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CHAPTER 1
1
INTRODUCTION
The conventional energy sources are limited and have pollution to the environment, so the main
mission is to short out the global energy shortage and pollution problem by utilizing renewable energy
resources. The wind energy is the fastest growing and most promising renewable energy source among
them, but the disadvantage is that the wind power is intermittent and depending upon weather conditions.
Steady of thesis is relates to tackle the intermittency problem of wind power by means of a short – term
energy storage to get a smooth power output from a wind turbine.
Compared with the wind turbine based on synchronous generators, the doubly fed induction
generator based wind generation technology has several advantages such as flexible active and reactive
power control capabilities, lower converter costs and lower power losses.
In the DFIG concept, the induction generator is directly connected to the grid at the stator
terminals, but the rotor terminals are connected to the grid via a variable-frequency AC/DC/AC
converter. When the wind speed varies, controlling two back-to-back four-quadrant power converters
connected between the rotor side and the grid side can make the rotor flux rotate from a sub-synchronous
speed to a super-synchronous speed. The DFIG can produce and inject constant-frequency power to the
grid by controlling the rotor flux. The power converter only needs to handle a fraction (typically 20-30%)
of the total power to achieve full control of the generator. With this configuration, the power converters
could be rated at lower power levels, compared with traditional configuration where the battery is directly
tied to the DC link of the rectifier-inverter pair connected to the generator terminal. The energy storage
device is controlled so as to smooth out the total output power from the wind turbine as the wind speed
varies. Control algorithms are developed for the grid-side converter, rotor-side converter and battery
converter, and the control strategies are tested on a simulation model developed in MATLAB/Simulink.
The model contains a DFIG wind turbine, three power converters and associated controllers, a DC-link
capacitor, a battery, and an equivalent power grid.
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1.1 Literature review
Renewable energy is increasingly attractive in solving global problems such as the environmental
pollution and energy shortage. Among a variety of renewable energy resources, wind power is drawing
the most attention from the government, academia, utilities and industry [1]-[9]. However, the
disadvantage is that wind power generation is intermittent, depending upon weather conditions. Shortterm energy storage is necessary in order to get a smooth power output from a wind turbine [10][13].Compared with the wind turbines based on synchronous generators, the doubly-fed induction
generator (DFIG) based wind generation technology has several advantages such as flexible active and
reactive power control capabilities, lower converter costs and lower power losses [14]-[17].
1.2 Thesis objective and overview
The goal of this thesis is to build a computer simulation of the integrated Wind power generation and
energy storage System, based on mathematical equations, through the use of MAT Lab Simulink.
1.3 Thesis Organization
This thesis is divided into five Chapters and its outline is given below:
Chapter 1 gives general introduction, objective, overview and thesis organization.
Chapter 2 deal with integrated wind power System description and modeling of DFIG, Power Converters
and battery energy storage system.
Chapter 3 presents control of power converters.
Chapter 4 relates to the simulation results.
Chapter 5 presents conclusion and scope of future work, references and appendix.
3
CHAPTER 2
4
INTEGRATED WIND POWER GENERATION AND BATTERY STORAGE SYSTEM
2.1System description
Wind
P+JQ
isabc
Gear
Box
L2
Ps+JQs
Grid
L
o
a
d
Rotor
Pr+jQr
Stator
irabc
vrabc
Pg+jQg
VSC
i1
Vdc
i2
i3
ic
VSC
Pb
C_rotor
C_grid
ib
vb
Bat.
Convt
Figure (2.1) Schematic diagram of integrated wind power generation and energy storage system
Schematic shows the proposed integrated power generation and energy storage system for wind turbine
systems. The wind turbine is mechanically connected to the doubly fed induction generator through a
gear box and a coupling shaft system. The wound rotor induction generator is fed from both the stator and
rotor sides. The stator is directly connected to the grid while rotor is fed through two back-to-back four
quadrant PWM power converters (a rotor side converter and a grid side converter) connected by a DC-
5
link capacitor. The power flow from/to the rotor and the stator can be controlled both in magnitude and in
direction so that it is possible to generate electrical power at constant voltage ad constant frequency at the
stator terminal and inject it into the grid over a wide operating range. A battery energy storage system is
connected to the DC_link capacitor through a bi-directional DC to DC power converter.
The integrated wind power generation and energy storage system is regulated by a control system, which
consists of two parts, electrical control of the DFIG and mechanical control of the wind turbine blade
pitch angle. This thesis focuses on the control of the electrical power flow, which is achieved by control
of three power converters, i.e control of the rotor side converter, control of the stator side converter and
control of the battery converter.
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2.2 Modeling of Induction Generator
The induction generator considered in this study is a wound rotor induction machine fed by two back-toback converters. In terms of the instantaneous variables shown in the above figure 2.1, the stator and
rotor voltage equations in the stationary frame can be written in matrix form as follows
[vsabc] = [Rs][isabc] + ρ[λsabc]
(1)
[vrabc] = [Rr][irabc] + ρ[λrabc]
(2)
where ρ is a derivative operator.
Transforming to a synchronously rotating d-q reference frame the stator voltage equations (1) and rotor
voltage equation (2) become
Stator voltage equations in dq reference frame
vsd = Rsisd - ωsλsq + ρλsd
(3)
vsq = Rsisq - ωsλsd + ρλsq
(4)
Rotor voltage equation in dq reference frame
vrd = Rrird – (ωs –ωr ) λrq + ρλrd
(5)
vrq = Rrirq + (ωs –ωr ) λrd + ρλrq
(6)
Where ωs is the rotational angular speed of the synchronous d-q reference frame, ωr is the rotational
angular speed of the rotor and the stator and rotor flux linkages are given by
Stator flux linkage equation
λsd = Llsisd + Lm(isd + ird) = Lsisd +Lmird
(7)
λsq = Llsisq + Lm(isq + irq) = Lsisq +Lmirq
(8)
Rotor flux linkage equation
λrd = Llrird + Lm(isd + ird) = Lrird +Lmisd
λrq = Llrirq + Lm(isq + irq) = Lrirq +Lmisq
Where ωs e the stator and the rotor inductances and are the stator leakage,
Rotor leakage and mutual inductances, respectively. The electromagnetic
torque produced by the generator can be expressed as
Te = np (λsq isd - λsd isq) = np Lm(isd irq - isq ird )
(9)
Where np is the number of pole pairs.
