusoidal and at desired constant level with a sinusoidal source current. In this paper, a shunt active power filter is proposed to protect DFIG wind turbine from the destroying effects of the current harmonics caused by the connection of nonlinear loads at the point of common coupling (PCC). Simulation results using MATLAB/SIMULINK are shown to validate the robustness and effectiveness of the SAPF to mitigate current harmonics. II. DFIG MATHEMATIC MODEL A doubly fed induction machine is a wound rotor with back-back converter in the rotor circuit. A DFIG works as a generator or as a motor at both above and below the synchronous speed by controlling the power injected into the rotor. Fig 1 Detailed Conﬁguration of 3P4WUPQC In DFIG the rotor is supplied by PWM inverter, while the stator is directly connected to grid. The rotor current exciting frequency is controlled as the wind velocity is changed. The frequency of output power is fixed at grid frequency, which is given as follows: ωs=p m± ωr (1) Where ωs is the grid electrical angular speed, m is the mechanical angular rotor speed, r ω is the electrical angular speed of rotor variables, and p is the number of pole pairs. In sub- synchronous operation mode the sign in (1) is positive; otherwise it is negative in super- synchronous operation mode. Equation (1) establishes is the basis for VSCF. The mathematical equations of the DFIG in terms of stator, rotor voltages and flux are given as follows [3]: Vsd=Rsisd –ωs φsq + φsd (2) Vsq=Rsisq –ωs φsd + φsq (3) Vrd=Rrird –(ωs-ω) φrq + φrd (4) Vrq=Rrirq –(ωs-ω) φrd + φrq (5) The direct and quadrature stator and rotor flux components are given as follows [4]: φsd=Lsisd + Lmird (6) φsq= Lsisq + Lmirq (7) φsd=Lrird + Lmisd (8) φrq= Lrirq + Lmisq (9) The d-q steady-state equivalent circuit of the DFIG is depicted in Fig. 2 [5]. 249 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME DFIG Wind Turbine The DFIG wind turbine adopted here is shown in Fig. 2. It consists of a DFIG driven by a wind turbine and controlled on the rotor side through the Back-to-back PWM power converters. Back- to-back PWM converters consist of two converters, the stator-side converter and rotor-side converter, which are controlled independently of each other. The main idea is that the rotor-side converter controls the active and reactive power by controlling the rotor current components, while the stator-side converter controls the DC-link voltage and ensures a converter operation at unity power factor (zero reactive power). Depending on the operating condition of the rotor, the power is fed into or out of the rotor. In an over synchronous condition, power flows from the rotor via the converter to the grid, whereas power flows in the opposite direction in a sub- synchronous condition. In both cases, the stator feeds power into the grid at the point of common coupling (PCC) through a transformer [8]. Fig. 1 shows the proposed studied system configuration. This system consists of a DFIG wind turbine, grid supply, SAPF and nonlinear load all connected at the point of common coupling (PCC). The shunt active power filter is composed of three parts. Three legs voltage source converter (VSC) connected to the PCC through interfacing inductors, a DC link represented by a capacitor and a control system. The nonlinear load is a three phase diode rectifier feeding RL load. 4. Shunt Active Power Filter (SAPF) Shunt active power filter is a power converter utilized in order to compensate current disturbances (harmonics, reactive power and unbalance). In order to meet quality enhancement constraints proper control of its power switches is needed. Several topologies and configuration have been introduced in the literature and in commercial implementations for this filter that highlight different aspects of its compensation tasks. The most common topology of the shunt active power filter is shown in fig. 1. Its main components are voltage source converter, DC bus (in our situation is a capacitor), output passive filter and a control system. The most important objective of the SAPF is to compensate the current harmonics generated by non linear loads. The reference currents consists of the harmonic components of the load currents which the active filter must supply [9]. These reference currents are fed through a controller to generate switching signals for the power switching devices of the voltage source converter (VSC). Finally, the AC supply will only need to provide the fundamental component for the non linear load. 250 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME III. ROTOR SIDE CONVERTER CONTROL ALGORETHIM- HYSTERESIS CURRENT CONTROL In this paper, hysteresis control technique is used to control the current harmonics injected by SAPF into the