LCC Quick Notes, Turbine Governors for DCS-Operated Plants, Avoiding Costly Hardware Overlaps
Turbine governor systems have historically been controllers and not instrument information systems. With the availability of plant Distributed Control Systems (DCS) data from multiple plant control systems can be presented on workstations, control managed by soft buttons, and data displayed in both real time and historical trend formats. Some governors have also evolved to provide DCS-like features and capabilities. The specifiers of a modernization retrofit must take care not to duplicate expensive hardware and distributed capabilities for plants already operating full featured DCS.
(1) Which Does What Best?
Both DCS and Dedicated Governors have their strengths and weaknesses. These are listed below.
■ Common control and data display formats for multiple systems.
■ Multiple assignable work stations both in and out of control room with superb graphics.
■ Super historical data trending for condition monitoring programs, handles large data records well.
■ Too slow for closing fast control loops, needs dedicated distributed satellite systems.
■ Not for safety protection systems of turbine like overspeed and emergency trip.
■ Risk of common mode system failure increases with number of complex control loops.
■ Many turbine control modes and features new to platform... require new, unproven programming for implementation.
Dedicated Governor Strengths:
■ Very fast turbine speed loop closure
■ Correct, proven turbine safety systems management
■ Correct, proven turbine control routines for multiple control modes.
Dedicated Governor Weaknesses:
■ Not usually designed to handle sophisticated data management for trending and historical recall
■ Limited work station capabilities
■ Command and Diagnostic protocols often unique to manufacturer.
(2) Selecting on Strengths
It's not difficult to see the obvious fit is to have the DCS handle the operator interface through their advanced work stations, manage condition monitoring data and archive historical data, while delegating the fast loop control to a distributed dedicated governor. This also represents a great economy in governor selection and simplicity in installation. While the marriage of the DCS to distributed dedicated control systems was once fraught with compatibility problems, the emergence of standard protocols and the use of full function simulators which exercise the loops in real time well forward of plant installation have eliminated these concerns.
(3) Separating the Hype from the Type
Some DCS manufacturers make the claim that only their internal control link solutions using proprietary communications are alone practical, with general bus links to dedicated systems consuming "thousands of debug hours". One must question that if this is the case the DCS vendor can master all the sophisticated data management but can't cope with an Ethernet or Modbus link? Similarly some governor vendors tout elaborate operator display stations and data timestamp capabilities which although nice are strictly duplicate features better off handled by a DCS.
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