Download II. Wind Turbine Topologies

International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Design Implementation of Boost Converter for
Variable Speed Small Wind Turbine
(IJSETR)
Yi Lay Nge#1, Zaw Min Naing*2, Kyaw Soe Lwin#3, Maung Maung Latt*4

Abstract— This paper presents the boost converter for
variable speed small wind turbine. Because of the irregular
nature of wind, the wind turbine generator produces
variable-voltage and variable frequency. Several different
methods were used to transform the variable-voltage,
variable-frequency outputs to the reliable constant-voltage,
constant-frequency outputs. In variable speed operation, it is
necessary to use power electronics converter as an interface
between the wind turbine and the grid. The power converter
consists of an uncontrolled three-phase diode rectifier, a DC-DC
boost converter and a three-phase PWM voltage source inverter.
The DC-DC boost converter is modeled by using the blocks of
MATLAB SIMULINK. The simulation results are compared
with the theoretical results.
Index Terms—Wind turbine, variable voltage and frequency,
power converter, DC-DC boost converter, MATLAB
SIMULINK
I. INTRODUCTION
The wind turbine technology is one of the most emerging
renewable technologies. In wind energy conversion systems
(WECS), normally there are two operating modes of wind
turbine generators (WTG) system: fixed speed and variable
speed operating modes [1]. In order to extract maximum
power from the fluctuating wind, variable-speed operation of
the wind turbine generator is necessary. This requires a
sophisticated control strategy for the generator. Optimum
power/torque tracking is a popular control strategy, as it helps
to achieve optimum wind-energy utilization. Some of these
control strategies use wind velocity to obtain the desired shaft
speed to vary the generator speed.
A control strategy for the generator-side converter with
output maximization of a (permanent magnet synchronous
generator) PMSG-based small-scale wind turbine is
developed. The generator-side switch-mode rectifier is
controlled to achieve maximum power from the wind. The
method requires only one active switching device [insulated
gate bipolar transistor (IGBT)], which is used to control the
Yi Lay Nge, Department of Electronic Engineering, Mandalay
Technological University ,(e-mail: [email protected]), Mandalay,
Myanmar.
Zaw Min Nainge, Technological University(Maubin), Maubin,
Myanmar, (e-mail; [email protected]).
Kyaw Soe Lwin, Department of Electronic Engineering, Mandalay
Technological University , (e-mail: [email protected]).
Maung Maung Latt, Technological University(Taungngu),Taungngu,
Myanmar, (e-mail: [email protected]
generator torque to extract maximum power. It is simple and a
low-cost solution for a small-scale wind turbine [2]-[3].
Most modern WTGs are designed for variable speed
operation. Variable-speed wind turbines have many
advantages over fixed-speed generation such as increased
energy capture, operation at maximum power point, enhanced
efficiency and power quality, and reduced mechanical
stresses and audible noise at low wind speed. The reliability
of the variable-speed wind turbine can be improved
significantly by using a direct-drive permanent-magnet
synchronous generator (PMSG) [4]. PMSGs are widely used
in the wind energy conversion systems, especially in the small
or medium power range, which increases the conversion
efficiency and reduces the maintenance cost due to brushless
design [5].
This paper focuses on step by step validation of the boost
converter. Simulation results are obtained using MATLAB
SIMULINK. Although the presented model has been tested
on the dc-dc converter theory, it can also be intended in
grid-connected application or additional loads by suitable
modification of the dc-ac inverter. This paper is organized as
follows. The wind turbine topologies are described in Section
II, the proposed system is presented in Section III, and the
design implementation and simulation results are mentioned
in Section IV.
II. WIND TURBINE TOPOLOGIES
The most important technical information for a specific
wind turbine is the power curve, which shows the relationship
between wind speed and generator electrical output. A
somewhat idealized power curve is shown in Fig. 1.
According to the power curve, there are three types of wind
speeds.
Cut-in Wind speed (3-5m/s): The cut-in wind speed VC is the
minimum needed to generate net power. Since no power is
generated at wind speeds below VC, which portion of the
wind’s energy is wasted. Fortunately, there isn’t much energy
in those low-speed winds anyway, so usually not much is lost.
Rated Wind speed (between11m/s and 16m/s): As velocity
increases above the cut-in wind speed, the power delivered by
the generator tends to rise as the cube of wind speed. When
winds reach the rated wind speed VR, the generator is
delivering as much power as it is designed for. Above VR,
there must be some way to shed some of the wind’s power or
else the generator may be damaged.
Cut-out or Furling Wind speed (between 17m/s and 30m/s):
At some point the wind is so strong that there is real danger to
the wind turbine. At this wind speed VF , called the cut-out
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
wind speed or the furling wind speed (“furling” is the term
used in sailing to describe the practice of folding up the sails
when winds are too strong), the machine must be shut down.
Above VF, output power obviously is zero [6].
Shedding the wind
Power delivered (kW)
Rated Power
PR
Rated
windspeed
Cut-in windspeed
Cut-out
windspeed
electrical generator. The captured power of the wind (P v) for a
wind turbine is given by equation (1).
