INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET)

Transcription

1 INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) ISSN (Print) ISSN (Online) Volume 3, Issue 1, January- June (2012), pp IAEME: Journal Impact Factor (2011): (Calculated by GISI) IJEET I A E M E ENHANCEMENT OF POWER QUALITY IN GRID-CONNECTED DOUBLY FED WIND TURBINES INDUCTION GENERATOR ABSTRACT Haider M. Husen 1, Laith O. Maheemed 2, Prof. D.S. Chavan 3 Wind power technology is expanding rapidly, more and more wind farms are being connected to the power systems. Incorporation of large scale wind farms presents various challenges that must be tackled, such as system operation and control, stability, reliability and power quality. Power quality problems such as voltage flicker and harmonic distortion due to non-linear loads, sags due to faults are some major issues considered in this paper. Doubly fed Wind turbine induction generator connected to a grid is analyzed using MATLAB/ SIMULINK. Owing to continuously varying wind speed components, the active and reactive power along with terminal voltage fluctuates unremittingly. The proposed wind electric generators use good control techniques to extract maximum power at the every available speed and to mitigate harmonics and other power quality issues. Keywords: Wind Energy Conversion Systems, Power System, doubly fed induction generator, power quality. I. INTRODUCTION Electric energy can be produced by many means and wind power is being used as a clean and safe electric energy resource for nearly a hundred years and is expanding at a faster rate. Near the beginning wind farms had relatively smaller power rated generators with respect to conventional power stations. But currently, large power rated offshore wind farms are being installed to control power system data instead of conventional ones. The wind farms have different impacts and functions on the performance of the grid than conventional power plants, because of variation of wind speed in time. Doubly fed and squirrel cage induction generators are widely used in wind energy conversion systems. These generators are usually grid-coupled via power electronic converters in order to control the voltage, frequency and power flow during the variation of wind speed. As a consequence, wind turbines affect the dynamic behavior of the power system in a way that might be different from hydraulic or steam turbines [1]. This paper discus the significant issues related to wind power integration into modern power systems. Firstly, the wind model will be described; the impacts of wind farm on power quality issue are to be analyzed, then the technical requirements for wind farm grid connection will be 182

2 introduced and in final case studies. The possible operation and control methods to meet the specifications and to improve system quality are conferred. A MATLAB/ Simulation example is also presented to illustrate a power quality problems and the possible method for improvement are evaluated. In order to achieve such high power, the modern high-power turbines make use of adjustable speed generators (ASG). Simple pitch control, reduced mechanical stress, improved power quality and system efficiency, and reduced acoustic noise because of the ability to operate at low speeds, are the prime features of such generators [1]. The preferred topology derives high power output from Doubly Fed Induction Generator (DFIG). Using a DFIG, the converters need only be rated to the rotor power, which is typically one-fourth of the rated system power. The flicker emitted during operation of the DFIG is reduced by means of reactive power compensation. This can be achieved by using a STATCOM based rotor control technique to control the reactive power in the grid side converter. The simulation software MATLAB/ Simulation has been used to develop the DFIG presented in this paper. This model has been used to study the effect of non-linear loads with grid conditions under different case studies. II. WIND POWER GENERATION AND TRANSMISSION Conventional power stations are usually connected to the high voltage or extra-high voltage system. While Wind turbines may be connected to ac system at various voltage levels, including the low voltage, medium voltage, high voltage as well as to the extra high voltage system. The suitable voltage level depends on the amount of power generated. For example, for large onshore wind farms at hundreds of MW level, high voltage overhead lines above 100kV are normally used. For offshore wind farms with a long distance transmission to an on shore grid, a high voltage submarine cable may have to be used. Modeling the DFIG The doubly fed induction generator has an AC-ACconverter in the rotor circuit, popularly known as the Scherbius Drive. The power converter needs only to be rated to handle the rotor power which happens to be less than a quarter of the stator power. Vector control techniques are used to control the active and reactive powers in a decoupled manner. Most DFIG systems employ either a current fed DC link converter or cyclo-converter. Both of them have certain disadvantages, which can be overcome by using two back-to-back PWM converters that are voltage fed and current-regulated [1]. Fig. 1 shows the schematic diagram of a doubly fed induction generator using two back-to-back PWM converters. Modeling the DFIG involves modeling of the wound rotor induction machine, the rotor side converter, the grid side converter and the back-to-back PWM converters with the DC link between them, along with the aerodynamic model of the wind turbine. In this paper, vector controlled models of the rotor side and the grid side controllers are developed [5], [6]. For the wound rotor induction machine, the elaborate model from the simulation software PSCAD/EMTDC library is used. For the converters, standard electrical components models are used from the PSCAD/EMTDC library. The aerodynamic model of the wind turbine and the model of the wind profile are built with custom components made in PSCAD/EMTDC. 183

