DFIG System using Hybrid Renewable Sources-A Simulation ...DFIG System using Hybrid Renewable...

DFIG System using Hybrid Renewable Sources-A Simulation

Analysis

Prateek Nigam1, Dr. A. K. Kurchania2, Dr. S. K. Gupta3

1PhD Scholar, 2Professor, 3Professor

Department of Electrical & Electronics Engineering

RNTU University, Bhopal, Madhya Pradesh, India

[email protected]

ABSTRACT: There are numerous load centers which are segregated from the main utility grid. Some of them are distant villages, island, ships transport etc. They entail segregated electric supply through or by means of stand-alone generation system. In this research work a stand-alone changeable speed wind generation is suggested by means of a vector-controlled WRIG. The dynamic steady-state simulation model of the DFIG is built-up using MATLAB. The current component of d-axis supplies the machine-excitation and current component of q-axis supplies the torque to manage the machine speed. The speed of machine is managed to track the maximum-output power-point in accordance with changeable burden or load. The power which is generated will stored in battery bank on direct current side of the Pulse Width Modulation converter. The direct current power can be converted as ac supply by the Pulse Width Modulation inverter. Simulation analysis is performed to examine a DFIG with variable load, reactive power and real power.

Keywords: PWM, SOFC, DFIG, WECS, Induction Machine (IM)

1. INTRODUCTION In this research paper the stand alone generator based on a WRIM (Wound Rotor Induction Machine) with rotor-side control. In this electric-generator arrangement, the prime-mover speed is permitted to differ with-in a certain range (sub-synchronous and super-synchronous speed) and electrical power output is always uphold at a constant or invariable frequency and voltage by controlling slip power from terminals of rotor. Another primary aspect of this arrangement is that power converters have to handle slip power only and as a result their rating is merely a fraction of the entirety system power. It depicts a vector-control method or scheme for a wound rotor induction generator and describes the details. This is a work on to the super synchronous and sub-synchronous mode operation. In sub-synchronous operation mode, energy fed to the terminals of rotor through direct current link (using capacitor or battery). In super-synchronous operation mode, the energy stores in direct current link. Energy-storage can give a faster active-power compensation to diminish the power flicker and fluctuations. The rejoinder of output-power of wind-turbine with storage is more robust. A storage scheme can quickly recompense active-power to system. With escalating attention on renewable energy resources, Wind Energy Conversion System (WECS) are now a day’s one of the main well-liked subject that the studies are rigorously passed on. Because of the following reality, very much changeable velocity of wind, capricious speed invariable frequency wind generating scheme has distinct merits than the conventional constant or invariable speed invariable frequency wind

Compliance Engineering Journal

Volume 10, Issue 10, 2019

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generating scheme for instance lower mechanical stress, less power fluctuation and more effectual power capture.

The merits of using an IG as a substitute of a synchronous-generator are well known.

Decreased size and unit cost,

Brushless in squirrel cage,

Lack of separate direct current (DC) source,

Easiness of maintenance,

Ruggedness,

Self-protection against harsh over-loads and short circuits etc.

A. STAND-ALONE GENERATION SYSTEM

A block sketch of standalone generating scheme with rotor-side control is shown in Figure 1.1. The bi-directional capability of power flow is likely by the employ of two back-to-back potential source inverters amid a common capacitive direct current (dc) link. The converters are referred to as line-side converter & rotor-side converter. The line-side converter is too called frontend converter.

Figure.1.1 Standalone Generation amid Rotor-Side Control

Wind power system (electrical) are lately getting set of notice, for the reason that they are environmental clean, harmless renewable-power sources and price competitive, as evaluated nuclear power & fossil fuel generation. The cause for worldwide attention in expanding wind generation scheme plants is the hastily growing demand for electrical power energy and resulting depletion assets of fossil fuels i.e. coal, oil, etc. Many situates also don’t have the capability for creating hydel energy power. Nuclear energy power production was previously treated amid immense optimism, although with the comprehension of ecological risk with probable leak from nuclear energy power plants, mainly countries have make a decision not to install or establish them any longer.

