ANALYSIS AND COMPARISION OF SRG AND DFIG FOR WIND GENERATION APPLICATION

The main objective of this paper is to document the comparative and absolute advantages and disadvantages of using Switched Reluctance Generator (SRG‟s) and Doubly Fed Induction Generator (DFIG‟s) in wind generation. The comparative study is based on the machine parameters like inductance, flux linkage, torque, output power, weight and cost. Performance analysis has been carried out using Finite Element Analysis (FEA). The predicted results show SRG‟S has the optimal choice for Wind Generation.


INTRODUCTION
Wind powered generators must operate efficiently under variable speed conditions. Although the efforts are getting it from conventional machines, a devoted special machine is welcome. The switched reluctance machine represent one of the simplest types of electrical machines and Switched Reluctance Generator. SRG is a potential device for variable speed power generation. Investigation conducted to explore this new potion showed that it is promising due to the new advances in power electronics and in micro processing. The SRG is reliable efficient and present some specific characteristics that match the needs of wind power.
The induction generator is the most common generator in wind energy application due to its simplicity and ruggedness. More than 50 years life time, the same machine is used as motor or generator without modification, high power per unit mass of materials and flexibility in speed range of operation. The main drawbacks in induction machine are its lower efficiency and the need for reactive power to build up the terminal voltage. However the efficiency can be improved by modern design and the solid state converters can be used to supply reactive power required.
Induction generator has a widely acceptance in using with wind energy conversion for many reasons. Induction generator is very simple, reliable, cheap, light weight, long life time, produces high power per unit mass of materials and requires very less maintenance. Also, induction generator works with constant speed constant frequency system as well as variable speed constant frequency system.

DOUBLY FED INDUCTION GENERATOR
The induction generator is the most common generator in wind energy application due to its simplicity and ruggedness. More than 50 years life time, the same machine is used as motor or generator without modification, high power per unit mass of materials and flexibility in speed range of operation. The main drawbacks in induction machine are its lower efficiency and the need for reactive power to build up the terminal voltage. However the efficiency can be improved by modern design and the solid state converters can be used to supply reactive power required.
Induction generator has a widely acceptance in using with wind energy conversion for many reasons. Induction generator is very simple, reliable, cheap, light weight, long life time, produces high power per unit mass of materials and requires very less maintenance. Also, induction generator works with constant speed constant frequency system as well as variable speed constant frequency systems.
The induction motor can also run as a generator. This simply happens when you, instead of forcing the rotor to turn at a rotational speed lower than the synchronous speed, exceed this synchronous speed by applying an outside energy source, such as diesel motor or a set of wind turbine rotor blades. Once again, the greater the difference between the rotating magnetic field of the stator and the speed of the rotor, the greater the torque produced by the rotor. When it is working as a generator, the rotating field however acts as a brake in slowing the rotor. The stator experiences a variable magnetic field from the rotor that drags its rotting magnetic field and thereby induces an electrical current in the stator. In comparison to motor operation the induced currents in the rotor and the stator will flow in the opposite direction, which means that power will be sent to the grid. The faster the rotor turns in relation to the magnetic field of the stator, the greater induction in the stator and the greater the production of power.

2.3FINITE ELEMENT ANALYSIS
FEA consists of a computer model of a material or design that is stressed and analyzed for specific results. It is used in new product design, and existing product refinement. A company is able to verify a proposed design will be able to perform to the client's specifications prior to manufacturing or construction. Modifying an existing product or structure is utilized to qualify the product or structure for a new service condition. In case of structural failure, FEA may be used to help determine the design modifications to meet the new condition.
There are generally two types of analysis that are used in industry: 2-D modeling, and 3-D modeling. While 2-D modeling conserves simplicity and allows the analysis to be run on a relatively normal computer, it tends to yield less accurate results. 3-D modeling, however, produces more accurate results while sacrificing the ability to run on all but the fastest computers effectively. Within each of these modeling schemes, the programmer can insert numerous algorithms (functions) which may make the system behave linearly or non-linearly. Linear systems are far less complex and generally do not take into account plastic deformation. Non-linear systems do account for plastic deformation, and many also are capable of testing a material all the way to fracture.
FEA uses a complex system of points called nodes which make a grid called a mesh (Figure 2). This mesh is programmed to contain the material and structural properties which define how the structure will react to certain loading conditions. Nodes are assigned at a certain density throughout the material depending on the anticipated stress levels of a particular area. Regions which will receive large amounts of stress usually have a higher node density than those which experience little or no stress. Points of interest may consist of: fracture point of previously tested material, fillets, corners, complex detail, and high stress areas. The mesh acts like a spider web in that from each node, there extends a mesh element to each of the adjacent nodes. This web of vectors is what carries the material properties to the object, creating many elements