grid. According to reference current and the injected current, the hysteresis control determines switching signals for the inverter gates. Hysteresis current control is based on error signal between the injected current (If) and the reference current (Iref generated by p-q theory) which produces proper control signals. Assuming the DFIG is connected to the state power grid in which the grid voltage and frequency is constant. Fixing the d-axis of the synchronous frame on the stator voltage vector, a stator voltage oriented (SVO) control is obtained. Thus, the vector of the stator voltage is: Vs=Vsd +j0 (10) According to (10), the active and reactive power output from the stator side of the DFIG can be represented as: Ps=Vsd isd (11) Qs= -Vsd isq (12) Substituting (2) in (11) and (3) in (12) respectively, the active and reactive powers can be derived as follows: Ps= (13) Qs= (14) As seen from (13) and (14), the active and reactive powers are related to rotor currents ird and irq respectively. Therefore, the active and reactive power can be controlled via ird and irq respectively, which is possible through the control of Vrd and Vrq. There is Hysteresis Band (HB) above and under the reference current and when the difference between the reference and inverter current reaches the upper (or lower) limit; the current is forced to decrease (or increase) as shown in Fig.3. Fig 3 Hysteresis control loop with duty cycle waveform T1+T2=T= (15) The proposed control algorithm assumes converting the two reference signals ird_ref and irq_ref using Park’s transformation inverse into abc reference frame, then by comparing the three rotor current signals ira_ref , irb_ref and irc_ref with actual rotor currents. The error signals issued from comparison are applied to hysteresis controllers. The logical outputs of these controllers are the switching signals of power transistor in RSC. The proposed algorithm based on hysteresis controller is shown in Fig. 4. 251 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME Where fc is the switching frequency and has an inversely proportional relation to HB [11]. In comparison with other PWM methods, the hysteresis current control has a very fast response, a simple operation and a variable switching frequency [12]. 7. Simulation Results and Discussion The proposed system configuration of Fig.1 has been simulated by Simulink of Matlab as it is shown in Fig. 4. The line voltage at PCC is 380 V with line frequency of 50 Hz. The nonlinear load is a three-phase diode rectifier with rating of 80 kVA. SAPF is connected to the system through a three-phase link inductor with Lf = 0.1 mH and the dc bus capacitance is C=4.4 mF with reference dc voltage of VCd=850 V. The VSC is a voltage source full-bridge IGBT based inverter driven by hysteresis control. DFIG wind turbine of 500 kVA is connected at PCC. The following waveforms show the high efficiency of SAPF for mitigation of harmonics. Fig. 5 and Fig. 6 show one phase Voltage waveform at PCC and its spectral decomposition before and after harmonic compensation. From Fig. 5 we can see clearly that the PCC voltage is distorted and the total harmonic distortion (THD) parameter is 7.86 % which is according to the IEEE 519- 1992 standard is not tolerable because it exceeds the limit of 5%. Voltage distortion in this case is due to the passage of current harmonics through the impedance of the grid (Zs), that is why in its spectral decomposition we find the same harmonics rank (5, 7, 11, 13, ….) as that found in the current driven from PCC. So, this distortion will disappear when current harmonics are compensated by SAPF as it is shown in Fig. 6. After the mitigation of the current harmonics, the THD parameter of the PCC voltage is reduced from 7.86 % to 0.09 % and the spectral decomposition shows a strong attenuation of the magnitude (MAG) of all harmonics rank. Phase Current waveform driven from PCC and its spectral decomposition before and after the connection of SAPF is illustrated by Fig.7 and Fig. 8 respectively. Fig. 7 shows a distorted current waveform with high value of THD parameter reaching 26.87 % which is according to the IEEE 519-1992 standard is not tolerable because it exceeds the limit of 5%. The corresponding spectral decomposition shows a very important magnitude of all harmonics rank especially the 5th and the 7th. Once SAPF is connected it injects the same current harmonics into the grid but with opposite phase (Fig. 9) and consequently the current driven from PCC becomes practically sinusoidal as it is shown in Fig. 8. After the compensation of the current harmonics, the THD parameter of the current driven from PCC is reduced from 28.87 % to 0.91 % and the spectral decomposition shows a strong attenuation of the magnitude (MAG) of