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ELECTRO HYDRAULICCONTROLLER

HEMANT KUMAR

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EHC Speed measurement

Speed controller Load controller

Pressure controller Controller Selection

Position controller

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SPEED MEASUREMENT Four Nos of hall probes mounted around a

disk containing 120 magnets (alternative60 N-pole & 60 S-pole)

One rotation will produce 60 pulses.

Three probes are used for measurement

0.8 mm gap is maintained b/w hall probeand disc.

Hall probe output is a sine wave with peakvoltage 700 to 800 mV.

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SPEED MEASUREMENT Pulse converters will convert the sine waves tosquare wave of peak 10 V.

Each pulse converter card will generate three

isolated output out of which two are used. Speed measurement is done in two different

channels.

Both channels will receive signal from each pulseconverters. One channel is used for indication andLVM & the other one for controller. nACT is shownin digital indicator provided in UCB

range: (0- 3300 rpm)

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SPEED CONTROLLER SC is used for increasing speed from

Barring speed to rated speed & to assist

auto synchronizer in synchronizing the m/c. SC can be used to load the m/c fully but it

is used only for loading upto 10% as

frequency variation will affect the v/vopening.

v/v opening is directly proportional to thediff. b/w grid freq and speed set point.

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SPEED CONTROLLER Speed set point

Set point Tracking

Speed set point controller

Droop

dn/dt monitoring No load pressure correction

Speed controller loop.

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SPEED CONTROLLER

Speedcontroller

49

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Speed set point Speed reference nRCan be changed from

desk, auto synchronizer, ATRS, C&I cabinet& AVR console.

The set point changes at a gradient of2160 rpm/min up to a setting of 2820 rpm.

After 2820 rpm it changes at a rate of

360 rpm/min to have accurate control. Speed set point indicator is available in two

ranges 1) 0-3300 rpm

2) 2700-3300 rpm

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Set point Tracking If the frequency is between 49 Hz & 51 Hzand LC or PC in action speed set point willtrack grid speed by +15 rpm.

This ensures that SC will not interfere innormal operation when frequency changes.

After turbine trips speed set point will followthe actual m/c speed by -120 rpm.

This ensures that speed set point is less thanactual speed (barring speed). So nxt timedurin rollin n has to be increased.

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Speed set point controller nRwhich is a step signal will beconverted to time delayed ramp signaln

RTD

or nRLIM

by an integrator for whichgradient is decided by TSE upper margin.nRlim indicator is available in ucb(0-3300rpm)

This has three parts

1. Gradient

2. Stop reference

3. Fast balance

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Speed set point controller1.Gradient

This is the rate at which the out put of SSPC

changes. Max rate is set at 600 rpm/min.

The rate also depends on TSE upper margin.

If it is 30ok then rate is maximum. Ratereduces as margin comes below 300k linearly.Min rate is 108 rpm/min.

This is to ensure minimum thermal stress

during rolling.

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Speed set point controller2.Stop reference

The output of the SSPC will be blocked

under certain condition given below. Though nRcan be changed no change will

be there in nRTD.

The controller can be enabled again afterthe TSE influence is switched off and faultreset by C&I. Then made on and SPD set

point release push button is pressed in UCB.

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Speed set point controllerStop reference logic

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3. Fast balance

When the set point is in tracking mode

(follow up mode) there will beinstantaneous change in nRTD valuefor a change in nR.

The controller doesnt see the gradient inthis mode.

Speed set point controller

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DroopAfter synchronization ifnR is increased

then actual speed will not increase.Instead the control will open more.

The droop is set at 5%. As the rated speedis 3000 rpm,5% is 150 rpm.

The deviation of 150 rpm will correspond

to full opening or full closing of control v/v(with the assumption that full opening ofCV will give full load at rated parameters).

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Droop

Droop= %51003000

30003150

X

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dn/dt monitoring KWU turbine got critical speeds between

700 rpm and 2850 rpm.

It is not advisable to run turbine forprolonged time in this speed zone.

For preventing low acceleration rate dn/dt

monitoring is provided. When following conditions satisfy dn/dt

monitoring actuates and will bring the speed

to 600rpm.

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dn/dt monitoring nR is more than 600 rpm

ESVs open

G. bkr NOT ON

nact less than 2850 rpm

dn/dt less than 108 rpm/min SC in action

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No load pressure correctionAs the speed controller loop is PD(P)

controller an error in input must be thereto keep the control V/Vs open.

As the v/v opening required for runningm/c at rated speed is directly proportionalto steam pressure, a pressure dependent

correction signal is give to the speedcontroller loop.

Correction is approximately 20 rpm for

100 bar.

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LOAD CONTROLLER Load controller is used to maintain load set

by the operator or load dispatch center

(optional) LC comes into action only after

synchronization.

LC is also responsible for taking care offrequency changes and correcting load iffrequency influence is ON.

It is divided into following subsections.

