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A Comparative Analysis of Wind Turbine Technologies in Focus on the Grid Integration PEREIRA1 Heverton Augusto LIU2 Silas Yunghwa RAMOS2 Manoel Candido de Lima MENDES2 Victor Flores SILVA2 Sel nio Rocha 1Universidade Federal de Vi osa Electrical Engineering Department 2Universidade Federal de Minas Gerais Electrical Engineering Department E mail heverton pereira ufv br silasyl victor auto br selenios dee ufmg br Abstract The development of new technologies has provided a fast growth of the wind turbine industry Worldwide governments have presented financial programs of alternative energy sources The increase of wind turbines in electrical systems has led the system operators to make codes to regulate the integration of wind farms with the network setting minimum conditions for supportability The aim of this paper is to discuss the different technologies of wind turbines present in the world analyzing their main characteristics Therefore the simulation of some technologies during a three phase voltage sag is analyzed Index Terms Wind turbine grid integration voltage sag I INTRODUCTION Technological advancements achieved by the wind turbine industry and the increase in number of wind farms around the world have consolidated this alternative as a safe reliable and cost effective method for electric power generation Highlights are the evolution of the materials used in construction which makes the equipment lighter and stronger advances in the turbine aerodynamics and integration of power electronics and automation producing each year equipment with higher power and dimensions and performance compatible to the increasing demands of the network codes All this progress was possible due to the ideals of sustainability adapting the conveniences of progress of human activities to the needs of minimizing the impacts on the environment 1 In the end of 2009 the world total installed capacity of wind generation was 159 2 GW with prediction of reaching 203 5 GW by the end of 2010 according to the report released by WWEA World Wind Energy Association 2 Figure 1 illustrates the evolution of installed power in this decade and the forecast value for 2010 indicating that even with the global economic crisis of 2008 2009 the growth of wind power in this period has grown amazingly Brazil has yet to develop much of its wind power potential About 800 MW is installed and in operation in the country and more than 1 900 MW approved that have not yet started construction 3 The Leil o de Energia de Reserva Reserve Energy Auction of 2009 was a milestone for the future of wind energy in Brazil Characterized by the presence of national investors that have committed to an unexpectedly low energy price of R 148 30 MWh in average this auction has contracted the installation of 1 800 MW through June 2012 representing the inclusion of 71 new wind farms with the participation of a large number of manufacturers To integrate the wind farms to the grid they must meet the requirements imposed by the network codes that are standards drawn up by operators of electric systems worldwide Year Installed Capacity GW Actual Forecast Year Installed Capacity GW Year Installed Capacity GW Actual Forecast Fig 1 Evolution of the world total installed capacity of wind power Fig 2 Voltage tolerance boundary for wind turbine generators according to the Brazilian grid codes 4 2010 9th IEEE IAS International Conference on Industry Applications INDUSCON 2010 978 1 4244 8010 4 10 26 00 2010 IEEE In Brazil the Procedimentos de Rede Brazilian Grid Codes standards drawn up by ONS Operador Nacional do Sistema National System Operator 4 was launched in 2009 These are basic procedures for connection to the grid The sub module 3 6 entitled Requisitos t cnicos m nimos para a conex o rede b sica Minimum technical requirements for connection to the basic grid presents the minimum criteria that the wind generators must meet In the case of momentary voltage sags Fig 2 shows the relationship between the percentage of sinking and duration of absence for which the wind generator must remain connected to the grid The aim of these criteria is to ensure that the generation from wind farms which have increased the installed capacity with prospects for greater inclusion of this source to the energy network may contribute to the system to withstand a momentary voltage sag The attraction of new investments and incentives by government programs coupled with new standards for network connection further stimulated the arrival of new technologies of wind turbines in Brazil In this paper we present the existing technologies of wind turbines many of them not currently in Brazil and a comparative analysis of the performance of some of these technologies when integrated to the grid and subjected to voltage variations of short duration II WIND TURBINE TECHNOLOGIES This section presents the technologies of wind turbine that exist in the international market and the outstanding characteristics of each Seven wind turbine technologies are currently available one at fixed speed SCIG one with a narrow range of speed WRIG and five at variable speeds The latter group includes the most competitive technologies in terms of cost and performance energy use and robustness to disturbances and two technologies recently launched in the international market wind turbine with hydrodynamic gearbox from Voith 5 and doubly fed induction generator with permanent magnet exciter 6 Fig 3 Squirrel Cage Induction Generator connected to the grid Fig 4 Wound Rotor Induction Generator connected to the grid Fig 5 Doubly Fed Induction Generator connected to the grid Fig 6 Doubly Fed Induction Generator with Permanent Magnet Exciter connected to the grid Fig 7 Synchronous Generator with Excited Rotor connected to the grid Fig 8 Synchronous Generator with Permanent Magnet connected to the grid Fig 9 Synchronous Generator with Dynamic Multiplication Gearbox connected to the grid