INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET)

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1 INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) International Journal of Electrical Engineering and Technology (IJEET), ISSN (Print), ISSN (Print) ISSN (Online) Volume 3, Issue 1, January- June (2012), pp IAEME: Journal Impact Factor (2011): (Calculated by GISI) IJEET I A E M E POWER QUALITY IMPROVEMENT OF WIND ENERGY CONVERSION SYSTEM USING UNIFIED POWER QUALITY CONDITIONER Mr. Laith O. Maheemed 1, Prof. D.S. Bankar 2, Dr. D.B. Talange 3 ABSTRACT This paper presents a comparison between Unified Power Quality Conditioner UPQC controller and normal power converter to be used for DFIG control system. Implementation of unified power quality conditioner (UPQC) in wind energy conversion system (WECS) more performance improvement is observed than the normal power converter system. This work deals with two types of UPQC models which is developed for checking its control strategy validity in wind energy conversion system based Double fed induction generator (DFIG) since UPQC has the same structure of the DFIG converters. This paper mainly focuses on efficient control system of unified power quality conditioner that makes it possible to reduce the voltage fluctuations like sag and swell conditions, as well as current and voltage harmonics mitigation in wind energy conversion system. The UPQC which can be used at the PCC for improving power quality is modeled and simulated using proposed control technique and the performance is compared by applying it to a wind energy conversion system with UPQC and with normal power converter. With the help of MATLAB/SIMULINK environment Dynamic models of the DFIG and UPQC are developed. Key words: Double Fed Induction Generator (DFIG), unified power quality conditioner UPQC, Power Compensation, Total Harmonic Distortion (THD). I INTRODUCTION The increasing use of nonlinear loads is the main cause for increased current and voltage harmonics issues. As well as, the penetration level of different size of renewable energy systems based on solar energy, wind energy, fuel cell, nuclear, etc., installed at distribution, transmission levels is increasing significantly. This integration of renewable energy sources in a power system is further imposing new challenges to the electrical power industry to accommodate these newly emerging distributed generation systems [1]. Generally, in various system like power electronics, signal processing and other control systems, the nature of load characteristics have changed at all. These nonlinear loads draw non-linear current and disturb electric power quality. The quality degradation leads to several problems such as weak power factor, low efficiency, and increasing heat of transformers and so on [2]. Extensive research works have been carried out to check electric power networks having nonlinear loads and quantify the problems associated with. Conventionally passive L C filters were used to mitigate harmonics and capacitors were 288

2 employed to be used for power factor correction of the ac loads. However, passive filters have several advantages such as fixed compensation, large size, and resonance.[3] The increased distortion due to the harmonic pollution in various power networks has took the attention of power electronics and power system engineers to develop dynamic Fig. 1. Unified power quality conditioner (UPQC) system model. and adjustable solutions to the power quality issues. Such equipment, always known as active power filters (APF s), and are also called active power line conditioners (APLC s), instantaneous reactive power compensators (IRPC s), active power filters (APF s), and active power quality conditioners (APQC s). Recently, on load balancing neutral current compensation, harmonics, sags, swells, reactive power associated with linear and nonlinear loads many publications have also appeared [3]. Simultaneously, The Unified Power Quality Conditioner (UPQC) is one of the best solutions to solve problems related to both current and voltage in power system [4-5]. The UPQC illustrated in the general model which is shown in Fig. 1 [6]. II. UPQC Technique Now days, with the advancement in complex electronics industries, there are lots of problems associated with the power system and it has become necessary to provide a dynamic solution with high degree of accuracy and fast speed of response in order to mitigate and deals with these kinds of issues. The active power filtering has appeared as one of the best solutions for mitigation of major power quality problems [7]. In Parallel with advancement in the field of power electronic devices and automated control systems, it is very common to come across the situation where compensation of both current and voltage related problems are required. Recently, The UPQC which is integration of shunt and series APF is one of the most suitable as well as effective device in this concern [8]. A comprehensive review on the UPQC to enhance the electric power quality at distribution and transmission levels for various type of power generation system has been reported in [9]. Developments up to date, new designs and different aspects of UPQC in this area of research have been briefly addressed. An effort is made to put the UPQC interesting features in category through an acronymic organization list. These acronyms could be used to clearly identify particular application, utilization, configuration, and/or characteristic of the UPQC system under study. It is desirable that this review on UPQC will serve as a useful reference guide to the researchers working in the area of power quality enhancement utilizing APFs [9]. The main purpose of UPQC is to solve the problems coming from both source side and load side, such as voltage sag, voltage swell, distortion in the supply voltage, harmonic currents, reactive currents etc [10]. The general block diagram representation of a UPQC-based system is shown in Fig. 2. It basically consists of two series and shunt inverter connected back to back using a common dc bus capacitor. This paper deals with a novel concept of optimal utilization of a UPQC in wind energy conversion system [11]. 289