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LOAD CONTROLLER Load set point

Load set point controller

Frequency influence

Load limiter

Actual load measurement Load controller loop

Isolated grid

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LOAD CONTROLLER

Loadcontroller

TSE & loadgradient

UCBATRS

C&I panelCMC

Stop fastbalance

Load setPoint

Controller

Gradient

PR

PACT

CMC ON

PR

LC output0 to +10V

FI ON/OFF

Frequencyinfluence

MIN

PRTD

Loadlimiter

+

_

droop

nACT set freq(51Hz)

PRf

PRCMC

Pressurecorrection

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Load set point

Load set point can be changed from desk,C&I cabinet, ATRS & CMC

Set point changes at a gradient of 250MW/min

The gradient remains the same throughout

the range Load set point PR,indicator is available UCB

. range(0-600 MW).

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Load set point controller PRwhich is a step signal will be converted to

time delayed ramp signal PRTD or PRLIM by

an integrator for which gradient is decided byload gradient and TSE influence .

PRLIM indicator is available in ucb (0-600MW)

LSPC has three parts

1. Load gradient and TSE influence

2. Stop reference

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Load set point controller

1) Load gradient and TSE influence

Load gradient acts in parallel with TSE

margin with a minimum selector. Load gradient range is 0-60 MW/min and

can be set from UCB

TSE margin 0-30

0

k corresponds to0 to 60 MW/min

Both are having individual ON/OFF switch.The one which is ON will control the

output.

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If both are OFF then PRTDchanges at a fixed rate of60 MW/min.

If both are ON min of two will be selected for

controlling the output. Load gradient setting is common for increasing as

well as decreasing.

But in case of TSE , upper margin controls theincreasing rate and lower margin controls thedecreasing rate.

If TSE fault occurs, the gradient will be 30 MW/min(50%)

Load set point controller

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Load set point controller

2) Stop reference

The PRTD will freeze on any of following conditions.

i) TSE is ON and it develops fault. It can be resetby switching OFF TSE influence and fault resetby C&I. Then made ON and Load set point

release push button is pressed in UCB.ii) Limit pressure reached AND PC in action AND

Load set point raised.

iii) Load set point controller disabled

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Load set point controller

3) Fast balance

In this mode output of LSPC, PRTD, will follo

the actual load if any of the followingcondition exists

i) generator breaker is OFF

ii) Load controller OFFiii) Load shedding to less than station aux

load AND MW error> 5%

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Load set point controller

This ensures PRTD brought down to zeroafter GB opening.

After synchronization PRTD tracks the actualload so that more time is not wasted inbringing PRTD from zero to working value

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Frequency influence Frequency influence is used to correct PRTD.

The MW correction corresponding to frequency

deviation is added to PRTD. FI has a droop of 5% (it can be changed 2.5%

to 8.0% in steps of 0.5%). ie, a deviation of0.5 Hz will cause a load change of 100 MW.

FI can be switched ON/OFF from UCB.

UCB indicator for the correction signal is knownas PRf.(range:-200 to +200 MW).

Pressure correction is similar as that of SC

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Load limiter Load limiter is used to limit the load to a

certain value set by the operator. Load limitervalue is shown in UCB indicator.(range: 0-600 MW)

Final set point (sum of PRTD & PRf )is fed to aminimum selector with the load limiter.

As load limiter acts after PRTD it can be usedfor fast reduction of load in case ofemergency.

Final set point after load limiter is know as PR

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Actual load measurement

Three load transducers are used formeasurement.

Actual load 0-600 MW corresponds to 4-20 mA.

Median value of the three will be selected forprocessing.

If any value drifts from the other value by morethan 5%, the selective card discards thedefective channel.

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Load controller loop

Final control loop is a PI controller.

error signal= PRPACT.

Error is fed to a PI controller which producesan output of 0 to +10V which corresponds to0-600MW.

LC has provision for two follow modesi) follow above

ii) follow low

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Load controller loop

Follow lowmode is present when the GB is ONand speed controller is still in action

During this mode LC tracks the SC out put withan error of -150mV.

The LC output can go more than SC output and

hence LC can be taken into service. Due to this follow mode, the LC can be taken

into service anytime without any delay after

synchronization.

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Isolated grid

Isolated grid operation will act If,

i) PR Pact >10% (50MW) AND

ii) frequency > 51 Hz AND

iii) LC in action

In this condition the controller will be changed to

proportional (P) type for fast corrective action. This condition can be eliminated by adjusting

load set point near to actual load and pressing

the Isolated grid reset button in UCB desk.

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PRESSURE CONTROLLER Pressure controller maintains the throttle

pressure within working range at all loads.

Deviation= actual (PrAct) - set pressure (PrR) The operating range of deviation is 0 to+150mV

As long as deviation is more than +150mV ,the PC output remains at max saturation(+12V).

ie, for a deviation of 0 to+150mV, PC willproduce an output of 0 to+12V.

PC has two modes of operation.

