International Conference on Electrical, Electronics, and Optimization Techniques (ICEEOT) - 2016
Direct control technique for PMSG based Variable
speed Wind Applications
Indupuri Sai Babji Kumar
Bindu S
Department of Electrical and Electronics Engineering
Manipal Institute of Technology
Manipal, India
saibabjiindupuri@gmail.com
Department of Electrical and Electronics Engineering
Manipal Institute of Technology
Manipal, India
bindu.s@manipal.edu
Abstract—This paper deals with direct control technique to
control Permanent Magnet Synchronous Generator (PMSG)
based variable speed Wind Energy Conversion System (WECS).
This control technique is simple and has many advantages over
indirect vector control technique like dependency on lesser
parameters, number of controllers required will be less and there
is no requirement of rotor position sensor. Furthermore, the
system is unaffected to variation in parameters because stator
resistance is the only required criteria. This control technique is
implemented in SIMULINK/Sim power systems and the
simulation results shows that this suggested control technique
works well to achieve Maximum Power Point (MPP) tracking of
the system and to control torque, flux and speed parameters of
PMSG.
Keywords—Direct control; PMSG; variable speed wind turbine.
II. WIND TURBINE DESIGN AND CHARACTERISTICS
Wind turbine takes the input of wind speed and generates
torque, which is used by the electrical generator to produce
electricity.
A. Wind power curve
The power curve is associated with three wind speeds. They
are Cut-in, Cut-out and Rated wind speeds. Wind turbine will
start absorbing power from wind only when speeds higher than
Cut in wind speed. Wind turbine should stop producing power
at speeds higher than Cut-out speed. When speed is higher than
rated wind speeds, there is a need to control the blades so as to
see the power do not crosses its rated value. Fig. 1. shows the
typical wind-power curve [4].
I. INTRODUCTION
At present, our entire power production is heavily
dependent on fossil fuels. Fossil fuels are depleting day by day
and also fossil fuel power plants emits greenhouse gases into
the environment which leads to air pollution. So there is a need
to look for alternate resources for producing power. Recently
people are attracted towards producing power using clean
energy resources like solar, wind and tidal. The power plants
that use these clean energy resources do not have any
environmental impact and fuel cost. Wind energy is one of the
clean energy sources that can be trusted and readily available.
The WECS contains wind turbine, generator and control
techniques. It can either operate as fixed speed system or as
variable speed system. Variable speed WECS is the most used
one in the current world market since it has many benefits
compared to fixed speed wind turbines like operation at MPP
possibility, better efficiency, quality of power and increased
energy capture from the wind [1].
Many types of generators are possible to generate power in
WECS. In variable speed WECS, Doubly Fed Induction
Generator (DFIG) with gearbox is the most used generator for
power generation [2]. Other than DFIG, PMSG can also be used
in variable speed technology. It has the advantage that it do not
need rotor excitation. So the use of PMSG can increase the
performance of wind energy system [3].
Fig. 1. Wind speed vs Mechanical power curve
The power that is extracted from the wind energy by wind
turbine is written as [5]
(1)
ρ = Density of air (kg/m3)
ʋw =Velocity of wind (m/s)
A = Area enclosed by turbine blades (m2)
Cp = Power coefficient
B. Importance of Cp
The efficiency of WECS is described by the power
coefficient CP, It is dimensionless and nonlinear. Cp is a
978-1-4673-9939-5/16/$31.00 ©2016 IEEE
function of Tip Speed Ratio (TSR) and pitch angle (β) which is
given in (2).
0.5176 116
1
0.08
0.035
1
0.4
5
.
.
0.0068
(2)
So the two parameter variables that influence the efficiency
are β and λr.
The tip speed ratio is described as the ratio of the rotor speed
and wind velocity, and it is defined as
(3)
ωm = Speed at which turbine rotates
R = radius of wind turbine blades.
The wind turbine can extract maximum power from wind
only when Cp is at maximum value (Cp_opt) [6]. Cp value
depends on β and λr. Therefore, it is required to make the TSR
(λr) value always stays at an optimum value (λr_opt). If wind
speed is varying, the turbine speed must be adjusted
accordingly so as to see λr is at λr_opt. The relation between Cp,
λr, and β is shown in fig.2.
Fig. 3. Wind turbine power characteristics.
The mechanical torque Tm experienced by the wind turbine
is defined as [6]
(4)
The optimum TSR is given by
_
_
(5)
The optimum power is given by
0.5
_
_
_
⁄
(6)
_
The optimum torque is given by
_
Fig. 2. Cp Vs λ characteristics of the wind turbine.
From fig. 2 we can see that maximum value of Cp is possible
only when β = 0 and as pitch angle increases, the maximum
possible value of Cp decreases. So it is desirable that β = 0 in
order to get maximum power extraction from the wind with
higher efficiency.