=
(1)
The mechanical power (Pm) generated by the wind turbine
from captured power of the wind depends on the power
coefficient (Cp) of the wind turbine.
.
(2)
The Cp is a function of tip-speed ratio λ.
(3)
Therefore, the mechanical power and torque production
from wind turbine is given by the following equations:
ρ
(4)
(5)
VC
VR
VF
Windspeed (m/s)
Fig.1 Idealized power curve
The overall block diagram of a wind energy conversion
system is shown in Fig 2. Wind turbine converts wind power
into shaft power to drive an electrical generator. The output
power from the WT fed to the Permanent Magnet
Synchronous Generator (PMSG). The generated power of
continuously varying frequency is fed to local load through
suitable power converters, to ensure constant voltage and
constant frequency. This paper is focused on the shaded areas
of the overall system.
Wind
PMSG
Diode
Rectifier
Boost
Chopper
PWM
Inverter
Load
δ
Wind
Turbine
Controller
Fig.2 Block diagram of wind energy conversion system
A. Wind Turbine
Wind turbines can either be fixed speed or variable speed
turbines based upon their operating speed characteristics. The
most common conventional wind turbine systems uses
multistage gears coupled to a fixed speed squirrel cage
induction generator directly connected to the grid. Fixed
speed turbines are designed for optimum performance at a
fixed rotor rpm and hence its energy yield is low. Doubly fed
induction generator topology with converter on the rotor side
is the most popular variable speed wind turbine application.
To match the low speed turbine rotor and high speed
generator, gear box with appropriate gear ratio is used.
The modern trend in wind power generation is toward
gearless permanent magnet synchronous generator
incorporated direct drive system [4]-[6]. Many disadvantages
can also be avoided in gearless WTG. The noise caused
mainly by a high rotational speed can be reduced and also
high overall efficiency and reliability reduced weight and
diminished need for maintenance [7]. The wind turbine basic
principle is to convert the linear motion of the wind into
rotational energy. This rotational energy is used to drive an
Where: ρ = the air density
r = the radius of the wind blade
u = the velocity of the wind
Cp = the power coefficient
CT = the torque coefficient
ωm = the rotor speed
When wind speed changes, the rotational speed, ωm has to
achieve the best value of Cp.
B. Wind Turbine Generator
Both induction and synchronous generators can be used for
wind turbine systems. Induction generators can be used in a
fixed-speed system or a variable-speed system. The induction
type machine has the advantages of robustness, low cost and
maintenance-free operation. However, they have the
drawbacks of low power factor and need for an AC excitation
source. In the regular wind turbine generator it has an
induction generator that it can rotate at a speed of 1000 to
1500 rpm for normal operation and good efficiency.
Permanent magnet generator is chosen so as to eliminate
the drawbacks of induction generator. Synchronous
generators are normally used in power electronic interfaced
variable-speed systems. It provides an optimal solution for
varying-speed wind turbines of gearless or single-stage gear
configuration. Many disadvantages can also be avoided in
gearless WTG. The noise caused mainly by a high rotational
speed can be reduced and also high overall efficiency and
diminished need for maintenance.
Simplest synchronous generator model consists of a speed
controlled voltage source in series with impedance. Because
of the ease of handling the rotation dependent parameters,
direct and quadrature axis machine quantities in power system
study are transferred to arbitrary reference frames. Machine
dynamics is represented by differential equations using park
transformation with rotor axis as the reference frame [6].
(6)
+(
Where,
Ld, Lq
R
id, iq
vq, vd
)
]
(7)
(8)
d and q axis inductances
stator winding resistance
d and q axis currents
d and q axis voltage
2
All Rights Reserved © 2012 IJSETR
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
ωr
λ
p
Te
angular velocity of rotor
amplitude of rotor induced flux
pole pair number
electromagnetic torque
Where, D=duty ratio of the boost converter
(13)
C. Power Converter
Since the wind power fluctuates with wind velocity, the
generator output voltage and frequency vary continuously.
Therefore, a full power capacity AC-DC-AC power converter
is employed to convert the variable frequency variable speed
generator output to the fixed frequency fixed voltage.
The varying AC voltage is rectified into DC in a diode
bridge and the dc voltage is then regulated to obtain constant
voltage by controlling the duty ratio of a DC/DC boost
converter. The DC voltage is inverted to get the desired AC
voltage and frequency employing a PWM inverter. The duty
ratio, δ controls the Boost converter output voltage.
III. DESIGN CONSIDERATION
A. Three-phase Rectifier
The three-phase diode bridge rectifier converts the AC
generated output voltage, which will be varying in magnitude
and also in frequency, into DC voltage.
The average output voltage of the three phase diode
rectifier is obtained as follows:
Vdc =(3*Vm )/π
(9)
Here, Vm =
*Vrms
(11)
Here, the output voltage of PMSG is assumed about 400V.
