Direct Torque Control for Doubly Fed Induction Machine-Based Wind Turbines Under Voltage Dips and Without Crowbar Protection

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Abstract:- This paper proposes a rotor flux amplitude reference generation strategy for doubly fed induction machine based wind turbines. It is specially designed to address perturbations, such as voltage dips, keeping controlled the torque of the wind turbine, and considerably reducing the stator and rotor over currents during faults. In addition, a direct torque control strategy that provides fast dynamic response accompanies the overall control of the wind turbine. Despite the fact that the proposed control does not totally eliminate the necessity of the typical crowbar protection for this kind of turbines, it eliminates the activation of this protection during low depth voltage dips.

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Aim:- The main aim of the project is to analyze the performance of the double fed induction generator(DFIG) which is an integral part of the wind energy generation system under unbalanced grid fault condition. . And we have to control the speed of the induction generator to produce constant current even in voltage dips and un balanced load conditions by using a new method Direct torque control method with out using crowbar protections.

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INTRODUCTION Here we discuss about the analysis on the control of doubly fed induction machine (DFIM) based high-power wind turbines when they operate under presence of voltage dips. Most of the wind turbine manufacturers build this kind of wind turbines with a back-to-back converter sized to approximately 30% of the nominal power. This reduced converter design provokes that when the machine is affected by voltage dips, it needs a special crowbar protection in order to avoid damages in the wind turbine and meet the grid-code requirements.

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The main objective of the control strategy proposed in this project is to eliminate the necessity of the crowbar protection when low-depth voltage dips occurs. Hence, by using direct torque control (DTC), with a proper rotor flux generation strategy, during the fault it will be possible to maintain the machine connected to the grid, generating power from the wind, reducing over currents, and eliminating the torque oscillations that normally produce such voltage dips

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Conventional Methods:- Sensor methods Power electronic converters (VSI) Sensor less methods (Adaptive observer method) Crowbar protection

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Disadvantages:- Increases the size of the machine Noise pollution Drift effects are increased By using A.O techniques Require more memory space, Have to check for number of estimated values, Difficult to tune with number of values, More complex of calculating part.

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Proposed method:- Direct torque control method is one of the best control strategies which allow a torque control in steady state and transient operation of induction motor. The main aim of direct torque control strategies is to effectively control the torque and flux of induction motor. Direct torque control method made the motor more accurate and fast torque control, high dynamic speed response and simple to control.

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Advantages:- No need to use sensors. Efficient output. No chance of drift effects and power losses. No need to use a crowbar protection. Reduce the stator and rotor over currents during faults .

Wind Generation:

Wind Generation When wind strikes the stationary blade of the wind turbine forcely then it starts rotating.

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This blades are connected to hub it is connected to low speed shaft.

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This shaft starts rotate with the speed of wind and give dynamic energy to Gear box. This gear box connected to Generator with high speed level shaft to give more dynamic energy to the Generator This Generator converts this Gearbox M.E into E.E. This is the general process to generate electricity through Wind energy .

Why we use DFIG?:

Why we use DFIG? Generally in wind generation we use DFIG. The reason is the speed of the rotor is based upon the wind speed. Wind speed is varies at every time. Due to this variation of speed the rotor shaft will be damaged. To overcome this problem in wind generation we use a specially designed machine “ Double fed induction generator ” It can withstand under presence of voltage dips, Ability to control rotor currents. Allows for reactive power control and variable speed operation.

Methods of Speed Control of Induction motors:

Methods of Speed Control of Induction motors Stator voltage Control Stator Frequency Control Stator Current Control V/F Control Slip power recovery Control ( Wound Rotor Induction Machine) 15

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General control techniques to control the speed of the Induction Motor are Stator side Rotor side

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But it is difficult to control through Rotor side and Stator side. The next step is to control the I.M is by using Sensors.

Disadvantages of sensors:-:

Disadvantages of sensors:- If we use sensors to control IM, We have to put voltage and current sensors at both stator and rotor sides. So It increases the size of the machine and It increases the cost of the machine. If we use sensors in get some noisy, and sound polluted. Drift effects are increased. To over come this problems the next step to control the I.M we go for sensor less techniques .

