1. 10-E-REN-1691
A New Method of Maximum Power Point Tracking for DFIG Based
Wind Turbine
Shabani, A. Deihimi
Department of electrical engineering
Bu Ali Sina university, Hamedan, I. R. Iran
a.shabani@basu.ac.ir
Keywords: wind turbine, DFIG, mppt, rotational speed
Abstract electric networks. In the grid of today
In this paper, different operational regions of penetration of wind turbines (WTs) is
doubly fed induction generator (DFIG) based growing rapidly in size and number. This
wind turbine (WT), from viewpoints of rotor increase in the size of wind turbines leads to
speed, generated power, tip speed ratio (λ) further reduction in the cost of wind power.
and the angle of blades of the WT's rotor, is And on the other hand, the decrement of cost
studied and classified. Then a new fast and of wind power redound to intensification in
explicit method of maximum power point the use of WECSs.
tracking (mppt) will be proposed. The method Along different types of WTs, the DFIG
is based on the difference between optimum based wind turbine has caught most of
and current rotational speed of the shaft of interest. This is because of: 1) in comparison
WT. The proposed method is compared with with constant speed WTs, DFIG based WT
another method to unfold the superior one. operates in much wider range of wind
This comparison will be done based on the velocity and it's production has a better power
speed of operation and quality of generated quality. 2) in comparison with synchronous
power and the results shows the priority of the generator based WT (PMSG or WRSG),
proposed method. DFIG based WT has less manufacturing cost
[1].
In DFIG based WT by using back-to-back
1. INTRODUCTION PWM inverters between the grid and the rotor
The soaring use of fossil fuels and their circuit (see fig. 1), and employing vector
depletion over the last two decades combined control techniques, the active and reactive
with a growing concern about pollution of powers handled by the machine can be
environment have led to a boost for renewable controlled independently [2]. Since the stator
energy generation. This accelerated drive has is directly connected to the grid, the stator
led to a tremendous progress in the field of flux is constant over the entire operating
renewable energy systems during last decade. region. Therefore, the torque can be
Now a day wind energy conversion systems
(WECS) becomes an essential part of modern
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2. A New Method of Maximum Power Point Tracking for DFIG Based Wind Turbine
25th International Power System Conference
maintained at its rated value even above the 2. AERODYNAMIC POWER
synchronous speed [3]. Encountering of the wind with the blades of
However this type of WT is a variable speed the WT's rotor leads to creation of mechanical
one, but it's range of operational speeds is power in the rotor. The value of this power
restricted by the rate of power of it's rotor side depends on the velocity of the wind,
inverter. In usual, the rate of power of rotational speed of WT's rotor, the angle of
converter is between 0.1 through 0.4 of rotor's blade and the structure of blades. The
generator rated power, and hence the produced power may be expressed as:
maximum slip (Smax) of generator would be
0.1 through 0.4, too [4]. 1 (1)
Same as rotational speed, the rated power of . . . . ,
2
the DFIG based WT, is dictated by rated
power of generator and of course the rated Where PM is the mechanical power extracted
power of it's inverter [5]. from the wind in N.m/s; ρ is the air density
Based on the maximum value of speed and (1.225kg/m3); V is the wind speed (m/s) and
rated generating power, different controlling R is the wind turbine rotor radius (m). Cp is
regions may be exposed and in each region, the Efficiency coefficient which is formulated
an especial controlling law by use of the structural data of the WT's rotor,
blades angle (β) and tip speed ratio (λ) which
may be described as:
WT's rotor
. (2)
Back to back
converter
Gear
box Where ωwt stands for rotational speed of WT's
DFIG
rotor.
Fig. 1 a simple structure of DFIG based wind turbine Cp can be described as:
governs. These controlling algorithms are 116 (3)
, 0.22 0.4
described by maximum power point tracking
(mppt) controller [6]. 12.5
In recent years, many papers has been 5
published about different methods of mppt Where
algorithm, some of them used sophisticated
controlling method like sliding mode [6]-[7], 1 1 0.035 (4)
and some used adaptive control [8] or Rotor
0.08 1
Position Phase Lock Loop (PLL) [9], [10].
