By: William W. Terry,
Tennessee Valley Authority
In today's rapidly changing energy markets, every utility is continually looking for new ways to reduce costs in order to remain competitive. Since the fuel cost associated with hydro generation is basically free, the only means to substantially reduce costs is to either reduce operating and maintenance expenses or to improve unit efficiencies. To this end, the Tennessee Valley Authority (TVA) has embarked on a program to completely automate the operation of all of its 29 conventional hydropower plants over an eight-year period. TVA is currently in the fifth year of this effort.
TVA's winter net dependable generating capacity of 28,123 MW consists of coal-fired boilers, combustion turbines, nuclear plants and hydro facilities (Table 1). The 5,298 MW of hydro capacity provides both baseload power and peaking power.
TVA has implemented Supervisory Control And Data Acquisition (SCADA) at various remote hydro facilities to reduce operating costs by consolidating the control of multiple plants at one location. This type of control, however, still requires the attention of an operator even though that operator may be miles away at another plant site.
Several years ago, TVA's Hydro Operations Management commissioned a study to determine the feasibility of completely automating the control functions at all hydro facilities in the TVA system. The object of this study was to evaluate the cost of implementing an automated control strategy, compared to the economic benefits that could be realized.
The project team defined four major program objectives:
- Reduce operating costs by reducing the number of operating personnel required.
- Optimize water resources by producing the maximum amount of power with the minimum amount of water.
- Improve responsiveness to system changes by providing rapid automatic real-time dispatching of the generating units.
- Ensure system compatibility with future enhancements such as machine condition monitoring.
An economic analysis showed that the automation program could expect to realize $58.9 million in benefits through savings in operating expenses, improvements in system efficiency and improvements in unit efficiency (Table 2). The projected cost of the automation program was $50 million over an eight-year period, and the projected internal rate of return for the project was 30 percent. Based on these expectations, TVA approved the program in early 1997.
General Design Concepts
TVA realized that it needed a new approach to the specification development and design engineering for this project. TVA has internal engineering resources that could have been utilized to design and implement the automation program. However, while these engineering resources are familiar with the hydro plants and their design, they have never had experience with designing an automation system of this complexity. Conversely, there were numerous automation vendors available that have experience with designing such automation systems, but that lacked familiarity with the individual designs and operational concerns of the TVA plants.
In the past, TVA had always prepared detailed specifications and allowed potential suppliers to bid on them. While TVA had certain program objectives and specific ideas about how such a system should be designed, it recognized that the past experience of the system integrator would be extremely valuable in finalizing the design criteria. Instead of preparing a detailed specification as had always been done in the past, TVA selected a system supplier based on criteria such as experience, quality, design concept and cost. The supplier selected was Siemens Westinghouse Power Corporation and TVA entered into a long-term strategic alliance with this company. By combining the talents of TVA's and the system integrator's engineers, conceptual designs were developed which ensured that TVA would get an automation system meeting all its needs at a reasonable price.
Hydro Dispatch Control Cell Design
As noted earlier, TVA had specific ideas about some of the features the automation system should have, including the use of the TVA wide area network (WAN) as a primary communications path. TVA has an existing WAN that links every plant and office site in the company. The desire was to utilize this system in the control philosophy. Intelligent control systems placed at most of the plant sites enable all control functions to be generated locally. The local system is responsible for starting, stopping, protecting and loading the generating units. The corporate WAN is used to transmit schedules to the plants from the Hydro Dispatch Control Cell (HDCC) located in the Power Business Center (PBC) in Chattanooga. If the WAN is unavailable for some reason, the local control system simply continues to operate the plant according to the last schedule it received. Additionally, dedicated SCADA communication channels allow for manual control of the plants if the WAN is unavailable. The basic control components and design concept for the HDCC are shown in Figure 1.
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Monitoring and emergency manual control of the plants is conducted from either of the two triple-headed operating stations shown at the top of Figure 1. These operating stations are connected to a pair of servers that are configured in a hot-standby, redundant configuration by a bi-directional fiber optic local area network (LAN). The servers accumulate all the plant operating data and make it available to anyone on the WAN with authorization for viewing. Additionally, the servers provide the function of maintaining long-term data archives.
The servers are connected to multiple Siemens S7-400 series programmable logic controllers (PLCs) via a fiber optic LAN. There is one PLC per plant operating as the master unit for controlling data flow and commands required for remote operation of the plants. This LAN also connects the HDCC system to the TVA WAN. The PLCs are being used as communication hubs allowing direct communication with the plants utilizing an industry standard SCADA protocol. TVA chose this approach for compatibility with existing hardware and because the protocol supports a multiple master scheme that permits the division of transmission and generation areas of responsibility. By routing the control of the hydro switchyards to Transmission System Coordinators, the Hydro Production Coordinators in the HDCC need only concentrate on optimizing the value of the hydro system without the distraction of direct control of switchyards at the plants. All real-time operating data from the plants is routed to the HDCC over these SCADA links as well. As mentioned earlier, these SCADA links also allow for emergency manual control of the plants. In general, each plant has two physically separate SCADA communications paths from the HDCC to ensure reliability.
