A wind generator (WG) maximum power point tracking (MPPT) system is presented here. It comprises of a variablespeed wind generator, a highefficiency boosttype dc/dc converter and a control unit. The advantages of the aimed system are that it does not call for the knowledge of the wind speed or the optimal power characteristics and that it operates at a variable speed, thus providing high efficiency. The WG operates at variable speed and thus suffers lower stress on the shafts and gears compared to constantspeed systems. It results in a better exploitation of the available wind energy, especially in the low windspeed range of 2.54.5 m/s. It does not depend on the WG wind and rotorspeed ratings or the dc/dc converter power rating. Higher reliability, lower complexity and cost, and less mechanical stress of the WG. It can be applied to batterycharging applications.
1. Introduction
WIND GENERATORS (WGs) have been widely applied both in autonomous systems for power supplying remote loads and in gridconnected applications. Although WGs accept a lower installation cost equated to photovoltaic’s, the overall system cost can be further reduced applying highefficiency power converters, controlled such that the optimal power is assumed according to the current atmospheric conditions. The WG power production can be mechanically controlled by changing the blade pitch angle
[1]
However, WGs of special structure are required, which is not the usual case, especially in smallsize standalone WG systems.
A commonly used WG control system
[2

6]
is shown in
Fig. 1(a)
. This topology is based on the WG optimal power versus the rotatingspeed characteristic, which is usually stored in a microcontroller memory. The WG rotating speed is evaluated; then, the optimal output power is calculated and equated to the actual WG output power. The resulting error is applied to control a power interface. In a similar version detected in
[7]
, the WG output power is evaluated and the target rotor speed for optimal power generation is deduced from the WG optimal power versus rotorspeed characteristic. The target rotor speed is compared to the actual speed, and the error is used to control a dc/dc power converter. The control algorithm has been implemented in Lab VIEW running on a PC.
Control system based on rotating speed measurements
In permanentmagnet (PM) WG systems, the output current And voltage is proportional to the electromagnetic torque and rotor speed, respectively. In
[8
,
9]
, the rotor speed is calculated according to the measured WG output voltage, while the optimal output current is calculated
using an approximation of the current versus the rotationalspeed optimal characteristic. The error resulting from the comparison of the calculated and the actual current is used to control a dc/dc converter.
The disadvantage of all above methods is that they are based on the knowledge of the WG optimal power characteristic, which is usually not available with a high degree of accuracy and also changes with rotor aging. Another approach using a twolayer neural network
[10]
updates online the pre programmed WG power characteristic by perturbation of the control signals around the values provided by the power characteristic. However, under real operating conditions where the wind speed changes rapidly, the continuous neural network training required results in accuracy and controlspeed reduction.
A control system based on windspeed measurements
[2]
is shown in
Fig. 1(b)
. The wind speed is measured, and the required rotor speed for maximum power generation is computed. The rotor speed is also measured and compared to the calculated optimal rotor speed, while the resulting error is used to control a power interface. Implementations of fuzzylogicbased control systems transferring the maximum power from a windenergyconversion system to the utility grid or to a standalone system have been presented in
[11

