Introduction
(CHAPTER1)
In the human life every work is inter connected with the utilization of electrical power in the today s technological world. The concentration in the green house gas effect (GHG) will heat up the earth s atmosphere. The vital resource is fossil fuel utilization which accounts for 80% of the utilization of power globally. The conservative scientific effects indicates that, consecutively to maintain a 50% opportunity to prevention of catastrophic climatic change, the global temperature must not extended by further than 2 by 2050 analysis. It will be capable to achieve in the major cost-effective manner, GHG production should peak between this duration from 2017 to 2020 and then rapidly decline. But now-a-days the globe is ever present in the surrounding area of a realistic path to globally decarbonizes, the entire energy generation environment by 2050. So significant are these challenges that an urgent energy revolution is required, before the access to 2 C is closed . In the year 2012 declared as the worldwide year of maintainable power for every non-conventional resources, sequentially to mobilize the international co-operation on the clean energy reform is initiated by The UNGA (United National General Assembly). The empowerment by using this resolution United Nations Secretary and General (UNSG) Ban Ki-moon set out the Global Sustainable-energy -for-All (SE4ALL) challenge for 2030.
The universal energy services,
Twice the energy efficiency rate and
Twice the non-conventional resource in the universal power since 15 to 30%.
The Renewable source is a dignified self-assured manner to become a dominant soft energy path. It will provide increasingly decentralized and de-carbonized electricity to the 1.3 billion population currently living without electricity and will became a viable substitute to the pollute biomass energy is utilized by 2.7 billion people. Appropriate adjustments in the climate reliant on the inter governmental panel gives the reliable improvement of the conventional source generation which is forecasted by 2035 to diminish the GHG discharge by 21% and supply it up to 45% of the universal electricity, whereas by 2050 contribute up and about to 77% of the universal electricity. Therefore the renewable power sources motivation to a fundamental source measurement of the carbon in the forthcoming generations [1].
For a lengthy period to generate electricity, by using nonconventional re-sources for this reason that of their avenues advantages in the environmental society. Now-a-days the renewable power sources are increased popularly in present scenario, which is a part of our natural environment and eco-friendly. Sequentially to achieve the fast rising requirement for power due to quickly increasing economy with population, it is better to replace these polluted non-renewable energy sources with the clean renewable energy resources. As per World Energy Outlook (WEO)-2010 the prospects for renewable energy based electricity generation hinge critically on government policies to encourage their development. Worldwide the distribution of the nonconventional supply in the electrical energy increases from 19 percent in 2008 to 32 per cent in 2035.
In the New Scenario policies, it reaches only 23% into the Policy of current but 45 % in the 450 Scenario. According to these three scenarios, intensifying the fossil-fuel expenditure and declining the costs make renewable sources further activity by means of the conventional technologies. Hydro-power is the foremost conventional re-source of power for greater than a century. In the vicinity of upcoming generations, to congregate the growing electricity generation the non-conventional resources such as airstream energy with solar energy re-sources have produced opportunity between the policy producers together with the industrial requirements. The enhancement in the electrical energy fabrication is forecasted by the renewable sources during 2008 to 2035 [2].
1.1 RESEARCH MOTIVATION
It gives the ecological along with efficient compensation offered by the airstream power production. This is an important reason why electrical systems found on the current of air control scheme are receiving wide spread global attention.
1.1.1 Development of Installed Wind Turbine Energy
A considerable amount of effort is ready to produce electrical energy from new sources of energy. Wind energy is now achieving exponential growth and has great potential [3]. Fig 1.1 gives the entire established wind turbine power in regional wise consistent with this inaugurated capability of current of air influence arrangement has showed during the months from January to December up to 2016. As another example, Ontario had an airstream fabrication ability of 15MW in 2003. At the closing stages of 2008, that number will jump to over 1,300 MW and put Ontario in first place for wind speed production in Canada. [4].
Fig.1.1 Wind Annual Energy inaugurated Capacity by Region [5]
TABLE 1.1 worldwide Wind power installed Capacity
hjjj
Source: GWEC [5].
Fig.1.2 overall established Wind power capability in India [5]
TABLE 1.2: Wind Power Installation across the States in India up to 2016
Source: GWEC [5]
1.1.2 Environmental Merits of Wind Power
Nowadays, the environmental disasters are threatened our well-being and existence. Rising pollution levels and remarkable transformation in typical weather demand a diminution in environmentally damaging emissions. Fiery of vestige fuels is the major significant supply of environmental pollution in the power producing plants for the fabrication of electrical energy. Preferred solutions to prevent emissions are using renewable and cleaner energy sources. In fact for every 1kWh of electricity generated by wind, the emission of carbon di-oxide (CO2) is reduced by 1kg [6].
