The debate over the benefits of using digital bus networks as the communications backbone of new power plants is all but settled. The technology is maturing, and the reliability of digital hardware is superior to that of hardwired systems. Newmont Gold Mining’s 200-MW TS Power Plant is perhaps the power industry’s best example of how a plantwide digital controls architecture can provide exceptional reliability and be significantly less costly to install.
Newmont Gold Mining’s 200-MW TS Power Plant (TSPP) (Figure 1) was a POWER 2008 Top Plant, and a complete description of the plant’s design features can be found in the October 2008 issue. However, that article only devoted a single paragraph to describe what we believe is the most advanced digital bus architecture ever installed on a coal-fired power plant. This article provides details of the TSPP control architecture, equipment selection, and many of the lessons we learned during this project. It also demonstrates the cost and potential schedule improvement opportunities of using advanced digital architectures in future plants.
1. Out of sight. TSPP, located in Eureka Country, Nevada, gives new meaning to the words “remote I/O.” Courtesy: Fluor
Fluor Power was selected as the engineering, procurement, construction, and commissioning (EPCC) contractor to complete TSPP in July 2004. Newmont selected DTE Energy as the owner’s engineer to work with Fluor in developing the plant design specifications and for consultation in reviewing Fluor’s designs. DTE Energy was also contracted by Newmont to provide construction oversight services and began providing Newmont O&M services when the plant was commissioned in early 2008.
Digital Bus Networking Saves Time and Money
The traditional power plant distributed control system (DCS) architecture provides device control and monitoring via hardwired signals over a shielded twisted pair of wires and has been the standard in new plant design for decades. Electronic signals are sent to devices (transmitters, control valves, electric motor – operated valves, and the like) by varying the current through the circuit with a signal that ranges from 4-20 mA. This design requires each individual device signal wire to either "home run" back to the central plant DCS server room or a field-located DCS input/output (I/O) cabinet. A single plant may have thousands of these devices. Sometimes there are multiple signal and control cables from each device, with many devices even needing a separate power feed that further adds to the number of wires that must be individually installed.
Digital bus networking uses a similar means of signal transport over a shielded twisted pair of wires. In a digital burst, the signal is transmitted by varying the voltage on the two wires as opposed to an analog current signal, and multiple devices are allowed to share the same wires. This single cable is typically referred to as a "trunk" or "segment." The devices connected to the segments are called drops or spurs. Segment protectors are located along the trunk or segment as a point of connection for multiple instruments located on separate spurs. The segment protectors sustain the network should there be a loss of an instrument along the trunk line. The devices connected to the segments can communicate integrally, without requiring a DCS controller in between. In addition to transmitting signals, power to some devices is handled through the same shielded twisted pair of wires.
The major benefits of digital bus networks are the cable purchase savings and the follow-on material and labor savings associated with their installation, either in underground conduit or overhead tray (Figure 2). The potential savings can be significant: One control system supplier has suggested that life-cycle savings up to $20 million over conventional hardwired analog controls is possible on a greenfield 800-MW coal-fired power plant.
2. Old and new. This diagram compares a traditional analog architecture with the new digital bus architecture for power plant controls. Source: Fluor
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