3 The classification of UPQC is given in Fig. 3 it shows different types of UPQC and each one has special function to do according the nature of connection which must match the application and system environment. The UPQC generally classified in two main categories based on : 1) The physical structure and 2) The voltage sag compensation. Voltage sag compensation type is one of the important functionalities of UPQC [9]. III. DFIG MODELING We used the classical modelisation of the DFIG in the (d-q) Park reference frame. (1) (2) The electromagnetic torque is expressed by: And the electro-mechanical equation is: (3) (4) Fig.2 shown the vector control diagram of DFIG which comprises RSC and GSC vector control. IV. RSC Vector Control The RSC is used to regulate the DFIG rotor currents (Fig.2) we choose to set the stator flux vector aligned with the d axis. The grid is assumed to be stable and consequently Φ ds is constant. The DFIG stator resistance R s is neglected [12]. Consequently, (5) Then, the rotor voltages are expressed by: 290

4 (6) With σ the dispersion coefficient, defined by: (7) To achieve an independent control of active and reactive power, we established the rotor reference currents, linked to the electromagnetic torque and stator reactive power references by : (8) Fig.2.Vector Control diagram of DFIG The RSC control diagram presented is directly established from the equations of the DFIG model. For the speed control, a Proportional Integral (PI) corrector with anti wind-up loop has been designed. The three-phase reference rotor currents are generated by the RSC controller implementing a modulated hysteresis current controller. Fig.3. represent the RSC control developed with the help of Matlab/Simulink software to be used in our proposed system. Fig.3. RSC developed in MATLAB/SIMULINK 291

5 V. GSC Vector Control The GSC is used to regulate the voltage of the DC bus connected between the RSC and the GSC. The power factor is set to unity, or in a way to fulfill the command strategy. The direct axis current is used to regulate the GSC reactive power and the quadrature axis current is used to regulate the DC bus voltage. This method also gives the possibility to control independently the active and reactive power exchanged between the GSC and the electrical grid. PWM control has been used for this converter with a switching frequency of 10kHz [13]. Fig.4. represent the GSC controller implemented in Matlab / simulink software to be used for our proposed system. VI. DESIGN OF UPQC CONTROLLER Fig 4. GSC developed in MATLAB/SIMULINK The control system of UPQC controller is comprised of three following parts 14]: 1. Shunt inverter control 2. Series inverter control 3. DC link voltage control A. Shunt Inverter Control: The UPQC shunt inverter controlling block diagram shown in fig.3 using synchronous reference frame theory where the sensitive load currents are (I La, I Lb I Lc ). The measured currents of load are transferred into d q0 frame using sinusoidal functions through d q0 synchronous reference frame conversion. The sinusoidal functions are obtained through the grid voltage using PLL. Here, the currents are divided into AC and DC components. The active part of current is i d while i q represent the reactive one. AC and DC elements can be derived by a low pass filter. Controlling algorithm corrects the system's power factor and compensates the all current harmonic components by generating the reference currents as relation (2): 292