A SQUIRREL CAGE INDUCTION GENERATOR SCIG This wind turbine concept which can be seen in Fig 3 was the first one used on a commercial scale being very simple and not having any type of power control The squirrel cage induction generator operates at fixed speed determined by the grid frequency and is independent of the wind speed 7 To work as generator the machine has to rotate at a speed above the synchronous i e with negative slip As the wind turbine is directly connected to the grid any disturbances from the wind variations are transmitted leading to power quality problems The machines connected directly to the grid have characteristics of a rigid connection i e the effects on the machines are passed directly to the grid and vice versa 8 Another problem is the need of a mechanical gearbox that needs regular maintenance but this is a problem of more modern technologies as well Since the squirrel cage induction generator always absorbs reactive power from the grid this configuration uses a bank of capacitors for power compensation B WOUNDROTOR INDUCTION GENERATOR WRIG AND DYNAMIC CONTROL OF ROTOR RESISTANCE An evolution of the squirrel cage induction generator is the utilization of wound rotor induction generator with the insertion of a variable resistor in the rotor and controlled by a set of IGBT switches as can be seen in Fig 4 The aim of the resistive control is to help the absorption of mechanical transients and therefore by varying the resistance value the slip and the output power of the system are controlled The range of the speed control is typically 0 10 above the synchronous speed C DOUBLY FED INDUCTION GENERATOR DFIG In the last decade the doubly fed induction generator is the technology that showed more growth both in installed capacity as well as in the number of units sold 9 The machine as seen in Fig 5 is a wound rotor induction generator and it is fed by two circuits The stator is fed directly by the grid while the rotor is connected to the system through two static converters which enable the bidirectional flow of power The converters used have only a portion of the rated power of the machine that is usually around 25 30 This technology allows the control of the reactive power generated as well as the variation in speed in the range of 30 around the synchronous speed However this technology requires the mechanical gearbox to interconnect the turbine to the generator and the direct connection of the stator to the grid which makes it fragile against voltage sags D DOUBLY FED INDUCTION GENERATOR WITH PERMANENT MAGNET EXCITER XDFM In analogy with the conventional wind turbine with doubly fed induction generator this technology uses a power converter with 25 30 of rated output of the generator and in contrast to that a permanent magnet exciter with the power of 16 18 of main power generator as seen in Fig 6 The inclusion of this exciter allows you to connect the DFIG rotor to the synchronous machine stator without slip rings isolating the rotor circuit of the power grid With the PWM converter isolated from the network this technology offers a superior quality of energy and supportability as advocated by the manufacturer 6 E SYNCHRONOUS GENERATOR WITH EXCITED ROTOR SGER The use of synchronous generators in the implementation of power plants that operate at variable speed came on the market as an attractive alternative to eliminate the mechanical gearbox Connected to the grid by means of frequency converters as seen in Fig 7 these machines can operate at low rotational speed due to large amount of magnetic poles in the generator 10 The converter of the grid side consisting of IGBTs controls the dc bus voltage across the current injection onto the grid acting as an inverter The generator side converter controls the power by using a diode rectifier and a boost converter that connects the unregulated dc bus with the regulated dc bus at the input of the converter on the grid side This technology is dominated by the Germany company Enercon and can be distinguished by the use of a generator of six phases at low speed and operating without a gearbox F SYNCHRONOUS GENERATOR WITH PERMANENT MAGNET SGPM This second technology using the synchronous generator has as main features the use of permanent magnet generator and the configuration of the machine side converter as PWM rectifier as seen in Fig 8 These design strategies produce a better fit with the rotor of the wind generator that allows for a structure of reduced size and weight minimizing the nacelle and efforts on the structure of the turbine The use of PWM rectifier although involving marginally higher costs makes better use of the electric machine improving its operation on variable speed G SYNCHRONOUS GENERATOR WITH DYNAMIC MULTIPLICATION GEARBOX SGDG This technology was introduced by the development of the windrive gearbox of hydrodynamic control by Voith in partnership with DeWind who has already marketed various types of wind turbines This equipment consists of a wind rotor connected to a gearbox of fixed ratio and from this connects to the hydrodynamic gearbox that allows for the axis of high rotation speed to operate at a fixed speed and allowing the axis of slow speed operating at several speeds as seen in Fig 9 This technology marketed by Voith for several decades to variable speed pumps is successfully applied to design an equipment that avoids the use of static converters allowing the use of conventional synchronous generators of high rotation All these produce a series of advantages that can be summarized in the following Good integration to the net not generating harmonics conventional maintenance of reactive and supportability to disturbances Adequate onshore and offshore application in different climates Weight reduction in comparison with the synchronous generator Increased reliability for not using static converters This type of wind turbine cannot be easily compared with other technologies since their characteristics are close