13]
respectively. The controllers are based on a polynomial approximation of the optimal power versus the windspeed characteristic of the WG.
Apart from the accuracy reduction due to the approximation of the WG characteristics, an accurate anemometer is required for the implementation of the aforementioned methods, which increases the system cost. Furthermore, due to wind gusts of lowenergy profile, extra processing of windspeed measurement must be incorporated in the control system for a reliable computation of the available wind energy, which increases the control system complexity.
 1.1. Control system based on rotatingspeed measurements:
WG rotating speed is measured. Optimal output power is calculated and compared to the actual output power. Resulting error is used to control a power interface. WG output power is measured.
 1.2. Control system based on output power measurements:
Target rotor speed for optimal power generation is derived from the WG optimal power versus rotorspeed characteristic. Target rotor speed is compared to the actual speed. Resulting error is used to control a dc/dc power converter.
 1.3 Control system based on wind speed measurements:
Wind speed is measured. Required rotor speed for maximum power generation is computed. Rotor speed is measured and compared to the calculated optimal rotor speed. Resulting error is used to control a power interface. Require knowledge of the WG optimal power characteristics, which is not available with a high degree of accuracy and also changes with ageing. Less efficiency, under real operating conditions where, the wind speed changes rapidly. Use of anemometer increases the system cost. For lowenergy wind gusts, extra processing of windspeed measurements must be incorporated in the control system for a reliable computation of the available wind energy, which increases its complexity.
The incoming wind energy is converted to mechanical energy and then to threephase ac electricity by the wind turbine and generator. The dc is converted to ac by the threephase bridge rectifier. The dc voltage level is boosted by the boosttype dc/dc converter. The MPPT circuit is used to extract maximum power from the wind by monitoring the WG output power using measurements of the WG output voltage and current and directly adjusting the dc/dc converter duty cycle according to the result of comparison between successive WGoutputpower values The WG operates at variable speed and thus suffers lower stress on the shafts and gears compared to constantspeed systems. It results in a better exploitation of the available wind energy, especially in the low windspeed range of
2.54.5 m/s
. It does not depend on the WG wind and rotorspeed ratings or the dc/dc converter power rating higher reliability, lower complexity and cost, and less mechanical stress of the WG. This paper is organized as follows. The WG characteristics are described in Section II, the proposed system is analyzed in Section III, and the theoretical and experimental results are Presented in Section IV.
Control system based on wind speed measurements
2. Wind Turbine Modelling System
The tip speed ratio
λ
is defined as:
Where, ω, R and V represent turbine rotational speed, Turbine blade radius and the wind velocity respectively.
In general, the power captured from the wind turbine can be written as:
where
Cp ( λ
, β) is the power coefficient, ρ is the air density,
V
is the wind speed,
R
is the blade radius, β is the blade pitch angle and
λ
is the tip speed ratio. The power curve speeds of a typical wind turbine are shown in
Fig. 3
. The value of maximum wind turbine output power per unit can be obtained by putting zero pitch angle and Betz limit, when the velocity of wind turbine is 12 m/s.
Turbine Output Characteristics (Zero Pitch angle)
It is not suitable for real time application. In this proposed paper, maximum power can be captured in different wind turbine speeds. Considering the generator efficiency
η
G, the total power produced by the WG
P
is
The WG power coefficient is maximized for a tipspeed ratio Value
o
pt when the blades pitch angle is
β
= 0◦. The WG power curves for various wind speeds are shown in
Fig. 3
. It is observed that, for each wind speed, there exists a specific Point in the WG output power versus rotatingspeed characteristic where the output power is maximized. The control of the WG load results in a variablespeed WG operation, such that maximum power is extracted continuously from the wind (MPPT control). The value of the tipspeed ratio is constant for all maximum power points (MPPs), while the WG speed of rotation is related to the wind speed as follows:
Where Ω
_{n}
is the optimal WG speed of rotation at a wind Velocity
V_{n}
. Besides the optimal energy production capability, another advantage of variablespeed operation is the reduction of stress on the WG shafts and gears, since the blades absorb the wind torque peaks during the changes of the WG speed of rotation. The disadvantage of variablespeed operation is that a power conditioner must be employed to play the role of the WG apparent load. However, the evolution of power electronics helps reduce the powerconverter cost and increases its reliability, while the higher cost is balanced by the energy production gain. The torque curves of the WG, consisting of the interconnected windturbine/generator system, for various generator output voltage levels under various wind speeds, are shown in
Fig. 4
. The generator is designed such that it operates in the approximately linear region corresponding to the straight portion of the generator torque curves in
Fig. 4
, under any windspeed condition.
WG Power curves at various wind speeds
3. Proposed System
 3.1 MPPT algorithm
As mentioned in Section of
Fig. 5
, the MPPT process in the proposed system is based on directly adjusting the dc/dc converter duty cycle according to the result of the comparison of successive WGoutputpower measurements. Although the wind speed varies highly with time, the power absorbed by the WG varies relatively slowly, because of the slow dynamic response of the interconnected windturbine/generator system. Thus, the problem of maximizing the WG output power using the converter duty cycle as a control variable can be effectively solved using the steepest ascent method according to the following control law: Wherever Times is specified, Times Roman or Times New Roman may be used. If neither is available on your word processor, please use the font closest in appearance to Times. Avoid using bitmapped fonts. True Type 1 or Open Type fonts are required. Please embed all fonts, in particular symbol fonts, as well, for math, etc.
Block diagram of proposed system
Where D
_{k}
and D
_{k}
−1 are the dutycycle values at iterations
k
and
k
− 1, respectively (0 < D
_{k}
< 1); Δ
Pk
−1/ΔD
_{k}
−1 is the WG power gradient at step
k
− 1; and
C
1 is the step change. In order to ensure that this method results in convergence to the WG MPP at any windspeed level, it is adequate to prove that the function
P
(
D
), relating the WG power
P
and the dc/dc converter duty cycle
D
, has a single extreme point coinciding with the WG MPPs depicted in
Fig. 3
. Considering the WG power characteristics depicted in
Fig. 3
, it is obvious that at the points of maximum power production
where Ω is the WG rotor speed.
Applying the chain rule, the above equation can be written as:
Where
V
_{WG}
is the rectifier output voltage level and Ω
_{e}
is the generatorphasevoltage angular speed. In case of a bucktype dc/dc converter, its input voltage is related to the output (battery) voltage and the duty cycle
as follows:
where
V
_{o}
is the battery voltage level.
The windturbine rotor speed is related to the generator speed
as follows:
where
p
is the generator number of pole pairs.
The rectifier output voltage
V
_{WG}
is proportional to the generator phase voltage
V
_{ph}
; considering
Fig. 4
, it is concluded that
And
Considering (7)(11), it holds that
Thus, the function
P
(
D
) has a single extreme point, coinciding with the WG MPP, and the dc/dc converter dutycycle adjustment according to the control law of (5) ensures convergence to the WG MPP under any windspeed condition.
The power maximization process is shown in
Fig. 6
. Since the dutycycle adjustment follows the direction of
dP/dD
, the dutycycle value is increased in the highspeed side of the WG characteristic, resulting in a WGrotorspeed reduction and power increase, until the MPP is reached. Similarly when the starting point is in the lowspeed side, following the direction of
dP/dD
results in dutycycle reduction and the subsequent convergence at the MPP, since the WG rotor speed is progressively increased. The proposed method can also be applied to maximize the output power of the WG in case of alternative dc/dc converter configurations.
Mppt Tracking Process diagram