Ignoring the power loses in the stator and the rotor resistances; the active and reactive powers from the
stator are given by
7
Ps = -1.5 (vsdisd + vsqisq )
(10)
Qs = -1.5 (vrqisd - vsdisq)
(11)
And the active and reactive powers into the rotor are
Pr = 1.5 (vrdird + vrqirq )
(12)
Qr = 1.5 (vrqird – vrdirq)
(13)
The electromechanical dynamic equation is given by
np j ρωm = Tm – Te – Dm ωm
(14)
Where Tm is the mechanical torque provided to the machine, the electromagnetic torque Te is in opposite
direction of the mechanical torque, ωm is the rotational angular speed of the lumped mass shaft system
and ωm = np ωr, Dm is the damping of the shaft system. Since the rotor, magneto-motive force has to be
in synchronism with the stator magneto-motive force, the frequency of the rotor current, or the slip
frequency, ωslip, is
ωslip = ωs- ωr = s ωs
(15)
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2.3 Modeling of Power Converters:
Time average models are developed for two back to back voltage source converters in the rotting d-q
frames and for the DC/DC power converter in the stationary frame.
2.3.1 Rotor side converter
The rotor side converter is four –quadrant PWM converter connected to the rotor terminals through three
filter inductors. The voltage equations in the d-q frame rotating at the angular frequency of ω1 are as
follows.
vrd = dd1vdc + ω1L1irq – L1ρird
(16)
vrq = dq1vdc - ω1L1ird – L1ρirq
(17)
where ω1 = ωs - ωr is the angular frequency of the rotor currents. The rotor side
converter can be
represented by a current controlled voltage source as shown in figure below
ird
L1
ω1L1irq
i1
Vrd
+
dd1.Vdc
Vdc
L1
dd1ird
dq1irq
ω1L1irq
-
Vrq
dq1.Vdc
Figure (2.2) Time average model of the rotor side converter
9
2.3.2 Stator side converter
The stator side converter is a four quadrant PWM converter connected to the stator terminals through
three filter inductors. The voltage equations in the d-q frame rotating at the angular frequency of ω2
are as follows.
Vgd = dd2vdc + ω2L2igq – L2ρigd
(18)
Vgq = dq2vdc + ω2L2igd – L2ρigq
(19)
Where ω2= ωs is the angular frequency of the stator voltages. The stator side converter is presented by
a current controlled voltage source as shown in the fig. below
L2
ird
ω1L2igq
i2
Vgd
+
dd2.Vdc
Vdc
L2
dd2ig d
dq2ig q
-
ω2L2igd
dq2.Vdc
Figure (2.3) Time average model of the stator side converter
10
Vgr
q
2.3.3 Principle of space vector modulation
The circuit model of a typical three phase voltage source PWM inverter is shown below.
Figure (2.4) Three phase voltage source PWM inverter
S1 to S6 are the six power switches that shape the output, which are controlled by the switching variables
a, a’, b, b’, c and c’. When an upper transistor is switched on the corresponding lower transistor is
switched off. Therefore, the on and off states of the upper transistors S1, S3 and S5 can be used to
determine the output voltage.
The relationship between the switching variable vector [a,b,c]T and line to line voltage vector [Vab Vbc
Vca]T is given by the following.
Vab
1 -1 0
a
Vbc
= Vdc 0 1 -1
b
-1 0 1
c
Vca
(20)
Also,the relationship between the switching variables vector [a, b, c] T and phase voltage vector [Va Vb
Vc] T can be expressed below.
11
Van
Vbn
2 -1
= Vdc/3 -1
Vcn
2
-1 -1
-1
a
-1
b
2
c
(21)
There are eight possible combinations of on and off patterns for the three upper power switches. The on
and off states of the lower devices are opposite to the upper one and so are easily determined once the
states of the upper power switching devices are determined According to equations eight switching
vectors, output phase voltage and output line voltage in terms of DC- link Vdc are given in table and
figure shows the eight inverter voltage vectors (V0 to V7).
voltage
Switching vectors
Line to neutral voltge
Line to line voltage
vectors
a
b
c
Van
V0
0
0
0
0
0
0
0
0
0
V1
1
0
0
2/3
-1/3
-1/3
1
0
-1
V2
1
1
0
1/3
1/3
-2/3
0
1
-1
V3
0
1
0
-1/3
2/3
-1/3
-1
1
0
V4
0
1
1
-2/3
1/3
1/3
-1
0
1
V5
0
0
1
-1/3
-1/3
2/3
0
-1
1
V6
1
0
1
-1/3
-2/3
1/3
1
-1
0
V7
1
1
1
0
0
0
0
0
0
Vbn
The respective voltage should be multiplied by Vdc
12
Vcn
Vab
Vbc
Vca
Figure (2.5) the eight inverter voltage vectors (V0 to V7).
Space Vector PWM refers to a special switching sequence of the upper three switching devices of a three
phase power inverter. It has been shown to generate fewer harmonic distortions in the output voltage and
currents applied to the phases of an AC machine and to provide more efficient use of supply voltage
compared with sinusoidal modulation technique.
To implement the space vector PWM, the voltage equations in the abc frame can be transformed into the
stationary dq reference frame that consists of th horizontal (d) and vertical (q) axis as depicted in fig
13
Figure (2.6) the relationship of abc reference frame and stationary dq reference frame.
From this figure, the relation between these two reference frames is below.
fdq0 = Ks fabc
(22)
1 -1/2 -1/2
Where Ks = 2/3 0 √3/2 -√3/2
1/2 1/2
fdq0 = [ fd, fq, f0 ]T, fabc=[fa fb fc]T
1/2
(23)
where f denotes current or voltage variable.
As descried in the figure this transformation is equivalent to an orthogonal projection of [a b c] T onto the
to dimensional perpendicular to the vector [1 1 1]T (the equivalent d-q plane) in three dimensional coordinate system. As a result, six non zero vectors and two zero vectors are possible. Six non zero vectors
(v1-v6) shape the axis of a hexagonal as depicted in fig and feed electric power to the load. The angle
between any adjacent two non zero vectors is 600. Meanwhile, two zero vectors (V6 and V7) are at the
origin and apply zero voltage to the load. The eight vectors are called the basic space vectors and are
denoted by V0, V1, V2, V3, V4, V5, V6 and V7. The same transformation can be applied to the desired
output voltage to get the desired reference voltage vector Vref in the dq plane.
