Certain Investigations On A Grid Connected Introduction Generator With Voltage Control

Certain Investigations On A Grid C'onnected Irtduction Generator With \'oi tage Control V Subbiah

E; (it.erlli1

Professor Depanment of Electrical 6 Electronics Engneering PSG College of Technoioa Colnibatore 64 I 004 ISDl A

Rcse:irch Sc1iol:ir Drprtniiteni of Elccti ical & Electronics Ei\giiwe:ing PSC col leg^ of T e c ~ h i t ~ l o ~ Coimbntpre 04 I 004 INDIA

H,v ?\cr, n 1113JOr drawback of irtdiictioii Seiwatoi Abstncx -

With large number of w n d enera con\ersion systems , e J I ~i n

eurJM,shhHdin r,wmt

uril,tr

counlr).

ix

the grid

fadng

=\ere problmi of p w r p o w t factor at l o w w n d speeds. 1 his is because of the reactive pcwer tahrti by t h t induction generator from the grid.

The use of an

ac r d t ~ g e controller. ss in

suggwsloll oJ

,,,ethods

o,

paper. d d e d inv-tiptions

dminrtds si!;iiificant ai:tount of re;ictii. 2 p a t e r froill

IS that i t

Hcrice. tlic power Eictot

load conjlrlon.

1

e,

'it

reduced

undesirable cliarxtei istic

the o s e of an induction motor, powrr ,sctor.

tf, in,pro,e

,',thi5

a r e cnrried out on a grid connected

inducttan generator wth voltage c o n l r d .

The generator perfomiance

w s determined theoretird-ally under different operating conditions. Expenmcntsl i n \ e s ~ g a t i u n s carried out using autotransfotnier a3

pouer nat\:urk

and

plnccs

low e q x ~ i d l :it l ~ 11:111

I.>

wlilcj ;in

i n tiii II 011 otlwr

sped

'T1115

undue bttrderl oii tllc uttI1s connrcted

Wherr a fev, induct1011generators arc conilcctd to

13 I t

tilt.

grid,

the iiiipact of the supply of reaciibe power froni the gild

ma> not be felt vorq niuch However, wltcn a ciost?r of ~ t i i i d

v o l t a s controller confirm the thtwreticai results. Further, the fe3sibifity

farins \ilth illore 1 l t n C h l n C S are SOlltlilg Up. t h l S

01 uzinq a three phJse thyribtnrised 3c vdta?e cnnlrctller. u d in wind

be s e \ m

prOblt.111 14111

energy conversion system for the soft s t m t of rnduaion nischine ns U

motor, for the power factor itiiprovement of the s y s t ~ ISi studird. Here

SeLeral attempts have been niade to inlpro.i:! the

the p w U e m is that the \oitage applied to the indocoon generator cnn not be reduced M o w a pnrfjcular value.

The reason for lhis problem i s

s n d j s e d in this paper 2nd a solution suggest&.

powx factor of the grid corinected induction gencrators

The most popular method But,

III

IS

the u s e of fixed capacitors

app'i:atioiIs such as wild energy conversion s>stmls

where the r+xtive power demand varies with w n d 5pt.t-d.

1. INTRODUCTION

this cannot provide the required reactive pouer It nil

Also, self excitation problems The de~elopnirntof suitable generators. driven by renewble energy soiiices siicli :is w n d , 11:~s recently iissurned greatel signifiratice due to incrc'asing cost and rapid depletion of conveiitional e n e r a sources Remarkable

progress has 'wen itude III thc ccruixry i n the f b r m t i o n and erection of 31id connected \hind pouer statioiis in

Invariably

a11 these stations. induction generntcrs are bring used

in

pretereiice to synchronous generators because of their inh?rznt

advnnt'iges like asynchroiious opr.ltion, less i

iiinintennnce.

I ugged

0-7803-2795-0

and br ushiess rotor, low

iiriit

cost etc

resulting 11, undue Over voltages I

I*

tiiixs

be inore pionoti1lced,

series wth the generator is suggested in [2] as one of the methods to improve the power k t o r

Hence, investigations have beer? carried out to fiiid out the reason for tlie liiiiited range of voltage conrrol

The results of soiiie

the

in

I

experimental investigations carried out on a grid connected

case of induction generator wth static voltage controller

induction generator wth voltage contrdi are presented

Bj represenling the induction machine

[6,7]

However.