all harmonics rank. For the SAPF to work properly, it must have a good regulation of its DC-link voltage. The purpose of the DC-link voltage controller is to preserve the DC-link voltage Vcd at its reference 252 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME value Vcd ref = 850 V. This is accomplished by balancing the active power flow in the capacitor. The performance of the DC-link voltage controller is verified by simulations in Fig. 10. The DC- link voltage controller consists of a PI-controller, where the integral part reduces the steady state error of the DC-link voltage. This implies a faster response to changes in the capacitor current and thereby reduction of the DC-link voltage deviation during transients VI. Grid Side Converter (GSC) CONTROL ALGORITHM The proposed algorithm of GSC adopts the SVO technique to regulate DC-Link voltage and achieve a unity power factor. This strategy leads to getting the following active and reactive powers: Prec=Vd id (17) Qrec=-Vd iq (18) Thus, the current command of q-axis controls the reactive power and it is obvious that the current command of q-axis must be zero iq_Ref =0 for unity power factor operation. Whereas a current command of d-axis controls the active power, and consequently controls indirectly the DC-link voltage. From the above mentioned analysis, the d-axis must have two loops; inner one, which employed hysteresis controller to regulate the d-axis current; the outer loop; which uses proportional-integral controller to control the DC-bus voltage. The output of the PI controller generates id_Ref. The reference d-axis current, which is formed by PI controller and q-axis current, which set to zero are both transformed to abc reference frame using the Park’s transformation inverse. The error signals issued from the comparison between current reference values and actual ones are applied to PWM controllers. The logical outputs of these controllers are the switching signals of power transistor in GSC to maintain the desired currents. The proposed scheme is shown in fig. 6. In terms of the steady-state condition, Vdq= Vd+ j0 if the d-axis of the reference frame is aligned along the PCC voltage position. Assuming Vdq1= Vd1+ j Vq1 and neglecting the grid ﬁlter resistance, then, the current ﬂowing between the PCC and the GSC according to (3) is Idq= - (19) in which Xf stands for the grid ﬁlter reactance. Fig 5: proposed grid side controller (GSC) The following issues are considered in the design of the conventional nested-loop control system. 1) To prevent the converter from getting into the nonlinear modulation mode, a saturation mechanism is applied to the output voltage of the controller if the amplitude of the reference voltage generated by the inner current-loop controller exceeds the converter linear modulation limit. The general strategy is to set a limitation on but keeps unchanged as shown by (11) [13], [14], where and are d the q and 253 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME components of the modiﬁed controller output voltage and Vmax is the maximum allowable voltage. It is found that any other saturation mechanisms could cause more system oscillations and unbalances. Vmax . cos( = Vmax . cos( (20) 2) To prevent the GSC from exceeding the rated current, the -axis current reference is adjusted if the amplitude of the reference current generated by the outer control loop exceeds the rated current limit. The general approach is keeping the d–axis current reference unchanged to maintain dc-link voltage control effectiveness while modifying the q-axis current reference to satisfy the reactive power or ac bus voltage support control demand as much as possible as shown by (12) [13], [14] 2 =sign ( . Sqrt(( -( 2 (21) The overall control structure of the GSC is shown by Fig. 4, which consists of a -axis loop for dc- link voltage control and a d-axis loop for reactive power or grid voltage support control. Signal processing technology is applied to the measured dc-link voltage and d-and q-axis currents to prevent the high order harmonics from entering the controllers. The current-loop controller may integrate PI, fuzzy and adaptive control technologies to improve the dynamic performance of the GSC. The PI part of the controllers operates on a direct target control principle. The fuzzy and adaptive parts of the controllers adjust the PI parameters based on the error, between the controlled variable and its target value, and the change in error [18]. The initial values of the PI current-loop controllers are tuned according to the fundamental intelligent control principle, i.e., minimizing the rms error between the reference and measured values [15]. In addition, a nonlinear programming strategy as shown below is utilized to prevent