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    • N. R. Bhasme, Dr. W. Z. Gandhare / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue4, July-August 2012, pp.2181-2185 Power Converters for Grid Integration of Wind Power Systems N. R. Bhasme Dr. W. Z. Gandhare Asstt. Professor Principal Government College of Engineering, Aurangabad Maharashtra- IndiaAbstract: This paper presents the different power ample scope in India for the use of powerconverters for grid integration of wind power converters in wind power systems. During recentsystems. In wind power systems while years different converter topologies have beenintegrating with grid, power quality issues are investigated as to whether they can be applied inbecoming more predominant. This can solve by wind turbines such as Back-to-back, Multilevel,using power electronics converters like two level Tandem, Matrix, Resonant Converters. This paperback to back converter and matrix converters. is concerned of a wind energy system withConverters are compared on the basis of different topologies for the power converters,efficiencies, voltage conversion ratios and THDs. mainly a two-level converter, three level and matrix converter providing constant voltage and frequencyKey words: Wind energy systems, power quality, to the grid. Grid quality characteristics and limittwo level back to back converter, three level and values in accordance with DIN EN 50 160 and thematrix converter. VDEW give directives for in-plant generation. Converters are compared on the basis of1. Introduction efficiencies, voltage conversion ratios and THDs. World Market for Wind Turbines sets a Fig. 1 shows different converter applied in windnew record of 42 GW of new capacity in 2011, power systems.worldwide total capacity at 239 GW enough tocover 3 % of the worlds electricity demand.Renewable delivered close to 20% of globalelectricity supply in 2010 and by early 2011 theycomprised one quarter of global power capacityfrom all the sources. In several countries renewablerepresent a rapidly growing share of total energysupply. India added an estimated 2.7GW of gridconnected renewables during 2010 mainly fromwind but also from biomass, small hydropowerand solar capacity for a total of nearly 19GW byJanuary2011. Direct driven turbine designscaptured 18% of the globalmarket..The increasingshare of wind in power generation will change Fig 1. Different converters applied in wind powerconsiderably the dynamic behaviour of the power systemssystem and may lead to a new strategy for powersystem frequency regulation in order to avoiddegradation of frequency quality .Hence, network 2. Turbine and Electric Machineoperators have to ensure that power quality is not The mechanical power of the turbine iscompromised at consumer end. New technical given by: 1challenges emerge due to increased wind powerpenetration, dynamic stability and power quality. Pm   Au 3c p (1) 2Power electronic converters have been developed where Pm is the power extracted from the airflow,for integrating wind power with the electrical grid. ρ is the air density, A is the area covered by theThe use of power electronic converters allows for rotor, u is the wind speed upstream of the rotor, andvariable speed operation of the wind turbine and cp is the performance coefficient or powerenhancement in power extraction. In variable speed coefficient. The power coefficient is a function ofoperation, sophisticated control methods require the pitch angle of rotor blades θ and of the tipextracting maximum power from the turbine and speed ratioλ , which is the ratio between blade tipproviding constant voltage and frequency to the speed and wind speed upstream of the rotor. Thegrid is required. In India out of installed capacity computation of the power coefficient requires theonly 15% wind turbines are connected to the grid use of blade element theory and the knowledge ofthrough power electronics converters. There is a blade geometry. We consider the blade geometry using the numerical approximation developed in 2181 | P a g e
    • N. R. Bhasme, Dr. W. Z. Gandhare / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue4, July-August 2012, pp.2181-2185[7], assuming that the power coefficient is given torque in electric machine bearing,Tae is theby: resistant torque due to 18.4 the viscosity of the airflow in the electric machine,C p  0.73i e ii (2) and Te is the electric torque. The equations for modelling a permanent magnetic synchronous machine, PMSM, can be found in diverse literature;where i and λii are respectively given by: using the motor machine convention, the following 151 set of equations is considered:i   0.58  0.002 2.14  13.2 (3) ii did 1 1  (ud  pwe Lq iq  Rd id ) (9)ii  (4) dt Ld 1 0.003  diq 1 (  0.02 ) ( 3  1)  uq  pwe ( Ld id  Mi f )  Rqiq  (10) dt Ld  The maximum power coefficient is given for a nullpitch angle and is equal to: where i f is the equivalent rotor current, M is the mutual inductance, p is the number of pairs of poles; and where in d- q axes id and iq are theC p max  0.4412 (5) stator currents, Ld and Lq are the statorwhere the optimum tip speed ratio is equal to inductances, Rd and Rq are the stator resistances,opt  7.057 (6) ud and uq are the stator voltages. A unity power factor is imposed to the electric machine,The power coefficient is illustrated in Figure 1 as a implyinga null Qe . The electric power Pe is givenfunction