Variable Speed Wind Turbine Using the Squirrel Cage
Induction Generator with Reduced Converter Power
Rating for Stand-Alone Energy Systems
Trapp, J.G.
Federal Institute of Education, Science and
Technology Sul-Rio-Grandense - IFSul
Campus – Venâncio Aires
Venâncio Aires, Brazil
jordantrapp@yahoo.com.br
Abstract – This paper presents a new configuration for a
wind energy conversion system (WECS) with variable speed,
using a squirrel cage induction generator (SCIG) for standalone energy system applications. The Proposed configuration
utilizes converters with reduced power rating, operates with
MPPT to maximum wind energy extraction, uses battery bank
for energy storage and can operate with non-linear and
unbalanced loads. Energy storage characteristic is desirable for
wind energy conversion systems isolated from the mains since an
energy interruption does not stop of the entire system. Also, the
power rating of the converters is reduced compared to the
generator and load power rating due to the low power
requirements of the power converters to SCIG excitation control
and to supply the load. It is presented also the whole proposed
WECS and the system operation modes according the turbine
speed. It is presented also the whole proposed system and the
system operation modes according the turbine speed. The
simulation of the proposed WECS is performed in conjunction
with the dynamic wind turbine model of 1 kW and the results
confirm the effectiveness to supply unbalanced and non-linear
loads, the converters power rating reduction and operation with
variable turbine speed. Simulation results show the effectiveness
of the system to supply unbalanced and non-linear stand alone
loads.
Index Terms – Wind energy conversion system, SCIG,
stand-alone energy system, reduced converter power rating.
I.
INTRODUCTION
Nowadays alternative energy sources are being widely
used for both grid connected systems and stand alone
applications. Among the alternative sources in recent years
wind energy stands and assumes one of the most important
rules in power systems of several countries. In addition to the
environmental issues, wind energy can be used to supply stand
alone or grid connected loads with good energy quality and
safety. The variable speed wind turbines (VSWT) are more
efficient in regarding to the fixed speed ones (FSWT) and also
present a better dynamic response. The VSWT use power
converter interfaces between load and generator and they have
higher energy production at their MPPT [1,2]. The squirrel
cage induction generator (SCIG) is suitable for alternative
energy source applications because it is cheap, has simple
Farret, F.A., Fernandes, F.T., Corrêa, L.C.,
Wechenfelder, C.M.
Energy Processing Department
Federal University of Santa Maria - UFSM
Santa Maria, Brazil
fafarret@gmail.com
construction, good power/weight ratio, low maintenance
levels, and it is robust and easily replaceable. For these
reasons, the SCIG is being strongly considered as a good
option in conjunction with VSWTs for stand alone loads [4].
Using an energy storage system the power of stand alone
systems based on VSWTs using the SCIG becomes fully
practicable for powers up to 100 kW [3]. Battery energy
storage in the converters DC link associated to the SCIG is
much explored for stand alone applications and ensures a
stable operation at the MPPT. Furthermore, the battery bank
improves voltage stability [5,6] and, in case of breakdowns or
shutdowns, the battery bank allows the DC link voltage
provide a correct system startup [3,7].
SCIG is usually associated with converters to regulate both
the generated voltage and frequency and for load interface. In
the grup of, the static compensator (STATCOM) is the most
commonly used topology, which is able to compensate
reactive loads and keep the voltage stable under steady state
and transient periods [8]. Furthermore, the STASTCOM is
able to control the active and reactive power flow between
generator, DC link and load [9]. Also, the STATCOM features
reduced power rating in regarding to the SCIG power rating,
contributing to reduced converter costs. The back-to-back
topology is extensively explored and acts to transfer active
power from generator to the load. Although this configuration
is best suited to control bidirectional power flow, the cost is
higher and the overall efficiency is lower due to the two
series-connected converters operating at system full power
[10]. Currently the compensation of harmonics caused by nonlinear loads is explored through the control of the load
side/grid side converter when the back-to-back converter is
utilized for power interface [11,12].
On the stand alone SCIG based system the generate
voltage is strongly affected by the harmonic content of the
load current [13]. In this context, the load side inverter can be
used as a shunt active filter (SAF), satisfactorily reducing the
load harmonics experienced by the generator [11]. So, power
compensation theory should be used in conjunction with the
load side inverter. The P-Q theory applied to the SAF can
properly control active and reactive powers, while
compensating non-linear and unbalanced loads [12,14].
