CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION 1.1 GENERAL Electrical energy is a basic input for stimulating industrial and agricultural growth. India s rapidly growing economy and population leads to persistently increasing electricity demand. Despite the massive power generation capacity additions, the Indian government is struggling to keep up with growing demand. Commercial primary energy consumption in India has grown by about 700% in the last forty years. The per capita consumption in India is at the rate of 400 kwhr per annum. It has been estimated that 60% of total energy requirement in India is derived from conventional fuels and nonconventional fuels contributes the rest 40%. Total energy produced in the form of electricity is as follows: 60% from coal, 25% from hydel power, 4% from diesel and gas, 2% from nuclear power and less than 1% from nonconventional sources like solar, wind, ocean, biomass etc. In India, after independence, the emphasis has shifted from agriculture related activities to industry related activities with a view to achieve optimum benefits from both the sectors. Accelerated use of clean and green energy offers a great remedy for the three significant problems facing the world today that include depletion of fossil fuels, environmental sustainability and climate change. The search for new energy technologies has intensified in the last two decades. Wind power, with an average growth rate

2 2 of 30%, has emerged as the leading green energy technology. Harnessing of wind energy could play a significant role in the energy mix of each nation. The sun radiates 1 PW (Peta Watt) or 1014 kwhr, of energy every hour; as little as 3% of this if being converted into wind energy, will be able to meet the global power demand today. It is estimated that about MW of usable power is continuously available in the earth s winds. The magnitude of this vast potential is in striking contrast with that of the hydro power potential of the earth. Globally, wind power is growing at a record rate of more than 30% per annum. As per projections made by the Global Wind Energy Council, the present installed capacity is GW. The size of the global annual wind market is expected to reach 62.5 GW by 2014 and the cumulative capacity by 2014 would be around 409 GW. Wind Turbine Generators (WTG) of large capacities from 225 kw to 2.1 MW are often grouped together in wind farms because this is the most economical way to create electricity from the wind. Almost all the commercial WTG have the axis of rotation in horizontal direction and hence they are termed as horizontal axis WTG. A typical wind farm layout with the required transmission and distribution systems is illustrated in Figure 1.1. Most wind farms are located on land, but wind farms can also be offshore on the ocean or sea. Proper micrositing should be done to install the WTG to reduce the energy loss of about 2-5% due to Wind Park Effect. If multiple WTG are located too close to one another, their efficiency will be reduced. Each WTG extracts some energy from the wind and hence the downwind of a turbine will be slower and more turbulent. For this reason, WTG in a wind farm are typically installed five times the rotor diameter of WTG apart perpendicular to the predominant wind direction and seven times the respective rotor diameter apart parallel to the predominant wind.

3 3 The WTG are connected in parallel. The kinetic energy in the wind is converted into rotary mechanical energy by the wind turbine rotor. The mechanical energy is stepped up by suitable gear transmission, which couples the rotor and the generator. This is converted into electrical energy by the generator and the power thus generated may be supplied to the grid or to an isolated load through necessary switching and protection systems, transmission lines and transformer. Main Circuit Breaker Power Transformer Wind Turbine Generators Overhead Transmission Line State Electricity Board Sub-Station 11 kv / 22 kv / 33 / 66 kv Utility Grid Sub-Station 220 V AC Consumer Figure 1.1 General schematic of wind farm layout

4 4 1.2 WIND POWER IN INDIA In India, wind power is at the threshold of a new era. The wind power programme in the country was initiated towards the end of the sixth plan, in A market-oriented strategy was adopted from inception, which has led to the successful commercial development of the technology. The Indian wind energy sector has shown impressive growth in the past few years and investments into the sector have increased significantly. The total potential for wind power in India was first estimated by the Centre for Wind Energy Technology (C-WET) at around 45 GW, and was recently increased to 48.5 GW, which is presently adopted by the Government as the official estimate. Wind in India is influenced by the strong south-west summer monsoon, which starts in May-June, when humid air moves towards the land and the weaker north-east winter monsoon, which starts in October, when dry air moves towards the ocean. During the period March to August, the wind is uniformly strong over the whole Indian peninsula, except the eastern peninsular coast. The speed of the wind during the period November to March is relatively weak, though higher winds are available during a part of the period on the Tamil Nadu coastline. India is world s fifth largest producer of wind power after China, USA, Germany and Spain with a total installed capacity of MW as of July 30, Wind power has been concentrated in a few regions, especially the southern state of Tamil Nadu, which maintains its position as the state with the largest wind power installation, with 6160 MW installed till July 2011, representing 42% of India s total installed capacity. Tamil Nadu being the first state to introduce wind farms in the country has achieved tremendous success in harnessing renewable energy for

