International Journal of Engineering Trends and Technology (IJETT) Volume-4 Number-5 - October 216 Enhancement of Power Quality using D-Statcom Fed Induction Motor Drive ABSTRACT--- D-STATCOM is used to compensate the source currents which are affected by the harmonics due to unbalanced and nonlinear loads. Here a PV and WIND FARM based inverter is used as a shunt active power filter to mitigate the current harmonics. The theory of reference frame is used to generate the three phase reference currents. Hysteresis current controller (HCC) is used to generate the switching pulses for the gate drives of the grid interfacing inverter. The inverter acts as a shunt active power filter to inject the compensated current to the system. The total harmonic distortion (THD's) of the source currents are reduced by using shunt active power filter (APF). The THD's of the distribution system with and without APF are compared. The whole work is done in MATLAB/SIMULINK. Keywords- Total harmonic distortion, hysteresis current controller (HCC), Active power filter (APF), distribution static Compensator. I. INTRODUCTION Due to increasing pollution, heating concerns, decreasing fossil fuels and their increasing value have made it necessary to seem towards renewable sources as a future energy resolution [1]. There are several Renewable Energy Sources (RES) reminiscent of wind, solar, tidal power, biomass etc. Solar energy has great potential to produce energy with minimum impact on the surroundings, since it's clean and pollution free. To find solutions to beat a worldwide energy crisis, the Photo Voltaic (PV) system has attracted significant awareness in recent years. The govt is providing incentives for increasing the utilization of grid connected PV systems. Conventionally, grid connected Photo Voltaic energy conversion systems are composed of associate inverter. Renewable energy sources (RES)integrated at distribution system is also known as distributed generation (DG). After generation, we have to integrate it with already existing power system by using power electronic devices. Generally, current controlled voltage 1 B.Kishore Kumar Naik, M.Devika Rani 2 1 PG Student, Assistant Professor 2, Department Of EEE PVP Siddhartha Institute of Technology Vijayawada, India source inverters are used to interface the intermittent RES within the distributed system. A few control methods for grid connected inverters incorporating PQ resolution have been proposed. The inverter acts as an active inductor at a definite frequency to absorb the harmonic current [2]. However the precise calculation of network inductance in real time is tough and will decrease the control performance.[4] A control technique for renewable interfacing inverter based mostly on p-q theory is proposed. during this work, load and also the inverter current sensing is needed to compensate the load current harmonics. The intensive use of those non-linear loads causes harmonic injection into the system that affects the quality of the power supply [3]. These harmonics current causes problems like equipment heating, supplementary losses, EMI related problems and may harm devices etc. Harmonics are introduced into the system by diode or thyristor loads. These harmonics are to be filtered to make the system behavior as per the planned operation. So as to eliminate these harmonics and load devices by using some controlling techniques like p-q theory. during this work, a photovoltaic and wind farm based inverter is employed as a shunt active power filter (APF) to compensate these current harmonics and current unbalance due to unbalanced and non-linear loads. This APF is connected in shunt to the system as it injects current for harmonic compensation for increased power quality. The recent advances within the power semiconductor technology have led to the development of high power switches like IGBT s, GTO s and thyristor s that has enabled the practical implementation of active power filters. Different kinds of active power filters comparable to shunt, series and shunt series/series-shunt have evolved (Singh et ai, 1999). These filters applied to power distribution networks are referred as custom power devices [5], [3]. Here this PV and wind farm based inverter is connected in shunt with the system. Synchronous reference frame theory controlling technique is ISSN: 2231-5381 http://www.ijettjournal.org Page 251
International Journal of Engineering Trends and Technology (IJETT) Volume-4 Number-5 - October 216 employed to generate the reference signals. Hysteresis current controller is used to generate switching pulses to drive the gates of the PV and wind farm based grid interfacing inverter. Thus with the use of the HCC control technique, the inverter can keep the supply current balanced. Most of the power quality problems are due to harmonics, unbalance and low power factors within the load currents. Hence, these issues can be mitigated using D-STATCOM and are implemented practically by adding a Induction motor as another load we can check the performance of induction motor drive in industrial application. mechanical power. A generator can convert this mechanical power into electricity to power homes, businesses, schools etc. C. Voltage Source Current Controlled Interfacing Inverter A voltage source current inverter is a power electronic device which is connected in shunt with the system. The function of this inverter is to convert the dc voltage into a balanced 3 phase