Power Quality of Grid-Connected Wind Turbines with DFIG and Their Interaction with the Grid Sun, Tao

Transcription

1 Aalborg Univeritet Power Quality of Grid-Connected Wind Turbine with DFIG and Their Interaction with the Grid Sun, Tao Publication date: 2004 Document Verion Publiher' PDF, alo nown a Verion of record Lin to publication from Aalborg Univerity Citation for publihed verion (APA): Sun, T. (2004). Power Quality of Grid-Connected Wind Turbine with DFIG and Their Interaction with the Grid. Aalborg: Intitut for Energiteni, Aalborg Univeritet. General right Copyright and moral right for the publication made acceible in the public portal are retained by the author and/or other copyright owner and it i a condition of acceing publication that uer recognie and abide by the legal requirement aociated with thee right.? Uer may download and print one copy of any publication from the public portal for the purpoe of private tudy or reearch.? You may not further ditribute the material or ue it for any profit-maing activity or commercial gain? You may freely ditribute the URL identifying the publication in the public portal? Tae down policy If you believe that thi document breache copyright pleae contact u at vbn@aub.aau.d providing detail, and we will remove acce to the wor immediately and invetigate your claim. Downloaded from vbn.aau.d on: juli 10, 2018

2 Power Quality of Grid-Connected Wind Turbine with DFIG and Their Interaction with the Grid by Tao Sun Diertation ubmitted to the Faculty of Engineering & Science at Aalborg Univerity in partial fulfilment of the requirement for the degree of Doctor of Philoophy in Electrical Engineering Intitute of Energy Technology Aalborg Univerity, Denmar May 2004

3 Aalborg Univerity Intitute of Energy Technology Pontoppidantraede 101 DK-9220 Aalborg Eat Copyright Tao Sun, 2004 ISBN

4 Preface Thi thei i ubmitted to the Faculty of Engineering and Science at Aalborg Univerity in partial fulfilment of the requirement for the Ph.D. degree in Electrical Engineering. The reearch ha been conducted at the Department of Electrical Energy Converion which i part of the Intitute of Energy Technology, Aalborg Univerity. Four upervior have followed the project: Profeor Zhe Chen, Profeor Frede Blaabjerg, both from the Intitute of Energy Technology, Aalborg Univerity, Profeor Huizhu Dai, from China Electric Power Reearch Intitute, and Profeor Yihan Yang, from North China Electric Power Univerity. I would lie to than all of them for their upport and advice to my wor during the entire project period and epecially, Prof. Zhe Chen for the inpiration, encouragement and preciou comment. The Intitute of Energy Technology, Aalborg Univerity, ha funded the major part of the reearch leading to thi thei. Thi funding ha been vital for thi reearch project. I would lie to expre my gratitude to Profeor Frede Blaabjerg, for providing me the preciou chance of carrying out the whole project in the Intitute of Energy Technology, Aalborg Univerity. Before I came to Aalborg Univerity, I tudied a a Ph.D. candidate in China Electric Power Reearch Intitute in Beijing, China. During that time, I gained valuable nowledge, idea, and inpiration from all the colleague in the reearch group. I am very grateful to all of them, epecially Profeor Weiheng, Wang, for many fruitful, pirited dicuion. My colleague at the Intitute of Energy Technology, Aalborg Univerity are all given than for their help during thi reearch project. Finally, I owe pecial than to my whole family and my girlfriend, Li Fan, for their patience with me in the period where I have been apart from them. May 2004, Aalborg Tao Sun III

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6 Abtract Intitutional and governmental upport on wind energy ource, together with the wind energy potential and improvement of wind energy converion technology, ha led to a fat development of wind power generation in recent year. The continuou increae of the wind power penetration level bring a reult that wind power generation gradually become an important component of power generation in the grid, which mae the tudy on the wind power quality iue and the interaction between the wind turbine and the grid neceary and imperative. The reearch documented in thi thei examine power quality iue of grid-connected wind turbine and the interaction between wind turbine and the grid. The pecific goal of the reearch ha been to invetigate flicer emiion and mitigation of grid-connected wind turbine with doubly fed induction generator (DFIG) during continuou operation, and voltage recovery of uch ind of grid-connected wind turbine after the clearance of a hort circuit fault in the grid. A a bai of the reearch, a model of grid-connected wind turbine with DFIG i developed in the dedicated power ytem analyi tool PSCAD/EMTDC, which imulate the dynamic of the ytem from the turbine rotor, where the inetic wind energy i converted to mechanical energy, to the grid connection point where the electric power i fed into the grid. The complete grid-connected wind turbine model include the wind peed model, the aerodynamic model of the wind turbine, the mechanical model of the tranmiion ytem, model of the electrical component, namely the DFIG, PWM voltage ource converter, tranformer, capacitor, and the control ytem. The grid model and the electrical component of the wind turbine are built with tandard electrical component model from PSCAD/EMTDC library. The wind model, the aerodynamic model, the mechanical model and the control ytem are built with cutom component developed in PSCAD/EMTDC. Two control cheme are implemented in the developed grid-connected wind turbine model: peed control and pitch control. The peed control cheme i compoed by two vectorcontrol cheme deigned repectively for the rotor-ide and grid-ide PWM voltage ource converter. Cacade control i ued in the vector-control cheme. Two deign method, poleplacement and internal model control, are applied for deigning the PI-controller in the vector-control cheme. The pitch control cheme i employed to regulate the aerodynamic power from the turbine. The performance of the control cheme, repectively current control loop, power control loop, DC-lin voltage control loop and pitch control loop, are illutrated, which meet the deign requirement. Simulation reult how that the wind turbine i capable of providing atifactory teady tate and dynamic performance, which mae it poible that the wind turbine model can be applied to tudy the power quality iue of uch ind of grid-connected wind turbine and their interaction with the grid. To evaluate the flicer level produced by grid-connected wind turbine with DFIG, a flicermeter model i developed according to the IEC tandard IEC , which imulate the repone of the lamp-eye-brain chain and provide on-line tatitical analyi of V

