Application of Wind Power Generation in Grid Connected Power System.

Transcription

1 Application of Wind Power Generation in Grid Connected Power Sytem. Kenneth E. Okedu Ph.D. Department of Electrical and Electronic Engineering, Univerity of Port Harcourt, Nigeria ABSTRACT Thi paper preent application of wind power generation in a grid connected multi-machine power ytem. An overview of wind energy technology and the current world wind energy cenario are preented. Variou power generating unit were conidered in the prototype model of tudy. Merit and drawback of ingle and multi-machine ytem, fixed and variable peed wind turbine were alo preented. Focu wa made on the wind-dieel ytem in the multimachine model preented. The wind power impact on power quality wa alo dicued. In addition, the tranmiion ytem to hore (high voltage direct current - HVDC and high voltage alternating current - HVAC) were analyzed repectively. The proce of producing hydrogen ga a a mean of tranporting and balancing wind power production, through an electrolyzer unit connected to the variable peed wind turbine in the offhore wind farm wa analyzed. The variable peed drive mathematical imulink differential equation were alo preented. Simulation analye were carried out in power ytem computer aided deign and electromagnetic tranient including DC (PSCAD/EMTDC) to demontrate the repone of the prototype model of the tudy. (Keyword: variable peed wind turbine, wind energy, hydrogen ga, onhore network, offhore network.) INTRODUCTION Energy i an eential ingredient of ocioeconomic development and economic growth. Renewable energy ource like wind energy are indigenou and can help in reducing the dependency on foil fuel [1]. Currently, about 87% of total energy i generated from foil fuel (coal, oil, and natural ga), 6% i generated in nuclear plant, and the remaining 7% come from renewable ource (mainly hydro and wind power) [2]. Unfortunately, the world ha limited amount of foil fuel and nuclear power reource. According to current etimate, the natural uranium for nuclear power will lat only about 50 year; oil will lat no more than 100 year; ga, 150 year; and coal, 200 year. Alo, our overdependence on foil and nuclear fuel i cauing environmental pollution and afety problem, which are now becoming dominant iue in our ociety. The impact of the environmental pollution on global warming and the reulting climatic change can have diatrou conequence in the long run. Hence, at thi juncture, the world i turning more and more to environmental clean and afe renewable energy ource like wind, photovoltaic, and fuel cell. The world ha enormou reource of wind energy, and it ha been etimated that tapping barely 10% of the wind energy available could upply all the electricity need of the world [2, 3]. The recent technological advance in variablepeed wind turbine, power electronic, drive, and control have made wind energy competitive with coal and natural ga power. However, one of the problem of wind energy i that it availability i poradic, and, therefore, it need to be backed by other power ource. Photovoltaic ytem have the additional advantage of being tatic and barely requiring repair and maintenance. However, photovoltaic power i typically five time more expenive than wind power, though there i tremendou reearch and development effort to develop low-cot photovoltaic panel for widepread terretrial application. The Pacific Journal of Science and Technology 87

