Comparison Analysis Of Wind Energy System With Synchronous & Induction Generators

Comparison Analysis of Wind Energy System with Synchronous & Induction Generators

ABSTRACT

Wind energy, as an alternative to fossil fuels, is plentiful, renewable, widely distributed, clean, and produces no greenhouse gas emissions during operation. The world has enormous resources of wind power. It has been estimated that even if 10% of raw wind potential could be put to use, all the electricity needs of the world would be met. A phased programme to develop wind energy in India started as early as 1985, and today the total installed capacity has reached 1650 MW, saving about 935,000 metric tonnes of coal.

Wind electrical generation systems are the most cost-competitive of all the environmentally clean and safe renewable energy sources in the world. They are also competitive with fossil fuel generated power and much cheaper than nuclear power.

Traditionally, wind generation systems used variable pitch constant speed wind turbines (horizontal or vertical axis) that were coupled to squirrel cage induction generators or wound-field synchronous generators and fed power to utility grids or autonomous loads.

The recent evolution of power semiconductors and variable frequency drives technology has aided the acceptance of variable speed generation systems. Such systems can yield 20-30% more power than constant-speed generation systems.

CHAPTER 1

INTRODUCTION

The conventional energy sources are limited and pollute the environment. So more attention and interest have been paid to the utilization of renewable energy source such as Wind Energy, Fuel Cell, Solar Energy etc., Wind Energy is the fastest growing and most promising renewable energy source among them as it is economically viable.

1.1 WIND ENERGY IN INDIA

In 2015, India was the country that brought online the third largest amount of wind energy, after the US and China, and it now ranks fifth in total installed capacity with 19,645 MW of wind power installed at the end of 2015. A strong domestic manufacturing base has underpinned the growth of the Indian wind energy market.

The Indian wind turbine manufacturer Suzlon is now a recognised player on the global market and many international companies are established in India. India has a great untapped potential for wind energy.

A strong domestic manufacturing base has underpinned the growth of the Indian wind energy market.

India has a great untapped potential for wind energy. According to official estimates, the Country's total wind energy resource amounts to 100 GW of installed capacity, but some experts think that this figure is on the conservative side, and that technological improvements could significantly increase this potential.

The positive development of wind energy in India has mainly been driven by progressive state level legislation, including policy measures such as renewable portfolio standards and feed-in-tariffs. At the moment, there is no coherent national renewable energy policy to drive the development of wind energy. This is urgently needed to realize the country‟s full potential and reap the benefits for both the environment and the economy.

The Government of India is currently considering the introduction of a national renewable energy policy, so this report comes as a timely reminder of how important a role wind energy could play in securing India’s energy security, curbing its CO2 emissions, providing new employment and boosting economic development.

This also realizes how important a role wind energy could play in securing India's energy security, curbing its CO2 emissions, providing new employment and boosting economic development. As can be seen by the Indian Wind Energy Outlook, the wind industry, both domestic and international, stands ready to do its part in achieving an energy revolution in India.

1.2 ECONOMY OF WIND ENERGY IN INDIA

In the early 1980s, the Indian government established the Ministry of Non-Conventional Energy Sources (MNES) to encourage diversification of the country's energy supply, and satisfy the increasing energy demand of a rapidly growing economy. In 2006, this ministry was renamed the Ministry of New and Renewable Energy (MNRE). Renewable energy is growing rapidly in India. With an installed capacity of 13.2 GW, renewable energy sources (excluding large hydro) currently account for 9% of India's overall power generation capacity.

By 2012, the Government of India is planning to add an extra 14 GW of renewable resources in its 10th Five Year Plan. The Government of India had set itself a target of adding 3.5 GW of renewable energy sources to the generation mix. In reality, however, nearly double that figure

was achieved. In this period, more than 5.4 GW of wind energy was added to the generation mix, as well as 1.3 GW from other Resources.

The Indian Ministry of New and Renewable Energy (MNRE) estimates that there is a potential of around 90,000 MW for the country, including 48,561 MW of wind power, 14,294 MW of small hydro power and 26,367 MW of biomass In addition, the potential for solar energy is estimated for most parts of the country at around 20 MW per square kilometre of open, shadow free area covered with 657 GW of installed capacity.

1.3 WIND POTENTIAL

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. This figure was also adopted by the government as the official estimate.

