Journal of Research in Engineering and Applied Sciences
JREAS, Vol. 2, Issue 03, July 2017110
MAXIMUM POWER POINT TRACKING ALGORITHMFOR STANDALONE WIND ENERGY SYSTEM
Assistant Professor, Department of Electrical Engineering, YCCE, Nagpur, India
Due to high energy demand wind energy has become an elegant solution. But wind energy can extract maximum power at different operating condition for different wind speed. It is clean source of energy. To extract maximum amount of energy from this source various mechanism were suggested by different researchers. In this paper slope assisted maximum power point tracking (SA-MPPT) algorithm is used to obtain maximum value of Coefficient of Power (Cp) for different wind speed. The simulation work is carried out with MATLAB software and analysed for different wind conditions. Simulation results indicates the faster response for varying wind speed. Simulation results further gave faster stability.
Key Words : Coefficient of power (Cp), Slope assisted maximum power point tracking(SA-MPPT), wind energy.
Wind energy is becoming an excellent solution due to its ability of high energy generation. It has less environmental hazards. It produces large amount of clean power in minimum cost. In last few decades' wind energy has attained most of the attention due to its attractive features. Wind energy conversion system (WECS) converts the wind energy into electrical energy using wind turbine and the power conversion devices -. Nowadays power electronics is playing an important role due to its numerous advantages. Up till now wind energy system has been used for large power generation. But now-a days WECS is used for small standalone wind energy system also. These systems can be used in urban areas, residential application in remote areas where it is impossible to have a grid connection. Wind turbines are capable of supplying substantial amount of power due to improved aerodynamic designs and power electronics interface-.
Different types of generators are used in WECS like Squirrel cage Induction Generator, Doubly fed Induction Generator, Wound rotor Induction Generator, Permanent Magnet Synchronous Generator. Presently, Permanent Magnet Synchronous Generator is preferred over other types due to its numerous advantages like high efficiency, small size, less cost, less maintenance, high energy generation capability-.
To have the maximum amount of output power , maximum power point tracking (MPPT) control strategy is necessary. The MPPT techniques can be classified as, tip speed ratio(TSR) control, the optimum relation based control(ORB), the perturb and observe(P&O) control-.
Using the measured wind and turbine shaft speed, the turbine shaft speed is directly controlled to maintain optimal
TSR in TSR control. In ORB control, optimum relation between different system variable is used to track the maximum power point (MPP) . In P&O control algorithm The MPP is tracked by continuously changing the rotating speed to obtain maximum power for particular wind speed. Step size for variation in the voltage remains constant in the P&O method. Algorithm monitors changes in the power in regards of changes in Till the algorithm reaches the MPP the next perturbation size and the direction may be determined. Generally power speed relation of wind turbine has been used for P&O control. But nowadays authors use dc link voltage and duty cycle for P&O control. This reduces system cost and increases reliability. The cost reduces further because the wind speed measurement is not required. This method is more reliable and less complex because prior information about system parameter is not needed. But when the wind speed changes rapidly, this method shows less response. This is the disadvantage of this P&O method -.
This paper explains the Slope Assisted Maximum Power Point Tracking Algorithm (SA-MPPT). This MPPT algorithm uses slope relationship that relates the measured power and speed of turbine to system tip speed ratio(TSR). At all wind speed the maximum power capture occurs at single TSR. The generator speed values and the power is represented by using the slope relationships. By using the relationships , the algorithm remember the last position on maximum power
ISSN (Print) : 2456-6411, ISSN (Online) : 2456-6403
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curve(MPC) before the atmospheric change. When the wind speed and air density changes, the algorithm determine optimal tip speed ratio that allows it to search the exact location of MPP.
Section II explains the overall system when the SA-MPPT module is connected. In Section III the overall algorithm is explained by giving the equations and the respective flowchart. The simulation and the results are explained in Section IV. Conclusion is given in Section V and Section VI gives the References.
ISSN (Print) : 2456-6411, ISSN (Online) : 2456-6403
2. Description of Sa-mppt Scheme
Fig. 1 : Block Diagram of wind energy conversion system
Fig.1 represents the proposed scheme which initially consist of a wind turbine. The wind turbine gives the rotational speed which is given to the PMSG. The wind is nothing but the kinetic energy. The PMSG converts this mechanical energy into electrical energy. The three phase output of PMSG is then given to the filter circuit. The filter circuit will make the output ripple free. This output is given to the converter circuit. There are many possible configuration of converter circuit. It can be a diode bridge, controlled rectifier, multilevel converter etc. The output voltage and output current are given to the SA-MPPT module. The SA-MPPT module generate the reference speed. Then the reference speed and the rotor speed is compared. Using the PWM controller the switching pulses for chopper is generated.
