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Variable Speed Wind Energy Conversion System Engineering Essay

Abstract—In this paper, two methods of tracking the peak power in a wind energy conversion system (WECS) is proposed, which is independent of the turbine parameters and air density. The algorithm searches for the peak power by varying the speed in the desired direction. The generator is operated in the speed control mode with the speed reference being dynamically modified in accordance with the magnitude change of active power. The peak power points in the P–ω curve correspond to dP/d ω =0. This fact is made use of in the optimum point search algorithm. The generator considered is a wound rotor induction machine whose stator is connected directly to the grid and the rotor is fed through back-to-back pulse-width-modulation (PWM) converters. Stator flux-oriented vector control is applied to control the active and reactive current loops independently. Simulation results show that the performance of the control algorithm compares well with the conventional torque control method.

I.INTRODUCTION

Renewable energy systems, especially wind energy generation have attracted great interests in recent years. Large wind farms have been installed or planned around the world and the power ratings of the wind turbines are increasing. Wind energy generation equipment is most often installed in remote, rural areas. These remote areas usually have weak grids, often with voltage unbalances and under voltage conditions. A wind energy system can provide a cushion against electric power price increases [1]. Wind Energy, energy contained in the force of the winds blowing across the earth’s surface. When harnessed, wind energy can be converted into mechanical energy for performing work such as pumping water, grinding grain, and milling lumber. By connecting a spinning rotor (an assembly of blades attached to a hub) to an electric generator, modern wind turbines convert wind energy, which turns the rotor, into electrical energy [1]. The reason for the world wide interest in developing wind generation plants is the rapidly increasing demand for electrical energy and the consequent depletion reserves of fossil fuels, namely, oil and coal.

*Pankaj Shukla, S.R.Mohanty, P.K.Ray are with the Electrical Engineering Department, Motilal Nehru National Institute of Technology, Allahabad, India. (e-mail: pankajshuklajan@gmail.com, soumyaigit@gmail.com, pkraymnnit@gmail.com ).

Many places also do not have the potential for generating hydel power. Nuclear power generation was once treated with great optimism, but with the knowledge of the environmental hazard with the possible leakage from nuclear power plants, most countries have decided not to install them anymore [2].Wind energy has been the subject of much recent research and development. In order to overcome the problems associated with fixed speed wind turbine system and to maximize the Wind energy capture, many new wind farms will employ variable speed wind turbine. DFIG is one of the components of Variable speed wind turbine system. DFIG offers several advantages when compared with fixed speed generators including speed control. These merits are primarily achieved via control of the rotor side converter. Many works have been proposed for studying the behaviour of DFIG based wind turbine system connected to the grid. Most existing models widely use vector control Double Fed Induction Generator. The stator is directly connected to the grid and the rotor is fed to magnetize the machine [3].

The problem of wind energy conversion system output power control has been considered extensively [4–11]. Maximization of the wind energy conversion efficiency based on a brushless doubly fed reluctance generator is discussed in Ref. [4]. Ref. [5] maximizes power based on a standard V/Hz converter and controls the frequency to achieve the desired power at a given turbine speed.Ref. [6] maximizes power based on controlling the slip power, which is extracted from the rotor circuits and fed to the grid though a rectifier-inverter branch. The firing angle of the inverter is used to control the slip power. Ref. [7] presents a hill-climb searching (HCS) control for the maximum wind turbine power at variable wind speeds. Ref. [8] present control of the power smoothing system compensates for the effects of wind variation and load disturbances. Refs. [9–11] in investigate robustness and power quality performance of a simple wind–diesel system.

This paper implements two different way to track the maximum power as explained in the next Section.

II.MAXIMUM POWER POINT TRACKING

In this paper two different ways to track the maximum power were implemented. All these tracking characteristic process are previously implemented, but here these processes are compared and new one is implemented in different way. The variable speed control is done based on the optimal power curve that shows the relation between the maximum output of the system (output) and the generator speed (input), namely maximum power point tracking (MPPT). The wind speed control or the generator speed control is adopted for MPPT. At a given wind velocity, the mechanical power available from a wind turbine is a function of its shaft speed. To maximize the power captured from the wind, the shaft speed has to be controlled. For a given shaft speed turbine power increases with increase in wind velocity v. Also peak power points of turbine power occurs at different turbine speed for different wind velocity and shaft speed corresponding to maximum power increases with increase in wind speed. To trap maximum power from the wind some control algorithm should be incorporate such that rotational speed ω of the wind turbine adapts the to the wind speed v automatically leading to maximum power point operation. This is known as maximum power point operation of wind turbine, and the process of keeping track of peak Power points with change in wind speed is Maximum Power Point Tracking MPPT [12-13].

