Islanding Operation of DFIG for Rural Electrification Using Back to Back Converter

Presented ByPaper ID-236

M.Tech Scholar Rakhi SoniCo-authors- Monika Jain & Sachin Tiwari

OIST Bhopal (MP)

IV th International Conference on Advances in Energy Research Indian Institute of Technology Bombay, Mumbai

Contents

IntroductionDistributed generation Technologies of DGAdvantages of DGTypes of generator DFIG (Doubly Fed Induction Generator)Control schemes of DFIGResults & discussionconclusionReferences

INTRODUCTION

Wind turbines convert the kinetic energy present in the wind into mechanical energy by means of producing torque.

They are operated either at fixed speed or variable speed.

Generators driven by fixed speed turbines can directly be connected to grid.

Variable speed generators need a power electronic converter interface for interconnection with the grid.

Variable speed generation is preferred over fixed speed generation.

Types Of AC. Generators

AC GENERATOR

ASYNCHRONOS GENERATOR

SEIG DFIG

SYNCHRONOS GENERATOR

DFIG (Doubly Fed Induction Generator)

Wound rotor induction generator with slip rings.

Rotor is fed from a three-phase variable frequency source, thus allowing variable speed operation reduction of mechanical stress; higher overall efficiency, reduced acoustical noise.

The variable frequency supply to rotor is attained through the use of two voltage-source converters linked via BESS.

Advantage of DFIG

The variable speed machines have several advantages :-

They reduce mechanical stresses.Reduced convertor losses.improve power quality & system efficiency.

Control schemes of DFIGDFIG

Pm

ist(a)

Stator side converter

Rotor side converter

Battery

RC Filter

Inductive Filter

iLi(a)

iro(a)

Vp(ca)

ica

icb

icc

Pm

Cdc

Rbat Cbat

Rin

ibat

Vdc

Voc

a

b

c

balanced/unbalanced, resistive

and reactive

load

icn

Zig- Zag Transformer

ist(c)

ist(b)

Vp(bc)

Vp(ab)

iLi(c)

iLi(b)

iro(c)

iro(b)

Principle of Operation

The rotor currents and stator currents of the DFIG are controlled by stator-flux oriented control to decouple the active and reactive components of these currents and to achieve fast dynamic response. Torque Control- The electrical torque of the DFIG can be controlled to operate the wind turbine at the point of maximum aerodynamic efficiency by controlling the active power current component of the rotor.

Max. Power Point Control- To operate at maximum coefficient of performance for optimizing energy capture from the wind, the turbine should run at optimal tip speed ratio.

Continue…..

Voltage control- the voltage control at the stator terminals is achieved by controlling the reactive component of the stator current.

Frequency control- the stator frequency control is achieved by generating stator flux angle by integrating the reference stator frequency and using that angle for transformation of reference d-q stator quantities to reference three phase quantities.

Wind Turbine

The aerodynamic power generated by wind turbine can be expressed as by, P=0.5ρACpV3ω

 Where the aerodynamic power is expressed as a function of the specific density (ρ) of the air, the swept area of the blade ( A), and the wind speed (V3ω)

Stator side converter

a

Inductive filter

Stator current

measurem-ent

Vter

Ɵ st flux

dq/ abc

Hysteresis controller

Hysteresis modulating

signals

Zig- zag transformer

Connected to load

DFIG

PI Controller

Connected to stator

bcn

Idst*

Iqst*

i*

st(a)

i*

st(b)

i*st(c)

ist(c)ist(b)

ist(a)Battery

CdcVoc

Rbat

Cbatibat Rin

-Lm/Lst

2Πf* t

Vter *

Iqro

Cntd…

The terminal sensed voltage (Vter) is calculated by,

Vter={(Vab2+Vbc

2+Vca2)/3}1/2

Vter* which is taken as 415V.

Vterr(n) = Vter* -Vter

Vterr is fed to PI voltage controller with gains Kpv and Kiv.

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The magnetizing current requirement of the DFIG is provided by the rotor side converter. Any additional reactive power for the electrical loads and stator leakage reactance is supplied from the stator side converter.

