Variable Speed Wind Energy System using Squirrel-cage
Induction Generator
Surabhi Motghare1, Debanshu Das2, Deepanshu Gupta3, J.O.Chandle4
Department of Electrical Engineering, V.J.T.I., Mumbai, India
surabhimotghare@gmail.com1, debanshudas1992@gmail.com2
deepansu.gupta01@gmail.com3, jochandle@vjti.org.in4
Abstract-- This paper presents the
Hybrid energy systems (renewable coupled
fundamental
the
with conventional energy source) can
design of variable speed wind energy
significantly reduce the total life cycle cost
system using squirrel-cage induction
of a stand-alone system in many off-grid
generator and study the feasibility of
situations,
integrating this system with the main
providing a reliable supply of electricity
grid. This will facilitate reliability in
using a combination of energy sources.
considerations
for
supply with the added advantage of
harnessing wind energy to produce
electricity.
while
Renewable
at
energy
the
same
sources
time
have
attracted considerable interest due to
increments in cost, limited availability and
Index Terms—Wind energy, Renewable
adverse environmental impacts of fossil
energy,
fuels.
Hybrid
energy
systems,
In
addition,
technological
advancements and government incentives
Variable speed SCIG.
have provided a boost to the use of
renewable energy. As such, wind energy is
I. INTRODUCTION
Owing to the advent of Smart Grid and
synchronized
technology,
phasor
hybrid
power
measurement
generation
systems are gaining importance in the
gaining
importance
and
obtaining
electricity from wind offers the cheapest
economic
perspectives
of
renewable
energy sources, wherein, grid connected
induction generators are very popular. [1]
world energy scenario. These hybrid
Variable speed wind electric generators
power plants aim to overcome site specific
are a popular choice in the market because
bottlenecks of individual power plants.
of their capability to extract more energy
than
fixed
speed
machines,
reduced
III. CONFIGURATION OF
mechanical stress and aerodynamic noise.
The robust, relatively maintenance-free
and cheap induction machines have long
been used as a good choice as the electrical
generator in wind electric generation
systems, although those are fixed speed
systems. [2]
VARIABLE SPEED SCIG
In
variable-speed
squirrel
cage
induction generator (SCIG) wind energy
conversion systems (WECS), full-capacity
power converters are used to adjust the
speed of the generator in order to harvest
the maximum possible power available
II. WIND TURBINE
from the wind. In this type of system, a
TECHNOLOGIES
back-to-back power converter is usually
Wind kinetic energy is transformed into
mechanical energy by a wind turbine that
has several blades mounted on the rotor
hub. [3] The rotor hub is installed on the
low speed shaft (main shaft). The gearbox
matches the low speed of the turbine to the
higher speed of the generator. The
employed as shown in Fig.2. The system
comprises of the following components: (i)
horizontal axis wind turbine, (ii)gearbox,
(iii) three phase SCIG, (iv) generator side
converter, (v) DC link capacitor, and, (vi)
grid side converter synchronised to a three
phase grid.
mechanical energy is transmitted through
the drive train consisting of shafts,
bearings, and gearbox, and then to the
generator, which converts mechanical
energy into electric energy.
This conversion is usually assisted by a
power converter system which delivers the
Fig.1. Wind turbine connected to the grid
power from the generator to the grid
(Fig.1).
Most
of
the
wind
turbine
components are enclosed in a nacelle on
top of the tower. [4]
Fig. 2. Typical configuration of variable speed SCIG
wind energy system
The generator-side converter (rectifier)
and a condenser between both. The
is used to control the speed or torque of the
rectifier and the inverter are composed
generator whereas the grid-side converter
using suitable power electronic devices.
(inverter) is employed for the control of
[5]
DC link voltage and grid-side reactive
IV. PROBLEM STATEMENT
power.
If the induction generator is connected
directly to the grid, then a capacitor bank
is connected in stand-alone systems to the
stator to provide the magnetizing current
for the reactive power. However, for grid
connected systems the reactive power is
drawn from the grid. Induction generators
are able to use back to back converters.
This leads to the formation of several
configurations. The converter can be
coupled
to
the
SCIG,
because
this
decoupling of the generator and the grid
allows
us
to
work
with
different
frequencies on each side. Therefore, the
usual operation topology of SCIG can be
The purpose of using wind turbine
generators is to generate power using the
kinetic energy of the wind and supply this
useful power to the grid. The speed of the
rotor (ωr) of the squirrel-cage induction
machine is a related to the synchronous
speed (Ns). The speed of a squirrel-cage
induction machine can be maintained
constant with a small variance of slip (s).
For
rotor
speed
greater
than
synchronous speed, the turbine operates in
the generating mode. On the other hand,
for rotor speed less than the synchronous
speed, the turbine operates in the motoring
mode.
changed due to the advantage of working
-
For ωr > Ns
Generation
with
-
For ωr < Ns
Motoring
different
frequencies
without
worrying about the rotor speed. This
enables a variable-speed operation using
the SCIG. To take advantage of the
variable
speed
operation
a
power
electronic interface must be provided
between the machine terminals and the
grid. The back to back converter is a
suitable option for cage induction machine
in wind power application. The back to
back is formed by a rectifier, an inverter
the
This implies that during the generating
mode, the machine supplies power to the
grid whereas during the motoring mode,
the machine consumes power from the
grid. Our aim is to utilise the generation
capacity and prevent the occurrence of
motoring action. In order to achieve this,
two cases are possible: either the rotor
speed
must
be
increased
or
the
synchronous speed must be decreased with
respect to the rotor speed.