2 Solar power converion efficiency i typically around 16%, and it availability i alo poradic. The primary fuel for fuel cell energy i hydrogen or a foil fuel type like gaoline or methanol, with a reformer. Fuel cell are tatic and have high converion efficiency of about 60%. However, they are bulky and expenive and have poor tranient repone in their current tate. Fuel cell how tremendou promie for the future, particularly for electric car, although a tremendou amount of reearch and development i needed to achieve thi goal. Thi paper addree application of wind power generation in a grid connected multi-machine power ytem. An overview of wind energy technology and the current world wind energy cenario are preented. Variou power generating unit were conidered in the prototype model of tudy. Merit and drawback of ingle and multi-machine ytem, fixed and variable peed wind turbine were alo preented. Focu wa made on the wind-dieel ytem in the multimachine model preented. The wind power impact on power quality wa alo dicued, in addition to the tranmiion ytem to hore (high voltage direct current - HVDC and high voltage alternating current - HVAC). The mean of producing hydrogen ga for tranporting and balancing wind power production, through an electrolyzer unit connected to the variable peed wind turbine in the offhore wind farm wa analyzed. The variable peed drive mathematical imulink differential equation were alo preented. Simulation analye were carried out in power ytem computer aided deign and electromagnetic tranient including DC (PSCAD/EMTDC) to how the repone of the prototype model conidered. OVERVIEW OF WIND ENERGY TECHONLOGY Wind i the indirect form of olar energy and i alway being replenihed by the un. Wind i caued by differential heating of the Earth` urface by the un. It ha been etimated that roughly 10 million MW of energy are continuouly available in the Earth wind [4-6]. Wind energy provide an environmental friendly option and national energy ecurity at a time when decreaing global reerve of foil fuel threaten the long-term utainability of global economy. The wind turbine technology ha a unique technical identity and unique demand in term of the method ued for deign. Remarkable advance in the wind power deign have been achieved due to modern technological development. Since 1980, advance in aerodynamic, tructural dynamic, and micrometeorology have contributed to a 5% annual increae in the energy yield of the turbine [7-11]. Current reearch technique are producing tronger, lighter, and more efficient blade for wind turbine. The annual energy output for turbine ha increaed enormouly and the weight of the turbine and the noie they emit have been halved over the lat few year. More power can be generated from wind energy by etablihment of many wind monitoring tation, election of wind farm ite with uitable wind generator, improved maintenance procedure of wind turbine to increae the machine availability, ue of large capacity machine, low wind regime turbine, higher tower height, wider wept area of the rotor blade, better aerodynamic and tructural deign, fater computer-baed machining technique, increaing power factor and better policie from government. Even among the other application of renewable energy technologie, power generation through wind ha an edge becaue of it technological maturity, good infratructure and relative cot competitivene. Wind energy i expected to play an increaing important role in the future national energy cene [1, 3 and 12]. Wind turbine convert the kinetic energy of the wind to electrical energy by rotating the blade. Greenpeace tate that about 10% electricity can be upplied by the wind by the year At good windy ite, it i already competitive with traditional foil fuel power tation. With thi improved technology and uperior economic, expert predict wind power would capture 5% of the world energy market by Advanced wind turbine mut be more efficient, more robut and le cotly than current wind turbine. WORLD WIND ENERGY SCENARIO Detail on the world wind energy cenario in November, 2011, baed on the world wind energy aociation (WWEA) report [13] are a follow. Worldwide capacity reached 196,630 MW, out of which 37,642 MW were added in 2010, lightly le than in The Pacific Journal of Science and Technology 88

3 World Total Intalled Capacity [MW] Figure 1: World Total Intalled Capacity of Wind Energy [13]. Wind power howed a growth rate of 23.6%, the lowet growth ince 2004 and the econd lowet growth of the pat decade. All wind turbine intalled by the end of 2010 worldwide can generate 430 TWh per annum, more than the total electricity demand of the United Kingdom, the ixth larget economy of the world, and equaling 2.5% of the global electricity conumption. The wind ector in 2010 had a turnover of 40billion Euro and employed 670,000 peron worldwide. China became number one in total intalled capacity and the center of the international wind indutry, and added 18,928 MW within one year, accounting for more than 50% of the world market for new wind turbine. Major decreae in new intallation can be oberved in North America and the USA lot it number one poition in total capacity to China. Many Wetern European countrie are howing tagnation, wherea there i trong growth in a number of Eatern European countrie. Germany keep it number one poition in Europe with 27,215 MW, followed by Spain with 20,676 MW. The highet hare of wind power can be found in three European countrie: Denmark (21%), Portugal (18%) and Spain 16%). Aia accounted for the larget hare of new intallation (54.6%), followed by Europe (27.0%) and North America (16.7%). Latin America (1.2%) and Africa (0.4%) till play only a marginal role in new intallation. In Africa, North Africa till repreent large hare of intalled capacity, and wind energy play hardly a role yet in Sub-Sahara Africa. Nuclear diater in Japan and oil pill in Gulf of Mexico will have long-term impact on the propect of wind energy. Government need to urgently reinforce their wind energy policie. The World Wind Energy Aociation (WWEA) ee a global capacity of 600,000 MW a poible by the year 2015 and more than 1,500,000 MW by the year 2020 [13]. In the year 2010, the wind capacity reached worldwide MW a hown in Figure 1, after 159,050 MW in 2009, 120,903 MW in 2008, and 93,930 MW in 2007 [14-17]. Invetment in new wind turbine aw a decline in many part of the world. For the firt time in more than two decade, the market for new wind turbine wa maller than in the previou year and reached an overall ize of 37,642 MW, after 38,312 MW in China accounted for more than half of the world wind market Without taking into account China, the world market hrank by one third and decreaed from 24,512 MW to 18,714 MW. Still and in pite of the lowdown, the intalled wind capacity ha been more than doubled every third year. In the year 2010, altogether 83 countrie, one more than in 2009, ued wind energy for The Pacific Journal of Science and Technology 89