7 VI Abtract the flicer ignal and the final reult. Baed on the developed model of grid-connected wind turbine with DFIG and the flicermeter model, the flicer emiion during continuou operation i tudied. The influence factor that affect flicer emiion of grid-connected DFIG wind turbine, uch a wind characteritic (mean peed, turbulence intenity) and grid condition (hort circuit capacity, grid impedance angle) are analyed. The effect of the influence factor are compared with previou reearch reult related to the fixed peed wind turbine. In particular, the effect of mean wind peed, turbulence intenity and grid impedance angle are different from that in the cae of the fixed peed wind turbine. It i poible to regulate the reactive power flow on the connection line o that the voltage fluctuation caued by the active power flow can be compenated by that caued by the reactive power flow. Baed on thi principle, two effective meaure are propoed to mitigate the flicer level produced by grid-connected wind turbine with DFIG, repectively by wind turbine output reactive power control and uing STATCOM. Simulation reult demontrate that thee two meaure are effective for flicer mitigation regardle of mean wind peed, turbulence intenity and hort circuit capacity ratio. The voltage recovery tudy i tarted with grid-connected wind turbine with dynamic lip control, which are imple, cot-effective, partially variable peed wind turbine, for gaining a good undertanding of tranient repone of induction generator in an external hort-circuit ituation. The model of a variable peed wind turbine with dynamic lip control in the imulation tool of PSCAD/EMTDC i preented, and the control cheme, repectively dynamic lip control and pitch control, are decribed. The tranient proce of grid-connected wind turbine after an external hort-circuit fault i analyzed in detail. It i concluded from the analyi that increaing the electromagnetic torque or decreaing the aerodynamic torque help to recover the voltage after the clearance of an external hort-circuit fault. For the wind turbine with dynamic lip control, after the clearance of an external hort-circuit fault, the electromagnetic torque may be trengthened by adjuting the generator lip, and the aerodynamic torque may be reduced by regulating the pitch angle, which helped to low the rotor peed down and re-etablihed the voltage at the wind turbine terminal. Simulation reult demontrate that pitch control, dynamic lip control and combined control are effective meaure for voltage recovery of grid-connected wind turbine with dynamic lip control. An emergency pitch regulation cheme i developed and applied in the cae of pitch control. Baed on the acquired nowledge, the voltage recovery of grid-connected wind turbine with DFIG i tudied. Two ind of ituation are tudied which depend on whether the rotor protection device in the DFIG are triggered or not. When the ituation after an external hort-circuit fault i not eriou enough to trigger the rotor protection device, the control cheme of the DFIG operated a normal and are capable of forcing the rotor peed down and re-etablihing the voltage at the wind turbine terminal after the clearance of the hort-circuit fault, which are demontrated by imulation reult. The performance of the wind turbine a well a the control cheme are illutrated in detail. If the ituation after an external hort-circuit fault i eriou enough, the protection device in the rotor circuit will be triggered which yield a reult that the generator rotor i hortcircuited and the rotor-ide converter i deactivated. In thi ituation, a control trategy i propoed to re-etablih the voltage at the wind turbine terminal and retore the wind turbine normal operation after the fault clearance, which i verified by imulation reult. The control trategy, which tae advantage of the benefit of the rotor circuit protection device and the emergency pitch control cheme, are performed in three tep, repectively protection device activation, voltage recovery aited by pitch control, and normal operation retoration of the wind turbine with DFIG.

8 Content Preface Abtract III V Part I Preliminarie 1 1 Introduction Bacground and motivation Literature tudy Flicer Voltage recovery Problem tatement Outline of the thei Bibliography Part II Modelling and control of grid-connected wind turbine with DFIG Model of grid-connected wind turbine with DFIG Introduction Wind model Par cale wind model Rotor wind model Aerodynamic model Power extraction from the air tream Aerodynamic model Aerodynamic power control Mechanical model DFIG model Steady tate equivalent circuit Operation principle Dq-model in the arbitrary reference frame Dq-Model in the Rotor Fixed Reference Frame PWM voltage ource converter model Summary Bibliography VII

9 VIII Content 3 Control cheme and performance of grid-connected wind turbine with DFIG Speed control cheme Vector-control cheme of grid-ide PWM voltage ource converter Vector-control cheme of rotor-ide PWM voltage ource converter PI-controller deign Pitch control cheme Performance of the wind turbine Wind turbine decription Wind imulation Performance of the control cheme Performance of the wind turbine Summary Bibliography Part III Flicer emiion and mitigation of grid-connected wind turbine with DFIG 73 4 Flicer emiion of grid-connected wind turbine with DFIG The reaon of flicer Flicermeter model Flicer emiion of grid-connected wind turbine with DFIG The influence factor of flicer Mean wind peed Turbulence intenity Short circuit capacity ratio Grid impedance angle Summary Bibliography Flicer mitigation of grid-connected wind turbine with DFIG The principle Flicer mitigation by control of wind turbine output reactive power Flicer mitigation uing STATCOM Simplified model of PWM voltage ource converter Control cheme for STATCOM Flicer mitigation uing STATCOM Summary Bibliography Part IV Voltage recovery of grid-connected wind turbine at an external hort-circuit fault Voltage recovery of grid-connected wind turbine with dynamic lip control Modelling and control of grid-connected wind turbine with dynamic lip control Wind turbine model Control cheme Tranient analyi...113