The C-WET study was based on a comprehensive wind mapping exercise initiated by MNRE, which established a country-wide network of 105O wind monitoring and wind mapping stations in 25 Indian States. This effort made it possible to assess the national wind potential and identify suitable areas for harnessing wind power for commercial use, and 216 suitable sites have been identified.

However, the wind measurements were carried out at lower hub heights and did not take into account technological innovation and improvements and repowering of old turbines to replace them with bigger ones At heights of 55-65 meters, to replace them with Bigger ones. At heights of 55-65 meters, the Indian Wind Turbine Manufacturers Isolation (IWTMA) estimates that the

potential for wind development in India is around 65-70 GW. The World Institute for Sustainable Energy, India (WISE) considers that with larger turbines, greater land availability and expanded resource exploration, the potential could be as big as 100 GW. Wind power in India has been concentrated in a few regions, especially the Southern state of Tamil Nadu, which maintains its position as the state with the most wind power, with 4.1 GW installed at the end of 2008, representing 44% of India‟s total wind capacity.

1.4 WIND FARMS IN INDIA

1. Muppandal–Perungudi (Tamil Nadu)

With an aggregate wind power capacity of 450 MW, the Muppandal –Perungudi region near Kanyakumari in Tamil Nadu has the distinction of having one of the largest clusters of wind turbines. About Rs 2500 crores has been invested in wind power in this region.

2. Kavdya Donger, Supa (Maharashtra)

A wind farm project has been developed at Kavdya Donger at Supa, off the Pune–Ahmednagar highway, about 100 km from Pune. This wind farm has 57 machines of 1-MW capacity each. Annual utilization capacity of up to 22% has been reported from this site. The farm is connected through V-SAT to project developers as well as promoters for online performance monitoring.

3. Satara district (Maharashtra)

Encouraging policy for private investment in wind power projects has resulted in significant wind power development in Maharashtra, particularly in the Satara district. Wind power capacity of about 340 MW has been established at Vankusawade, Thosegarh, and Chalkewadi in Satara district, with an investment of about Rs.1500 crores.

CHAPTER 2

WIND ENERGY-GENERATING SYSTEMS

2.1 WIND TURBINES:

Wind turbines produce electricity by using the power of the wind to drive an electrical generator. Passing over the blades, wind generates lift and exerts a turning force. The rotating blades turn a shaft inside the nacelle, which goes into a gearbox. The gearbox adjusts the rotational speed to that which is appropriate for the generator, which uses magnetic fields to convert the rotational energy into electrical energy. The power output goes to a transformer, which converts the electricity from the generator at around 700V to the appropriate voltage for the power collection system, typically 33 kV.

A wind turbine extracts kinetic energy from the swept area of the blades. The power contained in the wind is given by the kinetic energy of the flowing air mass per unit time.

That is

Pair = 0.5ρAV3

Where Pair is the power contained in wind (in watts) , ρ is the air density (1.225 kg/m3 at 15°C and normal pressure), A is the swept area in (square meter), and V is the wind velocity without rotor interference, i.e., ideally at infinite distance from the rotor (in meter per second).

Although the above equation gives the power available in the wind, the power transferred to the wind turbine rotor is reduced by the power coefficient, C

C = P wind turbine / Pair

P wind turbine = 0.5ρCAV3

Maximum value of Cp is defined by the Betz limit, which states that a turbine can never extract more than 59.3% of the power from an air stream. In reality, wind turbine rotors have maximum Cp values in the range 25-45%.

Solidity: The solidity of a wind rotor is the ratio of the projected blade area to the area of the wind intercepted. The projected blade area is the blade area met by the wind or projected in the direction of the wind.

Solidity has a direct connection with the torque and speed. High-solidity rotors have high torque and low speed, and are employed for pumping water. Low-solidity rotors, on the other hand, have high speed and low-torque, and are usually suited for electrical power generation

SPECIFIED RATED CAPACITY:

Specified Rated capacity (SRC) is an important index which is used to compare a variety of wind turbine designs.

It varies between 0.2 (for small rotors) and 0.6 (large rotors)

2.2 CHARACTERISTICS OF WIND TURBINE:

Various Characteristics of wind turbine are plotted to have a better understanding.