4 electricity generation. 52 countrie increaed their total intalled capacity, after 49 in the previou year. The turnover of the wind ector worldwide reached 55 billion US$ in 2010, after 70 billion US$ in the year The decreae i due to lower price for wind turbine and a hift toward China [13]. The decreae in new intallation outide China can be een a a reult of inufficient political upport for wind energy utilization. In a paradox ituation, more and more policy maker are declaring their upport for increaed ue of wind energy, but uch tatement do not follow with the neceary political deciion. While the year 2009 had een two major miletone the firt North American feed-in law in Ontario and the introduction of the firt feed-in tariff in Africa, the year 2010 did not bring comparable breakthrough deciion in national or international policie. Epecially in the USA, there i major regulatory uncertainty and not enough focu on renewable energy. Alo in many developing countrie there i till a huge policy gap and there i not yet enough tability and reliability in market framework, next to a lack of financial reource. In addition, the neceary international framework for renewable energy have not yet been etablihed. The year 2010 howed the econd lowet growth rate, 23.6%, of the lat decade. The growth rate i the relation between the new intalled wind power capacity and the intalled capacity of the previou year. Before 2010, the annual growth rate even had continued to increae ince 2004, peaking in 2009 at 31.7%, the highet rate ince The highet growth rate of the year 2010 by country can be found in Romania, which increaed it capacity by 40 time. The econd country with a growth rate of more than 100% wa Bulgaria (112%). In the year 2009, till four major wind market had more than doubled their wind capacity: China, Mexico, Turkey, and Morocco. Next to China, trong growth could be found mainly in Eatern European and South Eatern European countrie: Romania, Bulgaria, Turkey, Lithuania, Poland, Hungary, Croatia and Cypru, and Belgium. Africa (with the exception of Egypt and Morocco) and Latin America (with the exception of Brazil), are again lagging behind the ret of the world in the commercial ue of wind power. PROTOTYPE MODEL OF STUDY Figure 2 how the prototype model conidered for thi tudy. Two wind farm uing fixed peed wind turbine (FSWT) and variable peed wind turbine (VSWT) are connected to the multimachine power ytem. The FSWT wind farm i onhore, while the VSWT wind farm i offhore. Some highlight about ingle and multi-machine ytem are given below. Single Machine Sytem Ha only one type of machine. Simple in tructure. Not realitic cae for tudy becaue i normally connected to an infinite bu, hence cannot be ued to actually judge ytem performance. Multi-Machine Sytem Ha different type of machine. Complex in tructure. Give a realitic cenario of a tudy and can be ued to effectively judge performance of a propoed ytem or controller. Alo, ome of the merit and drawback of FSWT and VSWT are itemized a follow. Fixed Speed Wind Turbine (FSWT) Ha fixed peed operation, thu the power captured i limited. Technology ha limited ability to provide voltage and frequency control. Ha uperior characteritic uch a bruhle and rugged contruction, low cot, maintenance free, and operational implicity. Require large reactive power to recover the air gap flux when a hort circuit fault occur in the power ytem. The intallation of reactive power compenation device to upport the fixed wind peed generator increae the overall cot of the ytem. The Pacific Journal of Science and Technology 90

5 Variable Speed Wind Turbine (VSWT) Ha variable peed operation making it poible to achieve a high efficiency of energy converion compared to contant peed operation epecially in low wind peed area. Ability to decouple control of active and reactive power rapidly and independently by econdary excitation control. Thu, ytem tend to be more table during network diturbance. The reduction of mechanical tree and acoutic noie. Improvement of the power quality of the ytem without external reactive power compenation device. The required capacity of the power converter for econdary excitation can be le than half for the cae of a DFIG ytem, and thu the total cot decreae. Wind-Dieel Sytem Figure 2 how a larger AC coupled wind-dieel power ytem in ection B, C, and D repectively. A technically effective wind dieel ytem upplie firm power, uing wind power to reduce fuel conumption while maintaining acceptable power quality. In order to be economically viable, the invetment in the extra equipment that i needed to incorporate wind power, including the wind turbine themelve, mut be recouped by the value of the fuel aving and other benefit [18]. A the ratio of the intalled wind capacity to the ytem increae, the required equipment needed to maintain a table AC grid alo increae, forcing an optimum amount of wind power in a given ytem. It hould be noted that other dieel retrofit option do exit and thee include the ue of other renewable technologie, uch a bioma, or imply better control of the dieel generator [19]. A typical iolated dieel power upply ytem ha the following characteritic [18-20]. It ha only one or few dieel generating et. By uing a number of dieel genet of cacading ize an optimal loading of dieel genet may be obtained, thu increaing the efficiency of the dieel plant. The exiting power ytem ha quite imple controller, often only the governor and voltage regulator of the dieel generator poibly upplemented by load haring or elf-ynchronizing device. The local infratructure may be limited and there may be no readily available reource for operation, maintenance and replacement. Fuel i generally expenive and i ometime carce and prone to delivery and torage problem. The dieel engine provide adequate frequency control by the adjutment of power production to meet the load and voltage control by modifying the field on the generator. In technical term, an iolated power ytem for a large community that incorporate wind power (only ection B, C, and D in Figure 2) will be defined a a wind-dieel ytem when both the ytem layout and operation are ignificantly influenced by the preence of wind power in term of: Frequency control, tability of ytem voltage and limited harmonic ditortion. The operating condition of the dieel generator, epecially with regard to minimum load, Proviion for the ue of any urplu wind power. Operation, maintenance and repair of any ytem component. Wind-dieel power ytem can vary from imple deign in which wind turbine are connected directly to the dieel grid with a minimum of additional feature (a hown in Figure 2), to more complex ytem [21, 22]. The important complication of adding wind power to dieel plant i that the production of energy from the wind turbine i controlled by the wind, meaning mot turbine cannot control either line frequency or voltage and mut rely on other equipment to do o a will be hown in the imulation reult analyi. With only mall amount of wind energy, the dieel engine can provide thi control function, but with larger amount of wind energy, other equipment like the energy capacitor ytem (ECS), uper magnetic energy torage (SMES) ytem, etc. are neceary. The Pacific Journal of Science and Technology 91

