Are the input filter line voltage and current

STUDY OF WIND TURBINE DRIVEN DFIG USING AC/DC/AC CONVERTER

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS OF THE DEGEREE OF Bachelor of Technology
In
Electrical Engineering
By
ASHISH KUMAR AGRAWAL (10502066)
BHASKAR MUNSHI (10502049)
SRIKANT KAYAL (10502054)
Under the guidance of Prof. K. B. Mohanty

CERTIFICATE

This is to certify that the thesis entitled, “Study Of Wind Turbine Driven Induction Generator Using AC/DC/AC converter” submitted by Ashish Kumar Agrawal, Bhaskar Munshi and Srikant Kayal in partial fulfillment of the requirements for the award of Bachelor of Technology Degree in Electrical Engineering at the National Institute of Technology, Rourkela (Deemed University) is an authentic work carried out by them under my supervision. And to the best of my knowledge, the matter embodied in the thesis has not been submitted to any other University/Institute for the award of any Degree or Diploma.

National Institute of Technology Rourkela

CERTIFICATE

ACKNOWLEDGEMENT

We would like to articulate our deep gratitude to our project guide Prof. K. B. Mohanty who has always been source of motivation and firm support for carrying out the project.

ABSTRACT

In recent years, wind energy has become one of the most important and promising sources of renewable energy, which demands additional transmission capacity and better means of maintaining system reliability. The evolution of technology related to wind systems industry leaded to the development of a generation of variable speed wind turbines that present many advantages compared to the fixed speed wind turbines. These wind energy conversion systems are connected to the grid through Voltage Source Converters (VSC) to make variable speed operation possible. The studied system here is a variable speed wind generation system based on Doubly Fed Induction Generator (DFIG). The stator of the generator is directly connected to the grid while the rotor is connected through a back-to-back converter which is dimensioned to stand only a fraction of the generator rated power.

Pr ,Qr are the rotor-side active and reactive powers, respectively
RON ,ROFF are the IGBT ON and OFF resistances, respectively
D, J are the moment of inertia and damping coefficient, respectively P are the Number of poles
M1,M2 are the stator and rotor modulation depths, respectively
Vtri is the triangular Voltage Signal
R,L are the resistance and inductance of input filter, respectively V1, I1 are the input filter line voltage and current, respectively
E is the DC-link voltage
s is the Laplacian Operator
C is the DC-Link capacitance
PDC is the DC-link active power
J Combined rotor and wind turbine inertia coefficient

Ws Rotational speed of the magnetic flux in the air-gap of the generator, this speed is named synchronous speed. It is proportional to the frequency of the grid voltage and to the number of generator poles
.

Chapter 1
INTRODUCTION

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In a more detailed approach, actual converter representation with PWM-averaged model has been proposed, where the switch network is replaced by average circuit model, on which all the switching elements are separated from the remainder of network and incorporated into a switch network, containing all the switching elements. However, the proposed model neglects high frequency effects of the PWM firing scheme and therefore it is not possible to accurately determine DC-link voltage in the event of fault. A switch-by-switch representation of the back-to-back PWM converters with their associated modulators for both rotor- and stator-side Converters has also been proposed. Tolerance-band (hysteresis) control has been deployed. However, hysteresis controller has two main disadvantages: firstly, the switching frequency does not remain constant but varies along the AC current waveform and secondly due to the roughness and randomness of the operation, protection of the converter is difficult. The latter will be of more significance when assessing performance of the system under fault condition. In order to resolve the identified problems, a switch-by-switch model of voltage-fed, current controlled PWM converters, where triangular carrier-based Sinusoidal PWM (SPWM) is applied to maintain the switching frequency constant. In order to achieve constant switching frequency, calculation of the required rotor voltage that must be supplied to the generator is adopted. Various methods such as hysteresis controller, stationary PI controller and synchronous PI controller have been adopted in order to control current-regulated induction machine. Among which, synchronous PI controller has been acknowledged as being superior.

Power quality is actually an important aspect in integrating wind power plants to grids. This is even more relevant since grids are now dealing with a continuous increase of non-linear loads such as switching power supplies and large AC drives directly connected to the network. By now

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Doubly Fed Induction Generator
Wind turbines use a doubly-fed induction generator (DFIG) consisting of a wound rotor induction generator and an AC/DC/AC IGBT-based PWM converter. The stator winding is connected directly to the 50 Hz grid while the rotor is fed at variable frequency through the AC/DC/AC converter. The DFIG technology allows extracting maximum energy from the wind for low wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on the turbine during gusts of wind. The optimum turbine speed producing maximum mechanical energy for a given wind speed is proportional to the wind speed. Another advantage of the DFIG technology is the ability for power electronic converters to generate or absorb reactive power, thus eliminating the need for installing capacitor banks as in the case of squirrel-cage induction generator.

2.1 Operating Principle of DFIG

The mechanical power and the stator electric power output are computed as follows:

For a loss less generator the mechanical equation is:

	=–		= – = –s

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2.2 Dynamic simulation of DFIG in terms of dq-

Stator Voltage Equations:

	= p	+ω	+ …………………(1)
	= p	−ω

= (−) …………………….. (6)

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