Maximum Power Point Tracking Of Wind Energy Conversion System With Permanent Magnet Synchronous Generator

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 Gorakhpur-273010 MMMEC Gorakhpur-273010

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)

1. 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 variable-speed 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 wind-power fluctuations. Recently, PMSG-based directly driven variable-speed WECS are becoming more popular due to the elimination of gear box and excitation system [2][3][4]. Generally shaft-mounted speed sensors are used, resulting in additional cost and complexity of the system. To alleviate the need of these sensors, several speed-estimating algorithms based on motional electromotive force (EMF), flux-linkage 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 low-voltage 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].

2. System description

   The system under consideration employs PMSG- based variable speed WECS consisting of three phase full converter and load with a common dc-link. 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 PMSG-based variable speed WECS

   1. 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.

      1. 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 aero-turbine 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 aero-turbine, 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.

      2. Wind Turbine

The wind power captured by wind turbine depends on its power co-efficient (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 above-mentioned 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].

1. Generator

   WECS need a low-speed gearless generator, especially for off-shore 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 low-speed PMSG requires:

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

   2. Big rotor diameter for the high wind turbine

      Fig. 2 charecteristi of wind

      torque

      1. 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 magnetic-flux 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;

      2. Equivalent Circuit And Modelling of PMSG:

      Since the back-EMF 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 state-space-equivalent 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 dc-link [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 line-to-neutral voltages are defined as

      The corresponding line-to-line 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

   3. 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 power-speed characteristic. For this purpose, the power at dc-link is used to obtain reference speed by using the power-speed 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 q-axis reference current component (torque controlling current component) can be derived using

      The d-axis 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 generator-side control diagram is shown in Fig. 7.

      fig.7 Proposed model

   4. Simulation and results

      The proposed control strategy for PMSG-based 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

   5. 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.

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