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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
NPTI 20072010
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
NPTI 20072010
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
NPTI 20072010
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NPTI 20072010
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.)
NPTI 20072010
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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
NPTI 20072010
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
LP Heater 2 level control
LP Heater 3 level control
LP Heater 4 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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NPTI 20072010
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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NPTI 20072010
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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NPTI 20072010
LPH 3 LEVEL CONTROL;-
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NPTI 20072010
LPH 3 LEVEL CONTROL;-
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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NPTI 20072010
LPH 3 LEVEL CONTROL;-
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 %
LPH2 level High
Protection close
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NPTI 20072010
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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NPTI 20072010
HOTWELL LEVEL CONTROL;-
I/P
Main condensate valveCEP discharge to LPH
Recirculation valve
CEP discharge to Hotwell
Condensate Extraction Pump
Hotwell
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NPTI 20072010
HOTWELL LEVEL CONTROL;-
Hotwell level(0-356 mmWc)
Set Point(0-356 mmWc)
PI
A /M
I /P
Z/I
(X)K
os
Recirculation valve
CEP discharge to Hotwell
Z/I
(X)K
Main condensate valveCEP discharge to LPH
LPH2 level High
Protection close
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NPTI 20072010
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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NPTI 20072010
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