Multiple Boost Converter Based Wind Energy Conversion System Engineering Essay

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Wind Energy Conversion Systems are on an established technology for Electrical energy generation and they are used an alternative energy source to providing additional functions such as reactive power supply, voltage control and active power regulation .This type of functions are possible owing to the improvement of the WECS components and to increasing sophistication of the control system .Wind machines are typically used for mechanical applications like water pumping, grinding, woodcutting, or for ac or dc power generation in grid connected or isolated mode. Wind power is an alternative to fossil fuels, is plentiful, renewable, widely distributed, clean, produces no greenhouse gas emissions during operation, and uses little land. In operation, the overall cost per unit of energy produced is similar to the cost for new coal and natural gas installations. The construction of wind farms is not universally accepted, but any effects on the environment from wind power are generally much less problematic than those of any other power source.

The wind energy conversion system with a rated output in MW is proposed and deeply discussed which has many advantages as compared with traditional ac-dc and dc-dc converter. It has 3-phase permanent magnet synchronous generator, 3-phase Rectifier Bridge and multiple boost converter. All the devices are designed to limit harmonic and produce unity power factor and high purity sine wave current to the diode bridge. In case of the usage of permanent magnet synchronous generator, 3-phase diode rectifier with boost chopper is more cost effective solution for ac-dc converter than 3-phase IGBT PWM converter. The use of direct drive and robust low speed rotor design system results in minimum wear, reduces noise levels, improves efficiency, reduced maintenance requirements, lower life cycle costs, and a long life time.

This wind energy conversion system has many advantages, such as low harmonic distortion, low EMI, no generator slip rings required, able to with stand wide variations of grid voltage and frequency, no torque peak on grid fail as usual with the case of asynchronous and double feed systems. Long distance between generator and converter is possible by the use of boost converter with constant voltage at its output terminal. Boost converter has several advantages such as simple structure, continuous input current and regulated output voltage. Conventional boost converters cannot provide such a high dc voltage gain, even for an extreme duty cycle, this is a main disadvantage of boost converter. It also may result in serious recovery problems and increase the rating of all devices. As a result, the conversion efficiency is degraded and the electromagnetic interference (EMI) is severe under this situation. To overcome this problem multiple boost converter is used.

As the important part of the MW-level wind energy conversion system, the three boost converters are used to provide constant voltage. All the theory of the Multiple boost converter are confirmed by the simulation results. Multiple boost converter maintain a constant dc voltage at the load terminal or inverter dc terminal.

Literature Review

In literature there are many methods available for controlling a wind power by power electronic system. for example A DSP based controller for high-power interleaved boost converters brings out clearly a dual-loop average current method which is used to achieve the fast transient response, power-factor corrected boost converter that exhibits voltage-doubler characteristic etc. Also there are different improved power quality converters with their configurations, design features, control approaches, selection of components and other related considerations like as step-up, step-down, choppers, inverters, cycloconverters etc. In general, to lead better technologies, take advantage of the strengths of each methodology and at the same time overcome some of the limitations of the individual techniques.

Xiong Xin and Liang Hui [1], brings out clearly a multiple boost converter controlled by DSP with average current-control method. As the voltage and frequency of turbine generators output vary along the wind speed change, the multiple boost converter is utilized to maintain constant DC voltage level high enough for the PWM inverter to transmit energy to the network. Xudong Huang et al. [2], brings out clearly a dual-loop average current method which is used to achieve the fast transient response. a DSP-based fully digital control is implemented for an interleaved high power dc/dc.Chuanwei Yang et al. [3], brings out the wind turbine which connected directly to a generator .which referred the Direct-Drive concept for this conversion system. One advantage of the Direct-Drive concept is the removal of the losses associated to the gearbox and the ac-dc-ac converter system is preferred to transform the energy compatible to the grid in Direct-Drive wind turbines application.

Dos Reis et al. [4], brings out comparative simulation study between three different approaches applied to harmonic mitigation in PMSG WECS. The studied techniques are harmonic trap filters, single-switch three-phase boost rectifier and three phase boost type PWM rectifier (PWMREC).T. Ahmed et al. [5],brings out the proposed converter using three winding coupled inductor to provide high step up voltage at high frequency circuit. Fernando Valenciaga and Paul F. Puleston [6],brings out wind energy conversion system for maximization of the wind energy conversion efficiency and minimization of the machine copper losses. They proposed the multiple input multiple-output (MIMO) robust controllers and a general design method is used for MIMO nonlinear affine systems.