5 5 generation of grid quality power. The state is endowed with three prominent passes having high wind potential namely Aralvaimozhy pass in Kanyakumari District, Shengottah pass in Tirunelveli District and Palghat pass in Coimbatore District. The average annual wind velocity for the above locations is assessed as km/hr, km/hr and km/hr respectively. Recent study has identified some wind potential sites in coastal area near Chennai, Rameswaram, Palani and Theni. The Indian Government expects wind energy to contribute 10% of the total power generation capacity by the year 2012 and have 4% 5% share in the electricity mix. The details of wind power projects implemented in different states are given in Table 1.1. Table 1.1 Details of wind power projects in India as on March 2011 State Gross Potential (MW) Total Installed Capacity (MW) till March 2011 Andhra Pradesh Gujarat Karnataka Kerala Madhya Pradesh Maharashtra Orissa Rajasthan Tamil Nadu Others 4 Total (All India) 48,561 14,158 Courtesy: Indian Wind Energy Association The state-wise data of so far generated units from wind farms are shown in Table 1.2. A total unit of Billion kwhr until January 2011 has been generated from the wind power. Due to the spectacular growth of wind power projects in Tamil Nadu, the total units generated is around Billion kwhr which comes about 52% of India s total wind power generation.

6 Table 1.2 State-wise unit generation data (Billion kwhr) from wind power projects as on January 2011 Sl. No. State Upto Mar 2005 Year-wise Generation (Billion kwhr) Cumulative Upto Generation Jan 2011 (Billion kwhr) 1 Andhra Pradesh Gujarat Karnataka Kerala Madhya Pradesh Maharashtra Rajasthan Tamil Nadu TOTAL (Billion kwhr) Courtesy: Ministry of New & Renewable Energy 6

7 7 In the last ten years, India has emerged as a major wind power market in the world. As of March 2011, its cumulative installed capacity is 14,158 MW, with the wind power market growing at an average rate of 23% over the past 3 years. Its development benefits range from fiscal and financial incentives, including provision of 80% accelerated depreciation, a ten-year tax holiday and favourable provisions on wheeling, banking and third party sale. Presently there are about 23 approved wind turbine manufacturers by the C-WET with different technologies in harnessing the wind power. Courtesy: Indian Wind Energy Association Figure 1.2 Development of wind power in India and Tamil Nadu The development of wind power in the whole of India and only Tamil Nadu during the last six years is illustrated in Figure 1.2. The World Institute of Sustainable Energy predicts an annual market of 5000 MW of wind power by the year Emerging technologies like repowering, offshore wind, etc., are also being explored. The fabulous growth of wind farms in Tamil Nadu is attributable to early efforts of Government in assessing the wind resource potential in the state, setting up demonstration wind farms and the encouraging policies

8 8 adopted to attract private investors. The various factors that influence the investment in wind power in Tamil Nadu are: i. Conducive and consistent policies of the state Government / Tamil Nadu Electricity Board (TNEB) such as attractive wheeling and banking facilities on charges of 5% each. ii. Favourable terrain in potential locations with easy accessibility. iii. Higher plant load factor of 30% and therefore higher power generation per MW. iv. Reasonable power Rs. 3.49/- per kwhr and regular payment by TNEB. v. Adequate infrastructure for power evacuation including approval for investors to put up their own sub-stations. Even though the wind turbine technology appears to have become mature, most of the European designs have not been fully adapted to suit the weak grid conditions prevailing in India. Large capacity WTG in the MW range which were introduced in Europe in late 90 s are now popular, but the medium range WTG from 225 kw to 750 kw sizes are still in demand. The simpler design with asynchronous generators continues to be the preferred option. There is no systematic study for evaluating the performance of wind farms. Electrical losses within the wind farm during medium and low wind periods appear to be high and abnormal. The main reasons for electrical losses include inappropriate VAR compensation technology for the wind turbine driven induction generators, switching losses in power converters, losses in power transformer etc. Hence a detailed study on the performance of the wind farms such as power generation, power consumption and power factor