POWER-SPEED CHARACTERISTICS:

Fig: 2.1 Typical Power versus speed characteristics of a wind turbine

TORQUE –SPEED CHARACTERISTICS: The typical torque versus speed characteristics of horizontal axis (two blade propeller Type) wind turbine is shown:

Fig: 2.2 Torque versus speed characteristics

The curve shows that for any wind speed the torque reaches peak value at a definite rotational speed, and this maximum torque varies in the order of the square of rotational speed. Generally the load torque depends on the electrical loading. The torque can be made to vary as the square of the rotational speed by choosing the load properly.

Different control techniques such as Pitch angle control, Stall control (active and passive), Power electronic control and Yaw control are used to control the wind turbines.

CHAPTER 3

LITERATURE SURVEY

The performance and controllability of IG are excellent in comparison with SG systems; they capture more wind energy, they exhibit a higher reliability gear system, and high quality power supplied to the grid. It saves investment on full rated power converters, and soft starter or reactive power compensation devices (fixed speed systems). Modern wind farms, with a nominal turbine power up to several MWs, are a typical case of IG application. Besides this, other applications for the IG systems are, for example, flywheel energy storage system, stand alone diesel systems, pumped storage power plants, or rotating converters feeding a railway grid from a constant frequency utility grid. In practical applications, the IG is gradually maturing as a technology for variable speed wind energy utilization. Although topologies of new systems with improved performance are emerging both in academia and industry, IG is the most competitive option in terms of balance between the technical performance and economic costs. The following sections discuss about the literature survey of mathematical modeling, designing of and various controller techniques such as proportional integral, resonant, direct torque, direct power and direct current controllers of IG.

Lie Xu et al (2007) presented an analysis and control design of a IG based wind generation system operating under unbalanced network conditions [13]. Variations of stator active, reactive powers and generator torque are fully defined in the presence of negative sequence voltage and current. A rotor current control strategy based on positive and negative dq reference frames is used to provide precise control of the rotor positive and negative sequence currents. The proposed control strategy, the enhanced system control and operation such as minimizing oscillations in active power, electromagnetic torque, stator and rotor currents are achieved. Yi Wang et al (2010) investigated the control and operation of DFIG and FSIG based wind farms under unbalanced grid conditions [14]. The behaviors of the DFIG and FSIG systems under unbalanced supplies described using a mathematical model. The performance of IG based wind farms can be improved by regulating the negative sequence current to eliminate torque, output power, and DC voltage oscillations. The coordinated control, for compensating voltage unbalance and torque ripple are presented. The proposed IG control system improved not only its own performance, but also the stability of the FSIG system with the same grid connection point during network unbalance.

Alvaro Luna et al (2011) presented the fault ride through (FRT) capability of IG in wind power applications. A simplified model of the IG is extracted from the classical 5th order model [15]. The mathematical models of such generators enabled to analyze their response under generic conditions. However, their mathematical complexity did not contribute to simplifying the analysis of the system under transient conditions and not help in finding straightforward solutions for enhancing. Also, accurately estimate the behavior of the system while significantly reducing its complexity is discussed.

SUMMARY

This chapter discussed about the literature review of modeling of IG, behavior analysis of IG . Based on this, mathematical modeling of IG at steady state, dq model of arbitrary and rotor reference frames are discussed in the next chapter.

CHAPTER 4

Induction generator

An induction generator or asynchronous generator is a type of AC electrical generator that uses the principles of induction motors to produce power. Induction generators operate by mechanically turning their rotor in generator mode, giving negative slip. In most cases, a regular AC asynchronous motor is used as a generator, without any internal modifications.

PRINCIPLE OF OPERATION:

Induction generators and motors produce electrical power when their rotor is rotated faster than the synchronous frequency. For a typical four-pole motor (two pairs of poles on stator) operating on a 60 Hz electrical grid, synchronous speed is 1800 rotations per minute. Similar four-pole motor operating on a 50 Hz grid will have synchronous speed equal to 1500 rpm. In normal motor operation, stator flux rotation is faster than the rotor rotation. This is initiating stator flux to induce rotor currents, which create rotor flux with magnetic polarity opposite to stator. In this way, rotor is dragged along behind stator flux, by value equal to slip. In generator operation, a prime mover (turbine, engine) drives the rotor above the synchronous speed. Stator flux still induces currents in the rotor, but since the opposing rotor flux is now cutting the stator coils, active current is produced in stator coils, and motor is now operating as a generator, and sending power back to the electrical grid.

Grid and stand-alone connections:

In induction generators the magnetizing flux is established by a capacitor bank connected to the machine in case of stand-alone system and in case of grid connection it draws magnetizing current from the grid.

