Course 234

8/11/2019 Course 234

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8/11/2019 Course 234


234.00-5

b

There must be a speed sensor ;

c

There must be a

con t ro l re l ay to

admit

hydrau l i c

f l u id to the governor

steam

va lve power p i s t ons ;

d

There

must be a method of feedback

from

the govern

o r

steam

va lve pos i t i on to th e con tro l

re l ay ;

e

There

must be

a

method

of

rap id ly clos ing

the

gov

ernor steam

valves to

shu t o ff steam in

the

event

th e

genera to r load i s lo s t ;

f There must be a

method

of shut t i ng the governor

steam valves dur ing c er ta in c a su a lt ie s .

The

r e la t ionsh ip

o f these

components

i s shown in

Figure

5.1 .

Mechanical Governor

C

Governor

Steam

Valve

P i l o t

O il

Regulating

Valve

From

Lub r i c a t i ng

O i l

System

Con t r o l

Relay

P i l o t

O il

System

Handwheel

Basic

Mechanical Hydraul ic Governing System

Figure 5 .2

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234.00-5

Basic

Mechanical

Hydraulic Governor

Figure 5.2

shows a

b a s i c mechanical

governing system.

Control o i l

i s

supplied

a t

a cons tant

r a t e through

a P i l o t

Oil Regulating Valve

and discharged

a t a v a r i a b l e

r a t e

through t h e

Governor

Oil

Cylinder .

The p r e s s u r e i n t h i s

P i l o t

Oil System i s thus a funct ion of

the

r a t e

a t

which

p il o t o il

i s

dra ined

from

th e

system.

Depending

on

the

open

ing

i n the governor

o i l c y l i n d e r

the p i l o t o i l pressure may

vary

from about 200

KPa g

t o

about

600 KPa g .

I t

i s the

value of

t h i s p i l o t

o i l

pressure which i s the

c o n t r o l

s i g n a l

f o r

governor steam

valve

operat ion .

I f

p il o t o il p r e s s u r e

i n c r e a s e s o i l p r e s s u r e forces

th e

Control P i s t o n

up

a g a i n s t spring t e n s i o n .

The

le v e r p iv ots

on th e

top o f the

power p i s t o n

C

and p u l l s up

the

spool of

th e c o n tr o l r e la y . This adm its

Power

o i l t o

the

unders ide of

th e power p i s t o n

and

forces up a g a i n s t

spr ing

t e n s i o n .

The l e v e r

now

p i v o t s on

the top

of

the

c o n t r o l

p i s t o n B and

pushes

th e

spool

of

th e c o n tr o l r e l a y

back

t o

the

n e u t r a l

p o s i t i o n t h u s s h u t t i n g o f f power o i l t o

the

power p i s t o n .

Therefore,

the

system

r eg ai ns equ il ib ri um , with the governor

steam valve more f u l l y

open

and passing more steam.

The

reverse

occurs when

p i l o t

o i l

p r e s s u r e

i s

reduced.

As o i l p r e s s u r e

under

th e c o n t r o l p i s t o n decreases , sprin g

t e n s i o n pushes the p i s t o n

down.

The

l e v e l p i v o t s on the top

on

th e power p i s t o n C and

pushes down

th e spool of the

con

t r o l r e l a y .

This allow s power o i l t o d r a i n from the under

s i d e

of th e power p i s t o n

and

the power

p i s t o n

moves

down.

The l e v e r

now

p i v o t s on

the top

of the c o n t r o l

p i s t o n B

and

p u l l s th e spool of

the

c on tr o l r el a y back t o the n e u t r a l pos

i t i o n

thus ,

s h u t t i n g

o f f

the

d r a i n

of

o i l

from

the

power

p i s t o n . Therefore ,

the

system

rega ins equi l ibr ium, with the

governor

steam

valve l e s s f u ll y

open

and pass ing l e s s steam.