1.2 OBJECTIVE OF THESIS
The victorious investigation of the DFIG-system supported in the company of wind-turbine in to the aggressive wind promote inspire to revise the special working and serious circumstances applied to observe of the DFIG-system. It is furthermore entitled as WRIG (wound-rotor induction generator). Doubly Fed means, it having both the stator along with rotor of a machine by means of a wound-rotor is associated to electrical sources. This is popular for the power production appliance to present generations with limited variable speed range. [11]. This DFIG-system permit a changeable speed operation in excess of a large range by injecting a rotor current through a changeable frequency into the BTB converter; it will be engaged to compensate the diversity connecting the grid-frequency and the mechanical angular frequency. The behavior of this generator is governed by the semi-conductor devices and its controllers are operated in two conditions such as normal and faults operations. The BTB converter having two converters they are the machine-side converter along with grid side converter, which are controlled independent of one another. The key scheme at the back of the DFIG-system with back-to-back converters is by scheming the machine current components of the machine-side converter, controls the re-active in addition to active power where as the GSC manage the dc- link voltage and it also ensures that the procedure at the unity p.f [12]. By changing the slip (s) value of a machine, the control along with the function of DFIG-system is capable to work in three different modes.
The control of power-flow in the rotor the technique of process of the generator is explained in the diagram 1.3. Depending on the control is feed keen on otherwise beyond the rotor, which shows in the casing of super-synchronous mode and after that it runs as of the machine s rotor via inverter to the utility-grid. In addition to the sub-synchronous mode, it runs in the opposite direction.
Fig 1.3 the DFIG Power Flow Diagram
(Sub-synchronous, Super-synchronous generating mode)
The DFIG with wind turbine having the following, advantages in the study state condition.
To decouple the dynamic active power along with the re-active power control can survive acquired by calculating the rotor energized current independently in the generator system.
The DFIG-system can not only magnetized by power grid but also it magnetized by the machine route of a generator. It also having the capability to produce reactive power via the GSC that will be distribute to the stator. However, the GSC operates normally at unity p.f with is not concerned in the exchange of the re-active power between the airstream turbines in addition to the utility-grid.
In weak grid applications, wherever the voltage fluctuations occurred the DFIG- system might be there forbidden to generate or consume the quantity of re-active energy in the direction of the grid otherwise from the grid through the utility of the voltage control.
The essential purpose of this thesis is the investigation of the DFIG-system in support of an airstream turbine through filter topology used in back-to-back converter application both during steady state operation and transient operation such as
Voltage drop
Voltage swell
Change in wind speed
Sudden change of re-active power insist by the engine
Sudden change of supply frequency in the utility-grid.
So as to evaluate the function of the entire DFIG-system during steady state plus transient state operation, both the modeling and controlling of the scheme are important issues. Hence the control and modeling (to be discussed detail in next chapters) are also the part of this dissertation work [13]. In simulation work the arrangement is modeled by designing the filter topology associated at the converter side plus grid-side accustomed to amplify the power excellence of the entire DFIG system. By using MATLAB environment, the overall system is simulated and all the results are enclosed in the chapter 5.
1.3 HISTORICAL BACKGROUND OF WIND-ENERGY
In the 19th century, the airstream energy was meant for the fabrication of electrical energy owed to the low price of the fossil fuels for example smear with oil and coal efficiently distasteful. Look into the contemporary WECS was revived in 1973 for the reason that of the natural sources adversity. At first the research was concentrated on making the modern wind turbines, with larger capacity which led to the growth of the enormous machines as researched from this survey. Next to this, the manufacturing of the airstream turbines was slowed down by so many scientific troubles in conjunction with the elevated rate of manufacturing [7] consequently the concentration was twisted to building low price turbines. These systems were encompassing of a tiny airstream turbine an asynchronous generator and a gearbox and a straightforward control scheme.