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By using this AO Techniques we get accuracy output, but the main disadvantages are Require more memory space, Have to check for number of estimated values, Difficult to tune with number of values, More complex of calculating part . To overcome this problems we use Crow-Bar protection.

Crowbar protection.:

Crowbar protection. It is used to mitigate the high voltages and high current ratings. A crowbar thyristor is connected across the input dc terminals. A current sensing resistor detects the value of converter current. If it exceeds preset value, gate circuit provides the signal to crowbar SCR and turns it on in a few microseconds.

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If we use Crow-Bar protection if any fault occurs we have to replace the fuse and Thyristor. The circuit became complex to design The size and cost of the equipment increased. Externally we have to add another device to reduce voltage dips. In order to overcome these problems and to reduce the faults with in the transmission here we use a new method Direct Torque Control Method.

Principles of Vector Control:

Principles of Vector Control The basic conceptual implementation of vector control is illustrated in the below block diagram: Note : The inverter is omitted from this diagram.

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The motor phase currents, i a , i b and i c are converted to i ds s and i qs s in the stationary reference frame. These are then converted to the synchronously rotating reference frame d-q currents, i ds and i qs . In the controller two inverse transforms are performed: 1) From the synchronous d-q to the stationary d-q reference frame; 2) From d * -q * to a * , b * , c * .

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There are two approaches to vector control: 1) Direct field oriented current control - here the rotation angle of the i qs e vector with respect to the stator flux qr ’ s is being directly determined (e.g. by measuring air gap flux) 2) Indirect field oriented current control - here the rotor angle is being measured indirectly, such as by measuring slip speed.

Direct Vector Control:

Direct Vector Control In direct vector control the field angle is calculated by using terminal voltages and current or Hall sensors or flux sense windings. A block diagram of a direct vector control method using a PWM voltage-fed inverter is shown on the next slide.

Direct Vector Control (cont’d):

Direct Vector Control (cont’d)

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The principal vector control parameters, i ds * and i qs * , which are dc values in the synchronously rotating reference frame, are converted to the stationary reference frame (using the vector rotation (VR) block) by using the unit vector cos e and sin e . These stationary reference frame control parameters i ds s * and i qs s * are then changed to the phase current command signals, i a * , i b * , and i c * which are fed to the PWM inverter.

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A flux control loop is used to precisely control the flux. Torque control is achieved through the current i qs * which is generated from the speed control loop (which includes a bipolar limiter that is not shown). The torque can be negative which will result in a negative phase orientation for i qs in the phasor diagram. How do we maintain i ds and i qs orthogonality ? This is explained in the next slide.

Why FOC ?:

Why FOC ? IM is superior to DC machine with respect to size, weight, inertia, cost, speed DC motor is superior to IM with respect to ease of control High performance with simple control due de-coupling component of torque and flux FOC transforms the dynamics of IM to become similar to the DC motor’s – decoupling the torque and flux components

Basic Principles DC machine:

Basic Principles DC machine Current in Current out a f By keeping flux constant, torque can be controlled by controlling armature current T e = k I f I a

Basic Principles of IM:

Basic Principles of IM a b b’ c’ c Stator current produce stator flux s r Interaction between stator and rotor fluxes produces torque Space angle between stator and rotor fluxes varies with load, and speed Stator flux induces rotor current produces rotor flux

FOC of IM drive:

FOC of IM drive Torque equation : In d-q axis :

FOC of IM drive:

FOC of IM drive In d-q axis : Choose a frame such that:

FOC of IM drive:

FOC of IM drive Choose a frame such that:

FOC of IM drive:

FOC of IM drive Choose a frame such that: q s d s As seen by stator reference frame:

FOC of IM drive:

FOC of IM drive Choose a frame such that: q s d s d r q r Rotating reference frame:

FOC of IM drive:

FOC of IM drive To implement rotor flux FOC need to know rotor flux position: (i) Indirect FOC Synchronous speed obtain by adding slip speed and rotor speed Rotor voltage equation: Rotor flux equation:

FOC of IM drive - indirect:

