A Novel Variable-Speed Wind Energy System Using Induction Generator and Six-Switch ACIAC Converter M. Heydaril, Student Member, IEEE, A. Yazdian Varjani2, Member, IEEE and M. Mohamadian3, Member, IEEE Power Electronic and Protection Laboratory (PEP Lab.), Faculty of Electrical and Computer Engineering Tarbiat Modares University, Tehran, Iran 2 3 Emails:
[email protected]. yazdian@ modares.ac.if.
[email protected] Abstract- Nowadays using of the wind energy has a growing trend in the world due to their numerous advantages. The wind energy
is
free,
inexhaustible
and
it
produces
no
waste
or
greenhouse gases. Furthermore, it is a good method of supplying
J
energy to remote areas. In this paper, a new variable-speed wind energy
conversion
induction
generator
systems (SCIG)
(WECSs) and
a
with new
a
squirrel
six-switch
cage
AC/AC
converter as power electronic interface between SCIG and network
is
proposed.
Characteristics
of
six-switch
wind
speed
and
delivering
power
to
the
grid,
simultaneously. This configuration uses only six active switches and six diodes and has the lowest number of active switches among three-phase to three-phase AC/AC converters proposed hitherto in the literature. This converter offers sinusoidal input and output, unity power factor and more importantly, low manufacturing cost. Two simulations are performed for proposed configurations and results are presented.
The results verify
effectiveness of the proposed interface configuration. Keywords; Adjustable speed generation system; Squirrel Cage Induction Generator
(SCIG);
Wind energy conversion system
(WECS); Six-Switch ACIAC Converter
I.
c
J
�
Output Terminal
Grid
AC/AC
converter are used for maximum power tracking control under different
�
L,
INTRODUCTION
Nowadays more of countries place great importance on using renewable energy sources due to their numerous advantages. As a renewable and non-polluting energy source, wind power has a role to play in reducing harmful emissions and the impact of climate change. Greater use of wind power also makes us less reliant on increasingly scarce fossil fuels like coal and oil, which are subject to huge price fluctuations and must often be imported from volatile parts of the world [ 1]. With growing application of wind energy conversion systems (WECSs), various technologies are developed for them. Squirrel cage induction motors are the most commonly used electrical machine in AC drives, because they are robust, cheap and have low maintenance cost. These advantages make the induction machine very attractive for wind power applications both for fixed and variable speed operation [24]. To take advantage of the higher energy capture and increase in the system compliance resulting from variable speed operation a power electronic interface must be provided between the machine terminals and the grid. The back-to-back PWM inverter based power electronics interface is a suitable option for cage induction machine in wind power applications. Fig. 1 shows a conventional configuration of ac-dc-ac topology for squirrel cage induction generator (SCIG).
Fig. 1.
Connection of wind power generation system to grid through back to back inverters
This configuration includes a back-to-back PWM rectifier inverter [5]. The PWM rectifier is controlled for maximum power point tracking (MPPT) and inverter is controlled to deliver high quality power to the grid [2]-[6]. This topology requires 12 active switches and 12 diodes. Recently, study of low cost power converters has become a growing trend towards cost reduction in power electronics based systems. In addition to the cost reduction in power electronics based systems, reducing the system size and weight as well as providing a high degree of reliability is of utmost importance. These objectives could be achieved by reducing the number of active switches in power converters. The resultant configurations are called reduced switch count converters. A recent reduced switch count structure for WECEs proposed in [7] as three-phase to three-phase ACIAC converter is achieved by combining the B6 inverter and B6 rectifier stages via sharing an entire row of switches between them (Fig. 2). This converter which is called nine-switch converter was first suggested as multi output inverter for control of two motors [8]. However, due to higher voltage stress and switching loss in comparison with conventional three phase back-to-back converters, this topology is more suitable for low voltage and low power applications. In this paper, a new SCIG-based WECS with six-switch three phase ACIAC converter is proposed to deliver the wind power to the grid. The proposed topology is shown in Fig. 3. This topology has cost advantages compare to back-to-back WECS and nine switch WEC S. The proposed topology reduces number of switches by 33% and 50% respectively compared to back-to-back and nine-switch converters without any change in the objectives of WECS. Six-switch converter was first presented in [9] as dual output inverter for control of two three phase loads and also in [ 10] as three-phase ACIAC converter. This configuration has the lowest number of active switches among three-phase to three phase ACIAC converters proposed hitherto in the literature.
