Asian Research Consortium

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1 Asian Research Consortium Asian Journal of Research in Social Sciences and Humanities Vol. 6, No. 6, June 2016, pp Asian Journal of Research in Social Sciences and Humanities ISSN A Journal Indexed in Indian Citation Index DOI NUMBER: / Category: Science and Technology A STATCOM-Control Scheme for Grid Connected Wind Energy System for Power Quality Improvement using PI Controller G.Balaji*; A.Sibichakravarthy** Abstract *Department of Electrical and Electronics Engineering, Paavai Engineering College, Namakkal. **Department of Electrical and Electronics Engineering, Paavai Engineering College, Namakkal. The grid-connecting wind energy generation system for power quality improvement by using STATCOM-control scheme is simulated with the help of MATLAB When the wind power is connected to an electric grid affects the power quality. The effects of connecting wind turbine into the grid system covers power quality areas are the active power, reactive power, harmonics and variation of voltage, and electrical performance of switching operations. The installation of wind system with the grid creates the power quality problems which can be determined by studying this paper. By using the Static Compensator (STATCOM) with using. battery energy storage system (BESS) at the point of common coupling to improving the power quality of the grid The battery energy storage used to keep real power from varying wind power. At lower power demand hours the generated power can be stowed in the batteries. The combination of battery storage with wind energy generation system will even out the grid system by absorbing or injecting reactive power and enable the real power flow essential to the load. This recreates the main supply source from the reactive power demand of the load and induction generator in this proposed scheme. Keywords: International electro-technical commission (IEC), power quality, wind generating system (WGS), battery energy storage system, STATCOM. 53

2 I. Introduction Balaji & Sibichakravarthy (2016). Asian Journal of Research in Social Sciences and Humanities, To have sustainable enlargement and social progress, it is necessary to meet the energy need by utilizing the renewable energy assets like solar, wind, biomass, hydro, co-generation, etc In sustainable energy system, energy conservation and the use of renewable energy source are the key paradigm. The need to integrate the renewable energy like wind energy into the power system is to make it possible to minimize the environmental impact on conventional plant [1]. The integration of wind energy into present power system presents a technical challenges and that requires consideration of voltage regulation, constancy, power quality problems. The power quality is an essential customer-focused measure and is mostly affected by the operation of a distribution and transmission network. The issue of power quality is of great importance to the wind turbine [2].There has been an wide growth and quick development in the exploitation of wind energy in recent years. The individual units can have large capacity up to 2 MW, feeding into distribution network, mainly with customers connected in close proximity [3]. Today, moreover wind generating turbine are successfully functioning all over the world. In the fixed-speed wind turbine operation, all the fluctuation in the wind speed where transmitted as oscillations in the mechanical torque, and electrical power on the grid and leads to large voltage fluctuations. During the normal process, wind turbine creates a continuous uneven output power. These power variations are mostly caused by the effect of turbulence, wind shear, and tower-shadow and of control scheme in the power system. Thus, the network requests to manage for such fluctuations. The power quality issues are viewed with respect to the wind generation, transmission and distribution network, such as voltage sag, swells etc. However the wind energy generator presents instabilities into the distribution network. One of the simple methods of running a wind energy generating system is to use the induction generator linked directly into the grid system. The induction generator has inherent benefits of cost effectiveness and robustness. However; induction generator has require reactive power for magnetization. When thus generated active power of an induction generator is varied due to the variation of wind, absorbed reactive power and incurable voltage of an induction generator can be significantly affected. A proper control system in wind energy generation system is essential under normal operating condition to allow the proper control of the active power production. In the event of growing grid disturbance, a battery energy storage system for an wind energy generating system is usually required to compensate the fluctuation generated by wind turbine. An STATCOM based control technology has been planned for improving the power quality which can technically maintain the power level associates with the commercial wind turbines. The proposed STATCOM control scheme for a grid connected wind energy generation for power quality improvement has the following objectives. Unity power factor at the three phase source side. STATCOM only supports the reactive power to wind Generator and Load. STATCOM use simple bang-bang controller to achieve fast dynamic response. The paper is prearranged as fallows. The Section II presents the standards power quality, issues and its consequences of wind turbine. The Section III presents the grid coordination rule for grid excellence limits. The Section IV describes the topology for power quality improvement. The Sections V, VI, VII describes the control scheme, system performance and conclusion respectively. 54

