Figure 1. The North Omaha power station can generate up to 700 MW of power. It is operated by the Omaha Public Power District, which provides some 2200 MW to a 13 county area in southwest Nebraska. Nebraska is the only all-public-power state in the USA, and OPPD is a subdivision of the Nebraska state government
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In early 2000, the Omaha Public Power District (OPPD) embarked on an instrumentation and control system upgrade project at its North Omaha power plant. By 2002, the $20.9 million project will be complete, helping to increase reliability, improve unit response, and lower operation and maintenance costs at the 40-year old coal-fired plant.

The North Omaha Station is situated on a 40 ha site on the Missouri River, about 9.5 km from downtown Omaha, Nebraska, USA. Its five coal-fired units can generate up to 700 MW. Four of the units are operated with pneumatic control systems that are more than 35-45 years old.

The project scope includes removing the pneumatic controls and installing a state-of-the-art distributed control system (DCS) for each of the plant’s four units, a master plant historian, relay protection, burner front instrumentation, and field instrumentation and control system simulator.

Management and control

Figure 2. Marking the end of an era for the North Omaha facility is its pneumatic control room with its switches, gauges, and round recorder charts. Kept in excellent condition, the control room will be altered to make way for CRT operator consoles and large screen displays
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Each DCS system will have the following sub-systems: boiler control system, burner management system, data acquisition system, alarm management system, sequence of events system, turbine auxiliary system, motor control system, foreign device interfaces, and an electrical auxiliary system.

ABB, which received the contract for the project, will replace the pneumatic controls with its Symphony Enterprise Management and Control Systems, which are advanced DCS. Installation will require the gutting and remodelling of the existing control room which will house operator stations for the four units. The control room space will be effectively separated into two areas; one area, some 186 m2, set aside for operator and workstations assigned to Units 1 and 2; and another 204 m2 designated for stations for Units 3 and 4, with similar operator interface.

The control room will also house stand-alone systems, not incorporated into the DCS, but which may at some point interface with them. The stand-alone systems include sootblowers and fly ash systems. The systems that will be incorporated into the DCS are a static exciter panel, ID booster fan controls, an energy management system, protective relaying, and transducers. Also to be located in the control room, but not intended for DCS interface, are fire detection panels and continuous emissions monitoring terminals.

One historical aspect of the project is the heritage of the pneumatic and DCS systems. Unit 1’s pneumatics, installed in 1954, Unit 2’s in 1957, Unit 3’s in 1959, and Unit 4’s in 1963, all came from the Bailey Meter Company of Ohio, the predecessor of the ABB operation providing the Symphony systems.

Improving operations

Figure 3. The new look awaiting control room operators is similar to this photo of a demonstration control room at ABB. The data available on one operator console would be equal and exceed in many ways the existing pneumatic control room’s data display capability
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Among the project’s key areas for operating improvement are Area Management and Control, Human Systems Interface, and System Engineering and Maintenance.

Area Management and Control encompasses closer boiler control and burner management, and data acquisition solutions such as distributed trending. Software programs designed to carry out these functions provide a more accurate measurement of conditions and quicker response than pneumatics can offer.

Human Systems Interface concerns the operator stations and engineering workstations. Whereas pneumatic controls are mainly gauge and chart based, DCS is primarily a CRT display with a variety of display possibilities that can be static or dynamic. Graphics depicting the unit in operation, complete with measurement data from sensing devices, are ‘windows’ commonly seen in DCS control rooms.

Symphony Conductor VMS Series workstations were selected for the North Omaha project and use Compaq’s Digital Modular Computing Components product line with a 64-bit 21604A-based Alpha processor operating at 500 MHz. While designed for large applications, up to 30 000 tags, this console has proven to be popular on systems of various sizes due primarily to the use of the X-Window standard.

Four operator stations per unit will be installed in the control room. They will each consist of two CRTs and a DuPont Mylar operator keyboard. Additionally, the control room will be equipped with a large, 37-inch (94 cm) CRT screen for each unit in order to display graphics or other data that unit operators select.

