International Journal For Technological Research In Engineering
Volume 2, Issue 8, April-2015
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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 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 www.ijtre.com
Copyright 2015.All rights reserved.
ISSN (Online): 2347 - 4718
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International Journal For Technological Research In Engineering
Volume 2, Issue 8, April-2015 ISSN (Online): 2347 - 4718 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. 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
Theoretically for space-vector modulation the dc-link voltage converter by an outer low-speed control loop. This reference, should not be less than the peak maximum value of the linevdc_ref, has been determined according to the generator to-line ac voltages. Considering the practical limitations such speed and the required minimum voltage for proper as turn-on and turn-off times of the switches the dc-link operation of the grid-side inverter. The v dc_ref voltage has to be maintained slightly larger than the characteristic that has been employed in the prototype. The theoretical value. The same principle governs the operation of voltage across the added extra capacitor, vdc2, has been the generator-side inverter. The wind generator provides a regulated by the embedded isolated SEPIC converter and its small voltage at the cut-in speed of the wind turbine and it reference value, v dc2_ref has been determined so that v dc1 has to be boosted up to a much larger value which is the dc, has a proper value for the generator-side converter for full link voltage. This requires a high conversion ratio for the range of generator speed, This is because v dc equals v generator-side converter as it has to operate with the dc-link dc_ref +vdc2_ref at steady-state, assuming the controllers voltage that is dictated by the grid-side converter. Due to the operate with zero steady-state error. The reference values practical limitations such as turn-on and turn-off times of the switches the duty ratio required to boost the small cut-in should be properly chosen to avoid impracticable duty ratio for the SEPIC converter. In this work, the variation of duty speed voltage to the dc-link voltage is not achievable with the ratio for the SEPIC converter throughout the whole speed 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 range has been shown in Fig 3.1. The turns ratio of the isolating transformer has been selected to be n=0.5.
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
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

 generator-side converter cannot be realized due to practical limitations.
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
The system cannot harvest energy at low cut-in speeds.
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
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 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.
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 www.ijtre.com
Fig. 2. The proposed power electronics solution
Copyright 2015.All rights reserved.
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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 www.ijtre.com
Copyright 2015.All rights reserved.
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International Journal For Technological Research In Engineering
Volume 2, Issue 8, April-2015 ISSN (Online): 2347 - 4718
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 www.ijtre.com
Copyright 2015.All rights reserved.
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
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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
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
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.
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[5] M. Chinchilla, S. Arnaltes, J.C. Burgos, "Control of permanent-magnet generators applied to variablespeed wind-energy systems connected to the grid,"
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[6] Z. Wang, L. Chang, "A DC voltage monitoring and control method for three-phase grid-Connected wind turbine inverters," IEEE Trans. Power
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[7] F. Blaabjerg, M. Liserre, K. Ma, "Power electronics converters for wind turbine systems," IEEE Trans.
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[8] S. Alepuz, S. Busquets-Monge, J. Bordonau, J.
Gago, D. Gonzalez, J. Balcells, "Interfacing renewable energy sources to the utility grid using a three-level inverter," IEEE Trans. Industrial
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