But drawback of these method is their
complicated methods used by them. And λopt is a value of λ which make the maximum
meanwhile none of them describes the value of Cp (see fig. 2).
controlling algorithm of all operational Fig. 3 illustrates mechanical power curves for
regions. each wind speed versus ωwt with use of
In this paper, controlling method of whole equations (1) through (4). And effect of β on
operational regions are described and a new the extracted power is shown in fig. 4 [1].
simple and explicit mppt algorithm is
proposed.
2

3. A New Method of M
Maximum Power Point Track
king for DFIG Based Wind Turbine
G d
25t Internation Power Sys
th
nal stem Confere
ence
th rotor or from the rotor to th grid) is
he r he s
us
sually betw
ween 0.1 t through 0.4 of rated
4 d
po
ower of gen nerator and this ratio determines
d s
th maximu
he um slip o generato So the
of or. e
ro
otational sp
peed of g generator should laid
s d
be
etween the rrange of [6] :
]
1 | | )
(5)
1 | |
Fig. 2 C versus λ .
Cp Where ωrated is the synchronous speed of
W s f
ge
enerator.
The rated pow of WT may be rep
wer presented as
s
a function of stator or ro rated po
f otor ower:
1 (6)
)
1 | |
Where Pt is t delivere power of WT to the
W the ed f e
gr Ps is the power del
rid, e livered just by stator of
f
ge
enerator and Pr is the p
d power delivvered by the e
ro circuit [5].
otor
Fig. 3 harves mechanic power vers ωwt , spee
sted cal sus eds Pt
of
o the wind fro 4m/s to 22
om 2m/s. I II III
Pmax
Vw
ωr
ωrmax
Vw
Cp
Cpmax
Fig. 4 diffe
erent mechanic power for constant wind
cal d
speed of 12m which are results of fluc
m/s ctuations in β (in
de
egree) Vw
Var.
V ωr Co ωr
ons. Co ωr
ons.
OPERATI IONAL RE EGIONS Opt. λ V λ
Var. Co Pt
ons.
Β=0 Β=0 Β>0
Β
As can be seen in fig 1, the ro
g. otor circuit of
generator i connected to the util grid via a
is d lity a Vcut in Vr max Vrated Vcut out
back to back conv verter. By using th
y his
converter, the generat power o WT may b
ted of be
controlled. The maxi
. imum powe which can
er Fig. 5 DF based WT controlling regions
FIG T r
flow throu the con
ugh nverter (from the grid to
m
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4. A New Method of Maximum Power Point Tracking for DFIG Based Wind Turbine
25th International Power System Conference
It is evident that the limits of maximum This means that in this region, the λ is not at
allowable generating power and maximum it's optimum value any more and would be
and minimum allowable rotational speed of reduced (see fig. 2).
generator should not be exceeded. Based on In this region the value of β is still zero.
these limitations, different operational regions
may be introduced. C. Region III
Fig. 5 shows different operational regions This condition is the last situation that a WT
which may be considered in controlling of a would operate in. the lower wind speed limit
DFIG based WT. of this region is Vrated and the upper limit is
As can be seen in fig. 5, there are three major Vcut out . if the wind speed exceed this limit the
controlling regions, which are discussed here: operation of WT would be stopped and the
braking system would engaged because
A. Region I mechanical damage is expected.
This region is between Vcut in and Vr max. When In this region, the rotational speed of WT is
the wind velocity is less than Vcut in , the WT still kept constant at it's maximum value. λ
doesn't produce any electrical power and the and Cp are not on their optimum value and
incoming mechanical power is wasted in further more, the angle of blades has
rotational components of WT. At speed of cut increased from zero to kept the mechanical
in, which depends on the type of WT is power at it's rated value (see fig. 4).
between 2.5 m/s through 4 m/s, producing the The increment command of angle of blades is
electrical power which is commanded by issued by mppt algorithm.
