DCS-to-EMS Data Link Improves Electric Utility`s Systemwide Operations
By Darryl Haag, MAX Control Systems, and Stanley Krutsick, Pennsylvania Power & Light Co.
There has been a long standing requirement for power plants to respond to load dispatch signals from centrally located power control centers. Likewise, control centers need to know what is happening at the power plants to enable good dispatch decisions to be made and often to forward information from the plants to other departments in the company. The communication methods between power plants and control centers have evolved over the years, but have often not taken advantage of new technology and its benefits. With distributed control system (DCS) emergence and new digital communications technology, many power companies are now in a position to rethink and redesign the communications between their power plants and headquarters. One power company, Pennsylvania Power & Light (PP&L), is taking advantage of the new available technology. PP&L has implemented a reliable and flexible data link between its power plants` DCS with the company`s energy management system (EMS).
Basic Network Configuration
PP&L is an electric utility serving 2.1 million customers in a 10,000-square-mile area in central eastern Pennsylvania. The utility generates electricity from 12 coal-fired units, two nuclear units, two hydroelectric stations and two oil- and gas-fired units. The company recently added natural gas to its two 850 MW oil-fired units at Martins Creek. PP&L`s EMS communications network has evolved over the years to improve reliability and capacity. The central hub of the EMS communications network is the Power Control Center (PCC) in Allentown, Pa. This network is a collection of routers, computers and circuits, interconnecting sites and communication nodes within the EMS. The network configuration provides each major plant site with two physically independent communication paths. PP&L is geographically divided into five operating divisions, each forming one of five spokes from the PCC central hub. Plants are typically connected to the network through the two closest divisions. At a two-unit site, each unit is connected to a separate division. This configuration together with the EMS communications software provides these plants with redundant communications routing. Figure 1 is a current network diagram that highlights redundant paths into the PP&L`s Montour plant.
During an EMS upgrade project in the early 1980s, PP&L upgraded computer systems and communications technology throughout the network. At about the same time, the utility was planning to replace obsolete plant computers and control systems with DCS at three of its major plants: Martins Creek, Montour and Brunner Island. As the EMS upgrade progressed, PP&L recognized that the DCS installations coincident with the EMS upgrade provided the opportunity to design a new plant-to-EMS data link. The EMS project team designed its communication network so that each generating unit would be a node within the EMS network. A point-to-point link was designed to connect each node to a router located at one of the EMS remote sites. The design allowed each generating unit`s DCS to make a logical connection to the PCC over the EMS network. Plant DCS integration with the EMS network became the goal.
The new EMS and plant DCS interface addressed the original system`s problems and limitations. For example, with the original system, a single component`s loss would halt communications until the failed component was located and repaired, which could range from several hours to several days. The new design`s redundant communications components have virtually eliminated the problem by improving communication path reliability at the plant level. The new design also allows text string, or “free-form” information, to be transferred between the dispatcher and the plant; the original system did not. In addition, any DCS data can be sent from the plant to the PCC, or vice versa, with no additional hardware, wiring or programming.
Design, Development and Testing
The data link design focused on original computer RTU emulation replacement and enhancement. The following requirements for update rate, throughput, reliability, performance and protocol were established: plant to PCC operating data would be updated every two seconds; each plant logical link would have a capacity to transmit 50 points, either analog or status, at a two-second scan rate or 60 points at a 10-second scan rate or 20 points at an hourly scan rate plus 800-character text strings; the link would be fault tolerant at the plant level so that no single component failure would interrupt data transfer. The design, developed for a two-unit site, was first installed at PP&L`s Martins Creek plant, Units 3 and 4. The basic link design consists of a server for each generating unit, accessed by the PCC client. The plant end of the communications link includes a pair of modems and routers, a digital equipment workstation (DECstation) and a MAX 1000 DCS for each unit. The workstation acts as a server for the PCC. A MAXPORT 486 is used as a gateway to the DCS, directly connecting to the DCS data highway. The routers, servers and gateways share a common Ethernet link and are configured so that either router, either gateway and either workstation can communicate with both units, providing the desired redundancy. Figure 2 shows the Martins Creek configuration. A program running continuously on a PCC computer controls all communications with the plants. When the PCC client requests a connection, the plant`s server accepts the connection and waits for commands over it.