The objective of space vector PWM technique is to approximate the reference voltage vector Vref using
the eight switching patterns. One simple method of approximation is to generate the average output of the
inverter in a small period, T to be the same as that of Vref in the same period.
14
.
Figure (2.7) Basic switching vectors and sectors
Therefore, space vector PWM can be implemented by the following steps:
Step1. Determine Vd, Vq, Vref and angle (α)
Step2. Determine time duration T1, T2, T0
Step3. Determine the switching time of each switching device (S1 to S6)
Step 1. Determination of Vd Vq Vref and angle (α)
Figure (2.8) Voltage Space Vector and its components in (d, q).
15
From above figure, Vd, Vq Vref and angle (α) can be determined a follows:
Vd = Van – Vbn .cos60 –Vcn cos60
= Van – 1/2 Vbn -1/2Vcn
(24)
Vq = 0 + Vbn.cos30 – Vcncos30
= Van + √3/2Vbn - √3/2Vcn
Vd
2
1 -1/2
(25)
-1/2
Van
=─
Vq
Vbn
3
0 √3/2
Vcn
-√3/2
(26)
│Vref│= sqrt (Vd2 + Vq2)
(27)
Vq
Α = tan-1
─
=ωt =2πft, where f = fundamental frequency
Vd
(28)
Step 2: Determination of time duration T1, T2, T0
Figure (2.9) Reference vector as a combination of adjacent vectors at sector 1
▪ Switching time duration at Sector1
T
T1
Vref = V1dt
0
0
T1+T2
+
T1
V2 dt
Tz
+ Vo
T1+T2
(29)
16
Tz . Vref = (T2 .V1 + T2 V2 )
(30)
Cos α
cosπ/3
1
Tz . │Vref│.
= T1 .2/3 Vdc .
Sin α
+ T2 . 2/3 .Vdc
(31)
sinπ/3
0
Where, (0 ≤ α ≤ 600)
Sin(π/3 - α)
T1 = Tz .a
(32)
Sin (π/3)
Sin(α)
T2 = Tz . a
(33)
Sin(π/3)
T0 = Tz – (T1 + T2),
│ Vref│
where, Tz = 1/Fz and a =
(34)
2/3 Vdc
▪▪ Switching time duration at any Sector
√3 Tz │Vref│
π
n-1
sin ─ - α + ─ π
T1 =
Vdc
3
√3 Tz │Vref│
=
sin
Vdc
(35)
3
nπ
─-α
(36)
3
17
√3 Tz │Vref│
nπ
nπ
sin ─ . cos α – cos ─ . sinα
=
Vdc
3
√3 Tz │Vref│
T2 =
3
n-1
sin
Vdc
α-─ π
(38)
3
√3 Tz │Vref│
=
(37)
-
n -1
n -1
cos α . sin ─ π + sinα .cos ─
Vdc
3
π
(39)
3
where, n=1 through 6 ( that is sector 1 to 6
T0 = Tz –T1 – T2,
(0 ≤ α ≤ 600)
(40)
18
Determination of the switching time of each switching device (S1 to S6)
Sector
1
2
3
4
5
6
Upper switches
Lower switches
S1 = T1+T2+ T0/2
S4 = T0/2
S3 = T2+ T0/2
S6 = T1+ T0/2
S5 = T0/2
S2 = T1+T2+ T0/2
S1 = T1+ T0/2
S4 = T2+ T0/2
S3 = T1+T2+ T0/2
S6 = T0/2
S5 = T0/2
S2 = T1+T2+ T0/2
S1 = T0/2
S4 = T1+T2+ T0/2
S3 = T1+T2+ T0/2
S6 = T0/2
S5 = T2+ T0/2
S2 = T1+ T0/2
S1 = T0/2
S4 = T1+T2+ T0/2
S3 = T1+ T0/2
S6 = T2+ T0/2
S5 = T1+T2+ T0/2
S2 = T0/2
S1 = T2+ T0/2
S4 = T1+T0/2
S3 = T0/2
S6 = T1+T2+ T0/2
S5 = T1+T2+ T0/2
S2 = T0/2
S1 = T1+T2+ T0/2
S4 = T0/2
S3 = T0/2
S6 = T1+T2+ T0/2
S5 = T1+ T0/2
S2 = T2+ T0/2
19
2.4.4 Battery Converter
The battery converter is used to support the DC-link voltage. The rotor side converter draws current i1
= dd1ird + dq1 irq from the DC link, while the stator side converter draws i2 = dd2 igd + dq2 igq from the
DC link. Neglecting the switching and conduction losses in these converters and power losses in the
DC link, the battery converter and DC link dynamics are governed by
vb = Lb ρ(ib) + (1-db)vdc
(41)
ic = i3 – i1 – i2
(42)
= (1-db)ib – i1- i2
(43)
= C ρ(vdc)
(44)
The battery converter is represented by a voltage controlled current source, as shown in the figure given
below
i3
Lb
ib
+
i1
i2
+
-
Vdc
+
-
Vb
(1-db ) . Vdc
(1-db ) . ib
Figure (2.10) Time average model of the battery converter
At steady state,
Pb = Pr + Pg
(45)
And
i3 = i1 + i2
(46)
Thus the DC link voltage Vdc stays constant with negligible ripples. However, when a disturbance
occurs, the relationship
Pb = Pr + Pg, is broken and the current flowing through the DC link capacitor, ic = i3 – i1 –i2 is not zero,
which results in fluctuations of the DC link voltage Vdc, The battery provides or absorbs an appropriate
amount of current to support the DC link voltage.
20
2.4.5. Modeling of battery energy storage system
The battery is modeled as a non linear voltage source whose output voltage depends not only the
current but also on but also on the battery state of charge, SOC, which is a nonlinear function of the
current and time as given by
vb = v0 – Rb . ib – K(Q/Q-∫ibdt) + A. exp(-B∫ibdt)
(47)
SOC = 100(1-(∫ibdt/Q)
(48)
Where, Rb is the internal resistance of the battery, V0 the open circuit potential (V), K the
polarization voltage (V), Q the battery capacity (Ah), A the exponential voltage (V), and B the
exponential capacity (Ah.)
