National Power’s Drax Generating Station in the Selby coal field, with a 4,000 MW capacity, is the largest coal-fired power plant in Europe.
When the United Kingdom Privatized its Power Industry in 1990, National Power Plc Became One of several energy suppliers to its main customer, the National Grid Co. In the new competitive marketplace, the company saw the need to change how it viewed its extensive power plant operations. For example, to meet business goals, units originally designed for baseloading would now have to be shifted up and down, sometimes rapidly, to meet peak demands. Such “two-shift” operation in the past required a high degree of manual operator intervention during transitional loading periods. Additionally, the more frequent start-ups and shutdowns would increase wear and tear on plant equipment, thereby increasing maintenance costs. To accommodate these changes, National Power implemented advanced process control and plant management systems at all of its plants.
Historically, coal-fired plants in the United Kingdom contained supervisory level data acquisition and control systems, backed up by hard-wired control desks. Power stations often contained different generations of software and hardware, and hardware implementations were not standardized.
The existing control systems performed well enough for steady-state loading, but were not designed to automatically control plant start-ups, shutdowns and load transitions. Operators were required to make manual control adjustments during changing load conditions. The results often depended on the skill of individual operators and were not amenable to the needs of privatization.
In 1994, National Power formed several teams to review the status of process controls at its various power plants. The teams comprised a cross-section of corporate, financial, commercial and power plant interests. The review process evaluated the suitability of the company’s current control systems for future operating regimes and forecasted market changes. Other National Power in-house programs identified operational and commercial inadequacies and devised methods for overcoming them. Automating these methods would provide benefits such as extended plant life, reduced plant failures and enhanced operational consistency.
Based on this system-wide review, National Power decided to upgrade the technology associated with each generation unit on two levels. One level of information technology would guide the business and commercial aspects of plant operation. The other level would provide a higher degree of automation. The goal was to integrate the two levels, optimizing both business and process control considerations. A higher degree of process control automation, for example, would help minimize stresses on plant equipment associated with more frequent start-ups and shutdowns. More consistent, automated load transitions would decrease maintenance costs and increase plant life, which would improve the business side of the equation.
Birth of APMS
From these initial studies, National Power’s Advanced Plant Management System (APMS) emerged to address the corporate, financial and commercial issues that would be vital in the emerging competitive market. APMS would serve as a platform for developing strategic business-related applications for the company. On the energy generation level, APMS would maintain a comprehensive real-time database of a unit’s process control and business data. Using an open architecture, APMS would also integrate legacy and updated control systems and provide a flexible operator interface.
Because the review found no existing information technology packages or control packages on the market capable of meeting all the desired APMS functions, National Power decided to develop its own system, using off-the-shelf products and open technologies. After a study of platform possibilities, the company chose the Syseca Open PMS software package, which is an extension of the Hewlett-Packard Real Time Applications Platform (RTAP) software. National Power and Syseca worked together to complete development of core APMS software for power plant applications.
As shown in Figure 1, APMS coordinates and integrates interactions between the power plant, operator stations, business functions and customer contracts. Existing and updated process control systems readily complement APMS, providing functions such as sensing and data acquisition, programmed logic control (PLC), control actuation, and all those functions normally associated with a conventional distributed control system (DCS).
New System Architecture
Figure 2 depicts a DCS upgrade in a power station, combined with APMS overall plant management. The APMS incorporates a real-time database within a DEC dual Alphaserver, as shown. The database completely represents a coal-fired plant, containing the values of up to 30,000 analog variables and two-state logic variables.
For example, the database mirrors plant conditions for the fuel handling, boiler, turbine-generator, condensate and feed systems, essential auxiliaries, and electrical systems. But it also contains business-related data such as the results of efficiency and environmental calculations, contract obligations and energy prices. Examples of support data in the database include alarm and object classes. Additional functions have been developed that are specific to the power industry, such as enthalpy and entropy calculations.
Unit data originates from combinations of the legacy and updated process control systems. Via the dual DCS Ethernet data network, the APMS host servers interconnect with a wide variety of process control systems. These systems include the legacy Cutlass and Swepspeed systems, as well as several PLC and DCS units from a variety of vendors, including Allen-Bradley and Moore Process Automation Solutions.