For low wind speed, the turbine speed becomes low when
operating with MPPT. For this situation the frequency
becomes lower and the SCIG cannot supply directly the load
for fixed or quasi-fixed applications. Under variable wind and
variable turbine speed is the same. Therefore, a series
converter configuration (rectifier-inverter or back-to-back
converter) must be used to decouple frequency of the
generator and the load [2,6,10]. However, the converter power
rating in these situations cannot be reduced.
In contrast, if the turbine speed is close or equal to their
rated values and no series converter is used, the load can be
supplied directly from the generator (if the frequency of the
generated voltage is close or equal to the nominal frequency of
the load). The SCIG excitation can be controlled by a
STATCOM with reduced converter power rating, with
approximately 30% to 50% of generator rated power [16].
Also, for low wind speeds the power of the wind turbine is
below to the nominal level. In this situation, if a series
converter configuration is used, this must process about 50%
to 60% of rated turbine power, approximately, depending of
the specific turbine dynamic features.
In both situations the converters have reduced power rating
and using the STATCOM it is allowed voltage and frequency
variation within acceptable safe limits. These statements are
only valid for isolated systems, which can operate at a variable
frequency close to its nominal value without compromising
the load [4]. Furthermore, the SCIG operates with high
efficiency if the frequency varies freely with the speed [15].
This paper proposes a wind energy conversion system
based on VSWTs using SCIGs. Proposed system uses a
STATCOM with reduced converter power rating for generator
excitation and active power control. A shunt active filter based
also on reduced converter power rating to compensate
unbalanced and non-linear loads and battery energy storage
across the converters dc link is utilized. The proposed system
is suitable for stand-alone wind energy generation, operates
under MPPT mode using tip speed ratio (TSR) control, and
the maximum available active power requires low active
power of the converters to control active and reactive power
flows.
In this configuration, the SCIG is able to supply the load at
variable rotor speed and ensures load and DC link voltages
stability in all its operational modes. P-Q theory is applied to
the load side converter for operation as SAF. Simulation
results are presented and prove the effectiveness of the
proposed WECS to supply stand-alone loads under turbine
variable speed and with variable voltage and frequency closed
to nominal load values.
II.
PROPOSED WECS
The proposed WECS is based on the SCIG and VSWT and
is suitable for stand-alone energy systems. Two fully
controlled three-phase converters are utilized to control the
power flow. One of the power converters operates as a static
compensator controlling the generator excitation, generated
voltage and battery charge/discharge. The other one operates
as a shunt active filter when the load is directly connected to
the generator by a by-pass switch. Generator and load are
connected directly when the turbine is able to maintain the
minimum generator speed and, consequently, the load
frequency at minimum acceptable and safe values. These
values were defined in this paper at 55 Hz for the lower limit
and 65 Hz for the upper limit. Load disconnection occurs for
low wind regimes and, in this case, the frequency will be
below of 55 Hz. When the rotation is below the limit of direct
generator/load connection, the by-pass switch is opened and
the SAF is now operating as an inverter, setting a series
energy conversion system (back-to-back converter). When the
load side converter is operating as an inverter, the synthesized
voltage is compensated to amplitude maintenance. Fig. 1
shows the proposed system and the interconnections between
generator, dc-link, converters and load.
Fig. 2 shows a typical 1 kW wind turbine curves of power
versus turbine speed for various wind speeds. Fig. 2 is based
on the PSim wind turbine model using 1.2 m diameter blades,
655 RPM and 1 kW nominal values at 10.4 m/s wind speed.
These data were obtained from the Work Wind manufacturer,
with a turbine of 1kW available in the CEEMA – UFSM
laboratories.
Figure 1. Proposed WECS based on the SCIG.
1600
1400
Power [W]
1200
1000
800
600
400
200
0
0
100
200
300
400
500
600
700
800
900
1000
Turbine speed [RPM]
Figure 2. Power versus turbine speed of a typical 1 kW wind turbine for various wind speeds.
For different turbine aerodynamic designs, the curves of Fig.
2 may differ. The maximum power curve keeps the trend
shown in Fig. 2, but the variation rate of this curve may
change for each model of wind turbine. This occurs if one
considers that different nominal speeds are obtained for the
same wind speed regime. Typically, the nominal power
reduction at rated speed, in regarding to the available power
at minimum speed to maintain the connection generator/load
is around 40% to 50%. It is clear that the wind turbine should
be designed for desired nominal turbine and wind speeds.
Each wind farm location has different annual average wind
speeds and this should be taken into consideration to turbine
design for the correct application of the proposed system.
A. Operation Modes
Proposed wind energy conversion system operates at two
modes and is defined according to the turbine speed. The two
operation modes are shown in Fig. 3 and define the back-toback converter or STATCOM/SAF configurations. To prevent
turbine over speeds the aerodynamic or mechanical break
must be used. Fig. 3 explains the reduction of the converter
power rating and the two operation modes with variable speed.