Wei Li et al. [7], brings out a use of bidirectional IGBT boost converter which often selected to interface the super capacitor or battery ESS with the dc voltage bus. For conventional single-unit topologies, the converter has discontinuous current on the high voltage side and other limitations. This paper proposes a parallel structure of bidirectional buck or boost converters and its power-tracking control scheme for wind ESS applications.W. Sudmee and B. Neammanee [8], brings out the proposed control algorithms to regulate the DC link voltage in a vector controlled boost type PWM converter with particular applications to a wind energy conversion system.Li Jianlin et al. [9], brings out proposed control method which can correct the power factor of the PMSG without increasing the system cost, decrease the current harmonic contents, and increase the real power output ability and the overall system efficiency.

Bing Hu et al. [10], brings out proposed and developed an innovative single-phase, single-stage, flyback-based, buck-boost converter for renewable energy systems, especially for wind turbine systems in grid-connected applications.R.K. Thakur, and V. Agarwal [11], brings out the uses an interval analysis based computation method for the calculation and analysis of various machine parameters and variables in WECS. Yiming He and Xianyi Qian [12], brings out the controlling of boost converter for tracking the maximum power wind turbine. Single-phase inverter used current for the inner loop and voltage for the outer loop of the dual closed-loop control.

Anubhav Sinha et al. [13], brings out a dc-dc boost converter stage has been proposed to be used in such a situation to robustly hold the output dc voltage to the desired constant value needed for the dc link of the PWM inverter. A.Karthikeyan et al. [14], brings out proposed scheme which reduced control complexity because the inverter is operated with a fixed PWM and hence the control is restricted mainly to switching the bi-directional dc-dc converter. Tonny Wederberg et al. [15], brings out scheme for increasing power in wind turbines some manufactories introduce synchronous generators. Akie Uehara and Toshihisa Funabashi [16], brings out the proposed a control method for output power smoothing of a wind energy conversion system (WECS) with a permanent magnet synchronous generator (PMSG) using the inertia of wind turbine and pitch control.

M. Singh et al. [17], brings out study of grid interconnection of a permanent magnet synchronous generator (PMSG)-based wind turbine with harmonics and reactive power compensation capability at the point of common coupling (PCC).Y.Y. Xia et al. [18], brings out the investigate methods to reduce this electromagnetic torque ripple, from both the viewpoints of the circuit topology and the control strategy.Venkata Yaramasu and Bin Wu [19], brings out the proposed a new MV topology using diode rectifier, three-level boost (TLB) and NPC converter has been proposed to further reduce the cost and size.

1.2 Objective of the Dissertation

The main objective of dissertations is to obtain reduced harmonic distortion and maintain a constant dc voltage at the load or inverter input terminal. As the voltage and frequency of turbine generators output vary along the wind speed change, the multiple boost converter is utilized to maintain constant dc voltage high enough for the dc load or PWM inverter to transmit energy to the network.

Wind energy conversion system generates the ac power. The output of this system is applied to the variable speed constant frequency wind energy system (VSCF).It uses the ac-dc and dc-dc Power Converter to dc-level the high-power energy from Permanent-Magnet Generators to the load .The main section of dc energy delivering link is Multiple Boost Converter with closed loop control strategy . At the output of the multiple boost, an appropriate dc voltage is produced at the load. This output is also used for inverter dc terminals to enable the three-phase PWM inverter to drive the desired current into the grid for the optimal real power Transfer and reactive power regulation.

1.3 Organisation of the Dissertation

Chapter 1 deals with the general introduction of multiple boost converter based on wind energy conversion system and advantages associated with them. This chapter also includes the literature review of the reference papers.

Chapter 2 deals with wind energy conversion system as well as their advantages and their disadvantages, main circuit and wind power characteristics. Further describe the summery of all types of main problems related to output of Multiple boost converter.

Chapter 3 deals with the types of DC-DC converter.This chapter also describe the advantage and disadvantage of different types of DC-DC converter.

Chapter 4 deals with the multiple boost converter based on wind energy conversion system. The chapter also describe about characteristics of the boost converter. The brief introductions of state space model and control algorithm for boost converter are also discussed.

Chapter 5 deals with the multiple boost converter based on wind energy conversion system analysis. This chapter also described open loop and closed loop system.

Chapter 6 deals with conclusion of the multiple boost converter based on wind energy conversion system as well as scopes for the future. Further the references which are taken in this dissertation are also discussed.

Chapter-2

WIND ENERGY CONVERSION SYSTEM

Wind is an indirect solar energy source. Wind results from air in motion and this arise from pressure gradient. One primary forcing function causing surface wind from poles towards the equator is convective circulation. The circulation of air on the earth caused by a non uniform heating of earth surface by the sun, its characteristics can be summarized as follows.

