Various Controls in Power Plant


Various controls of Power Plant


1. Flow Control

2. Pressure Control

3. Level Control

4. Temperature Control


Flow Control


The flow control is a fast control. It consists of PI control. The flow control includes


1. Feed water Flow

2. Condensate Flow

3. Combustion Air (PA and SA) Flow

4.  Fuel Flow Controls.

5. CEP Recirculation Flow 


The flow is basically a PI control. Low P and high I will make the control smooth, but the control will be slower. On the contrary High P values make the control fast, but very high P and I values tends to create high over and undershoots creating oscillation in the process value.


Process to Tune:


First keep the P and I value 0.5 and 1 and see the behaviour of the system. If the action seems to be very slow then increase P value as well as I value in range 0.8 and 1.4. Below are some basic values you can try with.

Start           0.5 and 1

Increase     0.8 and 1.4

Increase     1.2  and 1.8

Increase     1.2  and 2.3

Increase     1.6 and  3.2

Increase     2 and  3.5


For very Fast systems


Increase to 2.5 and 5 but in this case you should vary the P and I with respect to the deviation or the error signal.


If the system is slow then the PI values goes as follows

Start 0.5 and 1

Decrease 0.3 and 0.8

Decrease 0.2 and 0.5

Decrease 0.2 and 0.3

Do not decrease the values below that will cause no effect on the system.


Pressure Control 


Pressure Control Loops in boilers


1. Furnace Pressure (ID and FD Control)

2. Main Steam Pressure Control

3. Feed water Pressure control (Scoop Control of BFP)

4. Auxiliary Steam Pressure Control

5. HP Bypass Steam Pressure Control


Pressure control is same as flow control, but most of the time it is a very fast control, unlikely flow can be slow control also.


Level Control


1. De-Aerator Level Control

2. Hot well Level Control

3. Drum Level Control

4. LPH Level


The above mentioned are some of the typical level controls of power plant. Most of the level control includes PI only but I personally think introduction of D variable makes it a much better control.

The level control is very critical in Power Plants and needs to be taken very seriously.


Drum Level Control:


This control is divided into 2 types.


1. Single Element Control

2. Three Element Control


1.        Single Element Control


PV = Drum Level (%)

Setpoint = Drum Level (%)

Error = PV - SP

Error is fed to the PID / PI controller which controls the drum level.


2.      Three Element Control


Here there are 2 PID Controller


1st PID/ PI controller


PV = Drum Level

SP = Drum Level

Error = PV – SP


The error is fed to the 1st controller which generates a set point for the next controller 2nd Controller.


PV = FW Flow (Compensated)

SP = Signal from 1st Controller + MS Flow

Error = PV – SP


Tuning of these controllers are very critical and time taking .It is to be noted that the PV i.e. the drum level should be damped signal.


De-aerator Level Control






ERR is fed to PI controller and the output is modified with the rate of change of Condensate and FW Flow. The rate of change of Condensate and Feed water flow is used to have a feed forward effect in the controller. By adding this parameters we can take care of the level before hand.


Condenser Level Control


PV = Hot well LVL 

SP = Hot well LVL



ERR is fed to PI controller and the output is modified with the rate of change of Condensate Flow. By adding this parameters we can take care of the level before hand.


Temperature Control


This control is the critical control in Power Plant. Major loops under this control are as follows


1. RH temperature Control

2. SH Temperature Control

3. Main Steam Temperature Control

4. Auxiliary Steam Temperature Control

5. HP Bypass Steam Temperature Control


The temperature control is a very typical control and here we have to use PID loops. There are many new controllers where we can feed process dead time and process lag time. I f we can use such controllers then it will be very helpful to tune this controllers.

Temperature controllers tuning process is different as per different plant parameters.