 For a grid connected system, frequency and voltage of the machine will be dictated by the electric grid, since it is very small compared to the whole system.

 For stand-alone systems, frequency and voltage are complex function of machine parameters, capacitance used for excitation, and load value and type.

GRID CONNECTED INDUCTION GENERATOR

Grid connected induction generators develop their excitation from the Utility grid. The generated power is fed to the supply system when the IG is run above synchronous speed. Machines with cage type rotor feed only through the stator and generally operate at low negative slip. But wound rotor machines can feed power through the stator as well as rotor to the bus over a wide range known as Doubly Fed Induction Machines [2].

FIXED SPEED GRID CONNECTED WIND TURBINE GENERATOR:

The structure and performance of fixed-speed wind turbines as shown in Fig. 3.1 depends on the features of mechanical sub-circuits, e.g., pitch control time constants etc.

Fig 4.1: fixed speed wind turbine with directly grid connected squirrel-cage

induction generator

The reaction time of these mechanical circuits may lie in the range of tens of milliseconds. As a result, each time a burst of wind hits the turbine, a rapid variation of electrical output power can be observed. These variations in electric power generated not only require a firm power grid to enable stable operation, but also require a well-built mechanical design to absorb high mechanical stress, which leads to expensive mechanical structure, especially at high-rated power.

Variable Speed Wind Turbine Generator:

A way to make more convenient turbines is variable speed turbines. Variable speed turbines have become the most dominating type of the yearly installed wind turbines as they can store

some of the power fluctuations due to turbulence by increasing the rotor speed, pitching the rotor blades, these turbines can control the power output at any given wind speed.

Fig. 3.2 shows a variable speed turbine connected to a Squirrel- Cage Induction Generator SCIG. Although these direct-online systems have been built up to 1.5 MW, but presence of power inverter causes lots of disadvantages such as:

a) This power converter, which has to be rated at 1 p.u. of total system power, is expensive.

b) Converter efficiency plays an important role in total system efficiency over the entire operating range.

Fig 4.2: variable speed wind turbine with Synchronous generator

Another way is using Doubly Fed Induction Generator DFIG, as shown in Fig.3.3 It consists of a stator connected directly to grid and a rotor – via slip rings – is connected to grid through four-quadrant ac-to-ac converter based on insulated gate bipolar transistors (IGBTs)

This system offers the following advantages:

1. Reduced inverter cost, because inverter rating is typically 30% of total system power.

2. Improved system efficiency.

3. Power-factor control can be implemented at lower cost.

4. It has a complete control of active and reactive power.

Fig 4.3: Variable speed wind turbine with Synchronous generator

The Synchronous Generator with a power converter shown in Fig. 3.3 is a simple and highly controllable way to transform the mechanical energy from the variable speed rotor to a constant frequency electrical utility grid. The main reason for the popularity of the doubly fed wind induction generators connected to the national networks is their ability to supply power at constant voltage and frequency while the rotor speed varies.

Synchronous generator

Currently DFIG wind turbines are increasingly used in large wind farms. A typical DFIG system is shown in the below figure. The AC/DC/AC converter consists of two components: the rotor side converter Crotor and Grid side converter Cgrid .These converters are voltage source converters that use forced commutation power electronic devices (IGBTS) to synthesize AC voltage from DC voltage source. A capacitor connected on DC side acts as a DC voltage source. The generator slip rings are connected to the rotor side converter, which shares a DC link with the grid side converter in a so called back -to-back configuration. The wind power captured by the turbine is converted into electric power by the IG and is transferred to grid by stator and rotor windings. The control system gives the pitch angle command and the voltage commands for Crotor and Cgrid to control the power of the wind turbine, DC bus voltage and reactive power or voltage at grid terminals.

Fig 4.4 : A Synchronous generator and wind turbine system

OPERATION:

When the rotor speed is greater than the rotating magnetic field from stator, the stator induces a strong current in the rotor. The faster the rotor rotates, the more power will be transferred as an electromagnetic force to the stator, and in turn converted to electricity which is fed to the electric grid. The speed of asynchronous generator will vary with the rotational force applied to it. Its difference from synchronous speed in percent is called generator‘s slip. With rotor winding short circuited, the generator at full load is only a few percent.

With the DFIG, slip control is provided by the rotor and grid side converters. At high rotor speeds, the slip power is recovered and delivered to the grid, resulting in high overall system efficiency. If the rotor speed range is limited, the ratings of the frequency converters will be small compared with the generator rating, which helps in reducing converter losses and the system cost.