3 2.1 Wind Turbine Model Fig 1: DFIG based wind turbine system connected to grid The aim of this paper is to study the effect of grid conditions on the electrical behaviors of the system, so the mechanical systems are simplified to some extent. The aerodynamic model of the wind turbine [6] is based on the following equation which gives the relation between the wind speed and the mechanical power captured by the turbine: It depends on power coefficient Cp given for wind velocity v. Cp is generally defined as a function of the tip speed ratio λ and the pitch angle β. Where ω is the rotational speed of the turbine, and r is the radius of the rotor. The wind turbine model used in this study takes the wind profile as input and generates an output mechanical power and torque. This mechanical power is set as the reference for the stator real power output. The wind source model has been used from the PSCAD/EMTDC library, and it provides settings for mean wind speed, gust, and ramp and noise components. When the wind speed crosses the rated wind speed for the generator the power reference sets to 1.0 p.u. This generates the maximum rated power in case of high wind speeds. The grid connection may include two parts, the local electrical connection within a wind farm at a medium voltage level and the connection from the wind farm to the electrical grid. If the wind farm is large and the distance to the grid is long, a transformer is used to step up the medium voltage in the wind farm to the high voltage at transmission level. III. IMPACT OF WIND FARMS ON POWER QUALITY A. Voltage variations On the local level, voltage variations are the main problem associated with wind power. This can be the limiting factor on the amount of wind power which can be installed. 184

4 In normal operational condition, the voltage quality of a wind turbine or a group of wind turbines may be assessed in terms of the following parameters [10]: Steady state voltage under continuous production of power Voltage fluctuations Flicker during operation Flicker due to switching The influence of connecting a wind farm on the gird voltage is directly related to the short circuit power level. The short circuit power level in a given point in the electrical network represents the system strength. If the voltage at a remote point can be taken as constant, Us, and the short circuit power level SSC in MVA is defined as Us2/Zk where Zk is the equivalent impedance between the points concerned. Fig. 3 illustrates an equivalent wind power generation unit, connected to a network with equivalent short circuit impedance, Zk. The network voltage at the assumed infinite busbar and the voltage at the Point of Common Coupling (PCC) are Us and Ug, respectively. The output power and reactive power of the generation unit are Pg and Qg, which corresponds to a current Ig. The voltage difference, U, between the system and the connection point is given by The voltage difference, U, is related to the short circuit impedance, the real and reactive power output of the wind power generation unit. It is clear that the variations of the generated power will result in the variations of the voltage at PCC. If the impedance Zk is small then the voltage variations will be small (the grid is strong). On the other hand, if Zk is large, then the voltage variations will be large (the grid is weak). However, strong or weak are relative concepts. For a given wind power capacity P the ratio RSC = SSC/P is a measure of the strength. The grid may be considered as strong with respect to the wind farm installation if RSC is above 20. Fig 2 A simple system with an equivalent wind power generator connected to a network. B. Voltage fluctuations Fluctuations in the system voltage (more specifically in its rms value) may cause perceptible light flicker depending on the magnitude and frequency of the fluctuation. This type of disturbance is called voltage flicker, or shortened as flicker. There are two types of flicker emissions associated 185

