MPPT Control Technique for DirectDrive FivePhase PMSG Wind Turbines with Wind Speed Estimation
AbdelRaheem Youssef^{1}, Mahmoud A. Sayed^{1}, M. N. AbdelWahab^{2}, Gaber Shabib Salman^{3}
^{1}Dept. of Electrical Engineering, Faculty of Engineering, South Valley University, Qena, Egypt
^{2}Dept. of Electrical Engineering, Faculty of Engineering, Suez Canal University, Ismailia, Egypt
^{3}Dept. of Electrical Engineering, Faculty of Engineering, Aswan University, Aswan, Egypt
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To cite this article:
AbdelRaheem Youssef, Mahmoud A. Sayed, M. N. AbdelWahab, Gaber Shabib Salman. MPPT Control Technique for DirectDrive FivePhase PMSG Wind Turbines with Wind Speed Estimation.International Journal of Sustainable and Green Energy.Vol.4, No. 5, 2015, pp. 195205. doi: 10.11648/j.ijrse.20150405.14
Abstract: This paper has presented comprehensive modeling of direct driven fivephase PMSG based gridconnected wind turbines along with the control schemes of the interfacing converters. Wind speed estimation has been achieved based on measured rotor speed. Fivephase to threephase interface power converter based backtoback common dclink converter has been used to achieve the system objectives. The machine side converter (MSC) is used to track the maximum power point at different wind speed. The grid side converter (GSC) uses a vector current controller to inject pure active power to the grid. The effectiveness of proposed control scheme is validated through extensive simulation results by using MATALB/SIMULINK.
Keywords: MPPT, FivePhase PMSG, MSC, GSC
1. Introduction
Wind power is today’s most rapidly growing renewable energy source. A wind turbine operates either at a ﬁxed or variable speed [1]. Most of wind turbine manufacturers are developing new different scale wind turbines based on variablespeed operation with pitch control using either a permanent magnet synchronous generator (PMSG) or a doubly fed induction generator (DFIG) [2]. Due to the intensiﬁed grid codes, a PMSG wind turbine with full VSCbased converters is becoming more favored by the wind power industry [3]–[7].The variable speed wind turbine with a multiphase PMSG and fullscale/fully controllable voltage source converters (VSCs) is considered to be a promising, but not yet very popular, wind turbine concept [3]. The wind turbines based multiphase PMSG conﬁguration have many advantages such as gearless construction [4], elimination of a dc excitation system [5], full controllability of the system for maximum wind power extraction and grid interface, and ease in accomplishing faultride through and grid support [6].Therefore, the efﬁciency and reliability of a VSCbased PMSG wind turbine is assessed to be higher than that based DFIG [7].
Recently, multiphase machines have gained much interest due to their advantages over conventional threephase machines. The use of multiphase permanent magnet synchronous generators PMSG to implement high power is an alternative to reduce the current rating of the converter power switches. Multiphase PMSG have many advantages such as reducing the amplitude and increasing the frequency of torque pulsations, higher reliability, and lowering the dc link current harmonics [8][11]. Therefore, multiphase PMSG are very suitable for the applications of high power, high reliability, and low dc bus voltage, such as renewable energy. These advantages have motivated the wind turbine manufacturers to use multiphase machines. For example, Spanish manufacturer Gamesa has developed a fullpower 4.5 MW wind turbine with 6 parallel converters and 18phase generator [13]. Some other topologies that use series connected generatorside converters have also been proposed to achieve medium voltage on the gridside [14], [15].
Maximum power point tracking control in most of the conversion systems is implemented using wind speed data obtained from wind speed sensors [16][19]. However, accurate measurement of wind speed is not easy especially in case of large size wind turbines. Anemometer installed on the top of nacelle provides limited measurements of wind speed only at the hub height and cannot cover the whole span of large blades [20]. Moreover, due to the interaction between the rotor and the wind, anemometer, usually placed on nacelles, leads to inaccurate wind speed measurements in both upwind and downwind turbines. Therefore, Speed control of wind turbine based sensorless algorithms has gained many interests due to its accuracy and simplicity in tracking the maximum power point during wind speed variations [21,22].
In this paper, maximum power point tracking for wind turbine based fivephase PMSG has been achieved by wind speed estimation technique. Estimation of the wind speed has been achieved measure rotor speed and the estimated load torque. A full scale power converter based fivephase has been used. The dclink, connecting the backtoback converters, allow fully decoupled control of the fivephase PMSG from the grid side. The MPPT has been achieved by controlling the PMSG speed at the generator side, whereas the grid side converter has been controlled to achieve unity power factor at the grid side. The effectiveness of the proposed control technique in addition to the efficient operation of the wind turbine system has been verified using Matlab/Simulink.
2. Wind Energy Conversion System
Wind energy conversion system (WECS) converts kinetic energy of wind to mechanical energy by means of wind turbine rotor blades; then the generator converts the mechanical power to electrical power. The resulting electrical power is being fed to the electrical network through power electronic converters. In this paper, the WECS consists of a gearless wind turbine coupled to a fivephase PMSG, interfaced with the grid through backtoback converters connected to each other through a common dclink capacitor, as shown in Fig. 1.
2.1. Wind Turbine Model
The mechanical power captured from wind turbine can be formulated as follows:
(1)
