Bonfring International Journal of Power Systems and Integrated Circuits, Vol. 1, Special Issue, December 2011
56
Active Power Control in Wind Driven Variable
Speed Squirrel-Cage Induction Generator
N.G. Greeshma and Sasi K. Kottayil
Abstract--- This paper describes active power control in a
grid connected variable speed wind electric generation system
(WEG) using squirrel cage induction machine. The chosen
variable speed WEG system consists of a wind turbine,
squirrel-cage induction generator, an AC-DC-AC interface
and the power grid. In this work the generator side converter
is controlled using vector control of induction machine and
the grid side converter is controlled using DC link voltage
control. Active power delivered by the WEG to the grid when
driven by a wind speed can be controlled, albeit within a
limited range, by varying the DC link voltage at the input of
the grid side converter. The WEG scheme is simulated using
system models in MATLAB-Simulink and the performance is
studied.
Keywords--- Wind Electric Generation System, SquirrelCage Induction Machine, AC-DC-AC Converters
I.
INTRODUCTION
I
NITIAL interest in renewable energy such as wind energy,
solar energy, fuel cell, tidal power and geothermal power is
due to the oil crises of the 1970s and fear of resource
depletion and political insecurity resulted in frenetic research
and development activity, impressive technological and bold
energy policy experiments. Among these, wind power
generation is relatively economic and hence developed
commercially. [12], [2],[11],[13]
connected variable speed WEG that employs squirrel cage
induction generator (SCIG), popularly known as VSIG. The
chosen system consists of an AC-DC-AC asynchronous link
(Comprised of a pair of DC linked converters) between the
SCIG and the grid. In this work the generator side converter
is controlled by vector control of induction machine and the
grid side converter is controlled by DC link voltage.
For a given wind speed, the DC link voltage decides the
active power transfer to the grid, however, subject to the rotor
speed and the related parameters. At any operating condition
the system will choose a rotor speed such that the power flow
equilibrium is maintained.
The system model is first developed by use of
characteristic equations of all the components and then
operation under a steady wind speed is simulated; regulation
of active power flow is obtained by varying DC link reference
voltage.
II.
THE VSIG SYSTEM
The VSIG is shown in Fig.1. The following are the system
components: (i) horizontal axis wind turbine, (ii) gear, (iii)
three phase SCIG, (iv) generator side converter, (v) DC link
capacitor, and, (vi) grid side converter synchronised to a three
phase grid.
Variable speed wind electric generators (WEGs) are
popular in the market because of their capability to extract
more energy than fixed speed machines, reduced mechanical
stress and aerodynamic noise. Induction generators operated
by vector control techniques have fast dynamic response and
accurate torque control which are advantageous in variable
speed operation [2], [3],[4], [6], [7], [8], [9].
The robust, relatively maintenance-free and cheap
induction machines have long been used as a good choice as
the electrical generator in WEG systems, albeit those are
fixed speed systems. The vector or field-orientated control of
induction generator yields high dynamic performance ideal
for variable-speed WEG systems too [9].
Figure 1: Schematic Diagram of VSIG
This paper presents active power control in a grid-
In Fig 1, P is active power, Q is reactive power, ωr is rotor
speed of SCIG, Idc is the output current of the generator side
converter, Vabc and Iabc are respectively the output voltage and
current of the grid side converter and Vdc is the voltage across
the DC link capacitor.
N.G. Greeshma, Department of EEE, Sree Narayana Guru College of
Engineering
and
Technology,
Payyanur,
India.
E-mail:
greeshmang0@gmail.com
Sasi K. Kottayil, Department of EEE, Amrita School of Engineering,
Amrita
Vishwa
Vidyapeetham,
Coimbatore,
India.
E-mail:
kk_sasi@cb.amrita.edu
Here the active power is flowing from the wind turbine to
the grid and reactive power from the grid to the generator. A
pair of back to back PWM converters is connected between
the SCIG and the grid for asynchronous operation. That
facilitates the variable speed operation of the wind power
generation system. The variable frequency variable voltage
ISSN 2250 – 1088 | © 2011 Bonfring
Bonfring International Journal of Power Systems and Integrated Circuits, Vol. 1, Special Issue, December 2011
power from the generator is rectified by the generator side
converter. This converter also supplies the magnetisation
current of the machine. The supply side converter supplies the
generated power to the utility grid.
III.
57
The DC link voltage control loop regulates the DC link
voltage to a predefined value Vdc*. The output of the DC link
voltage control loop is the id*, the reference for the converter
current control loop.[5]
MODELING OF WEG
The function of the wind turbine (WT) is to convert the
linear motion of the wind into rotational one that can be used
to drive a generator. Wind turbines capture the power from
the wind by means of aerodynamically designed blades and
convert it into rotating mechanical power. The amount of
mechanical power captured from wind by the turbine could be
formulated as,
(1)
where, PT is the power output of WT in W, CP is the
power coefficient of the WT and it depends on the
aerodynamic characteristics of the blades, is the air density
in kg/m3, A is the sweep area of WT and v is the wind speed
in m/s [1].
