 Open Access
 Total Downloads : 6753
 Authors : Krishna Kumar Pandey, Dr.A.N.Tiwari
 Paper ID : IJERTV1IS5198
 Volume & Issue : Volume 01, Issue 05 (July 2012)
 Published (First Online): 02082012
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Maximum Power Point Tracking Of Wind Energy Conversion System With Permanent Magnet Synchronous Generator
Krishna Kumar Pandey Dr.A.N.Tiwari
Student(M.Tech)Power electronics and drives Associate professor Electrical Dept. MMMEC Gorakhpur273010 MMMEC Gorakhpur273010
Abstract
This paper presents a control strategy of a variable speed wind turbine and gets maximum power point tracking which is connected with multipole permanent magnet synchronous generator (PMSM) and fully controlled three phase converter.
The simulation results show that theoretical analysis and the validation of the proposed strategy.
Keywords Permanent magnet synchronous motor (PMSM), Wind Energy conversion system (WECS), maximum power point tracking (MPPT)

Introduction
One of the most commonly used renewable energy; wind power is the most promising for replacing the fossil fuel in the near future. The advancement in power electronics devices has further played an important role in the improvement of their reliability and controllability [2]. The variablespeed wind turbines are more attractive, as they can extract maximum power at different wind velocities, and thus, reduce the mechanical stress on WECS by absorbing the windpower fluctuations. Recently, PMSGbased directly driven variablespeed WECS are becoming more popular due to the elimination of gear box and excitation system [2][3][4]. Generally shaftmounted speed sensors are used, resulting in additional cost and complexity of the system. To alleviate the need of these sensors, several speedestimating algorithms based on motional electromotive force (EMF), fluxlinkage variation, However, the precise estimation of rotor position and speed is very difficult as most of these suffer because of simplified computations based on several assumptions, ignorance of parameter variations, and inaccuracy involved with lowvoltage signal measurement at lowerspeed, especially in case of
directly driven PMSG. The wind turbines with full scale converters will be preferred in future, their are maximum power is the cubic function of generator speed for a given tip speed ratio so for tracking MPPT [1]. We are tacking two close loop which are speed control loop and current control loop [17][19].

System description
The system under consideration employs PMSG based variable speed WECS consisting of three phase full converter and load with a common dclink. The block diagram of variable speed WECS is shown in Fig.1 and the main components of system with their important characteristics are discussed below:
Fig.1 Block diagram of PMSGbased variable speed WECS

Wind Energy and Wind Turbine
The circulation of air in the atmosphere is caused by the non uniform heating of the earths surface by sun. In general, during the day the air above the land mass tends to heat up more rapidly than the air over
water. In coastal regions this manifests itself in a strong onshore wind. At night the process is reversed because the air cools down more rapidly over the land and the breeze blows off shore.

Power in wind energy
The power in the wind can be computed by using the concept of kinetics. The wind mill works on the principal of converting kinetic of the wind to mechanical energy [18][20]. We know that power is equal to energy per unit time. The energy available is the kinetic energy of the wind. The kinetic energy of any particle is equal to one half its mass times the square of its velocity, or . The amount of air passing in unit time, through an area A, with velocity V, is A.V, and its mass m is equal to its volume multiplied by its density of air, or
Substituting this value of the mass in the expression for the kinetic energy,
Equations tell us that the wind power is proportional to the intercept area. Thus an aeroturbine with large swept area has higher power than a smaller area machine [5][6][7]; Since the area is normally circular of radius r in horizontal axis aeroturbine, then
So the equation of wind power is converted to
Characteristics of wind power depend on the cubic function of wind velocity, which is sown in below.

Wind Turbine


The wind power captured by wind turbine depends on its power coefficient (Cp) which is given by the relation
But, for a given turbine is not always constant. The most common parameters for are the tip speed
ratio and the pitch angle . The tip speed ratio is given as
Here the power coefficient is a function of both parameters. Consequently, different wind speeds will require the optimal values of tip speed and pitch angle to achieve a high and therefore giving the highest power output at all available wind speeds. The abovementioned aspects make it very clear that to extract maximum power out of the varying wind we need to have a wind turbine that allows the change in rotor speed to reach optimal aerodynamic conditions [8][9][10].

