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ANALYSIS OF VARIOUS COMPENSATION DEVICES FOR POWER QUALITY IMPROVEMENT IN WIND ENERGY SYSTEM

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In recent trend, the renewable source of energy is increasingly used in the electric power generation, which leads to certain power quality issues. Hence some of the supplementary devices like capacitors, compensators or reactive power injection
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  http://www.iaeme.com/IJEET/index.asp 25 editor@iaeme.com International Journal of Electrical Engineering & Technology (IJEET) Volume 7, Issue 3, May  –  June, 2016, pp.25  –  39, Article ID: IJEET_07_03_003 Available online at http://www.iaeme.com/ijeet/issues.asp?JType=IJEET&VType=7&IType=3 ISSN Print: 0976-6545 and ISSN Online: 0976-6553 Journal Impact Factor (2016): 8.1891 (Calculated by GISI) www.jifactor.com © IAEME Publication ANALYSIS OF VARIOUS COMPENSATION DEVICES FOR POWER QUALITY IMPROVEMENT IN WIND ENERGY SYSTEM M. Thirupathaiah and P. Venkata Prasad   Department of EEE, Chaitanya Bharathi Institute of Technology,   Gandipet, Hyderabad, India V. Ganesh Department of EEE, JNTUA College of Engineering, Pulivendula, Y.S.R District, Andhra Pradesh, India ABSTARCT  In recent trend, the renewable source of energy is increasingly used in the electric power generation, which leads to certain power quality issues. Hence some of the supplementary devices like capacitors, compensators or reactive  power injection devices are added to the compensation system. With the advancement in power electronics, compensating devices such as Distribution Static Compensator (DSTATCOM), Unified Power Quality Conditioner (UPQC), Dynamic Voltage Restorer (DVR), Static Var Compensator (SVC) etc. are used. In this paper, the characteristics of three important compensating devices DVR, UPQC and DSTATCOM are analysed. Initially these devices are modelled and their performance is analysed with common  power quality problems such as voltage sag, swell and harmonics. The overall outcome suggests that the performance of the DSTATCOM is comparatively better than that of the other two devices, which become the most suitable device for power quality improvement in wind energy system. Key words: Distribution Static Compensator (DSTATCOM), Dynamic Voltage Restorer (DVR), Power Quality, Unified Power Quality Conditioner (UPQC), Wind Energy System. Cite this Article:  M. Thirupathaiah, P. Venkata Prasad and V. Ganesh, Analysis of Various Compensation Devices For Power Quality Improvement In Wind Energy System.  International Journal of Electrical Engineering & Technology , 7 (3), 2016, pp. 25  –  39 http://www.iaeme.com/ijeet/issues.asp?JType=IJEET&VType=7&IType=3  M. Thirupathaiah, P. Venkata Prasad and V. Ganesh   http://www.iaeme.com/IJEET/index.asp 26 editor@iaeme.com 1. INTRODUCTION Today’s technological world completely depends on electricity; however the availability of electric sources are low. The deficiency of electricity becomes the breaking point for developing countries like India. Hence the electric utilities are finding a suitable solution for providing uninterruptable electricity. In this situation the usage of renewable energy sources are the better solution, so these renewable energy sources are encouraged for electricity production [1, 2]. In India, the most available renewable source is wind and solar and the research on this area is under progress [3, 4]. The wind based energy acquisition is most encouraging research area because of its low complexity in installation and maintenance. In wind energy acquisition the wind turbine is used [5]. The technical challenges that a power system integrated with a wind power requires the analysis of power quality issues such as voltage regulation and stability [6, 7]. The wind turbine produces a continuous variable output. In wind power system, the wind turbine has a great importance to the power quality issues in the power system [8, 9]. The variations in the power output are caused due to wind groom and the disturbances in the power system [10]. Therefore, the system has to manage such variations so that the power quality issues such as voltage sag, voltage swell, flickers and harmonics can be considered w.r.to the generation, transmission and distribution systems of wind power [11]. But, the wind generation produces turbulence into the network. An induction generator directly connected to the grid system can be used to run a wind generation system [12]. Due to the variations in the wind, the active power generated by an induction generator varies which affects the reactive power absorbed by the induction generator and its terminal voltage [13]. In order to provide the control over the active power produced by the induction generator, the wind generation system should work with a précised control technique [14]. At point of common coupling (PCC), a compensator device such as Distribution Static Compensator (DSTATCOM), Unified Power Quality Conditioner (UPQC), Dynamic Voltage Restorer (DVR) & Static Var Compensator (SVC) etc. can be connected for improving the power quality which can manage the challenges of wind turbines [15]. 2. POWER QUALITY ISSUES IN WIND ENERGY SYSTEM The guidelines for measurement of power quality of wind turbine are developed by The International Electro-technical Commission (IEC) in coordination with the Technical Committee-88.This commission explained the methodologies for measuring the power quality characteristics of a wind turbine [16]. For grid connection, the base for the analysis is provided by the data sheet with electrical characteristic of wind turbine. 2.1. Power Quality Issues  2.1.1. Voltage Variations In wind generation system, the velocity of wind and the torque developed by the generator results in the variations in the voltage. This voltage variation results in real and reactive power variations. The fluctuations in the wind power occur during the normal operation of the wind turbine. The magnitude of these fluctuations depends on the impedance, phase angle, power factor of wind turbine and the strength of the grid. Various types of voltage variations are given as follows:  Analysis of Various Compensation Devices For Power Quality Improvement In Wind Energy System   http://www.iaeme.com/IJEET/index.asp 27 editor@iaeme.com    Voltage sag.    