In order to compete in today’s electric power market, steam plants are required to operate reliably over a wide range of outputs and for long periods of time. These demanding operational patterns have brought a need for modern control systems that can be retrofitted to existing plants.
Frederick W. Block and William Tabor, Siemens Westinghouse Power Corporation, Instrumentation & Controls Group, Alpharetta, GA, USA
Today, the American power generation industry is running its existing steam turbines for much longer periods of time than had been planned just two years ago. In order to compete in today’s American power market place, steam power plants are required to operate reliably over a wider range to be competitive and profitable. Many utilities and industrial companies are looking for ways to extend the life of their existing turbines. This has increased the demand for turbine control products specifically designed for the modernization of these vintage, but still very serviceable, turbines. In order to achieve improved flexibility and availability, steam turbine owners can retrofit and upgrade their steam turbine control systems with modern digital control platforms.
Many older steam turbines still operate with the original mechanical governor for speed sensing, mechanical linkages and low pressure servo motors powered from the turbine bearing lube oil system for hydraulic valve actuation. Later steam turbines had analog or early digital hydraulic control systems provided by their respective turbine original equipment manufacturer (OEM). Although these systems were reasonably reliable over a remarkably long service life, they have become increasingly difficult and more costly to maintain. Replacement parts, if available, are becoming expensive and often have long delivery times. In addition, maintenance of these older mechanical and electronic systems requires a high level of knowledge, which may be no longer readily available.
Market in mind
With the American turbine controls modernization market in mind, Siemens Westinghouse has designed the Turbine Control System using S7 (TCS7). The TCS7 is an innovative and cost effective turbine control system, which has high performance, used for the modernization of steam turbines. The TCS7 uses the Siemens S7-300 or S7-400 control platforms, which are ‘state of the art’ products with a proven record in service worldwide. The TCS7 is capable of controlling most industrial or utility steam turbines and their associated auxiliary systems. The TCS7 can be simplex or redundant to provide the required controller availability for almost any steam turbine. The TCS7 application software is also modular and totally configurable by menu-driven graphic displays from a local human machine interface (HMI) station. The software and hardware are fully integrated even down to the servo valve controls for the hydraulic actuators. The TCS7 can be interfaced to most existing plant control systems.
TCS7 has demonstrated stable turbine operation throughout a wide operating range and in all modes, i.e. during unit start up, shutdown, parallel operation, island mode, etc. The TCS7 can also control the turbine during unanticipated conditions, such as full load rejections created by a sudden separation from the grid and prevent an over-speed condition and possible collateral damage to the turbine.
Figure 1. The TCS7 is designed to be located near the turbine
The TCS7 has built-in high performance servo valve control modules for fast, precise, reliable control of the hydraulically actuated turbine control valves. A new addition to older turbines is a high availability servo valve controller option, which includes dual servo controllers, dual valve position sensors, dual coil servo valves and other associated mechanical adjustments. This TCS7 option, when combined with the redundant S7 turbine controllers, yields a high availability system that can withstand an initial failure and still maintain turbine operation. Most faults are identified by diagnostics and repairs can often be made while the turbine is in operation, which can greatly improve turbine availability.
The TCS7 application software is modular and has been pre-tested and proven by simulation and field applications, which results in improved software reliability. The simulation is used to test specific turbine applications prior to actual start-up and has the added advantage of being available for operator training. The software modules are configurable for each specific unit application by filling data into simple configuration displays. The user is not required to become a software engineer, which allows him to focus on the core business of generating power or a steam supply. The software modules not only duplicate the basic functionality of the existing controller, but may also offer improved operation or functionality not possible because of the technology limitation of the obsolete controller. A good example would be the fully implemented sequential control valve operation on large steam turbines, which includes high fidelity valve linearization for optimizing steam flow. The TCS7 can implement both partial arc and full arc admission during start-up, to conduct valve testing, and supports the shut down of an individual valve for repair of a component while maintaining unit operation.
Figure 2. The local operator HMI has a steam turbine overview display which provides an extensive summary of the turbine
Steam turbines are somewhat standardized and modular in mechanical design, but each one has its own individual characteristics. This ‘individuality’ can require adjustments in software during the integration, start-up, testing and tuning process. There are times when it is useful to consult with the OEM steam turbine designers to confirm application software changes and improvements prior to start-up.
The TCS7 HMI is an area where significant improvements have been made over the old turbine controllers. The HMI features the graphics that are familiar to the operator and similar in operation to the original operator control panels. Local operator panels are environmentally hardened devices that can be mounted in the field near the turbine or in almost any remote location. The HMI features graphic displays for operation, startup/shutdown, alarms, diagnostics and trends. The HMI also serves as an engineering station capable of changing the controller parameters. The engineering displays support servo valve calibration and an auto tuning routine with a trending display. The HMI is a secure device with multiple layers of passwords for protection. In the control room, the operator has a remote operator display with the same functionality as the local field panel.
The TCS7 turbine control system can be upgraded to a full distributed control system (DCS) that is capable of controlling the entire plant. The TCS7 application software remains the same, but S7 system elements are enhanced to include the features of a full scope plant wide DCS. There are significant advantages in having a common plant and turbine control system because operating, engineering, training and maintenance costs are minimized.
When modernizing controls on an existing steam turbine, it is required that the replacement turbine controller has a flexible design so it can be installed into the space available. The TCS7 has a very compact, flexible design. The controller has the environmental specifications that allow field installation, where space is available. Sections of the control system can be remotely located by using the Profibus fieldbus, which is usually installed using fibre optics. Cabinet designs are compact, rugged and standardized, and they be readily customized as required by the specific project.
A utility in the Midwestern US repowered its existing boiler with a new boiler, which cleanly burns low cost scrap coal. As part of this project, the plant needed to upgrade the turbine governor on its early 1960s Allis Chalmers steam turbine to allow remote operation from the plant DCS, while also improving the unit’s operating reliability. The original mechanical governor was replaced with the TCS7 system and dual servo valve controllers. A mechanical upgrade was made to the hydraulic high pressure actuator system, which included redundant position sensors and dual servo valve coils along with a new hydraulic supply unit. The plant has now been operating successfully for more than a year.
A municipal utility in the eastern US was experiencing unexpected shutdowns of its 180 MW Westinghouse reheat steam turbine. Some of these failures were caused by the turbine’s original analog control system, which was installed in 1974. New parts were not available for the obsolete analog electronic controller to restore the unit to reliable operation. The plant decided to replace the original controller with a high availability TCS7 to improve turbine availability. TCS7 has redundant processors and four dual, independent, high performance servo valve controllers with redundant position sensors. This unit has now been in operation for a year.
A fertilizer plant in the southeastern US was experiencing unreliable operation of an admission extraction, condensing steam turbine, which was a cogeneration unit that was critical to economic operation of the plant. The problems were due to the unreliability of the obsolete digital electronic controller. The plant elected to install a new TCS7 to improve reliability, and also make the operation and maintenance of the turbine easier for the plant staff. Another important reason for the TCS7 selection was that it could be upgraded to a DCS and expanded to control the rest of the plant. This plant has been in operation for over a year without turbine control problems.