IJESR/Nov 2012/ Volume-2/Issue-11/Article No-4/ ISSN International Journal of Engineering & Science Research

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1 International Journal of Engineering & Science Research WIND FARM TO WEAK GRID CONNECTION BY USING PI AND FUZZY BASED UPQC Parise Veeranjaneyulu* 1, VLN Sastry 2, Aswani Kumar Eedara 3 1 M Tech Student Scholar, Department of Electrical & Electronics Engineering, SASI Inst. of Technology and Engg, Tadepalligudem, W.G.Dist, A.P, India. 2 Assistant Professor, Dept. Electrical & Electronics Engineering, SASI Inst. of Technology and Engg, Tadepalligudem,W.G.Dist., A.P, India. 3 Assisant Professor, HOD, Dept. Electrical & Electronics Engineering, SASI Inst. of Technology and Engg, Tadepalligudem,W.G.Dist., A.P, India. ABSTRACT This paper deals with the using of unified power quality conditioner (UPQC) in wind energy conversion system (WECS). It presents effect of UPQC on wind energy conversion system (WECS) through showing its active filter behavior due to the nature connection of the both power controllers which look like same structure with the normal power convertors of Double fed induction generator (DFIG). This paper has represented a review paper for efficient control system for 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 without UPQC. With the help of MATLAB/SIMULINK environment Dynamic models of the DFIG and UPQC are developed at grid sid e. The internal control strategy is based on the management of active and reactive power in the series and shunt converters of the UPQC, and the exchange of power between converters through UPQC DC Link. This approach increases the compensation capability of the UPQC with respect to other custom strategies that use reactive power only. Simulations results show the effectiveness of the proposed compensation strategy for the enhancement of Power Quality and Wind Farm stability. Keywords: Speed control, hybrid PID plus fuzzy controller, induction motor, Ziegler- Nichols method. 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]. *Corresponding Author

2 Generally, in various systems 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 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 the power quality issues. Such equipment, always known as active power filters (APF s), 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. II. UPQC OVERVIEW Fig 1:General overview of UPQC Now a day, 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 kind 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 is 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 Copyright 2012 Published by IJESR. All rights reserved 1567

3 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]. 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. Fig 2: Unified power quality conditioner (UPQC) System configuration III. PROPOSED CONCEPT In this paper we propose and analyze a compensation strategy using an UPQC, for the case of SCIG based WF, connected to a weak distribution power grid. This system is taken from a real case [7].The UPQC is controlled to regulate the WF terminal voltage, and to mitigate voltage fluctuations at the point of common coupling (PCC), caused by system load changes and pulsating WF generated power, respectively. The voltage regulation at WF terminal is conducted using the UPQC series converter, by voltage injection in phase with PCC voltage. On the other hand, the shunt converter is used to filter the WF generated power to prevent voltage fluctuations, requiring active and reactive power handling capability. The sharing of active power between converters, is managed through the common DC link. Simulations were carried out to demonstrate the effectiveness of the proposed compensation approach. IV. SYSTEM DESCRIPTION AND MODELLING A. System description Fig.1 depicts the power system under consideration in this study. The WF is composed by 36 wind turbines using squirrel cage induction generators, adding up to 21.6MW electric power. Each turbine has attached fixed reactive compensation capacitor banks (175kVAr), and is connected to the power grid via 630KVA 0.69/33kV transformer. This system is taken from [7], and represents a real case. The ratio between short circuit power and rated WF power, give us an idea of the connection weakness. Thus considering that the value of short circuit power in MV6 is SSC 120MV A this ratio can be calculated Copyright 2012 Published by IJESR. All rights reserved 1568