The plant will contain four engineering workstations, one for each unit, for tasks other than operating the DCS. Such tasks include on-line and off-line configuration tuning and editing of control strategies; storing database information on CD disks, magnetic tape, or other movable media; and downloading stored database information, including selected graphics, logs, reports, and trending data. Workstations can also be used to monitor on-line operations.

Each workstation will be equipped with a colour CRT screen, CD-ROM drive, 3.5-inch floppy drive, mouse, configuration keyboard and Microsoft Windows NT 4.0 operating system.

System Engineering and Maintenance tools for the DCS include a mix of hardware and software used to configure control and monitoring strategies and maintain what is essentially a sophisticated computer system running the unit. The tool package for this installation is Symphony Wintools designed for a Windows operating system.

Wintools is a major asset to those configuring the screens for the operator stations which can display in addition to plant system and device graphics, trends of process and calculated data, and alarm summaries, all of which contain live process information.

The primary configuration elements of the operator console are the tag database and graphics displays. Other major configuration elements include display hierarchies, logging, archival storage, alarm management and password security. In addition to tools, there are separate, specific Symphony console configuration software packages operators can use.

ABB system graphic designers are working with OPPD counterparts at the power station to develop and configure the screens necessary to control all the functions of the DCS.

Monitoring performance

The master plant historian being provided for the project is sized at 8000 data points, with an on-line data retention of three months at the following resolution: 170 analog points and 600 discrete points at 2 s; 420 analog points at 10 s; and 6800 analog points at 30 s.

Estimated I/O requirements to operate each unit are 2170 I/O for Unit 1; 2496 I/O for Unit 2; 2496 for Unit 3; and 2604 for Unit 4. Each DCS will be equipped with a performance calculation software package to provide key data on major components of each unit to help ensure optimized operation and output. The approximate generating capacity for each unit is 75 MW for Unit 1, 110 MW for Unit 2, 110 MW for Unit 3, and 133 MW for Unit 4. Unit 5, which is not included in the project, has its own DCS and is rated at 214 MW.

The performance packages on Units 1 through 4 will provide data on the turbines’ overall efficiency and heat rate, boiler efficiency based on such measurements as ASME’s output and loss method, feedwater heater performance, condenser performance, air heater performance and other measurements. Calculation routines are supplied for data verification.

Each unit’s turbine auxiliary system will be upgraded with an ABB microprocessor-based system which is integrated into the DCS and uses the same hardware, software and configuration platform as the rest of the DCS sub-systems. The existing vibration monitoring devices, switches, and temperature elements will not be replaced, but will provide input into the new control system.

Data highway

Tying the DCS together is the project’s data highway which provides for a minimum 2 megabyte per second transfer of data, configuration programs and other pertinent information between controller modules in DCS cabinets, CRT stations, data acquisition computers and other system components.

The data highway used by the DCS and the master plant historian will consist either of a dual redundant coaxial or dual redundant fiber optic cable capable of accommodating at least 60 drops or devices and extend up to 10 000 feet (3048 m) in total length. ABB will install this protective relay communication system, along with the control room equipment, such as work and engineering stations.

Burner front instrumentation will include Safe Flame DFS flame rod igniter flame detectors supplied by ABB. Main gas burner valves are supplied by Forney. New side mounted horn igniters and corner modifications will be supplied by Alstom Power.

Field instrumentation supplied by ABB includes all pressure, flow, temperature, and differential pressure transmitters to replace the existing pneumatic transmitters. The new devices will be ABB Platinum Standard ‘smart’ transmitters with 0.01 per cent of span accuracy over the entire range. It is estimated that the project will require 500 transmitters. ABB is also supplying its AV positioners for all pneumatic drives and valves, which are inputs to the DCS. The positioners will provide a 4-20 mA feedback. It is estimated 160 positioners will be needed.