The budding consciousness of that troubles led to keen study attempts for budding option of energy resources for production of electrical power. The main enviable resource likely one that existing in profusion, nonpolluting & renewable and can be harnessed at an adequate price in both small-scale and large-scale system. The majority hopeful source fulfilling this intact prerequisite is wind energy, a natural source of energy. Small wind source energy schemes can be employed in association with an electricity distribution & transmission scheme (known as grid connected schemes), or in standalone purpose that are not associated to the utility-grid. A grid associated or connected wind-turbine can lessen expenditure of efficacy supplied electricity for lighting, equipment’s & electric heat.



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Wind energy production or generation apparatus is most frequently established in distant, rustic areas. These distant areas generally have feeble grids, regularly amid voltage unbalances and under voltage states. Energy (wind) has been the topic of a lot modern study and progress. In order to conquer the troubles related with set speed wind turbine scheme and to make the most of the wind energy capture, numerous fresh or novel wind farms will utilize variable-speed of wind turbine.

A wind turbine is in WECS, which transforms the wind energy into mech. energy and shaft of turbine is coupled with an electric generator at end, which changes mech. energy at shaft into electrical energy. The varieties of generator and turbine employed rely on various aspects for instance the wind characteristic, size of the power plant, and nature of application [7].

B. SELF-EXCITED INDUCTION GENERATOR

An Induction Machine (IM) can be functioned as a standalone generator. Capacitive self-excitation of IM (Induction Machine) has been recognized for over 80 years [1],[2]. Self-Excitation of Induction Machine (IM) is begun through the magnetism (residual) presented in machine core [3],[4]. These sorts of generators are then known as SEIG (Self Excited Induction Generators) [3],[5],[6],[7]. The primary supremacy of an Induction Generator is its capability to produce power at CVCF (Constant Voltage Constant Frequency) when obsessed by a changeable source of speed.

As a result, in the application of wind power, the majority of generators are IGs (Induction Generators), which were grid attached [8]. Though, there have been only some instances for standalone (not grid attached) applications of Induction Generator because of few significant shortcomings of this technique. The major shortcoming of an IG is it’s power (reactive) demand for excitation. Consequently, capacitors should be attached across the terminals of generator. An additional shortcoming is that they have deprived frequency and potential regulations under changeable wind speed and loads. This kind of process necessitates power (active & reactive) balances consistently [7]. Power (Reactive) balance necessitates erratic capacitance, which can be provided with circuits (power semiconductors). Power (Active) balance conversely necessitates outer elements to divert the extreme power from system, when source power surpasses the quantity requisite by load. Extreme power can be engrossed by the resistors attached to terminals of stator or by resistors attached to terminals of rotor.

An additional shortcoming is that, the machine de-magnetizes and discontinues generating potential either when the speed of wind falls below or load augments ahead of certain values. Afterward, even with speed of wind and load recurring to rated-values, Induction Generator can’t initiate working yet again devoid of assist of a secondary source of energy & a controller [6]. Consequently these shortcomings should be believed through the design phase.

The power (Reactive Volt-Ampere) requisites of Induction Generator and burden or load are given by way of VAR-Generator attached to the terminals of stator.

A variety of prospects to produce reactive power;

a. Sync Condenser b. The amalgamation of saturated reactor and capacitors



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c. Static-exciters

2. IG CLASSIFICATION

On the basis of rotor construction, self-excited induction generators are two types (i.e., the wound rotor induction generator and squirrel cage induction generator). Depending upon prime movers used (variable speed or constant speed) and their positions (near to the power network or at isolated locations);

Generating schemes can be broadly classified as under:

a. Constant Speed Constant Frequency (CSCF); b. Variable Speed Constant Frequency (VSCF); c. Variable Speed Variable Frequency (VSVF).