Cntd…

13

Cntd…

The stator frequency control is achieved by

өstatorflux=2πfnTsamp

14

Cntd…

15

Rotor side converter

ωter

ωter *

dq/ abc

Hysteresis controller

Hysteresis modulating

signals

Zig- zag transformer

Connected to load

PI Controller

Connected to stator

bc

n

Idro*

Iqro*

Ɵ slip

i*ro(a)

i*ro(b)

i*ro(c)

Iro(c)Iro(b)

Battery

CdcVoc

ibat

Inductive filter

a

Rbat

Cbat

Iro(a)

DFIG

Stator voltage

measurem-ent

Rotor current

measurent

Vter

imsaturated

(p/2) Ɵro

Ɵ st flux

Iro(a)Iro(b)Iro(c)

The reference for the d component of the rotor current (Idro

*) is taken as the rated magnetizing current (lmsaturated),

Imsaturated = √2 Vter

* / √3 Xm

Where, Vter* is 415V,

Xm is the magnetizing reactance of the DFIG.

Cntd…

17

Cntd…

18

Cntd…

For dq to abc transformation, the angle between d-axis and rotor axis (ϴSlip) is required. ϴSlip can be generated as,

ϴslip = ϴstatorflux - ( p/2 )ϴro

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Cntd…

20

Battery Specifications

The terminal voltage of the equivalent battery Vbat is given by,

Vbat= (2√2/√3) V

Where VLi is the line rms. voltage (V = 415V). A slightly higher round-off value of 750 V is considered.

the equivalent capacitance can be given by,

Cbat = (kwh * 3600 * 103) / 0.5 (V2ocmax – V2

ocmin)

Cbat = (7.5*10*3600*103) / 0.5 [(7602) – (7402)]

Cbat = 18000 F

Ratings……

A. Machine parameters: 7.5 kW, 415V, 50Hz, Y -connected, 4-pole, Rs = 1Ω, Rr=0.77Ω, Xlr= Xls= 1.5Ω, J= 0.1384kg-m2

B. Wind turbine parameters: Wind rating= 15 kW, Wind Speed Range = 9 -11 m/sec, Inertia = 3 .5 kg_m2, r = 3.55 m, gear ratio= 7.516.

C. Controller parameters: Kpf= 10, Kif = 50, Kpd = 0.02, Kid = 0.0025.

Ratings……

D. Battery parameters: Lf = 3mH, Rf = 0.1Ω and Cdc = 4000µf, R1=10K, Ro=0.01 Ω, Cbat=36000 F

E. Zigzag Transformer specification: Three phase zigzag transformer, 50 Hz, 150 V/ 415 V, kVA rating =10 kVA.

F. Consumer Loads: Resistive load= 2.5 kW single phase loads. Reactive load = 2.5kW, 1.875 KVAR 0.8PF lagging single Phase loads.

Performance of DFIG with BESS Feeding Balance / Unbalanced Resistive Load

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.5

-500

0

500

Stator V

ab

c

Stator voltage

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.5

-20

0

20

i ab

c

Stator current

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3-20

020

Icabc1

Controller current 1

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.5-20

0

20

I L

n

Load netural current

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.5-20

0

20

I L a

bc

Load current

Cntd…

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.545

5055

f

Frequency (Hz)

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3455055

f 2

Frequency (Hz) 2

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3455055

f 3

Frequency (Hz) 3

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.5500

550

600

V

te

r

Terminal voltage

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3500550600

Vt2

Terminal voltage 2

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3500550600

Vt3

Terminal voltage 3

-10

010

I bat

1

Battery Current

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.58

10

12

Time

Wind

Wind

Performance of DFIG with BESS Feeding Balance/ Unbalanced Reactive Load

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.5

-500

0

500

Stator

V

ab

c

Stator voltage

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.5

-20

0

20

i abc

Stator current2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3

-200

20

Icab

c1

Controller current 1

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.5-20

0

20

I L

n

Load netural current

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.5-20

0

20

I L a

bc

Load current

Cntd…

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.545

5055

f

Frequency (Hz)

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3455055

f 2

Frequency (Hz) 2

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3455055

f 3

Frequency (Hz) 3

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.5500

550

600

V

te

r

Terminal voltage

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3500550600

Vt2

Terminal voltage 2

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3500550600

Vt3

Terminal voltage 3

-10

010

I bat 1

Battery Current

2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.58

10

12

Time

Wind

Wind

Conclusion

There are many isolated locations which cannot be connected to the grid and where the wind potential exists, for such locations wind system are beneficial.

