Cscade control based on powerplant

1. Ashvani Kumar Shukla
Dy. Manager(C&I)
BGR ENERGY SYSTEMSLTD.

2. Introduction
Requirement
Block diagram
Application in power Plant
Drawbacks'
Advantages

3.  In cascade control a process is controlled by
two controllers in such way that both are acting
for each other.
 Cascade control is technique which contain
two closed loop control cascaded to each other
in such way that first loop controller output
will be set point for second loop controller. It is
called remote set point for second controller.
 In cascade control first loop called master and
second loop called slave controller.

4. Master controller generate the set
point for the slave controller.
Finally slave controller control the
process depends upon the remote set
point provided by master controller.

5. Requirement
Why we required the cascade control ?
In simple closed loop control, controller is controlling single measured variable at one point ,so it is
called single point closed loop control. It is monitored and controlled by single controller so process
has only one stage information for process control and less effective with respect to change in set
point.
In cascade control we are controlling the single measured variable at two point control. So we have
two stage process information . We have effective control of process with respect to disturbance and
set point change.
Primarily at master station where we measure the controlled variable and generate the controlling set
point for slave controller. Master controller generate the remote set point for slave controller , it
works as set point for slave controller and basis of that controller generate the output to control final
control element.
So cascade control are used where we require precise and critical control . In a process where load
changes disturb the process and simple closed lop are unable to control . Cascade control are able to
compensate the load changes of process and maintain at desired set value of any physical quantity.

6. Controller
1
Controller
2
Process 2
Process 1
d
O 2
SP
FEEDBACK
FEEDBACK
RSP
MV 1 MV 2
O 1

7. SP ,Called set point or reference
value.
RSP, Called remote set point.
d, Called disturbance created by
external source.
MV, Called measured variable.
 O1 & O2 are output 1 & output 2.

8.  Cascade control are used in steam power
plants at……..
 BOILER………
 In boiler we are using various types of closed
loop control and cascade controls. some
cascade loops are given below.
 1. Drum level control
 2.Steam temperature control.

9. LIC 101
S.P.
P.V.
2OO3
FIC 101
1*30%
FCV 1
1*100% FCV 2
2OO3
2OO3
P.V.
P.V.
R.S.P.
Feed water
flow TxSteam flow TxDrum level Tx
scale
0
100
0
2000
1E/3E
Single Element
Control
Three Element
Control

10.  Drum level control is typical three/Single element
control consisting of drum level, feed water flow and
main steam flow parameter. It is used to maintain
drum level by sensing steam load variation in main
steam flow and by regulating Feed water flow
through feed water control valves.
 The feed water controls valves that both typically
have 100 % capacity can be substitute each other
with one 30% capacity control valve (start up control
valve).The feed water start up valve controller
represents the single element portion of the feed
water control loop that is used during the start up of
the boiler. When the boiler generates steam load of
30 % MCR, the feed water control is switched to three
element mode (1E to 3E switch over is manual) and
controls shall be done with 100% control valves.

11.  Feed water Start up Valve Control 30 %( FCV-
103)
 The controller objective is to maintain drum
water level at a required set point during the
start up of the boiler. A pressure compensated
drum water level measurement is used as the
process variable for the controller.
 The feed water start up valve controller (LIC-
101) compares the drum water level
measurement to the operator adjustable set
point and generates an output that sets the
position of the feed water start up control valve(
30 % ) resulting in the change in feed water flow
rate and drum water level.
 Controller (LIC-101) action is reverse type.

12.  Feed water Main Control Valve (FCV-02) 3 Element Drum Level
Control.
 The Master controller objective is to maintain drum water level
at a required set point during the operational load i.e. (>30%) of
the boiler. A pressure compensated drum water level
measurement is used as the process variable for the controller.
 Master controller(LIC101) compare the compensated drum level
with the operator set point and generate the output depends
upon the water level.
 The feed water main valve controller (FIC-101) is slave controller
which receive the remote set point from the out of summation
block.
 Remote Set Point = O/P of LIC101+Main Steam Flow-50%(Bias of
steam flow)
 Slave controller compares the feed water flow with remote set
point and generates an output that drive the FCV02 depends
upon the controller. Opening/Closing of valve resulting in the
change in feed water flow rate and drum water level.

13.  Cascade control makes the system more
complex.
 Cascade control required more instruments and
equipments which leads the cost of process.
 Tuning of cascade controllers are more difficult
then close loop control.
 it requires an additional measurement (usually
flow rate) to work.
 Cascade control should generally not be used if
the inner loop is not at least three times faster
than the outer loop, because the improved
performance may not justify the added
complexity.

14.  Reduce the dead time and phase lag time in
the control system.
 Can be combined with feed forward and
other type of controllers.
 Improved dynamic response and performance
and provide limit on secondary variable.
 Cascade control should always be used if you
have a process with relatively slow dynamics
(like level, temperature, composition,
humidity) and a liquid or gas flow, or some
other relatively-fast process, has to be
manipulated to control the slow process.