It is environmentally clean source of energy.

Wind energy systems avoid fuel provision and transport.

Like all forms of solar energy, wind power systems are nonpolluting, so it has no adverse influence of the environment.

Its availability is unpredictable.

It is a renewable source of energy.

Wind machines are used for mechanical applications like water pumping, grinding, woodcutting, or for ac or dc power generation in grid connected or isolated mode. For most wind energy projects, electrical energy which is produced by the turbines passes through a substation where it is metered and the voltage is increased to match the voltage of the utility grid. Plant isolation breakers, power quality monitors, and protective equipment are also used in the substation to protect the electrical grid and the wind turbines. A system of switches and overhead infrastructure is used to connect the substation to the utility's power lines. The major components of a wind energy conversion system include a wind turbine, a generator, interconnection apparatus, and control systems. Wind turbine works by transforming the Wind Energy into mechanical power that can be used for conversion to electricity. At the present time, generators for wind turbines will be synchronous generators, permanent magnet synchronous generators, and induction generators, including the squirrel-cage type and wound rotor type. For small to medium power wind turbines, permanent magnet generators are often used because of their reliability and cost advantages. Permanent magnet synchronous generators and wound field synchronous generators are currently used in various high power wind turbines.

The cost of wind power generating system has continued to decline through technological development, increased production level, and the use of larger turbines. Interconnection apparatuses are devices to achieve power control, soft start, and interconnection functions. Very often, power electronic converters are used as such devices. Wind power is the nothing but a conversion of wind energy into a useful form of energy, such as using wind turbines to make electricity, windmills for mechanical power, wind pumps for water pumping , or sails to propel ships.

2.1 Permanent Magnet Synchronous Generator

PMSG's have the highest advantages from all the generators that are used in wind turbines ,because they are stable and secure during normal operation and they do not need an additional DC supply for the excitation circuit. A Permanent magnet synchronous generator is that type of generator where the excitation field is provided by a permanent magnet instead of a coil. PMSG are the majority source of electrical energy. They are commonly used to convert the mechanical power output of steam turbines, hydro turbines and wind turbines into electrical power for the grid. The speed of the rotor must always match the supply frequency so they are known as synchronous generators. In a permanent magnet synchronous generator, the magnetic field of the rotor is produced by permanent magnets. The direct current is given to the rotor field winding which is fed through a slip-ring assembly or provided by a brushless exciter on the same shaft.

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Figure 2.1: Permanent Magnet Synchronous Generator for Wind Power Generation

Permanent magnet generators do not require a DC supply, slip rings and contact brushes. But, large permanent magnets are costly which restricts the economic rating of the machine. The flux density of high performance permanent magnets is limited so the air gap flux is not controllable, so the voltage of the machine cannot be easily regulated.

2.2 Main Circuit

Wind energy system consist of permanent magnet synchronous generator, diode rectifier bridge, multiple boost converter, filter and PWM inverter .Permanent magnet synchronous generator gives the unity power factor because there is no field winding in the rotor and permanent magnet is used in a rotor circuit so its avoid the use of slip rings, hence it is simpler and maintenance free. The condensers are not required for maintaining the power factor in synchronous generator, as it is required in induction generator.

Boost converter are connected in parallel to maintain constant voltage at inverter terminal for high power level wind energy conversion system. Control signal of each boost converter is shifted with each other.ac filter are also connected to limit the harmonic generated by the switching of semiconductor switches. For controlling the power factor, boost converter is used, which gives continuous input current that can be manipulated by different techniques. The inverter output current can also be control by different control strategy, so harmonics can be reduced. Figure 2.1 shows the wind energy conversion system.

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Figure 2.2: Main circuit of wind energy conversion system

ac power generated by permanent magnet synchronous generator is rectified to dc power by

3-phase diode rectifier circuit. Following the generator there are three filter capacitors to supply the generator with capacitive current which can improve power capability and stabilize the generator voltage with current variations. Synchronous generator gives ac voltage; this voltage is converted into dc voltage by the use of Rectifier Bridge. The use of diode rectifier with boost converter is most cost effective solution for ac to dc converter.

In the MW wind energy conversion system, if the voltage in the system is higher than the lost in the system will be lower. Multiple boost converter is used in the dc link whose duty ratio is controlled by PI controller in closed loop control method. . The three boost converters are operated in parallel with control signal each boost converter shifted in order to reduce harmonic distortion and inductor size, furthermore, the current rating of the power device is reduced. The output of multiple boost converter is given to the load (R, RL, dc motor etc) or inverter dc terminal. The output of inverter is connected to the grid.