Voltage swells.    Voltage flicker.    Short interruptions.    Long duration voltage variation.  2.1.2. Harmonics The harmonics in the wind power system is due to the usage of power electronic equipment. At the connection point of the wind turbine to the system, the harmonic voltages and currents should be within their limits. To ensure the voltage harmonics within the limit, the current harmonics should contribute in a limited manner.  2.1.3. Wind Turbine Location & Self-Excitation of Wind Turbine The quality of the power generated by the wind energy system depends on the way of connecting the generation system into the network. Thus the location of the wind turbine in power system influences the operation of the power system. In general, the Wind Turbine Generating System (WTGS) is provided with a capacitor which results in the risk of self excitation. The capacitor connected to the generator provides the reactive power compensation. In Wind Turbine Generating System (WTGS), the self excitation is provided by a synchronous generator immediately after disconnecting the Wind Turbine Generating System (WTGS) with the load. But the major disadvantages of self excitation are the imbalance between real and reactive power and the safety [17]. 2.2. Consequences The issues mentioned above are the major causes to reduce the power quality of the grid. The voltage variations such as voltage sag, swells, flickers, short and long interruptions and harmonics causes the disoperation of the programmable logic controllers and microprocessor based control systems. Also it results in tripping of protective equipment such as circuit breakers, relays and contactors. Since the control system consists of sensitive equipments like computers, microprocessors and PLCs, the variation in the voltage leads to malfunction and sometimes even damage to this sensitive equipment .  Finally, due to this disoperation and malfunctioning of the equipment the process may get stopped. 2.3. Grid Coordination After the blackout in the United States in August 2003, the grid code was developed by The American Wind Energy Association (AWEA) at the distribution level for stable grid operation. The guidelines of grid operation of wind generating system at the distribution system are defined as-per IEC-61400-21. The organization and operation of interconnected system is governed by the operator of transmission grid [18].  2.3.1. Voltage Swell/Rise At the Point of Common Coupling (PCC), the voltage rise can be approximated as a function of maximum apparent power max S   of the turbine, the phase angle    and the grid impedances R and X at the point of common coupling [19], given in eqn. (1).  M. Thirupathaiah, P. Venkata Prasad and V. Ganesh   http://www.iaeme.com/IJEET/index.asp 28 editor@iaeme.com   2max  sincos U  X  RS u        (1) Where u   - Voltages rise max S  - Maximum apparent power,    - Phase difference, U   - Nominal voltage of grid. The limiting voltage rise value is < 2 %  2.3.2. Voltage Dip/Sag The voltage dip or sag is a sudden reduction in the voltage due to the start of wind turbine. It is mentioned in relative percentage voltage. The decrease in voltage is given in eqn. (2). K nu S S K d     (2) Where d  - Relative voltage change u K  - Sudden voltage reduction factor n S   - Rated apparent power K  S   - Short circuit apparent power. The limiting value of voltage dips is  %3     2.3.3. Flicker The voltage flicker measurements are done with more number of switching operations of wind turbine, as given in eqn. (3).   K nK lt  S S C P    (3) Where    K  C     - Flicker coefficient lt  P - Long term flicker The Limiting Value for flicker coefficient is 4.0  [20]  2.3.4. Harmonics At the point of common coupling, the harmonic distortion is measured for variable speed turbine with an electronic power converter [21]. The total harmonic voltage distortion of voltage is given as in eqn. (4): %100 402 12     hnTHD V V V   (4) Where, V  n  - nth harmonic voltage The THD limit for voltage is < 3 % The THD of current THD  I  is given as in eqn. (5):  Analysis of Various Compensation Devices For Power Quality Improvement In Wind Energy System   http://www.iaeme.com/IJEET/index.asp 29 editor@iaeme.com %100 1     I  I  I   nTHD  (5) Where, n  I  - n th  harmonic current The THD limit for current is < 2.5 %  2.3.5. Grid Frequency In India, the frequency of the grid is maintained in the range 0f 47.5 to 51.5 Hz for wind power systems. 3. POWER QUALITY IMPROVEMENT IN WIND ENERGY SYSTEM The power quality problem can arise in many ways in the wind turbine system, however any of the suitable compensator is well enough to solve the problems and improve the power quality in wind based power system. The grid connected system for improving the power quality at Point of Common Coupling (PCC) is shown in Fig. 1. Mainly it consists of a compensation device such as Distribution Static Compensator (DSTATCOM), Unified Power Quality Conditioner (UPQC), Dynamic Voltage Restorer (DVR) etc., induction generator, source and a non-linear load. The control system of the compensation device injects a harmonic free current into the grid system. To improve the power factor, the reactive part of the load current and the harmonics in the induction generator current are cancelled out by injecting the inverter output current of the compensating device, thus improving the power quality of the grid. To achieve these, the grid voltages are synchronised while generating the current command for the inverter in the control system of the compensating device. Figure 1 Grid connected system for improving Power Quality Because of the cost effectiveness, robust and simplicity the induction generator is used in this method. It can be used for constant and variable loads and also has a natural protection against short circuits. It is assumed that the wind generators in this configuration are working based on constant speed topography and pitch control methods for turbine. (DVR/UPQC/ STATCOM)V s , I s V i , I i V L , I L Non linear loadInduction GeneratorSourceCompensation Device 3-Phase 415 V, 50 Hz  
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