of the tip speed ratio. by: T Pe  ud  uq u f  id  iq if   (11) The output power injected in the electric network characterized by P and Q in α-β axes is given by:  P   u u  i  Q    u u  i  (12)       where in α-β axes, iα and iβ are the phase currents, uαand uβ are the phase voltages. The apparent output power is given by: S   P2 H2 1/2 Q2 (13) Fig. 2. Power coefficient curves versus tip speed where H is the harmonic power. ratio 3. Two-Level and Three Level ConvertersThe mechanical power extracted from the wind is The two-level converter is an AC/DC/ACmodelled by (1) to (4). The equations for modelling converter, with six unidirectional commandedrotor motions are given by: IGBTs Sik, used as a rectifier, and with the samedwm 1 number of IGBTs, used as an inverter. The rectifier  (Tm  Tdm  Tam  Telas ) (7) dt Jm is connected between an electric machine and a capacity bank. The inverter is connected betweendwe 1  (Telas  Tde  Tae  Te ) (8) this capacity bank and a filter, which in turn is dt Je connected to an electric network. A three-phase symmetrical circuit in series models the electricwhereωm is the rotor speed of turbine, Jm is network. The configuration of the system that willturbine moment of inertia, Tm is the mechanical be simulated is shown in Figure 3.torque , Tdm is the resistant torque in the turbinebearing, Tam is the resistant torque in the hub andblades due to the viscosity of the airflow, Telas thetorque of the torsional stiffness, we is the rotorspeed of the electric machine, Je is the electricmachine moment of inertia, Tde is the resistant 2182 | P a g e
    • N. R. Bhasme, Dr. W. Z. Gandhare / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue4, July-August 2012, pp.2181-2185 Fig. 3. Wind energy system with two-level converter The groups of two IGBT’s linked to the same phase Fig.4. Comparison of PWM waveforms for a) constitute a leg k of the converter. For the two-level Conventional two level switching b) Three level converter modelling we assumed that: 1) The switching IGBT’s are ideal and unidirectional, and they will never be subject to inverse voltages, being this situation Figure 4 compares waveforms of conventional guaranteed by the arrangement of connection in pulse width modulation (PWM) of two level anti parallel diodes; 2) The diodes are ideal: in switching voltage and three level switching conduction it is null the voltage between its voltage. In the two level switching voltage terminals and in blockade it is null the current that waveform, the line-neutral voltage is switched passes through it; 3) The continuous voltage in the between two voltage levels: positive Dc bus exit of the rectifier should always be vdc > 0 ; 4) voltage and the negative DC bus voltage. While in Each leg k of the converter should always have one the three level switching voltage waveform, the IGBT in conduction. For the switching function of line-neutral voltage is switched among voltage each IGBT, the switching variableγ k is used to levels of positive voltage of DC bus, negative identify the state of the IGBT i in the leg k of the voltage of the bus voltage and the mid-point converter. The index i with i ∈ {1,2} identifies the voltage of DC bus. The major disadvantages of the IGBT. The index k with k ∈ {1,2,3} identifies the multilevel conversion are its complexity of bus bar leg for the rectifier and k ∈ {4,5,6} identifies the interconnection. A larger number of semiconductor leg for the inverter. The two valid conditions for devices are required. As the number of voltage the switching variable of levels increases, the bus bar structures of the each leg k are as follows: multilevel DC link converter become more complex and difficult to fabricate. The capacitor 1, S  1 k   ik for i 1, 2 and k 1,   ,6 (14) voltage balancing problem in the multilevel 0, Sik  1 converter is also an eminent disadvantage. Several solutions have been proposed to overcome this The topological restriction for the leg k is given by 2 problem. S i 1 ik 1 k  1,   , 6 (15) 2- Level DC Link Converter Hence, each switching variable depends on the conduction and blockade states of the IGBT’s. The phase currents injected in the electric network are modelled by the state equation: dik 1   uk  Rcik  usk  k  4,5,6 dt  Lc  L f  (16) The output continuous voltage of the rectifier is modelled by the state equation: dvdc 1  3 6  (a)     k ik    k ik  (17) dt C  k 1 k 4  3- Level DC Link Converter Hence, (14) to (17) model the two-level converter. 2183 | P a g e
    • N. R. Bhasme, Dr. W. Z. Gandhare / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue4, July-August 2012, pp.2181-2185 conduction in order to achieve continuity in the voltage between the phases [10]. The configuration of the system that will be simulated is shown in Figure 2. The IGBT’s commands Sij are given in function of the on and off states as follows: 1,(on) Sij   i, j 1, 2,3 (18) 0,(off ) (b) Fig.5. Efficiencies of 3-phase Dc link Converters a) two level switching b) three level switching The switching technique employed for theconverter giving the pattern of the voltages acrossthe link capacitor. The waveforms based on PWMswitching technique for the conventional two leveland three level switching from the figure.5 showsthat the converter efficiencies are dominated by theefficiencies of the rectifier parts of the converters.In addition, at lower operating power or