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Under normal operating conditions, generation schedules are dispatched over the WAN to all the plant automation computers utilizing standard TCP/IP protocol. These schedules instruct the local automation systems at the plants to provide control according to various parameters including real power, reactive power, voltage, water flow, water level, etc. The plant automation computer receives the generation schedules and determines the proper mix and loading for the generators to achieve maximum efficiency. The schedules contain, at a minimum, operating instructions for the next 24-hour period, but may be updated and redistributed at any time, allowing for rapid response to any change in system requirements. (Note: While the schedules can contain up to 48 hours of generating information, 24 hours is the most the Production Coordinator enters in most cases.)
Plant Automation Systems
As indicated earlier, TVA already had a number of hydro plants that were operated by remote control utilizing standard SCADA concepts. The rest of the plants were manually controlled locally at the plant site. This difference in control philosophy throughout the TVA system prompted the design team to divide the hydro plants into three categories for automation implementation. The designations used for these differences are simply Type 1, Type 2, and Type 3. Any plant manually controlled onsite by a local operating staff is designated as a Type 1 plant. Little or no automatic control functions were implemented at these sites. The remainder of the TVA plants were all under SCADA control from one of the Type 1 plants. This means that a moderate amount of automatic control had already been implemented at the site through the use of a Remote Terminal Unit (RTU), which interfaces with the plant and unit input/output devices (I/O). These SCADA-controlled plants were subdivided into the remaining two categories according to economic benefit as determined by plant capacity factor.
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Since there is no economic benefit from staffing reductions at these sites, the only benefits available were from unit and system efficiencies. TVA decided that the plants with historically high capacity factors would have automation system equipment installed to capture the efficiency benefits. These plants were designated as Type 2. The remaining plants were designated as Type 3. The Type 3 plants would continue to be operated using the current SCADA philosophy, with one exception. The automation system at the Type 1 plants would now act as the controlling entity for the Type 3 plants.
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The automation philosophy for a Type 1 plant is shown in Figure 2. All offsite communications are handled by a pair of dual-redundant, hot-standby Siemens PCS 7 systems, designated as plant common PLCs. In addition to handling offsite communications, these PLCs contain the logic for control of all plant common functions. Some examples of plant common functions are station service control, headwater and tailwater elevation monitoring, switchyard control, and control of a Type 3 plant. The control logic for generating units resides in another pair of dual-redundant, hot-standby PCS 7 systems, designated as unit PLCs. The number of units controlled by each set of PLCs is determined by the complexity of the control logic for those units and may vary from plant to plant. This design provides for high reliability at both the plant and unit interfaces.
Inter-plant communications between the various components of the automation system are accomplished through one of three LANs. Communications between the local operating workstation and the automation hardware is over an Ethernet fiber optic LAN configured similar to those at the HDCC. This LAN also provides communications via the TVA WAN back to the HDCC for transmission of generation schedules and data that are not time critical. In addition to a firewall that restricts access to the automation system, all the Ethernet addresses on this LAN and the Machine Condition Monitoring (MCM) LAN are configured such that access to and from the Internet is not possible. The MCM LAN, which also utilizes Ethernet connections, is used to allow the transfer of data between third party machine condition monitoring systems and the automation system. The third LAN in the automation system is the Profibus LAN that allows communication between the various PLCs and their remote I/O. This LAN is entirely internal to the plant. Generation schedules for control of the Type 3 plants are sent to the plant common PLCs along with the schedules for the Type 1 plant. However, the control of the Type 3 plant is not as comprehensive as that for a Type 1 or 2 plant in that it utilizes a traditional SCADA philosophy and is limited in the amount of information available to the controlling computer.
The automation philosophy for a Type 2 plant, shown in Figure 3, is simpler and less expensive. Since a large majority of the plant and unit I/O was already interfaced with a SCADA RTU at these sites, it was only necessary to communicate with the RTU to implement most control functions. Some more critical functions are being removed from the RTU and directly interfaced with the automation system I/O in order to achieve more accurate control of the units. Some examples of these are governor speed adjust position feedback and governor control. Additionally, plant data that was not included in the SCADA control philosophy is being added to the automation system such as the direct monitoring of unit Resistance Temperature Devices (RTDs). The SCADA philosophy only monitored a high temperature alarm contact without discrete temperature information being available. By having the automation system monitor actual RTD inputs, the system can anticipate high temperature conditions and take appropriate action before a serious problem occurs.
At this point in time, hardware and software designs are complete and plant automation systems are up and running at seven hydro facilities with an eighth site approximately 95 percent complete.
Author: William W. Terry is the Manager of Hydro Automation Design for the Tennessee Valley Authority in Chattanooga, Tenn. He has been with TVA for 25 years and has held various engineering and management positions with the company. Terry holds a bachelor's degree in electrical engineering from the University of Mississippi. He is a Registered Professional Engineer in the state of Tennessee and is a Senior Member of the Institute of Electrical and Electronics Engineers.