Refit for repurpose

Quantum PLCs and Unity Pro software from Schneider Electric have provided an ideal foundation for the Drax power station's new control architecture.

Drax is one of the largest power stations in the UK, supplying around seven per cent of the UK's electricity with six generator sets, each rated at 660MW. Construction of the plant began in the mid 1960s, with the first phase coming on line in the 1970s and the second phase in the 1980s, with it being in continuous operation ever since.

The control systems on the original three generators were refurbished in the 1990s, but a decade later the control systems on the remaining three generators started to show their age. In particular, they were inflexible and they were becoming increasingly difficult and costly to maintain.

The decision was taken to refit the entire process control installation. Capula Limited, one of the UK's leading system integrators and a company with extensive experience in large-scale systems, was appointed to handle this major project. The engineers at Capula had, however, to contend with many special demands.

Hot standby

Among the technical requirements was the need for the highest possible level of availability for the new system. This necessitated the use of a 'hot standby' approach, with redundant architecture running simultaneously with the primary architecture, so that no single failure could shut down the system.

In addition, the new system would have to provide open connectivity to communicate with the station's high-end SCADA installation, and it needed to use IEC61131 compliant software. This would not only allow easy programming but would also help with the migration of the control algorithms used on the existing hardware platforms, as well as ensuring that the system would be easy to modify and maintain throughout its working life.

On top of these demands, the project included a punishing delivery schedule, as the new systems needed to be installed, commissioned and future proofed during a fixed 12-week shutdown window. It was essential that the equipment utilised was sourced from a supplier that could credibly provide support for the estimated minimum 20-year life of the system, and the hardware itself had to use current technologies with a good life expectancy.

In order to address these requirements, the engineers at Capula carried out an extensive survey of potential equipment suppliers. Their conclusion was that the projects needs would best be met by Quantum programmable controllers from Schneider Electric, used in conjunction with Unity Pro programming software.

Among the factors that influenced this decision was the ease with which Quantum controllers can be used in a dual-redundant hot-standby configuration that offers true bumpless transfer. In effect, if one of the controllers in a pair fails, its duty is almost instantly taken over by the other, with no discernable effect on the process being controlled.

As a further aid to maximising availability, Quantum controllers also support redundant remote I/O and permit hot swapping of I/O cards. Furthermore, they provide isolated analogue I/O channels, as well as support for high-speed Ethernet networks and open architecture, including OPC servers.

As finally implemented, the new process control system for each generator set comprises 17 hot-standby pairs of Quantum PLCs, supporting a total of almost 8,000 I/O points. The I/O, all of which is remote, is linked to the PLCs via dual redundant connections, and the PLCs communicate with each other by means of a self-healing Ethernet fibre-optic ring. Connections to legacy equipment are implemented in Modbus TCP and Modbus, both of which are supported as standard in Quantum products.

Use is made of the Quantum PLC's OPC connectivity, and its compatibility with OPC Factory Server, to provide a convenient interface with the site's high-level SCADA installation.

Modular approach

Development of the software for the project was greatly assisted by the simplicity, power and flexibility of the Unity Pro software package. This has allowed the control programs to be written in a modular form that reduced the amount of work involved, and also simplified testing. Further, this modular approach makes the installation easy to maintain, and will facilitate the incorporation of future modifications.

Despite the very tight schedule, the installation and commissioning of each of the new control systems was completed on time, fully meeting or, in some cases, exceeding the design requirements.

In addition to enhanced reliability and availability, the new Schneider Electric based control installation provides the operators of Drax Power Station with other important benefits. Among these are the ability to accurately log the stations emissions of sulphur dioxide, nitrogen oxide and particulates, which is now a regulatory requirement; and greatly enhanced operational flexibility.

Variable quality

This flexibility is particularly important as Drax was originally built to use coal from a single source - the nearby north Yorkshire coalfield. This coalfield is, however, now exhausted, so fuel has to be brought in from numerous different sources, and it varies considerably in quality. The new control system allows the combustion process to be fine-tuned to match each fuel type, thereby maximising efficiency and minimising emissions.

Capula has now delivered process control refits, all of which are built on Schneider Electric equipment, for three of the generator sets at Drax, plus the common services. Based on modern technology, and designed for reliability, the new control systems are expected to provide solid service for decades to come.

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