Figure 2: CP -λ Characteristics of a Wind Turbine for a given
β
The wind turbine can be characterized by its Cp -λ curve
as shown in Fig.2 where λ is the tip speed ratio and is defined
as the ratio between the linear speed of the tip of the blade to
the wind speed. That is,
(2)
where, ω is the turbine rotor speed in rad/s, R is the radius
of WT in m.
Cp of a WT may be expressed as a function of λ as well as
β, the blade pitch angle in degrees [14].
(3)
The inverter output current is converted from abc to dq,
here the d-axis has the active component and q-axis the
reactive component. Therefore the inverter current
component id is used to control the active power (through DC
link voltage), while iq is used to control the reactive power.
For unity power factor operation iq* is maintained at zero.
(4)
The outputs of current regulators are vd and vq. Using
these voltages three phase modulating signals are generated.
These are the input to the PWM generator, which supplies the
gate signals that drive the grid side converter.
where,
(1) to (4) together form the model of WT and it is
implemented in MATLAB Simulink for the present study.
The SCIG model is chosen from Simulink library. A gear
transmission ratio of 1:6 is used between WT and SCIG. The
system specifications are given in Appendix.
IV.
Figure 3: Grid Side Converter Control
GRID SIDE CONVERTER CONTROL
Fig.3 shows the grid side converter control scheme
adopted for the VSIG system. It contains two PI control loops:
a DC link voltage control loop and a converter current control
loop.
V.
GENERATOR SIDE CONVERTER CONTROL
Fig. 4 shows the generator side converter control scheme
for VSIG. The three phase stator currents are converted to
direct axis and quadrature axis components with the help of
abc to dq transformation. These are then compared with the
corresponding reference values.
Idref sets the machine flux level which is maintained
constant. For power flow control, Idref is derived from
generator side converter output current. Iqref is derived from
generator speed. The two control loops provide vd and vq
ISSN 2250 – 1088 | © 2011 Bonfring
Bonfring International Journal of Power Systems and Integrated Circuits, Vol. 1, Special Issue, December 2011
58
respectively. Using these voltages three phase modulating
signals are generated and sent to the PWM generator that
provides gate signals to drive the generator side converter.
Figure 7: Generator Output Power of VSIG for v=12m/s and
Vdc*=800V
Figure 4: Generator Side Converter Control
VI.
SIMULATION RESULTS
The system operation is simulated for different wind
speeds and the feasibility of active power control by varying
DC link voltage is checked. The results confirmed that active
power control is feasible within certain range as decided by
WT characteristics.
Fig.5 shows the SIMULINK model of VSIG. Fig.6 to
Fig.9 show the results obtained from the simulated operation
of the system for a constant wind velocity of 12m/s while Vdc*
is kept at 800V.
Figure 5: SIMULINK Model of VSIG Induction Machine
Figure 8: Voltage across DC Link Capacitor for v=12m/s and
Vdc*=800V
Figure 9: Active Power Supplied to Grid for v=12m/s and
Vdc*=800V
The system behavior with variation in DC link voltage
while wind speed is maintained constant is evident from
Table 1, in which changes in various parameters like WT
power output (PT), generator shaft speed (N), λ, Cp, power fed
to the grid (Pg) are given at the wind speed of 12 m/s while
Vdc* is varied from 700 V to 900 V.
Table 1: Performance of VSIG with different Vdc* for v
=12m/s
λ
N
(rpm)
2490
0.2171
4.635
1517
2500
0.2185
4.646
1521
2450
0.2148
4.616
1511
Pg
(W)
(W)
700
3119
800
3138
900
3085
(V)
Figure 6: Rotor Speed of SCIG for v=12m/s and Vdc*=800V
Cp
PT
Vdc*
The system adopts a different shaft speed in each case in
order to establish power flow equilibrium, as a result of which
ISSN 2250 – 1088 | © 2011 Bonfring
Bonfring International Journal of Power Systems and Integrated Circuits, Vol. 1, Special Issue, December 2011
λ and Cp vary; variation in PT is because of that in Cp. The
difference between PT and Pg is the total power loss in SCIG
and the link.
VII.
CONCLUSION
Dynamic modelling and simulation of a grid connected
variable speed WEG using squirrel cage induction machine
has been carried out. The simulation results suggest that the
DC link voltage control is a feasible method to control active
power in a grid connected WT-SCIG system. This method
can be further extended to develop maximum power tracking
in VSIG for wind speeds below the rated wind speed. The
method will also be useful for restricting generation when
wind speed goes high during periods of lean demand, as
stipulated in grid code for wind farms; compared to pitch
angle control the electronic control is faster and cheaper.
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APPENDIX: VSIG DESIGN PARAMETERS
Sweep diameter of WT : 4.2m
Rated Power of SCIG
: 4kW
Terminal voltage of SCIG : 400V
Gear ratio
: 1:6
ISSN 2250 – 1088 | © 2011 Bonfring
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