Cascade control

Cscade control based on powerplant

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

2. introduction 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.

3. 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.

4. Block diagram Controller 1 Controller 2 Process 2 Process 1 Cascade control block diagram d d O 2 SP FEEDBACK FEEDBACK RSP MV 1 MV 2 O 1

5. Legends of the block diagram symbols 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.

6. Application in power plant 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.

7. Drum level control LIC 101 S.P. P.V. 2OO3 FIC 101 1*30% FCV 1 1*100% F CV 2 2OO3 2OO3 P.V. P.V. R.S.P. Feed water flow Tx Steam flow Tx Drum level Tx scale 100 2000 1E/3E Single Element Control Three Element Control

8. Control loop description 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.

9. Continue………… 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.

10. Continue…………… 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.

11. Drawbacks 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.

12. advantages 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.