FOC of IM drive - indirect d component q component

FOC of IM drive - indirect:

FOC of IM drive - indirect d component q component

FOC of IM drive - indirect:

FOC of IM drive - indirect T* * 2/3 1/s i r sq * i r sd * i sq * i sd * i a * i b * i c * CC VSI slip r + + Rotating frame Stationary frame e j

FOC of IM drive:

FOC of IM drive (ii) Direct FOC Rotor flux can be estimated by: Rotor flux estimated from motor’s terminal variables Express in stationary frame

FOC of IM drive:

FOC of IM drive (ii) Direct FOC d q

FOC of IM drive - direct:

FOC of IM drive - direct T* r * 2/3 i sq * i sd * i a * i b * i c * CC VSI TC FC i r sq * i r sd * e j T e r Rotating frame Stationary frame

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Working:- Stator is directly connected Grid Rotor is connected supply through Back-Back converters We control the speed of the I.M by changing the firing angles at different level by taking the reference of Direct torque. DTC that controls the machine’s torque ( Tem ) and the rotor flux amplitude ( | ψr |) with high dynamic capacity, and a second block that generates the rotor flux amplitude reference, in order to handle with the voltage dips.

What is MATLAB?:

What is MATLAB? MATLAB (Matrix Laboratory) MATLAB is developed by The Math Works, Inc. MATLAB is a high-level technical computing language and interactive environment for algorithm development, data visualization, data analysis, and numeric computation. MATLAB can be install on Unix, Windows

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About MATLAB/Simulink: Matlab is a high-performance language for technical computing. It integrates computation, visualization, and programming in an easy-to-use environment where problems and solutions are expressed in familiar mathematical notation .

History of MATLAB::

History of MATLAB: Fortran subroutines for solving linear (LINPACK) and eigen value (EISPACK) problems Developed primarily by Cleve Moler in the 1970’s Later, when teaching courses in mathematics, Moler wanted his students to be able to use LINPACK and EISPACK without requiring knowledge of Fortran MATLAB developed as an interactive system to access LINPACK and EISPACK. MATLAB gained popularity primarily through word of mouth because it was not officially distributed. In the 1980’s, MATLAB was rewritten in C with more functionality (such as plotting routines)

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The Mathworks , Inc. was created in 1984 The Mathworks is now responsible for development, sale, and support for MATLAB The Mathworks is located in Natick, The Mathworks is an employer that hires co-ops through our co-op program

Strengths of MATLAB:

Strengths of MATLAB MATLAB is relatively easy to learn. MATLAB code is optimized to be relatively quick when performing matrix operations. MATLAB may behave like a calculator or as a programming language. MATLAB is interpreted, errors are easier to fix. Although primarily procedural, MATLAB does have some object-oriented elements.

Other Features:

Other Features 2-D and 3-D graphics functions for visualizing data Tools for building custom graphical user interfaces Functions for integrating MATLAB based algorithms with external applications and languages, such as C, C++, Fortran, Java, COM, and Microsoft Excel

Weaknesses of MATLAB:

Weaknesses of MATLAB MATLAB is NOT a general purpose programming language. MATLAB is an interpreted language (making it for the most part slower than a compiled language such as C++). MATLAB is designed for scientific computation and is not suitable for some things (such as parsing text).

Components of MATLAB Interface:

Components of MATLAB Interface Workspace Current Directory Command History Command Window

SIMULINK:

SIMULINK It is a commercial tool for modeling, simulating and analyzing multidomain dynamic systems . Its primary interface is a graphical block diagramming tool and a customizable set of block libraries. Simulink is widely used in control theory and digital signal processing for multidomain simulation and Model-based design.

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Generally there are three ways to open Simulink 1.By using start in Matlab 2.By typing Simulink in Command prompt. 3.By clicking Simulink icon in toolbar .

Cont……:

Cont…… Simulink is widely used in control theory and digital signal processing for multidomain simulation and Model-based design.

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Simulation diagram

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Simulation results

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Simulation results

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Conclusion:- we control the speed of the induction generator to produce constant current even in voltage dips and un balanced load conditions with out using any crowbar protections and by using a new method Direct torque control method.

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