ac voltage. If the inverter output voltage is greater than the existing system voltage then the inverter acts in capacitive mode. The switching device employed in this voltage supply inverter is an IGBT. D. Control Technique for Grid Interfacing Inverter as Shunt Active power filter Fig.1.Block diagram of three phase four wire shunt active power filter II. SYSTEM DESCRIPTION The above system consists of a photovoltaic cell and wind farm as a RES connected to the dclink of a grid interfacing inverter as shown in Fig.l. The voltage source inverter (VSI) interfaces the renewable energy source to the grid and delivers the generated power. A. Photo Voltaic Panel PV cell is an energy conversion device, which is used to convert the solar energy into an electrical energy and the amount of electrical energy produced depends upon solar irradiation and temperature. The turn ON and turn OFF instants of the inverter switches should be such that the load and the connected RES could be appeared as a balanced load to the system. For this type of control, we need to monitor the output of dc link capacitor continuously and is compared with the reference voltage V * dc. The difference between the reference and actual voltages will go through a voltage regulator, whose final output gives an active current component I m. By multiplying this peak value (I m ) with three unit sine vectors (U a, U b and U c ) which are in phase with the three source voltages will generate the reference current (I a *, I b * and I c *). The reference grid neutral current (I n *) is set to zero being the instantaneous sum of balanced grid currents. The synchronizing angle (9) is obtained from phase locked loop (PLL) [I] is used to generate unity vector template as U a Sin U 2 b Sin( 3 U c Sin( 2 ) 3..(1) )..(2).....(3) B. Wind Farm The terms "wind energy" or "wind power" describe the process by which the wind is used to generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind into ISSN: 2231-5381 http://www.ijettjournal.org Page 252
International Journal of Engineering Trends and Technology (IJETT) Volume-4 Number-5 - October 216 Fig.3. Wave-forms of Hysteresis Current Control Fig.2. Block diagram of grid interfacing inverter control. The reference grid currents of the three phase system is given as I a * Im *U a...(4) I b * Im *U b... (5) I c * Im *U c... (6) The neutral current is taken as I n * =...(7) The reference grid currents(i a *,Ib *, Ic * ) are compared with actual grid currents(i a, I b, I c )to compute the current error as I aerr I a * Ia.. (8) I berr I b * Ib.. (9) I cerr I c * I c. (1) E. Hysteresis current control In this work, the hysteresis current control operation is used to control the operation of the VSI. The gate control signals for the grid interfacing inverter to act as a shunt active filter for compensating current harmonics is given by hysteresis current control signals. An error signal is used to control the switches in a voltage source inverter. The difference between the desired current Ia' and the current being injected by the inverter Ia is taken as error. If the error exceeds the upper limit of the band, the upper switch of the inverter is turned OFF and the lower switch is turned ON, which implies that the current starts decreasing. If the error crosses the lower limit of the band, the lower switch of the inverter is turned OFF and the upper switch is turned ON, which implies that the current starts increasing and gets back into the band. The upper and maximum values of the error signal are e min and e max respectively. The range of the error signal is e max - e min directly controls the amount of ripple content in the current output from VSI. F. INDUCTION MOTOR Induction Motors are cheap compared to DC and Synchronous Motors. During this age of competition, this is often a prime requirement for any machine because of its economy of procurement, installation and use, the Induction Motor is usually the primary selection for an operation. Induction Motors have high efficiency of energy conversion. Additionally they're terribly reliable owing to their simplicity of construction, Induction Motors have terribly low maintenance costs. Induction Motors have very high starting torque. This property is beneficial in applications wherever the load is applied before beginning the motor. Another major advantage of the Induction Motor over different motors is the ease with which its speed is controlled. Different applications need different optimum speeds for the motor to run at. Speed control is necessary in Induction Motors because of the subsequent factors: i. Smooth operation. ii. Provides torque control and acceleration control. ISSN: 2231-5381 http://www.ijettjournal.org Page 253
