Controlweb was engaged to refactor the control philosophy of a hydro electric generator onto a modern control system platform. The previous controller of the hydro’s design dated from the 90’s . Its scan time and functionality was very slow in comparison to newer versions. It is a synchronous hydro generator that required the speed to be controlled very closely to the grid frequency on start up. With no load being drawn from the generator it speeds up quickly which means this control has to be done within the first 4% of the guide vane’s operating range. The old controller struggled to meet this requirement and the hydro often sped up or took too long to reach synchronous speed. The problem is exacerbated by changing upstream pressures which in turn changes the rate at chich the hydro spins up. To top all this off the guide vanes (which control the flow through the hydro) are controlled by a split range proportional hydraulic valve. Keeping the output constant to the valve causes the guide vanes to creep open or closed. The problem was solved using cascade PID loops. The master loop controls the speed and the slave loop controls the guide vane position. We also used gain scheduling on the speed controller to adjust the gain based on the head pressure.
The newer PLC had the ability to scan the instrumentation a lot faster as well as the ability to accommodate object orientated programming. We utilized S88 (intended for batching systems) structures in the programming. The hydro was treated as a Unit and the process software was was written using software similar to Phases structured into a Unit Procedure. All equipment was encapsulated in Equipment Modules.
Wonderware’s System Platform was used as a SCADA system. It utilizes templates that dovetail perfectly into the objects created in the PLC software.
The whole system was simulated in an offline environment and commissioned in three days, two of which consisted of I/O loop testing and one for process commissioning.
The satisfactory result is a hydro that starts up quicker, runs more often and when a fault occurs it is quite clear what the cause was.
SCADA – Supervisory Control and Data Acquisition
PLC – Programmable Logic controller
S88 – A standard that used in Batching systems that delineate the boundaries and naming between equipment and process software modules
I/O – Inputs and Outputs to the PLC, meaning instrumentation connected to it.