Advantages of wind power

The greatest advantage of electrical energy generation from the wind is that, it's a renewable source of energy and not depleted with the use like fossil fuels.

The wind is free and with the use of modern technology it can be captured efficiently.

Remote areas which are not connected to the electricity, power grid can use wind

Turbines to produce their own supply.

There is no fuel cost and low maintenance cost.

Wind farms occupies a large land area so it is possible to use of this land for other purposes like vehicle testing ground, tourist park etc.

2.4 Disadvantages of wind power

Favourable winds are available only in few geographical location so all location are not suited for this purpose.

The speed of the wind is not constant all the time. This means, wind turbines do not able to produce the same amount of electricity all the time. There will be time when they produce no electricity.

Wind turbines are noisy in operation so produced noise.

Wind farms require vacant land and this land should be flat and free from forest.

The construction of wind turbine can be expensive and costly to surrounding wildlife during the build process.

2.5 Wind Power Characteristics

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Figure 2.3: Relationship between power and rotational Speed

Available wind power is calculated as follow:

Wind power

A: swept area

Air density

D: rotor diameter

V: wind velocity

This shows that maximum power available from wind turbine varies according to the square of the diameter of the intercept area.(. The output energy is obtained by the use of power coefficient of wind turbine, if the swept area, air density and wind velocity are constant.

2.6 Conclusion

The chapter discuss about the wind energy conversion system, main circuit and wind power characteristics. The advantages and disadvantages of such type of system are also discussed. The major problems which are taken concerned at the output of multiple boost converter is to maintain a constant voltage are also summarised in this section. The whole discussion gives the brief idea of wind energy conversion system.

Chapter-3

DC-DC CONVERTER

A DC-DC converter is an electrical circuit which converts fixed dc input voltage to a variable dc output voltage directly. It is class of power converter, DC to DC converters are used in portable electronic device such as cellular phone and laptop etc. DC-DC converters can be used as switching mode regulators which converts unregulated dc voltage to a regulated dc voltage. Switching device is normally MOSFET, IGBT, or BJT, and regulation is achieved by PWM techniques at a fixed frequency and switching regulators are available as a integrated circuits.

DC-DC converters are classified as following:-

1. Buck converter

2. Boost converter

3. Buck-Boost converter

4. Cuk converter

3.1 Buck Converter

In buck converter, the average output voltage is less than the input voltage .buck is a very popular converter. The buck converter requires only one switch (IGBT,BJT or MOSFET), and has high efficiency greater than 90% . The di/dt of the load current can be limited by inductor L. However, the input current is discontinuous and a smoothing input filter is normally required to continuous the input current. The main disadvantage of buck converters cannot provide input to output isolation, there is also a potential to over voltage the output ,if the switch shorts so it requires a high side switch drive and it has high input ripple current. Buck converter can be operated in continuous mode and discontinuous mode depending upon type of operation.

It provides only one polarity of output voltage and unidirectional output current. It also requires a protection circuit in case of possible short circuit across the diode path.

The circuit operation of buck converter can be divided into two modes.Mode-1 begins when switch S is switched on at t = 0.The input current rises and flows through inductor L, capacitor C and load. Mode 2 begins when switch S is switched off at t =and freewheeling diode D conducts due to energy stored in inductor L.Inductor current continoues flows through L,C, and diode D.The inductor current falls until switch S is switched on in the next cycle.this converter gives an output voltage smaller than the input voltage . It is based on the circuit of figure 3.1 and waveforms are shown in figure 3.2.

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Figure 3.1: Buck converter

The relation between duty cycle k and output voltage

The relation between duty cycle k and output current

Peak-to-Peak ripple current

Peak-to-Peak ripple voltage of the capacitor

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Figure 3.2: Waveforms of current and voltage in a buck converter operating in continuous mode

Features of the Buck Converter

Continuous input current, one switch requires.

It has high efficiency greater than 90 %.

Output voltage is always less than input voltage.

3.1.2 Advantages

It has many advantages such as simple structure, continuous input current and improved power factor.

Disadvantages

The main disadvantage of conventional buck converters cannot provide input to output isolation, there is also a potential to over voltage the output ,if the switch shorts so it requires a high side switch drive and it has high input ripple current.