1) Boost converter:

VWG= (1−D)VO,dVWG/dD=Vo≠ 0.

2) Buckboost converter:

VWG=Vo(1 −D) /D,dVWG/dD= −(1/D2)Vo≠ 0.

3) Cuk converter:

VWG=Vo(1 −D) /D,dVWG/dD= −(1/D2)Vo≠ 0.

4) Flyback converter:

VWG=Vo(1 −D) /D,dVWG/dD= −(1/D2)Vo≠ 0.
In order to reduce the impact of the sensor accuracy on the Generated power, the control law of (5) has been implemented based on incremental WG power measurements, rather than absolute measurements, as follows:
where Δ
D
_{k−1}
is the dutycycle change at step
k
− 1;
P
_{in,k−1}
and
P
_{in,k−2}
are the converter inputpower levels at steps
k
 1 and
k
− 2, respectively;
C
_{2}
is a constant determining the speed and accuracy of the convergence to the MPP; and the function sign(
x
) is defined as
 3.2 Powerelectronic interface
The detailed diagram of the proposed system is depicted in
Fig. 6
. The WG ac output voltage is first converted to dc form using a threephase fullwave bridge rectifier. The rectifier output capacitor value
C
r is calculated as follows:
Where
R_{L}
is the WG load resistance,
f
is the WG output voltage frequency, and RF is the rectifier output voltage ripple factor. A bucktype dc/dc converter is used to convert the high dc input voltage to the 24V battery voltage level. The fly back Diode
D
is of fastswitching type, while four power MOSFETs are connected in parallel, to comply with the converter power capability requirements. A power MOSFET is used to switch on and off a 10Ω resistive dummy load, thus limiting the WG speed of rotation under severe conditions. The power inductor
L
and the input and output capacitor values,
C_{in}
and
C
, respectively, are calculated as follows
[14
,
15]
Where
f_{s}
is the dc/dc converter switching frequency,
D_{cm}
is the duty cycle at maximum output power of the converter, Δ
I_{Lm}
is the peaktopeak ripple of the inductor current,
V_{om}
is the maximum of the dc component of the output voltage,
I_{om}
is the dc component of the output current at maximum output power,
RF_{o}
is the output voltage ripple factor (typically
RF_{o}
≤ 2%),
RF_{in}
is the input voltage ripple factor (typically
RF_{in}
≤ 2%), and
V_{WGm}
is the converter input voltage at maximum power.
The control unit is supplied by the battery and consists of an Intel 80C196KC microcontroller unit with an external erasable programmable ROM (EPROM) and a static RAM (SRAM), the interface circuits comprising of sensors and amplifiers connected to the onchip A/D converter, as well as the power MOSFET IC drivers. A 39.2kHz 8bitresolution on chip pulse width modulation (PWM) output is used to control the power MOSFETs of the buck converter through the IR2104 driver IC, while an I/O port pin controls the power MOSFET that switches the dummy load through the IR2121 driver IC. Another I/O port is used to drive a liquid crystal display (LCD) showing various parameters of the system operation.
The WG and battery voltages are measured by means of voltage dividers interfaced to operationalamplifier (opamp)  based voltagefollower circuits. The dc/dc converter input current is equal to the average value of the power MOSFET current, which has a pulsetype waveform and is measured with a unidirectional current transformer.
The flowchart of the control algorithm is shown in
Fig. 7
. The battery voltage is monitored and when it reaches a predefined set point, the MPPT operation is suspended in order to protect the battery stack from overcharging. The PWM dutycycle value is stored in an 8bit register of the microcontroller, taking values that correspond to dutycycle values 0%  99.6%. The WG output power is calculated and compared to the WG output power at the previous iteration of the algorithm. According to the result of the comparison, the sign of the dutycycle change Δ
D
is either complemented or remains unchanged. Subsequently, the PWM output duty cycle is changed appropriately, thus implementing the control law described by (13).
MPPT Process Algorithm
After the dutycycle regulation, the WG voltage is checked; if it is higher than the maximum preset limit, the dummy load is connected to the dc/dc converter input in order to protect the
Fig. 7
. MPPT process algorithm. WG from over speeding. The dummy load is disconnected when the WG output voltage falls below the lower preset limit. They hysteresis introduced by the maximum and minimum preset limits is necessary to avoid the dummy load continuous on/off switching.
4. Theoretical and Experimental Results
The prototype MPPT system was developed based on the Method described above
[16