6 Here, the system currents are: Fig. 5: Shunt inverter control block diagram. Switching losses and the power received from the DC link capacitors through the series inverter can reduce the average value of DC bus voltage. Other distortions such as unbalance conditions and sudden changes in load current can result in fluctuation in DC bus voltage. Here PI controller is necessary in order to track the error between the measured and desired capacitor voltage values. The resulted controlling signal is applied to current control system in shunt voltage source inverter which stabilizes the DC capacitor voltage by receiving required power from the grid. idc, the output of PI controller is added to the q component of reference current and so the reference current would be as relation (4): As shown in fig. 4, the reference currents are transferred into abc frame through reverse conversion of synchronous reference frame. Resulted reference currents (IFa * IFb * and IFc*) are compared with the output currents of shunt inverter (IFa, IFb and IFc) in PWM. Required compensation current is generated by inverter applying these signals to shunt inverter's power switch gates [14]. shunt power inverter control implemented in MATLAB is shown in fig.10 for our proposed which use UPQC controller. B. DC Link Voltage Control As mentioned above in last section a PI controller function is tracking the error exists between the measured and desired values of capacitor voltage in order to control the D.C link voltage as Fig. 4. Very large increasing in proportional gain make the control system unstable and so much reduction decreases the responding speed of control system. Integral gain of controller corrects the steady state error of the voltage control system. If this gain value is selected large, the resulted error in steady state is corrected faster and too much increase in its value ends in overshoot in system response [14]. Fig. 6: DC Link Voltage Control Block Diagram. 293

7 C. Series Inverter Control The controlling circuit of series inverter is shown in Fig. 5. SPWM method is used to optimize the response of series inverter [14]. Sinusoidal voltage controlling strategy of load is generally used to control the series part of UPQC. Here, the series inverter of UPQC is controlled the whole voltage distortions and maintains load voltage 3-phase balanced sinusoidal through compensation duty. The synchronous reference frame theory is applied to achieve that aim. In this method the desired value of load phase voltage in d-axis and q-axis is compared with the load voltage and the result is considered as the reference signal. The controlling circuit of series inverter is shown in Fig. 5. SPWM method is used to optimize the response of series inverter [14].Series power inverter control implemented in MATLAB is shown in Fig.9 for one of our proposed system that use UPQC controller. VII. UPQC and WECS Fig. 7: Series inverter control block diagram. Large numbers of research interests in DFIG systems environment in the literature have concentrated on the grid-connected wind power applications. In this case, the control scheme and operation of the DFIG are mainly concerned on active and reactive power control by modeling of DFIG [15], In this paper, we suggest the UPQC technique to be used instead of normal power convertors which are normally used in DFIG control system. Fig. 2 shows a unified power quality conditioner, UPQC (also known as a universal AF), which is a combination of active shunt and active series filters. The dc-link storage element (either inductor or dc-bus capacitor is shared between two current source CIS or voltage-source bridges VIS operating as active series and active shunt compensators. A voltage source inverter having IGBT switches and an energy storage capacitor on DC bus is implemented as a shunt APF. With the help of AF behavior of wind energy conversion system, compensation of harmonics, reactive power and elimination of the unwanted effects of non ideal ac mains supplies only unity power factor sinusoidal balanced three-phase currents. It is considered an ideal AF which eliminates voltage and current harmonics and has the capability of giving clean power to critical and harmonic-prone loads, such as computers, medical equipment, etc. It can balance and regulate terminal voltage and eliminate negative-sequence currents. The demerits of such equipment is the large cost and structure complexity because of the large number of solid-state devices involved [16]. The Structure of UPQC shown in Fig. 1. is similar to a DFIG normal converters except the grid side converter is connected in series with the stator before it is connected to the grid through a connection transformer. The RSC rotor side converter controls the active and reactive power 294

