npti 20072010

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Introduction to

Auto Control Loops in Thermal Power Plant

Sith.Ananda Kumar,Assitant Executive Engineer,

Thermal Power Station-II,

Neyveli Lignite Corporation.

A presentation for NPTI, Neyveli

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Automation

Preprogrammed execution

Less or no manual intervention

Time driven or event driven

Control loop

Auto / Manual

Closed / Open

Critical / Non-Critical

Local / Remote

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Controller Action

On / Off

Proportional / Integral / Derivative

Feedback

Feed forward

Ratio Control

Split Range

Cascade

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Controller Types

PLC / microprocessor / Microcontroller / Electronic

Distributed / Centralised

Hydraulic / Pneumatic / Electrical

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Elements of Control Loop

Process

Measurement System

Final Control Element

Human Machine Interface

Controller / Program

Necessity for Control Loop

For Optimal operation of process within range

Sequential execution cannot be applied always

Quick response / High sensitivity

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Controller(Decision)

Measurement(Process Variable) Action

(Final Control Element)

Process

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Terms related to Control Loop

Set Point (Desired Value)

- Auto- Depends on Process flow

- Manual

Set by operator

Controlled Variable (Process Variable)

- Parameter in process taken for consideration

- e.g.,Deaerator Pressure / Hot well Level

Manipulated Variable

- Parameter chosen to vary in order to achieve desired set point

- e.g., Damper position / TCV position

Control Deviation- Error signal that describes difference between SP and PV

Command Output

- Output to Final control element to correct the deviation

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Desk Control Tiles;-

0

10050

VALVE POSITION

%

-50

+500

SET POINT

mmWc

0

10050

COMMAND / DEV

%

-20

+200

AUTO / MAN

-20 0 +20

10 %

0 100

48.15 %

168 175

PV SP

M A

CLS A/M OPN

HPH6 to DEA / LPH4

2LCV553_063

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Execution of a Control Loop

Control Task

(e.g., SH / RH steam temperature control)

Variables to be measured in order to monitor stable performance of loop

(e.g., Drum level & Pressure / Feed water flow & Temperature /

Steam flow, Pressure & Temperature ) Selection of manipulated variables

(e.g., FCV position / Scoop Position)

Tuning of control loop / best control configuration

(Gain adjustments- Proportional / Integral)

Adjusting the manipulated variable based on information/signal received

(Command output from controller based on error signal.)

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Control LoopsSteam Generator

Furnace draft control

Secondary air header pressure control

Fuel oil supply header pressure control

Fuel oil temperature control

Fuel oil flow control

Atomising steam header pressure control

Secondary air flow to oil burner control

Hopper air flow control

Mill outlet temperature control

Feedwater flow control

Air heater average cold end temperature control

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Superheated steam temperature control

Reheated steam temperature control

Load and combustion control

Auxiliary steam pressure control

Auxiliary steam temperature control

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Control LoopsSteam Turbine

BFP Scoop Control

LP Heater 1 level control




BFP recirculation control

Deareater level control

HP Heater 6 level control

HP Heater 7 level control

Hot well level control

Deareater pressure control

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CEP minimum recirculation flow control

MST to Ejector pressure control

BFP Seal water temperature control

Gland steam pressure control

Gland steam temperature control

H2 cooler cooling water temperature control

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Boiler Drum Level Control ;-

Feed Water Flow

- Steam Flow

Set Point

Control desk / OSDrum Level(Compensated)

Blow down

constant

kM

A

X

28% - Steam flow

Zero

PI

A /M

I /P

Z/I

(X)K

PI

A /M

I /P

Z/I

(X)K

FCV 1 FCV 2

Correction factor

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Boiler Drum Level Control ;-

Feed Water Flow

- Steam Flow

Set Point

Control desk / OSDrum Level(Compensated)

Blow down

constant

kM

A

X

28% - Steam flow

Zero

PI

A /M

I /P

Z/I

(X)K

PI

A /M

I /P

Z/I

(X)K

FCV 1 FCV 2

Correction factor

Cascade

Feed forward

Feedback

Redundant

controller

Redundant Final

control element

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LPH 3 LEVEL CONTROL;-

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LPH3 level(0-356 mmWc)

Set Point(0-356 mmWc)

PI

A /M

I /P

Z/I

(X)K

os

LPH3 to LPH20-50 %

Z/I

(X)K

LPH3 to FT50-100 %

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LPH3 level(0-356 mmWc)


PI

A /M

I /P

Z/I

(X)K

os

LPH3 to LPH20-50 %

Z/I

(X)K

LPH3 to FT50-100 %

LPH2 level High

Protection close

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HOTWELL LEVEL CONTROL;-Main condensate valve

CEP discharge to LPH

Recirculation valve

CEP discharge to Hotwell

0 30 100%

Hotwell Level

V

a

l

v

e

p

o

s

i

t

i

o

n

100% open

0% close

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HOTWELL LEVEL CONTROL;-

I/P

Main condensate valveCEP discharge to LPH

Recirculation valve


Condensate Extraction Pump

Hotwell

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HOTWELL LEVEL CONTROL;-

Hotwell level(0-356 mmWc)


PI

A /M

I /P

Z/I

(X)K

os

Recirculation valve


Z/I

(X)K

Main condensate valveCEP discharge to LPH

LPH2 level High

Protection close

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Loop Consideration;-

Controller tuning

Steady process / manual mode / No load change

Vary proportional / Integral / Derivative gain Observe decay ratio / dead time / integral windup / Control deviation

Increasing Integral action causes CLCS more sensitive

Dead time leads to instability of loop response

Implement override control

(e.g, Scoop min 30%, LPH3- LPH2 level high protection close)

Include secondary measurements if necessary

(e.g., for MST flow compensation Pressure & Temperature also measured).

Different gain adjustment (e.g., during set point variation High / Low ramp)

Measurement / Control transmitters for high resolution

(e.g., Hotwell level 0-1600 mmWc / 0-356 mmWc)

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Concerns;-

Sensitivity and response time of the sensors and transducers.

Modes matching to application / controller (Dual loop for SH/RH temp control)

Tuning for specific condition

More Dead time (SH / RH attemperation control) & Hysteresis

Final control element size / shutoff / response / power to close againstdifferential pressure (e.g., DP in Feed water control valve)

Closed loop compatibility with control system

The existence of a control loop does not always mean that the system is capable of

controlling to the degree required.

Criteria for evaluating good control;-

Stability of process

Minimum or no overshoot on startup

Minimum time to reach the desired set point

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