3. CONVERTERS

A rigorous research in the field of variable-speed AC drives has been carried out over the past four decades. For a long time the importance of the study has been put on the motor inverter and its control, while the Alternating Current to Direct Current rectification has been accomplished by an irrepressible diode rectifier or a line commutated phase controlled thyristor bridge. Even though both these converters suggest a high reliability and easy structure they also have main inherent drawbacks. The output voltage of the diode rectifier cannot be controlled and the power flow is unidirectional. In addition, the input current of the diode rectifier has a comparatively high distortion. By controlling the firing angle, the Direct Current (DC) voltage of the Thyristor Bridge can be regulated. In addition, power flow from the Direct Current (DC) side to the Alternating Current (AC) side is possible, but for the reason that the polarity of the DC voltage must be reversed for this to happen, a thyristor bridge is not an appropriate rectifier for applications where a fast dynamic response is needed. Actually, the DC voltage polarity change is not even acceptable due to the electrolytic capacitors in general used in the DC link of a voltage source converter. By connecting two thyristor bridges anti-parallel, bidirectional power flow is possible without DC voltage polarity reversal, but, as a result, the number of the power switches is doubled. In addition, the power factor of thyristor bridge rectifier reduces when the firing angle augments and the line current distortion can be an even poorer trouble than that of the diode rectifier.

4. WOUND ROTOR INDUCTION GENERATOR

A squirrel cage induction machine or motor acting similar to a generator if driven faster than synchronous speed. This is a singly-fed IG, having electrical links only to the stator windings. A wound rotor induction machine or motor may too act as a generator when driven beyond the synchronous speed. Since there are links to both the rotor and stator, such a machine is called as a Doubly-Fed Induction Generator (DFIG).



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Figure.4.1 Rotor Resistance allows Over-Speed of Doubly-Fed Induction Generator

The singly-fed IG only had a useable slip range of one percent when driven by troublesome wind torque. As the speed of a wound rotor induction machine or motor may be controlled over a range of 50 to 100 percent by introducing resistance in the rotor, we may anticipate the same of the doubly-fed induction generator. Not only can us sluggish the rotor by 50 percent, we can also in excess of speed it by 50 percent. That is, we can change the speed of a DFIG by ±50 percent from the synchronous speed. In actual practice, ±30 percent is more practical.

If the generator over-speeds, resistance placed in the rotor circuit will absorb excess energy while the stator feeds constant 50 Hz to the power line. From the above figure, In case of under-speed, negative resistance introduced into the rotor circuit can make up the energy shortfall, still permitting the stator to feed the power line with 50 Hertz power.

Figure.4.2 Converters Recovers Energy from DFIG rotor

In genuine practice, the rotor resistance may be changed by a converter (Shown in figure above) absorbing power from the rotor, and feeding power into the power line in its place of dissipating it. This improves the efficiency of the generator.

Figure.4.3 Converter Borrows Energy from Power Line for Rotor of DFIG, allowing it to function well under Synchronous Speed.



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The converter may “scrounge” power from line for under-speed rotor, which passes it on to stator. From the above figure, the scrounged power, together with the larger shaft energy, passes to the stator which is attached to the power line. The stator appears to be supplying 130 percent of power to the line. Remember that the rotor “scrounges” 30 percent, leaving the line with 100 percent for the hypothetical lossless DFIG.

Wound rotor induction motor qualities.

Excellent starting torque for high inertia loads.

Low starting current contrasted to squirrel cage induction motor.

Speed is resistance variable over 50 percent to 100 percent full speed.

Higher preservation of brushes and slip rings compared to squirrel cage motor.

The generator version of the wound rotor machine is called as a DFIG, a variable speed machine.

A. OPERATION OF DFIG

In order to have a variable speed with improved operating covering the operating region 0.7

s < r < 1.3 s , the rotor inverter must be bidirectional like the one in figure 4.4 with

smooth transition through s= 0. That is, at synchronous speed the rotor needs DC currents,

and the system behaves like a pure synchronous generator. For medium power application

back to back double converter (CSI or VSI) are adequate, provided that a controlled resistor is

introduced at the rotor terminals for starting the system in motor mode. If the rotor converter

injects the reactive power (jQ) in accordance with eq.4.1, the stator reactive power can be

programmed as indicated by eq.4.2.

.

3 3( ) (( . ))

2 2ROTOR qdm qdr qdr e qdr qdrQ i v i s j i (4.1)

2.

3 3( ) ( )

2 2ROTOR

STATOR qdm qdS qdS e ls qds

QQ i v i s L i

S (4.2)

If reactive power is injected into the rotor, it will be subtracted from the reactive power injected into the stator. Theoretically, a leading power factor Q < 0 is possible at the cost of a lagging power factor in the rotor. However, as the rotor voltage is very low it is very difficult solution to implement. Figure 4.4 shows the operating range of the doubly fed induction generator with a torque-slip curve for sub synchronous and super synchronous modes. The actual torque can be calculated from a transient d-q model.