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Considering the structure of six-switch converter, special PWM technique is needed for that as shown in FigA. In each leg, there are two modulating waveforms, one for the rectifier side (Vxr) and one for the inverter side (Vxi), (x=A, B). The key point in the modulation of the proposed converter is that the interference of the rectifier and inverter reference waveforms should be avoided, i.e. Vxr> Vxi• This is achieved by adding two offset signals to the reference signals of both sides and limiting their modulation indices to prevent over-modulation [ 1 1]. Calculating the suitable offset values and adding them to the inverter and rectifier side reference waveforms, the two modulating waveforms of each leg are obtained. The modulating waveforms are then compared with a common triangular carrier signal. The gate signal for upper switch of each leg is positive logic value generated by Vxi and Carrier. The gate signal for lower switch of each leg is negative logic value generated by Vxr and Carrier. The gate signal for mid switch is logic value generated by the logical XOR value of the gate signals for upper and lower switches. Applying this scheme, there are always two ON switches in each leg. The converter acceptable switching states and the resultant output and input phase voltages of the proposed converter are shown in Table I. Since the two phases of the three-phase AC source and the two phases of the three-phase load are connected to the converter legs and the third remaining phase of them is connected to the joint of the split capacitor bank, the voltage references of rectifier and inverter should be chosen so as to achieve three-phase balanced voltage at the input and output terminals of the converter. The reference signals which are related to rectifier and inverter terminals can be expressed by (4) to (7). (4) v�r =mrsin(mrt+t>r)+offietr
Nine-Switch ACIAC Converter , .. -------------------------------.,
,
Temunal: ,
-_- _-___ - -+ -__ - _-__ -_I ,_L __ __-__ __-__ __ __ __ ...J ___) -_-+
Fig. 2.
Schematic diagram of the nine-switch converter in WECSs Proposed Six-Switch ACIAC Converter ".---------------------------., , ,
v�r =mrsin(mrt+t>r -rpr)+offietr V�i =misin(mit)+offieti V�i misin(mit + rpJ+ offieti =
Fig. 3.
Schematic diagram of the proposed topology
Section II of this paper describes operation of six-switch AC/AC converter. Then, power delivery and MPPT control of system are explained. Finally, simulation results are presented to verity the performance of the proposed system. II.
SIX-SWITCH AC/AC CONVERTER
The six-switch AC/AC converter is shown in Fig.3. As it is seen in the figure, this structure has two legs with three power switches in each one and it is comparable to a B4 rectifier as the active front end of a three-phase B4 inverter though a row of switches is shared between them. Two phases of the three-phase induction generator and two phases of the grid are connected to the two converter legs and the remaining phase of the source and load is connected to the joint of the split capacitor bank. Compared to its counterpart in Fig. 2, the proposed structure not only retains all the desirable features, but also reduces the number of required switches by 33% and hence proves more attractive in low voltage and power ratings applications. The amount of DC link capacitors' voltage are dependent on three factors of a, b and c. These coefficients are determined so as to provide balanced three-phase outputs without any DC offset. VCdJ =aVvc
VCd3 =cVvc
( 1) (2)
(5) (6)
(7)
where m,., m; are respectively the rectifier reference amplitude and the inverter reference amplitude, OJ,., OJ; are angular frequencies and ((J,., ((J; are respectively phase difference between voltage references of the rectifier and phase difference between voltage reference of the inverter. offset, and offset; are respectively the offset value of rectifier reference and offset value of inverter reference and are related to implementation of switching method and usually are set to -0.5 and 0.5. In this case, m, and m; are limited to 0.5. For the inverter side of the six-switch converter, using the similarity of triangles in Fig. 5 and assuming the voltage references to be as (4) to (7) and considering table I, yield the fundamental values of the inverter input phase voltages (8- 10) [ 10]. VA;
a
=
a
b
c
.