Therefore, from the DFIG-system the beam/shaft of the airstream turbine revolves at an invariable speed with the asynchronous producer is a correct selection to produce sufficient power to the electric grid. This wind-turbine machinery made the price reasonable even for those to acquire their utility. A new invention of electrical energy from the airstream power arrangement was developed on a larger scale from the result of the successful research on WECS (wind-energy alteration systems). From the previous days to track the more power rating by the invention of the wind turbines has grow in size and power rating income that the diameter of the generator s rotor, generator rating and the tower height have all increased. From the year 1980, the wind turbines having the span of the rotor with reference to 10 to 15m along with the total generated power charge at 10 kW to 65 kW will be used in the distribution systems. After 1980s the wind turbines were developed rapidly with the diameter of the generator with reference to 15 to 25 m in addition to the generators rated power up to 200 kW. According to AWEA, now-a-days bulky airstream turbines manufacture to the extent that 120 times more electricity than early turbine designs, with process with continuation costs only modestly higher, thus dramatically cutting O&M costs per kWh. Compared with the small wind turbines, bulkier airstream turbines do not turn as fast and produce less noise in the arrangement. From the literature survey, the introduction of new types of machines used in airstream systems another modification has been discussed. Since 1993 the minority makers are replaced by the conventional generator in their airstream turbine proposed in the midst of a synchronous-generator while other industries were using asynchronous machines [8]. From the above discussion, the airstream turbine scheme is developed by introducing new electrical converters and its control methods. From the concept of sophisticated PE (power electronics) in the airstream generator arrangement aimed with introduction of the new idea that is changeable velocity of wind gives the development in the electricity scenario. The quick progression of PE (power electronic) devices place a key responsibility due to higher energy-handling ability with lesser price/kW [9], the applications of the PE in the airstream turbines is predictable to enlarge in the next generations. The velocity of the airstream turbine shaft is able to proscribed by means of some control technologies for the huge wind energy turbines w.r.to the variable pitch blades, and this pitch control conception is applied from the last 14 years [10].
1.3.1 Wind Turbines
These turbines generate electrical energy by using the power of the wind to drive an electrical generator. The shaft of the rotor is turned by using the rotating blades inside the nacelle, which is goes into a gearbox due to the passage of wind over the blades generating a mechanical force. By using this gearbox, the rotational speed of the generator is developed and this rotational energy is converted into the electrical energy by using magnetic fields which is a function of this system. The output power received by a transformer which will convert the electricity from the generator to the power system is around 700 volts to the appropriate voltage typically 33kV.
Fig.1.4 Horizontal Axis Wind Turbine
The power produced from the wind is given by the product of half of the air mass per unit time and square of the wind velocity [25], Which is shown in the given below
Pair = (air mass per unit time) (wind velocity) 2
= ( AV ) (V ) 2 (1.1)
= AV 3
Where, Pair = the wind power in watts, = the air density ( at the standard condition is 1.225 kg/m3 at 15 C along with normal pressure), A = turbine blades swept area in mts, and V = wind velocity in meter per second.
From the above Equation provides the total air power which is available in the wind, the power coefficient (Cp) is used to reduce the total power, which is transferred to the wind turbine rotor.
Cp = P wind turbine / P air (1.2)
P wind turbine = Cp Pair = Cp AV 3 (1.3)
The value of Cp is defined by the Betz limit, which states that a turbine can never extract more than 59.3% of the wind power from an air stream but in reality wind turbine rotors have maximum Cp values in the range between 25-45%. The tip speed ratio is represented as follows
= R/ V (1.4)
Where, = the rotational speed of rotor in rpm and R = the radius of the swept area in meter. The tip speed ratio as well as the power coefficient Cp are the dimensionless.
1.4 DFIG WITH BACK TO BACK CONVERTER IN WIND ENERGY
1.4.1 Doubly Fed Induction Generator (DFIG)
Generally a wound rotor motor is also called as a doubly fed induction generator (DFIG). The term Doubly-fed gives the meaning as, it having the rotor along with stator of an induction machine with a wound rotor coupled to the electrical sources. This is popular for generation applications now-a-days with limited variable speed range. The DFIG can generate or absorbs power from both the stator and the rotor of a machine depending upon the speed of the shaft [27-28].
By changing the slip (s) value of an induction machine, the control and operation of a doubly fed induction generator (DFIG) system can be operated in three different modes as shown in [1].
_r< _s s >0…, Sub-synchronous mode
_r > _s s <0…, Super-synchronous mode
_r = _s s =0…, Synchronous mode
Fig.1.5 DFIG System Block Diagram [16]
The operation and control of a Doubly Fed Induction Generator (DFIG) system is introduced in the multi-MW wind turbines; it is more efficient to power generator. The aero-dynamic wind turbine system must be able to operate with wide range of wind speeds in place to attain an excellent aero-dynamic capability by fallowing the tip speed ratio. Consequently, the rotor of an induction generator should be capable to run at a variable rotational speed. Along with a rotor speed range around the synchronous speed of the rotor, an induction generator can be operated in both the modes such as sub and super synchronous modes. The stator of an induction generator is coupled with the grid, where as the rotor with 3-phase back to back (BTB) converter connected via with slip-rings. In the variable wind speed systems having small speed ranges typically 30% of the synchronous speed, the DFIG offers an adequate performance and it is also sufficient for the speed range required to exploit the typical wind energy resources [15].
In an induction generator, the power produced from the rotor is typically 30 percent with the formal generator power of an induction machine and the power electronic devices, which are used in the DFIG system, can be rated to handle a fraction of the total power produced from the generator. Consequently the converter devices can produce losses in the system; they can be reduced by using different control techniques such as direct and in-direct space vector controls. Due to the power electronic devices, the system cost is lower and the converter has to handle the entire power in system [16].
