Optical fiber for clear communications

Oct. 19, 2006
Optical fiber improves plant safety and reliability.

Industrial plant environments are hostile to instrumentation and control (I&C) systems. Electromagnetic interference, electrostatic discharge (ESD), extreme temperatures, vibration, high-current faults and high voltages can interrupt communications and create hazards.  Industrial plants can benefit from lessons learned in electric power generation, transmission and distribution sites, where electronics must function properly in high voltages.

In power stations, utilities use optical fiber to communicate with safety, monitoring and control devices.  Optical fiber’s intrinsic immunity to radio frequency interference and electromagnetic interference (RFI/EMI) makes it ideal for harsh electrical environments. In contrast to metallic wire and wireless radio connections, optical fiber is not susceptible to RFI/EMI from electrical arcs, motors, generators, short circuits (“faults”), switching, ESD or communications equipment. Signal-ground loops, which are another source of interference in metallic wire systems, do not affect optical fiber.

Reduce hazards

In the event of a ground fault, high currents travel through the resistance of the ground, causing a difference in electrical potential from one location to another.  This change in potential can damage equipment and injure people.  Optical fiber provides the necessary electrical isolation to drastically reduce the risks to people and equipment. Fiber-optic communications links do not provide a conductive path for lightning. Their immunity to interference helps prevent operational errors from RFI/EMI bursts.

Fiber-optic data links

Some fiber-optic transceiver suppliers make optical fiber as easy to use as copper. An Electronic Industries Alliance standard EIA-232 connector, which is approximately the same size as a cable connector shell, conducts power to the transceivers through pins. For example, in a standard 9-pin serial port connector, one can use the request-to-send (RTS) pin as a power pin, instead of using a hardware-handshake line, by setting the device to hold it in a “high” state.   Some vendors sell prepackaged EIA-232 cables that include transceivers and terminated optical fiber.  Just connect a prepackaged fiber-optic cable to the EIA-232 ports, and receive the EMI immunity and isolation advantages of fiber. The typical length limitation of EIA-232 wiring is 50 feet. With multi-mode fiber, it is possible to use EIA-232 port-powered, fiber-optic links for distances to 15 kilometers. Single-mode fiber allows for distances to 80 kilometers.

Choosing optical fiber and connectors

Many of the same parameters that you consider when specifying metallic cables apply to fiber-optic cables, including the cable’s outside diameter, bend radius, pull strength, fire rating (e.g., plenum or riser) and sensitivity to sunlight.  Note that a cable with a metallic strength wire will not have the safety advantages of fiber.

Additional parameters, which are specific to each optical fiber, include diameter, termination methods and signal attenuation.  In general terms, larger diameter fibers (>62 microns) are used for shorter distances, and are easy to terminate with specialized hand tools. For example, 200-micron fibers and connectors are available with a single hand tool that cleaves the fiber and crimps the connector without any polishing or other special actions. Most I&C engineers use glass or silica fibers, rather than plastic fibers, for their strength, durability and longevity.

Tiny six- or nine-micron fibers are often applied in applications with longer distances. These links have a lower cable cost, but require more expensive transceivers and more complex termination equipment and training. Within a plant facility, distances are generally less than 15 kilometers, so single-mode fibers are not appropriate.

Transceiver vendors provide information on receiver sensitivity and transmitter output power and wavelength. Fiber-optic cable suppliers provide attenuation per foot for specified light wavelengths. The difference between transmitted output power and receiver sensitivity is the “power budget” for the system. Sum the attenuation of the fiber and couplings, and make sure it does not exceed the power budget for the application. Or, use simple algebra to solve for the maximum distance for defined fibers and transceivers.

Protection and I&C

Microprocessor-based protective relays sense electrical conditions and rapidly trip a breaker to protect people, property and the power system.  They include internal diagnostics, so they can remain in service without interruptions for periodic testing.  Beyond their primary protection functions, they include:

  • Automatic control to restore service.
  • Metering and demand metering reports.
  • Sequential event reports.
  • Event reports with fault type, fault location and oscillographic data snapshots. 

The same I/O connections the relay uses for protection also provide control and indication capability to system and plant operators. 

To enable the use of low-cost, fiber-optic technology, point-to-point serial communications links connect the relays to a processor, forming a star topology network. Star communications topologies allow low-cost, point-to-point, fiber-optic connections to electrical station devices.  Compared to metallic EIA-232 serial connections, fiber-optic links provide:

  • Improved immunity from electromagnetic interference.
  • Isolation from dangerous rises in ground potential.
  • Isolation from I&C system interference arising from signal ground loops.
  • Longer cable lengths.

At some sites, the relays, optical fiber and a communications processor hub comprise the entire I&C system; at other sites, they are a subsystem of a larger I&C system. 

These links are the mechanism to retrieve data from the relays, and to provide direct control and instructions to the relays.  It is useful to have the internal relay clocks synchronized so that time-tagged reports from different relays can be analyzed in the proper time sequence.  Each link between a relay and a communications processor uses two optical fibers.  Each optical fiber pair provides full-duplex serial communications. Some fiber-optic transceivers include a mechanism that allows the use of one fiber to send an IRIG-B time synchronization signal from the communications processor to the relay, so that all of the internal clocks in the station are synchronized to within a few milliseconds.

One optical fiber cable to each relay provides IRIG-B time synchronization plus a full-duplex link for real-time data and control, and virtual terminal report retrieval and interaction.

Electric motors

Electric motor manufacturers embed resistance temperature detectors (RTDs) in their motors to sense the temperature of the electrical windings.  A module with a built-in optical port can be connected to an RTD and a motor relay, using optical fiber cable.  If the motor winding insulation fails, damaging currents can flow through the RTD wiring.  The optical fiber link insulates the relay and the protection and control system from the RTD wiring.  The relay includes motor protection and control, plus valuable monitoring and reporting functions that provide information to help prevent process outages due to motor failures.  Often the motor relays also are connected to an I&C system with fiber-optic transceivers and fiber.

Remote I&C

Fiber-optic links provide reliable, secure and safe communications with remote sites. For example, an observatory uses fiber-optic transceivers for the link to a weather instrumentation site.  This link extends the distance beyond the EIA-232 copper length limitation and eliminates concerns about lightning or signal ground loops.  The broad temperature range of the transceivers and fiber cables makes them ideal for this application.

For more information, contact Gary Scheer at Schweitzer Engineering Laboratories. He can be reached at 509-336-4429 or mailto:[email protected]. For information about Lucent’s Optical fiber, call 860-678-0371, mailto:[email protected]/specialtyfibergroup or visit http://www.lucent.com. 

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