CSW discovers simple is better
By Milt Neher and Jim Glegg, Central and South West Corp.
Central and South West Corp. (CSW) began its control system upgrade program in the mid-1980s with distributed control system (DCS) technology at many of its larger gas units. The early upgrades involved new boiler/turbine/generator (BTG) boards that looked just like their predecessors but with newer hardware and a couple of cathode ray tube (CRT) displays. These BTG upgrades cost about $1.5 million apiece.
In the late 1980s the company began to look at CRT-based operator interfaces as technology progressed and operator acceptance of CRTs increased. The savings in hardware, engineering and construction associated with BTG board replacement lowered the average cost of retrofit projects to about $1 million. This seemed a reasonable figure, and CSW engineers felt proud of shaving $500,000 off the price tag. Unfortunately, that euphoria was short lived.
From the perspective of West Texas Utilities Co. (WTU), a CSW subsidiary with many small units, the $1 million price tag to upgrade combustion controls was still completely out of the question. It felt like it would be lucky to find funding for a single loop controller (SLC) project, let alone the funds for a DCS upgrade. WTU challenged designers to develop a concept that could deliver DCS functionality at SLC prices.
It was impossible to scale down the DCS with the hardware available at the time. The major factors seemed to be the high cost of the man-machine interface (MMI) and the high price of the control processor. Then CSW controls engineers rediscovered the PLC. The big attraction was the ability to scale the system down in size to compete with the SLC and still provide the required control algorithm complexity. Additionally, the platform provided flexibility for future expansion of both input/output (I/O) cards and distributed nodes. The remaining challenge was to overcome the common conception that “you can`t do this with a PLC.”
The end of outsourcing
CSW management decided to perform the procurement, system engineering and installation engineering work it had previously outsourced. This put company engineers closer to the project and reduced project costs by eliminating duplication of activities. It also eliminated one more set of interfaces and potential confusion. CSW reduced the upgrade costs by 20 to 30 percent because of these added efficiencies.
In comparing PLC and DCS technology, system designers determined that I/O capabilities were fairly even, with PLCs having the edge in digital I/O and DCSs having an edge in analog I/O. In addition to familiarity and availability of several MMI options, PLC hardware was lower in cost, locally available and could be packaged at a local panel shop.
PLCs have long been recognized for their discrete control capability. However, there remained two significant issues:
1. Programming of analog control loops. Most ladder logic programs could do simple proportional-integral-derivative control, but tracking and complex analog strategies were very cumbersome. However, development of vendor and third-party function block programming languages has unlocked the PLC`s capability and given it the same abilities as the DCS. There is currently more variability between DCS vendors in programming technique, flexibility and documentation than there is between DCSs and PLCs. The system is really a DCS running on a PLC platform.
2. MMI software. Recent developments in vendor and third-party software packages have provided the PLC with the same look and feel as the DCS utilizing the same software people have at their desktops and at their homes. These improvements have been so successful that most DCS vendors are now offering a PC MMI solution as an alternate to the high-end UNIX workstation solution.
CSW can now integrate the capabilities of the new MMI software packages and the PLC into a fully capable, redundant, regulatory control system with supplementary, field-proven batch, on/off and sequencing roles. Usually, heavy PLC users have looked elsewhere (DCS, SLC) for their regulatory control needs. CSW now has a combined tool set that provides this type of functionality from a PLC and that allows the control designer to standardize on the PLC as the backbone of a plant control and information system.
There are numerous vendor and third-party software packages that provide MMI graphics to allow the operator to interact with the control system and the process. These packages run under Windows 95 or Windows NT and provide the operator with similar functionality to that offered by the larger, more expensive DCS UNIX workstations. In fact it is virtually impossible to tell which operating platform is resident from the look and feel of the MMI. In addition, the technical and operations personnel are familiar with the operating system, so they adjust better to maneuvering through the graphic displays and tools available to them.