1.5 MODELING CONSIDERATIONS OF BTB
Fig.1.6 Schematic Diagram of Back-to-Back Converter
1.5.1 The Rotor-Side Converter (RSC)
The rotor of an induction generator can be excited by using this RSC control converter and the main aim of this RSC converter is to maintain the currents produced from the rotor. At the shaft of an induction generator, the required torque can be maintained depending upon an optimal orientation of flux position of a rotor with respect to stator. RSC provides a torque controller which is used to control the active as well as reactive powers generated at the stator terminals of the machine. The output power can be regulated by using power speed characteristics of the wind turbine is used to extract the maximum power tracking.
The active power (output power) produced at an induction generator workstation, will increases the losses such as electrical as well as mechanical losses. These power losses were differentiating with the reference power, which is acquired from the turbine characteristics. Generally a PI regulator can be employed to decrease the rotor speed error to zero at the outer loop. This PI regulator s output must be introduced into the rotor of an induction generator by using RSC control as a rotor reference current i _(rq (ref)) and also T_e is controlled by the quadrature axis (q-axis). The actual rotor current component (i_rq) is differentiated with the reference rotor current component ( i_(rq (ref))) and an error produced by these two components can be reduced to zero by a proportional integral current regulator at the inner current loop. The voltage v_rq is the output of this current controller generated by the RSC. Similarly the required 3-phase voltages applied to the rotor winding are obtained [16].
1.5.2 The Grid-Side Converter (GSC)
The Grid side converter aims to regulate the voltage of the dc bus capacitor. Furthermore, the grid side converter can permit the production or consumption of reactive power for the voltage stability requirements. Grid side converter controller having two control loops such as inner and outer regulation loops. Firstly an outer regulation loop having a DC voltage regulator and it gives an output current i _(cd (ref)), which is further used as a reference current for the current regulator. Secondly, the inner current regulation control loops having a current regulator which is used to control the magnitude as well as phase voltage produced by the back to back converter [16].
1.5.3 Converter losses
The converter having two types of losses, they are switching losses and conducting losses. Due to the turn on and turn off of the converter switches the switching losses were generated. For the diodes the switching losses mainly consist of turn off losses i.e., reverse recovery energy. The switching losses for the transistor and the reverse recovery energy loss for a diode can be found from data sheets. The conduction losses arise due to the passage of the current through the transistors and diodes. The transistor and the diode can be modeled as constant voltage drops, and a resistance in series. The transistor switching losses can be considered to be directly proportional to the current for a given dc-link voltage. For a given dc capacitor link voltage and switching frequency, the switching losses of the IGBT and the diode can be modeled as a constant voltage drop, which is independent of the current rating of the valves (Petersson, 2005).
1.5.4 DC-link model
This DC capacitor link is connected between both the converters such as GSC and RSC, and it provides a voltage at constant efficient inversion. It describes the DC capacitor link voltage variations as a function of the DC link input power. The energy stored in the dc capacitor is
W_dc= P_dc dt=1/2 CV_dc^2 (1.5)
Where v_dc is the dc capacitor link voltage, w_dc is the energy stored in the capacitor link, and P_dc is the input power to the DC link [16]. The voltage and energy derivatives are
(dV_dc)/dt=P_dc/(CV_dc ),(dW_dc)/dt=P_dc (1.6)
The p_dc can be measured by using this formula which is represented as given below
P_dc = P _in – P_c (1.7)
Where
The input power P_in is produced from the RSC converter where P_c is the output power from the GSC. The input power to the DC link, P_dc is obtained from the difference between input and output powers and it is constant, when P_dc=0 [16].
1.6 THESIS LAYOUT
Following the chapter on Introduction, the rest of the thesis is delineated as follows.
Chapter 2 gives the information about the literature survey on the fundamental concepts of wind electricity generation along with DFIG-BTB system used for the power generation. And it also gives explanation on the diminution of harmonics in back-to-back converter by using filter topology to extract the maximum mechanical power as of the wind.
Chapter 3 give details on the arrangement of the DFIG-system with two LCL filters and also explains the constraints on the designing of LCL filter. In this chapter, it explains the problem identification of LCL filters with DFIG system is explained.
Chapter 4 presents the proposed solution of an LLCL filter. It provides the modeling considerations and step by step design procedure of LLCL filter.
Chapter 5 gives all the simulation results which are found by using MATLAB/ SIMULINK environment, and also includes the results which are validated experimentally.
Chapter 6 gives the review along with ending of this work, undertaken in this thesis. And also acknowledge about the future work. The references, which I have used for the intention of research work, are also showed within this section.