INTERNATIONAL JOURNAL OF ELECTRICAL ISSN 0976 – 6545(Print), International Journal of Electrical Engineering and Technology (IJEET),ENGINEERING & ISSN 1, January-June (2012), © IAEME 0976 – 6553(Online) Volume 3, IssueTECHNOLOGY (IJEET) ISSN 0976 – 6545(Print) ISSN 0976 – 6553(Online) Volume 3, Issue 1, January- June (2012), pp. 365-376 © IAEME: www.iaeme.com/ijeet.html IJEET Journal Impact Factor (2011): 0.9230 (Calculated by GISI) www.jifactor.com ©IAEME WIND FARMS RESPONSE IMPROVEMENT USING STATIC COMPENSATOR CONTROL Haider Muhamad Husen1, Laith O. Maheemed2, Prof. D.S. Chavan 3 ABSTRACT This paper presents one of the effective control strategy to enhance the terminal voltage and current of the wind energy system (WECS) using static compensator method . The paper deals with the DFIG turbine generator due to the wide use of it and shows its dynamics response under steady state condition and constant wind speed . In this paper fixed wind speed is used because Voltage stability is a major issue achieved the uninterrupted operation of these types of wind turbines. A detailed analysis of the operating characteristics of the various inverter topologies are suggested in the paper to analyze the performance of the STATCOM connected in shunt with the wind energy conversion system. The complete digital simulation of the wind energy conversion system is performed in the MATLAB/SIMULINK environment and the results are presented to be under study of the proposed topology. Key words: Wind Energy system (WECS) , Double fed induction generator (DFIG) , STATCOM , Power compensation. I. INTRODUCTION Wind energy is one of the top growing renewable energy technologies in recent years in the world because of its clean and removable characters. In 2008, wind energy comprised 4% of the European Union’s electrical generation with this figure expected to reach 12% by 2020  . The Fig (1) shows that it grew over the period between1997-2011. Electrical utilities and heavy industries face a number of challenges related to reactive power. Heavy industrial applications can cause phenomena like voltage unbalance, distortion or flicker on the electrical grid. Electrical utilities may be confronted with phenomena of voltage sags, poor power factor or even voltage instability. Reactive power control can resolve these issues. Wind power very often is a source of voltage fluctuating and flicker. The dominating type of wind generators are induction generators, since they are robust and cost effective. But they do not contribute to voltage regulation because they are substantial absorbers of reactive power. Voltage control problems are remedied by installation of fixed and Mechanically Switched Capacitors (MSC). Regular voltage flicker and voltage fluctuations caused by varying output wind generators cannot be 365 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME solved using conventional methods . To solve this problem dynamic reactive power compensation must be provided which improves the voltage stability and makes the wind farm to meet the grid code requirements. To achieve this, FACTS controller STATCOM  plays a vital role in the voltage stability in power system network. STATCOM provides superior voltage support capability with its nature of voltage source . Fig.1.Wind energy growth over the period between 1997-2011 Power factor correction and reactive power control are the other two major characteristics of the proposed system. II. WIND ENERGY CONVERSION Wind turbines capture power from the wind by means of aerodynamically designed blades and convert it to rotating mechanical power . The number of blades is three. As the blade tip-speed should be lower than half the speed of sound the rotational speed will decrease as the radius of the blade increases. For multi-MW wind turbines the rotational speed is typically 10-15 rpm. The most weight efficient way to convert the low-speed, high torque power to electrical power is to use a gear- box and a standard fixed speed generator as illustrated in Fig. 2. The gear-box is optional as multi-pole generator systems are also possible solutions. Between the grid and the generator a power converter can be inserted. The possible technical solutions are many and a technological roadmap starting with wind energy/power and converting the mechanical power into electrical power is shown in Fig. 5  and . The electrical output can either be AC or DC. In the last case a power converter will be used as interface to the grid. The development in wind turbine systems has been steady for the last 25 years and four to five generations of wind turbines exist and it is now proven technology. It is important to be able to control and limit the converted mechanical power at higher wind speed, as the power in the wind is a cube of the wind speed. The power limitation may be done either by stall control (the blade position is fixed but stall of the wind appears along the blade at higher wind speed), active stall (the blade angle is adjusted in order to create stall along the blades) or pitch control (the blades are turned out of the wind at higher wind speed)  and . 