3 Ii. Power Quality Standards, Issues and its Consequences 1. International Electro Technical Commission Guidelines The guidelines are providing for measurement of power quality of wind turbine. The International standards are developed based on the working group of Technical Committee-88 of the International Electro-technical Commission (IEC), IEC standard , gives the procedure for determining the power quality characteristics of the wind turbine [4]. The standard norms are specified. 1. IEC : Wind turbine generating system, part-21. Measurement and Assessment of power quality characteristic of grid coupled wind turbine. 2. IEC : Wind Turbine measuring procedure in determining the power behavior. 3. IEC : Assessment of emission limit for changing load IEC : Wind Turbine performance. The data sheet with electrical characteristic of wind turbine provides the base for the utility assessment regarding a grid connection. 2. Voltage Variation The voltage deviation issue results from the wind velocity and generator torque. The voltage variation is directly linked to real and reactive power variations. The voltage variation is commonly classified as under: Voltage Sag. Voltage Swells. Short Interruptions. Long duration voltage variation. The voltage flicker issue defines dynamic deviations in the network caused by wind turbine or by varying loads. Thus the power fluctuation from the wind turbine occurs during incessant operation. The amplitude of voltage fluctuation rest on grid strength, network impedance, phase-angle and the power factor of the wind turbines. It is defined as the fluctuation of voltage in a frequency Hz. The IEC stipulates a flicker meter that can be used to measure flicker directly. 3. Harmonics The harmonic arises due to the operation of power electronic converters. The harmonic voltage and current should be limited to the acceptable level at the point of wind turbine linked to the network. To ensure the harmonic voltage within limit, each source of harmonic current can allow only a restricted 55

4 contribution, as per the IEC guideline. The rapid switching provides a large reduction in lower order harmonic current compared to the line commutated converter, but the output current will have higher frequency current and can be easily filter-out. 4. Wind Turbine Location in Power System The way of connecting the wind generating systems into the power system highly influences the power quality. Thus the operations of power system depend on the structure of the adjoining power network. 5. Self Excitation of Wind Turbine Generating System The self excitation of the wind turbine generating system (WTGS) with an asynchronous generator take place after release of wind turbine generating system ( WTGS ) with indigenous load. The risk of self excitation arises especially when WTGS is prepared with compensating capacitors. The connection of capacitor to induction generator compensating the reactive power. The disadvantages of self excitation are the safety aspect and balance between real and reactive power [5]. 6. Consequences of the Issues The voltage variation, flicker, harmonics causes the fail of equipments namely microprocessor based control system, logical controller; adjustable speed drives, flickering of light and screen. It may leads to tripping of contractors, tripping of protection devices, strike of sensitive equipments like personal computer, programmable logic control system and it may stop the process and even damage of sensitive equipments. Thus it degrades the power quality in the grid. III. Grid Coordination Rule American Wind Energy Association (AWEA) lead the effort to develop its own grid code for stable operation as per IEC for the interconnection of wind plant life to the utility systems, after the block out in United State in August According to these, operation of transmission grid is responsible for the organization and operation of interconnected system. 1) Voltage Rise (u) The voltage increase at the point of common coupling can be approximated as a function of maximum apparent power Smax of the wind turbine, the grid impedances R and X at the point of common coupling and the phase angle, given in Eq. 1. Δu = smax (RcosΦ-XsinΦ) (1) ² Where voltage rise, smax max. apparent power, 56

5 Φ phase difference U nominal voltage of grid. The Limiting voltage rise value is <2 % 2) Voltage Dips (d) The voltage dips is reduction in voltage due to startup of wind turbine and it causes a sudden reduction of voltage. It is the relative some % voltage change due to switching operation of wind turbine. The decrease of nominal voltage change is given in Eq. 2. D = Ku (sn/sk) (2) Where d is relative voltage change, sn is rated apparent power, sk is short circuit apparent power, and Ku is sudden voltage reduction factor. The acceptable voltage dips limiting value is <3%. 3) Flicker The measurements are made for maximum number of specific switching operation of wind turbine with 10min period and 2-h period are specified, as given in Eq. 3. P = c (ψk )sn/sk (3) Where P Long term flicker. c (ψk ) Flicker coefficient The Limiting Value for flicker coefficient is about 0.4, for average time of 2 h. 4) Harmonics The harmonic distortion is assess for variable speed turbine with a electronic power converter at the point of common connection. The total harmonic distortion voltage of voltage is given as in Eq. 4. Where Vn is the nth harmonic voltage and V1 is the fundamental frequency (50) Hz. 57