The performance of proposed controller demonstrated under balanced/unbalanced linear loads. The simulated results verify the effectiveness of the controller under various consumer loads. It has been observed that the proposed controller has been found to regulate the magnitude and frequency of isolated system. It has also been found that controller is capable to function as load balancer, load leveler and harmonic eliminator as well as the capability of MPT.

Zigzag transformer is used for harmonics eliminator having voltage boost capability. The implementation of this technology reinforces the use of such system in remotely located villages by locally available energy sources.

References

[1] Peña, R., Clare J. C., and Asher G. M. (May 1996) “Doubly Fed Induction Generator using back to-back PWM converters and its application to variable speed wind-energy generation,” Proc. Inst. Elect. Eng., Elect. Power Appl., vol.143, no. 3, pp. 231–24.[2] Murthy S. S., (2007) “A Comparative Study of Fixed Speed and Variable Speed Wind Energy Conversion Systems Feeding the Grid,”IEEE conference.[3] Singh Bhim, (2010) “Performance of wind energy conversion system using a doubly fed induction generator for maximum power point tracking” IEEE conference.[4] Blaabjerg, F., Chen, Z. and Kjer S.B. (2010) “Power electronics as efficient interface of renewable source”, IPEMC conference.[5] K. Goel Puneet, “Modeling and Control of Autonomous Wind Energy Conversion System with Doubly Fed Induction Generator,”IEEE Conference.[6] K. Goel Puneet, Singh Bhim, (April 2011) “Isolated Wind–Hydro Hybrid System Using Cage Generators and Battery Storage,” IEEE Transactions On Industrial Electronics, vol. 58, no. 4, pp. 1141-1152.

Continued……[7] Mullane A. Lei Y, Lightbody G., and Yacamini R. (Mar. 2006) “Modeling of the wind turbine with a doubly-fed induction generator for grid integration studies,” IEEE Trans. Energy Conversion, vol. 21, no. 1, pp. 257-264.

[8] Kasal Gaurav Kumar, (June 2008) “Voltage and frequency controller for a Three-phase Four wire Autonomous wind energy conversion system,” IEEE Transactions on Energy Conversion, vol. 23, no. 2, pp. 509- 516.

[9] Singh Bhim, (2010) “Performance of Wind Energy Conversion System using a Doubly Fed Induction Generator for Maximum Power Point Tracking” IEEE conference.

[10] Verma Vishal, (2011) “Decoupled Indirect Current Control of DFIG for Wind Energy Applications,” IEEE Conference.

[11] K. Goel Puneet, (July/August 2011 ) “ Parallel Operation of DFIGs in Three-Phase Four-Wire Autonomous Wind Energy Conversion System,” IEEE Transactions on Industry Applications, vol. 47, no. 4,1872-1883.

[12] Marek Adamowicz* and Ryszard Strzelecki, (2008) “Cascaded Doubly Fed Induction Generator for Mini and Micro Power Plants Connected to Grid,” IEEE conference.

Continued……[13] Ganti Vijay chand, (2010) “Quantitative Analysis and Rating Considerations of a Doubly Fed Induction Generator for Wind Energy Conversion Systems,” IEEE conference.

[14] Kasal Gaurav Kumar, (2008) “Voltage and frequency control with Neutral current compensation in an Isolated Wind Energy Conversion System,” IEEE conference.

[15] Muller S., Deicke M. and De Doncker R. W. (May/Jun. 2002) “Doubly fed induction generator systems for wind turbines,” IEEE Ind. Appl. Mag., vol. 8, no. 3, pp. 26–33.

[16] Petersson A. Harnefors, L. and Thiringer T. (Jan. 2005) “Evaluation of current control methods for wind turbine using doubly-fed induction machine,” IEEE Trans. Power Electron., vol. 20, no. 1, pp. 227–235.

[16] Slootweg, J. G., Haan, H. W. S., Polinder, H. and Kling, L. W. (Feb. 2003 ) “General model for representing variable speed wind turbines in power system dynamics simulations,” IEEE Trans. Power Syst., vol. 18, no. 1, pp. 144– 151.

[17] Simoes M.G. and Farret F. A., (2004) “Renewable Energy Systems: Design and Analysis With Induction Generators”. Orlando, FL: Fl-CRC.

[18] Blaabjerg F., Chen Z., Teodorescu R. & Lov F., (2006) “Power electronics in wind turbine systems,” IEEE Int. Conf. on Power Electronics and Motion Control, pp. 1-11.

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