9 9 improvement techniques are required to identify suitable methodologies for reducing the losses. This has been the motivation for this thesis work. 1.3 LITERATURE REVIEW Wind Turbine driven Induction Generators are widely used nowadays due to low cost, minimal operation and maintenance, ruggedness and asynchronous operation. Bansal et al (2003) provides a complete review of 257 papers on the application of induction generators in renewable energy systems. Detailed literature review pertaining to the following aspects is done: Issues in commercial wind farms Performance analysis of self-excited induction generator in wind turbines Control of isolated wind energy conversion systems Doubly-fed induction generators in wind turbine applications Application of matrix converters for wind turbine generators Issues in Commercial Wind Farms The interest among private investors in setting up of grid connected commercial wind farm projects is a notable feature in India. As per the information given by Tamil Nadu Energy Development Agency (TEDA), the private investment in wind power exceeds Rs crores. Majority of these commercial wind farms are equipped with horizontal axis WTG. As most of the existing WTG are of European designs, several problems are being faced during their operation in Indian grid conditions. Generally the stability of Indian power systems is not sufficient to cater to the needs of increase in wind farms in a few rural areas as the existing transmission and distribution grids are very weak. Hence the power quality issues such as steady state voltage,

10 10 power factor, flicker and harmonic distortion are also to be analyzed. The power quality of the grid influences the performance and safety of the WTG and the life time of mechanical and electrical components. Wind turbines can operate in either constant speed or variable speed modes. Variable speed wind turbines are mostly preferred due to reduced mechanical stress and possibility of active and reactive power control, (Ackermann et al 2002). A power electronic interface is used in these WTG, which makes it possible to control the rotor speed. Thus the power fluctuations caused by variations in wind velocity can be minimized. In terms of energy capture, all studies come to the same result that variable speed turbines will produce more energy than constant speed turbines (Carlson et al 1996 and Zinger et al 1997). Reactive power management in wind farms has become an important criterion in Tamil Nadu as TNEB is insisting penalty for the wind turbine promoters who are not maintaining the power factor above In a wind farm, the induction generator and transformer contribute to lagging reactive power and capacitors contribute to leading power factor. The burden of severe VAR lagging load on the weak grid must be relieved by connecting balanced shunt capacitors across the induction generator terminals. These draw leading current or equivalently feed lagging magnetizing current of the generator (Tarek Ahmed et al 2006). To handle dynamic disturbances, the reactive power control has to be fast in order to provide effective voltage and power flow control and thereby a significant improvement in system stability. In the past, contactors were used for switching the capacitors in and out of the system manually (Mohan Mathur et al 2002). Presently to avoid penalty from TNEB, several reactive power compensation schemes are employed. Venkatesh et al (2000) detailed the comparison of various compensation schemes.

11 11 Reactive power is a key element in maintaining voltage and synchronous stability and ensuring proper power system performance. Two aspects of reactive power in today s power systems are the best design and operating practices and the effect of industry restructuring (Lin et al 2006). Hence a detailed case study in an existing wind farm equipped with several reactive power compensation schemes is required to evaluate the active and reactive power flow analysis Self-Excited Induction Generator The process of self-excitation is found to be significant in isolated power systems. Several literature deals with the concept of self-excitation phenomena in wind turbines employing induction generators. Elder et al (1983) elaborates the physical phenomena of self-excitation process. The necessity of residual magnetism and the required capacitor bank across the generator terminals are clearly explained. For an induction generator to get excited by itself, a proper combination of speed, load and capacitance is required (Chakraborty et al 1998). To build up voltage across the generator terminals, minimum capacitance is required. The concept of occurrence of excitation failure along with the determination of critical capacitance dealt in detail by Mohammed Orabi et al (2004). To simulate the wind turbine characteristics through a suitably controlled drive in the laboratory, a DC shunt motor coupled with a three phase induction machine is used for prototype studies. Therefore the DC motor characteristics are modified to match the characteristics of the wind turbine. The output power of the wind turbine, which equals the input power to the induction generator shaft, now becomes the corresponding output

12 12 power of the DC motor simulating the wind turbine (Yegna Narayanan and Johnny 1986). The steady state and dynamic modeling of the self-excited induction generator are presented and analyzed by several authors. Krause and Thomas (1965) proposed the d-q (direct-quadrature axis) reference model initially and several modifications have been made further. For dynamic analysis of self-excited induction generator, d-q axis model based on the generalized machine theory is employed (Natarajan et al 1987). A novel control scheme, which decouples the regulation of DC link current and the reactive power minimization, is also described in detail Isolated Wind Energy Conversion System In isolated systems, squirrel cage induction generators with selfexcited capacitors, are very popular (Bansal, 2005). They are directly used to feed the local loads where the utility grid does not exist. Seyoum et al (2003) describes the loading analysis of an isolated induction generator and how the operating frequency and generated voltage are affected by the change in operating slip value for regulated and unregulated rotor speed. Also the effect of magnetizing inductance on selfexcitation is discussed. The generalized dynamic modeling of a variable speed wind energy conversion scheme employing self-excited induction generator with AC-DC- AC converter is explained by Kumar and Kishore (2006). The transient characteristics for various operating conditions are presented and analyzed. To obtain an output voltage with constant magnitude and frequency, a sinusoidal pulse width modulated control strategy is proposed. The required variation in capacitance for a given load to maintain constant output voltage is