a

comprehensive analysis

in

on the

its

rbTth

RLE

equit'nlent circuit, the authors are able to pinyidc

;I

performance of the grid connected induction generators with

satisfactoy explanation to this problem and also a soiution

voltage control has not yet been reponed

to overcome this difficulty

intsstigitions closely agree

Hence, a detailed study

IS

Here again, the exptlrimeiii:il bb51

1

the rriialvt~calresu!is

undertaken by the

authors on the perforinance of grid connected induction I1. ANALYSIS WITH CONVENTIONAL ~ ClRCUIT

generators with voltage control at ltght load conditions The effecr of w l q e control

equivalent circiiit

is

analysed using the conventional

~

~

I

V

Investigations are carried out under

different operating conditions such as constant current,

The effect of voltage coiitrol on the perfornimce of

constant slip. constanf power output and constant poww

grid connected induction generator IS atialysed using the

input

I n all the operating canditions. tlie power factor

found to iiiiprove wth appropriate reduction

In

IS

conventional equivalent circuit grid connected systeni

applied

IS

The power factor of

tlic

dcteriiiiucd under iiifferciit

voltage to the generator Experimental invetigatrons, carried

operating conditions sucli as coilstatit c w w t t . u x i s i w i slip,

out using autotransformer as the voltage controllet, confirm

constant power output and constant power

the theoretical results

1OOV. 10A. 3-phase induction inactiine \-\.as used for the

Invstigations lnvesttgations on the performance of induction generators

\\qth

static voltage controllers. have also been

cam& out by fabricating a three phase thyristorised ac voltage controller As

in

the previous case. the performance

of the induction generator is observed under different More or less the same tniprovenient

operating conditions

1iIQlit

1 SkiV,

To determine the machine paraiiieters at

differerit operating voltages, no load tests at various Yoltages and blocked rotor test were conducted.

The macIiine

paranieters under different applied voltages are esriinatcd and @\en

in

table I

As lire t c :mge is reduced. the rronloss

as ne11 as tlie reactive current taken by the generator get reduced contributing to improvginent i n power factor

in p o w r factor could be achieved w t h appropriate reduction in

voltage

However. the control range of the static voltage

controller was found to be limited and as a result the voltage could not be reduced below a particular value.

With

tlie

help

of

these

equivalent

parmeters, the power factor of tlie grid connected induction generator for different applied voltages

The above problem has already been reported in

circtiit

IS

estiniated

Analysis has been carried out under different operating

conditions like constant cur rent operation , constant slip

[7] For a 1 5kW. 14OV. 8 pole machine, it is reported that the voltage could be reduced to nearly 350V only and for a

operation, constant power output operation and constant

7kW. JOOV, 6 pole machine the voltage could be reduced to

power input operation The curves illustrating the variation

-

nearly 310V only.

However, the reason €or such a liiiiited

of power factor wth voltage are given

control range has not yet been reported.

440

in

Fig 1-4

A

~

~

111: EXPERIMENTAL VERIFICATIONS

To

supplenient

the

theoretical

analysis.

experimental investigations have been carried out on the test machine

10-

09-

06-

The perforniance of the induction generator is tested under all the above operating conditions using autotransformer for controlling the

applied voltage

.

0 7.

0 6.

\ -

-0 0 5 -

The experimental observations. p e n in Fig 1-4, closely

:OL,

fh*ortlirol

-----Practical

agree K i t h the theoretical results In the light of the above discussions, it can be 0

concluded that the power factor of the grid connected induction generator can be improved by appropriate control over the applied voltage.

0T-O);

300

t60

5bO

Voltagr

441

100

ZOO

100

Vollapr

400

500

When the back emf leads the supply voltage, the

+

+

control range of ~1 wdl be from 0 y to x @ and when i t lags the range of CL will be Ron) -@ + y to X - @ Hence. To observe the behavior of the system w t h static

when the back en$ lags the supply voltage. the oueput

voltage controller, a thyristorised 3-phase voltage controlIer

voltage x n be brought to zero before a reaches ?T, whereas in the case of back en$ leadmg the supply voltage. has to be varied beyond x to bring the output voltage to zero For example. for a back emf of half that of the supply voltage

is fabncated and tested.

Comparison of the performance of

the generator wth rated voltage condtion shows an improvement in powerfactor of about 15% at 0.8 per unit load wth 0 85 per unit voltage and about 50% at 0 35 per

unit load with 0.8 per unit voltage

However as reported in the earlier work [7], it

and l a g g n g by 30" out pur voltage can be brought to zero at

a

= I52 degrees, and when i t leads by 30" the output voltage can be brought to zero at =208 degrees

is

For a three phase ac voltage controller. the above

observed that the voltage applied to the induction generator

investigations are valid if the load is star connected and the

canna be reduced below a particular value w t h the static

neutral of the load is connected to the neutral of the supply

voltage controller Out.