PC is a PI controller

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PRESSURE CONTROLLER

PrAct

Pressurecontroller

&

PrR Limitpressureselected

15ksc

15ksc

+

+

_

Pr

Limitpressure

reached

PC output0V to +12V

+

_

Limitpressureselected

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PRESSURE CONTROLLER1)Initial pressure mode

In initial pressure mode PC will try to

maintain the throttle pressure. Any deviation in pressure will be adjusted

by opening or closing the control v/v.

2)Limit pressure mode

In limit pressure mode boiler storagecapacity is utilized.

There wont be any change in PC out put

till Pr deviation becomes -15 ksc.

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PRESSURE CONTROLLER

Once the throttle pressure falls below the setvalue (15 kgc) then further fall in pressure isprevented by varying the control v/v positionand hence load.

p indicator is provided in UCB

Range:(-30 to +30 kgc)

Minimum EHC output is set at 30% to avoidcomplete closing of the control v/v whendeviation isve.

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PRESSURE CONTROLLER

Initial pressure mode will be selected on following

conditions.1. Boiler control on manual

2. Turbine follow mode

3. IP mode selected from console

Limit pressure mode will be selected on followingconditions.

1. Boiler follow mode

2. CMC selected3. LP mode selected from console

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CONTROLLER SELECTION

M

A

X

A

D

D

M

I

N

M

I

N

+10V

SC out put

-10V to +10V

LC out put0V to +10V

PC out put0V to +12V

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CONTROLLER SELECTION

SC output is -10V to +10V for 0 to 150 rpm.

LC out put is 0 to +10V for 0 to 600MW.

PC output is 0V to +12V for 0 to +150mV. SC output and LC output passes through a MAX

gate.

The value selected is passed to MIN-I gate. Theother input to the MIN-I gate is the adder outputgot by adding 10V(100%) to SC output.

The MIN-I output is fed to MIN-II gate for which

the other input is PC output.

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CONTROLLER SELECTION

The MIN-II gate out put is fed to the positioncontroller.

Adder is provided to prevent over speeding ofthe turbo set.

In normal condition SC output will be +1V as

nR is tracking grid speed by 15 rpm. During over speeding, nRwill stop tracking at

3060 rpm(51 Hz). At this point nRwill be

3075 rpm.

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CONTROLLER SELECTION If frequency further increases at 51.25 Hz

(3075 rpm) SC output will become 0V. From thispoint onwards SC output will beve for increase

in frequency which will reduce adder output.

Suppose if m/c is at 500MW (LC active), LC outputwill be 8.33 V. If adder output comes less than this

value SC will become active and m/c will startunloading.

At 53.75 Hz (3225 rpm) SC output will be -10Vand adder output will be 0V. ie, m/c will be fullyunloaded.

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POSITION CONTROLLER Position controller is a PID controller

Error signal is generated by comparing the

selection circuit output with the positionfeedback obtained from Collins transformer.

There are two Collins transformers which givesoutput voltage corresponding to piston position.

The output of the two Collins transformer passesthrough a MIN gate (logical max selector as itsout put isve) and the selected value is fed to

the controller.

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POSITION CONTROLLER

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POSITION CONTROLLER

Plunger coil is an integrator type.

The balance position of the pilot v/v piston is

achieved if -1.0V is fed to the plunger coil. For moving the piston in open direction the voltage

has to be increased. Say -0.9V is fed to the coil,then piston will keep on moving towards openingdirection till the voltage again made -1.0V.

Similarly if the voltage is made -1.1V, then thepiston will keep on moving towards closing

direction till voltage is again brought back to -1V.

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POSITION CONTROLLERPlunger coil

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POSITION CONTROLLER

Collins transformer is a differential transformerand produces output from -8.0V to +8.0V which

is converted into 0.0 to -10.0V. 0.0V output corresponds to the position at which

first control v/v just starts opening.

-10.0V corresponds to the position at which lastcontrol v/v is just full open.

A constant bias of +200mV is given for secondCollins txr so as to avoid frequent changeover

from one Collins to the other.

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POSITION CONTROLLER

If at any time the difference b/w the twoCollins exceeds 700mV a fault alarm isgenerated.

There is provision to switch ON/OFF thevoltage supply to plunger coil. This can bedone only if the output of position controller

is more than +5.0V. This ensures that at the time of switching

OFF, the piston is full open and there will

not any jump after switching OFF.

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POSITION CONTROLLER

If at any time during operation plungercoil wire breaks, the piston will go to

full open position and plunger coil faultalarm will be generated.

As the piston goes to full open position,

the control v/vs also will open fully.

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POSITION CONTROLLER

EHC fault will occur on following condition1. Starting device faulty

2.

Collins-1 faulty3. Collins-2 faulty

4. Plunger coil faulty

5. nAct-1 ~ nAct-2 >5%

6. Turbine tripped7. Power supply faulty

8. EHC faulty

9. Speed acquisition faulty

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