366 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME Fig. 2. Converting wind power to electrical power in a wind turbine The basic output characteristics of these three methods of controlling the power are summarized in Fig.6. A fixed speed wind turbine has the advantages of being simple, robust, reliable, well proven and with low cost of the electrical parts. Its direct drawbacks are the uncontrollable reactive power consumption, mechanical stress and limited power quality control. Due to its fixed speed operation, wind speed fluctuations are converted to mechanical torque fluctuations, beneficially reduced slightly by small changes in generator speed, and transmitted as fluctuations into electrical power to the grid. The power fluctuations can also yield large voltage fluctuations in the case of a weak grid and thus, significant line losses  and . The variable speed wind turbines are designed to achieve maximum aerodynamic efficiency over a wide range of wind speed . By introducing the variable speed operation, it is possible to continuously adapt (accelerate or decelerate) the rotational speed of the wind turbine to the wind speed, in such a way that tip speed ratio is kept constant to a predefined value corresponding to the maximum power coefficient. Contrary to a fixed speed system, a variable speed system keeps the generator torque nearly constant, the variations in wind being absorbed by the generator speed changes. Seen from the wind turbine point of view, the most important advantages of the variable speed operation compared to the conventional fixed speed operation are: reduced mechanical stress on the mechanical components such as shaft and gearbox, increased power capture and reduced acoustical. Additionally, the presence of power converters in wind turbines also provides high potential control capabilities for both large modern wind turbines and wind farms to fulfill the high technical demands imposed by the grid operators , such as: controllable active and reactive power (frequency and voltage control); quick response under transient and dynamic power system situations, influence on network stability and improved power quality. III. WIND TURBINE MODEL Wind turbines convert the kinetic energy present in the wind into mechanical energy by means of producing torque. Since the energy contained by the wind is in the form of kinetic energy, its magnitude depends on the air density and the wind velocity. The wind power developed by the turbine is given by the equation (1) : where Cp is the Power Co-efficient, ρ is the air density in kg/m3, A is the area of the turbine blades in m2 and V is the wind velocity in m/sec. 367 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME Figure 3 . wind turbine characteristics Figure 4. MATLAB/ Simulink model for wind turbine The power coefficient Cp gives the fraction of the kinetic energy that is converted into mechanical energy by the wind turbine. It is a function of the tip speed ratio λ and depends on the blade pitch angle for pitch-controlled turbines. The tip speed ratio may be defined as the ratio of turbine blade linear speed and the wind speed: Substituting (2) in (1), we have: The output torque of the wind turbine Tturbine is calculated by the following equation (4). Where R is the radius of the wind turbine rotor (m) There is a value of the tip speed ratio at which the power coefficient is maximum . Variable speed turbines can be made to capture this maximum energy in the wind by operating them at a blade speed that gives the optimum tip speed ratio. This may be done by changing the speed of the turbine in proportion to the change in wind speed. Fig.3 shows how variable speed operation will allow a wind turbine to capture more energy from the wind and fig. 4 shows the simulink model of the wind turbine. As one can see, the maximum power follows a cubic relationship. For variable speed generation, an induction generator is considered attractive due 368 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME to its flexible rotor speed characteristic in contrast to the constant speed characteristic of synchronous generator. IV. REACTIVE POWER AND VOLTAGE CONTROL Steady-state analysis shows that the proposed application has two main effects on WECS operation: 1) Power loss increase: harmonic current flow results in additional winding and solid-state switches loss; 2) Voltage distortion: harmonic current flow causes harmonic voltage drop on the line connecting the WECS to the PCC. This condition leads to peak voltages at the stator and power converter terminals exceeding the rated values. The power loss increase requires WECS derating for wind speeds above the design value (vw,n =12 m/s for the