21
CHAPTER 3
22
CONTROL OF POWER CONVERTERS
The objective of the rotor side converter is to manage the active power and reactive power from
the stator terminals independently, while the objective of stator side converter is to manage the active
power and reactive power from the stator side converter independently. The battery converter is used to
keep the
DC-link voltage constant regardless of the magnitude and direction of the rotor and stator
powers. The goal of these three converters is to maintain a constant total power output from the wind
turbine generator.
3.1 Control of rotor side converter
The rotor side converter control scheme consists of two cascaded control loops. The inner current
control loops regulate independently the d axis and q axis rotor current component, idr and iqr according
to a synchronously rotating reference frame where the voltage oriented vector control is used. The outer
control loops regulate the active power and reactive power output from the machine stator independently.
In the stator voltage oriented reference frame, the d axis is aligned with stator voltage vector,
namely, vs. The stator flux linkage vector λs is then aligned with q axis, i.e., λsq = λs = Lmims and λsd = 0.
Considering equation 3-10, we can gt the following relationships
Isd = -Lmird/Ls
(49)
Isq = Lm (ims - irq)/Ls
(50)
Ps = 1.5 ωsLm2imsird/Ls
(51)
Qs = 1.5 ωsLm2ims(irq - ims ) / Ls
(52)
Vrd = Rrird + σLrρird – (ωs - ωr)( σLrirq + Lm2ims/Ls)
(53)
Vrq = Rrirq + σLr ρirq + (ωs - ωr)Lr ρ ird
(54)
Where ims is the exciting current and ca be calculated from the equation
Ims =(Vsd – rs isd)/ ωsLm
(55)
And
σ is a constant and is equal to
( LsLr-Lm2 )/ LsLr
(56)
23
Equation (51) and (52) indicates that with the stator voltage oriented control scheme Ps and Qs are
directly related to the d axis and q axis component of the rotor current respectively and can be
independently controlled y regulating the rotor current d axis and q axis components, ird and irq
respectively. Consequently, the reference values of ird and irq can be determined from the outer power
control loops.
3.1.1 Inner current control loops
Here rotor windings are used as filter of rotor side converter and d axis and q axis voltage produced by
the converter are proportional to the corresponding duty cycles of the pulse width modulation signal. I
equation (53) and (54) ims , ird and irq are cross coupling terms for vrd and vrq respectively. These terms
are regarded as disturbances and considered separately, it is possible to define
v’rd = rrird + σLrρird
(57)
v’rq = rrirq + σLrρirq
(58)
Equation (57) and (58) indicates that ird and irq independently respond to vrd’ and vrq’ respectively and
act as two linear first order dynamic system. If a proportional integral (PI) control scheme is adopted to
produce the references for vrd’ and vrq’ based on a current feed back control, i.e.
vrd' = krdp + krdi / s
i*rd - ird
(59)
vrq' = krqp + krqi / s
i*rq - irq
(60)
then the following control law can be derived for the rotor side converter by substituting (59) and (60)
into the equations (53) and (54)
vrd* = krdp + krdi / s
i*rd - ird
– (ωs - ωr)( σLrirq + Lm2ims/Ls
(61)
vrq* = krqp + krqi / s
i*rq - irq
+(ωs - ωr)σLrird
(62)
3.1.2 Active power control
Equation (28) suggests that the active power exchanged between the stator and the grid is proportional to
ird. The following control law can be used for the rotor side converter to regulate the active power.
i*rd =( kpsp + kpsi / s) (P*s - Ps)
(63)
vrd* = ( krdp + krdi / s) ( i*rd - ird ) – (ωs - ωr)( σLrirq + Lm2ims/Ls)
(64)
24
3.1.3. Reactive power control
Equation (52) suggests that the reactive power exchanged between the stator and the grid is proportional
to irq. The following control law can be used for the rotor side converter to regulate the reactive power.
i*rq = ( kqsp + kqsi / s ) (Q*s - Qs)
(65)
Figure below show the overall vector control scheme of the rotor side converter. The compensated
outputs of the two current controllers vrd and vrq are used by the PWM module to generate the gate
control signals to drive the IGBT switches.
Ps
-
Active power
Controller
Ird
-
Id
(ωs-ωr)Lm2Ims/Ls
Controller
Vrd*
+
Ps_ref +
Ird +
-
Irq
Qs
DClink
(ωs-ωr)σLr
S
V
P
W
M
(ωs-ωr)σLr
Ird
+
Qs_ref +
+
Reactive power
controller
+
Irq
Iq
Controller
Vrq*
Figure (3.1) Vector control scheme of Rotor side converter
25
V
S
C
3.2 Control of stator side converter
The stator-side converter control scheme also consists of two cascaded control loops. The inner current
control loops regulate independently the d axis and q axis components of the stator side converter AC
output current, igd and igq in the synchronously rotating reference frame. The outer control loops
regulate the active power and reactive power exchanged between the stator side converter and grid.
3.2.1 Inner current control loops
With the d-axis aligned to grid voltage vector vs (vsd = vs, vsq = 0), the following d-q vector
representation can be obtained for stator-side converter
vs = dd2 vdc + ωs L2 igq – L2ρigd
(66)
0 = dq2vdc - ωs L2 igd – L2 ρigq
(67)
Following the same procedure as in (57) – (64), the references for vgd and vgq can be obtained by the
following feedback loops and PI controller
v*gd = ( kgdp + kgdi /s) (i*gd - igd) –ωs (L2igq + vs)
v*gq = ( kgqp +
kgqi
(68)
) (i*gq - igq) +ωsL2igd
(69)
where i*gd and i*gq are the reference values of the converter output current obtained from the outer power
control loops.
