INTEGRATED TOOLS transforming power

Engineering data acquisition and control systems for CCGTs has come a long way BY tilman tutken and vinod vaswani

Power plant control continues to be influenced by the rapid advances in technology. Computer performance doubles every year; new operating systems and graphical user interfaces require less expertise; faster and better applications can be built and run; multi-media is available; and object-orientated programming means software that’s easier to maintain and adapt.

The roots of solid-state C&I systems in power plant control were established way back, with relay-based control and protection systems and pneumatic controllers. Commands and setpoints reached the controllers via push-buttons and alarm lamps. Not all control loops were accessible in the control room. Remote control rooms and panels were utilised, and many items were manually controlled – handwheels and hanging chains.

Much higher availability

Today’s control systems now allow us to design for a much higher degree of automation, resulting in higher reliability and plant availability and ensuring more fuel-efficient operation. Three main fields of change are apparent – control room, process station and operator station, the latter including application packages and the engineering environment.

Although the changes in control system technology are broad, it’s the revolutionary changes in MMIs that indicate just how much this industry has been revolutionised. Today’s advanced computer technology means that process systems can now be controlled by a single operator. Information is displayed on screen, and the separation of data logging and closed loop control has effectively been consigned to the history books. Operators get plant overviews, optimised alarm handling and diagnosis, and also response assistance.

Plants can be controlled by simple mouse-clicking at the multi-purpose operator station. Related process objects appearing automatically on the screen are handled in the same way. Hard-wired backup is no longer required. The complete plant, including electrical distribution, can today be controlled from the same operator’s workplace. Operators can resolve disturbances using on-line instructions offered on the screen, supported by application packages.

In India, for example, at the GVK Jegurupadu power plant, a 235MW Combined Cycle Gas Turbine (CCGT) uses a large screen for control – process and electrical. One operator can start up and run this highly automated plant using just the desk-mounted large screens.

The increased complexity of power plants demands powerful applications to assist the operator. Applications available include: lifetime monitoring systems for large components; expert systems to support operators in detecting significant disturbances; and fuel and maintenance optimisation packs.

Most perform comparisons of actual plant behaviour with reference models, allowing accurate prediction and understanding of plant behaviour. One recent example is the model-based feed-forward controller for unit load. This typically runs in an advanced controller, such as ABB’s Advant Power system.

Performance calculation applications are another innovation. At GVK Jegurupadu, they provide for very accurate detection of plant efficiency deviations, thus reducing operational cost of the plant. Such energy management packages allow optimised operation of multi-unit power stations.

Optimised I/O processing and multi-processing are other new features now used in turbine controllers – including open loop control in protection applications. In the single cycle gas turbine plant at Birr, which has an innovative GT26 gas turbine, a combination of processor module redundancy for closed loop control, combined with a three channel protection design with two-out-of-three voting, was designed.

The gas turbine, which has two annular combustor chambers, contributed to the development of the control structure. A 32-bit controller, PM645, with an on-board speed input, has plenty of performance reserve. Cycle times down to 5msec can be chosen for each control task. For the GT, ABB’s Egatrol, which has been used in over 200 projects, was adopted.

The GT control was designed on Matlab and Simulink, and tested with a GT mathematical model. Each Egatrol-8 controller is tested prior to delivery against a dynamic model, and again during commissioning. The system’s Windows-based MMI allows parallel operation of the Excel spreadsheet for reports, etc. An additional touch-panel for back-up is included, also increasing availability for the GT.

Moving on, the Aquaba power plant (dual 130MW) has a flat system structure, where operator and control stations are directly connected to the system bus. Multi-head workplaces, with each operator using two screens and a common keyboard/mouse, define the MMI configuration. This technology only became available a few years ago; it enables the operator to use one screen for overview or sequence of events, while operating on the second screen.

Optimised packages

Meanwhile, an ABB steam turbine will be controlled using another optimised and standard package, called Turbotrol, again based on Advant Power. The burner management and boiler protection system can be set-up here in a dual or triple redundant configuration.

Today’s systems are also scaleable – so large advanced control system power can also be brought to small power plants. Industrial power plants, in particular, require a highly reliable control solution with an easy to use operate interface. Further, fast industrial turbines require a very fast-reacting turbine control system.

Similarly, control system retrofits can be addressed. One example is the coal fired power plant at Naga in the Philippines, where the boiler protection, turbine control and the main control system had to be replaced. Advant Power allowed all user requirements to be met. A smaller retrofit might only mean refurbishing a few major control loops – in the water steam cycle, for example. This would result in a small, cost-efficient solution, offering higher efficiency and increased plant availability.

Data acquisition and analysis are also candidates for refurbishment. A modern history database and information management system can support the shift and plant managers in supervising the plant.

Open interfaces to the real time database, via SQL or DDE, allow flexible reporting to be set-up. Application programming interfaces (APIs) allow further integration.