Generator
WECS need a lowspeed gearless generator, especially for offshore wind applications, where the geared doubly fed induction generator will require regular maintenance because of tearing wearing in brushes and gear box [5][6]. Both the brushes and the gear box can be eliminated from WECS by using directly coupled low speed generators. Further, the elimination of the gear box can increase the efficiency of wind turbine by 10%.
The lowspeed PMSG requires:

Higher number of poles to obtain suitable frequency at low speed;

Big rotor diameter for the high wind turbine
Fig. 2 charecteristi of wind
torque

E.M.F. Equation of an alternator
The expression for induced emf and torque is' derived for a machine with P poles, Zp coil sides in series per phase in a field with a flux per pole of , T turns per phase, f frequency and rotating at n rps(N rpm). Since the flux per pole is , each stator
conductor cuts a flux P . The average value of generated voltage per conductor
The average voltage generated per conductor
is convenient to model PMSG in this frame. The voltage equations of PMSG are as follows
……… (1)
We know that
Substituting the value of Pn in eq (1),we get
Since there are Zp conductors in series per phase, the average voltage generated per phase is given by
Since one turn or coil has two sides, Zp=2Tp, and the expression for the average generated voltage per phase can be written as
For the voltage wave, the form factor is given by
For a sinusoidal voltage, kf=1.11. Therefore, the
r.m.s. value of the generated voltage per phase can be written as
For rotating machine the winding factor (Kw) is involved in E.M.F. equation which is denoted by
Fig. 3equvalent circuit diagram of PMSM
where V ,V are stator terminal voltages, Ra is stator resistance, XS is stator inductance, i, i are output currents, and E, E are back EMFs, which can be given as
Here, r , r , and m are rotor speed, rotor position, and magneticflux linkage, respectively
On rearranging (1) and (2), and rewriting them in matrix form, we have
Where
For full pitch coil, Kc=1; For concentrated winding, Kd=1;

Equivalent Circuit And Modelling of PMSG:
Since the backEMF is the function of rotor position in stationary reference frame, therefore, it
Hear the transfer function modelling is arrange in a equation
where the dot indicates the estimated variables and
p>A typical inversion mode of operation is shown in Figure 2.8. Note that this corresponds to a second quadrant operation of the dc motor drive.
,
, ,
T 5
, and
Three phase
T
T
T
ac 1 2 3
supply
iac
–
+
–
+
Vdc
Ra
T6
The statespaceequivalent diagram of equation is T 4+
shown in below E –
Fig.5 Three phase controlled converter
Fig.4 state space model of PMSM
2.3. Power electronics interference
The proposed system consists of fully controlled converters decoupled by a dclink [21][22]. The converters have been realised by using six switches for converter. The thyristors require small reactors in series to limit the rate of rise of currents and snubbers[11][12], Which are resistors in series with capacitors across the devices are commutated.
2.3.1 Mathematical modelling and circuit diagram
If the linetoneutral voltages are defined as
The corresponding linetoline voltage are,
Fig. 6 wave form of controlled rectifier
The average output voltage is found from
The maximum average output voltage for delay angle , is
And the normalized average output voltage is
The rms value of the output voltage is found from


Development of proposed control strategy
In a variable speed WECS, the maximum power at different wind velocities is almost a cubic function of generator speed as shown in Fig.7. Therefore the generator speed is controlled in order to follow the powerspeed characteristic. For this purpose, the power at dclink is used to obtain reference speed by using the powerspeed curve of the generator [14][15]. The error of this reference speed and actual speed are then given to the proportional integral (PI) regulator to obtain reference torque of the generator expressed as
Where , are proportional and integral gains for generator speed control. The qaxis reference current component (torque controlling current component) can be derived using
The daxis reference current component can be set to zero in order to obtain maximum torque at minimum current and therefore to minimise the resistive losses in the generator [16] . The generatorside control diagram is shown in Fig. 7.
fig.7 Proposed model

Simulation and results
The proposed control strategy for PMSGbased variable speed WECS is simulated on MATLAB/SIM POWER SYSTEM in different operating conditions. The simulink model shows that, the stator current fluctuation in nearly proportional to fluctuation occurred in wind speed, due to the closed loop scheme. Three phase stator current converted into dc through the three phase fully controlled rectifier as shown in figure 9. The DC output power as shown in fig 11 and fig 12 converted in to wind speed in equivalent of wind power then by comparing with the generator speed and given error signal to the 2 phase to 3 phase converter as shown in fig.13. Then the output passed through the hysteresis controller to obtain the pulses in convertor as shown in fig.14.
Fig. 8 wind speed
Fig.9 Three phase current of generator
Fig.10 Rotor speed, angle & torque
Fig.11 DC output current and voltage
Fig.12 DC power
Fig. 13 speed Comparator
Fig. 14 Pulses for the controlled rectifier

Conclusions 1
This paper shows that the maximum power is traced on implementing closed loop scheme to remove fluctuation occurred in the output by maintaining the switching in the form of pulses on application of controlled rectifier.