to those used in hydroelectric and thermoelectric generators conventionally used III SUPPORTABILITY CHARACTERISTICS OF SOME WIND TURBINE TECHNOLOGIES To analyze the features of supportability of some wind turbine technologies they were submitted to a three phase voltage sag to 50 with duration of 300 ms With this dip it was possible to discuss the main features of the technologies by analyzing their strengths and weakness Figure 10 presents the sag to which the turbines were submitted All the simulations were made using the Matlab Simulink program and the performance of protections was not considered A SQUIRREL CAGE INDUCTION GENERATOR As mentioned earlier this was the first wind turbine technology to spread the market and accounted for 70 of the generators installed in 1995 worldwide In 2005 this technology accounted for 20 of all generators installed worldwide This reduction occurred mainly by developments that gave rise to other more robust and higher power producing technologies During the three phase voltage sag the turbine with SCIG shows an increase in the velocity as can be seen in Fig 11 With regards to power during the dip the active power reduces to around zero and the reactive power has peaks and then reduces to around zero as can be seen in Fig 12 and 13 respectively These reductions are consistent with the values of remaining voltage The unbalance between the mechanical power and the generated active power converts in the acceleration of the rotor During voltage recovery the active power returns to its rated value despite some oscillations and the reactive power shows a peak due to the generator remagnetization Regarding the generator stator currents they are increased during the studied sag to greater than three times the rated value as can be seen in Fig 14 And during the voltage recovery another current increase occurs These current variations can trigger the overcurrent protection defining the fragility of the technology B WOUNDROTOR INDUCTION GENERATOR AND DYNAMIC CONTROL OF ROTOR RESISTANCE The analysis of the WRIG and dynamic control of rotor resistance made through Fig 15 to Fig 17 shows the evolution of this technology compared to the squirrel cage generator By controlling the rotor resistance it is possible to notice a reduction in reactive power during the sag as well as after the Fig 10 Voltage sag that the wind turbine have to ride through according the Brazilian grid procedures Fig 11 SCIG speed during the voltage sag Fig 12 SCIG active power during the voltage sag Fig 13 SCIG reactive power during the voltage sag Fig 14 SCIG stator current RMS during the voltage sag recovery of the voltage as seen in Fig 16 Consequently a reduction in the generator stator currents occurs as seen in Fig 17 C DOUBLY FED INDUCTION GENERATOR The DFIG has several converter control topologies The study uses the control of velocity and reactive power using the dq transformation It was decided to control the reactive power to maintain the power factor of 0 93 at the pre fault system During the three phase dip the speed also rises However the machine is capable of maintaining the power production as seen in Fig 18 and Fig 19 The active and reactive powers show oscillations and the active power presents significant ripple The reactive power control is capable of keeping the value even during the disturbance One of the biggest problems during the disturbance is represented by the overvoltages and overcurrents induced in the rotor 11 12 Since the stator is directly connected to the grid the voltage and current peaks due to the sag are significant The currents reached almost twice the rated value as can be seen in Fig 20 However in more severe dips this value can reach three to four p u Since this technology uses a converter connected to the rotor it is of utmost importance to protect this converter and ensure the generation of power during the sag The system also suffers high torque peak during the dip and crowbar is usually employed D DOUBLY FED INDUCTION GENERATOR WITH PERMANENT MAGNET EXCITER According to the manufacturer the xDFM technology does not demand the use of crowbar to limit the torque peak during the voltage sag The converter remains active during all the dip because the exciter is a permanent magnet synchronous machine and the voltage is kept Hence the converter is able to keep the control of rotor currents during the sag In comparison with the synchronous generator the xDFM has the capacity of providing higher currents about five times the rated current than full converter technologies about twice the rated current E SYNCHRONOUS GENERATOR WITH EXCITED ROTOR In the dc bus of the SGER that connects the converters as seen in Fig 23 a brake chopper is used to dissipate the excess energy of the capacitor The chopper is triggered each time the voltage on the capacitor exceeds a certain threshold balancing variations in power generated by the machine with the limitations of power flow for the network as seen in Fig 21 and Fig 22 The chopper increases the supportability of the wind turbine facing sags and can be considered as an important measure of ride through 13 Fig 15 WRIG active power during the voltage sag Fig 16 WRIG reactive power during the voltage sag Fig 17 WRIG stator current RMS during the voltage sag Fig 18 DFIG active power during the voltage sag Fig 19 DFIG reactive power during the voltage sag Fig 20 DFIG rotor current during the voltage sag F SYNCHRONOUS GENERATOR WITH PERMANENT MAGNET Of the viewpoint of the supportability to voltage sags the SGPM is completely similar to the SGER G SYNCHRONOUS GENERATOR WITH DYNAMIC MULTIPLICATION GEARBOX The main characteristic of a synchronous generator directly connected to the grid without power electronics is associated to its ride through capability during voltage sags due to its capacity in remaining synchronized during and after voltage dip This characteristic is known as transitory stability During sags the synchronous generator provid