Fig. 3 [6] shows the non-linear power-speed curves of the
Wind Turbine. Each power–speed curve has a unique turbine
speed which corresponds to the MPP for that wind speed. From
which it is noticeable that if system is operating at MPP of the
curve, the power extracted from the wind would be maximum
even when wind speed is varying. Hence, the turbine rotor
speed has to be adjusted in such a way that λ corresponds to
MPP.
_
(7)
_
Optimum values are the values at which turbine can extract
maximum energy from the varying wind speeds and thus
producing maximum power from the generator [6].
Table I gives the parameters of the wind turbine considered
for the study.
TABLE I.
PARAMETERS OF WIND TURBINE
Nominal mechanical power output (kW)
4kW
Power coefficient of wind turbine, Cp
0.43
Pitch angle,β
00
Base wind speed ( m/s )
12
Tip speed ratio, λ
8.2
III. MODELING OF PMSG
The PMSG is basically wound rotor synchronous generator
where the rotor is replaced with permanent magnet. Because of
permanent magnet, rotor does not require any exciting current
for maintaining air gap flux. So the rotor excitation losses will
be absent. So wind energy can be used efficiently for producing
electric power [7].
To analyze PMSG, the machine is modeled in d-q reference
frame. The frame synchronously rotates with the rotor. The daxis is along the magnetic axis and q-axis is orthogonal to it.
and flux will be within the hysteresis bands so as to get the
required flux and torque response [7]-[11]. The required voltage
vectors for switching the converter, are selected according to
the switching table V.
The d- and q-axis voltages of PMSG are written as [7]
(8)
(9)
The d- and q-axis stator fluxes are written as
(10)
(11)
The torque developed by the PMSG is given by
(12)
Rs = Resistance of the stator.
ωr = Speed at which generator rotates
λM =Magnetic flux
P = Pole pairs.
p = d/dt operator
Fig. 4. Direct control technique for the PMSG
A. Advantages of this control technique
•
Where the remaining terms are d- and q- axis components
with respect to the stator.
Various parameters of PMSG considered for simulation
analysis is given in Table II.
TABLE II.
PARAMETERS OF PMSG
Rated power
4Kw
Rated torque
24Nm
Rated speed
1600 rpm
Rated voltage
415 V rms
Rated current
9.6 A rms
Magnetic flux linkage
18.237mH
q-axis inductance (Lq) per phase
49.239Mh
Stator resistance
1.56 ohm
No. of poles
6
Rotor inertia
0.0049 kg-m2
Viscous damping
B. Controler design
According to the wind speed variations, it is needed to
control the switches of generator side converter. Here, we are
using Space vector modulation (SVM) technique to control the
switches of generator side converter. Three phase rectifier
connected to PMSG is shown in fig. 5.
0.525723Wb
d-axis inductance (Ld) per phase
Static friction
•
•
•
Coordinate transformations are not required since
every computation is done in stator reference frame.
It does not require any rotor position (θr) sensor.
Require less number of parameters.
Number of controllers are reduced when compared to
indirect vector control technique.
0.637 Nm
0.237 Nm/krpm
IV. DIRECT CONTROL TECHNIQUE
The direct control method for PMSG is shown in the below
fig.4. Here in this control technique, it is not necessary to
control the stator currents. The torque and stator flux can be
regulated independently and directly by using two separate
hysteresis controller bands for flux as well as torque. The
selection rule is made in such a way that errors present in torque
Fig. 5. Rectifier connected to PM synchronous generator.
By making the use of a series of switches, from the 3 input
legs, AC is converted into a controlled DC. Here total of eight
switching vectors are possible for the rectifier, in that 6 are
active switching vectors and 2 are zero vectors. These switching
vectors are given in table III.
TABLE III.
Vector
Sa
Sb
SWITCHING VECTORS
Sc
Vab
Vbc
(15)
(16)
Vca
V0 {000}
0
0
0
0
0
0
Zero vector
V1 {100}
1
0
0
+Vdc
0
- Vdc
Active vector
V2 {110}
1
1
0
0
+Vdc
- Vdc
Active vector
V3 {010}
0
1
0
- Vdc
+Vdc
0
Active vector
V4 {011}
0
1
1
- Vdc
0
+Vdc
Active vector
V5 {001}
0
0
1
0
- Vdc
Vdc
Active vector
V6 {101}
1
0
1
+Vdc
- Vdc
0
Active vector
V7 {111}
1
1
1
0
0
0
Zero vector
+
C. Control of stator flux linkage
The stator voltages for a three phase machine in the form of
voltage vector is given by (13)
/
/
(13)
D,
Q = d- and q-axis stator fluxes.
=
stator
flux linkage.