6 Available wind peed Section C Section A Control ytem Hydro turbine AC Fixed peed wind turbine DC bu DC AC AC bu (FSWT) AC Rotary converter Battery Section E Dipatched load Nuclear plant AC Sytem grid voltage Dieel AC (ection B) Available wind peed Onhore network Section D Onhore ubtation SVC unit Variable peed wind turbine (VSWT) AC Shore line XLPE cable Electrolyzer Tranformer Offhore ubtation SVC unit Offhore wind farm Figure 2: Prototype Model of Study. Conidering the above factor, two overlapping iue trongly influence ytem deign and it required component. The firt i the amount of energy that i expected from the renewable ource (known a ytem penetration), while the econd i the ability of the power ytem to maintain a balance of power between generation and conumption (otherwie known a the primary ue for energy torage). The Pacific Journal of Science and Technology 92

7 WIND POWER IMPACT ON POWER QUALITY In ytem with turbine connected to the AC bu, the critical conideration i the variation of the power output from wind turbine and it impact on the operation of the power ytem and on power quality. Thi impact increae a the level of penetration increae [18]. The influence on power quality i mainly noted in the level and fluctuation of ytem voltage and frequency, the hape of the voltage ignal or power harmonic and it ability to manage a reactive power balance. It i alo important that the voltage of the ditribution network remain contant, epecially when interconnected to the wind turbine. The tranient and dynamic impact of wind turbine in ingle and multi-machine ytem have been tudied by the author in [23-40]. In all power ytem that produce AC power, three critical parameter mut be maintained: Sytem frequency: the balance of energy being produced and energy being conumed. If more energy i being produced, the frequency will increae. Active or real and reactive power: depending on the type of load and device being ued, a balance of active and reactive power mut be maintained. In addition, ome load will require large amount of reactive power, uch a any inductive motor, and the equipment operating the ytem will need to provide thi reactive power. Sytem voltage: table voltage from the ource i required to enure the proper operation of many common load. Different device can be ued to control different apect of ytem power quality. The need for different device will depend on the intantaneou penetration of renewable (i.e., wind energy that i expected). The higher the penetration, the more one mut worry about how thi will impact power quality and the more device that mut be added to the ytem to enure high quality electrical power. TRANSMISSION SYSTEM FOR OFFSHORE WIND FARMS The tranmiion ytem for offhore wind farm ue either high voltage alternating current (HVAC) or high voltage direct current (HVDC) connection. For the HVDC connection, there are two technical option. The line commutated converter (LCC) baed HVDC and the voltage ource converter (VSC) baed HVDC technology. An HVAC tranmiion ytem conit of the following main component [18, 41, and 42]: an AC baed controller ytem within the wind farm; an offhore tranformer tation including offhore reactive power compenation; three-core polyethylene inulation (XLPE) HVAC cable to hore, and onhore, and a tatic VAR compenator (SVC) unit a hown in Figure 2. With an increaing ditance to hore, reactive power compenation will be required at both end of the cable (offhore and onhore). Thi reactive power would be provided by the SVC unit. The main diadvantage of the of the HVAC ytem i that with increaing wind farm ize and ditance to hore, load loe will increae ignificantly. Alo, an increae in the tranmiion voltage level will lead to larger and more expenive equipment like tranformer and cable. However, an increae in the voltage level i jutifiable if an increae in capacity i required. The advantage of line commutated converter (LCC) baed high voltage direct current tranmiion (HVDC) technology i primarily for bulk power tranmiion over long geographical ditance and for interconnecting power ytem with different iland ytem [43]. A thyritor baed LCC HVDC tranmiion ytem conit of the following main component [44, 45]: an AC baed collector ytem within the wind farm; an offhore ubtation with two three-phae two-winding converter tranformer a well a filter and either a tatic ynchronou compenator (STATCOM) or a dieel generator that upply the neceary hort circuit capacity; DC cable; and an onhore converter tation with a ingle phae threewinding converter tranformer a well a relevant filter. The LCC HVDC technology require comparatively large converter tation onhore and offhore, a well a auxiliary ervice at the offhore converter tation. The auxiliary ervice at the offhore terminal require keeping trong AC ytem at the offhore converter in order to enable the operation of the line-commutated converter even during period with no or very little wind. The Pacific Journal of Science and Technology 93