20]
The WG used in the experiments has A threephase output rated at 100V rms thus, the dummyload Connection and disconnection voltage levels are set at 140 and 100 V, respectively. The dc/dc converter was designed according to the methodology analyzed in Section III. The power switch consists of four MOSFETs rated at 200 V and 30 an each, while the fly back diode has a 200ns reverserecovery time. The calculated input and output capacitor values are 470 and 4700
μ
F, respectively. The output inductor value is 45
μ
H and is wound on a Siemens E65/21 ferrite core with a 3mm air gap. The converter operates in continuous conduction mode, and the switching frequency is proximately 40 kHz. The dc input voltage value in this case is
V
WG = 66.8 V, and the dc output voltage value is
V
O = 33.9 V. Such a high value of the output (battery) voltage appears in case that the battery is fully charged and a sudden increase of the converter input power follows. If this is the case, the MPPT process is suspended according to the MPPT algorithm flowchart shown in
Fig. 7
. The dc/dc converter efficiency is defined as
Where
P
in and
P
o are the dc/dc converter input and output power, respectively, and
P
d is the power loss consisting of the MOSFET and diode conduction and switching losses, the inductor core and copper losses, and the control system power Consumption.
The theoretical values were calculated using data given by the manufacturers of the circuit elements. It is observed that the efficiency is quite high and relatively constant for a wide output power range. This is important in WG systems since the generated power depends strongly on the atmospheric conditions and varies over a wide range. The wind speed, the WG output power, and the corresponding rotor speed of rotation, measured during a 22min time period and sampled with a 0.1Hz rate, are depicted in
Fig. 10
. It is observed that the WG power production follows the changes of the wind speed. In order to further evaluate the MPPT performance, the wind speed, the WG power, and the WG rotational speed were measured during a 4h test period with a 0.1Hz sampling rate. The measured WG rotationalspeed range was divided in intervals of 10r/min width each, and the ensemble average (
Pi
,Ω
i
) of the WG power and rotationalspeed measurements corresponding to each interval was calculated as follows
[2]
:
Block diagram of proposed hardware system module
Where Ω
ij
is the
j^{th}
rotationalspeed measurement in the
i^{th}
Interval,
P_{ij}
is the
j^{th}
power measurement in the interval,
i^{th}
And
n_{i}
is the number of data sets in the
i
th interval. The resulting ensemble averages (
Pi
,Ω
i
) were used to build the diagram shown in
Fig. 11
. It can be concluded that, using the proposed MPPT method, the output power follows the optimal power versus the rotationalspeed characteristic. The maximum deviation from the optimal line is approximately 7%, mainly due to a lower number of measurements in the power range of 400600 W, while a 30% of this deviation is attributed to the rectifier power loss. For comparison purposes, the output power of a WG directly connected to a 24V battery through a rectifier is also indicated in the figure. The power produced in that case is much lower compared to that with the proposed MPPT method. For a further investigation of the WGoutputpower behaviour at various windspeed levels, the measured windspeed range was divided in intervals each of 1m/s width. The ensemble average (
P_{k}
,
V_{k}
) of the WG power and windspeed measurements corresponding to each interval is calculated as follows:
Boost chopper based hardware system module
Where
V_{kp}
is the
p
th windspeed point in the
k
th interval,
P_{kp}
is the
p
th power point in the
k
th interval, and
n_{k}
is the number of data sets in the
k
th interval. The proposed MPPT algorithm is modeled and simulated the wind turbine model is built from the SIMULINK library in
Fig.8
. The theoretical and measured efficiency for various output power levels is shown in
Fig. 9(a
,
b)
.
Simulation of a proposed system
(a), (b) Simulation of proposed system results
The switching frequency was chosen to be
25
MHz. It was seen that the dutycycle of the gate pulse of the dc/dc converter varied with the variation in the wind speed. Thus, maximum power was continuously extracted from the wind and the efficiency increased. The experimental hardware with the control setup of high performance based MPPT drive circuit is shown in
Figs. 10
and
11
.
It is noticed that the WG output power follows the optimal WG power versus windspeed characteristic with a maximum deviation of approximately 6.5%, while the rectifier power loss is responsible for 30% of this deviation. The power production of a WG directly connected to a batteryrectifier load is also indicated in the same figure. The WGoutputpower benefit using the proposed MPPT method compared to the batteryrectifier configuration is 11%50% in the power range of 100600 W. the
Fig. 12
. is clearly concluded that the proposed method results in a better exploitation of the available wind energy, especially in the low windspeed range of 2.54.5 m/s.
Boost chopper based hardware system results
The power transferred to the battery bank is derived consider in the dc/dc converter efficiency, the WG output power, and the power loss. The use of the proposed method improves the power transferred to the battery by 7%45% in the power range of 100600 W, compared to the simple batteryrectifier configuration. The experimental results of the pastproposed WG MPPT methods either have been obtained with laboratorybuilt WG simulators, thus the MPPT performance under real conditions has not been exhibited, or their performance has not been adequately investigated so as to indicate the deviation from the optimal power production.
 4.1. Abbreviations