Since the mechanical torque applied to the rotor is positive for power generation and since the rotational speed of the magnetic flux in the air gap of the generator is positive and constant for a constant frequency grid voltage, the sign of the rotor electric power output is a function of the slip sign. Crotor and Cgrid have the capability of generating or absorbing reactive power and can be used for controlling the reactive power or the grid terminal voltage. The pitch angle is controlled to limit the generator output power to its normal value for high wind speeds. The grid provides the necessary reactive power to the generator.

Steady state characteristics:

The steady state performance can be explained using Steinmetz per phase equivalent circuit model as shown in figure where motor convention is used. In this figure vs and vr are the stator and rotor voltages, is and ir are the stator and rotor currents, rs and rr are the stator and rotor resistances (per phase), Xs and Xr are stator and rotor leakage reactance‘s, Xm is the magnetizing reactance and s is slip.

TORQUE-SLIP CHARACTERISTICS

Fig: 4.5 Torque-slip characteristic when the angle of Vr is 0|Vr| is changing from -0.05 to +0.05 pu.

Fig: 4.6 Torque-slip characteristic when |Vr| is 0.05 pu.

The angle of Vr is changing from −90◦ to +90◦.

CONTROL STRATEGIES FOR A PMSG:

1. Vector control

2. Magnitude and frequency control

VECTOR OR FIELD ORIENTED CONTROL THEORY:

The complete control strategy of the machine is divided in two ways, one is scalar control and the other is vector control. The limitations of scalar control give a significance to vector control. Though the scalar control strategy is modest to implement but the natural coupling effect gives sluggish response. The inherent problem is being solved by the vector control. The vector control is invented in the beginning of 1970s. Using this control strategy an IM can be performed like dc machine. Because of dc machine like performance vector control is also known as orthogonal, decoupling or Tran‘s vector control. Different Vector control strategies have been proposed to control the active and reactive power of an induction generator.

The basic of the vector control theory is d-q theory. To understand vector control theory knowledge about d-q theory is essential.

D-Q THEORY:

The d-q theory is also known as reference frame theory. The history says in 1920, R. H. Park suggested a new theory to overcome the problem of time varying parameters with the ac machines. He formulated a change of variables which replace the variables related to the stator windings of a synchronous machine with variables related with fictitious winding which rotates with the rotor at synchronous speed.

Essentially he transformed the stator variables to a synchronously rotating reference frame fixed in the rotor. With such transformation (Park‘s transformation) he showed that all the time varying inductances that occur due to an electric circuit in relative motion and electric circuit with varying magnetic reluctances can be eliminated. Later in 1930s H. C. Stanley showed that time varying parameters can be eliminated by transforming the rotor variables to the variables associated with fictitious stationary windings.

In this case the rotor variables are transformed to the stationary reference frame fixed on the stator. Later G. Kron proposed transformation of stator and rotor variables to a synchronously rotating reference frame which moves with rotating magnetic field. Latter, Krause and Thomas had shown that the time varying Inductances can be eliminated by referring the stator and rotor variables to an arbitrary reference frame which may rotate at any speed [5].

CHAPTER 5

Induction Generators for Wind Energy Conversion Systems:

Constant Voltage, Constant frequency Generation (Line excited Induction Generator):

These derive its excitations from the utility bus. It is also called line excitation. Rotor is driven by the prime mover at super synchronous speeds. Generator draws lagging VAr from the bus and feed real power to the supply system. The cage rotor induction generators feed only through the stator and operate at low negative slip. Wound induction generators can feed power through the stator as well as rotor and operate over a wide speed range.

Reactive Power and Harmonics:

The grid connected induction generator draws its excitation from the power line to setup its rotating magnetic field and demands lagging reactive power. Such reactive power given may adversely affect the voltage level particularly in weak public utility networks and increases system losses. For large wind turbines driving induction generators, the voltage fluctuation and flickering arising from power output variation may exceed the saturation limit of the utility system.

This lagging reactive power is drawn from the supply through stator, thus reducing KW output for same current loading. If the rotor side converter is force commutated or uses IGBTs or BJTs, it can meet the reactive power demand. If firing angle is made > 180₀ for sub synchronous speed operation. Thus it generates the leading VAr. Firing angle of Converter2 is always < 180₀ and for that reason it draws leading VAr from supply through the transformer. However force commutation of IGBT, unity or lagging p.f operation is possible and overall p.f of the system can be improved.