References


MORREN J and DE HANN S.W : Ride through of wind turbines with doublyfed induction generator during a voltage dip, IEEE Trans.
Energy Convers., 2005 , 20 , (2) , pp. 435441

Connection of wind turbines at the grid under100 kV, Technical regulations T.F. 3.2.6, Eltra/Elkraft,July2004.Internet, http://www.eltra.dk

SENJYU T., KINJO T. and FUJITA H., AICHI
: Analysis of terminal voltage and output power control of wind turbine generator by series and parallel compensation using SMES, IEEE 35th Ann. Power Electronics Specialists Conf., June 2004, vol. 6, pp. 42784284.

SACCOMANDO G., SVENSSON J., SANNINO A : Improving voltage disturbance rejection for variablespeed wind turbines, IEEE Trans. Energy Convers., 2002, 17, (3), pp. 422428

MILLIGAN M : Measuring wind plant capacity value, (National Renewable Energy Laboratory, Golden Colorado, USA)

T. Senjyua, S. Tamakia, E. Muhandoa, N. Urasakia, H. Kinjoa, T. Funabashib, H. Fujitac, and
H. Sekinea, Wind velocity and rotor position sensorless maximum power point tracking control for wind generation system, in Proc. 30th Annu. Conf. IEEE Ind. Electron. Soc. (IECON 2004), vol. 2, pp. 19571962.

S. Bhowmik and R. SpÂ´ee, Wind speed estimation based variable speed wind power generation, in Conf. Rec. IEEE IECON 1998, Aachen, Germany, pp. 596601.

Capacity factors at Kansas wind farms compared with total state electrical demand, July 2007 to June 2008, http://www.kcc.state.ks.us/energy/charts/Wind Capacity Factors at Kansas Wind Farms Compared with Total State Electrical Demand.pdf

BOCCARD N : Capacity factor of wind power realized values vs. estimates, Elsevier Trans. Energy Policy, 2009, 37, pp. 26792688

G. Saccomando, J. Svensson, and A. Sannino, Improving voltage disturbance rejection for variablespeed wind turbines, IEEE Trans. Energy Convers., vol. 17, no. 3, pp. 422428, Sep. 2002

MULJADI E. and BUTTERFIELD C.P : Pitchcontrolled variable speed wind turbine generation, IEEE Trans. Ind. Appl., 2001, 37, (1), pp. 240246

J.Morren, S.W. de Hann, Ridethrough of wind Turbines with doublyfed induction generator during a voltage dip, IEEE Trans. Energy Convers., vol. 20, no. 2, pp. 435441, Jun. 2005

HEIER S : Grid integration of wind energy conversion systems (Wiley, Chicester, UK, 1998)

B. K. Bose, Modern Power Electronics and AC Drives, Upper Saddle River, NJ: Prentice Hall, 2002.

JOHNSON G : Wind energy systems (PrenticeHall, Englewood Cliffs, NJ, 1990)

ASIMINOAEI L., BLAABJERG F. and HANSEN S : Harmonic detection methods for active power filter applications, IEEE Ind. Appl.
Mag., 2007, pp. 2233

WESTLAKE A.J.G., BUMBY J.R. and SPOONER E : Damping the powerangle oscillations of a permanent magnet synchronous generator with particular reference to wind turbine applications, IEE Proc. Electr. Power Appl., 1996, 143, (3)

LEONHARD W : Control of electric drives (Springer, New York, 1997)

KRAUSE P.C., WASYNCZUK O. and SUDHOFF S.D : Analysis of electric machinery (IEEE Press, 1994)

SLOOTWEG J.G., DE HAAN S.W.H., POLINDER H. and KLING W.L : General model for representing variable speed wind turbines in power system dynamics simulations, IEEE Trans.
Power Syst., 2003, 18, (1), pp. 144151

CARRASCO J.M., FRANQUELO L.G., BIALASIEWICZ J.T. and ET AL : Power electronic systems for the grid integration of renewable energy sources: a survey, IEEE Trans.
Ind. Electron., 2006, 53, (4), pp. 10021016

ENSLIN J.H.R. and HESKES P.J.M : Harmonic interaction between a large number of distributed power inverters and the distribution network, IEEE Trans. Power Electron., 2004, 19, (6), pp. 15861593