8 output to the grid according to the maximum power tracking curve designed for the generator. The GSC grid side converter has two functions. During the steady state operation, it injects appropriate voltage in-phase with the grid and the stator voltage in order to transfer appropriate power from the rotor to the grid through the DC link capacitor, thereby maintaining a constant DC link voltage[17]. Fig.12 & 13 describe the capacitor voltage (Vdc) for both proposed system under study which are shown in Fig. 8 and 11 respectively, where Fig 8 represent the first overall proposed system using UPQC controller and Fig.11 represent the second overall proposed system using normal power converters. The authors in [18], [19], [20], and [21] explain the UPQC system modeling in detail. The expected behavior of UPQC in system like this we proposed is the shunt inverter plays an important role in achieving required performance from a UPQC system by maintaining the dc bus voltage at a set reference value. In order to cancel the harmonics generated by a nonlinear load. Similarly, the series inverter of UPQC is controlled in voltage control mode such that it generates a voltage and injects in series with line to achieve a sinusoidal, free from distortion and at the desired magnitude voltage at the load terminal. Table I shows the capability of UPQC to play efficient role in harmonics mitigation, power compensation and power factor correction in wind energy conversion system due to reduction of odd harmonics components and total harmonic distortion THD. VIII. MODELLING AND SIMULATION OF WIND CONVERSION SYSTEM Wind turbine generators, control systems, power factor correction equipments, transformers, wind farm substation, inner loops for connections and transmission lines can be listed as wind farm sections that should be modeled for power system studies. The models of the grid and the wind energy conversion system have to comply with common requirements of the simulation platforms for getting accurate results. Computer simulation makes it possible to investigate a multitude of properties in design and application phase. The correctness of a computer simulation depends on the quality of the built-in models and of the applied data. In order to investigate the effects of wind energy conversion systems on power system and vice versa; it is necessary to develop accurate models of both systems. When the aim is to investigate grid integration of wind turbines, there are three main interests; steady-state voltage level influence, rapid voltage fluctuations (flicker), and response to grid disturbances. There are two types of flicker emissions associated with wind turbines. In order to predict the rapid power fluctuations from fixed-speed turbines, there is a need to represent the wind field arriving at the turbine, since the flicker emission during continuous operation is mainly caused by fluctuations in the output power due to wind speed variations, the wind gradient and the tower shadow effect. Switching operations like start, stop and switching between generators or generator windings, also produce flicker. When the limits of the flicker emission are given, the maximum allowable number of switching operations in a specified period can be examined by appropriate models in simulations. Model of a single wind energy conversion system must take into consideration soft starter, capacitor group, pitch control, and wind speed variations. Comparison of the systems with UPQC and normal PI/PID rotor and grid side controllers under wind speed and pitch angle variation is shown. The non linear load with resistance and inductance of 5Ω and 5mH are analyzed in the system with DFIG generation and grid side voltages are 575V at 60 HZ frequency and the converters are IGBT with diode and various wind speed and pitch angle. The waveforms and spectrum of the two proposed systems with UPQC and normal converters are shown in Fig 11 to 19. In this paper, the wind generator is supplying certain power and remaining 295

9 by the grid load to a non-linear RL load (steel plant type). The waveforms of both rotor side and grid side were depicted in Fig 14 to 17 for both types. Fig 8: Simulink diagram of the proposed system using UPQC controller Fig.9 series inverter control implemented in MATLAB Fig.10. shunt inverter control implemented in MATLAB 296

10 The rotor and grid side waveforms in Fig 15 & 17 are more uniform than others in Fig 14&16. With these waveforms and by simple comparison, The UPQC has played efficient role more than normal power converter in harmonics mitigation, power compensation and voltage regulation through total harmonic distortion THD reduction. Therefore, from the above, the control techniques on rotor side is used to maintain sinusoidal waveform and controlled voltage while grid side is used to absorb produced harmonics and to optimize the current flow from the grid to load. Fig.11: Simulink diagram of the proposed system using normal power converter Fig.12. Vdc capacitor voltage of UPQC controller Fig.13. Vdc_capacitor voltage of normal power converter Respectively, Fig 14 and 15 shown the rotor side voltage and current from DFIG for both proposed systems as well as Fig 16 and 17 shown the grid side voltage and current from DFIG for both proposed systems also. 297