3.2.2 Active power control
Ignoring the power loses in the converter, the active and reactive powers from the grid side converter are
Pg = 1.5 (vgdigd + vgqigq) = 1.5 vgd igd
(70)
Qg = 1.5 (vgqigd - vgdigq) = 1.5vgdigq
(71)
Since d axis voltage is maintained constant, the active power exchanged between the stator side
converter and the grid is proportional to igd. The following control law can be used for the rotor side
converter to regulate the active power.
i*gd = (kpgp +
kpgi / s ) (P*g – Pg)
(72)
26
3.2.3 Reactive power control
Equation (71) shows that the reactive power exchanged between the stator side converter and the grid is
proportional to –igq. The following control law can be used for the stator side converter to regulate the
reactive power.
i*gq = -(kqgp + kQgi /s ) (Q*g - Qg)
Pg
-
(73)
Active power
Controller
Igd
-
Id
Controller
Vs
+
Vgd’
+
Pg_ref +
+
Igd_ref
+
Igq
Qg
-
DClink
ωsLg
S
V
P
W
M
ωsLg
Ird
Iq_ref
+
Qg_ref
+
+
Reactive power
controller
+
Igq
Iq
Controller
Vrq’
Figure (3.2) Vector control scheme of stator side converter
27
V
S
C
3.2.4 Control of battery converter
Neglecting the loses in the power converters, the battery, filtering inductor and harmonics due to
switching actions, the power balance of the integrated wind power generation and energy storage system
is governed by
Pb – Pr –Pg = vdcic = Cvdc .dvdc / dt
(74)
The objective of the battery converter is to maintain a constant voltage at the dc link, so the ripple in the
capacitor voltage is much les than the steady state voltage. The equation (74) can be rewritten as
Pb – Pr –Pg = vdcic ≈ Cvdc . dvdc/ dt
(75)
If the powers injected to the two back to back voltage source converters are assume constant at any
particular instant, the power from the battery is responsible to change the capacitor voltage. Therefore,
the transfer function from Pb to vdc is given by
{Vdc(s) / Pb(s) = 1/ Cvdc.s
(76)
Considering the battery voltage can be assumed constant t any instant and
Pb = vbib = Vbib
(78)
Equation (76) becomes
{Vdc(s) / ib(s)} = { Vb/ vdc }{1/Cs}
(79)
Equation (79) suggests that a linear first order dynamics exists between the battery current and the DC
link voltage. Therefore the reference values of ib can be generated using a voltage feedback control and a
PI control scheme as follows.
Ib* = ( kvp + kvi/ s ) (v*dc - vdc)
(80)
The inner current control loop is to force the battery current to follow the reference produced in equation
(80) using a PI scheme, as given by
db = ( kbp + kbi/s ) (ib* - ib)
(81)
28
PWM
Vdc
_
Lb
Voltage
Controller
Current
Controller
Vdc_ref
Gain (-i)
PWM
Vdc
Vb
Ib
Figure (3.3) Control scheme of battery converter
29
CHAPTER 4
30
SIMULATION RESULTS
To verify the control strategies designed above, a single machine infinite bus power system is used for
simulation studies in MAT LAB Simulik. A 2MW DFIG wind turbine system is connected to the stiff
grid through a step-up transformer and transmission line. The proposed integrated power generation and
energy storage configuration is used. The parameters of the DFIG wind turbine are given in the
Appendix. Three scenarios are studies where the output power to the grid and the wind speed are varied.
4 I Variation in load: Change of load from 500 KW to 1 MW at 5 seconds at constant wind speed of
11m/s
The wind turbine operates at a constant wind speed of 11 m/s with DFIG input mechanical power Pm=2
MW. The reference output power from the stator is set as Ps_ref = 1.8 MW and Qs_ref = 0 Mvar
respectively. The reference output power from the grid side converter is initially set as Pg_ref = 0.24 MW
and Qg_ref = 0.1 Mvar. This case is simulating the operation condition where there is a change in the
load while the power from the stator terminals is maintained constant.
As shown in the figure (4.a) below the DFIG’s input mechanical power and the stator’s output
power are kept constant, according to the control method discussed before the DFIG rotor current is
constant correspondingly, as shown in the figure 4 c.The battery initially discharges a small amount of
power, as shown in figure 4 a, at a low current figure 4 d, through the rotor side converter to meet the
difference between the stator output power and input mechanical power. As the stator side converter
increases its output power and the input mechanical power, the battery discharges more power to balance
the increased power difference. However, when the output power from the stator side converter decreases
to 0.2MW, the battery start to be charged figure 4 d and a certain amount of power is delivered to the
battery. The power of the stator side converter is controlled by its d axis current, as shown in c, which
agrees with equation 47. The change in the output reactive power fig 4 b does not affect the battery
current figure 4 d but the q axis current of the stator side converter fig 4 c. Figure 4 d shows that the
voltage across the DC link capacitor undergoes very slightly disturbance as the battery current changes,
which suggests that the battery can help stabilize the DC link voltage.
