First USC gets new DCS
Students of power plant history know that, as much as the term ultra-supercritical (USC) is bandied about today, Unit 1 of the four-unit Eddystone Generating Station of Exelon Corp. (formerly Philadelphia Electric Co.) still holds the record for the highest commercial steam conditions: 5,300 psig and 1,210F. Although it came on-line in 1960, Unit 1 remains to this day one of the world's most efficient coal-fired units (although today's operating steam parameters are somewhat lower than the design points). To keep Eddystone running another 15 years, Exelon determined that it needed a new brain.
According to a paper presented at last year's Power-Gen conference by Mitchell Goldberg, Eddystone's production manager, and Roger Leimbach of Metso Automation, part of the justification for the controls modernization has to do with the competitive market: PJM Interconnection requires the unit to follow load-dispatch commands, or automatic generation control (AGC).
As a result, Eddystone's units, which used to run in turbine-follow mode with the governor valves wide open above 70% load, now are required to follow load at a rate of 6 MW/min. After 44 years of operating under a "direct energy balance" (DEB) control concept, Exelon and Metso Automation (the successor company to Leeds & Northrup, the control system original equipment manufacturer [OEM]) embarked on a project to provide the flexibility required without unduly stressing the equipment and while consolidating information monitoring, which had previously been dispersed throughout numerous hard-wired recorders and indicators.
The updated DEB system for Eddystone (Figure 1) decouples generation control (turbine output) from boiler control. Generation control makes maximum use of stored energy to balance turbine output with AGC demand. Unlike conventional coordinated control, in which an arbitrary feed-forward rate is used to overfire or underfire the boiler, the new DEB system calculates in real time stored energy requirements plus load change and applies that to the control of firing rate. The concept thus maximizes use of the once-through boiler's stored energy.
1. Separating generation and boiler control. The direct energy balance (DEB) control strategy at Eddystone Generating Station was updated as part of modernizing the plant's distributed control system. Source: Metso Automation
Generation control includes a triple-cascade control loop with the following input parameters: megawatt output, turbine first-stage pressure, frequency, and aggregate governor valve position. This provides a linear megawatt response at maximum rate.
Two special algorithms allow for maximum rate of change at all times:
- A constraint coordinator ensures that all critical variables—including steam temperature, waterwall metal temperatures, and feedwater flows—are kept within allowable limits as the rate change is being implemented. When the rate of change is at the maximum level, operators don't have to worry about exceeding any process limits. Mechanical and electrical limits on fan dampers, feeders, spray injection valves, boiler feedpumps, and turbine valves also are monitored and "constrained."
- A demand limit regulator monitors various process errors, including airflow, fuel flow, and feedwater flows. During a load ramp, if any of these parameters is exceeded, boiler demand is reduced to a point that supports the limited parameter.
Other notable aspects of the modernization project are the following:
- The schedule for Unit 1 was seven months from contract signing to shipment, followed by an eight-week outage to remove the old equipment and install the new.
- Prefabricated cables were installed between existing terminal strips and the new input/output (I/O) modules. Existing termination cables were used where possible. In some cases, new remote I/O was installed in areas near field devices to reduce the need for new cabling.
- Some 16 of the 92 original manual/auto control stations were included on the new console for emergency situations on critical loops.
- HART signals are transmitted for those new final control elements that are so equipped.
- The plant information subsystem provides reporting, alarming, historical data storage, and performance calculations, all of which can be transmitted back to Exelon's engineering offices. A plant information system is included in the MaxDNA platform.
- Burner management and windbox damper operation are assigned to a programmable logic controller, which receives demand signals from the DCS via a serial communication link.
- Few plant components were replaced. Exceptions include the gravimetric coal feeders (to improve their feed-forward response when governor valves are wide open) and new process transmitters.
- A training simulator provides direct emulation of the configuration actually loaded into the control distributed processing units.
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