wind energy conversion system with enhanced power

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International Journal For Technological Research In Engineering
Volume 2, Issue 8, April-2015
ISSN (Online): 2347 - 4718
WIND ENERGY CONVERSION SYSTEM WITH ENHANCED
POWER HARVESTING CAPABILITY USING MLI FOR
HARMONIC REDUCTION
1
2
Devi M , Kapil Dev P
1
2
Associate Professor, PG Scholar
Department of EEE, Thangavelu Engineering College, Chennai-600 097
Abstract: This paper proposes a new technique for
enhancement of energy extraction capabilities in wind
energy conversion systems with low cut-in speed. The dclink voltage is maintained by the grid-side converter in wind
energy systems equipped with back-to-back three-phase
inverters. This voltage is required to be higher than a
certain value to ensure proper operation of the grid-side
converter. On the other hand, at low generator voltages,
switching times for the generator-side converter cannot be
realized due to practical limitations. Accordingly, the system
cannot harvest energy at low cut-in speeds. A power
electronics system consisting of an isolated SEPIC
converter along with an upper-hand control scheme has
been introduced and employed to alleviate the
aforementioned power extraction issue. The proposed
solution, allows for excellent power extraction even at low
cut-in speeds by maintaining an appropriate dc-link voltage
at various operation conditions. Therefore increasing the
overall renewable generation capability in wind-energy
systems. The proposed solution can be incorporated in
existing wind energy conversion systems with back-to-back
three-phase inverters with slight hardware and software
modifications. The integrated isolated SEPIC converter
handles a fraction of the rated power, therefore leads to
reduced cost and size compared to existing systems with
integrated full-rated boost converters.
I. INTRODUCTION
Wind power is the conversion of wind energy into a useful
form of energy, such as using wind
turbines to
produce electrical
power, windmills
for
mechanical
power, wind
pumps for water
pumping or drainage,
or sails to propel ships. Large wind farms consist of hundreds
of individual wind
turbines which are connected to
the electric
power
transmission network. For
new
constructions, onshore wind is an inexpensive source of
electricity, competitive with or in many places cheaper than
fossil fuel plants. Offshore wind is steadier and stronger than
on land, and offshore farms have less visual impact, but
construction and maintenance costs are considerably higher.
Small onshore wind farms can feed some energy into the grid
or provide electricity to isolated off-grid locations. A wind
farm is a group of wind turbines in the same location used for
production of electricity. A large wind farm may consist of
several hundred individual wind turbines distributed over an
extended area, but the land between the turbines may be used
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for agricultural or other purposes. A wind farm may also be
located off shore. Almost all large wind turbines have the
same design a horizontal axis wind turbine having an upwind
rotor with three blades, attached to a nacelle on top of a tall
tubular tower. In a wind farm, individual turbines are
interconnected with a medium, power collection system and
communications network. At a substation, this mediumvoltage electric current is increased in
voltage with
a transformer for connection to the high voltage electric
power transmission. Induction generators, often used for
wind power, require reactive power for excitation so
substations used in wind-power collection systems include
substantial capacitor banks for power factor correction.[28]
Different types of wind turbine generators behave differently
during transmission grid disturbances, so
extensive
modeling of the dynamic electromechanical characteristics
of a new wind farm is required by transmission system
operators to ensure predictable stable behavior during system
faults. In particular, induction generators cannot support the
system voltage during faults, unlike steam or hydro turbinedriven synchronous generators.
Doubly fed machines
generally have more desirable properties for grid
interconnection. Transmission systems operators will supply
a wind farm developer with a grid code to specify the
requirements for interconnection to the transmission grid.
This will include power factor, constancy of frequency and
dynamic behavior of the wind farm turbines during a system
fault.
II. EXISTING SYSTEM
Renewable energy sources have been extensively deployed
in the last decade to reduce the reliance of electric power
generation on fossil fuel. Wind energy posses a significant
share in renewable power generation. Application of power
electronics converters is the state-of-the-art solution for
energy harvesting from wind turbines. The back-to-back
three-phase bridge inverter has been extensively used in this
area, particularly with induction and permanent magnet
synchronous generators. In this topology, the power
electronics interface has a grid-side and a generator-side
inverter. The generator-side inverter is responsible for
extraction of maximum available power from the source and
to ensure safe operation of the generator. The extracted
power is injected to the dc-link. The grid-side inverter injects
this power to the grid by regulating the dc-link voltage. The
grid-side inverter is also responsible for maintaining the
power quality standards regulated by the utility grid. The
Copyright 2015.All rights reserved.
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International Journal For Technological Research In Engineering
Volume 2, Issue 8, April-2015
grid-side converter usually operates at unity power factor
operation condition. Since, the grid voltage is regulated by
the utility network, the dc-link voltage, should not be less
than a certain value to ensure proper operation of the gridside inverter. This value depends on the maximum amplitude
of the grid-voltage and also the switching algorithm.