OIL

CYLINDER MOVEMENT

:: >

SPEEDER

GE R

T T H ~

HERE

Oil ylinder

~ z Z Z

Drain

t

Gover or

t Plot.o

o

t

9 r -

Dill

Governor

Pis ton

and

Cylinder

Figure 5.3

- 3 -

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234.00-5

wi l l

now

turn our

a t ten t ion

to

how

the governor var-

ie s p i lo t o i l pressure . The method

by

which the mechanical

governor senses turbine speed is by

the

pos i t ion

of

the Fly-

ba l l s . The f lyba l l s

are

connected to th e

Governor

Pis ton.

As

the

governor pis ton moves, var ies

the

s ize of the open

ing from the p i l o t o i l system to the governor o i l cyl inder

Figure 5 .3 .

I f the

f lyba l l s move

out

speed increas ing ,

the

pis ton

i s withdrawn,

more

o i l

dra ins

out ,

the

p i lo t o i l

pressure decreases , and

the

governor s te am valve moves in the

shut

di rec t ion . I f the f lyba l l s move

in

speed decreas ing ,

the pis ton

i s inse r ted

fu r ther in to th e cy lin de r, less

o i l

dra ins out, the

p i l o t

o i l pressure

increases , and

the gover

nor steam

valve moves in the

open

di rec t ion .

In addi t ion

to

the ef fec t of speed

on

p i l o t

o i l

pressure

v ia the f lyba l l s

and

governor

p i s ton ,

the operator

can vary

p i l o t o i l pressure through the

pos i t ion

of

the

governor

s leeve . The governor

s leeve i s moved by the Speeder Gear

which i s

powered by an

e lec t r i c

motor.

A cont ro l s ignal

in i t i a t ed

by

the

opera to r or the

computer

dr ives

the

motor

to

move

the

governor s leeve . This increases

or

decreases the

s ize of the d ra in por t , thereby varying p i lo t o il pressure .

I t i s

impor tant

to keep

in

mind t ha t a l l the governor

can do

i s

vary p i l o t

o i l

pressure ,

and

t ha t a l l

varying

p i l o t

o i l pressure can do i s open or

shut

the governor

steam

va lve. The ef fec t produced by varying governor

steam

valve

pos i t ion varying steam

flow

is

dependent on whether the

generator

i s synchronized or n ot syn chron ize d

to

the

gr id ,

and has nothing to do with

the

design of

the

governor. For

example,

ra i s ing

p i lo t o i l pressure wil l open up on the gov

ernor steam

valves and

admit more

steam

to the

turbine.

Whether

th i s

increases

speed

or

increases

load has nothing

to

do

with the governor but i s

a

funct ion

of

the

external

oper

a t ing

condi t ions .

You w ill re ca l l from our

discuss ion

of governor opera

t ion

in

the l a s t lesson t ha t speed droop i s

a

funct ion of the

design of

the governor . Figure

5.4

shows

a

speed

droop

curve

for

a t yp ica l

la rg e tu rb in e

genera tor .

8/11/2019 Course 234


to

0

\D

N

:r:

IJ l

N

to

r l

0

t:l

\D

Q

0

0

f. l

to

r l

Q

0

N

.a

0 \

IJ l

E t

IJ l

l -

I -

r l

2 3 4 . 0 0 - 5

20

40

Sp eed Droop

Cur ve

F i g u r e

5 .4

80

G e n e r a t o r Load

( o f Fu ll Power)

C o n s i d e r

th e

c a s e o f

a tu rb in e g en era to r

w i t h t h i s

s p e e d

d r o o p c u r v e

s u p p l y i n g

a load . As

the

l o a d

i s increased , th e

h y d r o - m e c h a n i c a l g o v e r n o r wi l l

r e s p o n d

a s fol lows:

a )

load

in crea ses cu rre nt

t h r o u g h

genera tor

a r m a t u r e

i nc reases ;

b )

genera to r

counte r torque T

=

K ~

i n c r ea se s ;

c )

tu rb ine /genera to r

s l o w s

down;

d ) th e f l yba l l s move in c en tr i fu ga l force l e s s than

spr ing

tens ion ;

5

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234.00-5

e) the

governor plunger i s in se r te d in to the

governor

s leeve , reducing the ra te

of

dra in

from

the

p i l o t

o i l

system:

f ) p i lo t o il p re ss ur e i nc re a se s:

g)

the governor

steam

valve opens

fu r ther ,

admit t ing

more

steam.