turbine assumed in the present study, Fig. 3). Voltage distortion and consequent peak voltages that exceed the rated value require a conservative choice of WECS components, in particular of the solid-state devices.. Both the RSC and the GSC can be applied to control the reactive power of the DFIG . In the d-q synchronous reference frame, the RSC and GSC reactive power controllers generate the reference signals for the inner-loop current controllers of the RSC and GSC, respectively. The commands of the reactive power controllers can be generated by a supervisory controller of the wind farm, which in turn can be designed to control for example the power factor or the voltage at the grid connection point of the wind farm at a desired value. The RSC and the GSC of the DFIG can also be applied to control directly the voltage at the grid connection point of each individual wind turbine . This consideration is reasonable because the VFC rating is only 25-30% of the generator rating and the VFC is primarily used to supply the active power from the rotor to the power grid. Moreover, if the DFIG feeds into a weak power network without any local reactive compensation, both the RSC and the GSC can be applied to control the terminal voltage of the DFIG. This control mode however needs control coordination between these two converters. The use of voltage control can mitigate terminal voltage fluctuations of the DFIG caused by the variations of the wind speed, and therefore, improve the power quality when the wind turbine is connected to a weak power network . The voltage control of the GSC is also useful to help reestablish the grid voltage during a grid fault when the RSC has been blocked . In some power networks, the controllability of DFIG wind turbines is sufficient to meet grid code requirements on power factor or voltage (reactive power) regulation. However, many wind turbines are installed in remote, rural areas. These areas usually have electrically weak power grids, characterized by low short circuit ratios and under-voltage conditions. In such grid conditions and during a grid fault, the DFIGs may not be able to provide sufficient reactive power support. Therefore, it would be necessary to use external reactive compensation (e.g., STATCOM) to assist the wind turbines with reactive power and voltage support in order to maintain required power factor and voltage stability . In addition, in order to achieve certain optimal operating performance and economical benefits, it is necessary to coordinate the reactive power or voltage control actions of the wind farm and the external reactive compensator in an intelligent way . 369 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME VI. STATCOM CONTROL STRATEGY OVERVIEW The STATCOM is based on a solid – state voltage source implemented with an inverter and connected in parallel to the power system through a coupling reactor . A STATCOM consists of an inverter and a DC capacitor as shown in Fig.5. With this arrangement, this device can supply both leading and lagging Vars with a small energy storage device. The AC current is the result of interaction of the converter output voltage with the AC system and can have any phase relationship with respect to the voltage. It is always a combination of two controlled switches and one uncontrolled switch conducting and vice versa. The converter supplies real power from its DC capacitor to the AC system if the converter output voltage leads the corresponding AC system voltage whereas the converter absorbs real power from the AC system if the converter output voltage is made to lag the AC system voltage. The amount of exchanged real power is typically small in steady state; hence, the firing angle is also small. The real power that is being exchanged by the transmission system must be supplied or absorbed at its DC terminals by the DC energy storage. In contrast, the reactive power exchange is internally generated or absorbed by the voltage-sourced converter, without the DC energy storage device playing any part in it. Fig.6 shows the block diagram of phase angle control scheme implemented in STATCOM. A STATCOM is configured to keep the reactive power delivered by the source at a zero value as long as the reactive demand is within the STATCOM rating . The control of STATCOM reactive power is by pure α control. The source side voltages and currents are sensed and the reactive power is calculated by analog/pulse circuitry. This calculated value is compared with the desired value (usually zero) and the error is processed in a Proportional-Integral Controller . Fig .5 STATCOM Block Diagram The error output decides the phase shift needed in the inverter output in order to develop the required DC bus voltage such that the inverter output voltage magnitude will be sufficient to make the Inverter deliver the var required by the load. The inverter is gated by a fixed PWM pattern optimized