Where,

ρ air density, inkg/m3

β pitch angle, indegrees

V wind speed, inm/s

R blade radius, inm

A rotorswept area, inm2

Ω WG rotor speed, inrad/s

ηG generator efficiency
5. Conclusion
In this paper, the development of a novel WG maximum power tracking control system is presented, comprising of a highefficiency boosttype dc/dc converter and a microcontrollerbased control unit. The advantages of the proposed MPPT method are as follows: 1) no knowledge of the WG optimal power characteristic or measurement of the wind speed is required and 2) the WG operates at variable speed and thus suffering lower stress on the shafts and gears compared to constantspeed systems. The proposed MPPT method does not depend on the WG wind and rotorspeed ratings or the dc/dc converter power rating. Experimental results of the proposed system indicate that the WG output power is increased by 11%50%, compared to the case where the WG is directly connected via a rectifier to the battery bank. The proposed method results in a better exploitation of the available wind energy, especially in the low windspeed range of 2.54.5 m/s, where the power production of the batteryrectifier configuration is relatively low. The proposed method can be easily extended to include battery charging management or additional RES control, while it can also be modified to control a dc/ac converter in the case of a gridconnected windenergyconversion system.
The laboratory results from the implementation of the simple high performance based MPPT Control algorithm indicate that the proposed system has advantages. These include high reliability, cost effectiveness, and a wide speed range for variablespeed windturbine controllers.
BIO
S.Sutha received B.E from Government College of Engg. Tirunelveli, Monomania Sundarar University in 1996, M.E. from College of Engg. Guindy, Anna University Chennai in 2000 and Ph.D. from Anna University Chennai in 2008. Presently, she is working as Assistant Professor of ANNA UNIVERSITY (Panruti Campus) Tamil Nadu, India.
K.Kannan, received B.E course from R.V.S College of Engineering and Technology in 2006, M.E[PED] from Sri Venkateshwara College of Engineering Sriperumbhdur in 2009. Now, Research Scholar of Anna University and presently working as Assistant Professor in Department of Electrical and Electronics Engineering in R.V.S College of Engineering and Technology, Dindigul, Tamil Nadu, India.
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