The

turbine i s

now

supplying the

increased

load.

How-

ever , the

speed

does

not

re turn to

the

or ig ina l value r a ther

some

lower value . This i s ,

of

course, speed droop and

i s a

function

of

re la t ive

value of the spr ing force and f lyba l l

cen t r i fuga l

force.

I t

i s ,

however, des i rab le to

keep

the

turbine/generator

operat ing a t

a

cons tant frequency. In order to re s to re

the

speed of

the

turbine

frequency) to

i t s or ig ina l value,

the

speeder gear is moved in the ra ise speed di rec t ion .

This

moves the governor

s leeve

to close

off

the dra in por t , in -

r ~ s i n g

p i lo t o i l pressure and

opening

up

on

the

governor

steam

valves

to

supply

the

load

a t

the

or ig ina l

speed

f re -

quency). This is shown in Figure

5 .5 .

N

l/ )

-

II

Xl

r- i

IX

0

1,0

C

0)

0) 0

0..

0

U) co

r- i

0)

s

·roi

II

.Q

l-l

CI

l/ )

Eo

B

/ )

l

I -

M

20

40

60 80

100

Generator

Load

Effec t of

Load

Change (

of

fu l l

power)

Figure

5 .5

6

8/11/2019 Course 234


234.00-5

I f

a tu rb ine genera tor with

th i s speed

droop is supply

ing

th e

Ontar io

grid system, th e grid frequency

wi l l

not

vary

subs tan t i a l ly

from 60

Hz.

This

means t ha t

under normal con

di t ions

the speed

of

the t u rb ine /genera to r wi l l remain

sub

s tan t i a l ly

a t

1800 rpm

fo r

a pole

genera tor .

In t h i s case genera tor synchronized

to

an in f in i t e

bus ,

the

e f f ec t

of

increas ing

steam flow

i s

to

inc rease th e

power out of the genera tor . Since

speed

cannot change, i f

more

steam

power

i s

pu t in more

must be

supplied by the gen

e ra to r .

Load

Reject ion

I f

the generator

output breaker

t r i p s

open

due

to

an

e lec t r i ca l fau l t the

speed

of

the tu rb ine wi l l

s t a r t

to in

crease .

The

f ly ba lls w il l

be

thrown

outward , w i thdrawing the

governor

pis ton

from

the

governor

o i l

cyl inder .

This

dumps

p i lo t o i l to

dra in . As p i lo t o i l pressure i s

reduced, the

cont ro l pis ton i s

pushed down under spr ing

t ens ion .

This

lowers

the

spool

of the

con t ro l

re lay and dumps power o i l to

dra in . This shu ts the governor steam valve .

The mechanical

hydraul ic

governor

has

two

inherent

weak

nesses which decreases i t s ab i l i ty to handle an overspeed

fo llowin g loa d re jec t ion:

1.

The dead

t ime

associa ted

with the

re se rvoi r e f fec t of

low pressure lUbr icat ing o i l . The

t ime necessary

to

move the

l a rge volume

of p i lo t o i l

and

power

o i l to shut

th e

governor

steam

valves ,

r e su l t s

in excessive

over

speeds .

2. When the governor f ina l ly gets cont ro l

of

tu rb ine speed,

wi l l

at tempt to

con t ro l speed

a t the no load speed

determined by the

speed

droop curve.