In the residual converter, SCR with 120₀ mode of conduction are used. They inject low order harmonics into the supply system. Converter1 also injects low order and turbine current harmonics to mmf waves. Using PWM Technique with both the converters harmonics spectrum is shifted form low order to high order which can be easily filtered with PWM converters. AC line current can be made to quasi sinusoidal with appropriate phase shift related to supply voltage. Thus p.f is improved and harmonics are eliminated. The smoothing reactor in the dc link reduces the ripple in DC link current. However it is expensive and makes the system bulkier.

The other option is to use a capacitor across the dc link instead of an inductor. IT reduces ripples in dc link voltage making it steady dc. When Converters 1 & 2 are PWM voltage fed type and use IGBTs following characteristics can be obtained.

Reactive Power Compensation:

In double output induction generators with slip power control, the reactive power demand of generators is generated by converter 1. Its firing angle is made > 180₀ for sub synchronous speeds and < 360₀ for super synchronous speeds. Thus converter acts like a variable capacitor providing the VAr requirement of Induction generator while transferring the real power to utility grid via dc link. However in squirrel cage induction generators, the reactive power demand is made by a bank of capacitors or other VAr compensators. VAr compensators improve voltage stability, increase network capability and decrease losses. Various types of VAr compensators are Thyristor controlled reactor, static VAr compensator.

Figure 5.1 – Various Devices used for reactive power compensation

Thyristor switched capacitors

A bank of capacitors are switched ON and OFF in response to pre-set voltage levels. Contractors used in old systems for switching are replaced by thyristors for faster control. Continuous control of VAr is not possible with TSC as capacitor would remain in the circuit for full cycle before thyristor switches off when current limiting reactors are used in series with capacitors to limit the current that may arise due to the difference between the supply and capacitor voltages at the switching ON instant. In a 3 phase system, capacitor banks are delta connected to avoid triple n harmonics in line currents.

2. Thyristor Controlled Reactor: The disadvantage of thyristor switched capacitor is discontinuous control of VAr is eliminated using Thyristor controlled reactor. Fixed capacitor bank rated at full load VAr demand of the induction generator is connected in parallel with the line variable VAr is realized by varying the firing angle between 90₀ and 180₀. The excess reactive power from the capacitor bank at reduced load is absorbed by the inductor when delay angle approaches 90₀. TCRs in a 3 phase system are Δ connected to avoid triple n harmonics in compensating line currents.

3. Static VAr compensator:

It is the recent trend in reactive power control using voltage source PWM inverters. IT is the static realization of the synchronous condenser. Inductors are connected in series with AC supply. The inverters generate or absorb reactive power depending on its AC output voltage which in turn controlled to witching of IGBTs. The inverter produces a set of balanced voltages at the output terminals whose fundamental component VR is in phase with corresponding AC system voltage Vs. So only reactive power flows between the converter and system. When inverter output voltage VR is greater than AC system voltage VS, inverter acts as a capacitor generating lagging VAr. If VR < VS inverter acts as an inductor absorbing lagging VAr. In a practical inverter to supply inverter losses, the inverter output voltage VR is made to lag behind AC system voltage in case of capacitor operation and lead the AC system Voltage in case of inductor operation.

Function of dc link capacitor is to eliminate or reduce the ripples in dc link voltage.

STATCOM:

A STATCOM or Static Synchronous Compensator is a regulating device used on alternating current electricity transmission networks. It is based on a power electronics voltage-source converter and can act as either a source or sink of reactive AC power to an electricity network. If connected to a source of power it can also provide active AC power.

A STATCOM works by rebuilding the incoming voltage waveform by switching back and forth from reactive to capacitive load. If it is reactive, it will supply reactive AC power. If it is capacitive, it will absorb reactive AC power. This is how it acts as a source/sink. Uses:

Usually a STATCOM is installed to support electricity networks that have a poor power factor and often poor voltage regulation. There are a number of other uses for STATCOM devices including, wind energy voltage stabilisation, and harmonic filtering. However, the most common use is for voltage stability.