Fig. 2 : Typical power coefficient curve
Fig.1. shows typical power coefficient curve. It can be seen from the curve that maximum power efficiency C p maxis obtained at the single tip speed ratio denoted by l . The optpower extraction efficiency is totally dependent on C pfunction and TSR values in fixed speed wind energy conversion system. The amount of power extracted is affected by the air density but the air d
3. Sa-MPPT Algorithm
The algorithm has the feature of wind change detection which minimizes the MPP search error. This algorithm stores the current slope value and with respect to that it calculates the optimal value of tip speed ratio. The system parameter may
get shift due to aging or any other atmospheric change but this algorithm get adapted to these situation also-. If there is any wind change then the algorithm calculate the power speed change ratio (PSCR) where the current step is represented by subscript n. The PSCR decreases and then becomes zero and the algorithm operates normally when there is no change in wind speed. The speed change polarity becomes positive if there is any increase in turbine speed and is found to be negative when the turbine speed decreases. Fig.3. shows the flowchart of SA-MPPT algorithm. Initially the algorithm calculates the slope values given by(1)
mc =3 P
= mlk (1)v The change in speed is given by (2)
se =v n -v ref
(2) v ref
The percent difference change in power is given by (3)
=Pn - Pn-1 (3)
The percent difference change in speed is given by(4)
schange = v n -v n-1
If there is no change in speed then power speed change ratio(PSCR) is calculated
Pn - Pn-1
v n -v n-1
The difference between the SA-MPPT method and the conventional method is that the SA-MPPT method calculates the tip speed ratio values which reduces the time for maximum power point. The step size required is also less in this algorithm. If there is decrease in power
JREAS, Vol. 2, Issue 03, July 2017112ISSN (Print) : 2456-6411, ISSN (Online) : 2456-6403
and there is no wind change then the location for MPP is searched and is moved towards the last operating point and hence the oscillation at the MPP reduces-. When the wind speed change is detected then the algorithm will continue to search for the MPP. To represent the last operating point, the algorithm calculates the tip speed ratio. It is done for the new wind condition and again the search for MPP starts from where it is left off. The algorithm not only evaluates the change in tip speed ratio but also evaluates the change in power . This algorithm is very robust to atmospheric change and is able to detect the wind speed change. At the convergence point, the algorithm checks for-
a) P change e p,MPP ,
b) Pchange -e p,MPP
c) change in polarity of wind ( s pol ).
Fig. 3 : Flowchart of Slope-Assisted MaximumPower Point Tracking algorithm
4. Simulation Results
Fig. 4 : Wind turbine PMSG model for SA-MPPT
Fig.3. shows the system created in MATLAB. The wind turbine model is also modeled in MATLAB. The C p function
is designed on the basis of relationships from wind turbine model which is given in MATLAB Simulink. The wind turbine profile is given by (1).
(6) l+ 0.08b
b 3 +1
The turbine model is described by (2),
+ c6 l (7) C p (l, b ) = c1 -
where c1 =0.5176 , c2 = 116, c3 = 0.4, c4 = 5, c5 = 21,
c = 0.0068 .6
Fig.4 shows the wind turbine PMSG model for SA-MPPT which is simulated in MATLAB. Wind speed, pitch angle and the generator speed is given to the wind turbine. The output of wind turbine is given to the PMSG. As the converter used is an three phase diode rectifier and hence filter is not required. If the rectifier is an controlled rectifier then the filter is required necessarily. The output voltage and output current of the rectifier is multiplied and this power is given to the SA-MPPT module. The reference is generated from this module. This reference speed is compared with the rotor speed and is then given to the PI controller. This PI controller converts the speed
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into the voltage. This becomes the reference wave for the PWM technique. This PWM technique will generate the pulses for the switch of boost converter. The algorithm controls the duty cycle of the boost converter.
Fig. 5 : (a) ,(b) Behaviour of Slope- assistedMaximum Power Point Tracking Algorithm
Fig.5.a shows the performance of SA-MPPT algorithm.. Initially the wind speed is 8m/s. As the cut in speed is 3m/s
therefore power is generated at 8m/s. Later on the speed is made 7m/s. At that time the power is 800W. At 9m/s the power is 1238W. As the wind speed is constant at 7m/s, the power also becomes constant.The generator speed and the reference speed try to becomes constant as the speed becomes constant.
Fig.5.b is the performance for different wind condition. It initially starts with 3m/s which is cut in speed. Later on at 7m/s the wind speed becomes constant and hence power becomes constant. In this case the different power condition is for more time period. As the cut in speed is 3m/s hence very less amount of power is generated.
SA-MPPT algorithm is simulated and analyzed using MATLAB software. This algorithm gives more constant power at different wind conditions. When the wind becomes constant, the power becomes constant and the generator and reference speed tends to become constant.
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