The link`s initial implementation included only the basic control and data transfer functions (Table 1). The new link design makes these signals and functions very reliable and accurate. Analog signals, previously used to transfer dispatch signals between the plant computer and analog control system, have been eliminated. The link also supports new control functions. At Martins Creek, the DCS is equipped with an algorithm for dynamic on-line, maximum allowable unit change rate calculations. The allowable change rate can be dynamically reduced when high change rate stresses are imposed on the unit. The algorithm can be independently activated or deactivated for each parameter. When active, rate limiting is effective in all unit operating modes except manual. If a parameter exceeds preset limits, the ramp rate is dynamically reduced from the operator-entered maximum value until the value of stress is reduced or until the desired load level is reached. The active rate limit and the limit value are displayed to the operator.
Enhancements and Additions
Since implementation, PP&L has made several link software enhancements. Some initial link design software was hard coded for each specific plant site, requiring the utility to maintain different software versions at each of the three original installations. The software has since been rewritten in a more modular and generic form, allowing one version to be used at any present and future installation. The three sections of code–I/O code, link support code and plant specific code–have been segregated into modules. A scan table, created and maintained in the PCC, defines all of a plant`s/unit`s link points. Modifications to the scan list are sent over the link to the plant server, eliminating the need to be at the plant when updating the server and making link point changes. The new version also supports dual fuel operation, an important expansion that supports Martins Creek 3 and 4`s recent conversion to oil and gas co-firing. In addition to generation targets, the units receive a fuel ratio target from the PCC. The economic dispatch algorithms have been revised to utilize composite cost curves for both gas and oil when co-firing. The units report back generation limit conditions and flow for both fuels. Once the links were installed and operating and the EMS upgrade was completed, plant and system operators began to appreciate the ease with which link points could be added. Increased awareness of the need to be competitive and the impact of dispatch economics on plant operation have prompted requests for additional data. This has been satisfying for the designers, since data transfer expansion was anticipated during link development. PP&L is now utilizing the link to transfer real-time transmission and distribution system dispatch information to plant sites, allowing operators and engineers to evaluate unit status within power pool operations.
There are plans to convert the EMS network from DECnet to TCP/IP. At the time of the EMS upgrade project, there was a commitment made to DECnet. Since then, TCP/IP has become more widely used. The conversion is relatively easy and will not impact link operations. The relationship between the EMS communications network and the PP&L corporate WAN was considered during initial design and is again being discussed. While the DCS-to-EMS physical connections are shared in part with the corporate WAN, the EMS data links are independent applications with dedicated hardware. In other words, the DCS-to-EMS data link is not part of the corporate WAN. Another interesting issue being discussed is real-time plant performance data reporting for the purpose of updating dispatch information. It was thought that the DCS-to-EMS data link would be used to send this data to the PCC. It is more likely, however, that the data will be analyzed at the plant before it is passed on the PCC.
Development of the link required a joint effort by the plant and EMS project teams. The strong desire to use current technology was the driving force which allowed the team to solve problems and achieve its goals. The new data link has proven to be very successful. The hardware and software have been reliable, and the link has met its design and performance goals. PP&L has experienced increased plant data accuracy resulting from DCS integration, enabling PCC to fine tune automatic generation control. Automatic generation control is now more reliable and its availability has increased. In addition, the link can be easily expanded, allowing for future dispatch algorithm adaptations for plant emissions and/or performance criteria.
Darryl Haag received a bachelor`s of science degree in mechanical engineering from Drexel University in 1980. He is a member of the Instrumentation Society of America, American Society of Mechanical Engineers and the Electric Power Research Institute I&C Advisory Committee. He is a registered professional engineer and is employed at MAX Control Systems in Lansdale, Pa., as a principal engineer. Prior to joining MAX Control Systems, he worked for 15 years at Pennsylvania Power & Light Co. (PP&L). He may be reached at firstname.lastname@example.org (e-mail).
Stanley Krutsick received a bachelor`s of science degree in electrical engineering from Villanova University in 1980. He is a registered professional engineer in Pennsylvania. Krutsick is employed at PP&L as an instrument and controls engineer. He may be reached at email@example.com (e-mail).
1. -Richard W. Radtke and Olson, Conrad C., “Coordination of Unit Equipment Capabilities into Load Management Systems,” ISA, 1988.
2. -Gary A. Cohee, Broske, Dave, and Hubby, Robert N., “Enterprise Coordination at Entergy`s Lewis Creed Units 1 and 2 and Nelson Unit 4,” ISA, 1995.
3. ICCP Version 5.1, EPRI, November 1994.
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