CHAPTER 6

Synchronous Generators for Wind Energy Conversion Systems (PMSG):

In recent years, the electrical power generation from renewable energy sources, such as wind, is increasingly attraction interest because of environmental problem and shortage of traditional energy source in the near future. Nowadays, the extraction of power from the wind on a large scale became a recognized industry. It holds great potential showing that in the future will become the undisputed number one choice form of renewable source of energy. The force that pushes this technology is the simple economics and clean energy. As a consequence of rising fossil fuel price and advanced technology, more and more homes and businesses have been installing small wind turbines for the purposes of cutting energy bills and carbon dioxide emissions, and are even selling extra electricity back to the national grid. The kinetic energy in the wind is converted into mechanical energy by the turbine by way of shaft and gearbox arrangement because of the different operating speed ranges of the wind turbine rotor and generator. The generator converts this mechanical energy into electrical energy. Then generator side PWM converter convert this AC power into DC power, grid side converter convert this DC power into AC power and send to grid. However, as wind is an intermittent renewable source, the wind source extracted by a wind turbine is therefore not constant. For this reason, the fluctuation of wind power results in fluctuated power output from wind turbine generator. From the point of view of utilities, due to the fluctuation of generator output, it’s not appropriate for the generator to be directly connected to the power grid. In order to achieve the condition that the generator output power is suitable for grid-connection, it is necessary to use a controller to manage the output produced by the wind turbine generator. Permanent magnet machines are characterized as having large air gaps, which reduce flux linkage even in machines with multi-magnetic poles. As a result, low-rotational-speed generators can be manufactured with relatively small sizes with respect to its power rating. Moreover, the gearbox can be omitted due to low rotational Speed in the PMSG wind generation system, thus resulting in low cost. To increase the efficiency, to reduce the weight of the active parts, and to keep the end winding losses small, direct-drive generators are usually designed with a large diameter and small pole pitch. Compared with the traditional electrically excited synchronous generator, the requirement of a larger pole number can be met with permanent magnets which allow small pole pitch . In addition, permanents magnet synchronous generators (PMSGs) have the high torque density and the absence of excitation losses. Furthermore, the performance of PM’s is improving and the cost of PM is decreasing in recent years, the direct-drive permanent magnet wind generators have recently received increasing attention, especially for offshore wind energy. Thanks to the application of high energy PM materials such as neodymium-iron-boron, the volume and cost of this type of machine can be dramatically reduced. The distinct advantages offered by PMSGs are simple rotor design without field winding. The absence of field windings also results in higher efficiency since heat dissipation is avoided. PMSG is gaining a lot of attention for WECS due to compact size, high reliability higher power to weight ratio, reduced losses and robustness.

Fig 6.1 Wind Turbine

MODEL OF PMSG WIND TURBINE

(A)Structure of PMSG Wind Turbine: The basic of PMSG wind turbine structure shown on Figure The wind turbine generates torque from wind power. The torque is transferred through the generator shaft to the rotor of the generator. The generator produces an electrical torque, and the difference between the mechanical torque from the wind turbine and the electrical torque from the generator determines whether the mechanical system accelerates, decelerates, or remains at constant speed.

The generator is connected to a three-phase inverter which rectifies the current from the generator to charge a DC-link capacitor , The DC-link feeds a second three-phase inverter which is connected to the grid through a transformer. Through the control system, the information of wind speed, pitch angel, rotor RPM, and inverter output is accepted to compare with the grid-side data. Therefore, this information is solved by using a digital signal processing system to produce the correct signal to control these components. The main goal is to synchronize with utility grid and to export

power to it.

(B).Model of Wind Turbine: The wind turbine analyzed is a classic three-bladed horizontal-axis (main shaft) wind turbine design with the corresponding pitch controller. The output mechanical power available from a wind turbine can be expressed through the following algebraic relation

Where, ρ – air density

V – Wind Speed

Cp – Coefficient of Performance (or Power Coefficient) of the wind turbine

A – Area swept by the rotor blades of the wind Turbine

The power coefficient Cp is a nonlinear function of the blade pitch angle β and the tip-speed ratio by

Where, ωm – Angular speed of the turbine rotor, R-Radius of the turbine Blades

The power coefficient Cp can be expressed as,

The given figure shows that the various power curve of wind turbine with various speedFig.3. The curve of power wind turbine coefficient.

CONTROL OF PMSG WIND TURBINE

(A)Generator side inverter control:- The genera tor side inverter is controlled to catch maximum power from available wind power. To control electromagnetic torque we have to control q axis current iqs with the assumption ids is zero. The tip speed ratio λ taken into account. The error ω(ref) is produced. Therefore the error of w(ref) and w(s) is rescued to PI controller to produce q-axis current component iqs(ref) which put into SVPWM. The d-axis current ids(ref) is set to zero because d-axis current control is adopted.