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-1-0.8-0.6-0.4-0.200.20.40.60.81-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

slipto

rque

Figure 4.4 Injected slips Rotor Power on the Torque-Slip Characteristics of a DFIG

B. POWER CONTROL OF DFIG

The phase sequence of Alternating Current (AC) voltage generated or produced by the rotor side converter is positive for sub-synchronous speed and negative for super-synchronous speed. The frequency of this potential is the same to the product of the grid frequency and the utter value of the slip. The rotor-side and grid-side converter have the ability of generating or absorbing reactive power and could be used to control the reactive power or the potential at the grid terminals. The rotor-side converter is used to control the wind turbine output power and the potential (reactive power) calculated at the grid terminals. The grid side converter is used to regulate the voltage of the DC battery.

C. APPLICATION OF INDUCTION GENERATOR

IG, generally, have the applications in the solar, wind and micro hydro power plants to produce or generate power for different critical situations as given below:

Electrification of far flung areas

For feeding critical locations

As a portable source of power supply

5. MODELLING OF WOUND ROTOR INDUCTION GENERATOR

A. DYNAMIC d-q MODEL

Consider a three phase induction machine with stationary stator winding axes as-bs-cs at 2π/3-angle apart, with voltages Vas,Vbs,Vcs and with respect to the stationary ref. frame (ds-qs), the voltages are referred as Vs

ds , Vsqs as shown in figure 5.1. Our aim is to transform

stationary reference frame to synchronously rotating reference frame and vice versa.



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Figure 5.1. Stationary Frame a-b-c to ds-qs Axes Transformation

Let Vas makes an angle θ with Vsqs . Assume that the (de-qe) axes are oriented at an angle θ as

shown in figure. The voltages Vsds - Vs

qs can be resolved into as-bs-cs components and can be represented in matrix form as:

(5.1)

The corresponding inverse relation is

(5.2)

Where VSos

is added as zero sequence component, which may or may not be present. We have considered voltage as the variable. The flux linkages and current can be transformed by alike equations. It is convenient to set θ=0, so that the qsaxis is aligned with the as axis.

Figure 5.2. shows the synchronously rotating de-qe axes, which rotate at synchronous speed ωe with respect to the ds-qs axes and angle θe= ωet. The voltages on the ds-qs can be converted (or resolved) into de-qe frame (synchronously rotating frame):

Figure 5.2. Frame (Stationary) ds-qs to Synchronously Rotating Frame de-qe Transformation

cos sin 1

cos( 120 ) sin( 120 ) 1

cos( 120 ) sin( 120 ) 1

sqsas

sbs ds

sbs os

vv

v v

v v

cos cos( 120 ) cos( 120 )

cos( 120) sin( 120 ) sin( 120 )

0.5 0.5 0.5

sqs as

sds bs

scsos

v v

v v

vv



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The voltages on the ds-qs can be converted into de-qe frame (synchronously rotating frame):

(5.3)

(5.4)

Resolving the rotating frame parameter into stationary frame the relations are:

(5.5)

(5.6)

Let, assume that the three-phase stator voltages are sinusoidal and balanced, and are given by

(5.7)

(5.8)

(5.9)

From equations we get,

(5.10)

(5.11)

From equations,

(5.12)

(5.13)

cos sine s sqs qs e ds ev v v

sin cose s sds qs e ds ev v v

cos sinsqs qs e ds ev v v

sin cossds qs e ds ev v v

cos( )as m ev v t

2cos( )

3bs m ev v t

2cos( )

3cs m ev v t

cos( )sqs m ev v t

sin( )sds m ev v t

cosqs mv v

sinds mv v



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Equations (5.10)-(5.11) shows that Vqss and Vds

s are balanced, two-phase voltages of equal peak values and the latter is at π/2 angle phase lead with respect to the other component. Equations (5.12)-(5.13) verify that sinusoidal variables in a stationary frame appear as dc quantities in synchronously rotating reference frame. The vector magnitudes in rotating and stationary frames are equal that is,

(5.14)

6. VECTOR AND CONVERTER CONTROL

Fundamentals of vector control can be explained with the help of figure given below where the machine model is represented in a synchronously rotating reference frame. Inverter is omitted from the figure assuming that it has unity current gain that it generates current ia, ib and ic as indicated by the corresponding command current ia

*, ib* and ic

* from the controller. The machine model with internal conversion is shown on the right. The machine terminal phase currents ia, ib and ic are converted to ids

s and iqss components by 3φ/2φ transformation.