10 Content IX 6.3 Voltage recovery of grid-connected wind turbine with dynamic lip control Voltage recovery aited by pitch control Voltage recovery aited by dynamic lip control Voltage recovery aited by combined control Summary Bibliography Voltage recovery of grid-connected wind turbine with DFIG Studied ytem Tranient analyi of the pot-fault proce During the fault After the clearance of the fault Control trategy for voltage recovery Protection device activation Voltage recovery Normal operation retoration A imulation cae with high wind peed Summary Bibliography Part V Concluion Concluion Summary of the thei Concluion and new contribution Future wor Part VI Appendice 145 A Wind turbine model in PSCAD/EMTDC 147 A.1 Introduction A.2 Turbine A.3 DFIG A.4 PWM voltage ource converter A.5 Grid- and rotor-ide PWM converter control bloc Bibliography B Sytem parameter 153 C Complete reference lit 155 D Publihed paper 161

11 X Content

12 Part I Preliminarie

13

14 Chapter 1 Introduction The reearch documented in thi thei relate to power quality iue of grid-connected wind turbine and the interaction between wind turbine and the grid that they are connected to. In particular, thi wor concentrate on flicer emiion and mitigation of grid-connected wind turbine with doubly fed induction generator (DFIG) during continuou operation, and voltage recovery of uch ind of grid-connected wind turbine after the clearance of a hort circuit fault in the grid. The bacground and the motivation for the reearch are preented in thi chapter, a well a a tudy on the previouly publihed relevant reearch wor. Alo, the problem tatement i given and an outline of the individual chapter conclude the chapter. 1.1 Bacground and motivation Riing pollution level and worrying change in climate, ariing in great part from energyproducing procee, demand the reduction of ever-increaing environmentally damaging emiion. Additionally an emerging awarene of finitene of the earth foil fuel reerve ha caued many people to loo for alternative. Generating electricity, particularly by maing ue of renewable reource, allow the attainment of notable effect. Thereby the immene potential of wind energy on the earth aume great importance. The worldwide potential of wind power mean that it contribution to electricity production can be of ignificant proportion. In many countrie, the potential for wind energy production exceed by far the local conumption of electricity. Good propect and economically attractive expectation for the ue of wind power are indiviibly lined to the incorporation of thi weather-dependent power ource into exiting power ytem. In the pat decade, the wind power generation ha experienced a very fat development. Table 1.1 how the intalled wind turbine capacity worldwide at the end of 2002 [1], although it i obviou that with uch a rapid growth in ome countrie data of thi ind become out of date very quicly. The reaon that reulted in the fat development of wind power are quite complex. Important factor include the immene potential of wind energy on the earth, the political and economic upport from the government and the development of wind turbine technology. 3

15 4 Power quality of grid-connected wind turbine with DFIG and their interaction with the grid Table 1.1 Intalled wind turbine capacity throughout the world, the end of 2002 [1] Location Intalled capacity (MW) USA 4,685 Canada 238 North America 4,923 Germany 12,001 Spain 4,830 Denmar 2,880 Italy 785 Netherland 688 UK 552 EU Total 23,056 Other Europe 235 India 1,702 China 468 Japan 415 Other Total 2,914 World Total 31,128 Wind turbine technology, dormant for many year, awoe at the end of the 20th century to a world of new opportunitie. Development in many other area of technology were adapted to wind turbine and have helped to haten their quic emergence. A few of the many area which have contributed to the new generation of wind turbine include material cience, aerodynamic, power electronic, computer cience, teting and analytical method. The main option in wind turbine deign and contruction include [2]: axi of rotation: horizontal or vertical number of blade (commonly two and three) rotor orientation: downwind or upwind of tower blade material, contruction method, and profile hub deign: rigid, teetering or hinged power control via aerodynamic control (tall control) or variable pitch blade (pitch control) fixed or variable rotor peed orientation by elf-align action (free yaw), or direct control (active yaw) ynchronou or induction generator gearbox or direct drive generator Today, the mot common deign of wind turbine i the horizontal axi, three-bladed, upwind wind turbine.

16 Chapter 1. Introduction 5 Wind turbine can either operate at fixed peed or variable peed. Compared with fixed peed operation, variable peed operation ha a number of benefit including the reduction of wear and tear on the wind turbine and potential operation of the wind turbine at maximum efficiency over a wide range of wind peed, yielding increaed energy capture [2]. The benefit are lited in detail a follow [3]: below rated wind peed, the rotor peed can be made to vary with wind peed to maintain pea aerodynamic efficiency. the reduced rotor peed in low wind reult in a ignificant reduction in aerodynamically-generated acoutic noie noie i epecially important in low wind, where ambient wind noie i le effective at maing the turbine noie. the rotor can act a a flywheel, moothing out aerodynamic torque fluctuation before they enter the drive train thi i particularly important at the blade paing frequency. direct control of the air-gap torque allow gearbox torque variation above the mean rated level to be ept very mall. both active and reactive power can be controlled, o that unity power factor can be maintained it i even poible to ue a variable peed wind farm a a ource of reactive power to compenate for the poor power factor of other conumer on the networ; variable peed turbine will alo produce a much lower level of electrical flicer. Mot of the major wind turbine manufacturer are developing new larger wind turbine. Thee wind turbine are all baed on variable peed operation with pith control. Three main type of variable peed wind turbine are illutrated a follow [4]: wind turbine equipped with quirrel cage induction generator, connected to the grid through a tator converter cacade wind turbine equipped with DFIG, connected to the grid through a rotor converter cacade wind turbine equipped with ynchronou generator and a tator DC-lin cacade for networ connection Nowaday, DFIG are mot commonly ued by the wind turbine indutry for larger wind turbine [5]. The mot ignificant reaon for the popularity of DFIG i the relatively mall ize of power converter approximately 10-25% of nominal turbine power which i a cot efficient olution in order to obtain variable peed [6]. The acnowledgement of the increaing intalled capacity of grid-connected wind turbine with DFIG i the bacground for thi reearch wor. Fig. 1.1 how the bloc diagram of a grid-connected wind turbine with DFIG, where PCC repreent the Point of Common Coupling. A the wind power generation, which depend on the weather, i integrated into the grid continuouly and quicly, the influence of wind turbine on the grid power quality i becoming an important iue, epecially the influence on voltage fluctuation and flicer. Flicer i defined a an impreion of unteadine of viual enation induced by a light timulu, whoe luminance or pectral ditribution fluctuate with time [7], which can caue conumer annoyance and complaint. Furthermore, flicer can become a limiting factor for integrating wind turbine into wea grid, and even into relatively trong grid where the wind power penetration level are high.

