01 November 2003

Digital Critical

By Daniel Calandrini

In electric power plants, the 4–20 mA signal has reached its limits

Energy in the form of electricity cannot be stored.

The right amount of power must generate at the right time. This is in-market availability and is crucial for power plants. Fieldbus technology can help increase in-market availability and reduce total cost of ownership.

Digital technology is increasingly driving changes in the power industry. Its pervasiveness is evident in all sectors of the power industry, including instrumentation. The communications capability of devices and continuous, transparent information flow are requisite components of cutting-edge automation concepts. Advances in fieldbus technology follow similar trends. The introduction of fieldbus technology for instrumentation is closing the digital data communication gap between automation systems and field devices. The result is more accurate values and transmission of values, bidirectional communication, and flexible structures with fewer components.

The 4–20 mA signal is the standard used in the power industry today for the transmission of process values from the sensors to the control system and from positioners to actuators. The introduction of this standardized current signal resulted in a process automation revolution, saving users time and money with instrumentation. The 4–20 mA signal, however, has reached its transmission capacity limits. Sensors convert process information into digital signals for the instrument microprocessor. After the signal processes in the microprocessor, the signal reconverts into analog for the transmission. This inefficient conversion process costs money and reduces signal accuracy. By contrast, a pure digital signal transmission offers broader bandwidth for plausibility checks and status signals.

The digitization of process signals and the associated decentralization represents the most important development that paves the way for the application of a fieldbus system. The subsequent introduction of the fieldbus in power plants has changed the structure of control systems. Fieldbus technology allows shifting central functions into field devices and leads to decentralized structures in the automation systems. These decentralized structures reduce the costs for wiring and assembly and increase the availability. Improved maintenance and diagnosis through smart instrumentation also increase availability. Additional cost improvements can happen for installation, commissioning, operation, and also space requirements.

The introduction of the fieldbus technology for instrumentation is closing the gap of digital data communication between automation systems and field devices.

Fieldbus in power plants

Distributed automation solutions based on open fieldbuses are the current standard for many sectors of the manufacturing industry and more recently, for process engineering. Fieldbus technology provides more information about the process and the field instrument itself. Only fieldbus permits full utilization of the functional advantages of digital communication, such as improved resolution of measured values, diagnostic capabilities, and remote operation. For the selection of a fieldbus system, it is not only important to consider cost saving potentials, but also to select the "correct" system for your plant.

An important characteristic of networks is open communication. Open communication is typical for standardized networks where specifications are public. This assures the interoperability of hardware and software components from different manufacturers. Closed systems are manufacturer-specific protocols for proprietary applications and products. The use of components from different manufacturers requires additional development for interfaces and drivers. Digital communication has been around for a long time but only as proprietary protocols. In the mid-1980s several companies and universities started working on a standard protocol. The efforts resulted in two networks: Profibus and Foundation fieldbus. Profibus is a standardized, open, digital communication system for all areas of applications in manufacturing and process automation. The Profibus protocol emanates from the international standards EN 50170 and IEC 61158. The technology is suitable for replacement of discrete and analog signals.

Benefits of fieldbus

Fieldbus offers advantages in many areas. The standard has led to a variety of similar products from which customers can select the solution that best fits their needs.

Fieldbus has decentralized the structure of control systems. The control system consists of many control islands, sometimes with a distributed small controller. Single faults do not affect the entire system, and therefore do not affect the plant availability. The higher the degree of automation is, the higher the availability. To increase availability requirements, most components can assume redundant structure. Redundancy can enter at various stages, starting with partial redundancy up to a fully redundant structure including redundant modules.

Availability can also happen by using the diagnostic information from the smart devices. Fieldbus devices can provide more information than just the process value. Digital communication can also send diagnostic information such as process temperature, electronic temperature, operating hours, status bytes, or plausibility checks for the transmission. Other benefits of fieldbus devices include:

preventive maintenance of instruments
early prediction of sensor failures and damage to equipment
fast and precise error recognition of the field devices
predictable outages to increase in-market availability

The improved availability increases the life cycle of the plant and is therefore important in reducing the total cost of ownership.

Other factors that contribute to this reduction include increased performance and quality improvement. The benefits arise through configuration, cabling, engineering, documentation, assembly, and commissioning, as well as through plant operation. P

Behind the byline

Daniel Calandrini is a process automation consultant with Siemens Westinghouse Power Corporation. For the full text of this piece, additional references, and a closer examination of Profibus PA go to the ISA Web site: www.isa.org/InTech/ProtocolNov03/Profibus

Redundant communication paths

The I/O count for a power plant can range from 500 for a single turbine island to over 25,000 for a coal-fired power plant. The combined-cycle power plant is currently the most commonly built power plant.

This standard design consists of about 4,000 I/Os (1,100 analog I/Os) that are distributed to several power control centers (PCCs—α in this figure) and remote field locations.

Power plant I/O

Field locations can be the turbine hall (σ), the cooling fans (δ), or next to the heat recovery steam generator (HRSG—Φ) at ground level and at the 100-feet level next to the drums (γ).

Field locations have the advantage of reducing the space requirements in the PCCs. In these cases, fiber-optic cable carries the redundant Profibus for faster communication over the greater distance and avoids problems with lightning strikes.

The field locations also help increase the availability due to the redundant Profibus. Classic designs have I/O cabinets in the PCCs next to the controller with a nonredundant 4–20 mA communication out to field instruments.

With this design most of the communication is redundant, while only a small communication path from the remote I/O to the instruments is nonredundant.

Profibus is also being used to install small I/O clusters in operating equipment such as the turbine generator (η) and the continuous emissions monitoring system. The protocol has proven itself to be cost-effective and reliable in over 100 operating plants.

Commissioning is performed in a timely manner due to the simplicity and the limited number of wires used in power plants. The reduced number of wires decreases erroneous connections and accelerates troubleshooting during commissioning.

Certain applications, such as burner management systems or continuous emission monitoring systems, require separate controllers. Profibus DP can integrate these applications for a fully integrated control system.

Highway Addressable Remote Transducer (HART) can also integrate into systems by using HART-multiplexers or HART-capable distributed I/Os for applications where 4–20 mA is a better fit.

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