V. THE SIMULATION
A
squirrel-cage
induction
generator
Case (i): The rotor speed can be improved
connected to the grid through a back-to-
by
algorithm
back power electronic convertor (PEC) has
(Maximum Power Point Tracking). To
been modelled on SIMULINK (Fig.3). The
harvest the maximum amount of energy
phasor-type induction generator available
from the wind, the wind turbine must have
in the SIMULINK library has been
a particular rotation speed to maintain the
employed. The various parts of the
optimum tip-speed ratio. The purpose of
subsystem have been modelled so that a
the MPPT is to maintain the tip-speed ratio
Discrete POWERGUI can be employed. A
of the wind turbine as close as possible to
three-phase AC-DC-AC PWM generator
the optimal tip-speed ratio.[6]
has been created to model the PEC. The
employing
the
MPPT
Case (ii): In order to decrease the
synchronous speed with respect to the
rotor speed, electrical control is required.
In this scheme, we aim to set the
synchronous speed at a value which will
always be lower than the speed at the shaft
of the generator corresponding to the
current wind speed. This will ensure that
generation action takes place and the
machine supplies power to the grid.
The objective is
to
successfully
achieve Case (ii) in conjunction with Case
(i), that is, reduce the synchronous speed
so as to achieve generating action for any
wind speed at any point of time.
AC-DC-AC system can accept variable
frequencies
from
the
turbine
side.
However, it produces a fixed 50 Hz supply
on
the
grid-side.
Fig.4.
shows
the
simulation results for real power (P) and
reactive power (Q) at the output of this
system, along with the speed of the turbine
(ωr), the wind speed (ω1), and the pitch
control. The FFT analysis for different
input frequencies is shown in Fig.5 and
Fig.6.
Hence by varying the generator side
PWM
frequency,
the
effective
synchronous speed is kept below the
generator speed at the shaft corresponding
to the current wind speed.
VI. ADVANTAGES OF USING
THIS SCHEME
Generation of power using a variable
speed scheme facilitates better energy
capture as compared to the fixed speed
scheme. SCIG is preferred because it is
Fig. 3. SCIG connected to the grid through a back-toback PEC
mechanically simple, has high efficiency
and low maintenance cost. Variable speed
SCIG wind energy system offers strengths
like
simple,
rugged,
and
robust
construction, as well as easy maintenance.
Moreover, the power factor can be
controlled over a wide range. In addition,
the wind gusts, which can be buffered due
to the mechanical inertia, can also be
buffered in the energy storage of the PEC.
Another significant advantage of this
Fig.4. Simulation results for real power (P) and
reactive power (Q) on the grid-side
scheme is that the intermediate PEC
decouples the SCIG from the grid. This
decouples the SCIG from the disturbances
of the grid and vice-versa. [7] The size of
the PEC used depends on the stator ratings
and thus determines the cost of the PEC.
Although, the high cost of PEC is a
limitation of using this scheme, the scheme
Fig.5. FFT analysis of output signal for of 40-50 Hz
offers excellent benefits to improve the
generation of power using the wind
energy.
VII.
CONCLUSION
A complete model for variable speed wind
energy
system
using
squirrel
cage
induction generator connected to the
Fig.6. FFT analysis of output signal for 60-50 Hz
electric grid has been modelled and
2008 – May 31, 2011, The University
developed in SIMULINK. The advantages
of Texas at Austin, Texas, October
of using such a system have been outlined
2011
and the output waveforms have been
obtained which show power generation for
a wide range of wind speeds. The source
and load waveforms for the proposed
power electronic converter to make the
configuration
possible
have
been
generated. The results of the FFT analysis
conducted have shown that the grid side
[4] Bin Wu, Yongqiang Lang, Navid
Zargari,
Samir
Kouro,
Power
Conversion and Control of Wind
Energy Systems, Institute of Electrical
and Electronics Engineers (IEEE), Inc,
2011, John Wiley & Sons, Inc.,
Hoboken, New Jersey.
frequency for various generator side
[5] JL Dominguez Garcia, “Modelling and
frequencies is maintained at 50Hz with the
control of Squirrel Cage Induction
least Harmonic Distortion.
Generator with Full Power Converter
applied to windmills” Master thesis,
REFERENCES
Universitat Politècnica de Catalunya,
[1] A.B.Raju, B.G. Fernandes, Kishore
Chatterjee, “Modelling and Simulation
of a Grid connected Variable Speed
Wind Energy Conversion System with
Low Cost Power Converters”, Dept. Of
Electrical Engineering, Indian Institute
of Technology-Bombay, Mumbai.
[2] N.G. Greeshma and Sasi K. Kottayil,
“Active Power Control in Wind Driven
Variable
Speed
Squirrel-Cage
Induction
Generator”
Bonfring
International Journal of Power Systems
and Integrated Circuits, Vol. 1, Special
Issue, December 2011
[3] Mohit
Singh,
Surya
Santoso,
“Dynamic Models for Wind Turbines
and Wind Power Plants” January 11,
Spain.
[6] Márton Örs, , “Maximum Power Point
Tracking
for
Small
Scale
Wind
Turbine With Self-Excited Induction
Generator”, Technical University of
Cluj-Napoca Department of Automatic
Control, Romania Vol.11, No.2, pp.
30-34, 2009
[7] Joshua Earnest, Tore Wizelius, Wind
Power
Plants
and
Project
Development, PHI Learning Pvt. Ltd.,
2011