S
The stator flux phasor can be written as,
| |
(17)
tan
(18)
Fig. 7 shows the regions to control magnitude and direction
of the stator flux [8].
Depending on the position of these 3 switches (Sa, Sb, Sc), the
primary voltage vectors va, vb, vc are defined. The 6 non-zero
voltage vectors are displaced 600 from one another. These eight
voltage vectors can be written in single equation as
,
/
,
/
(14)
VD = 2/3Vdc
Vdc = dc link voltage
In the above equation, by substituting values of switching
states, we can find the values of these 6 non-zero voltage
vectors. These voltage vectors can be represented as in table IV
and fig. 6.
TABLE IV.
THE SIX VOLTAGE VECTOR VALUES
V1
V2
V3
V4
V5
V6
Vd
VD
0.5VD
-0.5VD
-VD
-0.5VD
0.5VD
Vq
0
0.866VD
0.866VD
0
-0.866VD
-0.866VD
Fig. 6. Vectorial representation of the stator voltage vectors.
D. Control of Magnitude and direction of stator flux
The stator flux is defined as integration of the difference
between the input voltage and the voltage drop at the stator
resistance. The stator flux can be written as in (15), (16)
Fig. 7. Control of the magnitude and the direction of the stator flux.
In fig. 7, the vector plane of the voltages is partitioned into
six regions 1– 6 in order to select the required voltage vectors
of the converter to control the amplitude and direction of the
stator flux. In every region, two neighboring voltage vectors
have to be chosen depending on hysteresis commands. When s
is in region 1, V2 is selected to decrease the amplitude of s and
V3 is selected to increase the amplitude of s. That means
amplitude of s is controlled by making error value to stay
within hysteresis band limits. In this way, the controller works
by choosing the switching vectors properly for the converter.
By controlling the direction of rotation of s, the
electromagnetic torque can be controlled, this is with respect to
the equation (12). In anti-clockwise functioning, if the torque
error is positive that means actual is less than reference, the
appropriate switching vectors are chosen accordingly to make
s rotate in the same direction. This makes θ to decrease and so
actual torque to increase. s rotates in the same direction till the
actual torque become more than the reference torque. When
actual torque is more than the reference, error becomes negative
and voltage vectors of opposite axes are selected to keep s
rotating in the reverse direction. This makes θ to decrease and
so actual torque to decrease. By choosing the switching vectors
in this pattern, s is rotated in all directions and the rotation of
s is controlled by the commands given by the torque hysteresis
controller.
The voltage vector switching table to control the amplitude
as well as direction of s is given in Table V. λ and τ denotes
the hysteresis controller outputs of stator flux and torque,
respectively.
TABLE V.
SIX-VECTOR SWITCHING TABLE TO CONTROL THE
CONVERTER
(c)
V. PITCH ANGLE CONTROL
The pitch angle control, controls the rotor blade angle of the
wind turbine in mechanical way so as to regulate the output
power of the wind turbine. It is needed so as to protect the wind
turbine from over power and over torque conditions due to
sudden wind gusts at higher wind speeds. The pitch angle is
usually fixed at velocities lower than the rated wind velocity.
On the contrary, at the speeds greater than the rated wind
velocity, the pitch angle control will be activated to decrease
the amount of power captured by the wind turbine by turning
the rotor blades through some angle from the direction of
striking wind.
(d)
VI. RESULT ANALYSIS
Case 1: By varying wind speed as shown in fig. 8(a)
performance of this direct control technique is observed. It can
be seen that for varying wind speeds, torque, flux, turbine speed
and mechanical power are following the references. TSR and
power coefficient are also following their optimum values.
(a)
(e)
(f)
(g)
(b)
Fig. 8. Performance of this control technique: (a) Wind velocity, (b) Generator
speed and the reference speed, (c) torque developed (Tg) and the reference
torque (Tg*) and (d) flux developed and the reference flux, (e) Mechanical
power and its reference,(f) Tip Speed Ratio, (g) Power coefficient.
Case 2: In this case pitch angle control strategy is used. We
observed the responses of pitch angle, generator speed and
generated power by changing Wind velocity from 12m/s to
13.5m/s at t=3s. It can be observed that pitch angle is 00 till t=3s
and when wind speed is changed to 13.5m/s, pitch angle control
is activated and pitch angle increased to required value to make
mechanical power and turbine speeds not to raise above their
rated values.
(a)
(b)
(c)
Fig. 9. (a) pitch angle (b) generator speed (c) generated power
VII. CONCLUSION
The Advanced control scheme to control PMSG based
variable speed wind turbine is developed and It is seen that this
controller is capable to maximize the power output from
variable speed wind turbine system under varying wind
velocities. Simulation results shows that the controller works
well by achieving controlled torque and speed under fluctuating
wind.
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