Control system simulator

Unlike the early years of DCS technology, when elaborate training routines were necessary to help control room operators bridge the gulf from a control architecture using gauges to a new one using screen displays, today’s operators are, for the most part, familiar with operator station CRT format concepts. Most homes, libraries, schools and colleges have computers, computer graphic courses are taught in many elementary schools, and the average consumer is familiar with ATMs and command sequences. Twenty years ago, when the first DCS installations were made, it was not unusual for the operator screen display to mimic the pneumatic or digital display it was replacing. That is not necessary today, but training still goes on.

ABB is providing operator training, including use of a simulator shipped to the site. The simulator is an exact duplicate of the operator stations, with displays to be used to run the units, and will be installed in the plant for current and future operator training. OPPD personnel will be taught to be simulator instructors and complete an extensive curriculum which includes overviews of hardware and software, overview of the simulator model and the process and the controls, overview of modifying the model, review of instructor functions, and hands-on practice using the system and building exercises for students.

Unit 4 will serve as the basis for simulator information. Simulator models will include all process systems controlled from that unit’s DCS. Systems and components to be simulated can include the boiler/furnace system, boiler air and gas system, fuel burning and boiler control system, condensate system, feedwater system, station electrical system, circulating water system, cooling water system, auxiliary steam system, and the DCS control system.

The simulator will be able to mimic abnormal and emergency events, including malfunctions and equipment failures to demonstrate inherent plant responses and automatic plant control functions. While simulating normal operations, an instructor can insert a malfunction. The instructor’s station has a selection of approximately 200 malfunctions to choose from.

A new world of possibilities

Crossing over from pneumatics to state-of-the-art distributed control opens up a whole world of optimization possibilities for the operators. Introducing improved control and monitoring strategies using pneumatics requires rerouting tubing, involving a significant cost and time investment. Also, the very pneumatic medium limits how creative a strategy can be. On the other hand, DCS not only allows operators to easily change strategies electronically, but it also provides the opportunity to introduce advanced software packages now available and under development. Both ‘off-the-shelf’ software originally developed for business use, and specific, proprietary industrial application programs can be loaded into the simulator, evaluated, and if accepted, applied to the DCS.

Software making its marketing debut include packages for simplifying control and monitoring strategy configuration, plant-wide asset optimization programs complete with predictive maintenance capabilities and analysis report preparation for emission monitoring use. This kind of software, which makes up Industrial IT solution suites from ABB, and similar offerings from others, can take advantage of ‘Information Age’ breakthroughs where data can be ‘mined’, processed, refined, and applied for improved productivity far beyond the capabilities of pneumatics.

Similarly, the advanced DCS being installed at the North Omaha facility expands the communication reach and variety of information more readily than pneumatics could, and integrates it to a single operator station, instead of a set of gauges and charts.

While pneumatic signals converted to electronic ones can be transmitted by wireless or cable connection, interpretation of the signal is usually limited to measurement purposes.

The control systems for the North Omaha plant, on the other hand, allow not only measurement data to be conveyed, but also remote diagnostic system testing and web and e-mail access to retrieve and receive technical bulletins. Interface with enterprise-wide business and marketing information systems is easily facilitated, as is interface with suppliers for order placement and electronic billing.

A new challenge

As DCS technology has increasingly taken hold of power plant control, a new challenge faces users, and that is the risk of lost opportunity. Whereas pneumatics is limited in its ability to improve efficiency and apply itself in such areas of communications, the DCS approach, like the Symphony system, is just the opposite.

Consequently, users need to be knowledgeable of new software being introduced to provide for yet higher levels of optimization. Additionally, the ability of enterprise-wide control systems to tie into different modes of communications means taking advantage of the latest developments, such as a host of wireless devices.

The control systems that are being installed at the North Omaha site are designed with ‘evolution without obsolescence’ architecture, so as later generations of hardware and software products and communications capabilities are introduced, they can be incorporated into the system.

Instead of being limited by pneumatics capabilities, users will now have to be stay abreast of more and more opportunities to enhance plant performance.