�
a
b
c
.
�
a
b
c
.
Vdcl( + + )m;sm(l1lt+u;)-( + + )l1I;sm(l1lt+u; -rp;) 3 3 3 6 6 6 b
c
a
b
c
+ - - +( + + )ojJset;l 6 6 6 6 6 6 a
b
c
.
�
�
VB; VdcH. + + )111; sm(l1lt+ u;)+( + + )111;sm(l1lt+ u; -'ll) 6 6 6 3 3 3 a
=
b
c
a
b
c
(8)
(9)
+ - - +( + + )ojJse l � 6 6 6 6 6 6
a b c . . a b e . V(.'; = VdcH. + + )m;sm(l1lf + 0) -( + + )m; sm(l1lf + 0 -'ll) a
b
c
6 6 6 a
b
c
..
-3 +3+3-(3+3+3)off.1eO
(3)
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6 6 6
( 10)
SaH 0 0 1 1 1 1 1
SbH 0 1 0
1 1 1 1
! SaL 1 1 1 0 0 1 1
SbL 1 1 1 0 1 0 1
TABLE I
VA;
-(b+e)VdJ3 (-a-2b-2e)VdJ3 (2a+b+e)VdJ3 aVdJ3 aVdJ3 aVdJ3 aVdJ3
SWltc h'mg States, nput Phase Voltages and Output Phase Voltages • • • • VAr VC; VB;
.
-(b+e)VdJ3 (2a+b+e)VdJ3 (-a-2b-2e)VdJ3 aVdJ3 aVdJ3 aVdJ3 aVdJ3
Considering the above equations, in order to produce balanced voltages, there should be f{ir=nI3. Moreover, a, band c coefficients are determined so as to remove DC offset of the input voltages of rectifier. For instant if offsetr=O.5, then a =1/4 , b =112 and c =114. Fig. 5 illustrates the DC link voltage coefficient values versus rectifier offset variation. ' v .� v
'
.,
I---------,�'*--------_/_�____j I-----#===;;;;=='*------_/_--�
.
2(b+e)VdJ3 (-a+b+e)VdJ3 (-a+b+e)VdJ3 -2aVdJ3 -2aVdJ3 -2aVdJ3 -2aVdJ3
0 .4
0.3
----:t---oiI
0.1
01
Fig. 5.
---E BI---
03 02 Upper Offset
a. c b 0.4
0.5
Dc link voltage coefficients versus rectifier offset
Two modes of operation are defined for the six-switch converter: 1- Variable frequency (VF) mode in which frequency and amplitude of the inverter output and rectifier input voltages can be different and 2- Constant frequency mode (CF) in which frequency of induction generator and grid is similar whereas the amplitudes of the input or output voltages may vary. Considering input and outputs of the six-switch WECS can have different frequencies and amplitude, therefore variable frequency mode should be chosen for that. To eliminate DC component and to obtain balanced input and output in, two strategies was suggested in [9]. These two different strategies are also considered for determining three coefficients of a, b and c. The first strategy allows using dc sources of unequal voltage levels in the dc link. These levels are constant and do not vary with modulation index. The second strategy provides the best possible dc bus voltage utilization by changing dc link coefficients according to modulation indices. This strategy is in fact a method for enhancing DC bus utilization by increasing the maximum limit of modulation index. For this purpose, offsetr and offset; are determined according
2eVdJ3 2eVdJ3 2eVdJ3 -2(a+b)VdJ3 (-a-b+e)VdJ3 (-a-b+e)VdJ3 2eVdJ3
mr_ -'...
__
(11)
mr +m;
Limits of ojJsetu, ojJsetL' mr and m; and also DC link voltage coefficients are tabulated in Table II for both strategies in VF mode. TABLE II.
Dc link voltage coefficients
Rectifier and inverter offset limits
Modulation indices limits
OPERATION DETAILS USING TWO STRATEGIES OF DC COMPONENT ELIMINATION VF MODE Second strategy
First strategy
-offset /2 b = 1/2 c = off"et i /2
(I-offset, )/2 = offset,. c = (I-offset, )/2 a =
a
b
r
III, III,
=
r
offset,. +ojJsel; SI offset; = I-a
Os offset; SI
-I S off'et,. sO offlet -offlet i 111, +111, slo.f/set,I+loffset,1
ojJset,.