17 6 Power quality of grid-connected wind turbine with DFIG and their interaction with the grid Turbine Gearbox DFIG Tranformer PCC Grid 3 Z Power converter Figure 1.1 Bloc diagram of a grid-connected wind turbine with a DFIG. A mentioned above, variable peed operation of the rotor ha the advantage that the fater power variation are not tranmitted to the grid but are moothed by the flywheel action of the rotor, and will alo produce a much lower level of electrical flicer in the grid. Even though, the propect of achieving a better undertanding of the flicer produced by grid-connected wind turbine with DFIG i one baic motivation for thi reearch wor, which will benefit integrating more wind power generation into the exiting grid. The continuou increae of the wind power penetration level bring a reult that wind power generation gradually become an important component of power generation in the grid, which mae the tudy on the interaction between the wind turbine and the grid neceary and imperative, epecially in ome critical ituation lie hort-circuit fault, voltage dip, etc. The ytem operator, who are reponible for maintaining power ytem tability and reliable power upply, have formulated pecification regarding grid integration of wind power generation. According to the pecification in Denmar [8], at a hort-circuit fault in the external grid, the voltage at the wind turbine terminal hould be re-etablihed after the fault clearance without any power lo caued by diconnection of wind turbine. The imilar requirement can be found in the pecification in Germany, the Netherland, England and Wale [9]. The reaon i, when the wind power penetration level i high, the protective diconnection of a large amount of wind power will be an unacceptable conequence that may threaten the power ytem tability. To invetigate the voltage recovery iue of grid-connected wind turbine with DFIG after the clearance of an external hort-circuit fault i another baic motivation for thi reearch wor. 1.2 Literature tudy A the intalled capacity of grid-connected wind turbine increaed continuouly and rapidly in recent year, the power quality iue of grid-connected wind turbine and the interaction between the wind turbine and the grid have attracted coniderable interet. Thi ha reulted in a numerou cientific paper and report, which are mainly from academia. An in-depth review of thi prior wor will become quite voluminou and, therefore, only the main contribution, which are tightly related to the flicer produced by grid-connected wind turbine during continuou operation and the voltage recovery iue of grid-connected wind turbine, are outlined and commented.

18 Chapter 1. Introduction Flicer The reaon of flicer Grid-connected wind turbine may have coniderable fluctuation in the output power, a the wind i a weather-dependent power ource. Reference [10] indicate that the grid uffer voltage fluctuation and flicer a the wind turbine output power, which flow into the grid, varie. The flicer produced by grid-connected wind turbine during continuou operation i mainly caued by fluctuation in the output power due to wind peed variation, the wind gradient and the tower hadow effect [11]. A a conequence of the combination of wind peed variation, the wind gradient and the tower hadow effect, an output power drop will appear three time per revolution for a three-bladed wind turbine. Thi frequency i normally referred to a 3 p. For fixed peed wind turbine with induction generator, power pulation up to 20% of the average power at the frequency of 3 p will be generated [12]. The tower hadow effect i produced becaue the wind turbine tower offer reitance to the wind flow, and it diturb the wind flow both uptream and downtream. Far from the tower influence, the wind peed i unchanged, while it increae when approaching the tower and decreae when coming cloer. A Fourier erie with harmonic multiple of 3 p frequency can repreent thi hadow effect [13]. The tower hadow effect i more important to the wind turbine having their blade downwind of the tower. The wind hear phenomenon alo produce torque ocillation caued by the wind peed gradient along the height of the area wept by the blade. The wind peed gradient may be decribed in polar coordinate centred at the hub elevation by the binomial erie [13]. A the rotor ample the incoming wind, it ee the wind profile a a periodical varying function of the time with harmonic multiple of 3 p frequency. The output power of grid-connected wind turbine have been analyed in the frequency domain [14, 15]. The reult how that, in addition to the dominating periodic component 3 p, the 6 p, 9 p, 12 p and 18 p component are viible too. A poible reaon for the exitence of the 1 p component i that the rotor may be unbalanced. Another poibility i that one of the blade produce a higher torque than the other one. The tower reonance frequency i alo detectable which i aumed originating from a ide-way ocillation of the turbine. Influence factor There are numerou of factor that affect flicer emiion of grid-connected wind turbine during continuou operation, uch a wind characteritic (e.g. mean wind peed, turbulence intenity), grid condition (e.g. hort circuit capacity, grid impedance angle) and type of wind turbine (e.g. fixed peed, variable peed). The flicer level increae at higher wind peed due to higher turbulence in the wind. For fixed peed wind turbine, the flicer level increae around three time from lower to higher wind peed. For variable peed wind turbine, the flicer level increae with a rie in the wind peed, until the wind peed reache it rated value. A the wind turbine reache it rated power, the variable peed ytem will mooth out the power fluctuation and, thereby, limit the flicer [11].