Via the dual plantwide Ethernet, APMS interconnects with minicomputers and workstations supporting the overall plant infrastructure. The infrastructure network VAX contains three important subsystems, two of which create ties with business considerations:
- The operational information system (OIS) provides a one-year on-line archive of plant data for analyzing trends.
- The integrated load management (ILM) system compares and checks the delivery of energy with the National Grid contract, informing the operator of any variation.
- The electronic dispatch and logging (EDL) system optimizes the requirements of all customer contracts.
National Power created a special engineering interface to simplify configuration and maintenance of the APMS database. Using this interface, the system manager can develop objects within an Oracle software database and link associated data variables with the process control and business systems.
APMS connects to the operator and engineering display workstations, referred to as “soft desks,” via a high bandwidth fiber distributed data interface (FDDI) network (Figure 2). In all of its power stations, National Power installed an APMS and replaced hard-wired control centers with the soft desk concept.
A soft desk consists of multiple computer monitors, each with a keyboard and mouse. A series of projectors provide large-screen displays of key overview graphics, which can be seen from a distance by several people (see photo on page 69). A single mouse can move across the range of computer monitors, which can display any graphics format. Soft desk displays provide the operator with plant overviews, details, schematics and trends. Alarms displayed on the screens signify plant status changes and suggest corrective actions. The system also logs events for future analysis.
The philosophy behind the display environment is that it is physically impossible to take all the indications and controls available on the conventionally wired desk, transfer them to a set of visual display units (VDUs) and then again make them all visible to the operator within his range of vision. Thus, the operator must have an alternative arrangement to allow him to effectively understand the process state at any time and the operator has to have facilities to make proper process changes.
The large overview displays, therefore, are fixed format with graphical symbols to connote physical equipment such as fans, pumps and pulverizers. These symbols are arranged to provide the operator with various equipment status indications: condition, availability, etc. Having observed a process disturbance, the operator can use the overview displays to immediately take control of the associated plant item (via drop-down menus), or he can call up more detailed process displays on the smaller VDUs.
For example, a drum level high alarm would show as a flashing boiler drum level symbol on the overview. By clicking on this item with a pointing device, the operator can bring up on one or more smaller displays a series of predefined, related displays-such as drum level modulating control panels, feed pump speed controls and boiler filling valve controls. The operator does not have to search for these items; the process has become “task oriented.” Differing process conditions associated with the same piece of plant equipment will bring up a different group of related displays.
National Power took several factors into consideration when implementing APMS in an individual power plant. The implementation team, together with the station staff, evaluated the condition of major equipment and estimated remaining plant life. The team also looked at plant efficiency and the need to respond to changing demand. Another factor was the level of sophistication of existing process control systems. At major coal-fired stations, National Power updated instrumentation, including progressive replacement of distributed control systems, sensors and actuators.
The Drax Power Station, in the Selby coal field, is the largest coal-fired station in Europe, containing six 660 MW units. Although originally designed for baseload operation, Drax will increasingly experience two-shifting and frequency regulation to meet competitive pressures. (National Power is in the process of selling Drax to AES Corp.)
In the completed APMS system for Drax Unit 2, a fully automated soft desk replaces the previous hard-wired control desk. Nine 21-inch color monitors provide normal operator plant control displays. These are augmented by a “video wall” of three large (67-inch) back-projection overview displays.
APMS has simplified communication with the system, and has given the operators the ability to change procedures quickly and easily. Operators have noticed improvements in fine controller adjustments, along with tighter control of environmental emissions. Operators now control Drax in a highly standardized manner, which helps to maximize start-up and shutdown efficiencies, save energy and minimize plant damage. Serious problems, such as a feedwater heater trip, automatically shut down the unit with no operator intervention. In the past, such events required the attention of four staff members.
Drax significantly refurbished its process control systems to accommodate APMS. Since APMS has an open architecture, distributed control systems from several different vendors could have been used. Several key factors led National Power to select Moore’s Advanced Process Automation and Control System (APACS) for Drax and several other plants.
First, users can efficiently configure APACS, which has a flexible programming language based on the IEC 61131-3 standard-consisting of a mixture of function blocks, sequential function charts, ladder logic and structured text. The engineer picks the configuration technique that fits his preferences, experience or process control requirements. This approach cuts project costs by reducing configuration time.
Second, APACS combines the capabilities of a distributed control system and a PLC in one fully integrated suite of hardware and software.