31
Load Current
IL
0.5
0
-0.5
2
3
4
5
6
time(s)
7
8
9
10
8
9
10
8
9
10
Wind speed
12
m/s
11.5
11
10.5
10
1
2
3
5
8.4
4
5
6
time(s)
7
Mechanical Power Input(Pm)
x 10
Pm(W)
8.38
8.36
8.34
8.32
1
2
3
4
32
5
6
time(s)
7
6
1.4
Stator active power(Ps)
x 10
1
0.8
0.6
1
2
3
4
5
6
time(s)
7
8
9
10
8
9
10
Rotor side conv. Power(Pr)
4
2
0
-2
1
2
3
4
4
4.6
5
6
Time(s)
7
Grid side convt.Power(Pg)
x 10
4.55
Pg(W)
Ps(W)
1.2
4.5
4.45
4.4
1
2
3
4
5
6
time(s)
Figure 4 I a
33
7
8
9
10
6
Stator Reactive Power(Qs)
x 10
-0.6
Qs(Mvar)
-0.8
-1
-1.2
-1.4
1
2
3
-7
5
6
time(s)
7
8
9
10
Rotor side convt.Reactive PowerQr)
x 10
1
4
Qr(Mvar)
0.5
0
-0.5
-1
1
2
-6
Qg(Mvar)
1.7
3
4
5
6
time(s)
7
8
9
10
9
10
Grid side converter reactive Power (Qg)
x 10
1.6
1.5
1.4
1
2
3
4
5
6
time(s)
Figure 4 I b
34
7
8
Rotor side controller D axis current(irg)
ird
0.5
0
-0.5
1
2
3
4
5
6
time(s)
7
8
9
10
rotor side controller q axis current(iqr)
iqr
0.5
0
-0.5
1
2
3
4
5
6
time(s)
7
8
9
8
9
10
grid side controller d axis current igd
1
igd
0.5
0
-0.5
-1
1
2
3
4
5
6
time(s)
35
7
10
-3
grid side convt.q axis curent(igq)
x 10
6
4
igq
2
0
-2
-4
1
2
3
4
5
6
time(s(
7
8
9
10
Figure 4 I c
Rotor side cont. current(i1)
0.8
0.78
i1
0.76
0.74
0.72
0.7
1
2
3
4
5
6
time(s)
7
8
9
10
Grid side convt Current(i2)
0.73
0.72
0.715
0.71
1
2
3
4
5
6
time(s)
7
8
9
10
Battery convert.current(i3)
20
10
i3
i2
0.725
0
-10
-20
1
2
3
4
36
5
6
time(s)
7
8
9
10
DC Link Voltage(Vdc)
1202
Vdc(V)
1201
1200
1199
1198
1
2
3
4
5
6
time(s)
7
8
9
10
Figure 4 I d
Case II: Change in Wind Speed
The wind turbine operates at varying speeds, which means input mechanical power to the DFIG is
changing. . The reference out put power from the generator stator terminals is set as Ps ref = 1.8 MW and
Qsref = 0, respectively. The reference output power from the stator side converter Pgref= 0.24MW and
Qgref =0.1Mvar.The simulation results are shown in fig 4 II. Initially the battery discharges a very low
current through the rotor side converter fig 4 II d because the DFIG input mechanical power is slightly
less than the total output power , as shown in fig a . As the mechanical power is decreases to 1.7MW, the
battery discharges more power to balance the increased power difference. However, when the input
mechanical power is increased to 2.4 MW, the battery starts to be charge and certain amount of power is
delivered to the battery Fig 4 II d. The rotor side converter outputs power from the rotor. The active
power of the rotor side converter is controlled by its d axis current, as shown in fig 4 II c which agrees
with equation (28). The change in the input mechanical power affects the reactive power slightly as
shown in fig 4 II b, which is reflected in the change q-axis current fig 4 II c. As soon the input
mechanical power change back to 2 MW the battery starts to be discharge again. Fig 4 II d shows that the
voltage across the DC link capacitor undergoes slightly disturbances as the battery current changes,
which suggests that the battery can help stabilize the DC link voltage. When the DFIG operates in the sub
synchronous mode, the generator rotor accepts active power active power from the DC link and the
required power will be supplied by the battery, which means that the rotor side converter operates in the
inverter mode. On the other hand, as the DFIG works in the super synchronous mode, active power will
be delivered from the rotor to the DC link. In this mode, whether the battery is charged or discharged will
depend on the difference between the rotor output power and the grid side converter output. If the rotor
output power is larger than the grid side converter output power, the battery will accept the remaining
power. While the rotor output power is less than the grid side converter output power, the battery will
export the rest of power. Since the DC link is controlled by the battery converter, the output power of the
stator side converter will not depend on the rotor side converter output power. That explains why the
stator side converter output power is not affected by the rotor side power variations.
37
Load current(iL)
0.4
iL
0.2
0
-0.2
-0.4
1
2
3
4
5
6
time(s)
7
8
9
10
wind speed
wind speed(m/s)
15
10
5
1
2
5
9
3
4
5
6
time(s)
7
8
Mechanical Input power(Pm)
x 10
8
Pm(W)
7
6
5
4
3
2
4
6
time(s)
38
8
10
9
10
5
Stator Power(Ps)
x 10
10
6
4
2
1
2
3
4
5
6
time(s)
7
8
9
10
Rotor side convert. Active Power (Pr)
Pr(W)
0.5
0
-0.5
1
2
3
4
5
6
time(s)
4
1
7
8
9
10
converter power(Pb)
x 10
0.5
Pb(W)
Ps(W)
8
0
-0.5
-1
1
2
3
4
5
6
time(s)
39
7
8
9
10
Grid side convt active power(Pg)
1
Pg(W)
0.5
0
-0.5
-1
1
2
3
4
5
6
time(s)
7
8
9
10
Figure 4 II a
5
Qs(Mvar)
-7
stator reactive power(Qs)
x 10
-7.5
-8
-8.5
1
2
3
4
5
6
time(s)
7
8
9
10
grid side convt. reactive power(Qg)
1
Qg(Mvar)
0.5
0
-0.5
-1
1
2
3
4
5
6
time(s)
Figure 4 II b
40
7
8
9
10
rotor convert. d axis current)idr)
idr
0.5
0
-0.5
1
2
3
4
5
6
time(s)
7
8
9
10
rotor convt.q axis current(iqr)
1qr
0.5
0
-0.5
1
2
3
4
5
6
7
8
9
time(s)
grid converter daxis current(igd)
1
Igd
0.5
0
-0.5
-1
1
2
3
4
5
6
time(s)
41
7
8
9
10
10
-3
grid converter q axis current(igq)
x 10
6
4
igq
2
0
-2
-4
1
2
3
4
5
6
time(s)
7
8
9
Figure 4 II c
rotor side convert current( i1)
0.73
i1
0.725
0.72
0.715
0.71
1
2
3
4
5
6
time(s)
7
8
9
grid side convt current (i2)
0.73
i2
0.725
0.72
0.715
0.71
1
2
3
4
5
6
time(s)
7
42
8
9
10
10
10
battery convt curent (i3)
20
0
-10
-20
1
2
3
4
5
6
time(s)
7
8
9
10
DC Link Voltage(Vdc)
1205
Vdc
i3
10
1200
1195
1
2
3
4
5
6
time(s)
7
8
Figure 4 II d
43
9
10
CHAPTER 5
44
CONCLUSION AND FUTURE SCOPE OF WORK
Conclusion
An integrated power generation and energy storage system has been presented for doubly fed induction
generator based wind turbine system. A battery energy storage system is connected to the DC-link of the
back-to-back power converters of the doubly fed induction generator through a bi-directional DC- to-DC
power converter. The battery is charged if there is excess power and can supply power to the load if the
power demand is higher than the input mechanical power from the wind. The energy storage device is
controlled so as to smooth out the output power as the wind speed varies or maintain a desirable power
output. Control algorithms are developed for the grid side converter, rotor side converter and battery
converter and the control strategies are tested on a simulation model of a 2 MW DFIG wind turbine
system developed in MATLAB/ Simulink. The model contains a DFIG wind turbine, three power
converters built with individual IGBT switches, a DC-link capacitor and several controllers. Simulation
results show that the integrated power generation and energy storage system can supply steady output
power as the wind speed changes. As the wind speed is constant, the wind turbine can output varying
power due to the existence of the battery. This study suggests that the integrated power generation and
energy storage is good for intermittent wind power generation.