19 8 Power quality of grid-connected wind turbine with DFIG and their interaction with the grid The turbulence intenity ha a ignificant influence on flicer emiion of grid-connected wind turbine. The flicer level ha an almot directly proportional relation with the turbulence intenity [16, 17]. The flicer level i approximately inverely proportional to the hort circuit capacity at the PCC of wind turbine [16, 18]. The grid impedance angle i o important that, if a proper value i choen, the voltage change from the varying active power flow will be cancelled by that from the varying reactive power flow and, therefore, the voltage fluctuation and the flicer level are reduced [16, 18, 19]. The determining factor i the difference between the grid impedance angle and the wind turbine power factor angle [16]. When the difference approache 90 degree, the flicer emiion i minimized. Different type of wind turbine produce different flicer level. The flicer level produced by variable peed wind turbine i coniderably lower than that produced by fixed peed wind turbine. A 4 time reduction of the flicer level can be realized through variable peed operation [16]. The flicer contribution from the 3 p component i quite ignificant for fixed peed wind turbine. However, variable peed wind turbine have the ability to reduce the 3 p pulation in the output power [11]. Mot of the reference lited above are focued on flicer emiion of fixed peed wind turbine. However, no pecific wor have been concentrated on flicer emiion of variable peed wind turbine with DFIG and the correponding effect of the above-mentioned influence factor. Flicer mitigation The flicer mitigation of grid-connected wind turbine during continuou operation may be realized by application of auxiliary device, uch a reactive power compenation equipment and energy torage equipment. The Static Var Compenator (SVC), a commonly ued device for flicer mitigation, i ued to reduce the voltage fluctuation and flicer produced by wind power generation [20]. Some reearch reult have hown that the STATCOM i uperior to the SVC with repect to flicer mitigation [21, 22]. Therefore, the STATCOM, which receive much more attention recently, i alo applied to mitigate the voltage fluctuation and flicer caued by wind power generation [23, 24]. Some other reactive power compenation device, uch a SVC Light [25] and UPFC [26], are alo employed for flicer mitigation of grid-connected wind turbine. Reference [27] applie an inductive torage in the DC-lin in a wind turbine with a tator AC-DC-AC power converter to mooth the output power, thu, to relieve the voltage fluctuation and flicer. However, the tudied wind turbine in [27] i not popular in reality Voltage recovery The tranient tability iue of grid-connected wind turbine at a hort-circuit fault in the external grid ha received much more attention in the pat decade. Mot of the reearch wor are focued on grid-connected wind turbine with cage induction generator and gridconnected wind turbine with DFIG.

20 Chapter 1. Introduction 9 Reference [28] illutrate in detail the tranient proce of grid-connected wind turbine with induction generator at a hort-circuit fault in the external grid. There are numerou of factor that influence the tranient tability of grid-connected wind turbine with induction generator at an external hort-circuit fault. The reearch wor in [29] examine the effect of wind turbine haft tiffne, ma moment of inertia of the wind turbine and generator rotor, length of the interfacing line, nominal voltage level of the interfacing line, wind turbine operating power factor, and the type of fault. Reference [30, 31] illutrate the influence of wind turbine mechanical parameter on the tranient tability. In [32], the influence of grid hort-circuit capacity, X/R ratio, fault type, and wind turbine power factor correction on the tranient tability are demontrated. The effect of wind peed, wind turbine mechanical parameter, reactive power compenation, and grid hort-circuit capacity are hown in [33]. Regarding the improvement of tranient tability of grid-connected wind turbine with induction generator at an external hort-circuit fault, [25, 31, 34] applie different meaure, uch a adjutment of wind turbine protective relay etting, uing the control ability of wind turbine, dynamic reactive compenation, to re-etablih the wind turbine terminal voltage after the fault clearance. Reference [32] propoe everal meaure to help recovering the wind turbine terminal voltage after the fault clearance, uch a increaing the generator electrical torque, reducing the turbine mechanical torque, dynamic reactive compenation, and controlling the wind turbine active power. However, the paper doe not provide the effect of the propoed meaure. Grid-connected wind turbine with DFIG have hown better behaviour concerning tranient tability at hort-circuit fault in the external grid in comparion with wind turbine with cage induction generator [33, 35, 36]. Reference [37] analye the tranient proce of grid-connected wind turbine with DFIG at a hort-circuit fault in the external grid. However, thi paper doe not dicu the improvement of tranient tability and the voltage recovery after the fault clearance. Reference [38] tudie the tranient proce of grid-connected wind turbine with DFIG at an external hort-circuit fault, and applie two meaure to improve the tranient tability and re-etablih the wind turbine terminal voltage after the fault clearance. The applied two meaure are repectively regulating the parameter of control cheme and fat-acting reactive power control. However, in thi paper the DC-lin voltage i aumed to be contant during the tranient proce, which will bring a dicrepancy into the reearch reult. 1.3 Problem tatement Although many reearch wor, a mentioned in the literature tudy, have been carried out on flicer emiion and mitigation of grid-connected wind turbine during continuou operation, no pecial wor are ytematically concentrated on flicer emiion and mitigation of grid-connected wind turbine with DFIG, one of the mot popular wind turbine worldwide. To have a good undertanding of the flicer produced by grid-connected wind turbine with DFIG during continuou operation may benefit the development of wind power generation. Furthermore, baed on the tudy of flicer emiion of grid-connected wind turbine with DFIG during continuou operation, ome effective meaure may be put forward to mitigate the flicer level. Therefore, the firt aim of thi reearch i to tudy the flicer emiion and mitigation of grid-connected wind turbine with DFIG during continuou operation. The flicer level and the influence factor that affect the

