Getting all fired up over control by PLCs

Combined Heat and Power (CHP) plants have been a major success – and not just because of their efficiency. Another key benefit is their flexibility. But it poses challenges for control of the generator sets. Centrax’s generator sets are based on Allison gas turbines. Control currently comprises a PLC system and an electronic governor. The […]

Combined Heat and Power (CHP) plants have been a major success – and not just because of their efficiency. Another key benefit is their flexibility. But it poses challenges for control of the generator sets.

Centrax’s generator sets are based on Allison gas turbines. Control currently comprises a PLC system and an electronic governor. The governor handles the start-up sequencing, and the critical task here is to control the fuel input as the gas temperature and turbine speed are raised. Processor cycle times are around 10-20msec.

`Firing temperature and shaft speeds must be scanned at a similar frequency. Conventional analogue modules are too slow’, says Harry Trump, from Centrax.

The PLC-based control system handles sequencing, control and protection of all generator set equipment, and acts as a back-up for the engine protection functions handled by the governor.

Trump: `While the governor stays in control of the fuel input, we use the PLC to alter setpoints on the governor. This determines what temperature and power level the engine is to run at – or the control modes, such as changing import or export of power from or to the grid.’

Centrax’s main PLC is a Siemens Simatic S5-135U, with an S5-95U as fail-safe back-up for critical functions, like speed, temperature and fire protection.

The two PLCs receive the same inputs for critical functions and run the same software. Each monitors the other’s outputs. If there’s a disagreement, one or other will shut the system down.

Now, Centrax is looking at Siemens’ S5-95F fail-safe PLC, which would be linked via Profibus into the rest of the system. `This will provide the same facilities, but will mean the main controller is freed from handling critical parameters’, says Trump.

Profibus will also minimise plant wiring. Previous hard-wired installations required some eight man weeks of cabling on site for the three panels used with each generator set. `We could now break up the panels and locate the controls where needed’, says Trump. This still leaves scope for remote control using Profibus, such as a connection to the SCADA system, or generator controls.

Future plans could include closer integration of the the start-up and sequencing governor, via a Profibus interface, or a backplane system for the PLCs. A further idea is to feed the I/O through the PLC interface, with a card communicating over the backplane.

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