Some of the attractive features of the PLC system are: Familiarity. Many plant technicians are familiar with the PLC hardware platform, which reduces learning curves and callouts from factory personnel and allows plants to capitalize on previous PLC training; Cost. The ready availability of PLC hardware eliminates the need for extensive travel, training and field service specialists from the factory; Expandability. The system can be added to at any time (by card or by rack) by the end user; Spare parts. Local PLC vendors stock hardware, eliminating the need to stock spares; Reliability. The systems contain field-hardened hardware that just doesn`t quit; Maintenance. Configuration changes are easy to make using the function block software, and graphics are easily revised with the graphics builder. The PLC hardware is modular, rugged and relatively maintenance free; Simulation. Full simulation capabilities ease troubleshooting and operator training; and Extras. Redundant processors, MMI, data highway, power supplies, etc. enhance the system`s usefulness, reliability and value.
To eliminate construction contracts, CSW uses multidiscipline project teams from the plants, regional technical support and the Dallas-based Advanced Control Division. These teams combine their talents and expertise to upgrade plant controls for a fraction of the costs of previous projects. The teams divide responsibility based on practical factors like location and experience:
z Regional Office–overall project coordination, installation design, checkout and startup;
z Plant–construction, checkout labor, startup; and
z Advanced Controls Division–system procurement and assembly, control system configuration, graphics development and implementation, operator training, checkout and startup, and tuning.
Application complexity and consistency
Before going the “do-it-yourself” route, CSW`s project management approach was to write the specification, then police its implementation. This can be quite time-consuming, with the result being a compromise between vendor software constraints, the vendor`s control engineers` skills and interpretation and the amount of time available to correct any deviations. Alternatively, one could take a completely hands-off approach and purchase “one lot, control system.” That method`s results can be anywhere from excellent to disastrous. The in-between approach is to write a performance specification with enough degrees of freedom to allow the control engineer a little latitude to achieve performance. The tough part of this method is to set reasonable control performance goals that are not equipment constrained. Too often these are accompanied by squabbles over penalties and goals achievement.
After a couple of system upgrade projects, CSW found that the amount of time taken to configure the controls is not significantly different from the time required to write the specification. The functional block AutoCAD control configuration drawings are about the same as the drawings CSW used to submit with system specifications.
The CSW Advanced Control Division has refined its core designs with each successive project, adding more features and assembling a library of ancillary control applications. It is now not necessary to reinvent the coordinated front end each time. Hybrid floating pressure is done. The way the operator changes mode and what he sees has been defined by experience and feedback from operators. The do-it-yourself philosophy has paid off in that the nuances and complexities are retained effortlessly and are executed consistently from project to project. That makes for very effective long-term support.
Application of this technology to the CSW upgrade program has reduced project costs from about $1 million to $500,000 without reducing functionality. Company management has adopted this solution for all small- to medium-size projects and has applied it in 22 power plant projects to date. CSW has concluded that PLC technology is a viable option to be considered when selecting the architecture for a specific project:
z The solution is robust, with excellent reliability in the harshest field environments.
z The solution has excellent initial cost and superior life-cycle cost characteristics.
z The program execution cycles are extremely fast.
Normal execution cycles for a complete combustion control system program are about 70 milliseconds (ms), which is about 14 times faster than the normal DCS execution cycle. That shouldn`t be a surprise since PLCs evolved to solve motion control and other applications that require precise positioning, fast program execution and a large amount of I/O.
The CSW group has executed do-it-yourself projects that include combustion control, burner management, automatic dispatching, coordinated front end, hybrid floating pressure, water treatment systems, coal- and ash-handling systems, and performance monitoring according to ASME`s Power Test Code. CSW has expanded its capability as an integrator by adding control systems for other industrial processes, including wood yard conveying, switchgear, balance-of-plant and motor control center controls. Specific examples follow.
Northeastern Unit 2
Configuration of Northeastern Unit 2 has added the complexity of a supercritical unit to the already impressive advanced control features implemented in the “standard” CSW control system. The new system takes advantage of full range pressure reducing valves between the primary and secondary superheaters. Much like the “floating pressure” systems CSW has installed on drum units, the 401 valves or “division valves,” as they are sometimes called, enable the boiler to maintain critical pressure while operating the secondary superheater at lower pressure.
The configuration utilizes a version of the standard hybrid sliding-pressure design. This feature avoids high-frequency disturbance rejection adjustments of the 401s by allowing the turbine governor valve to take care of MW regulation. It also stabilizes the secondary superheater inlet pressure and, consequently, the temperature, which improves temperature control. Participation of the 401 and governor valves in the pressure drop to first stage pressure is also balanced, which keeps Joule-Thompson effect cooling balanced between superheat and reheat stages. The result is near 1,000 F temperatures over the entire load range without spraying, with accompanying increases in efficiency.