45
REFERENCES
1.M. V. A. Nunes, J. A. Pecas Lopes, H. H. Zurn, U. H. Bezerra, and R. G.Almeida, “Influence of the
variable-speed wind generators in transient stability margin of the conventional generators integrated in
electrical grids,” IEEE Trans. Energy Conversion, vol. 19, no. 4, pp. 692-701, Dec. 2004.
2. H. Akagi, H.Sato, “Control and performance of a doubly-fed induction machine intended for a
flywheel energy storage system,” IEEE Trans.Power Electron, vol. 17, no. 1, pp. 109-116, Jan. 2002.
3. F. M Hughes, O. Lara, N. Jenkins, and G. Strbac, “A power system stabilizer for DFIG-based wind
generation,” IEEE Transactions on Power Systems, Vol. 21, No. 2, pp. 763 – 772, May. 2006.
4. Y. Lei, A. Mullane, G. Lightbody, and R. Yacamini, “Modeling of the wind turbine with a doubly fed
induction generator for grid integration studies,” IEEE Trans. Energy Conversion, vol. 21, no. 1, pp. 257264, Mar. 2006.
5. V. Akhmatov, “Analysis of Dynamic Behavior of Electric Power Systems with Large Amount of Wind
Power,” Ph.D. dissertation, Technical University of Denmark, Kgs. Lyngby, Denmark, Apr. 2003.
6. J. Morren and S. W. H. de Haan, “Ridethrough of wind turbines with doubly-fed induction generator
during voltage dip,” IEEE Trans. Energy Conversion, vol. 20, no. 2, pp. 435-441, Jun. 2005.
7. T. Petru and T. Thiringer, “Modeling of wind turbines for power system studies,” IEEE. Trans. Power
Systems, vol. 17, no. 4, Nov. 2002, pp. 144-151.
8. J. G. Slootweg, S. W. H. de Hann, H. Polinder, and W. L. Kling, “General model for representing
variable speed wind turbines in power system dynamic simulations,” IEEE. Trans. Power Systems, vol.
18, no. 1, pp. 1132-1139, Feb. 2003.
9. L. Xu, P. Cartwright, “Direct active and reactive power control of DFIG for wind energy generation,”
IEEE Transactions on Energy Conversion vol. 21, Iss.3, pp.750 – 758 Sept. 2006.
46
10. S. S. Choi, K. J. Tseng, D. M. Vilathgamuwa, T. D. Nguyen, “Energy storage systems in distributed
generation schemes”, Proceedings of 2008 IEEE Power and Energy Society General Meeting, pp. 1 – 8,
20-24 July2008.
11. E. Spahic, G. Balzer, A. Shakib, “The Impact of the "Wind Farm - Battery" Unit on the Power System
Stability and Control”, 2007 IEEE Lausanne Power Tech, pp. 485 – 490, 1-5 July 2007.
12. C. Abbey, K. Strunz, J. Chahwan, and G. Joos, “Impact and Control of Energy Storage Systems in
Wind Power Generation”, Proceedings of the 2007 Power Conversion Conference - Nagoya, pp. 1201 –
1206, 2-5 April 2007.
13. E. Spahic, G. Balzer, B. Hellmich, W. Munch, “Wind Energy Storages Possibilities”, 2007 IEEE
Lausanne Power Tech, pp. 615 – 620, 1-5 July 2007.
14. S. Muller, M. Dei, and R. W. Doncker, “Doubly fed induction generator systems for wind turbines,”
IEEE Industry Applications Maganize, vol.8, Iss. 3, pp. 26 – 33, May-June 2002.
15. S. Towito, M. Berman, G. Yehuda, and R. Rabinovici, “Distribution Generation Case Study: Electric
Wind Farm with Doubly Fed Induction Generators,” IEEE 24th Convention of Electrical and Electronics
Engineers in Israel, pp. 393 – 397, Nov. 2006.
16 S. Muller, M. Deicke, and R. W. De Doncker, “Doubly fed induction generator systems for wind
turbines,” IEEE Ind. Appl , Mag, vol. 17, no.1, pp. 26-33, May-Jun. 2002.
17.R. Datta and V. T. Ranganathan, “Variable-speed wind power generation using doubly fed wound
rotor induction – a comparison with alternative schemes,’ IEEE Trans. Energy Conversion, vol. 17, no. 3,
pp. 414-421, Sept. 2002.