The conventional method is to generate a control law to produce the target generator torque Te, which provides wind turbine with sufficient acceleration or deceleration torque to attain particular angular velocity leading to maximum power point operation. Irrespective of the generator used for a variable speed wind energy conversion system the output energy depends on the method of tracking the peak power points on the turbine characteristics due to fluctuating wind. The generator is operated in speed control mode with the speed reference being dynamically modified in accordance with the magnitude and direction of change of active power. If we operate at a peak power point a small increase or decrease in turbine speed would result in no change in output power because necessary condition for the speed to be a maximum power point is dP/dw =0.

A. First Method using Power Point Tracking Characteristics

The ABCD curve shown in Fig.1 represents the power point tracking characteristics. The actual speed of the turbine ωr is measured and the corresponding mechanical power of the tracking characteristic is used as the reference power for the power control loop. The tracking characteristic is obtained over four points. From zero speed to speed of point A the reference power is zero. Between point A and point B the characteristic is a straight line, the speed of point B must be greater than the speed of point A. Between point B and point C the tracking characteristic is the locus of the maximum power of the turbine. The tracking characteristic is a straight line from point C and point D. The power at point D is 1 pu and the speed of the point D must be greater than the speed of point C. Beyond point D the reference power is a constant equal to 1 pu.

Fig.1 Power point tracking characteristics.

B. Second Method using MPPT curve implemented as look-up table

From the above discussion it can be conclude that for the maximum power characteristic divided in different region, then using slop equation manipulate the value of power which is used as reference power for the simulation. Here same characteristics is used as look-up table shown in Fig.2 ,where the power only measured only some few wind velocity like at A,B,C & D but at other point it is not implemented i.e. other velocity in-between these point are not considered.

LOOK_UP Table:

Fig.2 Look up table.

III. SIMULATION RESULTS

(A) First Method using Power Point Tracking Characteristics

The simulation results give the information about the active and reactive power control. Here objective is to achieve zero reactive power, maximum active power and constant DC link voltage. Based on this study it is flexible to extend the design. Simulation result shows the controlled reactive power and active power, and constant DC link voltage.

Fig.3: Simulation results for method-I.

Detailed results of simulations performed are given in Fig.3. It can be seen from the Fig.3 that in the case of method-I, the maximum active power of 0.18 pu and zero magnitude of reactive power (pu) is achieved. Here the simulation of Va shows the phase voltage (469.48 volt). This simulation shows the DC link voltage response is strongly varying for 0.1 seconds then becomes stable near to reference voltage (1200 Volts).

(B) Second Method using MPPT curve implemented as look-up table

Simulation results of DFIG shown in Fig.4 represent that the method-II gives the controlled reactive power and active power, and also maintains the DC link voltage constant. The second method achieves the reactive power output zero faster than the first method but this method works only for some particular wind velocities.

Fig. 4: Simulation results for method-II.

It can be seen from the Fig.4 that in the case of method-II the maximum active power is 0.2 pu and zero magnitude of reactive power (pu) is achieved. DC link voltage response is highly varying whereas it remains fairly stable after few seconds in the method-I.

IV.CONCLUSION

This present work presented two different methods for the active and reactive power control strategy for a DFIG system. The method selects appropriate voltage vectors based on the stator flux position and active and reactive power errors. The simulation test confirms the reactive and active power obtained by proposed controller. The use of the PWM techniques for drives inverter, allows it possible to obtain perfectly sinusoidal currents on the level of the stator, therefore the energy provided by the machine to the net work is a clean energy without harmonics. The strategy is able to decouple the coupling effect of the d-q current control, allow the converter to manipulate in a decoupled fashion the active power flow as well as the reactive power flow. The simulation results are verified in MATLAB/ Simulink software which valid the proposed control strategy itself and power control under emergent operation conditions.

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