ANALYSIS OF THE SWITCHED RELUCTANCE GENERATOR
Switched Reluctance Machines (SRM) can work as motor or as generators just by changing their switching angles. SRG has a mechanical input and an electrical input. Its output is the electrical power supplied to the load.

Fig 3.1 Stator of SRG Fig 3.2 Rotor of SRG Fig 3.3 Switched Reluctance Generator
The voltage equation for a phase of 8/6 SRG is ' R resistance of the phase a winding ' L self ind uctance of phase "a" (Henry) ' ' L self inductance among phase "a" and "b"(Henry) ' ' ' L self inductance among phase "a" and "c"(Henry)  rotor angular position (deg) The induced electromotive force f is given by The supply is applied to the stator winding. When the back emf is positive electric power is converted to mechanical power and the machine works as motor. When the back emf is negative, it increases the current converting mechanical power into electrical power and the machine act as a generator.

DESIGN CALCULATION
The torque equation of SRG is given as Where, The angular speed of the SRG is given as Where,

H s -magnetic field induction of stator
Turns per phase of SRG is given as (7) Where,

FINITE ELEMENT ANALYSIS
FEA consists of a computer model of a material or design that is stressed and analyzed for specific results. It is used in new product design, and existing product refinement. A company is able to verify a proposed design will be able to perform to the client's specifications prior to manufacturing or construction. Modifying an existing product or structure is utilized to qualify the product or structure for a new service condition. In case of structural failure, FEA may be used to help determine the design modifications to meet the new condition.
There are generally two types of analysis that are used in industry: 2-D modeling, and 3-D modeling. While 2-D modeling conserves simplicity and allows the analysis to be run on a relatively normal computer, it tends to yield less accurate results. 3-D modeling, however, produces more accurate results while sacrificing the ability to run on all but the fastest computers effectively. Within each of these modeling schemes, the programmer can insert numerous algorithms (functions) which may make the system behave linearly or non-linearly. Linear systems are far less complex and generally do not take into account plastic deformation. Nonlinear systems do account for plastic deformation, and many also are capable of testing a material all the way to fracture.
FEA uses a complex system of points called nodes which make a grid called a mesh ( Figure 2). This mesh is programmed to contain the material and structural properties which define how the structure will react to certain loading conditions. Nodes are assigned at a certain density throughout the material depending on the anticipated stress levels of a particular area. Regions which will receive large amounts of stress usually have a higher node density than those which experience little or no stress. Points of interest may consist of: fracture point of previously tested material, fillets, corners, complex detail, and high stress areas. The mesh acts like a spider web in that from each node, there extends a mesh element to each of the adjacent nodes. This web of vectors is what carries the material properties to the object, creating many elements

Parameters Value
Rated output power(hp) 3

Rated voltage 220
Given rated speed 1500 Type of load 12 Number In this the efficiency that get increases suddenly and then speed increase and then gradually the efficiency that get decreases.

.5 Torques vs Speed
The Fig3.5.5 that shows the graoh of torque vs speed. In this graph that explains at starting point the torque is very high and then it decreases the speed that get increases gradually. w w w . i j c t o n l i n e . c o m The Fig3.5.6 that shows the graph of current vs rotor position. This graph that shows the the current increases to the certain point and decreases to zero then the rotor that shows the position.

CONCLUSION
The main advantages of renewable are available, clean, low cost and continuous energy. The reasons for choosing induction generator in wind energy system are that its very reliable tends to be comparatively inexpensive, light weight and low maintenance. The generator also has some mechanical properties which are useful for wind turbines. So, the SRG is the most common generator in wind energy system applications due to its simplicity and ruggedness. The used formula to calculate the minimum values required for Switched Reluctance Generator is simple. The formula which used here gives typical results of the given parameters. This analysis and the waveforms are done by using the magnet software.