International Journal of Engineering Trends and Technology (IJETT) Volume-4 Number-5 - October 216 iii. Different processes need the motor to run at different speeds. iv. It compensates for fluctuating process parameters. All these factors present a strong case for the implementation of speed control or variable speed drives in Induction Motors. III.SIMULATION RESULTS The proposed simulation work is done in MATLAB/SIMULINK software. The system under various load conditions is shown as follows A. Balanced Nonlinear Load 1 Source current Fig.5.THD of source current before compensation. The source current of balanced Non-linear load before compensation is 3.15%. -1.5.1.15.2.25.3 Load Current 1-1.5.1.15.2.25.3 Inverter current 1-1.5.1.15.2.25.3 Source voltage 5-5.5.1.15.2.25.3 Time Fig.4. Simulation results of Balanced Non-linear Load (a) Source current (b) Load current (c) Inverter current (d) Source voltage. It can be observed from above Fig.4(a) that from to.1 sec the waveform of source current is non-sinusoidal due to balanced non-linear load. When the inverter is turned ON at.1 sec it will starts compensating the non-sinusoidal current into sinusoidal. The FFT analysis of the Balanced Nonlinear load before and after compensation can be shown in Fig.8.and Fig.9. These figures show that there is a reduction in the harmonic content, when the system is connected with APF. Fig.6.THD of source current after compensation The source current of balanced Non-linear load after compensation is 3.27%. 1 B. Unbalanced Nonlinear Load Source current -1.5.1.15.2.25.3 Load Current 2-2.5.1.15.2.25.3 Inverter current 1-1.5.1.15.2.25.3 Source voltage 5-5.5.1.15.2.25.3 Time Fig.7. Simulation results of Unbalanced Nonlinear Load (a) Source current (b) Load current (c) Inverter current (d) Source voltage. ISSN: 2231-5381 http://www.ijettjournal.org Page 254
International Journal of Engineering Trends and Technology (IJETT) Volume-4 Number-5 - October 216 It can be observed from Fig. 7. (a) That from to.1 sec the waveform of source current is nonsinusoidal due to unbalanced non-linear load. When the inverter is turned ON at.1 sec it will starts compensating the non-sinusoidal current into sinusoidal. The FFT analysis of the unbalanced Non-linear load before and after compensation can be shown in Fig.8 and Fig.9. TABLE:1 PERCENTAGE OF SOURCE CURRENT DUE TO VARIOUS LOADS LOADS WITHOUT WITH APF APF Balanced Non-linear 3.15% 3.27% Unbalanced Non-linear 16.5% 3.1% Fig.8.THD of source current before compensation. The source current of Unbalanced non-linear load before compensation is 16.5%. Comparison of the source current THD with and without STATCOM is shown in the above table. Here we can see that the THD Percentages of source currents with APF are less as compared to THD Percentages without APF. This shows that the active power filter is the best solution to mitigate the current harmonics due to unbalanced and Non-linear loads. The performance of the induction machine is observed by adding a induction motor in series as load in addition to existing load ie., multiple loads. The speed torque characteristics of induction motor are as shown in Fig.1. 4 2-2.1.2.3.4.5.6.7.8 2 1-1.1.2.3.4.5.6.7.8 1 5 torque -5.1.2.3.4.5.6.7.8 Time Fig.9.THD of source current after compensation. The source current of Unbalanced Non-linear load after compensation is 3.1%. A. THD Analysis of source currents with and without D-STATCOM Fig.1.Speed torque characteristics of Induction motor IV CONCLUSION The performance of a shunt active filter is studied by using multi-function grid interfacing inverter under various load conditions. The power quality issues like current harmonics, current unbalance due to unbalanced and non linear load connected to the PCC is compensated effectively by using shunt active power filter (APF). ISSN: 2231-5381 http://www.ijettjournal.org Page 255
International Journal of Engineering Trends and Technology (IJETT) Volume-4 Number-5 - October 216 The hysteresis current controller is used to generate the switching pulses for the gate drives of grid interfacing inverter. The THD s of the distribution system with and without APF are compared and the performance of Induction motor drive in industrial application and speed torque characteristics are observed. REFERENCES [ 1 ] Mukhtiar Singh, Student Member, IEEE, Vinod Khadkikar, Member,IEEE, Ambrish Chandra, Senior Member, IEEE and Rajiv K. Varma,Senior Member,IEEE "Grid Interconnection of Renewable Energy Sources at the Distribution Level With Power-QualityImprovement Features."IEEE Transactions On Power. [ 2 ] U. Borup, F. Blaabjerg, and P. N. Enjeti, "Sharing of nonlinear load in parallel-connectedthree-phase converters," IEEE Trans. Ind. Appl.,vol. 37, no. 6, pp. 1817-1823, Nov.!Dec. 21. [ 3 ] J. H. R. Enslin and P. J. M. Heskes,-Hannonic interaction between a large number of distributed power inverters and the distribution network, IEEE Trans. Power Electron., vol. 19, no. 6, pp. 1586-1593, Nov. 24. [ 4 ] J. P. Pinto, R. Pregitzer, L. F. C. Monteiro, and J. L. Afonso, "3-phase 4-wire shunt active power filter with renewable energy interface," presented at the Conf. IEEE Rnewable Energy & Power Quality, Seville, Spain, 27. [ 5 ] J. M. Guerrero, L. G. de Vicuna, J. Matas, M. Castilla, and J. Miret, "A wireless controller to enhance dynamic perfonnance of parallel inverters in distributed generation systems," IEEE Trans. Power Electron.,vol. 19, no. 5, pp. 125-1213, Sep. 24. [ 6 ] Karuppanan P,Kamala KantaMahapatrab-PLL Synchronization With PID Controller Based on Shunt Active Power Line FI L TER.International Journal of Computer and Electrical Engineering,VoI.3,No.l,February 211. [ 7 ] M. Aiello, A. Catalioti, S. Favuzza, G. Graditi,-Theoretical and Experimental Comparison of Total Harmonic Distortion Factors for the evaluation of Harmonic and Inter harmonic Pollution of GridConnected." IEEE Transactions On Power Delivery, Vol. 21, No. 3, July 26. [ 8 ] J. M. A. Myrzik, and M. Calais, Member, IEEE -String and Module Integrated Inverters for Single-Phase Grid Connected Photovoltaic Systems A Reviewll.Power Technical Conference Proceedings, 23 IEEE Bologna. ISSN: 2231-5381 http://www.ijettjournal.org Page 256