This no

load

speed

may

be

as much as 5

above

the normal operat ing speed.

This means the governor has a bu i l t - i n bias which works

aga ins t

holding

speed down

on

a

load

re

j ec t ion .

Even

though the speeder

gear

i s driven back to the

pos i t ion

corresponding

to

a

no-load

speed

of

1800 rpm, cannot

move f a s t enough to

el iminate

the e f fec t

of

speed

droop.

One method

of

at tempting to

el iminate

both of these pro

blems i s the use

of

an auxi l i a ry governor which operates in

pa ra l l e l

with the main

governor . This

auxi l i a ry governor has

no speed

droop and

i s

se t to sp i l l

o i l

a t

a constant

above

normal

operat ing speed.

On an

overspeed

following

a load

re

j ec t ion

the auxi l i a ry governor begins to sp i l l o i l

a t

above operat ing

speed

1818

rpm

fo r an 1800 rpm tu rb ine .

The auxi l i a ry governor not only aids the

main

governor

in

7

8/11/2019 Course 234


234.00-5

dropping p i lo t o i l pressure but also

wi l l

a t tempt to contro l

speed

a t 1 overspeed

un t i l the speeder gear has run

back to

main

governor. This e f fec t ive ly e lim in ates th e speed

droop

bias of the main

governor

on an overspeed fo llowin g loa d

re -

j ec t ion .

In the upper

r i gh t hand

corner

of

Figure 5 .6 you can

see

th e

main

and

auxi l i a ry

governor

of

th e

Picker ing

NGS

t u r -

bine . You can

a lso trace

th e

p i lo t

o i l system from th e p i l o t

o i l regula t ing valve to

the

con tro l re la ys of each of the

four gov ern or ste am valves .

The

unfor tuna te fac t

about

the

mechanical

hydraul ic

gov

ernor

i s t ha t on

la rge tu rb in es ,

the

dead t ime

produced

by

rese rvo i r e f f e c t in the g overn in g system i s unacceptably

long. The

main and auxi l i a ry

governors

together

are not cap

able of

l imi t ing

the overspeed

following

a load re jec t ion

to

acceptable l eve l s .

To

l im i t

the amount

of

overspeed the

governing system

must

be aided by an e lec t r i c

ant i c ipa tor .

The governor

system shown

in

Figure 5 .6

conta ins

two e l-

ec t r i c an t i c ipa to r s which

operate in

pa r a l l e l . I f th e

gener

a to r

output

breaker opens under load the

e lec t r i c

an t i c i pa t -

o rs a re tr ipped by

auxi l i a ry

contacts on the b reaker. The

e lec t r i c an t i c ipa to r dumps

power

o i l to dra in which shuts the

emergency

s top

valves .

At th e

same

t ime th e low power

o i l

pressure , t r i p s pressure switches which shut th e i n t e r c ep t

valves

and

open the re l ease valves.

Afte r f ive seconds th e main and auxi l i a ry governors have

regained cont ro l of tu rb ine speed. At th i s

t ime

the

e lec -

t r i c ant ic ipators

rese t ,

and

the

emergency s top valves

and

in te rcept

valves

reopen

and

the

re l ease

valves

shut .

The

tu rb ine wi l l thus end up with the steam

cont ro l

valves

ESV,

IV and RV in t he i r normal posi t ion , with the auxi l i a ry

gov

ernor cont ro l l ing

turb ine

speed. As

the speeder

gear

i s run

back the main governor wi l l eventual ly gain contro l of

the

go vernor steam

valves

and lower

speed from

101

aux i l i a ry

governor to 100 of opera t ing speed. Figure 5 .7 shows the

response

to a

load

re jec t ion of a t yp i c a l

mechanical

hydraul

ic governing

system.