,SUPPLY

In this regard. investigations carried

VOLIAGE

are reported in the subsequent sections NE1 V O L l A G E

.

AC VOLTAGE CONTROLLERS WITH RLE LOAD

In the case of single phase ac voltage controller

1

w t h a resistive load (R load), control of the load or output

/

volGe is possible for the entire range of the finng angle (

a ) from 0 to x However, w t h inductive load (RL load), the control of the output voltage is possible only when a is greater than the load inipedence angle, y Hence, the control range of a I S from y to TC When an ac back enlfis also present (RLE load), the control range of is hrther depndent on the emf and 6 , the phase drfference between

V

supply voltage and the back emf Some investigations on ac voltage controller w t h RE and LE loads are gven in [SI Proceedmg i n similar lines, investigations have been carried out for RLE loads The net voltage across the thyristors will be the Merenee between the supply voltage k d back emf

The phase difference of the net voltage with respect to the supply voltage is gwen by angle a.which depends on the nlagrutuQe and phase of the back emf When the back enif lags the supply coltage, the net voltage wll be leading and

vice versa The voltage and current waveforms are gwen in Fig 5 (a) Lk (b)

442

V. INDUCTlON MACHINE AS RLE LOAD

R

An induction machine, for steady state sinusoidal

t

operation. is often represented as an equiyalent passive inductance and resistance

How\er such a representation

is grossly inaccurate when harmonic components are

t

involved

in

the supply to the machine

E

Under such

con&tions.’it IS better to represent induction iliachine as an equivalent counter emf and a series impedence [9]

An

equivalent circuit of induction machine. farly valid for operation w t h both the fundamental frequency as well as harmonic ones is Sven in Fig 6 where the back emf is given by

VI. APPLICATION CONSIDERATlONS

where rl” = r2’ ( I-s)/s

In wind energy conversion system. a 3- phase

When the induction machine runs as a 11iotor. the counter

thyristorised ac voltage controller is being used for the soft

enif lags the supply voltage and when

s?anof the induction inachine as a motor The same voltage

it

mns as a generator,

controller can be effectively

the counter enif leads [ I O ]

11

inipro\enient of the system

de use of for the power factor

At light wind speeds

it

is

4 s mentioned in the previous section. the control

obsened thnt the power factor of the system is very poor.

“&e of a depends 011 the mA%nitudeand phase of the back en’f In the case Of ~ n d u c t l nri’Xhlne. the supply voltage

Under such condmons if the voltage applied to the generator can :ippropriutely controlled, tlie reactive volt alnpere

Is

vaned. both the

Lwles

been

r”agnltudeand

phase

Of

the back

ell1f

taken b? the generator can be reduced resulting

TaklilgInto accouilt this valatlon. the a&‘sls has =led

Out

It

found that whrle running as a

generator when cx = x the voltage across the generator is nearly 30OV The voltage can be reduced further whena IS vaned beyond x

power factor

in

a better

The release of the reactive burden in the

network could outweigh the harinonics introduced due to phase col,trol

VXI. COKCLUSION

Eltpenmentally also i t mas observed that when CC = x the voltage across the generator has around 300V and it was decreasing when C( \\as\aried beyond x

Analysis of an lnductlon generator wttli voltage control has been carried out using the conventional

In the case of an induction motor the voltage could

eqiiikale-.‘ circuit It was found that the power factor of the

generator could be improved by voltage control at light load

be brouglit to zero before a reaches x .

conditions

443

Expe_rinieiit,,.

investigations

with

a u t o t d o r t i i e r confirm the theoretical resuIts

However,

when tested with thyristorised 3- phase voltage controller, it that the voltage across the generator could not

below a particular value. lnvestlgations have

n m e d

out in this connection

by representing the

machine as an RLE load and found that the

voltage can be further reduced if the firing angle is varied beyond 180 degrees

Hence,

i t IS

concluded that the ac

controller in wind energy conversion system can be effectively utilised for improving the power factor at low w n d speeds

ACKNOWLEDGMENT

The authors thank the Principal and Head of the Department

ef Electrical B Electronics Engineenng,PSG College of Technology. Coimbatore. for providmg the research facilities

REFERENCES

444