mppt unit, would be begun. The transferring
ratio of gear box should be chosen in a way
that when the Cp has its maximum value, the 3. PROPOSED MPPT ALGORITHM
ωr cutin corresponds to Vcutin be as: Basis of all these controlling regions shown in
fig. 5, is on contrasting of mechanical and
. (7) electrical torque which with neglecting the
1 | | damping factor and spring constant, it can be
formulated as:
And the upper limit of this region would be as (8)
upper limit in (5). In whole this region, the λ
should have it's optimum value, which leads
to maximum of Cp and hence, for each wind Whit knowing that:
speed, the maximum extractable power would
yields (see fig. 2 and 3). . (9)
And the angle of blades would be kept zero,
because the maximum mechanical power has It is evident that with convenient adjustment
been not reached yet. of recommended power (Pset), changes of ω
can be controlled.
B. Region II Now controlling algorithm of different
This region laid between two wind speed of regions may be expanded.
Vr max and Vrated . in this region, the speed of
the generator's rotor A. Region I
Reached to it's maximum value and should As mentioned before, in this region, it is need
not be exceeded (should be kept constant at to follow the optimum λ and hence according
it's maximum value). So the controlling to (2), the ω should be changed linearly with
algorithm should operate in a way that change of wind speed. So with regards to (8)
increase of wind speed doesn't lead to and (9), if the ω should be increased, the
increase in rotational speed of WT's rotor. value of Pset must be set much lower than
4

5. A New Method of M
Maximum Power Point Track
king for DFIG Based Wind Turbine
G d
25t Internation Power Sys
th
nal stem Confere
ence
mechanica power (PM) and if the ω should b
al e be WT's rotating parts, and this lead to reduction
W g d n
decreased, the value of Pset sh
e hould be sset in ωr and it w last unti Δω acced to zero.
n will il de
much high than PM . the propos followin
her sed ng With proper choosing of K, its possible to
W r p o
equation w
would do it a well:
as co
ontrol the rrate and accuracy of fluctuations
f s
in produced p
n power.
(10)
B. Region II
B
Where ωopt is the rotati
ional speed In this regi
n ion, it is n
need to ke the ωr
ept
correspond
ding to λopt . co
onstant, so t value of Pset should be equals
the s
to PM, which means tha the rate of change of
o h at f
ωr should be zero.
C. Region II
C III
The controll ling algorit thm of reg gion II, iss
ta
aken in this region too, and in add
, dition, since
e
th mechanic power h reached to it's limit,
he cal has ,
th pitch con
he ntrol takes a part in the controlling
g
prrocess. As can be s seen in fig 4, with
g. h
in
ncreasing t the angle of blades (β), the
s e
haarvesting mechaniccal poweer wouldd
deecreased an hence in
nd nput power remains in n
it's upper lim
mit.
Fig. 6 disp
placement of o
operation poin in proposed
nt d
mppt method
t Siince the p pitch contr rol is a mechanical l
prrocess, has it's own dyynamic which depends s
Fig. 6 illus
strates the w equation (10) work
way ks. on the struct
n ture of win turbine. In WTs of
nd f
At first, assume that th wind spe is V2, an
he eed nd to
oday, the m maximum ch hanges in bl lade's angle
e
the mechan nical power has settled at P2 and ω
r d is about 10 d
s degrees per second [1] Structure
r ]. e
of WT is ω2. Now th here is an in
ncrease in the of pitch con
f ntroller can be found in [11] and
i d
wind speed from V2 t V3. Due to enormo
d to ous [1
12].
moment of WT's rotating compo
f onents, at fir
rst
instant, flluctuation of rotation speed is
nal 4. SIMULAT
. TION RES SULTS
neglectable and hen
e nce the o output pow wer All simulatio here ha been acc
A ons as complished
d
would be P23. Accord ding to (10 in the fir
0), rst in MATLAB
n B/simulink. The para
. ameters of
f
moment, th discrepan betwee PM and Pset
he ncy en WT which h been c
W has considered as the test
t
is K(ω3 - ω2 ), whi would b subtracted
ich be beench is av vailable in appendix. for more e
from PM. t differen between PM and Ps ,
this nce n set nformation about mode
in eling of WT one may
T, y
will led the ωr to increase. During th his re to [5] a [13]. the value of K in (10) is
efer and e s
process, PM moves f from P23 to
oward P3, an nd ch
hosen as 5e the othe algorithm suggested
e6. er m d
Pset tracks the trajec
s ctory of B to P3. Wi ith in [3], is com
n mpared wit proposed algorithm
th d m
approachin Pset to P3 the stateme K(ω3 – ωr
ng ent to unfold su
o uperior one. This algo orithm uses
s
) reduces to zero.