8 The advantage of an LCC HVDC connection i that there are comparatively low loe in tranmiion over long ditance. In addition, the higher tranmiion capacity of a ingle cable compared with HVAC or VSC baed tranmiion can be an advantage for very large offhore wind farm. Voltage ource converter (VSC) baed HVDC technology i gaining more and more attention. It i a comparatively new technology that ha only become poible a a reult of important advance in higher-power electronic, namely, in the development of inulated gate bipolar tranitor (IGBT). In thi way, pule-width modulation (PWM) can be ued for the VSC a oppoed to thyritor baed LCC ued in the conventional HVDC technology. A VSC baed HVDC tranmiion ytem conit of the following main component [41, 42]: an AC baed collector ytem within the wind farm; an offhore ubtation with the relevant converter; DC cable pair; and an onhore converter tation. The ytem link of a VSC baed HVDC doe not require a trong offhore or onhore AC network, thu, it can even tart up againt a non-load network. Thi i poible becaue in a VSC the current can be witched off, which mean that there i no need for an active commutation voltage. Beide, the active and reactive power upply can be controlled independently. Alo, the VSC baed HVDC connection i uually not connected to ground. Therefore, the ytem alway need two conductor or cable. However, the total efficiency of the two converter tation of a VSC baed HVDC ytem i le than that of a LCC HVDC. The advantage of a VSC baed HVDC connection i the capability of four-quadrant operation, the reduced number of filter required, black-tart capability and the poibility of controlling a number of variable uch a reactive power, apparent power, harmonic and flicker when feeding the power ytem from a VSC [46, 47]. HYDROGEN To avoid the pilling of renewable energy production, an energy torage medium need to be incorporated into the generation ytem in order to allow flexible uage of the power generated. Hydrogen offer everal intereting characteritic in thi context [18]: It can be reconverted to electricity with a reaonably high efficiency if it ued a fuel cell. It enable peak power production and load following, either from central intallation or from virtual power tation, invariably, it offer decentralized generation capacity. It can contitute an alternative mean of energy tranport, for intance by uing pipeline where electricity cable are undeirable and ame time offering high energy denity and low tranport loe. It can be old a indutrial ga outide the electricity market, thu it reduce market preure and develop alternative market for renewable energy ytem. Hydrogen i the element with the lowet atomic weight. The apect of hydrogen that are mot important in energy production are energy denity, torage denity, volume of torage, tranportation energy effort, ignition limit, diffuion coefficient and exploion energy a per afety concern. The important role hydrogen play in the context of renewable energy arie from the fact that it can be produced eaily from water and electricity by the proce of electrolyi. It can either be burned a a fuel a a ubtitute for gaeou foil fuel or be converted to electricity in fuel cell in an electrochemical proce that exceed the efficiency of conventional electricity generation. Within certain limitation, hydrogen can be ued for toring renewable electricity and can either be old off a a product in it own or be reconverted to electricity. In the prototype model of thi tudy hown in Figure 2, hydrogen can be generated directly from electricity by uing electrolyi. An electrolyzer i hown connected via a tranformer to the moothed output of the variable peed wind turbine. In an electrolytic olution, water i plit into it component i.e. hydrogen and oxygen at two electrode. The gae are produced eparately at the electrode and have a high purity. Electrolyzer can be operated at preure between ambient preure and 200 bar. The high preure procee are more efficient but alo lead to higher equipment cot and more complicated ytem. However, they can feed a pipeline ytem directly without any additional The Pacific Journal of Science and Technology 94