3.2 Boost Converter

A boost converter can step up the input voltage without a transformer. Due to a single transistor, it has high efficiency. This type of circuit is used to 'step-up' a dc voltage to a higher and regulated voltage. A boost converter is part of a subset of DC-DC converters called switch-mode regulator. The output voltage is regulated by adjusting the duty cycle. Boost converter does not use resistive components to dissipate extra power so the efficiencies are seen in the range of 80-95%.

Power can come from dc sources such as batteries, solar panels, rectifiers and dc generators. A process that changes one dc voltage level to a different dc voltage level is called dc to dc conversion. A boost converter is a dc to dc converter with an output voltage greater than the input voltage. The boost converter is the tendency of an inductor to resist changes in current which flow through it. When this inductor being charged it acts as a load and absorbs energy and when being discharged it acts as an energy source. The voltage which produces during the discharge phase is related to the rate of change of current and not to the original charging voltage, thus this allows different input and output voltages. Boost converter can be operated in continuous mode and discontinuous mode.

The boost converter operation can be divided into two mode. Mode 1 begins when switch S is closed and inductor L charge up. Mode 2 begins when switch S is open and inductor discharges through load and capacitor C. It is based on the circuit of figure 3.3 and waveforms are shown in figure 3.4.

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Figure 3.3: Boost converter

The relation between duty cycle k and output voltage.

The relation between duty cycle and output current

Peak-to-Peak ripple current

Peak-to-Peak ripple voltage of capacitor

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Figure 3.4: Waveforms of current and voltage in a boost converter operating in continuous mode

Features of the Boost Converter

1. Continuous input current and eliminates the input filter.

2. Pulsed output current increases the output voltage ripple.

3. Output voltage is always greater than input voltage .

3.2.2 Advantages

It has many advantages such as simple structure, continuous input current, and clamped switch voltage stress to the output voltage.

Disadvantages

Boost converters cannot provide such a high dc voltage gain, even for an extreme duty cycle. It may result in serious recovery problems and this can increase the rating of all devices. As a result, the conversion efficiency is being degraded and the electromagnetic interference is severe under this situation.

3.3 Buck-Boost Converter

A Buck-boost converter provides an output voltage that may be greater than or less than the input voltage hence the name buck-boost, the output voltage polarity is oppositively to the input voltage. Buck-Boost converter is also known as an inverting regulator. The circuit diagram of a buck-boost converter is shown in figure 3.5 .

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Figure 3.5: Buck-Boost Converter

The buck-boost converter operation can be divided into two modes, Mode 1 and Mode 2 and operating waveform in various modes is shown in figure 3.5. Mode 1 begins when switch S is switched on and diode D is reversed biased. The input current rises and flows through inductor L and switch S. Mode 2 begins when switch S is switched off and current which was flowing through inductor L , would now flow through L, C, D and the load. The energy stored in the inductor would be now transferred to the load and the inductor current would fall until switch S is switched on in the next cycle.

The relation between duty cycle k and output voltage

The relation between duty cycle and output current

Peak-to-Peak ripple current

Peak-to-Peak ripple voltage of capacitor

Figure 3.6: Waveforms of current and voltage in a buck-boost converter operating in continuous mode

Features of the Buck-Boost Converter

A buck-boost converter provides output voltage polarity reversal.

Output voltage is greater than or less than the input voltage.

It has high efficiency.

Pulsed output current can increases output voltage ripple.

3.3.2 Advantages

Output voltage polarity is opposite to input voltage without a transformer.

Under fault condition of switch the of the fault current is limited by the inductor L.

Output short circuit protection can be easy to implement.

Disadvantages

If the switch S ever shorts there is no way to limit the current and the buck-boost converter is difficult to control in continuous-conduction mode.

3.4 Cuk Converter

A Cuk converter provides an output voltage that may be greater than or less than the input voltage, the output voltage polarity is opposite to the input voltage .Cuk converter operation can be divided into two modes. Mode 1 begins when switch S is switched on, the current flow through inductor L rises and voltage of capacitor reverse biased diode D and turns it off. This capacitor discharges its energy to the circuit which is formed by and the load.Mode 2 begins when switch is switched off .Capacitor is now charged from the input voltage and inductor transferred energy to the load.

The schematic diagram for a Cuk converter is shown in figure 3.7 and waveforms for voltage and current are shown in figure 3.8.

The relation between duty cycle k and capacitor voltage

The relation between duty cycle k and capacitor voltage

The relation between duty cycle and output current

Peak-to-Peak ripple current of inductor

Peak-to-Peak ripple current of inductor

Peak-to-Peak ripple voltage of capacitor

Peak-to-Peak ripple voltage of capacitor

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Figure 3.7: Cuk converter

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Figure 3.8: Waveforms of current and voltage in a Cuk converter

.3.4.1 Features of the Cuk Converter

A Cuk converter provides output voltage polarity reversal.