Fig 6.2 Generator side inverter control

axis moderate to control active and reactive power.The inner current loop is controlled by PI controller similar to generator-side inverter.The out-put voltage loop produced PI controller for calculating the error between Udc and Udc(ref) produced Id(ref).Therefore q-axis current is set to zero to decoupling control of the active power P and reactive power Q by mode rating the d-axis current id and q-axis current iq.

(C)Pitch angle control:- The system of aerodynamic control plays an important role in regulating the mechanical power. Pitch angle controller is based on the principle which is changing the blades angle at the revolutions over the maximal generator speed as well as protecting the generator before overloading at high wind speeds. The optimal angle for the wind speed below the nominal value is approximately zero and then it increases with the wind speed growing

(D)MPPT control:-The MPPT is a control method which control the wind turbine rotor speed by controlling torque of the generator. The blade pitching drive is a mechanical equipment which has a delay in response time in rapidly changing wind condition. However in, order to maximize the power production the rotor speed of the generator can be controlled electrically. This is usually done by adopting the rotor speed to the optimum tip speed ratio.

(E)PLL Control:- It is a control system that generates an output signal whose phase is related to the phase of an input signal. It is easy to initially visualize as an electronic circuit consisting of variable frequency oscillator and a phase detector. The oscillator generates a periodic signal .The phase detector compares the phase of the signal with the phase of the input periodic signal and adjust the oscillator to keep the phase matched.

IV. BACK TO BACK PWM INVERTER

WECS consist of a PMSG connected to a AC-DC IGBT-based PWM rectifier and a DC-AC IGBT based PWM inverter, with a LCL filter. This filter increase quality of the current.

Results

Based on the designing procedure and mathematical description of DTC and DPC, the performances of IG and Synchronous generator in grid are analyzed. The performance of the system is analyzed by the following cases.

Case 1: Pulsation of IG and Synchronous generator parameters with the DTC and DPC control techniques

Case 2: Effects of 5th and 7th harmonics of stator current and grid voltage

Conclusion

This study analyzes the control strategies as well as models and designs of whole autonomous system of PMSG wind turbine feeding AC power to the utility grid. The results show that the combination of Pulsation of IG and Synchronous generator parameters with the DTC and DPC control techniques and Effects of 5th and 7th harmonics of stator current and grid voltage has good dynamic and static performance. The maximum power can be tracked and the generator wind turbine can be operated in high efficiency. DC-link voltage is kept at stable level for decoupling control of active and reactive power. Hence, the output will get the optimum power supply for the grid.

From above analysis the performance of Induction generator is better as compared to synchronous generator.

References:

I. India wind energy outlook 2009

II. Alternative energy systems , design and analysis with induction generators by m. Godoy simones and felix a. farret.

III. http://www.seminarprojects.com/Thread-doubly-fed-induction-motor-full-report ixzz14v8lm52E

IV. Renewable and Efficient electric power systems by Gilbert. M .Masters, Wiley Interscience

V. Intelligent Control of a Variable Speed Cage Machine Wind Generation SystemVOL. 12, NO. 1, JANUARY 1997 Marcelo Godoy Simoes,, Bimal K. Bose,, and Ronald J. Spiegel.

VI. P. G. Casielles, J. G. Aleixandre, J. Sanz, and J. Pascual, “Design, installation and performance analysis of a control system for a wind turbine driven self-excited induction generator,” in Proc. ICEM ‟90, Cambridge, MA, Aug. 1990.

VII. „Wind Electrical Systems by‟ S.N Bhadra, D.Kastha, and S. Banarjee , Oxford University Press, 2005

VIII. “Wind as a renewable source of energy & torque speed characteristics of a wind turbine driven induction generator” by Abhisek Das, Saurav Mallick, Sushmee Badhulika, NIT Rourkela, 2007

IX.Wind Power In Power Systems‟ by Thomas Ackerman, John Wiley and Sons, 2005

X. Renewable energy from Wikipedia, the free encyclopaedia

XI. NWTC Wind Resource Information

XII. Wind Energy Conversion System Connected With Grid Using Permanent Magnet Synchronous Generator (PMSG) International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization) Vol. 4, Issue 1, January 2015