9 compreion. Electrolyi require an input of dealinated and de-mineralized water. SIMULATION ANALYSIS Simulation were carried out uing tandard laboratory imulation tool called PSCAD/EMTDC R. The imulink differential equation for the variable peed drive and ome of the imulation reult obtained baed on the prototype model of Figure 2 are decribed in the ubequent ubection. VSWT Mathematical Modeling A doubly fed induction generator (DFIG) variable peed wind turbine (VSWT) machine i a wound rotor with back-to back converter in the rotor circuit. The rotor i upplied by pule width modulator inverter, while the tator i directly connected to the grid. The rotor current exciting frequency i controlled a the wind velocity i changed. The frequency of output power i fixed at grid frequency, which i given a: The direct and quadrature tator and rotor flux component are given a follow [51]: Φ d =Li d +Lmi dr (6) Φ q =Li q +Lmi qr (7) Φ dr =Lri dr +Lmi d (8) Φ qr =Lri qr +Lmi q (9) Where, V d, V q, are the d- and q-axi tator voltage, Φ d, Φ q, Φ dr, Φqr are the d- and q-axi tator and rotor winding flux, i d, i q, are d- and q- axi tator current, i qr and i dr are d- and q-axi rotor current (A) repectively, L and L r are the tator leakage and rotor elf-inductance (H), and V i the magnitude of the tator phae voltage (V). The d-q teady tate equivalent circuit of the DFIG i hown in Figure 3 below [52]: ω= pω m± ω r (1) R L Lr Rr Where ω i the grid electrical angular peed, Ωm i the mechanical angular rotor peed, ω r i the electrical angular peed of rotor variable, and p i the number of pole pair. Equation (1) etablihe the bai for variable peed contant frequency. V Φ Lm Φ r jωφ jωφ r r Vr The mathematical equation of the DFIG in term of tator, rotor voltage and flux are given a follow [48-50]: Figure 3: Equivalent Circuit of DFIG in Synchronou Reference Frame. d V =R ωφ i - + Φ (2) dt d d q d d V =R i ωφ + + Φ (3) dt q q d q d V =R iω- - ( ω)φ + Φ (4) dt dr r dr qr dr d V =R i ω- + ( ω)φ + Φ (5) dt qr r qr dr qr For independent control of active and reactive power output of the DFIG, vector control i normally ued. Thi control technique ha many advantage compared to other trategie becaue it make the control algorithm imple, robut and provide fat repone. For the rotor ide converter (RSC) algorithm, it i aumed that the DFIG i connected to the power grid in which the grid voltage and frequency are contant. Fixing the d-axi of the ynchronou frame on the tator voltage vector, a tator The Pacific Journal of Science and Technology 95

10 voltage oriented control i obtained. The vector of the tator voltage then become: - V =V + j 0 (10) d According to (10), the active power and reactive power output from the tator ide of the DFIG can be repreented a: P =V i (11) d d Q = -V i (12) d q controlled via poible through the control of i dr and i qr, repectively, which i V and V. Figure 4 how the grid ide converter (GSC) pule width modulator rectifier. The aim of the control ytem i to maintain the DC-bu voltage to the required level for the RSC, while the main input current hould be inuoidal and in phae with their counterpart voltage, for which the control ytem of DFIG maintain unity power factor condition. For modeling and control deign, the d-q ynchronou frame voltage component are [48]: dr qr Putting Equation (2) into (11) and (3) into (12) repectively, the active and reactive power can be derived a follow: V d(φ d - L i m dr ) P= L (13) d V = e + L ωl i + Ri i - dt d d rec d rec d rec q d V = e + L i ωl+ Ri i + dt q q rec q rec q rec d (15) (16) Q = - V d(φ q - L i m qr ) L (14) From Equation (13) and (14), the active and reactive power of the DFIG ytem are related to the rotor current i dr and i qr, repectively. Therefore, the active and reactive power can be Where R rec and L rec are reitance and inductance of boot inductor, repectively. Thi trategy lead to getting the following active and reactive power: P = V i (17) rec rec d d Q = - V i (18) d q i1 i2 Va Rrec Lrec ia T1 T3 T5 Rotor Vb Rrec Lrec ib ea eb C icp Vdc Side Converter Vc Rrec Lrec ic ec (RSC) T2 T4 T6 Figure 4: Structure of the Grid Side Converter of DFIG. The Pacific Journal of Science and Technology 96