Where the mechanical output power (Watt), ρ is the air density ( ), A is the swept area ( ), is the power coefficient of the wind turbine, is the wind speed (m/sec), λ is the tip speed ratio, β is pitch angle.
Consequently, the output energy is determined by power coefficient , swept area, air density, tip speed ratio (λ) and pitch angle (β). If β is equal zero, the turbine power coefficient and the tip speed ration λ can be formulated as follows:
(2)
(3)
Where is the rotor rotational speed (rad/sec), R is the radius of blade (m).
The relation between and λ when β equal zero degree is shown in Fig. 2. It can be noticed that the optimum value of is about 0.48 for λ equal 8.1. Maximum power extraction from wind turbine can be achieved when the turbine operates at the optimum . Therefore, it is necessary to control the rotor speed of the wind turbine at optimum and λ during wind speed variation.
Based on the relations given in eq.(1) & (3), the optimum output power of the wind turbine can by formulated as follows:
(4)
(5)
Fig. 3 shows the relation between the mechanical powers generated by the turbine and the turbine rotor speed at different wind speeds. It is cleared that the maximum power point changes with the variation of wind speed and there is a unique maximum power point at each wind speed. The maximum power extraction can be achieved if the controller can properly follow the optimum curve with variation of wind speed, as shown in Fig. 3.
2.2. FivePhase PMSG Model
The voltage equations of fivephase permanent magnet synchronous generator expressed in the rotor reference frame using an extended park transformation (d_{1},q_{1} and d_{2},q_{2}) axis can be described as follows:
(6)
(7)
(8)
(9)
Where and represent the stator voltages in the (d, q) axis, and represent the currents in the (d,q) axis, represent stator resistance, L represent armature inductance, represents the (d, q) axis inductance, (p is number of pole pairs, represent the turbine rotor angular speed and ψ is the permanent flux linkage).
The electrical torque of the fivephase PMSG can be formulated as:
(10)
The mechanical equation of PMSG is given as follows:
(11)
Where f is the friction coefficient, J the total moment of inertia and is the mechanical torque produced by wind turbine, is electromagnetic torque of PMSG.
3. Wind Speed Estimation Technique
Prior to explaining the wind speed estimation method, the nonlinear blade power coefficient curve needs to be approximated as third order polynomial [21] as follows;
(12)
Substituting (3) and (12) into (1) results in mechanical power as follows.
(13)
Based on (13) wind speed can be formulated as follows
(14)
Where ,
The numerical solution for (14) generates three values for the wind speed. The second answer value is the more accurate empirical solution [23]. The mechanical power in (15) can be estimated using the detected rotor speed and the calculated torque as follows:
(15)
4. Control of Machine Side Converter (MSC)
Since the PMSG is a fivephase machine, the machine side converter has been built using fiveleg of bidirectional IGBT switches, as shown in Fig. (4). The generator side converter is mainly used to control the wind turbine speed in order to extract maximum power . In this case, the turbine should operate at maximum power coefficient . Therefore, it is necessary to keep the generator rotor speed at an optimum value of tip speed ratio . The PMSG rotor speed should be adjusted to follow the change of reference speed based on the change of wind speed and consequently adjust the turbine speed at wind variations. The MSC allows the generator to rotate at specified reference speed depending on wind speed variation. Fig. (4) shows the schematic diagram of the generator side converter control scheme.
In order to understand the speed control concept, the PMSG dynamic model should be studied [24]. The PMSG motion equation is given based on (11) as follows:
(16)
The mechanical rotational speed of PMSG rotor is given by:
(17)
Where, electrical rotational speed of PMSG rotor (rad/s), turbine rotational speed and gear ratio (if existed). For gearless PMSG based wind turbine . According to the characteristic of wind turbine at any value of wind speed, the rotational speed of the turbine rotor is regulated to the value through generator side control hence:
(18)
Therefore, the turbine power coefficient is kept at its maximum value.
From (16), the speed control of generator can be achieved by the control of electromagnetic torque Te. From (10) the electromagnetic torque can be controlled directly by qaxis current component , therefore the speed can be controlled by controlling the axis current, as shown in Fig. (4). The reference axis current component can be formulated, based on (10), as follows.
(19)
()axis current components are set to zero to minimize the current and resistive copper losses for a given torque.
5. GridSide Converter Control
The objective of grid side converter control is to adjusts the DC link capacitor voltage at its reference value, and adjusts the active power and reactive power delivered to grid while wind changing. In grid side converter, a PI controller is used to stabilize the DC voltage reference value. The dynamic model of the grid connection, in reference frame rotating synchronously with the grid voltage, is given as follows [25]
(20)
(21)
Where L and R are the grid inductance and resistance, respectively. are the dq axis inverter voltage components. If the reference frame is oriented along the supply voltage, the grid vector voltage is:
(22)
Active and reactive power can be expressed as follows [25].
(23)
(24)
It could be seen from above equations that we can control the active and reactive powers by respectively changing the d and qcurrent components. Also in order to transfer all the active power generated by the wind turbine the dclink voltage must remain constant [26].
(25)
Where subscript ’g’ refers to the grid and ‘t’ refers to the wind turbine.