4 Values of r < 20 are considered as a weak grid connection B. Turbine rotor and associated disturbances model The power that can be extracted from a wind turbine, is determined by the following expression: Where ρ is air density, R the radius of the swept area,v the wind speed, and CP the power coefficient. For the considered turbines (600kW) the values are R = 31.2 m, ρ = kg/m3 and CP calculation is taken from [8]. Then, a complete model of the WF is obtained by turbine aggregation; this implies that the whole WF can be modeled by only one equivalent wind turbine, whose power is the arithmetic sum of the power generated by each turbine according to the following equation: Moreover, wind speed v in (1) can vary around its average value due to disturbances in the wind flow. Such disturbances can be classified as deterministic and random. The firsts are caused by the asymmetry in the wind flow seen by the turbine blades due to tower shadow and/or due to the atmospheric boundary layer, while the latter are random changes known as turbulence. For our analysis, wind flow disturbance due to support structure (tower) is considered, and modeled by a sinusoidal modulation superimposed to the mean value of v. The frequency for this modulation is 3 Nrotor for the three bladed wind turbine, while its amplitude depends on the geometry of the tower. In our case we have considered a mean wind speed of 12m/s and the amplitude modulation of 15%. The effect of the boundary layer can be neglected compared to those produced by the shadow effect of the tower in most cases [3]. It should be noted that while the arithmetic sum of perturbations occurs only when all turbines operate synchonously and in phase, this is the case that has the greatest impact on the power grid (worst case), since the power pulsation has maximum amplitude. So, turbine aggregation method is valid. Copyright 2012 Published by IJESR. All rights reserved 1569

5 C. Model of induction generator For the squirrel cage induction generator the model available in Matlab/Simulink SimPowerSystemsc libraries is used. It consists of a fourth order state space electrical model and a second order mechanical model [5]. D. Dynamic compensator model The dynamic compensation of voltage variations is performed by injecting voltage in series and active reactive power in the MV6 (PCC) bus bar; this is accomplished by using an unified type compensator UPQC [9]. In Fig.2 we see the basic outline of this compensator; the bus bars and impedances numbering is referred to Fig.1. The operation is based on the generation of three phase voltages, using electronic converters either voltage source type (VSI Voltage Source Inverter) or current source type (CSI Current Source Inverter). VSI converter is preferred because of lower DC link losses and faster response in the system than CSI [9]. The shunt converter of UPQC is responsible for injecting current at PCC, while the series converter generates voltages between PCC and U1, as illustrated in the phasor diagram of Fig.3. An important feature of this compensator is the operation of both VSI converters (series and shunt) sharing the same DC bus, which enables the active power exchange between them. Fig 4:Block Diagram of UPQC Fig 5: Phasor Diagram of UPQC We have developed a simulation model for the UPQC based on the ideas taken from [10]. Since switching control of converters is out of the scope of this work, and considering that higher order harmonics generated by VSI converters are outside the bandwidth of significance in the simulation study, the converters are modelled using ideal controlled voltage sources. Fig.4 shows the adopted model of power side of UPQC. The control of the UPQC, will be implemented in a rotating frame dq0 using Park s transformation (eq.3-4) Copyright 2012 Published by IJESR. All rights reserved 1570

6 Fig 6: Power stage compensator model. AC Side This transformation allows the alignment of a rotating reference frame with the positive sequence of the PCC voltages space vector. To accomplish this, a reference angle synchronized with the PCC positive sequence fundamental voltage space vector is calculated using a Phase Locked Loop (PLL) system. In this work, an instantaneous power theory based PLL has been implemented [11]. Under balance steady-state conditions, voltage and currents vectors in this synchronous reference frame are constant quantities. This feature is useful for analysis and decoupled control. V. SIMULATION & RESULTS Here the simulation is carried out in two cases. In the first case the UPQC connected to the wind farm is studied with PI controller. In the second case the the UPQC is connected to the wind farm where the PI controller is replaced with Fuzzy controller. Figure below shows the simulation circuit of the UPQC with PI controller. Fig 7: Simulink model of the UPQC connected to wind farm The below figure shows the simulation results of active power and reactive power demand at the side of grid Copyright 2012 Published by IJESR. All rights reserved 1571