Unit flexibility is also improved by a total revamp of the startup/flash tank system logic, which enables operation at operator-set load while on the flash tank. Operation of the unit has been extended well below the 130 MW or 825,000 pounds-per-hour minimum feedwater flow. Minimum loads carried at night are typically 50 MW, giving this unit new dispatch functionality, without the costly piping modifications typically required to achieve low load operation. The ability to ramp at operator set rates and hold any position on the flash tank has provided more flexible operation, along with a better understanding of this region of operation. Improved burner management (32 burners) with auxiliary boiler control also enhanced this project.
Gas yard control
CSW is using the PLC as a remote terminal unit to control the addition of multiple gas suppliers at a generating station. It will provide the operators an easy way to select between fixed and swing suppliers and to set burn rates. The hourly totals and blended heating value computations are sent via telecom to central dispatch for intra-day buying adjustments. A creative control strategy enabled the elimination of a set of flow valves on the original system. It was recognized early in the project that the additional degree of freedom was not required. Sophisticated switching logic, an additional controller (software) and well-thought-out graphics accomplished the same objective. This resulted in almost $2 million to $3 million in mechanical savings for the gas yards at five generating stations.
New opportunities: turbine control
After proving the usefulness of PLC systems for burner control and combustion control systems, CSW engineers have turned to the development of a turbine control package to make this approach consistent for the entire unit. This application will include turbine speed, load and flow control functions, protective functions, and sequence-of-events monitoring with 1 ms resolution. The designers are satisfied with the mix of I/O cards available, and they believe the PLC positioning card will provide all the functions necessary for interfacing with the turbine control valves.
The Advanced Control Division is in the process of designing the first installation of the turbine control system in conjunction with Novatech of Fort Collins, Colo. Completion of this project will give the company a set of designs that will handle all major power plant components. This will truly give the control designer an alternative to the DCS or integrating with hybrid systems. Combined-cycle plants may also benefit from this approach, since it provides a cost-effective systems approach at a low enough cost to be attractive.
Industrial boiler control
The Advanced Control Division and Standard Automation of Houston, Texas, are also in the process of developing a standard PLC control package to address the needs of the industrial boiler market. Due to budget constraints, this market usually had no choice but to apply low-end solutions. The PLC solution now allows industrial boiler users to consider a robust, redundant architecture with DCS functionality at a reasonable price.
Starting with one of the Microsoft EXCEL spreadsheets used by the plant support group, CSW Advanced Control Division engineers have automated data input with “live” plant data. This provides plant operators the option of either expanding this into a low-cost performance monitoring system, or implementing a commercially available performance monitoring package. Power production management is very interested in this low-cost alternative and may adopt it for many of its units. The first system will enter service later this year.
CSW engineers incorporated remote dial-in capabilities into their control systems as part of a continuing effort to support their customers` facilities. These systems use remote access features that allow engineers at the Dallas headquarters to help troubleshoot problems and to make changes to the control system software at client facilities. Since the same engineers were involved in putting together the control system at client facilities, they can support the ongoing operation without the expense of having outside vendors visit the site. It is difficult to maintain the plant technical staff`s proficiency in all aspects of the plant control system. CSW`s headquarters engineering group can alleviate that concern by providing qualified staff to support a plant`s control system hardware and software requirements. z
Milt Neher, P.E., is the manager of control projects for Central and South West Corp. (CSW). He designed one of the first plant control systems based on microprocessor technology and now leads CSW`s efforts to develop a control system engineering and system integration business unit for internal and external customers. Neher has a bachelor of science degree in electrical engineering from Ohio State University and has been with CSW for 17 years.
Jim Glegg, P.E., is a senior control system engineer with CSW. He has been with CSW for five years and has been actively involved in developing many advanced control system applications, including the Hybrid Floating Pressure Front End. Glegg has a degree in chemical engineering from McMaster University.
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Coordinated unit control is provided through a single display providing boiler and turbine control, operator-adjustable load and ramp rate limits, plus alarm and status feedback.
Graphic displays give pictorial information to aid in rapidly analyzing and applying process feedback.