for eliminating chosen harmonics (usually fifth, seventh, eleventh etc; triple harmonics need not be eliminated since they do not result in current flows in a three wire system). The PWM technique is commonly employed to generate high quality output waveforms by relatively low power converter used in wind power applications. With this technique, the output of each converter pole is switched several times during a fundamental cycle between the positive and negative terminals of the DC source.PWM requires a considerable increase in the number of switch operations ,thereby it 370 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME generally increases the switching losses of the converter. However, the always increasing switching frequency of modern solid-state power switches used in FACTS controllers made possible the use of PWM in high power applications . The aim of the PWM control scheme is to maintain constant voltage magnitude at the point of common coupling, under system disturbances. The control system only measures the rms voltages at the PCC, i.e., no reactive power measurements are required. With these converters, the ac output voltage can be controlled, by varying the width of the voltage pulses. The basic diagram of the wind energy conversion system to be analyzed on this paper is illustrated in Fig.7 . The doubly fed induction generator (DFIG) stator terminals are connected to the PCC through a feeder. The DFIG rotor is supplied by two back to- back connected converters: the rotor side converter (RSC) and the line side converter (GSC). The feeder connects the GSC to the PCC . The RSC and GSC solid-state switches are driven by means of pulse width modulation (PWM) . The dc-link is made of capacitor,. A load of 500 MW is connected to the same PCC through a feeder. On the left side of the three-phase diagram presented in Fig. 7 , the step-up transformer and supply line circuit is shown. The case simulated in this study is shown in Fig. 1. Fig.6 STATCOM Control Scheme VII. System Simulation Model The simulink based overall system is in Fig. 7 with rotor controller , STATCOM and grid side controller . 371 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME Fig .7. proposed system It represents a typical situation, where a wind farm with doubly fed induction generators represented by a single machine is integrated to the external power system represented by a constant voltage source connected in series with its Thevenin’s equivalent impedance, then Buses connected necessary for the Point of Common Coupling (PCC). The STATCOM power and control circuit developed in Matlab / simulink environment are shown in Fig 8, and Fig. 9 respectively. Fig. 8 STATCOM Simulation model 372 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME Fig .9 STATSCOM control system simulated in matlab IX . SIMULATION RESULTS AND DISCUSSION The Wind farm simulation are shown with the help of Matlab /Simulink .The Fig.10. show that how wind energy conversion system can regulate the terminal voltage distortion due to load connected to PCC as well as regulate the DFIG current with the help of STATCOM technique . The simulation result which we got it represent the DFIG performance with or without compensation action of the STATCOM . Dynamic model of the DFIG is developed and the simulation results demonstrating the power quality improvement in the system are presented under steady state condition . Fig .10 . Results from top to button : DFIG side voltage with STATCOM , DFIG side voltage without STATCOM , DFIG side current with STATCOM and DFIG side current without STATCOM. XI. CONCLUSION This paper deals with the Wind turbine generators, control systems, power factor correction equipments, transformers, wind farm substation, inner loops for connections and transmission lines which can be listed as wind farm sections that should be modeled for power system studies. The flicker emission produced by grid-connected wind turbines during continuous operation is mainly caused by the harmonics generating grid side voltage source. To evaluate the flicker levels and mitigation by grid-connected wind turbines with DFIG, a FOC based rotor control and SPWM based 373 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME grid controller were designed and analyzed. 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Bharati Vidyapeeth University, U College of Engineering, Pune , India . firstname.lastname@example.org 3. Prof . D.S. Chavan : Ph D (Registered), ME (Electrical), BE (Electrical), DEE Associate Professor, ordinator cell),Co-ordinator (PH.D. Programme management) BharatiVidyapeeth Deemed Co-ordinator (R&D cell),Co 411043. University College Of Engineering Pune 411043 He is pursuing Ph D. He received ME (Electrical)(Power systems) Achieved rank certificate in Pune Universit for ME .email@example.com University 376
"WIND FARMS RESPONSE IMPROVEMENT USING STATIC COMPENSATOR CONTROL"