6 The THD limit for 132 KV is < 3%. THD of current ITHD is given as in Eq. 5 Where In is the nth harmonic current and I1is the fundamental frequency (50) Hz. The THD of current and limit for 132 KV is <2.5%. 5) Grid Frequency The grid frequency in India is specified in the range of Hz, for wind farm connection. IV. Topology for Power Quality Improvement The STATCOM based current control VSI injects the current into the grid in such a way that the source current are harmonic free and their phase-angle regarding source voltage has a desired value. The injected current will cancel out the harmonic part and reactive part of the load and induction generator current, thus it improves the power factor and the power quality. To attain these goals, the grid voltages are sensed and are synchronized in generating the current command for the inverter. The projected grid connected system is implemented for power quality improvement at PCC, as shown in Fig. 1. The grid coupled system in Fig. 1, consists of wind energy generation system and battery energy storage system with STATCOM. Fig.1 Grid Connected System for Power Quality Improvement 58

7 1.Wind Energy Generating System In this arrangement, wind generations are based on constant speed topologies with pitch control turbine. The induction generator is used in the proposed system because of its simplicity, it does not require a separate field circuit, it can accept constant as well as variable loads, and has natural protection against short circuit. The available power of wind system is presented as under in below equation Where ρ (kg/m ) is the air density and A (m) is the area swept out by turbine blade, V wind is the wind speed in mtr/s. It is not possible to take out all kinetic energy of wind, thus it extract a fraction of power in wind, called power coefficient Cp of the wind turbine. 2. System Operation The shunt connected STATCOM with the battery energy storage is connected with the interface of the induction generator and non-linear load at the PCC in the grid connected system. The STATCOM compensator output is varied according to the controlled approach, so as to maintain the power quality norms in the grid system. The current control strategy is included in the control system that defines the functional operation of the STATCOM in the power system. A single STATCOM using insulated gate bipolar transistor (IGBT) is proposed to have a reactive power support, to the induction generator and to the linear as well as nonlinear load in the grid system. The main block diagram of the system operational scheme is shown in Fig. 2. Fig.2 System Operational Scheme in Grid System 59

8 3. Control Scheme of System Balaji & Sibichakravarthy (2016). Asian Journal of Research in Social Sciences and Humanities, The control scheme of Grid Connected Wind System approach is based on injecting the current into the grid using hysteresis band. Using such techniques controller keep the control system variables between the boundaries of hysteresis area and thus may gives correct switching signals. The control algorithm needs the measurements of several variables such as three-phase current source i sabc, DC voltage V dc, inverter current i iabc with the use of sensor. The current control block, receives an input reference current i* sabc and actual i sabc current are subtracted so as to activate the operation of STATCOM in current control mode Hence; the inverter maintains the continuous power for the critical load. 4. Hysteresis Based Current Controller Fig. 3. Control System Scheme The current control based hysteresis controller is used in this scheme. The reference current is generated as in and thus actual current is detected by current sensors that are subtracted for obtaining current errors for a hysteresis based controller. ON/OFF pulse signals for IGBT switches of inverter are derived from hysteresis current controller. When the generated measured current is higher than the generated reference current, it is necessary to get negative inverter output voltage so that equivalent switches are commutated. Thus output voltages are decreased so that the output current reaches the reference current. Also, if the measured current is less than the reference current, positive inverter output voltage are attained by commutating particular switch Thus output current increases to the reference current. Hence, the output current will be inside a band around the reference one. The switching function SA for phase a is expressed as follows: i sa (4) i sa (5) Where HB is a hysteresis current-band, likewise the switching function SB, SC can be derived for phases b and c, respectively [9]. The current control mode of an inverter injects the current into the grid in such a way that the source currents are mostly harmonic free and their phase-angles are in phase with respect to source voltage. The reactive and harmonic part of the load 60

9 side is cancel out by the injected current at shunt part. Thus, overall it reduces harmonic content and improving the source current quality at the PCC. As soon as battery energy system is fully charged with the help of micro-wind generator, and the power transfers take place [4]. The source voltage is sensed and synchronized in generating the preferred reference current command for the inverter operation. V. Simulink Model and Results of the Conventional System The proposed system is implemented using MATLAB Simulation. The proposed circuit is drawn in MATLAB and the output is obtained from the Static Compensator (STATCOM) by changing the duty cycle using PI controller. Figure 4 shown below is the MATLAB model of Conventional system. Fig.4 Simulation Model of Conventional System The system having one conventional Three phase source, wind turbine generating system, STATCOM with Battery Energy Storage System (BESS), IGBT pulse control subsystem and load. The power factor correction capacitor is connected with wind energy generation system shown in Figure 3. The capacitor connected to asynchronous generator provides reactive power compensation. A wind turbine is a device that may converts kinetic energy from the wind, also called wind energy, into mechanical energy; a process is known as wind power. If the mechanical 61