Output voltage is greater than or less than the input voltage.

Cuk converter is based on capacitor energy transfer as a result input current is continuous.

Advantages

1. Cuk converter has low switching loss.

2. It has high efficiency.

The advantage of Cuk converter is that the input inductor and output inductor creates a

smooth current at both side of this converter.

3.4.3 Disadvantages

Cuk converter has high number of reactive component and high current stresses on the switch.

Conclusion

The chapter discuss about the types of the DC- DC convertor. The working of buck, boost, buck-book and cuk are described here. Advantages and disadvantages associated with these converters also discussed. In dissertation work, the main purpose of using Multiple boost converter is to maintain a constant dc voltage at the load terminal or inverter dc terminal.

Chapter-4

MULTIPLE BOOST CONVERTER

Multiple boost converter consist of three single boost converter connected in parallel which controls the dc link voltage by varying the duty ratio of the boost converter. The three boost converters are operated in parallel with control signal of each boost converter shifted in order to reduce harmonic distortion this also reduced the inductor size. The current rating of the power device is also reduced. Output of multiple boost converter is given to the load or inverter dc terminal. Multiple boost converter maintain the constant voltage at the output of its terminal.

4.1 Operation Principle of the Boost Converter

Boost converter step up the input dc voltage without a transformer. Due to a single switch it has high efficiency. Boost converter operation can be divided into two modes, Mode-1 and Mode-2.

Mode 1-This begins when switch S is switched on. The input current rises and flows through inductor L and switch S.

Mode 2-This begins when switch S is switched off, the current that was flowing through switch S would flow through L,C, load, and diode D. The inductor current falls until switch S is switched on again.

The energy stored in inductor L is transferred to load. The circuit diagram of a boost converter is shown in figure 4.1.When a boost converter operates in continuous conduction mode, the current that flow through the inductor L never falls to zero. The output voltage is also calculated and this output voltage is greater than input voltage. If the ripple amplitude of the current is high then inductor may be completely discharged before the end of commutation cycle. In this case, the current which flow through the inductor L falls to zero during part of the period so discontinuous conduction is obtained.

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Figure 4.1: Mode 1: L Charges (S closed)

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Figure 4.2: Mode 2: L Discharges into C and load (S open)

The input current is the same as the inductor current as can be seen by the waveforms. So it is not discontinuous as in the case of buck converter and the requirements on the input filter are relaxed as compared to a buck converter.

Boost converters are also used in battery powered devices, where the electronic circuit need a higher operating voltage than the battery can supply such as notebooks, mobile phones and camera-flashes.

4.2 Operation Principle of the Multiple Boost Converter

Multiple boost converter consist of three single step-up chopper. Output voltage is controlled by varying the duty ratio of the each boost switch. The three boost converters are operated in parallel and control signal each boost converter shifted. Inductor size can be reduced by using this and harmonics are also reduced, furthermore, the current rating of the switches is reduced. The schematic diagram for a Multiple boost converter is shown in figure 4.3.

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Figure 4.3: The Multiple Boost Converter

At the output of the multiple boost converter, an appropriate dc voltage is produced at the load terminal or inverter dc terminals to enable the three-phase inverter to drive the desired current into the grid for the optimal real power transfer and reactive power regulation. Although it is connected directly to a variable-voltage dc link, a nearly constant dc voltage at the inverter terminal is preferred for high efficiency and best utilization of the semiconductors. So the multiple boost converter can provide constant dc voltage.

Switch is switched on at .Control signal of switch shifted and Control signal of shifted. Multiple boost converter used in the dc link and the duty ratio is controlled by PI controller in closed loop control method. Pulses are generated through a closed loop and these pulses are given to the switch so switch operated and output is generated.

So Multiple boost converter principle is same like a boost converter. Control signal of each boost converter shifted. Pulsed output current increases the output voltage ripple. Frequency of harmonic in the current increases so size of filter is reduced. It maintains the constant voltage at its output terminal.

4.3 Analysis and Characteristics

In the Boost converter circuit, On-state condition, the switch S is closed, resulting in an increase in the inductor current and In the Off-state condition, the switch is open and the inductor current is flow through the flyback diode D, the capacitor C and the load R. This results in transferring the energy accumulated during the On-state into the capacitor so output voltage is greater than input voltage.Waveforms of boost converter is shown in figure 4.4.