7 Fig 8 : Active and Reactive power demand at power grid side. Fig 9: PCC voltage Fig 10: Lower curve: WF terminal voltages Fig 11: Power of the capacitor in the DC Bus Fig 12: Voltage of the capacitor in the DC Bus Fig.14 shows the simulink model of the speed control of the DFOIM. In this paper PI, Fuzzy and PI plus Fuzzy controllers are used to control the speed of the Induction motor. Fig 13: Voltage at WF, at PCC Fig 14: series injected voltage at a phase Fig 15: Shunt and series converter active power; and DC bus voltage Fig. 15 shows the required and the actual speeds of the Induction motor, There is minor distortion of the actual apeed from the required speed by using the PI controller. Copyright 2012 Published by IJESR. All rights reserved 1572

8 Case 2: Replacing PI controller with Fuzzy controller Fig 16: Control circuit of Shunt controller using Fuzzy controller Fig 17: Active and Reactive power demand at power grid side with Fuzzy controller V. CONCLUSION Fig 18: PCC voltage with Fuzzy Controller In this paper, a new compensation strategy implemented using an UPQC type compensator was presented, to connect SCIG based wind farms to weak distribution power grid. The proposed compensation scheme enhances the system power quality, exploiting fully DC bus energy storage and active power sharing between UPQC converters, features not present in DVR and D Statcom compensators. The simulation results show a good performance in the rejection of power fluctuation due to tower shadow effect and the regulation of voltage due to a sudden load connection. So, the effectiveness of the proposed compensation approach is demonstrated in the study case. In future work, performance comparison between different compensator types will be made. Finally the PI controller is replaced with the Fuzzy controller and the results are studied. Copyright 2012 Published by IJESR. All rights reserved 1573

9 REFERENCES [1] Palsson MP, Uhlen K, Tande JOG. Large-scale Wind Power Integration and Voltage Stability Limits in Regional Networks. IEEE 2002; [2] Ledesma P, Usaola J, Rodriguez JL. Transient stability of a fixed speed wind farm. Renewable Energy 2003; 28: [3] Rosas P. Dynamic influences of wind power on the power system. Technical report RISØR Ørsted Institute. March [4] Dugan RC, McGranahan MF, Santoso S, Beaty HW. Electrical Power Systems Quality. 2nd Edition McGraw Hill, ISBN X. [5] Kundur P. Power System Stability and Control. McGraw-Hill, [6] Hingorani NG, Gyugyi YL. Understanding FACTS. IEEE Press; [7] Saad-Saoud Z, Lisboa ML, Ekanayake JB, Jenkins N, Strbac G. Application of STATCOM s to wind farms. IEE Proc. Gen. Trans. Distrib. 1998; 145(5). [8] Burton T, Sharpe D, Jenkins N, Bossanyi E. Wind Energy Handbook. John Wiley & Sons, ISBN [9] Ghosh A, Ledwich G. Power Quality Enhancement Using CustomPower Devices. Kluwer Academic Publisher, ISBN [10] Schauder C, Mehta H. Vector analysis and control of advanced static VAR compensators. IEE PROCEEDINGS-C, 1993; 140(4). [11] Sasso EM, Sotelo GG, Ferreira AA, Watanabe EH, Aredes M, Barbosa PG. Investigac ao dos Modelos de Circuitos de Sincronismo Trif asicos Baseados na Teoria das Potˆencias Real e Imagin aria Instant ˆaneas (p PLL e q PLL), In: Proc. (CDROM) of the CBA 2002 XIV Congresso Brasileiro de Automtica, pp , Natal RN, Brasil, 1-4. [12] International Electrotechnical Commission, INTERNATIONAL STANDARD IEC : Electromagnetic compatibility (EMC) Part 4: Testing and measurement techniques Section 15: Flickermeter Functional and design specifications. Edition [13] Akagi H, Watanabe EH, Aredes M. Instantaneous power theory and applications to power conditioning. John Wiley & Sons, ISBN Copyright 2012 Published by IJESR. All rights reserved 1574