These are then converted to synchronously rotating frame by the unit vector components cos θe and sin θe before applying them to de-qe machine model as shown. The controller should make the two inverse transformations, where the unit vector cos θe and sin θe in the controller

should ensure correct alignment of ids in the direction of Ψr and iqs at 90 ahead of it. Clearly, the unit vector is the key element for vector control.

The control of the rotor side converter for a doubly fed wound rotor induction machine using a current-controlled PWM voltage source converter has been reported widely [29]–[35].

e e

s s

d qtod q

cos e sin e

s sd qtoa b c

s s

a b ctod q

s s

e e

d qtod q

mod

e e

machine

d q

el

cos e sin e

*dsi

*qsi *S

qsi

*Sdsi

*ai

*bi

*ci

Sdsi

S

qsi q si

dsi

q si

dsi

control m ac h ine

transformationinverse

transformation

ai

bi

ci

min

machine

ter al

r

e

Figure 6.1. Vector Control Implementation

With a controlled converter at terminals of rotor, currents of rotor are controlled in frequency, amplitude and phase by applying appropriate rotor voltages from the rotor side converter. Field-oriented control is applied to attain fast dynamic response. The stator flux-oriented frame of reference is employed for decoupling the reactive and active current control loops.

mV V



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e

jRi e

Su

m si

q axis

d axis

Figure 6.2. Vector Diagram of Field Oriented Control

The d-axis is aligned along the grid flux axis, while the q-axis is aligned along the stator

voltage vector leading the d-axis by 90 (Figure 6.2). The q-axis current loop controls the torque of the machine by controlling the active power flow, while d-axis controls the flux of the machine by controlling the reactive power flow.

The dynamic equations governing the rotor currents in the stator flux coordinates [29], [32] are as follows:

( ) (1 )Rd Rd mSR Rd mS e R Rq R

R

di u diT i T i T

dt R dt

(6.1)

( ) ( )(1 )Rq Rq

R Rq mS e RRd mS e RmS

R

di uT i Ti Ti

dt R (6.2)

Where

21 m s rL LL

TR (= LR/RR) is the electrical time constant of the rotor circuit and σ (= 1 − (1/ (1 + σ S) (1 + σR) is the total leakage factor of the machine. If the stator resistance drop is ignored, these can be taken as valid for grid flux also [29].

These equations are employed to plan the current control loops. The cross-coupling terms

between the d-axis and q-axis ( mS − e ) R RqL i and ( mS − e ) R RdL i and disturbance

input ( mS − e ) (1 − σ) R msL i are cancelled by feed forward compensation and thus

independent control of q-axis and d-axis current loops is attained by adding Proportional Integration (PI) controllers in the loop.

For the application of stand-alone generators, the wide aims of the rotor side converter are:



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i. Establishment of a local grid; ii. Regulation of voltage and frequency; iii. Reference generation for active (q) component of rotor Current;

To achieve the fore mentioned objectives, a control strategy is proposed and will be described below. The core current loops will stay as stated earlier.

dqto ab

ab to 3 ph

Triangle comparison and PWM Generation

αβ to dq

3 ph to αβ

Unit vector generation

method

Ldhi

Connected toLine side converter

IGBTconveter

-

-

-

cosθ

sinθ

WRIM

( )ms e

z

1

s

Sin table

su

*su

-

( )ms e R RqL i

( )ms e R RdL i

-1

(1 )S

*Rqi

Rqi

( )(1 )ms e R msL i

R di

(1 )ssdi

sqi

ms

*ms1Ri

2Ri

3 ph to αβ

αβ to dq

RYu

YBu

sqi

sdi

1si

2si

Rqi

R di

integrator

( )

sin( )

cos( )

-

-*Rdi*msi

314

Figure 6.3. Control Block Diagram of Rotor Side Converter

The complete vector control schematic block diagram of the proposed control scheme for the rotor side converter is shown in Figure 6.3. The current components idr and iqr are controlled in the d-q axis through PI controllers. The Proportional Integration (PI) controller design and derivation of the terms compensating the rotor back emf. The rotor-side converter controls the reactive and active power of the DFIG independently.