=
=-a
m,.SI m,SI
SI- loffiet, 1
111,. +111,
SI-loff1et,1
III.
0.2
VCr
"
-eVdJ3 -eVdJ3 -eVdJ3 (a+b)VdJ3 (-a-b-2e)VdJ3 (2a+2b+e)VdJ3 -eVdJ3
-eVdJ3 -eVdJ3 -eVdJ3 (a+b)VdJ3 (2a+2b+e)VdJ3 (-a-b-2e)VdJ3 -eVdJ3
a=
Carrier-based PWM modulation scheme of the proposed converter
0.54I1oI.o-:;-:---,-----�---�---�-- __.&J
•
to rectifier and inverter modulation indices by a which is called DC bus utilization factor and is defined in (11). This factor specifies the rectifier utilization of DC link voltage.
v'•. _�=¥-------�,.._I� ----____j
Fig. 4.
VBr
SI
CONTROL SYSTEM
The structure of the control system is shown in Figs. 6 and 7. The control system is composed of three parts: 1) Control of power delivered to the grid, 2) Vector control for SCIG and 3) MPPT. These control parts generate Vxr & Vx;(x=A, B) for PWM block of six-switch AC/AC converter. A.
Control ofpower delivered to the grid
The relations for active and reactive power delivered to the grid are given by [12]:
P =� (vdid 2
+v,/ J
(12)
Q= % (v,/d-Vdiq )
(13)
where P and Q are active and reactive power respectively. v is grid voltage and i is the current to the grid. The subscripts and 'q' stand for direct and quadrature components, respectively. If the reference frame is oriented along the grid voltage, Vq will be equal to zero. Then, active and reactive power may be expressed as '
P
3
=-(Vdl.d) 2
Q = -�(Vdi 2
q)
d
'
(14) (15)
According to earlier equations, active and reactive power control can be achieved by controlling direct and quadrature current components, respectively. DC voltage is set by controlling active power (Fig. 6).
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Fig. 6. B.
Block diagram of control of power delivered to the grid Fig. 7.
Vector control for SCIG The torque equation ofSCIG is given by [13] ( 16)
1'.
=
Te
3 P ---
2 2mh
=
(
.
.
If/qr1dr -If/dr1qr
: (Aqridr -Adriqr )
3
)
( 17) ( 18)
where P is number of poles, A and i are the air gap flux and the stator current, respectively. The subscripts 'ds' and 'qs' stand for direct and quadrature components in synchronously rotating reference frame, respectively. If the direct axis is oriented along the rotor flux, the quadrature component of the rotor flux (ljfqr) will be equal to zero and the torque and flux equations are simplified as
generator speed is controlled using a PI controller. The command current i*qs is derived from the speed control. Considering the electrical torque of generator is proportional to air gap flux, the air gap flux is maintained constant at its rating value and the value of direct current components is determined according to this rating value using (22). Furthermore, the flux axis angle is calculated using (23) and (24).
(23) (24) The gate signal for mid switch is logic value generated by the logical NAND value of the gate signals for upper and lower switches. C.
mr -m
e
Xm
iqs Tr Adr
=---
(2 1)
The above equations are the basis for field oriented control [ 13]. This approach simplifies the induction machine control. The model is very similar to a separately excited dc machine where the flux depends on the field current and the torque is proportional to the flux and the armature current. The main problem associated with field oriented control is the requirement to estimate the flux axis angle. This is done either by measuring the flux at two different points (with 90 degrees displacement), or estimating through rotor speed measurement [4]. Fig. 7 shows the vector control block diagram for SCIG. In order to achieve the maximum output power of wind turbine, the
Block diagram of vector control for SCIG
Maximum power point tracking (MPPT)
Neglecting the friction losses, the mechanical power available to be converted by the generator is given by (25) [ 14]. 1 2 3 (25) PM pl[R CpVW1Nf) 2 =
-
where VWINDis the wind speed, p is the air density, R is the blade radius of turbine and Cpis the power coefficient. The power coefficient depends on the pitch angle, fJ , the angle at which the rotor blades can rotate along its long axis, and is the tip-speed ratio, A , defined by:
OJMR (26) VW1ND where OJM is the rotor speed (in rad/s). As it can be seen in Fig. 8, the maximum output power of wind turbine is obtained in different speed of the turbine due to incident wind speed. Moreover, Fig. 9 shows that Cp is a function of A and its maximum value is yielded at the particular tip-speed ratio (AI/om), which is determined from the blade design. Considering
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A
=
(25), the mechanical power is proportional to Cp and hence the maximum output power of wind turbine is obtained at CPmax' I 2 3 (27) �nax - p7rR CPmaxVWIND =
2
Then, the optimum speed of generator is determined as
Anum (28) R The optimum speed is used as reference speed for speed controller of SCIG. OJM *
1.2
-;-
j
co.