Third, APACS minimizes the time required to bring refurbished units back into service and to fine-tune the control strategy during initial unit start-up. An on-line editing capability reduces time required for the site acceptance test. Just as importantly, unit availability is maximized during initial start-up by using the APACS system’s ability to adjust the control strategy without shutting the unit down.
Finally, APACS supports a comprehensive system redundancy, which is important for critical power plant applications. The standard, conventional APACS system includes support for dual redundancy with a hot-standby capability. A pair of controller racks can be arranged to shadow one another by simply connecting an umbilical cable between the two processor modules and enabling the appropriate software.
National Power has installed APACS on Drax Units 1, 2 and 3, replacing the existing, inflexible, relay-based plant interlock equipment and the combustion control system. The soft desk at Drax has a single APACS engineering workstation. Not only is this economical but also it is more controllable to have just one access point for making system changes.
Plant items under control of the multiple APACS racks for Drax Units 1, 2 and 3 include:
- sequence control
- air heaters
- draft fans
- mill group (primary air fan, mill, coal feeder)
- gland steam and vacuum raising
- starting and standby feed pumps
- barring gear sequence
- feed suction pumps
- low pressure feed heater lift pumps
- condenser extraction pumps
- automatic boiler controls
- combustion chamber pressure control
- combustion air control (forced draft fans and secondary air dampers)
- mill outlet temperature control (mill hot /tempering air damper)
- boiler pressure/fuel control and pressure ramp (mill primary air fans and coal feeders)
- superheater steam temperature (two-stage attemperation)
- turbine lube oil pressure control
- plant interlocks
The APACS-based interlocking system ensures safe plant operation by providing permissive start signals and plant trip signals. The boiler-firing progressive trip and overall flame failure trip are also integrated with the boiler interlock system.
Using APMS and APACS, National Power has been able to locate the processors and local I/O racks in the small amount of free space on the level immediately below the control desks. Cabling between the racks and the desk is now little more than the network, video, keyboard and mouse connections. Previously, the cable space below the control desks was occupied by thousands of multi-core cables. Several system nodes, however, are remotely located and connected into the control desk by either fiber-optic or copper dual redundant data highways.
According to George Ecclestone, Process Control Support Manager at Drax, the APMS system has enabled the plant to minimize both energy use and plant damage during plant start-ups and shutdowns, while delivering many fringe benefits. Installation and commissioning was not 100 percent trouble-free, but with a total of 14 APACS racks per unit supporting around 6,000 I/O points, systems demands were high.
Michael Petit-jean is Managing Director of Ambrio Ltd., a company he set up in June 1996 after leaving National Power. During his employment at National Power, the concept of APMS evolved and a framework was put in place to “roll out” this platform to 22 generating units over a three-year period.
Phill Robinson is Energy Industry Manager for Moore Process Automation Solutions in the United Kingdom and Eastern Europe. He trained with the Central Electricity Generating Board, from which National Power emerged. Robinson has worked in the instrumentation field for more than 30 years, involved in system commissioning, support and sales.
Didcot Safety System
At National Power’s Didcot “A” Power Station, originally configured with four 500 MW coal-fired units, Units 1, 3, and 4 have been converted to dual fire on gas or coal, or a combination of the two, including on-line fuel changes. Safety and fuel management in such a plant is critical and complicated, focusing attention on the burner management system (BMS) and gas firing control system (GFCS).
The BMS system on each unit at Didcot controls 48 burners in four groups of 12 (two mills per group). These directly equate to the eight mill groups used for coal firing. The burners are started and stopped via signals from the GFCS. In turn, the GFCS controls the burner firing pattern using signals received from the APMS manual input operator screen (this will be automatic when an APACS-based combustion cross-limiting system is installed).
The GFCS also controls the pressure in the main gas burner header in accordance with burner demand. Connection to the APMS is via the dual fiber optic Ethernet link. The gas control system has a dedicated engineering work station for system maintenance.
For these critical safety applications, National Power selected QUADLOG, a PLC system offered as an APACS extension. While the plant controls and safety systems operate completely independent of each other, they reside on the same network. Thus, safety data can be used in control strategies and there is a single operator interface for both. Further, both systems use the same engineering tool, making configuration simpler and decreasing start-up time.