The name of the game is a flexible friend

Radical changes in the electricity market have led to increasing pressure on plant operators to optimise station efficiency. Advanced control technology can help to maximise production efficiency, provide information for competitive bidding and to adhere to emissions regulations. Advanced Plant Management Systems (APMS) are being increasingly installed to do this, the latest example being Rugeley power station in the UK.

Operation of a power plant is based on a combination of continuous control needs, such as temperatures, pressures and flow rates, and discrete control needs, such as on/off circuits for pumps. Traditionally, distributed control systems (DCSs) have been used to handle continuous control, while discrete control has been implemented through programmable logic controllers (PLCs). To improve flexibility and cost-effectiveness of plant operation, it is necessary to find a way of combining these two traditions.

Flexible operation will be an increasingly important issue for coal-fired plants in an increasingly competitive market. Operators must maximise production efficiency, provide information on competitive bidding, and adhere to emissions regulations. At the same time, they must optimise plant life, especially as stations will often be running under part-loading and two-shift conditions.

In the UK, with the onset of full deregulation, and following recent government initiatives, there is a particular burden on companies operating coal-fired stations to produce power cleanly and efficiently. This requires improved control systems, allowing power station output to be precisely coordinated with demand, minimising energy use and plant wear, and leading to lower emissions and improved efficiency.

Eastern Merchant Generation chose a consortium of Moore Process Automation Solutions, Syseca and Ambrio to design, configure and commission a new control system for the two 500 MWe units at Rugeley power station. The contract involves a total control and instrumentation refit across both generating units and common services.

Syseca is the project leader, and will be installing its Advanced Plant Management System (APMS), developed in collaboration with National Power, using Moore’s APACS+ hardware and sofware.

The display and command functions will be implemented using a ‘soft desk’ entirely via VDUs and keyboards. The soft desk is able either to provide broad overviews or rapidly focus to give a high level of detail on a specific item of plant. The system will also enable a more rapid start up and shut down of generating capacity, hence making the station more responsive to demand. Total input/output (I/O) count for the distributed hardware is approximately 5500 channels.

Tracking speed

When an important part of a plant shuts down, it is vital to know which of the many events associated with the trip was a cause rather than a consequence. Distributed systems communicating serially around a plant can typically only maintain clock synchronisation to several tens of milliseconds. This is not fast enough to monitor fast plant trips.

To solve this problem, Moore has introduced a sequence of events (SOE) recorder that gives time stamping to an accuracy better than 1ms, even when events are monitored by different parts of a distributed system. This system, to be installed at Rugeley, increases the ability of operators to determine the sequence of closely related events.

This is possible by imposing a step function synchronisation signal in the system. This enables any point in the system to be able to compare the difference in time estimation between itself and any other point in the system. When an event occurs, each part of the distributed system can make the necessary time correction.

Integration of control and safety systems

It has long been accepted that it is unsafe for plant safety systems to make use of information derived from the regular control system. This is because the safety system can be affected by any data corruption that might occur within the control system. This has traditionally resulted in separate architectures for the safety system and the control system.

The reliability of regular programmable electronic systems is better than that of traditional relays, although the predictability is inferior. This results in the situation that although fewer failures occur, the number of ‘fail dangerous’ conditions is higher than for relays. To avoid this problem, Triple Modular Redundancy (TMR) is used. TMR is very safe, but it is a uniquely safety-based architecture, being too costly for normal control applications.

Specialist architecture has led to specialist suppliers, which has resulted in a situation where integrating a control system with a safety system involves custom communications protocols between devices from different suppliers.

This is the situation the systems integrator has to deal with when trying to integrate a control system from one vendor with a safety system from another. Costs are incurred in understanding the communications protocols of both systems, creating drivers, specific message code, and possibly even extra hardware required to cope with two systems that use totally different communications technologies.

To resolve this communications problem, Moore developed its Quadlog suite of hardware and software extensions to the APACS control system and these have been implemented at Didcot, Drax, Longannet and Rugeley. Quadlog’s diagnostic channel detects and overrides output circuits that have failed to energise, and provides fail-safe outputs. This resolves the communications problem by sharing an underlying structure, and ensuring identical protocols.

Safety is retained because the control system derives information from the safety system, rather than vice versa. Information monitored for safety purposes can be shared cost effectively with the control system for real-time control purposes. Corrupted data does not impact on safety, as the data still goes directly to the safety system.