10 energy is used to produce electricity, the device may be called as wind turbine or wind power plant. The result of a millennium windmill development is now modern engineering. Fig.5 Three Phase source Voltages and Current The effectiveness of the proposed method is demon-started through simulation result of grid voltage and current shown in Figure 4. This is due to the reference derived from the grid voltage. Figure 5 shows Three Phase source voltages and Current from grid or other source. Where figure 6 three phase source voltage and current waveform of wind turbine thus may shows some oscillations due to startup of wind turbine and the load. Fig.6 Three Phase source voltage and Current waveform of Wind Turbine 62

11 Fig 7. The Output Voltage Waveform without STATCOM Figure 7 the output load voltage waveform without statcom shows oscillation and voltage dip upon 0.15 sec, similar changes may occur in source voltage and source current. It may stabilize using STATCOM. When STATCOM controller is made ON, without any change in load condition parameters, it starts to mitigate for reactive power demand as well as harmonic current. The dynamic performance is also conceded by step change in a load, when applied at 0.01 sec. This additional demand is achieve by STATCOM compensator. Thus, STATCOM can regulate the available real power from source. The inverter output voltage under STATCOM operation with load variation is shown in Fig. 8. and the generated current from wind generator at PCC are portrayed in Fig. 9. Fig.8 Statcom Output Voltage 63

12 Fig.9 Inverter Injected Current Fig 10. The Output Voltage Waveform with STATCOM 64

13 Fig 11. FFT analysis without STATCOM Fig 12. FFT analysis with Statcom 65

14 Thus figure 10 shows the output voltage waveform with STATCOM it may stabilize voltage sag. Then Figure 11 shows FFT analysis without STATCOM it has total harmonic distortion of 2.30%. Figure 11 shows FFT analysis with STATCOM it has total harmonic distortion of 1.91%. VI. Conclusion In this paper presents the STATCOM-based control scheme for power quality improvement in grid connected wind generating system and with load. The power quality issues and its consequences on the consumer and electric utility are presented. The STATCOM-BESS of the control system was developed in MATLAB/SIMULINK for maintaining the power quality is simulated. It has an ability to cancel out the harmonic parts of the load current. It maintains the source voltage and current in-phase and support the reactive power demand for the wind energy generator and load at PCC in the grid system, thus it gives an opportunity to improve the utilization factor of transmission line. The integrated wind generation and STATCOM with BESS have shown the outstanding performance. References A. Sannino, Global power systems for sustainable development, in IEEE General Meeting, Denver, CO, Jun K. S. Hook, Y. Liu, and S. Atcitty, Mitigation of the wind generation integration related power quality issues by energy storage, EPQU J., vol. XII, no. 2, R. Billinton and Y. Gao, Energy conversion system models for adequacy assessment of generating systems incorporating wind energy, IEEE Trans. on E. Conv., vol. 23, no. 1, pp , 2008, Multistate. Wind Turbine Generating System Part 21, International standard-iec , J. Manel, Power electronic system for grid integration of renewable energy source: A survey, IEEE Trans. Ind. Electron., vol. 53, no. 4, pp , 2006, Carrasco. M. Tsili and S. Papathanassiou, Areview of grid code technology requirements for wind turbine, Proc. IET Renew.power gen., vol. 3, pp , S. Heier, Grid Integration of Wind Energy Conversions. Hoboken, NJ: Wiley, 2007, pp J. J. Gutierrez, J. Ruiz, L. Leturiondo, and A. Lazkano, Flicker measurement system for wind turbine certification, IEEE Trans. Instrum. Meas., vol. 58, no. 2, pp , Feb Indian Wind Grid Code Draft report on, Jul. 2009, pp , C-NET. C. Han, A. Q. Huang, M. Baran, S. Bhattacharya, and W. Litzenberger, STATCOM impact study on the integration of a large wind farm into a weak loop power system, IEEE Trans. Energy Conv., vol. 23, no. 1, pp , Mar

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