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
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 s qr ’ 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 ControlIn 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.
The principal vector control parameters, ids* and iqs*, whichare dc values in the synchronously rotating reference frame, areconverted to the stationary reference frame (using the vectorrotation (VR) block) by using the unit vector cos e and sin e.These stationary reference frame control parameters idss* andiqss* are then changed to the phase current commandsignals, 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 By keeping flux constant, torque can be controlled by controlling armature current a Te = k If Ia Current in Current out f
Basic Principles of IM s a Stator current produce stator r fluxc’ b’ Stator flux induces rotor current produces rotor flux Interaction between stator and rotor fluxes produces torqueb c Space angle between stator and rotor fluxes varies with load, and speed
FOC of IM driveTorque equation : 3 p Te s is 2 2 3 p Lm Te r is 2 2 LrIn d-q axis : 3 p Lm Te ( rd i sq rq i sd ) 2 2 Lr
FOC of IM driveIn d-q axis : 3 p Lm Te ( rd i sq rq i sd ) 2 2 LrChoose a frame such that: r rd r rq r 0
FOC of IM driveChoose a frame such that: r rd r rq r 0
FOC of IM driveChoose a frame such that: r rd r rq r 0 qsAs seen by stator reference frame: 3 p Lm isTe ( rd i sq rq i sd ) i sq 2 2 Lr r rq i sd ds rd
FOC of IM drive Choose a frame such that: r rd r rq r 0 qs Rotating reference frame: r q is d r 3 p LmTe ( i ir r rdsq sq rq i sd ) r 2 2 Lr r i sq r i sd ds
FOC of IM drive To implement rotor flux FOC need to know rotor flux position: (i) Indirect FOCSynchronous speed obtain by adding slip speed and rotor speedRotor voltage equation: Rotor flux equation: g g d r g g L r ir g L m is g0 R i r r j( g r ) r r dt g Rr g L mR r g d r g 0 r i s j( g r ) r Lr Lr dt Rr L mR r d r 0 r i sd r ji sq r j( slip ) r Lr Lr dt
FOC of IM drive - indirect d component q component Rr L mR r d r L mR r0 r i sdr 0 i sqr ( slip ) r Lr Lr dt Lr Rr L mR r d r 0 r i sd r ji sq r j( slip ) r Lr Lr dt
FOC of IM drive - indirect Rr L mR r d r 0 r i sd r ji sq r j( slip ) r Lr Lr dt d component q component * 4 Te Lr i sqr * 3p r Lm Rr L mR r d r L mR r0 r i sdr 0 i sqr ( slip ) r Lr Lr dt Lr * L mR r r ( ) i sqr i sd * r slip * r Lr Lm
FOC of IM drive - indirectT* * r i r * sq isq* ia* i sdr * Lm 4 Te * Lr 2/3 ib* CC i sqr * i r * ej 3p r Lm sd isd* ic* VSI* L mR r ( slip ) * i sqr r Lr 1/s slip r + + Rotating frame Stationary frame
FOC of IM drive(ii) Direct FOCRotor flux estimated from motor’s terminal variablesRotor flux can be estimated by: Rr LmR r d r g 0 r is j r r Lr Lr dt Express in stationary frame 3 p LmTe r is 2 2 Lr
FOC of IM drive (ii) Direct FOC Rr L mR r d r g0 r is j r r Lr Lr dt Rr L mR r d( rd j rq )0 ( rd j rq ) ( i sd ji sq )i j r ( rd j rq ) Lr Lr dt d q Rr L mR r Rr L mR r i sd dt rq rq i sq r rd dtrd Lr rd Lr r rq Lr Lr rq 2 2 r rd rq rd
FOC of IM drive - directT* i r * sq isq* ia* TC 2/3 ib* CC ejr* isd* VSI i r * FC sd ic* r Te Rr LmR r d r g 0 r is j r r Lr Lr dt 3 p Lm Te r is 2 2 Lr 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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• 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.