loweroperating voltage, the efficiencies of the rectifierparts decrease more rapidly than those of theinverter parts. Because the voltage produced by thegenerator is proportional to the wind speed, the Fig. 6. Wind energy system with matrix converterreduced voltage produced by the generator under The assumptions in 3) and 4) are given bylow wind speed conditions causes the rectifier parts restrictions (19) on commands Sij: 3 S i  1, 2,3to work harder in stepping up the voltagemagnitude. This requires grater proportion of the ij 1 j 1indirect power. In the rectifier portion of the DC (19)link system, the indirect power consists of energy 3that is first stored in the inductors and later released S i 1 ij 1 j  1, 2,3to the DC link. In contrast, the direct powerconsists of power flows directly from the input side The vector of output phase voltages is related to theto the output side, without going through the vector of input phase voltages through theintermediate step of being stored. Hence the commandindirect power conversion incurs additional losses. matrix, as follows: v A   S11 S12 S13  va  va 4. Matrix Converter v    S S 22  v   S v  S 23   b     b  The matrix converter is an AC/AC  B   21converter, with nine bidirectional commanded  vc   S31    S32 S33   vc     vc   IGBTs Sij. It is connected between the electric (20)machine and a second order filter, which in turn is The vector of input phase currents is related to theconnected to an electric network. The second order vector of output phase currents through thefilter is an inductive load that avoids the command matrix, as follows:interruption of the output currents. A three-phase ia ic    S  iA iB iC  T T Tactive symmetrical circuit in series models the ib (21)electric network. For the matrix converter Hence, (18) to (21) model the matrix converter.modelling we assumed that: Matrix converters do not require intermediate1) the diodes are ideal: in conduction it is null the energy storage and have lower switching losses.voltage between its terminals, and in blockade it is Although the matrix converter has six additionalnull the current that passes through it; 2) the switching devices, compared to the back-to-back,elements of the command matrix of the converter two-level, DC-link converter, the absence of theare bidirectional switches in voltage and current; 3) DC-link capacitor may increase the efficiency ofthe command variables Sij for each i has for one j the converter. Also, the power semiconductorthe value one, i.e. only one switch is in conduction devices in the matrix converter are switched atin order to achieve continuity in the current in each average voltages lower than those in the two-level,phase; 4) the command variables Sij for each j has DC-link converter. The major disadvantage of thefor one i the value one, i.e. only one switch is in matrix converter is the limitation of the voltage 2184 | P a g e
    • N. R. Bhasme, Dr. W. Z. Gandhare / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue4, July-August 2012, pp.2181-2185gain ratio, which leads to poor semiconductor Matrix Converters for Wind Powerdevice utilization. Another drawback is the large Applications: Final Report”, pp 15-25,number of semiconductor devices required to make December 2006.the matrix converter functional. Although devices [8] Ren 21-Renewable2011 Global Statuswith smaller current rating can be employed, they Report, pp 16-21still lead to a large number of gate driver circuits.In addition, with the absence of the DC-link, thereis no decoupling between the input and outputsides. Any distortion in the input voltage isreflected in the output voltage at differentfrequencies; as a consequence, sub harmonics canbe generated. Many soft-switching techniques havebeen proposed to improve the efficiency of thematrix converter.5. Conclusion The increased wind power penetration inpower systems networks leads to new technicalchallenges, implying research towards morerealistic and physical models for wind energysystems. This paper presents a more realisticmodelling of generator, power electronics converterused in wind power systems. Mainly three powerconverter topologies integrating wind power withthe electrical grid: two-level, three level and matrixconverters are discussed in depth with its amplescope in wind energy sector in India. Technicalcomparison between the two level, three level andmatrix converter are also given.References: [1] J. M. Carrasco et al., “Power-electronic systems for the grid integration of renewable energy sources: a survey”, IEEE Trans. on Industrial Electronics, Vol. 53, No. 4, pp 1002-1016, August 2006. [2] J. A. Baroudi, V. Dinavahi and A. M. Knight, “A review of power converter topologies for wind generators”, Renewable Energy, Vol. 32, No. 14, pp 2369-2385, November 2007. [3] R.Melicio,V. M. F. Mendes and J.P.S. Catalao, “Wind energy systems and power quality: Matrix versus Two level Converters” [4] K. Ghedamsi, D. Aouzellag and E. M. Berkouk, “Matrix Converter based variable speed wind generator system” , Journal of Electrical and Power Engineering 2(1), 39-49,2008 [5] Thomas Ackermann, “A book on wind power in power systems” John wiley & Sons,Ltd. pp 53-77. [6] Siegfried Heier, “A book on grid integration of wind energy conversion systems” John wiley & Sons,Ltd. pp 199- 214. [7] R. Erickson, S. Angkititrakul, and K. Almazeedi, “A New Family of Multilevel 2185 | P a g e