The principle of field-oriented control is employed to attain high dynamic response. The line currents are controlled in the rotating frame of reference attached to the stator voltage vector. The q-axis is aligned along the stator voltage vector, while d-axis is lagging it by 90° to maintain compatibility with the rotor side converter control (Figure 6.4).

Figure 6.4. Vector Diagram of Line Side Converter



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In the stand-alone generator system, upholding the power quality of the grid is one of the major concerns. The system, automatically, works as a local grid; as a result, nonlinear loads affect power quality of the founded grid at the stator terminals directly. In the present scheme, synchronous reference frame (SRF) method is used for nonlinear load currents.

The complete schematic control block diagram of the line side converter is shown in Figure 6.5. In this scheme, load currents are measured and extracted in d−q frame attached to the stator voltages. In synchronous d−q frame, the basic positive sequence components are dc and all harmonic components are AC.

The stator side (front-end) converter vector control uses a reference frame oriented along the stator voltage vector position. Under stator voltage orientation, direct component of AC side converter current, id is proportional to the rotor power and is employed as actuating changeable in a DC-link voltage control loop, DC-link voltage reference being set by user from the PC.

fedi

feqi

1fei

2fei

feL

S fe feqL i

S fe fedL i

dcu

*dcu

Li

feqi

acqu

fedi

*feqi

*fedi

dcu

Li

Figure 6.5. Control Block Diagram of Line Side Converter

The quadrature current component, iq stand for the reactive power and directly find out the displacement factor of converter current. In a stand-alone application, any reactive current drawn by the load must be supplied either by the stator or the stator converter. If supplied by the stator, the rotor excitation current, id will naturally compensate under the action of the ims loop. Sourcing the reactive power from the stator converter can be achieved by forming the iq

* demand from the ims PI controller; this can be done after the iqr

* demand reaches a given limit or alternatively the ims PI controller output may supply both demands through a sharing algorithm. The best source of reactive power will depend on relative loss deliberations in the machine and the two converters.



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7. SIMULINK MODEL & RESULTS

The performance analysis of static compensator (STATCOM) based voltage controller for DFIG supplying nonlinear loads. The DFIG being a weak isolated system, its performance is very much affected by harmonics. The extra negative aspects of SEIG are poor voltage regulation and it necessitates modifiable reactive power source with varying load to maintain constant terminal voltage. A 3-phase Insulated Gate Bipolar Transistor (IGBT) based back to back current controlled PWM voltage source converter known as STATCOM. It is used for harmonic elimination and provides required reactive power for the DFIG with varying loads to maintain constant terminal voltage. A dynamic model of the DFIG STATCOM feeding non-linear loads using vector control based synchronous d-q axes reference frame is developed for predicting the behavior of the system under steady state and transient conditions.

The simulation model of DC motor with DFIG shown in figure 7.1. In this thesis another one thing analyze instead of wind turbine the prime mover of induction generator has been connected with DC motor.

Figure 7.1. Simulation Model of DFIG with Back to Back PWM Converter

Simulation experiments for fix speed operation were conducted on a 3- , 4-pole, 50Hz,

230V, 3.7KW, star /star connected wound rotor induction generator. A soft start-up method is followed to build up the stator voltage in a smooth manner and to evade unwanted transients. Initially a constant dc voltage is applied from rotor side converter. With the prime mover (wind turbine) in rotation, the induction machine will act as a synchronous generator. The output voltage & frequency are maintaining constant at different load condition.

The principle of field-oriented control is employed to attain high dynamic response. The line currents are controlled in the rotating frame of reference attached to the stator voltage vector.



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The q-axis is aligned along stator voltage vector, whereas d-axis is lagging it by 90 degree to maintain compatibility with the control of rotor side converter. On the basis of all the above Simulink of grid side model in MATALAB/SIMULINK is designed as shown below in figure 7.2.

