Stability Analysis of DFIG based Wind Energy System

This paper presents system stability analysis of DFIG based wind energy conversion system. Presently for variable wind speed DFIG is considered as efficient machine to generate the power. In addition to this advanced AC/DC/AC converters with closed loop control system has been presented. For stability analysis 1.5MW wind turbine has been considered. As the speed of the wind varies by ± 18% to 20% the voltage generated will also varies by ± 18 to 20%. With the aid of precise converter design it is possible to achieve stable output at the grid. From the simulation results it has been found that the method of achieving stable output at the grid is best suitable for wind energy conversion system. Keyword Wind turbine, wind energy conversion system, power converters, closed loop system, doubly fed induction generator

Wind energy is clean and renewable and in recent times its application has attracted worldwide attention. As a result, the design of wind energy conversion system has become one of the most popular research area. The wind turbine extracts the power from the wind which depends on the following factors.
 Availability of wind power.
 Machine ability to respond to the wind perturbations.
 The characteristics of the machine.
In 1890 first wind energy conversion system was operational in rural USA. Today wind turbine of capacity 1 to 3MW has been successfully installed worldwide. Also since 1980s capital cost and running costs of wind power technology is reduced by almost 80% globally, and presently grid integrated wind power generation system has been increased to 95%. As a result, wind power plant has become highly competitive over nonrenewable power plants [4][5][6][7]. Table 2 provides evolution in wind power technology.  Advancement in microelectronics and control and its cost.
 Improved reliability and efficiency.
 Development of precise system modelling and simulation Today with the advancements in large low cost blade design, wind generators and power electronics and control it is possible to produce wind power in large scale. The advancement in microelectronics and control is of essential significance for the innovation of wind energy conversion system. In particular speed control of the wind turbines and grid interfacing is of major importance. Presently, blade pitch control or power converter inversion technology has been incorporated by the wind generators to adjust the power output of the variable speed of the wind turbines [8][9][10][11][12]. Because the wind speed is fluctuating, the reliability and quality of the wind power requires detail evaluation and for power conditioning suitable control schemes are to be adopted. Types of wind power systems are standalone, hybrid and grid connected.

III. DESIGN CONCEPT OF WIND ENERGY SYSTEM
The wind turbine which produces the mechanical power, a cubic function of wind speed is given by Where power coefficient = C p , Area swept by rotor of the wind turbine, m 2 = A, Speed of the wind, m/s = U. The dynamic efficiency of the rotor is calculated by means of suitable expression for C p with a function of pitch angle for variable speed wind turbine.
The power coefficient C p is the ratio of power available at the shaft to the available wind power which is given by Power curve of the wind turbine is as shown in figure 1. The power generated by wind turbine doubles as the area swept by wind mill blade doubles. Therefore, as the wind speed doubles, output power will increment by eight times. The figure 2 shows different power curves of a wind turbine [13][14][15][16][17][18]. At various wind speeds, if the operating point is along the maximum power locus, which will be carried out by controlling the load on the wind turbine, then the wind energy system will be highly efficient. Figure 3 shows production of wind. The wind speed is disturbed and is represented as the Weibull probability density function as shown in Equation (3) and Figure 4 shows Weibull probability function.
Where k = shape parameter, v = wind speed and c = scale parameter. The air stream through the wind turbine is given by  Figure 3 shows the air flow through the wind turbine. The power drawn by the wind turbine is given by Where, ρ = air density A = rotor area v b = turbine wind speed v = inlet wind speed v d = blow-out wind speed. According Betz's law wind power is given by here R = radius of the windmill. The air density is given by The unit of ρ is kg/m 3 .

IV. GENERATORS FOR WIND SYSTEM
Induction Generators and Permanent Magnet Synchronous Generators (PMSG) are generally used for wind energy applications. This is because of their simplicity in construction, low maintenance, ruggedness and cost effectiveness. For fixed speed operation, wind turbine with synchronous generators is used, which can operate at any power factor. Whereas wind turbine with induction generators will supply only active power and at the same time it consumes the reactive power. Considering the similar size and high power rating, the synchronous generators are more expensive than induction generators. Also for grid connected applications, synchronous generators are connected through the power converters to the grid. There are various methods to operate the Variable Speed Wind Turbine (VSWT). One among them is dynamic slip control which uses doubly-fed induction generators (DFIG). But for operation of DFIG reactive power is required. A gear box is necessary for the wind turbine which makes use of induction generators. But the drawbacks are more cost, extra losses and regular maintenance.
 A wound rotor induction generator with rotor side control is useful for large scale wind power generation, because of variable speed constant frequency (VSCF) operation. The advantages of wound rotor induction generators for VSCF are Enhanced wind energy capture.  Without any modifications of construction, there will be increase in shaft power of the induction generator.  Turbulent wind energy can be captured by a variable speed turbine.
 Superior grid quality and efficient operation. Figure 6 shows the block diagram of the wind power system. Advancement in power electronics and control is of vital importance for implementing variable speed operation. It is because of this technology wind energy conversion system has made a significant advancement. In order to meet the requirements of the present power generating systems, the know-how of power electronics has to be developed right from the devices to the system network. The requirements of advances are as listed as below.
 Modular type power converters. Modular system with high power density will offer flexibility and efficiency for high power wind generation systems.  The requirement of integration of other renewable energy sources like photovoltaic and fuel cell with cooling technologies are to be dealt with.  Incorporating new superior switching technology with enhanced temperature capability.

V. ILLUSTRATION OF WIND ENERGY CONVERSION SYSTEM
The detail technical specifications of wind power system are shown in Table 3. The following parameters have been calculated for the above technical specification.

VI. DESIGN OF POWER CONVERTERS
The following conditions are considered for designing power converters for wind energy system.
 The unstable wind speed which will change at certain interval of time at certain speed range.
 The variations in output voltage and frequency of DFIG by ± 20%.
This unstable wind energy is converted into electrical energy and delivered to the grid. The design parameters of power converters for the above conditions are shown in Table 4.

SIMULATION RESULS
Simulation results are shown in figures 7a, 7b, and 7c respectively. It is shown that when the input voltage and frequency are changed by ±20%, both output voltage and frequency of the system remained stable.

VIII. CONCLUSION
The proposed wind energy conversion system with doubly fed induction generator is capable of achieving high power density, enhanced efficiency and reliability. It is found that design configuration with performance requirements of the power converters are capable to achieve stable output voltage and frequency when the wind speed is varied at different intervals of time.
Dr. Rajashekar P. Mandi is presently working as Director of School of Electrical Engineering, REVA University, Bangalore. Before joining REVA University, he worked in Central Power Research Institute, Bangalore for more than 26 years in research. He received M. Tech. degree in Energy Systems Engineering from Visweswaraiah Technological University, Belgaum with 3 rd rank and Ph.D in Power and Energy from NITK, Surathkal through research. He is a professional member of IEEE. He is accredited energy auditor from Bureau of Energy Efficiency (BEE), Govt. of India. He is presently chairman of Society for Energy Efficiency & Manager (SEEM) Karnataka Chapter