'5 � o co.
=
Parameter Nominal Mechanical Power Base Wind Speed Pitch Angle Base rotational speed (p. u. of base generator speed) Maximum power at base wind speed (pu of nominal mechanical power)
Value 16 kW
Parameter Stator Resistance (R.J Stator Inductance (L,J Rotor Resistance (R'r) Rotor Inductance (L ',,) Pole Pairs (P) Inertia (J) Nominal Power (P,J Nominal Voltage(L-L) (Vn ) Nominal Frequency ([oJ
Value 0.2761 n 2.191 mH 0.1645 n 2.191 mH
TABLE III
-- VW1ND
TABLE IV
Wind Speed
0.8
0.6
:t.i
0.2
��� °o�--.m���������--�� 0.2
Fig. 8.
0.4
Rotor speed 0.6
0.8
(pu)
0.4
0.3
0.1
o
Fig. 9.
A Power coefficient versus tip-speed ratio with the pitch angle parameter
IV.
as
SIMULATION RESULTS
To verify the performance of the proposed WECS, several simulation tests are performed by simulink® software. The simulated system parameters are listed in Tables III ,IV and V. These simulations were performed using methods mentioned in Section III. Because the input grid frequency is different from the output frequency, variable frequency operation mode is selected. Furthermore, the modulation space is equally divided between the rectifier and inverter sides and hence offsetr and ojJseti are set to 0.5 and -0.5, respectively and the DC link coefficients are set to a=1!4, b=112 and c=I!4. The proposed method in [10] is used for calculating the DC-link capacitor sizes.
2
0.01 kg.ml\2 15 kW 460 V 60 Hz Value 0.01 n 5 mH
C,II Cd2
4000 uF
208 V 2000 uF
Switching Frequency Vdc A.
0.73
SIMULATION PARAMETERS
CI3
0.5 "..----,--"
0 1.2
Parameter Grid Resistance (RJ Grid Inductance (LJ Grid Voltage (L-LJ
1.4
Mechanical power versus rotor speed with the wind speed as parameter
12 m/s
PARAMETERS OF SCIG
TABLE V
::l o " 0.4 I:
�
PARAMETERS OF WIND TURBINE
4000 uF 10 kHz 2200 V
(F:J
Operation of Constant Wind Speed
In order to evaluate the performance of the proposed system, simulation is ftrst performed using constant wind speed (10 mls ). Fig. 10 shows DC link capacitors' voltages. It can be seen that the voltages have low ripple and have followed their corresponding commands, i.e. VCd1= (1I4)Vm VCd2= (1I2)VDc and VCd3=(l!4)VDC' Total DC link voltage of the converter was shown in Fig. 11. Reactive power that is kept at zero (unity power factor) is also shown in Fig. 12. Fig. 13 shows active power delivered to the grid and the extracted mechanical power. The electrical power delivered to the grid is different from the extracted mechanical power due to electrical and mechanical losses. To obtain maximum power control, the rotor speed has changed using method mentioned in section III. 1200
1
1
1100 �� ���' Y���� 1000 900
: I 1 I I
:
: �Y��: ���:YY��' ��, ��
I l'... V I ) I I I C J ____ ____1____ J ____ L ___ _ I I I I I I I I I I - - -t - - - t- - - - -1- - -t - - - t- -
'E
________
----1 -
800 700 600
1 VC II � � 1
500 0.8
0.9
Fig. 10.