Fig. 3 explains the reduction of the converter power rating
and the two operation modes with variable speed. In the Fig.
3, speeds below 555 RPM force the circuit to operate at mode
1, as a back-to-back converter because the minimum
frequency is no longer guaranteed. In this condition the
maximum power of converters is 60% of rated power.
Between 555 RPM and 655 RPM the load remains connected
to the generator and the wind energy system operates at mode
2. These turbine speeds correspond in the simulations to 55 Hz
and 65 Hz, respectively, and are designed to operate that way.
Above 655 RPM, energy production is limited and
aerodynamic or mechanical brake is activated to over speeds
limitation. In this case, turbine speed remains around of 655
RPM. Below 250 RPM the proposed WECS is turned off. Fig.
4 presents the power circuit configuration at two operation
modes.
1200
1000
800
600
400
200
0
0
100
200
300
400
500
600
700
Figure 3. Maximum power curve and WECS operation modes.
800
Figure 4. Proposed WECS configuration for the two operation modes.
Power limit control through turbine speed limitation is the
pitch angle control or mechanical break. Pitch angle control
can be realized in conjunction with the MPPT control and will
be discussed in the subsequent text. Mechanical break should
be used to turbine protection and auxiliary speed control at
high wind speeds.
B. Principles of the SCIG Operation
The principles of SCIG operation can be explained by
torque curves, flux calculations or by the voltage/frequency
inter relation. The magnetization curve is an important utility
and demonstrate the voltage/frequency inter relation for
various frequency and, consequently, various generator
speeds. Fig. 5 shows magnetization curves of the SCIG used
in this paper and described in the Appendix. These curves are
detailed for better analysis in the region of operation.
Correspondent generator speed for 60 Hz is 1800 RPM for the
4 pole machine presented in the Appendix. On Fig. 5, if the
STATCOM synthesizes 335 V at 60 Hz and 1800 RPM, the
operation point is P3. In this case, the available SCIG
mechanical torque is the point T3 on the Fig 6.
Figure 5. Proposed WECS configuration for the two operation modes.
The point T3 determines the operation of the induction
machine as a motor because the torque is positive. However, if
the frequency of the STATCOM synthesized voltage is
reduced to 57.5 Hz, keeping the speed and voltage constants,
the operating point moves to the point P1 in Fig. 5 and T2 in
Fig. 6, setting operation of the induction machine as generator.
If the frequency is reduced to 55 Hz, has the operating points
P2 and T2. In this case, the available torque at SCIG remains
negative and becomes larger. That is, reducing even more the
frequency and maintaining the voltage and speed unchanged,
it is possible to extract more active power from generator. The
v/f control is explored in this article for the STATCOM.
However, for the same turbine speed, STATCOM synthesized
voltage is kept constant and only the frequency is changed
according to the turbine available power. This power is used
as reference to STATCOM control and is obtained from the
TSR-MPPT method. The control implemented for the two
converters is detailed in the following section.
III.
STATCOM, SAF AND INVERTER CONTROL
Proposed system utilizes all measured variables shows in
Fig. 1. STATCOM control is based on SCIG speed and
turbine available power. The speed is obtained by an encoder
at generator shaft. Available turbine power is obtained from
TSR-MPPT method, where the tip speed ratio is known and
generator speed is measured. TSR-MPPT method maintains
the wind turbine at optimal tip speed ratio and is guaranteed
the maximum energy extraction [17]. However, the maximum
power curve must be known. This paper makes the available
power calculation of the wind turbine using the maximum
power curve stored in a lookup table.
The load side converter operates as inverter or as shunt
active filter. Therefore, the inverter control and SAF control is
applied alternately to the converter, according to the generator
speed.
A. STATCOM Control
Actual synthesized voltage by STATCOM and frequency
can be calculated and compensated to control correctly the
SCIG. Through the anemometer, available turbine power Pturb
is obtained from a lookup table saved in DSP control unit.
Equation (1) calculates the actual tip speed ratio.
λ act =
RPM turb
0.1047 ⋅ Rturb ⋅ V w
(1)
where RPM turb is the turbine shaft speed, Rturb is the blades
length and Vw is the wind speed.
Turbine speed is calculated by (2).
RPM turb = RPM SCIG GBratio
Figure 6. Proposed WECS configuration for the two operation modes.
(2)
where RPM SCIG is the generator speed and GBratio is the
gear box ratio connected between generator and turbine.
Result of (2) is compared with the optimum tip speed
ratio λ opt , which results in actual turbine power error, given
by (3).