Theoretically for space-vector modulation the dc-link voltage
should not be less than the peak maximum value of the lineto-line ac voltages. Considering the practical limitations such
as turn-on and turn-off times of the switches the dc-link
voltage has to be maintained slightly larger than the
theoretical value. The same principle governs the operation of
the generator-side inverter. The wind generator provides a
small voltage at the cut-in speed of the wind turbine and it
has to be boosted up to a much larger value which is the dclink voltage. This requires a high conversion ratio for the
generator-side converter as it has to operate with the dc-link
voltage that is dictated by the grid-side converter. Due to the
practical limitations such as turn-on and turn-off times of the
switches the duty ratio required to boost the small cut-in
speed voltage to the dc-link voltage is not achievable with the
back-to-back three-phase bridge topology. Therefore, the
energy conversion system will not be able to harvest energy.
This indeed restrains the power generation capability of the
system. Fully-rated boost converters are employed at the dclink to deal with this issue. This solution imposes increased
cost and size for the wind energy systems. This project
introduces a new power electronics and control solution to
provide an appropriate dc-link voltage for the grid-side
converter even under very low generator speeds. The
proposed solution is based on integrating an isolated SEPIC
converter to maintain proper dc-link voltage for three-phase
six-switch inverters. It is shown that this converter needs to
handle a fraction of the rated power, which significantly
reduces the cost and size of such system compared to when a
fully rated boost converter is employed.
Limitations
 At low generator voltages, switching times for the
generator-side converter cannot be realized due to
practical limitations. 

 The system cannot harvest energy at low cut-in
speeds. 

 Cost and size of the existing systems is high. 
The existing power electronics system consists of a currentcontrolled isolated SEPIC converter and an upper-hand
power management scheme to provide proper reference
signals to the generator- and grid-side converters. The goal,
which is to provide the appropriate dc-link voltage, vdc1, for
the generator-side converter, has been achieved by
integrating an extra dc-link capacitor and controlling its
voltage, vdc2. The voltage, vdc2, has been controlled by the
added isolated SEPIC converter to the dc-link circuitry. The
dc-link voltage utilization may vary according to the
employed modulation technique. Table I, shows the
normalized line voltage versus the most popular modulation
techniques. For Space Vector Modulation (SVM) technique,
the peak amplitude of the line-to-line voltage cannot be more
than the total dc-link voltage. This means for a three phase
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ISSN (Online): 2347 - 4718
system of 208v, the dc-link voltage must be at least 295v to
ensure proper sinusoidal voltage generation. However, due to
practical limitations this theoretical limit cannot be reached
in practice. Therefore, a dc-link voltage more than 295v is
required for proper operation of the inverter. The total dclink voltage, vdc=vdc1+vdc2 is regulated by the source-side
converter by an outer low-speed control loop. This reference,
vdc_ref, has been determined according to the generator
speed and the required minimum voltage for proper
operation of the grid-side inverter. The v dc_ref
characteristic that has been employed in the prototype. The
voltage across the added extra capacitor, vdc2, has been
regulated by the embedded isolated SEPIC converter and its
reference value, v dc2_ref has been determined so that v dc1
, has a proper value for the generator-side converter for full
range of generator speed, This is because v dc equals v
dc_ref +vdc2_ref at steady-state, assuming the controllers
operate with zero steady-state error. The reference values
should be properly chosen to avoid impracticable duty ratio
for the SEPIC converter. In this work, the variation of duty
ratio for the SEPIC converter throughout the whole speed
range has been shown in Fig 3.1. The turns ratio of the
isolating transformer has been selected to be n=0.5.
Fig.1. Existing circuit diagram
III. PROPOSED SYSTEM
The proposed system introduces a new power electronics and
control solution to provide an appropriate dc-link voltage for
the grid-side converter even under very low generator
speeds. The proposed solution is based on integrating an
isolated SEPIC converter to maintain proper dc-link voltage
for three-phase six-switch inverters. Multi-level inverter is
implemented. So; we can reduce harmonics level without
distortion. Using battery energy storage, we can give power
into the transmission lines.
Fig. 2. The proposed power electronics solution
Copyright 2015.All rights reserved.