13 13 explained. For voltage sensitive loads, a neuro controller to regulate the output of the self-excited generator feeding local loads is proposed by Rajambal and Chellamuthu (2005). Detailed analysis is focused on maximization of power which is essential for heating loads rather than the regulation of output voltage. Wekhande and Agarwal (2001) propose a closed loop controller for the Pulse Width Modulated (PWM) inverter used in an isolated wind energy conversion system for regulating the output voltage. To extract maximum power obtained from the wind, a fuzzy logic controller to control the modulation index of the PWM inverter is explained by Hilloowala and Sharaf (1996). The modeling of wind turbine generator along with the network components like uncontrolled rectifier, filter and PWM inverter are presented in detail Doubly-Fed Induction Generator Ackermann and S oder (2002) stated that the variable speed WTG are mostly preferred for larger power rating in the range of 2.5 MW to 5 MW due to added advantages such as reduced mechanical stress and possibility of active and reactive power control. Pitch controlled variable speed wind turbines may be employed with Doubly-Fed Induction Generators (DFIG) with gear mechanism or direct driven synchronous generators (SG). The main advantage of DFIG as mentioned by Yazhou Lei et al (2006), Palo Ledesma and Julio Usaola (2005) and Tapia et al (2003) is that, the power electronic interface used in the rotor circuit has to handle only (20 30) % of the total system power. Generally two back-to-back pulse width modulated voltage source inverters with an intermediate DC link are used (Rajib Datta and Ranganathan 2002).

14 14 The speed or torque of the DFIG and the power factor at the stator terminals are controlled by the rotor side converter. By proper switching of both converters, the power flow between the rotor circuit and the grid / load can be controlled both in magnitude and in direction. This is effectively the same as connecting a controllable voltage source to the rotor circuit (Janaka Ekanayake et al 2003). The equivalent circuit of a DFIG can be expressed in different reference frames (Lie Xu and Yi Wang 2007). The significance of control on active and reactive power requires accurate modeling, control and selection of appropriate wind energy conversion systems as stated by Chitti Babu and Mohanty (2010) Matrix Converter Many theoretical studies are available on matrix converter but have found very few practical applications in wind energy systems. As stated by Venturini (1908), matrix converter has been known to offer an "all silicon" solution for direct AC-AC conversion, eliminating the DC link consists of energy storing elements in conventional AC-DC-AC converters. Many benefits are encompassed by this topology such as, the matrix converter fulfills the requirements to provide a sinusoidal voltage at the load side and on the other hand, it is also possible to adjust the unity power factor on the mains side under certain conditions. The topology of matrix converter was first proposed by Gyugyi and Pelly (1976). Single phase matrix converter was first realized by Zuckerberger et al (1997). Sunter and Aydogmus (2008), proposed a detailed analysis along with the required characteristics of a single phase AC-AC converter drive with particular emphasis on the harmonic content, input voltage utilization and variable speed performance of the motor load.

15 15 The main drawbacks of AC-DC-AC conversion are increased size and weight, low reliability of DC link capacitor; poor line power factor and significant harmonic distortion in line and machine currents (Vinod Kumar et al 2009). The IEEE Standard 519 (1991) severely restricts line harmonic injection. Bernet et al (2002) proves that the semiconductor losses are reduced to 22% in matrix converter compared to voltage source inverter and it is also possible to reduce current rating by 33%. A simplified version of the Venturini algorithm as discussed by Sunter (1995) is used here to simulate and analyze the performance of the matrix converter. Space vector modulated matrix converters are dealt by Ebubekir Erdem et al (2005) and Casadei et al (1993). It is also inferred that the space vector modulation algorithm eliminates the third order harmonics and allows maximum voltage ratio. 1.4 OBJECTIVES OF THE THESIS The main objective of the thesis is to investigate the performance of a horizontal axis WTG and its system components for varying operating conditions. The specific objectives are as follows: 1. Investigation of the issues related to reactive power compensation methodologies employed in commercial wind farms and to enhance the power yield from them by minimizing the major stoppages to evaluate wind farm losses by conducting a systematic survey