=230.94V
Vm=
*230.94=326.55V
*Vrms=
Vo =
Vin=
*311.83=623.66V
To meet the required boost converter design, the value of
L and C can be calculated based on the minimum overshoot
level and required voltage range. According to the boost
converter design equation from typical design, the proposed
converter can be evaluated with high performance. So, the
design specification for the proposed boost converter is
shown in Table 1.
TABLE 1
DESIGN SPECIFICATIONS FOR DC-DC BOOST CONVERTER
Input voltage range, Vin
280V ~ 310V
Output voltage, Vout
Above 600V
Output current, Iout
4 Amp
Input inductor L
Output resistor R
Output capacitors (c,c1)
1mH
200Ω
2000μF
(10)
And, phase-to-phase rms voltage=VL/
So, Vrms=400/
Since the duty ratio D is between 0 and 1 the output
voltage must always be higher than the input voltage in
magnitude. The variable output voltage is obtained by
regulating the ON time of the duty cycle. Here, it is used 0.5
as the duty cycle. From the above calculation, the output
voltage of the rectifier is 311.83V. It is the input of the
DC-DC converter. The output voltage is
IV. IMPLEMENTATION AND SIMULATION RESULT
The simulation performance is divided into two main parts
including three-phase rectifier and dc-dc boost converter.
Here, Fig. 3 shows the simulation model of three-phase
rectifier part. It is presented for 230V AC voltage with
variable frequencies (50~80Hz).
The DC output voltage=Vdc=(3*326.55)/π=311.83V
B. DC-DC Boost Converter
DC-DC converter is a device that accepts a DC input
voltage and produces a DC output voltage. Typically the
output voltage is at a different voltage level than the input. In
addition, DC-DC converters are used to provide noise
isolation, power bus regulation, etc. As the name implies, the
output voltage is greater than the input voltage.
The boost converter output voltage is obtained as
Vinton+(Vin-Vo)toff=0
(12)
Fig.3 SIMULINK model of rectifier circuit
Vinton+Vintoff-Votoff=0
Vin(ton+toff)-Votoff=0
VinT=Votoff
Fig.4 illustrates the simulation result of the three-phase
rectifier. According to the figure, it can be seen that the
theoretical result and simulation result are nearly the same.
=
Here, D=
3
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Voltage across switch
Voltage(V)
DC Voltage(Vdc)
Rectifier output voltage
Time(sec)
Time(sec)
Fig.7 Voltage across switch
Fig.4 The rectifier output voltage
Boost converter Output Voltage
DC Voltage(Vdc)
When the frequency changes between 50 Hz and 80 Hz, the
various dc output voltages are also changed as shown in Fig.5.
Time(sec)
Fig.5The results of variable frequencies
Fig. 6 is the simulation model of DC-DC boost converter.
Fig.8 DC output voltage of boost converter
Fig.8 presents the output voltage of DC-DC boost
converter. At the beginning, the voltage is increased to about
1000V. During 0 to 0.5s, the voltage is continuously
decreased to around 600V. After 0.5s, it is stabled over 600V.
Therefore, the simulation result is approximately same with
the theoretical result. Fig.9 illustrates the corresponding DC
output current.
Fig.6 SIMULINK model of DC-DC converter
According to the simulation, it can be seen that the voltage
across in MOSFET as shown in Fig. 7. As the duty cycle is
0.5, ton and toff are the same duration. For this simulation the
suitable duty cycle is 50%.
DC Current(Idc)
DC Output Current
Time(sec)
Fig.9 DC output current
4
All Rights Reserved © 2012 IJSETR
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
[10] R. Bharanikumar and A. Nirmal Kumar; “Analysis of Wind Turbine
Driven PM Generator with Power Converters”, International Journal of
Computer and Electrical Engineering, Vol. 2, No. 4, August, 2010
[11] Athimulam Kalirasu, Subharensu Sekar Dash; “Simulation of Closed
Loop Controlled Boost Converter for Solar Installation:, Serbian
Journal of Electrical Engineering, Vol. 7, No. 1, May 2010, 121-130
Fig.10 Input vs. Output voltage
Fig.10 shows the relation curve of input and output
voltages of boost converter. According to the curve, the boost
converter output over 600V can get from the input voltage
over 300V at 50 Hz. It can be used as part of a DC-AC
inverter. The desired voltage can also be obtained by
changing the duty ratio of the boost converter.
V. CONCLUSION
The DC-DC boost converter was considered by using the
MATLAB simulation blocks. The target of the system is to
get the DC output voltage above 600V. Here, AC 400V is
assumed to the PMSG output voltage. The voltage is fed to
the three phase diode bridge rectifier and then the output is
connected to the DC-DC converter. The input voltage range is
(280~310) V with variable frequencies (50~80) Hz.
Therefore, the variable output voltage is achieved within the
voltage range of 550~610V. In practical, a power MOSFET
with parameters of 1500V VDSS, current ID of 4A and low
resistance RDS(on) should be used as the simulation result of
output voltage is increased up to about 1000V. Consequently,
the boost converter design in this paper has to be found that
the system requirements are outfitted to implement the
reliable variable speed small wind turbine.
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