- 8 -

8/11/2019 Course 234


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10

8/11/2019 Course 234


234 00 5

E l e c t r i c a l Governor

Toothed

Wheel

Speed Probe

Governor

Steam

Valve

Narrow

Range

Speed

De te c to r

Var i ab l e

Droop

speed /

Load

c o n t r o l

Servo-

Ampl

Servo-

Ampl

I n t e r c e p t

Valve

Sequence

Basic

Elec t r i ca l Hydrau l i c

Governing System

Figure

5 .8

Bas ic E l e c t r i c a l

Hydrau l i c Governor

Figure 5 .8

shows

bas i c e l e c t r i c a l governing

sys tem

The

t u rb ine

speed i s sensed through

probe which counts

the

t e e t h

pass ing on too thed wheel a t t a ched

to

the s ha f t .

By

c a l c u l a t i n g th e r a t e a t which th e t e e t h pass the

probe

th e

speed o f th e t u rb ine

can be

c a l c u l a t e d . The output

of

11

8/11/2019 Course 234


2 3 4 . 0 0 - 5

the narrow range speed

s e n s o r

i s fe d

t h ro u g h

th e speed

droop

con t ro l to t he s er vo a m p li fi er s fo r th e g o v e r n o r steam v a l v e s

and

in te rcept

v a l v e s . A typ ica l

s e r v o

cont ro l system i s

shown in F i g u r e

5 . 9 . The

in te rcept

v a l v e s a re

sequenced

so

a s to be fu l ly open

any

t i m e t h e

g o v e r n o r

steam

v al v es a re

more t h a n 25 open and

to be

5

open

when th e

g o v e r n o r

steam v a l v e s a re fU l l y

s h u t .

The opening o f t h e g o v e r n o r steam v al v es and in te rcept

v a l v e s i s accomplished t h ro u g h

th e

s p e e d / l o a d cont ro l which

i s

t h e

e lec t r i ca l

g o v e r n in g

s y s t e m s

e q u i v a l e n t

o f

th e

s p eed

e r

g e a r . As

w ith

th e me c h a n ic a l

g o v ern o r

t h e in p u t

to

t h e

s p e e d / l o a d

cont ro l

may be

e i t he r manual computer l o ad i n g

o r

runback.

I n

th e

me c h a n ic a l hydraul ic g o v e r n o r th e

speed

droop i s

a

f u n c t i o n o f th e d e s ig n o f th e

g o v e r n o r

and

as

such

is

vi r -

tua l ly a

constant .

In th e e lec t r i ca l

g o v ern o r however

i s r e a s o n a b l y easy to v a r y

th e

spe ed droop to o b t a i n th e most

des irable

droop

se t t ing

for

a

par t icu la r

o p e r a t i n g c o n d i

t i on . The droop se t t ing i s variab le between 1 and 12 .

Basica l ly

t h e

droop i s h i g h

12 )

a t

o p e r a t i n g

speed when

n o t c o n n e c te d

to

th e grid . Th e droop is moderate

4 )

when

a t o p e r a t i n g sp e e d

and s y n ch ro n i zed

to

th e

gr id .

Th e droop

i s

lo w 1 )

on

a

l o ad re jec t ion .

F i g u r e

5.10 shows a s ch em a ti c b lo ck diagram

o f th e e lec-

t r i c a l hydraul ic g o v e r n in g

s y s t e m .

Th e wide r a n g e

speed

d e

t ec tor

i s used

to

c o n t r o l

th e emergency s t o p v alv es d u rin g

ru n up

to o p e r a t i n g sp e e d .

Once a t

o p e r a t i n g

sp e e d

th e

wide

ra n g e sp eed detec tor p l a y s no

role in normal

o r

abnormal

speed control .

