Distributed Control / Systems Integration

Aero-Derivative Advanced Control

What's APC and What's Just a Collection of Clever Programming Blocks?

By Dan Hebert

Lou Bertha of RDI Controls isn't sure, and he's the one who put together an "APC" system to control an aero-derivative combustion turbine. The process is basically a jet engine exhausting into a power turbine, with the power turbine coupled directly to a generator.

The system must control and limit multiple jet engines, coordinate the speed to 3600 rpm to synchronize with the electrical grid, and adjust load when online. Each of the engine controllers positions the fuel control valve to provide the required liquid or gas fuel requirements, and that's where it gets interesting.

With two engines operating, the system will determine which engine is the master. The slave engine will get an engine speed cascaded setpoint from the master. Either engine controller can be the master, and the control logic seamlessly switches masters as operational modes dictate.

The system is controlled with four Opto 22 rack-mounted SNAP PAC R-series controllers with I/O modules. Controllers are installed on each of the engines. The controllers communicate peer-to-peer with each other to ensure overall operational requirements are met.

"Both systems have multiple PID controllers, including cascaded control loops, which all vie for control of the fuel control valve," explains Bertha. "Tracking and coordination between these controllers can become quite complex, because we need the controllers to respond immediately when needed and not have to wind down."

In multiple engine configurations, they need each engine to control its fuel flow to maintain precise speed and load control of generator. Multiple modes of operation and tracking complicate things further. "We can put one engine in a test mode, which allows direct control of engine speed, or have one engine shut down due to engine problems while the other engine maintains the specified generator setpoints," Bertha notes.

The system uses advanced control techniques such as cascade control loops with and without feed-forward, adaptive tuning, anticipatory control adjustments and multiple controller coordination and tracking.

"Opto22 allows us to develop user-defined subroutines, which can range from simple to complex," says Bertha. "These subroutines are developed in the same flow diagram/scripting language used to develop the control logic. Since Opto22 provides both higher and lower level commands, we have the ability to develop the blocks or subroutines, such as rate limiters, function generators, monitors, Modbus commands and so on, to suit our needs."

But is this APC? "Some of these blocks may be considered standard on some DCS type systems, but the Opto22 setup allows us to develop and modify these subroutines to provide greater flexibility than the 'fixed' routines in other systems," Bertha explains. "You don't need to be a computer science major to figure out and modify these routines to suit your needs. Whether or not these are considered APC is in the eye of the beholder, but the tracking and coordination between PIDs and setpoints makes it a fairly advanced setup," says Bertha.