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1
I
-
-
1
r
1
-
I
1
T
1 1
� L
-,
�-V-ca3 1 1
1.1
lime
(s)
1.2
J 1
- -,
1.3
Capacitor voltages of DC-link
� T 1.4
---
- -
1.5
2400
I
2200
- � --- ' ---
2,000
� 21
1,000
::::;
I
I
I
I
400
200
0 0.4
I
Fig. 11.
0.9
0.8
0.7
Time
1.1
200
1< « "" c:
:.:J 0 0
- - - - - - -1- - - - - - - -t - - - - - - - t- --------I
I
_____
1500
I
I
1
1
I
1 1 _______ L _______,__ 1
1
------ �1 ------- r -------· -
1000
Time (5)
Fig. 14.
SCIG rotor speed and the obtained rotor speed from MPPT
SCIG rotor speed and the obtained rotor speed from MPPT are shown in Fig. 14. It is seen that SCIG rotor speed nearly equals with the rotor speed obtained from MPPT.
0.5 Time
Fig. 17.
(5)
1.5
2
DC link voltage under variable wind speed operation
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v.
�O r----.--.--'--r---�
-+
200 � «
G 100
I -;
� "
."
0..
�
a:
(3 '0
1 I
_
· 100
200
_
,-
·
I
L
__
_
r
·300 1'----: 1 ': 1 ::.8 ----:2:----: .4:--- -1:-'-.6::---� 2.:' 2 2. ----:1.L lime
Fig. 18.
(5)
Grid Reactive Power
15000 ,-------,----i' ,i--;:==============;l
1 -----
:t
, , I I � ,I I I 1 - _ - __ _ t J __I ______ .J ______ 1. ____ I : � I ,'1', I ,: :
10000
�
�
5000
o
Fig. 19.
- - - -
U ··· ·
o
g. (; 15 a:
: , : , '
I I I I 1
Q5
I
lime
(5)
:
_
REFERENCES [I]
fool'
[2]
1.5
-
/
I I
I
- - -,!- - - : ,.j'
o
I
- - - - -d ------:- ---- t� =_����_� [----If
50
40
: - - - - - � - --- -
:
I�---T--------,\ I
.'
Fig. 20.
(3 "
p-C·· A
11 :I ( I 100 ---- -/-: ----
o
�
1:\
! l
- -----: /.....\- ---------- -
-;;;- 150
� a:
I I I I
I I
In this paper, a SCIG-based WECS with six-switch ACIAC converter is proposed. Six-switch converter is used for maximum power tracking control and delivering power to the grid, simultaneously. The proposed system has cost advantages compare to conventional WECS with back-to-back converter or with nine-switch converter, because the number of switching semiconductors is reduced from 12 and 9 to 6 respectively. With this topology, not only 3 active switches and 3 diodes is omitted than nine-switch converter but also there is not any change in the objectives of WECS. The features of the proposed six-switch topology are: - The number of switches and hence drive circuits is reduced. - Despite higher voltage stress, manufacture cost can stilI become lower especially in low voltage designs. - Unity input power factor - High reliability - Less size and weight The performance of the proposed topology was confirmed by simulation.
Active power delivered to the grid and extracted mechanical power
200
1:1
Active Power Delivered to the Grid
--- Extracted Mechanical Power
I ,'
----
__
SCIG Rotor Speed
[3]
[4]
[5]
Rotor Speed Obtained from MPPT
I
- :-I
---
I
I
: -----�-
-
----
[6] ----
(5) SCIG rotor speed and the obtained rotor speed from
[7]
1.5
0.5
lime
MPPT [8]
'0
[9]
= £
[10] [II] I
- - - 1- ___I ___ 1. ___I_
I
I
- - - t- - - --t
1.3
-
I
1.4
1.45
1.5
1.55
1.6
I
[12]
- - + ---1-
(5) Injected three phase currents to the Grid
1.35
1.65
1.7
[13]
lime
Fig. 21.
CONCLU SION
[14]
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