21 10 Power quality of grid-connected wind turbine with DFIG and their interaction with the grid flicer level mut be tudied, and, baed on the reearch reult, ome effective meaure to mitigate the flicer level mut be put forward. Due to the flexibility provided by power electronic converter, the behaviour of gridconnected wind turbine with DFIG i quite different from that of wind turbine with cage induction generator. It i quite intereting to analye the tranient repone of the wind turbine control cheme in an external hort-circuit fault ituation, thu to tudy the tranient tability of grid-connected wind turbine with DFIG. Furthermore, it i nown from the literature tudy that no effective meaure have been propoed for voltage recovery of gridconnected wind turbine with DIFG. Therefore, the econd aim of thi reearch i to analye the tranient proce of grid-connected wind turbine with DFIG at a hortcircuit fault in the external grid, and in critical pot-fault ituation, to propoe effective meaure for voltage recovery of grid-connected wind turbine with DFIG after the fault clearance. The voltage recovery iue of grid-connected wind turbine with dynamic lip control i initially tudied, which are imple, cot-effective, partially variable peed wind turbine, for gaining a good undertanding of tranient repone of induction generator in an external hort-circuit ituation. 1.4 Outline of the thei The documentation of the reearch i accomplihed to fulfil the above two aim. The eight chapter in the thei are organized in five part a follow: Part I. Preliminarie Chapter 1: Introduction It i thi chapter. The bacground and the motivation for the reearch have been preented along with a brief decription of the publihed reearch wor in thi area. Next the problem tatement i given. Finally, thi outline of the individual chapter in the thei i given. Part II. Modelling and control of grid-connected wind turbine with DFIG Chapter 2: Model of grid-connected wind turbine with DFIG A whole model of a grid-connected wind turbine with DFIG uing bac-to-bac PWM voltage ource converter in the rotor circuit i built, which include the wind peed model, the aerodynamic model of the wind turbine, the mechanical model of the tranmiion ytem and model of the electrical component, namely the DFIG and PWM voltage ource converter. Chapter 3: Control cheme and performance of grid-connected wind turbine with DFIG Two control cheme, repectively peed control cheme and pitch control cheme, are implemented in the wind turbine model developed in Chapter 2. The peed control cheme i compoed by two vector-control cheme deigned repectively for the rotor-ide and gridide PWM voltage ource converter. Two deign method, pole-placement and internal model

22 Chapter 1. Introduction 11 control, are applied for deigning the PI-controller in the vector-control cheme. The pitch control cheme i employed to regulate the aerodynamic power from the turbine. The performance of the control cheme and the wind turbine are alo decribed in detail. Part III. Flicer emiion and mitigation of grid-connected wind turbine with DFIG Chapter 4: Flicer emiion of grid-connected wind turbine with DFIG A flicermeter model i built according to IEC tandard IEC [39]. Baed on the wind turbine model and the flicermeter model, flicer emiion of grid-connected wind turbine with DIFG i invetigated during continuou operation. The influence factor that affect the flicer level, uch a wind characteritic (mean peed, turbulence intenity) and grid condition (hort circuit capacity, grid impedance angle) are analyed. The effect of the influence factor are compared with previou reearch reult related to the fixed peed wind turbine. Chapter 5: Flicer mitigation of grid-connected wind turbine with DFIG Baed on the reearch reult in Chapter 4, two effective meaure, repectively controlling the wind turbine output reactive power and uing STATCOM, are applied to mitigate the flicer level produced by grid-connected wind turbine with DIFG, which are verified by imulation reult. Part IV. Voltage recovery of grid-connected wind turbine at an external hortcircuit fault Chapter 6: Voltage recovery of grid-connected wind turbine with dynamic lip control A model of a grid-connected wind turbine with dynamic lip control i developed. The tranient proce of uch ind of wind turbine at a hort-circuit fault in the external grid ha been invetigated. After the fault clearance, the voltage recovery may be realized by pitch control, dynamic lip control and combined control, which are demontrated in the imulation reult. Chapter 7: Voltage recovery of grid-connected wind turbine with DFIG The tranient repone of the control cheme and the wind turbine at a hort-circuit fault in the external grid are analyed in detail. In critical pot-fault ituation, a control trategy i propoed to help recovering the wind turbine terminal voltage and improving the ytem tranient tability, which i verified by the imulation reult. Part V. Concluion Chapter 8: Concluion The main concluion and contribution of the reearch documented in thi thei are highlighted with uggetion for future wor.

23 12 Power quality of grid-connected wind turbine with DFIG and their interaction with the grid Part VI. Appendice Appendix A: Wind turbine model in PSCAD/EMTDC The DFIG wind turbine model ha been developed in the dedicated power ytem analyi tool, PSCAD/EMTDC. Thi appendix decribe the function of the main bloc in the DFIG wind turbine model, e.g. turbine, DFIG, PWM voltage ource converter, grid- and rotor-ide PWM converter control bloc. Appendix B: Sytem parameter Thi appendix provide ome parameter that are ued in the imulation wor, uch a the impedance value in different grid condition, and the protection etting of the wind turbine. Appendix C: Complete reference lit All publication referred in thi thei are lited in thi appendix. Appendix D: Publihed paper A copy of the cientific paper publihed on the wor documented in thi thei i included in thi appendix. A complete lit of the author publication i alo given. Bibliography [1] European Wind Energy Aociation. (2003, March 3). Record growth for global wind power in 2002 [Online]. Available: [2] J. F. Manwell, J. G. McGowan, A. L. Roger, Wind energy explained: theory, deign and application, John Wiley & Son Ltd, Chicheter, [3] T. Burton, D. Sharpe, N. Jenin, E. Boanyi, Wind energy handboo, John Wiley & Son Ltd, Chicheter, [4] S. A. Papathanaiou, G. A. Voa, M. P. Papadopoulo, Ue of power electronic converter in wind turbine and photovoltaic generator, Proc. of the IEEE International Sympoium on Indutrial Electronic, vol. 1, pp , July [5] T. Acermann, L. Söder, An overview of wind energy-tatu 2002, Renewable and Sutainable Energy Review, vol. 6, no. 1/2, pp , [6] L. H. Hanen, P. H. Maden, F. Blaabjerg, H. C. Chritenen, U. Lindhard, K. Eilden, Generator and power electronic technology for wind turbine, Proc. of the 27th Annual Conference of the IEEE on Indutrial Electronic Society, vol. 3, pp , Nov./Dec [7] L. Roetto, P. Tenti, A. Zuccato, Electromagnetic compatibility iue in indutrial equipment, IEEE Indutry Application Magazine, vol. 5, Iue. 6, pp , Nov./Dec [8] ELTRA Tranmiion Sytem Planning, Specification for connecting wind farm to the tranmiion networ, 2nd ed., ELT a, ELTRA Tranmiion Sytem Planning, Apr