Load R e j e c t i o n

Th e

basic

di fference

between

th e

res p o n s e

o f th e e lec t -

r i c a l hydraul ic g o v e r n o r

and

th e me c h a n ic a l

h y d r a u l i c

g o v er

n o r on a l o ad

re jec t ion

i s

t ha t

th e e lec t r i ca l

h y d r a u l i c

g o v e r n o r i s s o

much

fas te r . Because o f

th e

u se o f e lec t r i ca l

s igna l s and h i g h p r e s s u r e FRF,

dead

t i m e

is

vi r tua l ly e l i m i n

ated . During a fu l l lo a d re jec t ion th e g o v e r n o r

can

cont ro l

th e

ent i re

o v e r sp e e d w i t h o u t o p e r a t i o n

o f

th e emergency

stop

v a l v e s .

Th e e l e c t r i c a l g o v e r n in g system h a s two features

which

come

in to o p e r a t i o n d u ri n g

a

lo a d re jec t ion:

1 . An a cc el er at io n d et ec to r s en s es th e

ra p i d in c re a s e

in

sp e e d from th e narrow ra n g e sp eed de tec tor . Th i s r a p i d

acce lera t ion i n i t i a t es

a

fa s t v a l v i n g

i n p u t

which feeds

an o v e r r i d i n g s ignal to

th e g o v e r n o r steam

v a l v e s and

in te rcept

v a l v e s .

Reg ard l es s o f

what o t h e r mode

th e

- 12 -

8/11/2019 Course 234


ca....

, a a c .

. fOLD

/

LO C K

y

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CTU Toa

0 1 1 .

r

nu

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SHUT

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SY SnM

F L

n

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- - ~ -

V

...

I. . . . .

-

LlY

I

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POStTION

I

I

INDICATO.

S n A M L_

Ca a . ICT _

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I

II D

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C

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l ilU

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ruT

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MULTIPLIER

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0

•

0

l

W

SHUT-Off

O K

HIGH

= ~ . ~ ~ o S U I t 4

J

G ov er no r S tea m V al ve C o n t r o l System

F i g u r e

5.9

8/11/2019 Course 234


-

--

lVacuum

Condenser

-

Toothed

Wheel

U

U

U

Probe

System

<::h H·

HP

LP1

LP2

LP3

ES ~ l v s

HP

o v ~ n i n g Valves

l

Interceptor

~ l ~ ~

0

1

1' 7

0 1 ~

Valves t

~

~ ] [

~ [

~ t ~ t

fO

1 2 3 4 6

I

0

2

r7

l (Re-

I

Boi l e r ill

IHeate

Press

tPr7

j

r7

-rrransd

- Ini t iate

,

Posn

~ S i g n a 1 s to non-

04

return release

y Servo

.........

-

Valve Control

Amos

Posn

Servo

-

I nmn

Fast r---,

H ~ I ~ ~ 1

1 ~ ; ~ o t

Va1ving- - . . c : J . o s u r ~

- -

Input a lJlIEU

- - -

- ~

I

Manua'

'; 0

ed J

E:ua1

1

ad I

nn ,

u t 1

I

I

. te

COllputer

Accn

t losurel

Ext Control

RWl Up

l

'I

Detec 1 rrimer 1

~

r

var

1

i

Select

I

1

In t

d

Dr

nnn P ' 'HLow

Seq

I

1 M a n u ~ l l

L. n ~ ~ J

ld/

1Input

I

~ O O P

hr , -

I

O Speed

Test Interlock

I

Computer

loading

Gar

Fast

Runback

Gag

Synch/Unsynch

l i u n b a c ~

I

Detect

........Pressr

l

L

'Sea

,

L.,;;acuum

L ~ P l 1

1

_.

__ .

Block Diagram

of

Elec t r i ca l Hydrau l i c Governing Sys tem

Figure 5.10

8/11/2019 Course 234


234.00-5

valves may

be

in t h i s

s ignal

fu l ly closes

the valves a t

the f a s t e s t possib le

r a t e .

The closure of the

in te rcept

valves and

g ov ern or ste am

valves i n i t i a t e s the s ig na l to

c lose the ext rac t ion steam non re turn

valves and open

the

re lease valves .