5 with wind turbines, the flicker emission during continuous operation and the flicker emission due to generator and capacitor switchings. Often, one or the other will be predominant. The allowable flicker limits are generally established by individual utilities. Rapid variations in the power output from a wind turbine, such as generator switching and capacitor switching, can also result in variations in the RMS value of the voltage. At certain rate and magnitude, the variations cause flickering of the electric light. In order to prevent flicker emission from impairing the voltage quality, the operation of the generation units should not cause excessive voltage flicker. IEC specifies a flickermeter which can be used to measure flicker directly [12]. The flicker measurement is based on the measurements of three instantaneous phase voltages and currents followed by using a flicker algorithm to calculate the Pst and Plt. where Pst is the short term flicker severity factor and measured over 10 minutes, and the long term flicker severity factor Plt is defined for two hour periods. The flicker assessments can also be conducted with simulation method [13]. Disturbances just visible are said to have a flicker severity factor of Pst = 1 The flicker emissions, Pst and Plt may also be estimated with the coefficient and factors, cf(ψk, va ) and kf(ψk) obtained from the measurements, which are usually provided by wind turbine manufacturers. The flicker emissions from a wind turbine installation should be limited to comply with the flicker emission limits. It is recommended [3] that Plt 0.50 in kv networks and Plt 0.35 in kv networks are considered acceptable. However, different utilities may have different flicker emission limits. The assessments of the flicker emissions are described below. 1) Continuous operation The flicker emission from a single wind turbine during continuous operation may be estimated by: Where cf (Ψk, va) is the flicker coefficient of the wind turbine for the given network impedance phase angle, Ψk, at the PCC, and for the given annual average wind speed, va, at hub-height of the wind turbine. A table of data produced from the measurements at a number of specified impedance angles and wind speeds can be provided by wind turbine manufactures. From the table, the flicker coefficient of the wind turbine for the actual Ψk and va at the site may be found by applying linear interpolation. The flicker emission from a group of wind turbines connected to the PCC is estimated using equation (6) Where cf,i (Ψk, va) is the flicker coefficient of the individual wind turbine; Sn,i is the rated apparent power of the individual wind turbine; Nwt is the number of wind turbines connected to the PCC. If the limits of the flicker emission are known, the maximum allowable number of wind turbines for connection can be determined. C. Harmonics Harmonic disturbances are a phenomenon associated with the distortion of the fundamental sine wave and are produced by non-linearity of electrical equipment. Harmonics causes increased 186

6 currents, power losses and possible destructive overheating in equipment. Harmonics may also rise problems in communication circuits. Harmonic standards are specified to set up the limits on the Total Harmonic Distortion (THD) as well as on the individual harmonics. Power electronic converters, which operation in an on-and-off way, are used in variable speed wind turbine systems [14, 15]. The Pulse Width Modulation (PWM) switching frequency, with a typical switching frequency of a few thousand Hz, shifts the harmonics to higher frequencies where the harmonics can be easily removed by smaller filters. In general harmonic standards can be met by modern wind turbines. IV. REQUIREMENTS OF CONNECTING WIND FARM INTO POWER SYSTEMS Integration of large scale wind power may have severe impacts on the power system operation. Traditionally, wind turbines are not required to participate in frequency and voltage control. However, in recent years, attention has been increased on wind farm performance in power systems. Consequently, some grid codes have been defined to specify the steady and dynamic requirements that wind turbines must meet in order to be connected to the grid. Examples on such requirements are capabilities of contributing to frequency and voltage control by continuous modulation of active power and reactive power supplied to the transmission system, as well as the power regulation rate that a wind farm must provide. Some specifications have been worked out with regard to the preparations for future large offshore wind farms as the following example [3]. Active power and frequency control: the active power is regulated linearly with frequency variation between a certain range (47 Hz -52 Hz) with a dead band (49.85 Hz Hz) and the regulating speed is 10 % of the rated power per second, The reactive power should be regulated within a control band, at a maximum level of 10% of rated power (absorption at zero real power and production at the rated real power), Wind turbine will generally operate in normal conditions (90%-105% voltage and Hz), however, it should also be able to operate outside of the above conditions within certain specified time limits, Under the condition of a power system fault, a wind turbine would experience a voltage variation. The severer degree of the voltage variation and the time period of such voltage variation will determine whether the wind turbine must not be disconnected (ride through) or may be disconnected or must be disconnected. Also the wind turbine has to be able to withstand more than one independent faults occurred in a few minute intervals. There are also requirements related to rapid voltage variations, flickers, harmonics and interharmonics. A series of special test conditions have been set and the wind turbines have to meet these conditions accordingly before they can be connected into the power system. The regulation ability of reducing the wind turbine production from full load to a level between 0 and 20 per cent in a few seconds is required. V. MODELLING AND SIMULATION OF WIND FARMS FOR POWER SYSTEM STUDIES Wind turbine generators, control systems, power factor correction equipments, transformers, wind farm substation, inner loops for connections and transmission lines can be listed as wind farm sections that should be modeled for power system studies. The models of the grid and the wind farms have to comply with common requirements of the simulation platforms for getting accurate results. Computer simulation makes it possible to investigate a multitude of properties in design and application phase. The correctness of a computer simulation depends on the quality of the built-in models and of the applied data. In order to investigate the effects of wind energy conversion systems on power system and vice versa; it is necessary to develop accurate models of 187