1417
International Journal For Technological Research In Engineering
Volume 2, Issue 8, April-2015
The isolated SEPIC converter maintains a proper value for v
dc2 so that v dc1 can have a small value at cut-in speed;
meanwhile, v dc1 +vdc2 is regulated by the grid-side
converter and battery stored using bidirectional converter
A. Principles of Operation
The proposed power electronics system consists of a currentcontrolled isolated SEPIC converter and an upper-hand
power management scheme to provide proper reference
signals to the generator- and grid-side converters. The goal,
which is to provide the appropriate dc-link voltage, vdc1, for
the generator-side converter, has been achieved by
integrating an extra dc-link capacitor and controlling its
voltage, vdc2. The voltage, vdc2, has been controlled by the
added isolated SEPIC converter to the dc-link circuitry. The
dc-link voltage utilization may vary according to the
employed modulation technique. Table I, shows the
normalized line voltage versus the most popular modulation
techniques. For Space Vector Modulation (SVM) technique,
the peak amplitude of the line-to-line voltage cannot be more
than the total dc-link voltage. This means for a three phase
system of 208v, the dc-link voltage must be at least 295v to
ensure proper sinusoidal voltage generation. However, due to
practical limitations this theoretical limit cannot be reached in
practice. Therefore, a dc-link voltage more than 295v is
required for proper operation of the inverter. The total dc-link
voltage, vdc=vdc1+vdc2 is regulated by the source-side
converter by an outer low-speed control loop. This reference,
vdc_ref, has been determined according to the generator
speed and the required minimum voltage for proper operation
of the grid-side inverter. The Vdc_ref characteristic that has
been employed in the prototype in this work has been shown.
The voltage across the added extra capacitor, vdc2, has been
regulated by the embedded isolated SEPIC converter and its
reference value, vdc2_ref has been determined so that vdc1,
has a proper value for the generator-side converter for full
range of generator speed, Fig 3.2. This is because vdc equals
vdc_ref+vdc2_ref at steady-state, assuming the controllers
operate with zero steady-state error. The reference values
should be properly chosen to avoid impracticable duty ratio
for the SEPIC converter. In this work, the variation of duty
ratio for the SEPIC converter throughout the whole speed
range has been shown in Fig 3.3. The turns ratio of the
isolating transformer has been selected to be n=0.5
TABLE 1 DC- Link Voltage Utilization
ISSN (Online): 2347 - 4718
IV. SIMULATION AND RESULTS
Sim Power Systems and other products of the Physical
Modeling product family work together with Simulink to
model electrical, mechanical, and control systems. Sim
Power Systems operates in the Simulink environment.
Therefore, before starting this user's guide, you should be
familiar with Simulink. The main Sim Power Systems power
library window also contains the Powergui block that opens
a graphical user interface for the steady-state analysis of
electrical circuits.
Fig. 3. Simulation model LC filter charging mode
Fig. 4. Simulation result charging mode withLC filter SEPIC
output Voltage
Fig. 5. Simulation result charging battery voltage
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1418
International Journal For Technological Research In Engineering
Volume 2, Issue 8, April-2015
Fig. 6. Simulation result Output voltage charging mode
with LC filter
Fig. 7. Simulation modelwith LC filter discharging mode
Fig. 8. Simulation result discharging mode with LCfilter
SEPIC voltage
Fig.9. Simulation result discharging mode DC voltage with
LC filter
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ISSN (Online): 2347 - 4718
Fig. 10.Simulation result discharging mode capacitor voltage
with LC filter
Fig. 11. Simulation model without LC filter charging mode
Fig. 12. Simulation result charging mode without LC filter
in output Voltage
Fig. 13. Simulation result charging mode without LC filter in
SEPIC voltage
Copyright 2015.All rights reserved.
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International Journal For Technological Research In Engineering
Volume 2, Issue 8, April-2015
Fig. 14. Simulation result Output of charging mode battery
voltage without LC filter
ISSN (Online): 2347 - 4718
Fig. 18. Simulation discharging mode capacitor voltage
without LC filter
V. CONCLUSION
The issue of power extraction in wind energy systems with
single phase converters and multi level inverter solution to
this issue was proposed in this project. The proposed power
electronics solution effectively allows for power harvesting
from wind generators. This accordingly escalates the overall
generation capability in wind energy systems. The proposed
system can be installed on existing wind energy conversion
systems with multi level inverter. The integrated SEPIC
converter handles a fraction of the rated capacity of the
system. This in turn reduces the size and cost of the system
compared to existing solution where a boost converter with
the full rating of the system is used to boost the dc-link
voltage. Bidirectional converter is added with dc link
capacitors .The proposed solution shows excellent
performance over a wide speed range of the wind generator.
Fig. 15. Simulation model without LC filter
discharging mode
Fig. 16. Simulation result Discharging mode without LC
filter SEPIC voltage
Fig. 17. Simulation result discharging mode without LC filter
output voltage
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REFERENCES
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International Journal For Technological Research In Engineering
Volume 2, Issue 8, April-2015
ISSN (Online): 2347 - 4718
March-April 2012.
[8] S. Alepuz, S. Busquets-Monge, J. Bordonau, J.
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