11 Thu, the current command q-axi control the reactive power, wherea a current command of d- axi control the active power, and conequently control indirectly the DC-link voltage. The above mathematical differential equation algorithm for the rotor ide converter and the grid ide converter, would now be applied to the VSWT in order to achieve the independent controllability of active and reactive power of the ytem. Simulation Reult and Dicuion Conidering the prototype model preented in Figure 2, ome of the ytem repone are preented in Figure In the imulation analyi, real wind peed data obtained in Hokkaido Iland, Japan a hown in Figure 5 are ued for the fixed peed wind turbine and the variable peed wind turbine in ection C and D repectively of the prototype model. The imulation wa run for 612 ec with a imulation timing tep of ec to demontrate the effectivene of the controller of the ytem. Wind peed[m/] Avaliable wind peed in ection C Avaliable wind peed in ection D 2 Time[ec] Figure 5: Available Wind Speed for FSWT and VSWT Sytem. Power coefficient (C p ) curve of VS wind turbine [pu] Rotor peed and optimum rotor peed of VS wind turbine[pu] Grid and electrolyzer energy [MJ] Cp curve of wind turbine in ection D 0.0 Time[ec] Figure 6: Power Coefficient Curve of VSWT Rotor peed of variable peed wind turbine in ection D Optimum rotor peed and variable peed wind turbine peed in ection D Time[ec] Figure 7: Rotor and Optimum Rotor Speed of VSWT Grid energy cloe to ection D Electrolyzer energy in ection D Time[ec] Figure 8: Energy of Grid and Electrolyzer Sytem. The Pacific Journal of Science and Technology 97

12 DC-bu voltage of VS wind turbine[pu] Sytem grid voltage[pu] DC-bu voltage of variable peed wind turbine in ection D 0.4 Time[ec] Figure 9: DC-Bu Voltage of VSWT Sytem voltage without ection D incorporated Sytem voltage with ection D incorporated 0.8 Time[ec] Figure 10: Sytem Grid Voltage. Frequency of ytem [Hz] Time[ec] Sytem frequency Figure 12: Frequency of the Network Sytem. Real power of electrolyzer, VSWT and grid ytem [MW] Real power of electrolyzer ytem Real power of variable peed wind turbine Real power of grid ytem Time[ec] Figure 13: Real Power of Electrolyzer, VSWT and Grid Sytem. Hydrogen ga quantity produced[nm 3 ] Amount of hydrogen ga produced in ection D 0 Time[ec] Figure 11: Amount of Hydrogen Ga Generated from the Sytem. Real and reactive power of FSWT and hydro turbine (MW and MVAR) Hydro turbine real power [MW] ection A Hydro turbine reactive power [MVAR] ection A Fixed peed wind turbine real power [MW] ection C Fixed peed wind turbine reactive power [MVAR] ection C -20 Time[ec] Figure 14: Real and Reactive Power of FSWT and Hydro Turbine. The Pacific Journal of Science and Technology 98

13 Hydro turbine governor and torque peed[pu] Hydro turbine governor peed ection A Hydro turbine torque peed ection A Time[ec] Figure 15: Hydro Turbine Governor and Torque Speed. The power coefficient repone of the VSWT i hown in Figure 6 during the wind peed change of the VSWT, while Figure 7 how the rotor and the optimum rotor peed of the VSWT. It could be oberved from Figure 7 that the rotor peed of the VSWT follow the optimum rotor peed of the VSWT, demontrating the effectivene of the controller ytem employed. Conequently, maximum and wide range of power can be captured ince the VSWT utilize variable peed operation, making it poible to achieve a high efficiency of energy converion compared to contant peed operation epecially when the wind peed goe below the cut-out wind peed deigned for the wind turbine. Figure 8 how the grid energy and the electrolyzer energy in MW for the ytem. A expected, the grid energy i higher than the electrolyzer energy. In Figure 9, the DC bu voltage wa maintained effectively at 1.0 p.u, baed on the control trategy of the VSWT ytem, depite the tochatic nature of the wind peed in Figure 5 for the VSWT ytem. At the ame time, the grid voltage of the ytem wa maintained contant at 1.0 pu, with ection D connected to the network ytem. However, if ection D i iolated from the network, the grid voltage of the network ytem cannot be maintained at 1.0 pu, rather, it follow the nature of the wind peed. Thi i due to the fact that there i no reactive power compenation device, which can help tabilize the grid voltage of the network ytem. Conequently, ince the preence of the VSWT ytem in ection D can help maintain the ytem grid voltage, the out of the wind generator that i connected to the electrolyzer ytem in ection D i alo moothed. Therefore, hydrogen ga could be produced in ection D a hown in Figure 11. The amount of hydrogen ga produced in Nm 3 gradually increae with time. A dicued earlier, the dieel plant and other ynchronou machine can control the ytem voltage and frequency to ome extent, when the amount of wind peed i not much. The ytem frequency i hown in Figure 12. Since there i no flexible AC tranmiion ytem (FACTS) device like the uper magnetic energy torage ytem (SMES) or the energy capacitor ytem (ECS) to contribute to the active power control, the ytem frequency cannot be controlled ordinarily. Thi i becaue, the conventional controller of the VSWT in ection D and the FSWT in ection C, doe not have the capability for frequency control. However, if a flywheel adjutable control trategy i incorporated in the VSWT ytem in ection D, the ytem frequency can be maintained, while at the ame time moothing the output of the wind generator in order to contain the high and low wind peed fluctuation. The repone of the real power of the electrolyzer, VSWT and the grid ytem are hown in Figure 13 for the dynamic analyi. The real and reactive power of the FSWT and the hydro turbine in ection C and A repectively of the model ytem are hown in Figure 14. The hydro turbine governor and torque peed in ection A of the model ytem are hown in Figure 15. CONCLUSION A prototype model of grid connected multimachine in a power ytem ha been tudied. Two wind farm were connected to the multimachine power ytem. The firt wind farm i made of only fixed peed wind turbine (FSWT) and i onhore baed. Variable peed wind turbine (VSWT) exit in the econd wind farm that i offhore baed. An electrolyzer unit wa connected to the moothed terminal of the VSWT ytem. It wa found that the ytem grid voltage could be maintained effectively depite the tochatic nature of the available wind in the wind farm, with the help of the VSWT. Another alient part of the tudy that wa alo reported i the production of hydrogen ga from the electrolyzer unit that i connected to the moothed output of the VSWT ytem. The Pacific Journal of Science and Technology 99