2. When the generator

becomes

unsynchronized by a

load re -

jec t ion

the

droop

con t ro l

s h i f t s

the

speed droop

to

1 .

This

serves the same funct ion

as

the auxi l i a ry gov

ernor

in

the mechanical hydraul ic governing system. The

s h i f t in

speed

droop

wi l l

insure the tu rb ine re turns

to

a no-load

speed of only

overspeed

u n t i l

the speed/

load cont ro l drives the no-load

speed

back to

1800

rpm.

Figure

5.11 shows

the

response

to

a load

re jec t ion of

a

t yp i c a l

e lec t r i ca l hydraul i c

governing sys tem .

Eccen t r i c

B olt T rip

r ·

.. l

2

.. l

c

>

.j..l

U1

0

sl

ll

I l

°

l f

0

.. l

ll

QJ

U

o

5

10

15

20

Time

Seconds)

El ec t r i ca l

Hydraul ic

Governing

System

on

Load

Reject ion

Figure 5.11

15

8/11/2019 Course 234


234.00-5

Eccentr ic

Bol t

Emergency

Trip Plunger

Regardless of whether the

t u r b i n e i s

equipped with

an

e l e c t r i c a l

or m echanical g ov ern in g system , t w i l l

have

an

e c c e n t r i c b o l t

emergency t r i p p i n g

device.,

This device w i l l

shutdown

the t u r b i n e on

overspeed

and s e t high enough

110 -112

o f

operat ing speed t o be

required

t o operate only

the o t h e r overspeed devices q o n t c o n t r o l the speed. That

i s , i f

the

normal overspeed devices operate

proper ly ,

the ec

c e n t r i c b o l t

emergency t r i p p i n g device

should never have t o

o p e r a t e .

rhe

tot: i ~ r ~ d n <

m echan ism of

t..b..e

eccen t t

i< : bo

t..

t.. t i );\ );\in c:;

device

i s shown i n Figure 5.12. Spring t e n s i o n holds

the

b o l t i n a g a in s t c e n t r i f u g a l

force .

At

the

t r i p

speed, cent

r i f u g a l force

overcomes

spr ing t ens ion

and

the b o l t i s thrown

out,

contact ing

a

t r i p pi n g l a t c h. This

a c t i o n operates a

plunger which

d i r e c t l y

or i n d i r e c t l y

dumps the

f l u i d holding

open the

steam

admission valves.

In

the

case of the mechanical hydraul ic governor Figure

5 . 6 , the

e c c e n t r i c

b o l t

t r i p p i n g device dumps power o i l

which closes the

emergency

s top valves and governor

steam

valves . The

decreasing power o i l p r e s s u r e

i n

t u r n t r i p s

p r e s s u r e switches

which

i n i t i a te s a ir s i g n a l s

t o

shut

the i n -

t e r c e p t valves and open

the

r e l e a s e

valves .

In

the case of the

e l e c t r i c a l

hydraul ic

governor,

the

e c c e n t r i c

b o l t t r i p pi n g

device dumps

FRF which

closes the em

ergency s to p valves ,

governor

steam

valves , i n t e r c e p t valves

and r e h e a t emergency s top valves .

The

s h u t t i n g

of

the

GSVs

and IVs

i n i t i a t e s

a s i g n a l t o open the steam r e l e a s e

valves

and

shut the

e x t r a c t i o n

steam

non -r et urn va lv e s.

Load Limiter

Figure 5.13 shows

a

t y p i c a l speed droop curve.

I f

g r i d

frequency were t o f a l l

due

t o

a

system disturbance, the gov

ernor steam valves

would

open

f u r t h e r .

This overpowering of

the

t u r b i n e

might cause

physica l

damage

t o

the

t u r b i n e .

I t

may

a l s o be necessary t o l i m i t

load

t o

some

value

below

100

maximum

continuous r a t i n g .

