Self-excitation in Generators

8/12/2019 Self-excitation in Generators

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www.hcmute.edu.vn

FACUTY OF ELECTRICIAL & ELECTRONICS

ENGINEERING

UNIVERSITY OF TECHNICAL

EDUCATION HO CHI MINH CITY

REPORTSUBJECT:TRANSIENT PROCESS IN ELECTRICAL POWER SYSTEM

TOPIC:

SELF-EXCITATION IN GENERATORS

:

HCMC, December/2010


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www.hcmute.edu.vn/feee/

FEEEEnsuring Enhanced Education

OBJECTIVE

Find out conditions that the synchronous

generator can self - excite without stimulus.


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CONTENTS

Introduction

Conditions that generator can self -excite :

Residual magnetism

Generators rotation

F ield Current

F ield resistance


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SELF-EXCITATION

When generator stops working, in

the stator core of main magnetic

poles and the yoke remain residual

megnetism.

If Field circuit is open (It=0 ),

n = nm, U=(2-3)%Um

If Field circuit is closed , (rt Field

resistance) creates t0t

t0

r

UI


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If is in the same direction with the voltage at

the generators terminals increases and if It rises

continuosly, the generator will self- excite.

If is in the reverse direction with generator

will be demagnetized;therefore, self-excitation can not

occur.

t0

t0

SELF-EXCITATION


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CONDITIONS THAT GENERATOR

CAN SELF EXCITE

In which:

Supposed Lt=constOmit Ras R


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CAN SELF EXCITE

n

tntt iaiaiaaE ....

2

210(2)

Supposed that self

excitation is carried out when n=nm ,for

simple study, no-load characteristic. U0 = E =f(i

t)

No-load characteristic as follows:

In which:

E: electromotive force in the armature

winding

a0: electromotive force generated by


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CAN SELF EXCITE

Solving equations (1),(2) we have:

The value itchanges from the value 0 to the set value

it= It. When itreaches the set value :

Voltage generated at generators terminals

U0=E= rt. It

0dt

dit


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Edt

diLir t

ttt

n

tntt iaiaiaaE ....2210

When it=0 with a00, E 0 => output voltage

If it=0 with a0=0 ,E=0 => no output voltage

Generator must have Residual magnetismso

that it can self- excite.


CAN SELF EXCITE


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E

dt

diLir t

ttt

When rotor rotates, armature winding is affected and

creates e.m.f and then causes It, current it produces

flux. If the flux is the same direction with , Field

Current will be increased and generator can self-excite.

If it >0, E 0 Output voltage

If it


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If the rotor rotates in the reverse direction, the

generator will be demagnetized; therefore self -

excitation can not occur

The rotor must rotate in the constant direction


CAN SELF EXCITE



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rt4 > rt(th) > rt2 >rt1U0=E

It

M

rt..it

dt

iLdtt )(

C It

Ed

4 3 2 1

At M, it=It=> U=it. rt


CAN SELF EXCITE



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0. tt

tirE

di

diL


It

M

rt..it

dt

iLdtt )(

C It

Ed

4 3 2 1

When current itincreases, generator can self-excite

When it < It, derivative:

derivative >0 it has increased

result generator can self-excite.

Created voltage is intersection M

at the position in which Ldit/dt =0

and it reaches the set value It


CAN SELF EXCITE



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It

M

rt..it

dt

iLdtt )(

C It

Ed

4 3 2 1

If rt increases line

U=it.rt has high slope

output voltage is low


CAN SELF EXCITE


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CONCLUSION

Residual magnetism must be existed in generator.

Field current must be increased.

The rotors rotation should be in the constant

direction so that flux can not be demagnetized.

Field resistance must be smaller than the value rth


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REFERENCES

1. Vu Gia Hanh, Tran Khanh Ha, Phan Tu Thu, Nguyen Van Sau,

ELECTRICAL MACHINES , Ha Noi Science and Technology

Publisher, 2006

2. Nguyen Trong Thang, Tran The San, ELECTRICAL

MACHINES AND CONTROL CIRCUIT, STATISTICS

Publisher, 2003


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Self-excitation in Generators

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