16 16 to analyze the criteria for low power production during low and medium wind speeds to identify an optimum solution for reactive power consumption in order to avoid the penalty for low power factor imposed by TNEB to enhance the performance of the existing machines by modifying the design of some components 2. Analyzing the performance of isolated wind energy conversion schemes to estimate the capacitance requirements for wind turbine driven self-excited induction generator to model the system components with necessary power converter and analyzing the performance for various operating conditions to design and implement an artificial neuro controller for regulation of terminal voltage 3. Dynamic modeling and analysis of doubly-fed induction generators to model a wind turbine driven doubly-fed induction generators along with the network components feeding a local load to study the transient performance for varying wind velocities and load conditions

17 17 4. Design and implementation of matrix converters for wind turbine applications to study and validate the characteristics of a sinusoidal pulse width modulated single phase matrix converter for variable frequency applications to compare and analyze the various configurations of three phase matrix converter using Venturini algorithm and space vector modulation technique through simulation studies to design and implement a prototype model of Field Programmable Gate Array (FPGA) based three phase matrix converter employing sinusoidal pulse width modulation 1.5 ORGANIZATION OF THE THESIS Chapter 1 briefs the significance of the performance measures in the existing commercial wind farms. It gives a general introduction and important features about harnessing wind power in India and Tamil Nadu. Chapter 2 deals with the basic concepts of horizontal axis WTG used in commercial wind farms; their components and the working principle have been briefed in detail. The aerodynamic power conversion in wind turbine rotor is explained with the theoretical concepts and the practical power characteristics are discussed. The various generators employed in different types of commercial wind turbines are elaborated with their salient features. Finally the essential requirements for installing a wind farm are presented.

18 18 In Chapter 3, the performance issues in commercial wind farms are discussed in terms of power quality and reactive power management. Since most of the wind farms are employed with induction generators, the effect of VAR consumption from the grid and hence the need for maintaining power factor above 0.85 to avoid penalty from TNEB are described. For various operating conditions, simulation results showing the real power generated from the WTG and the reactive power consumed from the TNEB grid are analyzed in detail. Due to the varying VAR demand with wind velocity, dynamic VAR compensation is proposed and the results are analyzed. A detailed survey in a wind farm employing different schemes of reactive power compensation methodologies is conducted and certain investigations in enhancing power yield from wind farms are presented. Also two investigations regarding the design modification in a group of existing WTG to overcome the machine stoppages and to increase the efficiency of the machines are presented. The results after implementation of the proposed design are also discussed in terms of power production and machine down times. Chapter 4 presents the dynamic modeling and analysis of a self-excited squirrel cage induction generator feeding a local load. The estimation of critical, maximum and minimum values of self-excitation capacitance is done in the laboratory model and a novel control strategy to emulate the characteristics of wind turbine on the DC shunt motor driving an induction generator in the laboratory model is designed. The variation of system voltage with the self-excitation capacitance and the load resistance is studied. Modeling and simulation of the stand-alone wind energy conversion scheme with the network components like rectifier and inverter are done in MATLAB and the simulated results are validated with the

19 19 experimental set-up. From the power flow analysis, it is observed that the generator voltage, frequency and hence the output power from the WTG gets varied for various input and loading conditions. Hence a rectifier and PWM inverter set-up is connected in between the generator and load. The effect of changing the modulation index of the PWM inverter and self-excitation capacitance is studied for various operating conditions. To cater to the needs of the voltage sensitive loads, an artificial neuro controller is designed and simulated to regulate the generator terminal voltage for various operating conditions. The results of the simulation and implementation of the complete system behaviour for various inputs with controller are consolidated and presented. Chapter 5 describes the dynamic modeling and analysis of wind turbine driven DFIG feeding a local load which has entered the Indian market recently. The system is modeled based on the mathematical equations using MATLAB software and integrated with the other network components to analyze the transient response for different working conditions. In chapter 6, a direct AC-AC converter concept is explored for horizontal axis WTG. Single phase matrix converters employed with sinusoidal pulse width modulation is studied for various output frequencies. Both simulation and hardware models are presented to describe the effectiveness of the converter in terms of the power quality issues. Three phase matrix converters are employed to minimize the losses in the DC link of the conventional AC-DC-AC converter. The different topologies and switching strategies are explained in detail. The simulation results of Venturini algorithm based and space vector modulated matrix converters are analyzed in depth in terms of power quality measures. A prototype model of FPGA based three phase matrix converter is designed, fabricated and tested in the laboratory for different operating conditions.

20 20 In the concluding chapter 7, the major contributions of the research work are summarized. It further discusses the possible future scope for extension in the related area.