Figure 7.2. Simulink Model of Stator Side Converter Controller

The control of the rotor-side converter for a doubly-fed wound rotor IM using a current-controlled Pulse Width Modulation voltage source converter has been detailed broadly [1]–[7]. With a controlled converter at rotor terminals, currents of rotor are controlled in frequency, amplitude and phase by applying appropriate rotor voltages from the rotor side converter. The rotor side controller, consisting of a reactive power controller and an active power controller, is commonly a two stage controller as shown in Figure 7.1 and also shown in figure 7.3 the Simulink model in MATALAB/SIMULINK. It works in either stator flux or stator-voltage oriented reference frame and hence the q-axis current component represents active power and the d-axis component stand for reactive power. The two controllers in machine side controller find out inverter d- and q- voltages by comparing d- and q-current set points to actual d- and q- currents of IM.

Figure 7.3. Simulink Model of Rotor Side Converter Controller



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The experimental validation of the scheme is presented first with the generator driven by a speed controlled DC motor at both sub- and super-synchronous speeds. Beneath steady speed conditions, output regulation performance is examined for impact changes in both reactive and active load. For investigating the recital beneath variable speed, an auxiliary load control system is presented and the Direct Current (DC) motor operated beneath torque control so as to imitate a wind turbine. Dynamic outcomes are presented exemplifying the regulation of output voltage and frequency under load or burden impacts. A schematic of the overall system feeding an isolated load is shown in Figure 7.1. The system shown is in fact a modified version of the isolated-connected system. To see the performance of load we have taken two types of results. First variation on no-load and second variation on full load.

Figure shows the Steady State waveforms of 3-phase generator voltages (Vabc), three-phase controller currents (Iabc), amplitude of AC terminal voltage and its reference (Vt/Vf), dc bus voltage and its reference (Vdc/Vdcref). To generated rated voltage of 230 V (380 V Peak) at no load to full load across DFIG, at 0.2 Sec. gate pulses are given to the Insulated Gate Bipolar Transistor (IGBT) and control action of current controlled voltage source inverter is made active. Controller behaves as a source of reactive power and draws active power from the generator to charge its DC bus battery at reference voltage (100 V). There is a small oscillation at switching in controller but damps out within a few cycles. We have taken two results on no-load and single result on full-load. Figure below showes first no-load output waveforms in 100W and second no-load output waveforms in 200W. Last figure 7.6. represnts waveforms of full-load 3000W.

OUTPUT IN NO LOAD OR 100W

Figure 7.4. Variation in Stator and Rotor Voltage, Current, Speed and DC Voltage Feeding Non-Linear 100W Load



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OUTPUT IN NO LOAD OR 200W

Figure 7.5. Waveform of DFIG Controller with Three Phases Resistive Load at NO-Load

OUTPUT IN FULL LOAD OR 3000W LOAD

Figure 7.6. Waveform of DFIG Controller with Three Phases Resistive at Full Load



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The simulation model of DFIG with back to back PWM converter is shown in figure 7.1. The parameters of the wound-rotor induction machine used in the simulation and the experimentation are given in appendix. The stator of the DFIG or DFIG is connected to the grid. The rotor is driven by wind generator. But in here DFIG is driven by a separately excited dc motor which represents the wind turbine. The speed of the DFIG can be varied by varying the armature voltage applied to the dc motor. The simulation is run for a sufficiently long time so the system reaches a steady speed. For a given rotor speed, the frequency of the inverter is adjusted so that the stator frequency is 50 Hz. Initially, the generator runs on no-load and after one second, it runs on full-load. The simulation waveforms from no- load full- load are shown in figure 7.5-7.6.

It is seen that the rotor voltage contains considerable amount of harmonics whereas the rotor and stator currents have lower and much lower harmonics, respectively. The closed loop implementation is simple. The output of the speed is used to control the frequency of the voltage to be injected into the rotor. It is also used to vary the dc link voltage applied to the inverter which controls the amplitude of the stator output voltage and maintains it at a desired level.

8. DFIG WITH FUEL CELL

The construction and functioning of a fuel cell are alike to that of a battery apart from that the fuel can be incessantly fed into the cell. The cell consists of two electrodes, anode (negative electrode) and cathode (positive electrode) separated by an electrolyte. Fuel is fed into the anode where electro-chemical oxidation takes place and oxidant is fed into cathode where electro-chemical diminution takes place to generate or produce electric current and water is primary product of cell reaction.