(
)
K ⎞
⎛
Perror = λ opt − λ act ⋅ ⎜ K p + i ⎟
S ⎠
⎝
(3)
The reference power to STATCOM control is given by (3).
Pref = Pturb − Perror
(3)
Using the Clarke transform, voltages and currents of the
generator are changed to dq axis. Through (4), the reference
voltage Vd _ ref is calculated.
Vd _ ref =
RPM SCIG ⋅ 380
RPM nom
(4)
where RPM nom is the generator nominal speed shown in the
Appendix.
By comparing the reference voltage calculated in (4) with
the measured voltage in dq axis, the voltage compensation to
the STATCOM PWM modulator is obtained. The variable
Vq is compared with zero to correctly reference generation.
From the speed RPM SCIG is calculated the frequency of the
generator in relation to its actual speed, given by (5).
f =
RPM SCIG ⋅ 60
RPM nom
(5)
The generator power is calculated by (6).
(
)
Pmed = Vd ⋅ I d + Vq ⋅ I q ⋅ 3 / 2
(6)
From the comparison between Pref and Pref , the power
drained from induction generator is compensated by
frequency through ∆f . Thus, the control acts on the
generator torque by frequency variation, as in Fig 6. The
reference frequency is given by (7).
f ref = f − ∆f
(7)
Through a resettable integrator is generated the reference
angle for controlling the STATCOM from f ref . This angle is
used in the Clarke and Park transform, making control simple
and with better dynamic response. Fig. 7 shows the block
diagram of the STATCOM control.
Va* , Vb* and Vc* are the compensation variables which are
used in the STATCOM PWM modulator.
Figure 7. Block diagram of the STATCOM control.
B. SAF Control
The shunt active filter control uses the PQ theory for
harmonics and phase unbalance compensation, and is similar
to the theory presented in [14]. However, it also uses the
control of active power to charge the battery bank. This
characteristic is exploited to limit the active power of the
STATCOM. Thus, the maximum power processed by the
STATCOM is 600 W, while the maximum power transferred
to the battery bank by SAF is 400 W. Through (8) is
calculated the SAF active power.
PSAF _ dc = Vdc ⋅ I SAF _ dc
(8)
where Vdc is the DC bus voltage of the converters and
I FA _ dc is the battery current of charge through SAF.
By comparing the reference power with maximum active
power drained to the dc link by the STATCOM, is obtained
the active power reference to control the SAF, given by (9).
PSAF = Pref − 600
(9)
Power PSAF is limited from zero up to 400 W. Thus, total
active power drains the battery bank does not exceed the
maximum total power of the wind turbine. The compensation
variable for the shunt active filter is determined through (10).
Pref _ SAF = Pref − PSAF _ dc
(10)
Applying the PQ theory and using the oscillating powers
~
~
P and Q are calculated the reference currents for the active
filter compensation, through (11) and (12).
Iα =
Iβ =
~
~
P ⋅ Vα + Q ⋅ Vβ
Vα2 + Vβ2
~
~
P ⋅ Vβ − Q ⋅ Vα
Vα2 + Vβ2
(11)
(12)
However, the SAF must be drain active power. Thus,
simply changing (11) and adding the compensation power
defined in (10) with the oscillating power of PQ theory.
Therefore (11) is rewritten to (13).
Iα =
(P~ + P
)
ref _ SAF ⋅ Vα
Vα2 + Vβ2
~
+ Q ⋅ Vβ
Figure 9. Block diagram of the inverter control.
The compensation currents in abc coordinates for the
PWM modulator are determinate using Park transform. Fig. 8
shows the block diagram of the P-Q theory based control
applied to SAF control.
(13)
Comparing the reference currents (12) and (13) with the
output currents of the SAF, are obtained compensation
currents I α* e I β* .
C. Inverter Control
The load side converter operates as inverter and the
synthesized output voltage remains at fixed voltage value
under load variation and wind turbine lower speeds. Fig. 9
presents the inverter control. The inverter control uses a
classical control implemented in dq axis, where the d axis
voltage Vd _ inv is compared with a constant value. The result
is two compensated variables in dq axis, Vd _ inv * and Vq _ inv * .
Frequency of synthesized voltage is constant and is equal to
the minimal frequency defined for the generator/load
connection. Furthermore, f inv is equal to 55 Hz. This
frequency determines the control angle generated by the
resettable integrator.
IV.
Figure 8. Block diagram of the SAF control.