the GSC from going over the rated current and to avoid the converter getting into a nonlinear modulation mode, where irated is the rated GSC phase rms current and is the reference reactive power absorbed from the grid by the GSC. The basic principle of the nonlinear programming formulation is that under GSC rated current and linear modulation limits, the system should operate to achieve the dc-link voltage control goal while minimizing the difference between the reference and actual reactive power as much as possible Minimize: | - | (22) Subject to: Vdc= (23) /(2 ) (24) The nonlinear programming strategy is implemented in the following way. If | | generated by the outer dc-link voltage and reactive power control loops exceeds the rated current limit, and are modiﬁed by (12). If | | generated by the inner current control loops exceeds the converter linear modulation limit, d-axis and q-axis the axis voltages are recalculated by (14). As it can be seen, the recalculation does not change the q-axis control voltage so that the q-axis control loop is not affected. Hence, according to (7), the effectiveness of active power or dc-link voltage control is maintained. But, the recalculation makes the d-axis control voltage does not follow the control voltage generated by the d–axis current-loop controller. Thus, the effectiveness of the reactive power or bus voltage support control, according to (7), would be affected. Under such conditions, the reactive power control is actually decided by the constraint of the converter linear modulation requirement but not the control rule 254 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME =sign( . ) = (25) V. SIMULATION All simulation studies presented in this paper have been done with MATLAB/ Simulink with use of the SimPowerSystems Toolbox. The generators are represented by a model of the electrical circuit and the mechanical part is neglected due to the small speed deviation during the time period considered. Therefore, the simulations were carried out with constant rotor speed. The IGBT converters are modeled as ideal switches with anti-parallel diodes. Distributed parameter models are used for lines and the transformer models consider saturation effects, but no hysteresis. Circuit breaker models are ideal and open exactly at the first current zero crossing after the open command. Fig 6 Simulation design of the system For the simulation scenarios a 15kVA wind farm is modeled by two equivalent wind turbines in scenario A and by one equivalent wind turbine. The performance of the three-phase four-wire shunt APLC system is evaluated through Matlab programs in order to program and test the system under unbalanced non-linear load conditions. The system parameters values are; Line to line source voltage is 440 V; System frequency (f) is 50 Hz; DC-link capacitor C=5000 µF ; Reference dc voltage 600 V; Interface inductor is 5 mH and 1 full bridge rectifier load 168 + j 16 . The conventional power circuit of the voltage source inverter based active power filter connected at the point of common coupling shown in Fig 6. The voltage source inverter has six power transistors with freewheeling diodes and two energy storages capacitor on DC-side that is implemented as a four-wire active power filter. The source draws non-sinusoidal or harmonic current due to the non-linear load. This nonlinear load current contains the fundamental signals and harmonic current components. Fig 7 & 8 shows the unbalanced non-linear load current or source current before compensation. It is indicate that the grid and wind generator source having voltage and current harmonic components due to the non-linear load, which is diode rectifier. 255 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME Fig 7. Simulation wind turbine side voltage and current without controller Fig 8. Simulation grid voltage and current without controller The total harmonic distortion (THD) of wind generator FFT analysis voltage and current waveforms were as shown in Fig 9 & 10 and grid side voltage and current waveforms were shown in Fig 11 & 12. Fig 9. Showing Wind generator voltage harmonics due to non-linear load without controller Fig 10. Showing Wind generator current harmonics due to non-linear load without controller 256 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME Fig 11. Showing grid voltage harmonics due to non-linear load without controller Fig 12. Showing grid current harmonics due to non-linear load without controller The 3-phase source voltages are converted to the 3-phase unit current(s) while corresponding phase angles are maintained. The unit current is defined as (26) These unit currents multiplied with peak reference current for generating the reference currents. The proposed PI-control scheme