Where k is the duty cycle.

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Figure 4.4: Waveforms of current and voltage in a boost converter operating in continuous mode

Assuming that the inductor L current rises linearly from to in time ,

and the inductor L current falls linearly from to in time ,

Where is the peak to peak ripple current of inductor L.

Substituting and then average output voltage,

Substituting kinto Eq.(6),we get

Assuming a circuit is lossless,

Average input current

The switching period T can be get from,

Peak-to-Peak ripple current

Peak-to-Peak ripple voltage of capacitor

Due to a single transistor used in the boost converter, it has a high efficiency.

4.4 State space model of Boost converter

State space models are very useful for dynamic modeling of power converter circuits. They provide the basis for applying various linear control techniques to the power circuits. Boost converter operates in two modes, Mode-1 and Mode-2.The behavior of boost converter circuits in these modes can be depicted by state space models. On-mode and Off-mode are shown in figure 4.5 and 4.6.

The general form of state space model is given by

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Figure 4.5: Boost Converter during ON mode.

In a Boost Converter, During 'On' Mode,

From KVL,

From KCL,

In State Space form

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Figure 4.6: Boost Converter during OFF Mode

During 'OFF' Mode:-

From KVL,

From KCL,

In State Space form

4.5 Control algorithm for Multiple boost converter

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Figure 4.7:-Block diagram of the current locked loop

The block diagram of the closed loop control is shown in Figure 4.7. The two current signal and are compared. is defined as a reference current calculated by the output power and input voltage at that time. is the average feedback current of the multiple boost which are sampled through the modules for the implementation of the closed loop control .

An error current proportional to - is integrated through a PI controller and the output of PI controller is transmitted to the driver circuit. The driver circuit controls the switch with a periodic rectangular pulse whose duty ratio is proportional to its input current. When the error current is different from zero, the input to the driver circuit is continuously increased or decreased which is depend on the sign of the error current so the duty ratio will be change in such a way to reduce the error current to zero.

4.6 Advantages of Multiple boost converter

It has many advantages such as simple structure, continuous input current, and it's provide a constant output voltage which is greater than input voltage. It also reduces the inductor size and harmonics distortion. It maintains a constant dc voltage at the load terminal or at the inverter dc terminal.

4.7Conclusion

The chapter discuss about the multiple boost converter. The characteristics of boost converter are also discussed. The brief introductions of state space model and control algorithm for boost converter are described here. The main advantages of the multiple boost converter are also discussed in this section.

Chapter-5

Simulation and Results

The simulation and result of the multiple boost converter based on wind energy conversion system are as follows

5.1 Open Loop Simulink Model

In open loop simulink model of multiple boost converter, pulse generator is connected for switching purpose of the IGBT. Display is taken for the showing input voltage, output voltage and output current magnitude. Voltage measurement is taken across the load R-L. The open loop model of the Multiple boost converter are shown in figure 6.1.

C:\Users\IRAM\Desktop\mnn.jpgFigure 5.1: Open Loop Simulink Model of Multiple boost converter

5.2 Closed Loop Simulink Model

The closed loop model of the Multiple boost converter are shown in figure 6.2. In this model PI controller is used as output voltage controller. Reference voltage is taken as constant for error signal generation. Saturation is used for limiting the switching signal given to the IGBT. The Voltage across the load R-L, input voltage of multiple boost converter, and output current are measured.

C:\Users\IRAM\Desktop\nn.jpgFigure 5.2: Closed Loop Simulink Model of Multiple boost converter

5.3 Simulation Results

The results of Multiple boost converter related to open loop and closed loop control are as follows:

5.3.1 Open Loop Input Voltage, Output Voltage and Current Waveforms

The input voltage, output voltage, output current of Multiple boost converter and Rectifier input current are shown in figure. This shows uncontrolled behavior of the Multiple boost converter. When the input voltage increases the output voltage shows very large variations.

Figure 5.3: Open Loop Input Voltage of Multiple Boost Converter

Figure 5.4: Open Loop Output Voltage of Multiple Boost Converter

Figure 5.5: Open Loop Output Current

Figure 5.6: Rectifier Input Current

5.3.2 Closed Loop Input Voltage, Output Voltage and Current Waveforms

The closed Loop controlled input voltage, output voltage and current waveform at different reference voltages are as given. The output voltages are controlled by hit-n-trial method of PI controller where the value of = 2.5 and = 40.The PI controller controls the output voltage from 77 V to 540 V. When the reference voltage changes beyond this limit output voltage becomes uncontrollable. The minimum reference voltage is 77 volt whereas maximum reference limit is 540 V.