SIMULATION RESULTS
The proposed system simulations were performed using
the dynamic model of squirrel cage induction machine
presented in [18]. This dynamic model presents magnetic core
saturation and is suitable for induction machine simulations as
generator. In conjunction with de SCIG was used the wind
turbine model of PSim software. This model uses the wind as
input variable and turbine maximum power curve storage in a
lookup table. Use of these two models allows a complete
simulation of wind energy conversion systems, including
converters and control. With variable wind, the variable
turbine speed and variable available power of the turbine are
obtained. Fig. 10 (a) shows the wind speed pattern used in the
simulations. As expected, the turbine and, consequently,
generator speed, varies according to the wind, but with a low
difference due to the inertia of the turbine and generator. This
result is show in Fig 10 (b). As expected, the reference power
of the wind turbine presented in Fig. 10 (c) changes according
to the wind speed and turbine speed.
Figure 10. Simulation results for: (a) wind speed in m/s, (b) generator speed in
RPM and (c) turbine reference power in W.
As shown in Fig. 10, the available power correlation between
wind speed and turbine speed correspond to the Fig. 2. Fig.
11(a) presents, in concordance, the turbine available power
and SCIG active power.
Figure 12. Simulation results for: (a) generated voltage in V, (b) STATCON
current in A, (c) SAF current in A, (d) load current in A and (e) battery current
charge/discharge in A.
It is observed in Fig. 11(b) that the aerodynamic brake is
effective to control the turbine over speeds. This over speed
limitation is reflected to the STATCOM reference voltage, as
presented in Fig. 11(e). Fig. 11(d) shows the ∆ f actuation to
control the SCIG power.
Figure 11. Simulation results for: (a) turbine reference power in W, (b) SCIG
power in W, (c) frequency of the generated voltage in Hz, (d) ∆f and (e) SCIG
peak voltage in V.
Figure 13. Simulation results for: (a) generated voltage in V, (b) SCIG current
in A, (c) load current in A.
Fig. 12(a) presents the voltage synthesized by
STATCOM. This voltage is according with the generator
speed. As presented in Fig. 10, all results show the
effectiveness of the TSR-MPPT control to maintain the wind
turbine at maximum power point. The proposed system also
operates satisfactorily to control the generated voltage and the
battery and load powers. The SCIG power is detailed in Fig.
11 and Fig. 12. Fig. 13 shows detailed results of the generated
voltage, SCIG current and load current, proving the
effectiveness of the proposed configuration to compensate
non-linear and unbalanced loads. Simulation results prove the
effectiveness of the proposed system to supply isolated loads
and to control the active power at load and battery bank.
V.
REFERENCES
[1]
[2]
[3]
[4]
[5]
CONCLUSIONS
A new configuration for an energy conversion system
using variable speed wind turbines to supply isolated loads
was presented. According to Fig 2 and Fig. 3 and by
simulations, it is clear the active power reduction that the
interconnection between load and generator provides for
frequencies above 55 Hz up to 65 Hz. The simulation results
prove the converters active power control according of
STATCOM, SAF and Inverter controls. Furthermore, adopted
control remains the wind turbine at maximum power curve. It
is observed in simulation that the voltage and frequency at the
load remains within of safe limits. Proposed energy
conversion system is adequate to supply isolated loads with
good energy quality and low cost. It also ensures excellent
energy production by using the MPPT in all modes of
operation.
[6]
[7]
[8]
[9]
[10]
ACKNOWLEDGMENT
The authors are grateful to CNPq, CAPES, Post-Graduation
Program in Electrical Engineering - PPGEE of the Federal
University of Santa Maria for their financial support and to
CEEMA-UFSM for the use of all its laboratorial infra-structure.
[11]
[12]
APPENDIX
Machine specifications
Rated voltage:
Rated current:
Frequency:
Rated speed:
Rated Power:
[13]
380 V (Y connected)
2.3 A
60 Hz
1760 RPM
1 kW
[14]
Parameters
[15]
Stator, rotor inductances (Ls, Lr): 0.0022 H, 0.0022 H
Stator, rotor resistance (Rs, Rr): 4.2 Ω, 4.32 Ω
TABLE I.
Current
[A]
0.00
0.08
0.12
0.18
0.26
0.35
[16]
MAGNETIZATION CHARACTERISTIC OF THE INDUCTION
MACHINE (RMS VALUES PER FASE).
voltage
[V]
0.00
22.73
33.90
50.50
72.30
96.40
current
[A]
0.52
0.71
0.85
0.97
1.00
1.05
voltage
[V]
137.20
176.80
201.20
220.60
225.60
229.80
current
[A]
1.23
1.31
1.65
1.96
2.30
3.00
voltage
[V]
251.20
262.00
290.00
311.00
328.60
360.50
[17]
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