estimates the peak reference current of an APF system. The two storage DC-side capacitor voltage is sensed and compared with a reference voltage. The error, e= Vdc_ref -Vdc (27) at the sampling instant is used as input for PI-controller. The following waveforms show the high efficiency of SAPF for mitigation of harmonics. Fig. 9 to 12, shows voltage and current waveforms at wind side generator and grid side source and its spectral decomposition before harmonic compensation. From Fig. 9 and 11, we can see clearly that the voltage is distorted and the total harmonic distortion (THD) parameter is about 56% and 58% which is according to the IEEE 519-1992 standard is not tolerable because it exceeds the limit of 5%. Voltage distortion in this case is due to the passage of current harmonics through the impedance of the grid (Zs), that is why in its spectral decomposition we find the same harmonics rank (5, 7, 11, 13, ….) as that found in the current driven from PCC. So, this distortion will disappear when current harmonics are compensated by SAPF as it is shown in Fig. 6. After the mitigation of the current harmonics, the THD parameter of the wind generator side voltage and current is reduced from 56.56 % to 1.50% and grid side voltage spectrum THD has decreased from 57.84% to 1.92% without and with compensation as shown in Fig . The spectral decomposition shows a strong attenuation of the magnitude (MAG) of all harmonics rank. 257 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME Fig 13 Simulation wind turbine side voltage and current with controller Fig 14. Simulation grid side voltage and current without controller Fig 15. Showing Wind generator voltage harmonics due to non-linear load without controller Fig 16. Showing Wind generator current harmonics due to non-linear load without controller 258 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME Fig 17 Showing grid voltage harmonics due to non-linear load without controller Fig 9. Showing grid current harmonics due to non-linear load without controller By using the proposed controller, the harmonics which were produced because of non-linear load were minimized by the wind generator system control circuit. This technique is very efficient in decreasing voltage harmonics to a very great extent. VI. CONCLUSION A new current decomposition technique, based on frequency domain and SRF theory, with indirect current control and reduced number of current sensors for prioritized selective compensation of different power qualities and their combinations has been investigated for the shunt APF of three-phase three-wire UPQC. A control strategy based on SRF theory is applied for the control of the series APF of UPQC. The observed performance of the UPQC has demonstrated the ability of the proposed control technique to selectively compensate the customer generated harmonics, the total source current harmonics, unbalanced loading, reactive power and voltage harmonics, based on priority to respect the limited power capacity of VSIs employed for the shunt and series APFs. In addition to this, by mitigation of customer generated harmonics only, the responsibility of the utility and customers at the PCC is attributed. It is also observed that the proposed control scheme has a fast response and is able to maintain the voltage and current harmonics levels, thus conforming to IEEE-519 standards. Further, the applied control scheme is able to self support the dc bus voltage of back to back connected VSIs of the UPQC. The control scheme of shunt APF has the advantage of ﬂexibility in the selection of the power quality indices for which the reference may be computed. In addition to this the shunt APF compensates the current based distortions even under distorted utility voltages, hence the operation of shunt and series APF are independent of each other. In case of a voltage sensitive load, the series APF may be switched on to mitigate the voltage harmonics present in the load voltages. 259 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME APPENDIX The system parameters used are as follows: Supply voltage and line impedance: 415V L-L, f=50 Hz, Rs=0.1 , Ls=0.05mH Ripple Filter: R=7 , C=5mF DC bus capacitance: Cdc=3000mF DC bus voltage of UPQC: Vdc=600V Series Transformer: 250KVA, 1.1KV/5.5KV Loads: 1) Three-Phase Rectiﬁer Load with R=25 on dc side, and 2) Three single phase load 10KW, 6KVar (lagging) in each phase. REFERENCES [1] A. Ghosh and G. Ledwich , Power Quality Enhancement Using Custom Power Devices, Kulwer International Series in Engineering and Computer Science, 2002. [2] N. G. Hingorani, “Introducing custom power,”in Proc. IEEE Spectrum, Vol. 32, pp. 41-48, Jun.1995. [3] A. Cetin, H.F. Bilgin, A. Acik, T. 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