47
APPENDIX
Parameters
Values(p.u)
Lls
0.171
Llr
0.156
Lm
2.9
L2
0.30
Rs
0.0071
Rr
0.005
Vdc nom
1200 Volts
C
0.06 F
Vnom
575 Volts
Pnom
10 MW
Vbattery
600 Volts
np
6
7.1 Table-1 System parameters used in simulation
48
7.2 Mat lab/ Simulink model of integrated wind power generation with battery energy storage system
Discrete ,
Ts = 0.0001 s
powergui
B
N
Wind _Speed
A
B
A
aA
A
a
aA
A
A
A
a
aA
B
bB
B
b
bB
B
B
B
b
bB
cC
C
C
30 km line
C
c
c
cC
C
C
C
2500 MVA B120 120 kV/25 kV
X0/X1=3 (120 kV)
47 MVA
C
120 kV
B 25
(25 kV)
A
B
C
N
a
b
c
Grounding
Transformer
X0=4.7 Ohms
cC
B575
(575 V)
25 kV/ 575 V
Conn1
aA
Conn2
bB
Conn3
cC
DFIG
wind turbine system
and battery energy storage
A
B
C
A
Filter
0.9 Mvar
Q=50
Initial load 500 KW,
3.3ohms after 5sec another 500 KW load
is switched on
DFIG Wind Turbine System And Battery Energy Storage System
7.3 Mat lab/ Simulink detailed model of integrated wind power generation with battery energy storage
system
1
A
2
B
3
C
aA
aA
bB
cC
B1
a
b
c
From 1
Iabc _g
From 6
[Vdc ]
From 5
Iabc _r
From 2
[wr]
wr
From 3
Vabc_B1
Iabc _g
Vdc
Iabc _r
B
CBgrid
m
Tm
A
a
bB
B
b
cC
C
c
Bstator
Scope 5
g
Vabc A
Vabc _B1
[Tm ]
Asynchronous Machine
pu Units
+
+
A
B
-
-
C
Universal Bridge
<Rotor s peed (wm)>
g
Vabc A
A
a
B
b
C
Universal Bridge 1
Vabc_B1
Iabc _g
c
-
Scope 6
B
C
Brotor
+
SVPWM _Grid_Convt
[wr]
v
Scope 8
[Vdc ]
Vdc
-CIabc _r
Constant
SVPWM _Rotor_Convt
wr
controllers
[wr]
Generator s peed (pu)
0
Pitch angle (deg) Tm (pu)
Vdc _ref
Vdc
i_battery
A1
A2
Battery conv
Constant 1
1
Wind _speed
Wind s peed (m /s )
Wind Turbine
49
[Tm ]
Scope 1
7.4 Mat lab/ Simulink detailed model of converter’s controller
abc
dq0
sin_cos
1
Vabc _B1
Vdq
Vgd
Idq
Vdq
puls e
Vgq
Freq
1
pulse _grid _conv
Subsystem 3
Rate Limiter
Sin_Cos
wt
Vdc _nom
freq
Subsystem 1
4
Vdc
Rate Limiter 1
Discrete
Virtual PLL
abc
dq0
sin_cos
Terminator 3
2
Iabc _g1
frequency
Rs
Rs
Rs
Lm
Lm
Vrd '
Lm
3
Iabc _r
Iabc _r
5
wr
wr
Vdq
Lls
Vdc _nom
Lls
Llr
Freq
puls e
Lls
Llr
Rotor_Angle
Sin_Cos
Discrete
3-phase PLL 1
Subsystem2
Vrq '
Terminator 1 Llr
Vabc (pu) wt
2
Pulses_rotor _conv
Vs d
Terminator 2
Subsystem
7.5 Mat lab/ Simulink detailed model of rotor side controller
5
wr
1.8
1
frequency
P _ref
PI
60
Product
Discrete
PI Controller
F_nom
2
ws
Qs
Rs
Ro
Divide
Add 1
Saturation 5
Rs
3
ird
Lm
PI
Lm
4
Iabc _r
Iabc _r
6
Lls
8
Rotor _Angle
9
Vsd
Discrete
PI Controller 1
Ps
7
Llr
1
Vrd '
Im s
Lls
irq
ws-wr
Llr
Ls
Ro
Lr
Rotor_Angle
Lr
Lm ^2
Vs d
Product 1
Product 2
Subsystem 1
irq
PI
Discrete
PI Controller 2
PI
0
Discrete
PI Controller 3
2
Vrq '
Qs_ref
Product 3
50
7.6 Mat lab/ Simulink detailed model of grid side controller
1.5
Constant
1.5wsLm^2ims
1
ws
ws
5
Ps
ird
2
Rs
Ls
8
Vsd
Ls
Saturation 6
ims
7
Rotor _Angle
Theta
isd
idq_r
4
Iabc _r
ird
3
ird
irq
1
Qs
iabc
-Lmird
Subsystem
3
Lm
Saturation 3
-1
4
Ims
Gain 2
Ls
5
Lls
Ls
7
Ls
Saturation 7
|u|
Lm
2
9
Lm ^2
Lm ^2
wsLm
ws
6
irq
Saturation 5
Ls
LsLr
Lr
6
Llr
lm ^2
Add
Saturation 1
2
Ro
8
Lr
7.7 Mat lab/ Simulink detailed model of battery controller
-K >=
Gain
1
Vdc _ref
PI
Subtract
2
Vdc
Discrete
PI Controller
PI
-K >=
Discrete
Subtract 1 PI Controller 1
Gain 1
g
C
1
A1
1
i _battery
E
Diode
+
i
g
C
Current Measurement
Series RLC Branch 4
E
Diode 1
2
A2
51
DC Voltage Source
7.8 Mat lab/ Simulink model of SVPWM
1
Vdq
Vdq
pulse
2
Vdc _nom
1/z
1
pulse
Vdc_nom
Subsystem
7.9 Mat lab/ Simulink detailed model for calculation of Vref, angle, Sector,T1, T2, and T0
2
97 .47
Re
1
dq
Im
|u|
u
-K -
Angle
f(u)
Saturation 1
Sector
sector
rad _angle
2
Vdc
Display _sector
Display _angle
f(u)
fcn
T1
3
Tz
f(u)
.5
T0
T2
Saturation 7
1
T1,T2& T0
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7.10 Mat lab/ Simulink detailed model for generation of gate pulses
f(u)
>=
Ta
NOT
1
T1,T 2&T0
f(u)
>=
Logical
Operator 2
Tb
NOT
f(u)
>=
Logical
Operator 1
Tc
NOT
Logical
Operator
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1
pulse
7.11DFIG _DATA.m file
%Matlab program for data initialization
%for grid model
NumberOfUnits=1;
% Asynchronous machine data
Pnom=2e6/0.9*NumberOfUnits;
Vnom=575;
Fnom=60;
Rs=0.0071;
Lls=0.171;
Rr=0.005;
Llr=0.156;
Lm=2.9;
H=5.04;
F=0.01;
p=3;
Ls=Lls+Lm;
Lr=Llr+Lm;
roh=Ls*Lr-Lm*Lm/Ls*Lr;
%DC link data
Vdc_nom=1200;
C_DClink=10000e-6*NumberOfUnits;
Fz=1e3;
Tz=1/Fz;
%Choke data
R_RL=0.30/100;
L_RL=.50;
Lg=L_RL*Vnom^2/Pnom/(2*pi*Fnom);
%Wind turbine data
Pmec=2e6*NumberOfUnits;
power_C=0.73;
speed_C=1.6;
% Nominal power (VA)
% Line-Line voltage (Vrms)
%Hz
%pu
%pu
%pu
%pu
%pu
%Inertia constant (s)
%Friction factor (pu)
%Number of pairs of poles
%Volts
%Farads
%pu
%pu
% Nominal power (W)
%pu
%pu
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