11 Fig14: Rotor side voltage and current from DFIG with normal power converter Fig15: Rotor side voltage and current from DFIG with UPQC controller Fig16: Grid side voltage and current near non-linear load with normal power converter 298

12 Fig17: Grid side voltage and current near non-linear load with UPQC controller The grid side voltage wave spectrum for the system with UPQC controller was shown in Fig.19; it is observed that load voltage was having about % total harmonic distortion (THD), While the grid side voltage wave spectrum for the system with normal controller were shown in Fig 18 ; it is observed that load voltage were having about % total harmonic distortion (THD). From this information, As a result UPQC controller has more efficiency than normal controller in wind energy conversion system and exactly in power quality issues improvement. The voltage and current waveforms of source and load were shown in Fig 8 & 10. When comparing these Fig18: load voltage spectrum with normal power converter Fig19: Load voltage spectrum with UPQC controller 299

13 Harmonic Comp. &THD With normal power Converter TABLE I With UPQC Controller H1 100 % 100 % H2 0.23% 0.00 % H % 0.00 % H4 0.48% 0.01 % H % 8.36 % H % 0.03 % H % 6.98 % H % 0.01 % H % 0.00 % H % 0.01 % H % 4.55 % H % 0.03 % H % 3.43 % H % 0.01 % H % 0.00 % H % 0.01 % THD % % IX. CONCLUSION UPQC topology has been proposed in this paper, which has the capability of compensating the load. The proposed method is validated through simulation studies in a wind energy conversion. This paper has compared harmonic mitigation, power compensation and voltage regulation in wind energy conversion system through reduction of total harmonic distortion due to the connection of UPQC and normal converters in it. It also discussed capability of the both shunt and series power inverter of the UPQC to achieve a green active and reactive power source with compensation capability and suggest the various type of UPQC to be under light. Simulation results demonstrated that active filter behavior has additional function to improvement impact on the system power factor and reduction of the power loss. X. Acknowledgement We would like to thank everybody help us by any effort to finish this work. REFERENCES [1] R. A. Walling, R. Saint, R. C. Dugan, J. Burke, and L. A. Kojovic, Summary of distributed resources impact on power delivery systems, IEEE Trans. Power Del., vol. 23, no. 3, pp , Jul [2] E.W.Gunther and H.Mehta, A survey of distribution system power quality, IEEE Trans. on Power Delivery, vol.10, No.1, pp , Jan

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15 [17] Jayanti, N. G. et al. : A new configuration and control of doubly fed induction generator (UPQC-WG). IECON 2008: 34th Annual Conference of IEEE on Industrial Electronics, Orlando, Nov., 2008, pp [18] R. Strzelecki, G. Benysek, J. Rusinski, and H. Debicki, Modeling and experimental investigation of the small UPQC systems, in Proc. Compat. Power Electron., Jun. 1, 2005, pp [19] L. M. Landaeta, C. A. Sepulveda, J. R. Espinoza, and C. R. Baier, A mixed LQRI/PI based control for three-phase UPQCs, in Proc. 32 nd Annu. Conf. Ind. Electron. Soc., Nov. 9 10, 2006, pp [20] Y. Rong, C. Li, H. Tang, and X. Zheng, Output feedback control of single-phase UPQC based on a novel model, IEEE Trans. Power Del., vol. 24, no. 3, pp , Jul [21] T. Zhili and Z. Dongjiao, Design of dc voltage controller for UPQC by using its small signal model, in Proc. Electr. Control Eng., Jun , 2010, pp Mr. Laith O. Maheemed, M.Tech. candidate, Electrical Engineering, Department of Bharati Vidyapeeth University, College of Engineering, Pune, India. eng.laithpower@gmail.com 2. Prof. D. S.Bankar, Associate Professor, Electrical Engineering Department of Bharati Vidyapeeth University College of Engineering Pune, India dsbankar@bvucoep.edu.in 3. Dr D. B. Talange, Ph.D IIT Bombay, Specialization in Systems and Control Engineering, currently working as Professor in Electrical Engineering Department of Govt. College of Engineering Pune (India) 302