7 both systems. When the aim is to investigate grid integration of wind turbines, there are three main interests; steady-state voltage level influence, rapid voltage fluctuations (flicker), and response to grid disturbances. There are two types of flicker emissions associated with wind turbines. In order to predict the rapid power fluctuations from fixed-speed turbines, there is a need to represent the wind field arriving at the turbine, since the flicker emission during continuous operation is mainly caused by fluctuations in the output power due to wind speed variations, the wind gradient and the tower shadow effect. Switching operations like start, stop and switching between generators or generator windings, also produce flicker. When the limits of the flicker emission are given, the maximum allowable number of switching operations in a specified period can be examined by appropriate models in simulations. Model of a single wind energy conversion system must take into consideration soft starter, capacitor group, pitch control, and wind speed variations. Comparison of the system with and without rotor and grid side controller under constant wind speed pitch angle. Fig 3 Simulink diagram of the system that is being studied 188

8 Fig 4 Rotor side controller- FOC technique Without controller- constant wind speed 8m/s and pitch angle of 5. The simulink based overall system is in Fig 3 with rotor controller in Fig4 and grid side controller is as in Fig 5. Fig 5 Grid side controller- SPWM The grid source IS ITSELF PRODUCING 2 nd and 5 th order harmonics, and the flicker mitigation is analyzed in the system with DFIG generation with and without STATCOM. The grid side voltages are 120kV at 60 HZ frequency and the converters are IGBT with diode and constant wind speed 10m/s and pitch angle of 5 respectively. The waveforms and spectrum of the above system without rotor and grid side controllers are shown in Fig 7 to 10. In this the wind generator is supplying certain power and remaining by the grid load to a non-linear RL load (steel plant type). The waveforms of both wind source at top and bottom were depicted in Fig

9 Fig6 DFIG top and bottom side voltage and current without STATCOM The load voltage and current wave spectrum without controller were shown in Fig 7; it is observed that load voltage and current were having about 26 and 12% total harmonic distortion (THD). Fig 7 Top wind source voltage spectrum without STATCOM 190

10 Fig 8 Bottom wind source voltage spectrum without STATCOM The top side wind source voltage and current spectrum in Fig 7 & 8; it is observed that source voltage and current were having about 26 and 12% total harmonic distortion (THD). Fig 9 Top wind source current spectrum without STATCOM 191

11 Fig 10 Bottom wind source current spectrum without STATCOM The bottom side is designed with field oriented control to maintain constant speed if there is change in the torque input from the DFIG. In our study, however, the wind speed is constant, thereby torque is also constant. The grid side is controlled by using SPWM technique for optimal power flow and to maintain minimum distortion waveforms. The voltage and current waveforms of source and load were shown in Fig 10. When comparing these waveforms in Fig 6, the voltage waveform is sinusoidal and the voltage waveform reaches steady state current is attained in nearly 0.01 seconds when using STATCOM and also the system quality of power has been improved. The waveforms for the system with STATCOM controller is being considered for the above flickering generator source. The top and bottom side wind source voltage and current waveforms and its wave spectrum are shown in fig 10 to 15. With these waveforms comparing with 7 to 9, the THD of both top and bottom voltage waveforms were less than 10% and 6% and current THD near is 3.55% and 3.97%. When comparing these figures without STATCOM, they were 28 and 12% for voltage waveforms and 6% and 9% for current waveforms. 192

