Cascade control
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This document discusses cascade control in a power plant boiler. Cascade control uses two controllers, a master and slave, to more precisely control a process. In a boiler, drum level is controlled using cascade control with drum level as the master controller and feedwater flow as the slave controller. This provides improved control over drum level as steam load changes are compensated for through remote manipulation of the feedwater flow setpoint. Benefits of cascade control include reduced lag time and improved dynamic response, while drawbacks include increased complexity, cost, and controller tuning difficulty.
Cascade control
- 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.