24 Chapter 1. Introduction 13 [9] Generic Proviion Woring Group. (2003, June). Bacground information to grid code conultation document D/03 [Online]. Available: [10] T. Achermann, K. Garner, A. Gardiner, Embedded wind generation in wea grid - economic optimization and power quality imulation, Renewable Energy, vol. 18, no. 2, pp , [11] Åe Laron, Flicer emiion of wind turbine during continuou operation, IEEE Tran. on Energy Converion, vol. 17, no. 1, pp , Mar [12] G. Gerde, F. Santjer, Power quality of wind turbine and their interaction with the grid, Proc. of Euro. Wind Energy Conf., pp , Oct [13] D. A. Spera, Wind turbine technology: fundamental concept of wind turbine engineering, ASME Pre, New Yor, [14] T. Thiringer, J. Dahlberg, Periodic pulation from a three-bladed wind turbine, IEEE Tran. on Energy Converion, vol. 16, no. 2, pp , June [15] T. Thiringer, Power quality meaurement performed on a low-voltage grid equipped with two wind turbine, IEEE Tran. on Energy Converion, vol. 11, no. 3, pp , Sep [16] M. P. Papadopoulo, S. A. Papathanaiou, S. T. Tentzerai, N. G. Boulaxi, Invetigation of the flicer emiion by grid connected wind turbine, Proc. of the 8th International Conference on Harmonic and Quality of Power, vol. 2, pp , Oct [17] P. D. Ladaao, M. G. Ioannide, M. I. Koulouvari, Aement of wind turbine impact on the power quality of autonomou wea grid, Proc. of the 8th International Conference on Harmonic and Quality of Power, vol. 2, pp , Oct [18] H. Amarí, C. Vilar, J. Uaola, J. L. Rodríguez, Frequency domain analyi of flicer produced by wind energy converion ytem, Proc. of the 8th International Conference on Harmonic and Quality of Power, vol. 2, pp , Oct [19] H. Sharma, S. Ilam, T. Pryor, C.V. Nayar, Power quality iue in a wind turbine driven induction generator and dieel hybrid autonomou grid, Journal of Electrical and Electronic Engineering, vol. 21, no. 1, pp , [20] Y. Kubota, T. Genji, K. Miyazato, N. Hayahi, H. Touda, Y. Fuuyama, Verification of cooperative control method for voltage control equipment on ditribution networ imulator conidering interconnection of wind power generator, Proc. of Tranmiion and Ditribution Conference and Exhibition 2002: Aia Pacific., vol. 2, pp , Oct [21] Z. Zhang, N. R. Fahmi, W. T. Norri, Flicer analyi and method for electric arc furnace flicer (EAF) mitigation (a urvey), Proc. of 2001 IEEE Porto Power Tech Conference, vol. 1, pp. 6/1-6/6, Sep [22] T. Laron, C. Poumarède, STATCOM, an efficient mean for flicer mitigation, Proc. of the IEEE Power Engineering Society 1999 Winter Meeting, pp , Jan./Feb

25 14 Power quality of grid-connected wind turbine with DFIG and their interaction with the grid [23] Z. Saad-Saoud, M. L. Liboa, J. B. Eanayae, N. Jenin, G. Strbac, Application of STATCOM to wind farm, IEE Proc. Gener. Tranm. Ditrib., vol. 145, no. 5, pp , Sept [24] J. E. Hill, A practical example of the ue of ditribution tatic compenator (D- STATCOM) to reduce voltage fluctuation, Proc. of the IEE Colloquium on Power Electronic for Renewable Energy, pp. 7/1-7/5, Jun [25] R. Grünbaum, SVC Light: A powerful mean for dynamic voltage and power quality control in indutry and ditribution, Proc. of the Eighth International Conference on Power Electronic and Variable Speed Drive, pp , Sept [26] A. Papantoniou, A. Coonic, Simulation of FACTS for wind farm application, Proc. of the IEE Colloquium on Power Electronic for Renewable Energy, pp. 8/1-8/5, Jun [27] P. S. Doopoulo, C. S. Dimoulia, I. M. Manouaridi, A. X. Patralexi, Improvement of power quality in a grid with wind turbine uing inductive torage, Wind Engineering, vol. 23, no. 4, pp , [28] S. K. Salman, I. M. Rida, Invetigating the impact of embedded generation on relay etting of utilitie electrical feeder, IEEE Tran. on Power Delivery, vol. 16, no. 2, pp , Apr [29] S. K. Salman, A. L. J. Teo, Windmill modeling conideration and factor influencing the tability of a grid-connected wind power-baed embedded generator, IEEE Tran. on Power Sytem, vol. 18, no. 2, pp , May [30] V. Ahmatov, H. Knuden, Modelling of windmill induction generator in dynamic imulation program, Proc. of International Conference on Electric Power Engineering, pp. 108, Aug./Sept [31] S. K. Salman, A. L. J. Teo, Improvement of fault clearing time of wind farm uing reactive power compenation, Proc. of 2001 IEEE Porto Power Tech Conference, vol. 2, pp. 6/1-6/6, Sept [32] L. Holdworth, N. Jenin, G. Strbac, Electrical tability of large, offhore wind farm, Proc. of the Seventh International Conference on AC-DC Power Tranmiion, pp , Nov [33] J. Uaola, P. Ledema, Dynamic incidence of wind turbine in networ with high wind penetration, Proc. of 2001 IEEE Power Engineering Society Summer Meeting, vol. 2, pp , July [34] V. Ahmatov, H. Knuden, A. H. Nielen, N. K. Poulen, J. K. Pederen, Short-term tability of large-cale wind farm, Proc. of the European Wind Energy Conference, pp , July [35] T. Gjengedal, Integration of wind power and the impact on power ytem operation, Proc. of 2003 Large Engineering Sytem Conference on Power Engineering, pp , May [36] L. Holdworth, X. G. Wu, J. B. Eanayae, N. Jenin, Comparion of fixed peed and doubly-fed induction wind turbine during power ytem diturbance, IEE Proc. Gener. Tranm. Ditrib., vol. 150, no. 3, pp , May 2003.