12 Fig 11 DFIG top and bottom side voltage and current without STATCOM Therefore from the above, the control techniques on rotor side is used to maintain sinusoidal waveform and controlled voltage and grid side is used to absorb produced harmonics and to optimize the current flow from the grid to load. Fig 12 Load voltage spectrum with controller 193

13 Fig 13 Load current spectrum with controller Fig 14 Source voltage spectrum without controller Fig 15 Source current spectrum without controller 194

14 The STACOM controller controls the harmonics flow by injecting or absorbing current using the voltage source inverter in order to maintain the system with overall voltage harmonics around 8% and current harmonics not exceeding 4%. VI Conclusions 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 grid controller were designed and analyzed. It is found that by suing these two techniques, the current waveform reaches steady state operation in nearly 0.1 seconds and maintains regulated voltage because of rotor controller and the grid side controller is helping in maintaining with tolerable harmonic free system voltage and current and also optimizes the current flow from the wind with current THD nearly 3.55 and 3.97% at source and the voltage THD at both sources are near 10%. REFERENCES [1] Åke Larsson, Flicker emission of wind turbines during continuous operation, IEEE Trans. on Energy Conversion, vol. 17, no. 1, pp , Mar [2] G. Gerdes, F. Santjer, Power quality of wind turbines and their interaction with the grid, Proc. of Euro. Wind Energy Conf., pp , Oct [3] T. Thiringer, Power quality measurements performed on a low-voltage grid equipped with two wind turbines, IEEE Trans. on Energy Conversion, vol. 11, no. 3, pp , Sep [4] H. Amarís, C. Vilar, J. Usaola, J. L. Rodríguez, Frequency domain analysis of flicker produced by wind energy conversions systems, Proc. of the 8th International Conference on Harmonics and Quality of Power, vol. 2, pp , Oct [5] T. Sun, Z. Chen, and F. Blaabjerg, Flicker study on variable speed wind turbines with doubly fed induction generators, IEEE Trans. Energy Convers., Vol. 40, No. 4, pp , [6] Gutierrez, J., Ruiz, J., Leturiondo, L. & Lazkano, A. Filcker Measurement System for Wind Turbine Certification, IEEE Transactions on Instrumentation and Measurement 57(12): , 2008 [7] IEC Ed. 2.0 (2010). Electromagnetic compatibility (emc) U part 4: Testing and measurement techniques - section 15: Flicker-meter functional and design specifications. [8] Ruiz, J., Gutierrez, J., Lazkano, A. & Ruiz de Gauna, S. (2010). A Review of Flicker Severity Assessment by the IEC Flicker-meter, IEEE Transactions on Instrumentation and Measurement 59(8): [9] Grainger, B.; Thorogood, T., Beyond the harbour wall, IEE Review, Volume: 47 Issue: 2, March 2001, Page(s): [10] Mutlu, Ö.S., Akpınar, E. and Balıkcı, A. Power Quality Analysis of Wind Farm Connected to Alaçatı Substation in Turkey. Renewable Energy 34 (2009) [11] Chen, Z. (2005). Issues of Connecting Wind Farms into Power Systems. Proc. of IEEE/PES Transmission and Distribution Conference & Exhibition 2005 : Asia and Pacific-China. [12] Mutlu, Ö.S.(2009). Effects of Wind Turbines on Power System Operation. Ph.D. Thesis. Dokuz Eylül University. 195

15 Mr. Haider M. Husen, M.Tech. candidate, Electrical Engineering, Dep BharatiVidyapeeth University, College of Engineering. Pune, India, Mr. Laith O. Maheemed, M. Tech. candidate, Electrical Engineering, Dept. Bharati Vidyapeeth University, College of Engineering, Pune, India. Prof. D.S. Chavan : Ph D (Registered), ME (Electrical), BE (Electrical), DEE Associate Professor, Co-ordinator (R&D cell),co-ordinator (PH.D. Programme management) BharatiVidyapeeth Deemed University College Of Engineering Pune He is pursuing Ph D. He received ME (Electrical)(Power systems) Achieved rank certificate in Pune University for ME.greenearth1234@yahoo.com 196