The

load l i m i t e r

can

be adjusted

t o

provide

an upper

l i m i t t o

steam flow

t o

the

turbine

by

l i m i t i n g the

opening

of

the

governor s te am

valves .

In the mechanical hydraul ic governor t h i s

i s

done

by

r e -

s tr i c ti n g p i l o t o i l

pressure

t o

a maximum value.

In the

e l -

e c t r i c a l h y d r a u l i c

governor

t h i s i s done by

a s e l e c t low

f e a t u r e which l i m i t s the g ov ern or steam valve

opening

s i g n a l

t o the lower of t h a t determined by the speed droop or

the

load

l i m i t e r .

- 16 -

8/11/2019 Course 234


CHANNEL A

Overspeed

Trip Mechanism

CHANlfEL B

Overspeed Trip MechanisM

To Valves

Over

speed

Bolts

f

•

Main Tripping

Plunger

Bypass

lunger

\

-

1

I

I

I

I

I

I

t

I

I

I

t f

I .L .L T

I

I

I

I

__

_ 1

I

I

I

I

I

I

I

=1:

Main

Tripping

Plunger

I

I

Overspeed

Bolts

From

R Pumps

Main

Tripping ircuit

i lo t

Tripping ircuit

Overspeed

Tripping

ircui t

Figure 5 2

8/11/2019 Course 234


234.00-5

o

Cl I

Ll

;;:

1

25

50

75

lOO

125

Anti-Motoring

Device

Speed

Droop

Curve

Figure 5.13

Generator Load

( o f Ful l Power

Small

t u rb ine

g en era to rs u su al ly have a reverse

power

t r i p

assoc ia ted

with

t h e i r

outpu t b reaker

which opens

the

breaker

on reve rse power to p reven t motoring . l a rge

t u r -

bine gene ra to rs motor i s permi t ted

and such

reverse power

t r i p s do

not ex i s t

There must ex i s t some f ea tu re however to preven t t r y -

ing

to s t a r t

th e

tu rb ine from th e

genera tor

end by

c los ing

the outpu t

breaker

when

th e

tu rb ine i s shutdown. The gener -

a to r

i s not designed

to

withs tand th e la rge overcurren ts

-

18

-

8/11/2019 Course 234


234.00-5

assoc ia ted with

s t a r t i ng the

u n i t with

th e genera to r

ac t ing

as

an induc t ion motor. The ant i motor ing

device

prevents

c los ing th e o utp ut b re ak er

the

emergency

stop

valves

are

shu t .

SSIGNMENT

1.

For a

mechanical hydrau l i c governing system,

ex pla in th e

p a r t the

fol lowing play

in

l imi t ing an overspeed fol low

ing load re jec t ion:

a

e l e c t r i c

a n t i c ipa to r

b main

governor ,

c aux i l i a ry

governor ,

d speeder gear ,

e

eccen t r i c

b o l t

emergency

t r ip

device .

2. For an e l e c t r i c a l hydrau l i c governing system, exp la in

the p a r t the fol lowing play in l im it in g an

overspeed

fo llo win g lo ad r e j e c t i o n :

a narrow range

speed

de tec to r

b acce le ra t ion

d e t ec to r

c var iab le droop contro l

d speed / load con t ro l

e emergency t r ip plunger .

3. w i s ac t ive

load

va r i ed :

a with

a

mechanical

hydrau l i c

governor?

b

with an e l e c t r i c a l

hydraul ic

governor?

4. Expla in

the

advantages

of

an e l e c t r i c a l hydrau l i c gover

nor

system

using

FRF over a

mechanica l hydrau l i c gover

nor

us ing l ub r i ca t ing

o i l .

5.

What

i s

th e fu nctio n

of

th e

load

l imi t e r?

6.

What

i s

the

funct ion

of t he a nti m o to rin g device?

R.O. Schuelke

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Course 234

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