Short-Circuiting Fieldbus Installation Problems

Tests by Evaluation International (www.evaluation-international.com), an independent instrumentation testing and evaluation service, confirm the advantages of the fold-back technique: When a spur is short-circuited, segment voltage actually increases because the device coupler removes the shorted device from the segment. This means multiple short circuits on a segment cannot deprive other instruments of power and cause a complete segment failure, as with conventional device couplers (See Figure 5).

Figure 5. Device couplers utilizing "fold-back" active spur protection automatically remove the faulted device from the segment and do not permit any current flow to the device until the fault is corrected.

With fold-back device couplers, users are also free to place more devices on fieldbus segments. A large industrial process operation may have hundreds, if not thousands of devices. If the "safety margin" approach is implemented, where the entire capability of fieldbus is not used, the cost of all the extra fieldbus segments can become substantial.

End-User Benefits

For Abengoa Bioenergy, the fieldbus-based DCS employing Trunkguard technology will deliver long-term competitive benefits. Fieldbus provided a "leaner" automation architecture containing less wiring and hardware than a traditional control system. Loop and wiring diagrams, panel drawings and cable schedules were greatly simplified. Plus, installation was easier than with a traditional system, since several devices could be multi-dropped on a single pair of wires.

The flexibility of the fieldbus architecture also allows the Ravenna plant to reconfigure its process automation scheme to meet product and sales demands without major reinvestments. It reduces I/O subsystem requirements and makes the plant control system very scalable. The system can be expanded or modified loop-by-loop as needed.

Thanks to the fold-back device couplers, which do not allow any excess current per spur under fault conditions, FeedForward's control system designers were free to configure fieldbus segments at their maximum capacity. Unlike the current limiting approach, which places additional load on the fieldbus segment upon detecting a short, the fold-back technique removes the failed device from the segment and uses a trickle current to determine when the short is eliminated. This, in turn, enables voltage on the segment to actually increase—minimizing the possibility of other devices dropping off the network.

The solution also expedited unit startup at the Abengoa facility by providing increased fieldbus status information. Green and red LEDs on device couplers helped technicians determine if there was proper voltage on the fieldbus spurs. The indicator lights also showed whether terminators had been applied at specific device couplers.

Lessons Learned

Abengoa's greenfield ethanol plant project provided valuable insights for process industry end users installing their first fieldbus control systems. Engineers face new challenges when designing fieldbus segments, which potentially can be brought down by a single short. For most process plants, this is unacceptable; they cannot afford an unexpected shutdown that would immediately affect their bottom line (See Figure 6).

Figure 6. Abengoa's greenfield ethanol plant project provided valuable insights for process industry end users installing their first fieldbus control systems.

Specific lessons learned from the Abengoa project include:

1. Don't become confused by the choice of fieldbus technologies. Rather, choose a solution that provides a satisfactory and functional control system for your particular application. In continuous operation process plants, Foundation fieldbus and Profibus-PA are the dominant fieldbus protocols. Most installations will use multiple fieldbuses to accomplish the many tasks required.
2. Foundation fieldbus and Profibus systems carry both DC power and the digital communications signal on the same wire pair. Thus, the segment power supply requires low pass conditioning to filter out that signal. This conditioning may be active (notch filters, etc.) or passive (series inductance).
3. Terminators are required at each end of the segment cable to prevent line reflection, which may otherwise result from open-ended cables, and also to source/sink the communications current. Careful installation management to ensure the correct number of terminators is essential, or the issue can be completely avoided by using device couplers that automatically provide correct signal termination.
4. Short-circuit faults on individual spurs will drag down the entire fieldbus segment. Hence, device couplers need to incorporate some form of spur short-circuit protection, which again may be active or passive in design. The best approach is auto-setting fold-back overcurrent protection, where any faulty spur is switched off and that load completely removed from the segment. This approach also allows system designers to use the maximum available power supply capacity without worrying about "headroom."
5. Do not ground the shield in the field. This can result in unnecessary complications and noise issues. Instead of grounding in multiple locations using a capacitive technique, installers should ground the bus one time only at the power conditioner level.

Conclusion

Thanks to a well-engineered fieldbus automation solution, Abengoa's Ravenna, Nebraska, ethanol production facility has achieved optimal process operating conditions that increase yields, while also cutting the amount of energy needed per gallon of ethanol produced.

Abengoa ensured the fieldbus installation was simple, practical and reliable by using the Trunkguard physical layer solution. On the Ravenna plant project, these innovative fieldbus device couplers spelled the difference between quick up-time and low maintenance, versus delayed start-up and frequent downtime.

Tim Wilson is the chief operating officer, Abengoa Bioenergy, and Jeff Marsh is senior project manager, FeedForward