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Figure 5.7: Closed Loop Input Voltage at Reference Voltage 310 volt

Figure 5.8: Closed Loop Output Voltage at Reference Voltage 310 volt

Figure 5.9: Closed Loop Output Current at Reference Voltage 310 volt

Figure 5.10: Rectifier Input Current

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Figure 5.11: Closed Loop Output Voltage at Reference Voltage 340 volt

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Figure 5.12: Closed Loop Output Voltage at Reference Voltage 420 volt

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Figure 5.13: Closed Loop Output Voltage at Reference Voltage 500 volt

Input Voltage

Reference Voltage

Output Voltage

20.0V

110

110V

41.9V

230

230V

56.5V

310

310.4V

76.5V

420

420.1V

91.1V

500

500.3V

Table 1: Output Voltage at Different Reference Voltages

The control action of PI controller is shown in Figure. The PI controller controls the output voltage frequently within the certain acceptable range.

Figure 5.14: The control signal of the PI controller

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Fig. 5.15: Variation of output voltage across the load with =100V,=200V,=300V,=400V and =500V

THD Analysis

THD Analysis of input current in open loop and closed loop is as follows:

Open loop System

THD analysis in input current of rectifier and its output is connected to the boost converter and Multiple boost converter is shown in figure 6.5 and 6.7.In Multiple boost converter ,all three boost converter are operated in parallel so harmonic distortion is reduced in comparision with single boost converter.Furthermore,inductor size is also reduced.In Boost converter,THD in input current is 5.32% and in Multiple boost converter,THD in input current is 1.53%.

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Figure 5.16: Open Loop Simulink Model of Boost converter (THD Analysis)

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Figure 5.17: Open Loop Simulink Model of Multiple Boost converter (THD Analysis)

5.4.2 Closed loop System

In Boost converter,THD in input current is 4.56% and in Multiple boost converter,THD in input current is 1.20%.so harmonic distortion is reduced .

C:\Users\IRAM\Desktop\hhhhhhhhhhhhhhh.jpg

Figure 5.18: Closed Loop Simulink Model of Boost converter (THD Analysis)

C:\Users\IRAM\Desktop\ggn.jpgFigure 5.19: Closed Loop Simulink Model of Multiple Boost Converter (THD Analysis)

Chapter-6

Conclusion and Future Scope

This chapter presents the conclusion and future work prospects for the Multiple boost converter based on wind energy conversion system as given below:

6.1 Conclusion

For wind energy conversion system, multiple boost converter is introduced in this dissertation. Closed loop control strategy for multiple boost chopper which deliver the high-level power energy produced by permanent magnet synchronous generator timely and precisely is also discussed. The three boost converters are operated in parallel in order to reduce harmonic distortion this also reduced the inductor size. The current rating of the power device is also reduced. Output of multiple boost converter is given to the load or inverter dc terminal. Multiple boost converter provides the constant voltage at the output of its terminal and maintaining the advantages of the boost converter, such as a continuous input current and a clamped voltage stress on switch. As the important part of the wind energy conversion system, the circuit and closed loop control theory of the multiple boost chopper is analyzed and deeply discussed.

The PI controller technique is used to control the output of the Multiple boost converter which gives the controlled variation of output voltage from 77V to 540V.

Furthermore, the implementation of the experimental system verifies the feasibility of the multiple boost converter used in the wind energy conversion system.

The following conclusions achieved in this dissertation are as

The multiple boost converter provides constant high dc voltage at the load terminal.

Multiple boost converter also reduce the harmonic distortion and inductor size.

Multiple boost converter maintains the advantages of the single boost converter, such as regulated output voltage.

Multiple boost converter provides the continuous input current with simple structure.

This wind energy conversion system has many advantages, such as low harmonic distortion, low EMI, no generator slip rings is required, able to with stand wide variations of grid voltage and frequency, and no torque peak on grid fail as usual with

the case of asynchronous and double feed systems.

6.2 Scope for Future Work

The theory developed in this dissertation was verified analytically and through simulation, which conclusively prove that the proposed approach provide reduced harmonic distortion and maintain a constant dc voltage at output of multiple boost converter. For further reduced harmonic distortion and inductor size, more than three boost converter is used so this needs to be investigated. When reference voltage is taken beyond the 540 volt, the settling time is increased rapidly. The reasons for occurrence need to be investigated. The detailed mathematical analysis and simulation results of Multiple boost converter based on wind energy conversion system with control circuit and current control strategy are yet to be done. This work can extend for hardware implementation in future.

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