在意大利大型组合循环床电厂中Foxboro自动化系统将控制集成在FF和其他协议上

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InfraServ Hoechst and Aventis Pharma of Germany performed a comparison study between an installation using conventional technology and a comparable installation using fieldbus. Their conclusion: Fieldbus reduces overall costs if used with FieldBarrier.

Traditional Fieldbus Topology

With conventional technology, if there is a power supply failure or a short circuit at any of the connected field instruments, signals from every device are lost. A new installation topology using FieldBarrier or Segment Protector solves this problem; and offers the following additional advantages:

* Redundant standard power supplies with higher output current

* Increased number of instrument connections

* Independent IS barriers for each spur increases instrument availability

* Failure at one spur (short circuit) does not affect other spurs or the trunk

* Beneficial in hazardous and non-hazardous applications The traditional fieldbus power supply is replaced with two redundant power supplies and a “failure-free” fieldbus power conditioner.

The FieldBarrier or the Segment Protector replaces the spur connections of the field instruments. The FieldBarrier is a field-mounted junction box that includes four integrated Intrinsic Safety (IS) barriers, while the Segment Protector is a junction box that includes short circuit protection for the spur outputs.

Fieldbus Power Conditioner (FPC)

This new approach divides the traditional fieldbus power supply into two components. A standard power supply and a filter are used to provide power to a segment. The FPC contains this filter. The filter is a passive device and contains only a few  discrete, fixed-value components (inductors, capacitors, resistors). Redundancy is not needed due to the passive design of the FPC.

Power Supply

Standard, redundant power supplies can be used, but they must be isolated from each other. This isolation is typically completed through decoupling diodes. Therefore, two redundant power feed modules are used and are supported by two independent power supplies. The fieldbus cable is energized from the FPC, which is then powered from the power rail. Up to four FPCs can be supported by the power feed modules.

Intrinsic Safety Applications

FieldBarrier FieldBarrier combines four IS barriers (channels) in a common IP66/Type 4 field housing. Typically one intrinsically safe (FISCO or Entity) field instrument is connected per channel. If there is a failure along any of the four channels, the other FieldBarrier channels remain unaffected. A FieldBarrier consumes a limited amount of current from the bus to operate, providing maximum power to the field devices. All outputs are galvanically isolated from the host segment.

Several FieldBarriers can be connected to one redundant power source. Most of the available

hosts support 16 devices per fieldbus segment. Newer FPCs provide nearly three times the amount of current than previous configurations. When delivering high current to hazardous areas, an increased safety protection method is used for the incoming bus connection. A Division 2 wiring method may be used for these applications as well. The maximum distance between instruments and FieldBarrier outputs is 120 m, regardless of the area classification (Zone 1 or Division 2

mounting). The FieldBarrier creates isolated spurs and all of the field instruments are directly connected to it. Built-in terminations are included. No additional junction boxes are necessary.

The Segment Protector for non-hazardous or Division 2 (non-incendive) applications provides

a topology similar to the FieldBarrier. To learn more about FPC, visit www.fieldconnex.info.