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PID (Proportional-Integral-Derivative) controller is control loop feedback mechanism that widely used in control system. PID calculating the error value that generated by difference between feedback Process Value (PV) and Set Point (SP) PID. When error value generating, PID controller adjusting output Measurement Value (MV) to minimize error near to zero value.

Let us give some example PID Controller on Booster Pump application like on the left picture. Feedback Process Value (PV) is Actual Pressure (Bar), Set Point (SP) PID is Set Pressure (Bar), and output Measurement Value (MV) is VSD Reference Frequency (Hz).  This is the mechanism. Whenever the difference between PV & SP negative, PID Controller increasing output MV. Otherwise when the difference between PV & SP positive, PID Controller decreasing output MV. How fast the PID Controller could execute error calculation are depend on cycle speed on each controller and type of PID function.  There are several PID function base on control actions, which are P, I, PI, PD. PI function is widely used for Booster Pump application.

This is the PID & Alternate control system on Booster Pump application. There are 1 unit Pressure Transmitter,  1 unit PLC and 2 units Booster Pump that controlled by 1 unit VSD. Because of most VSD have PID Controller, we design the PI controller in the VSD. These are the several case :

• At the first time start PID in VSD with Pump A. VSD adjusting output frequency (Hz) into Pump A so that Actual Pressure (Bar) close to Set Pressure (Bar).
• Case 1 : Pump A couldn’t reach the target of Set Pressure (Bar), then PLC set 50Hz into Pump A and start PID in VSD with Pump B.
• Case 2 : Actual Pressure (Bar) is too high so that VSD set 0Hz into Pump B. This case make PLC to stop Pump B and start PID in VSD with Pump A.
• Case 3 : Actual Pressure (Bar) still on high pressure so that VSD set 0Hz into Pump A. This case make PLC to stop Pump A. That mean booster Pump A and B on the sleep condition.
• Case 4 : Suddenly Actual Pressure (Bar) drop to below Set Pressure (Bar). This case make PLC start PID in VSD with Pump B because of the last running pump before sleep is Pump A. This mechanism important to make sure all pumps have same running hour.

With the PID & Alternate control system, we could just add booster pump without add more VSD  to increase efficiency and capacity on Booster Pump Station. Mostly, Booster Pump Stations are using PID & Alternate control system because it could saving more cost for electrical energy consuption.

All of animation and graphical developed by Wonderware ArchestrA 3.0.

Tags: Wonderware