Direct Torque Control for Doubly Fed Induction

Machine-Based Wind Turbines

Under Voltage Dips and Without Crowbar

Protection

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.

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.

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.

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

Conventional Methods:-

Sensor methods

Power electronic converters (VSI)

Sensor less methods (Adaptive observer method)

Crowbar protection

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.

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.

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

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

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

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?

• 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

(1) Stator voltage Control

(2) Stator Frequency Control

(3) Stator Current Control

(4) V/F Control

(5) Slip power recovery Control ( Wound Rotor Induction Machine)

15

• General control techniques to control the speed of the Induction

Motor are

– Stator side

– Rotor side

• 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:-

If we use sensors to control IM,

I. We have to put voltage and current sensors at both stator and rotor

sides.

II. So It increases the size of the machine and

III. It increases the cost of the machine.

IV. If we use sensors in get some noisy, and sound polluted.

V. Drift effects are increased.

To over come this problems the next step to control the I.M we

go for sensor less techniques.

By using this AO Techniques we get accuracy output, but the

main disadvantages are

a) Require more memory space,

b) Have to check for number of estimated values,

c) Difficult to tune with number of values,

d) More complex of calculating part.

To overcome this problems we use Crow-Bar 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.

• 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

The basic conceptual implementation of vector control is illustrated in the below block diagram:

Note: The inverter is omitted from this diagram.

The motor phase currents, ia, ib and ic are converted to idss and

iqss in the stationary reference frame. These are then

converted to the synchronously rotating reference frame d-q

currents, ids and iqs.

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*.

There are two approaches to vector control:

1) Direct field oriented current control

- here the rotation angle of the iqse 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

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)

The principal vector control parameters, ids* and iqs

*,

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 cose and

sine. These stationary reference frame control parameters idss*

and iqss* are then changed to the phase current command

signals, ia*, ib

*, and ic* which are fed to the PWM inverter.

A flux control loop is used to precisely control the flux. Torque

control is achieved through the current iqs* 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 iqs in the phasor

diagram.

How do we maintain idsand iqs orthogonality? This is explained in

the next slide.

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

Current in

Current out

a

f

By keeping flux constant, torque can be controlled by controlling armature current

Te = k If Ia

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

Torque equation :

sse i2

p

2

3T

srr

me i

L

L

2

p

2

3T

In d-q axis :

)ii(LL

2p

23

T sdrqsqrdr

me

FOC of IM drive

In d-q axis :

)ii(LL

2p

23

T sdrqsqrdr

me

Choose a frame such that:

rrdr

0r

rq

FOC of IM drive

Choose a frame such that:

rrdr

0r

rq

FOC of IM drive

)ii(LL

2p

23

T sdrqsqrdr

me

Choose a frame such that:

rrdr

0r

rq

qs

ds

si

r

sqi

rq

sdird

As seen by stator reference frame:

FOC of IM drive

rsqr

r

me i

LL

2p

23

T

si

)ii(LL

2p

23

T sdrqsqrdr

me

Choose a frame such that:

rrdr

0r

rq

qs

ds

r dr

qr

rsdi

rsqi

Rotating reference frame:

FOC of IM driveTo implement rotor flux FOC need to know rotor flux position:

(i) Indirect FOC

grrg

grg

sr

rmgr

r

r )(jdt

di

LRL

LR

0

rsliprr

sqr

sdr

rmr

r

r )(jdt

djii

LRL

LR

0

Synchronous speed obtain by adding slip speed and rotor speed

Rotor voltage equation:

grrg

grg

rr )(jdt

diR0

gsm

grr

gr iLiL

Rotor flux equation:

FOC of IM drive - indirect

dtd

iLRL

LR

0 rrsd

r

rmr

r

r

d component

rslipr

sqr

rm )(iLRL

0

q component

rsliprr

sqr

sdr

rmr

r

r )(jdt

djii

LRL

LR

0

FOC of IM drive - indirect

m

r

r

*e

sq LL

p3T4

*i r

dtd

iLRL

LR

0 rrsd

r

rmr

r

r

d component

rslipr

sqr

rm )(iLRL

0

q component

rsliprr

sqr

sdr

rmr

r

r )(jdt

djii

LRL

LR

0

m

*r

sd L*i r

r

sqr

*r

rmslip i

LRL

)(

FOC of IM drive - indirect

T*

*2/3

1/s

irsq*

irsd*

isq*

isd*

ia*

ib*

ic*

CCVSI

slip r

+ +

Rotating frame Stationary frame

m

*r

sd L*i r

m

r

r

*e

sq LL

p3T4

*i r

rsq

r*r

rmslip i

LRL

)(

ej

FOC of IM drive

grr

rs

r

rmr

r

r jdt

di

L

RL

L

R0

srr

me i

LL

2p

23

T

(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

grr

rs

r

rmr

r

r jdt

di

LRL

LR

0

(ii) Direct FOC

)j(jdt

)j(di)jii(

LRL

)j(LR

0 rqrdrrqrd

sqsdr

rmrqrd

r

r

dti

LRL

LR

rqrsdr

rmrd

r

rrd

dti

LRL

LR

rdrsqr

rmrq

r

rrq

d q

rd

rq

2rq

2rdr

FOC of IM drive - direct

grr

rs

r

rmr

r

r jdt

di

L

RL

L

R0

srr

me i

LL

2p

23

T

T*

r*2/3

isq*

isd*

ia*

ib*

ic*

CCVSI

TC

FC

irsq*

irsd*ej

Te

r

Rotating frame Stationary frame

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.

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MATLAB (Matrix Laboratory)– MATLAB is developed by The Math Works, Inc.– MATLAB is a high-level technical computing

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History of MATLAB:

Fortran subroutines for solving linear (LINPACK) and eigen value (EISPACK)

problems

Developed primarily by Cleve Moler in the 1970’s

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MATLAB developed as an interactive system to access LINPACK and

EISPACK.

MATLAB gained popularity primarily through word of mouth because it

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In the 1980’s, MATLAB was rewritten in C with more functionality (such as

plotting routines)

The Mathworks, Inc. was created in 1984

The Mathworks is now responsible for development, sale, and

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The Mathworks is located in Natick,

The Mathworks is an employer that hires co-ops through our co-op

program

Strengths of MATLAB• MATLAB is relatively easy to learn.

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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

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Cont……

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

Simulation diagram

Simulation results

Simulation results

• 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.