Based on (25), if the two powers (the wind turbine power and the grid power) are equal there will be no change in the dclink voltage. The grid side converter control scheme contains two cascaded loops. The inner loop controls the network currents and the outer loop controls the DClink voltage. The inner loop regulates the power flow of the system by controlling the active and reactive power delivered to the electrical grid. Further, unity power factor flow (zero reactive power exchange) could be easily obtained, unless the grid operators require different reactive power settings.
6. Simulation Results and Discussion
The model of wind turbine based fivephase PMSG in addition to the backtoback interface converters for grid connection have been carried out using Matlab/Simulink. The parameters of the system under study are given in appendix A. The proposed control scheme of the fivephase PMSG based variable speed WECS has been carried out using MATLAB/Simulink at different values of wind speed in order to investigate the wind speed estimation technique and the MPPT at the generator side in addition to the unity power factor control at the grid side and common dclink capacitor voltage control of the interface converters.
6.1. Ramp Change Wind Speed
Fig.6 shows the actual and estimated wind speed, error in wind speed, the reference and actual rotor speed, power coefficient, tip speed ratio, mechanical power, the mechanical and electromagnetic torque of the PMSG and the fivephase current of PMSG. According to wind turbine characteristic, the estimated and actual wind speed values coincide well and when wind speed varies the controller adjust PMSG rotor speed to follow the same value of . It is clear that the actual and reference rotor speed agree well and the error between them is very small. Moreover, the power coefficient and tip speed ratio are almost constant following their optimal values for the whole simulation period. This inturn prove that the MPPT has been achieved. The actual and estimated mechanical power agree well with the maximum power. The mechanical and electromagnetic torques of fivephase PMSG are varying according to the change in wind speed. Fig.7 shows the dclink capacitor voltage, grid voltage and current, dqaxis grid current, power factor, and injected active and reactive power. It is the clear that sinusoidal grid voltage and current are inphase for the whole simulation period to achieve unity power factor. The reference and actual dclink voltage coincide well. The actual and reference qaxis current at grid side is always controlled to be zero to achieve unity power factor. Therefore, the injected reactive power is zero during the whole simulation time, whereas the injected active power has a changes according to the change in the wind speed.
6.2. Random Wind Speed
The effectiveness of the proposed control techniques has been investigated with Random wind speed variation. Fig.8 shows the actual and estimated wind speed, error in wind speed, the reference and actual rotor speed, power coefficient, tip speed ratio, mechanical power, the mechanical and electromagnetic torque of the PMSG and the fivephase current of PMSG. It is clear that the difference between estimated and actual wind speed is very small, the turbine shaft speed is controlled to track its reference value. Achievement of MPPT is known, from the power coefficient, which is almost constant value at (0.48) and the change of the tip speed ratio that varies in a relatively small range around the optimal value of (8.1). The actual and estimated mechanical power agrees well with the maximum power. The mechanical and electromagnetic torques of PMSG coincides well. Fig. 9 shows the dclink capacitor voltage, grid voltage and current, dqaxis grid current, power factor and injected active and reactive power. It is the clear that sinusoidal grid voltage and current are inphase for the whole simulating period to achieve unity power factor. The reference and actual dclink voltage coincide well. The actual and reference qaxis current at grid side is always controlled to be zero to achieve unity power factor. The injected reactive power is zero during the whole simulation time, whereas the injected active power has a step change according to the change in the wind speed.
Simulation results prove that the wind speed estimation algorithm has the ability to estimate the wind speed, the MSC has the ability to control the PMSG to extract the maximum power based on MPPT control technique, and the GSC has the ability to achieve unity power factor at the grid side.
7. Conclusion
This paper has presented comprehensive modeling of direct driven fivephase PMSG based gridconnected wind turbines along with the control schemes of the interfacing converters. Wind speed estimation has been achieved based on measured rotor speed. Fivephase to threephase interface power converter based backtoback common dclink converter has been used to achieve the system objectives. The generator side converter has been used to achieve maximum power operation point at each wind speed. The grid side converter has been used to inject sinusoidal current inphase with the grid voltage in addition to controlling the common dclink capacitor voltage. Vector current controller has been employed on the grid side VSI to obtain unity power factor. Simulation results prove that the proposed control scheme has a great capability to obtain unity power factor at the grid side and to achieve sensorless maximum power point tracking of wind turbine based fivephase PMSG during wind speed variation.
Appendix
Appendix 1. Specification of Wind Turbine
blade radius 

Air density 

Optimal tip speed ratio 

Maximum power Coefficient 

Appendix 2. FivePhase PMSG Parameters
Pole pairs number 

Stator resistance 

Directaxis inductance 

quadratureaxis inductance 

Moment of inertia 

Flux linkage 

Appendix 3. DC Bus and Gird Parameters
dclink voltage 

Capacitor of the dclink 

Grid frequency 

Grid resistance 

Grid inductance 

Appendix 4. Machine Side Control
Appendix 5. Grid Side Control
References