26 Chapter 1. Introduction 15 [37] P. Ledema, J. Uaola, Minimum voltage protection in variable peed wind farm, Proc. of 2001 IEEE Porto Power Tech Conference, vol. 4, pp. 6/1-6/6, Sept [38] J. B. Eanayae, L. Holdworth, X. G. Wu, N. Jenin, Dynamic modeling of doubly fed induction generator wind turbine, IEEE Tran. on Power Sytem, vol. 18, no. 2, pp , May [39] International Electrotechnical Commiion, Electromagnetic Compatibility (EMC) --- Part 4: Teting and meaurement technique --- Section 15: Flicermeter --- Functional and deign pecification, IEC , International Electrotechnical Commiion, Geneva, Switzerland, Nov

27 16 Power quality of grid-connected wind turbine with DFIG and their interaction with the grid

28 Part II Modelling and control of grid-connected wind turbine with DFIG

29

30 Chapter 2 Model of grid-connected wind turbine with DFIG To invetigate the power quality iue of grid-connected wind turbine and their interaction with the grid, a proper model of grid-connected wind turbine hall be etablihed firt. The grid-connected wind turbine model imulate the dynamic of the ytem from the turbine rotor where the inetic wind energy i converted to mechanical energy, to the grid connection point where the electric power i fed into the grid. In thi chapter, the model of a grid-connected wind turbine with DFIG, developed in the dedicated power ytem analyi tool, PSCAD/EMTDC, i decribed in detail. Firt, a general introduction of the wind turbine model i given. Next, the wind peed model, the aerodynamic model of the wind turbine, the mechanical model of the tranmiion ytem and model of the electrical component, namely the DFIG and PWM voltage ource converter, are preented in equence. Finally, a ummary of the model of different component of grid-connected wind turbine with DFIG complete the chapter. 2.1 Introduction The grid-connected wind turbine conidered here applie a DFIG, uing bac-to-bac PWM voltage ource converter in the rotor circuit. Fig. 2.1 illutrate the main component of the grid-connected wind turbine, where P, Q are the wind turbine output active and reactive power. The complete grid-connected wind turbine model include the wind peed model, the aerodynamic model of the wind turbine, the mechanical model of the tranmiion ytem, model of the electrical component, namely the DFIG, PWM voltage ource converter, tranformer, capacitor, and the control ytem. The overall tructure of the grid-connected wind turbine model i hown in Fig The equivalent wind peed v eq repreent the whole field of wind peed in the rotor plane of the wind turbine. To include the patial variation of the wind peed field in the rotor plane, the wind model ue the turbine rotor poition θ WTR, which i fed bac from the mechanical 19

31 20 Power quality of grid-connected wind turbine with DFIG and their interaction with the grid model. The aerodynamic model ue an equivalent wind peed v eq, the wind turbine rotor peed w WTR and the blade pitch angle θ a input. It output i the aerodynamic torque T W. The input to the mechanical model are the aerodynamic torque T W and the electromagnetic torque T G. The output are the wind turbine rotor peed w WTR and the generator peed w gen. The generator peed w gen i ued by the control ytem for peed control. The electrical model provide the generator electromagnetic torque T G and ue the generator peed w gen a input. In the other end, the electrical model interface with the grid by the voltage U WT and current I WT on the wind turbine terminal. The electrical model alo output the active power P MS and reactive power Q MS, repreenting the meaured voltage and current of the control ytem. The control ytem provide a number of control ignal for the electrical model, including the control ignal α f to the PWM converter. The model of the grid-connected wind turbine with DFIG i developed in the dedicated power ytem analyi tool, PSCAD/EMTDC. The grid model and the electrical component of the wind turbine are built with tandard electrical component model from PSCAD/EMTDC library. The wind model, the aerodynamic model, and the mechanical model are built with cutom component developed in PSCAD/EMTDC. The control ytem of the wind turbine i alo built with cutom component developed in PSCAD/EMTDC, which will be dicued in the next chapter. Turbine Wind Gearbox DFIG 3 Tranformer P, Q Capacitor PWM converter PWM converter Figure 2.1 Bloc diagram of a grid-connected wind turbine with a DFIG. θ Control ytem α f,... P MS, Q MS Wind Aerodynamic T W Mechanical w gen Electrical I WT Grid v eq w WTR T G U WT θ WTR Figure 2.2 Overall tructure of the grid-connected wind turbine model.