o th changes of power t control the WT. as
he to t s
At the nex step, its assumed t
xt that the win nd th input, th wind spe assume to be 10
he he eed ed 0
speed has a reductio from V2 to V1. Lik
on ke m/s, in secon 70 it red
m nd duces to 9 m/s and in
n
pervious st tate, at the f
first momen ωr rema
nt, ain se
econd 150, it increases to 10 m/s again (see
s s e
unchanged and PM ha a step d
d as down from P2 fig. 7). The rated wi
e ind speed of WT is s
to P21. The Pset woul be P21+K 2 – ω1). In
en ld K(ω su
upposed to b 12 m/s.
be
this case Pset is more than PM, an deficiency
nd As can be se in fig. 8 the produ
A een 8, uced power
r
of mechan nical input p power will compensated of proposed algorithm (the bolt one) is
f d m s
by kinetic energy sto ored in rotaating mass of sm
moother tha the othe one and it's power
an er r
5

6. A New Method of Maximum Power Point Tracking for DFIG Based Wind Turbine
25th International Power System Conference
quality is better, too. Another drawback of outputs of this algorithm are shown. In fig. 9,
suggested mppt algorithm in [3]. the amplitude of λ, is shown which according
to fig. 2 should be about 7.2. fig. 10, shows
the rotational speed of WT's rotor. Fig.11,
illustrates the electrical torque of generator.
Fig. 12, presents the mechanical input power
(bolt one) which delivered by the WT's shaft
and electrical produced power delivered to the
utility grid. In these simulations, demanded
reactive power is supposed to be zero. As can
be seen in fig. 12, the generated electrical
Fig. 7 profile of wind speed, applied for comparison power has an overshoot at time 70s, and an
Fig. 8 comparison of produced power by proposed Fig. 11 generated electrical torque of WT.
mppt algorithm (the bolt one) and the mppt algorithm
suggested in [3].
Fig. 12 mechanical (bolt line) and electrical (thin line)
power.
Fig. 9 amplitude of λ.
Fig. 13 terminal out coming d and q currents.
Fig. 10 rotational speed of WT's rotor.
undershoot at time 150 s. these are because of
Now that the superiority of proposed the kinetic energy stored in rotating mass
algorithm has been unfolded, the other which unleashes during reduction in
rotational speed, and stores during increment
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7. A New Method of Maximum Power Point Tracking for DFIG Based Wind Turbine
25th International Power System Conference
. this affects terminal currents shown in fig. Ind. Electron. Soc., vol. 48, pp. 786–793,
13, which exposes terminal out coming d and Aug. 2001.
q currents. [3] R. Datta and V.T. Ranganathan," A
Method of Tracking the Peak Power
Points for a Variable Speed Wind Energy
5. CONCLUSION Conversion System", IEEE Trans, energy
Different operating and controlling regions of conversion, VOL. 18, NO. 1, MARCH
2003
DFIG based wind turbine from viewpoints of
[4] Ion Boldea, Variable Speed Generators.
rotor speed, generated power, tip speed ratio Boca Raton, FL: Taylor & Francis, 2006.
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maximum power point tracking algorithm is Regulation at a Remote Location," IEEE
presented which is based on Δω of rotor. This Trans, Power systems, VOL. 22, NO.
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Control of a DFIG-Based Wind Turbine
for Power Maximization and Grid Fault
Tolerance " Electric Machines and Drive
APENDIX Conference, 2009
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Kse=1.4035e4, De=500e3, Power and energy engineering conference,
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[8] Changhong Shao, Xiangjun Chen and
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Controller Based Adaptive Control
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