Figure 8.1. Schematic of an Individual Fuel Cell [6]

The archetypal anode and cathode reactions for hydrogen fuel cell are given by Equations

H2 ----------------2H+ +2e- (8.1)

½ O2 + 2H+ +2e- ------H2O (8.2)



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An individual fuel cell produces or generates not as much of than a volt of electric potential. A great number of cells are stacked on top of each other and attached in series (with bipolar connects) to produce higher voltages. Figure illustrates a cell stack which consists of replicating units, each comprising a cathode, anode, electrolyte and a bipolar separator plate. The number of cells depends on the needed power output.

Fuel cells are classified in keeping the type of electrolyte used. There are a variety of fuel cell types at dissimilar stages of development.

The various types of fuel cells are:-

Proton exchange membrane fuel cell (PEMFC)

Phosphoric acid fuel cell (PAFC)

Molten carbonate fuel cell (MCFC)

Solid oxide fuel cell (SOFC)

The modeling of SOFC is based on the following assumptions;

The temperature of fuel cell is supposed to be constant.

The fuel cell gasses are ideal.

Nernst’s equation applicable.

By Nernst’s equation dc voltage fc V across stack of the fuel cell at current I is given by the following equation.[6]

�� = �� +��

��

�.�

��

� − �� (8.3)

Where,

Vfc – Operating dc voltage (V)

E0 – Standard reversible cell potential (V)

Pi – Partial pressure of species i (Pa)

r – Internal resistance of stack (S)

I – Stack current (A)

N0 – Number of cells in stack

R – Universal gas constant (J/ mol K)

T – Stack temperature (K)

F – Faraday’s constant (C/mol)



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The main equations explaining the slow dynamics of a SOFC can be written as follows;-

�� = �� ∗ �� (8.4)

��

��=

�

�� [−�� + ��] (8.5)

��

��=

�

��[−��

�� + 2��

��∗

�

��] (8.6)

��

��=

�

��[−�� +

�

��

�� − 2��] (8.7)

��

��=

�

��[−�� +

�

��

�

��

− 2��] (8.8)

��

��=

�

��[−�� + 2

��

��] (8.9)

Where,

qH2 – Fuel flow (mol/s)

qO2 – Oxygen flow (mol/s)

τH2 – Response time for hydrogen (s)

τO2 – Response time for oxygen (s)

τH2O – Response time for water (s)

τe – Electrical response time (s)

τf– Fuel response time (s)

Uopt – Optimum fuel utilization

rHO – Ratio of hydrogen to oxygen

Kr – Constant (kmol/s A)

Pref – Reference power (kW)



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KH2O – Water Valve molar constant (kmol/s atm)

KH2 –Hydrogen Valve molar constant (kmol/s atm)

KO2 – oxygen Valve molar constant (kmol/s atm)

Figure 8.2. Block Diagram for Dynamic Model of SOFC

9. MODEL AND RESULTS OF DFIG WITH FUEL CELL

Figure 9.1. Grid Connected DFIG with Fuel Cell



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WHEN LOAD IS 7KW

Figure 9.2. Simulation Result (DC Voltage, Mechanical Torque, Electrical Torque, Active Power, Reactive Power and Rotor Speed)

WHEN LOAD IS 70KW

Figure 9.3. Simulation Result (DC Voltage, Mechanical Torque, Electrical Torque, Active Power, Reactive Power and Rotor Speed)



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10. CONCLUSION

The control of a slip-ring Doubly Fed Induction Generator (DFIG) supplying an isolated load at constant voltage and frequency has been presented. The scheme utilizes two back-to-back PWM converters in the rotor circuit resulting in low distortion currents, reactive power control and both sub- and super-synchronous operation. Power converter are usually controlled utilizing vector control techniques which allow the decoupled control of both active and reactive power flow to the isolated grid.

It has been observed that the developed mathematical model of three-phase DFIG-controller is capable of simulating its performance while feeding non-linear loads under Steady-state conditions. From the simulated results, it is found that DFIG terminal voltage remains constant and sinusoidal feeding the three-phase or single-phase load. When load is connected, Controller currents balance these unbalance load currents and generator currents and voltage remain constant and thus Controller acts as a load balancer.

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