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16 44. Cartwright, P., L. Xu, and C. Saae Grid Integration of Large Offhore Wind Farm uing Hybrid HVDC Tranmiion. Proceeding of the Nordic Wind Power Conference. March Chalmer Univerity of Technology: Stockholm, Sweden. 45. Kirby, N.M., L. Xu, M. Luuckett, and W. Siepman HVDC Tranmiion for Large Offhore Wind Farm. IEE Power Engineering Journal. 16(3): Burge, K., E.J. van Zuylen, J. Morren, and S.W.H. de Haan DC Tranmiion for Offhore Wind Farm-Concept and Component. Proceeding of the Second International Workhop on Tranmiion Network for Offhore Wind Farm. Royal Intitute of Technology: Stockholm, Sweden. 47. Konig, M., M. Luther, and W. Winter Offhore Wind Power in German Tranmiion Network. Proceeding of the Fourth International Workhop on Large-cale Integration of Wind Power and Tranmiion Network for Offhore Wind Farm. Royal Intitute of Technology: Stockholm, Sweden. 48. Va, P Senorle Vector and Direct Torque Control. Oxford Univerity Pre: New York, NY. 49. Yamamoto, M. and O. Motoyohi Active and Reactive Power Control for Doubly Fed Wound Rotor Induction Generator. IEEE Tran. Power Electronic. 6(4): Muyeen, S.M., M.H. Ali, R. Takahahi, T. Murata, and J. Tamura Damping of Blade-Shaft Torional Ocillation of Wind Turbine Generator Sytem. Electrical Power Component and Sytem. 36(2): Ghennam, T., E.M. Berkouk, and B. Francoi Modeling and Control of a Doubly Fed Induction Generator (DFIG) Baed Wind Converion Sytem. IEEE Tran. On Energy Converion Chondrogianni, S. and M. Barne Stability of Doubly Fed Induction Generator under Stator Voltage Oriented Vector Control. IET Renewable Power and Generation. 2(3): ABOUT THE AUTHOR Dr. (Engr.) Okedu, Eloghene Kenneth hold a Ph.D. in Renewable Energy Sytem Engineering, Department of Electrical and Electronic Engineering, Kitami Univerity of Technology, Hokkaido, Japan. He alo hold a B.Eng. and M. Eng. in Electrical and Electronic Engineering, Univerity of Port Harcourt, Nigeria. Dr. Okedu erve a a Lecturer in the Department of Electrical and Electronic Engineering, Univerity of Port Harcourt, Nigeria. Hi reearch interet are renewable energy, tability analyi of wind farm uing variable peed wind turbine, and integration and augmentation of renewable energy into power ytem. SUGGESTED CITATION Okedu, K.E Application of Wind Power Generation in Gird Connected Power Sytem. Pacific Journal of Science and Technology. 13(2): Pacific Journal of Science and Technology The Pacific Journal of Science and Technology 102