Product category:
Smaller-Sized PLCs
News Release from: Siemens Automation and Drives | Subject: Simatic S7 series
Edited by the Engineeringtalk Editorial
Team on 31 May 2005
PLC system answers power station upgrade
demands
When British Energy decided to replace ageing Reypak 99E PLCs used on the fuel routes at two of its power stations, it chose a Siemens solution based on the Simatic S7 series of PLCs.
Until recently, both Torness power station in East Lothian and Heysham II power station in Lancashire used a Reypak 99E PLC on their fuel routes The Reypak system, designed and supplied by NEI in the 1980s, is believed to be in use only at three sites in the world
This article was originally published on Engineeringtalk on 6 Mar 2001 at 8.00am (UK)
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British Energy decided to investigate a replacement, eventually opting for a Siemens solution based on its Simatic S7 series of PLCs.
British Energy knew that sourcing a replacement system that could meet its stringent criteria was going to be a challenge.
However, help was at hand in the form of Scomagg, a supplier of computer-based information solutions and services.
Engineering Manager, Gordon Fleming takes up the story: "Scomagg has a long term relationship with British Energy going back 20 years or so", he says.
"We became involved at the concept stage of this particular project and facilitated some technical sessions with Siemens and a number of other potential vendors, and we invited British Energy to attend".
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A typical Reypak system in the fuel route comprises a main rack containing the control CPU and application software, and a number of distributed I/O racks each communicating with the main rack via an RS422 serial line.
Distributed racks are situated at distances of up to 400m from the main rack, making the project logistically demanding.
Among the other challenges to be overcome by the equipment supplier/system integrator in replacing the I/O racks were: there must be no disturbance of the existing I/O wiring; replacement modules must fit the available space at the required density; the new system must replicate the Reypak I/O communications and plant interface requirements; the replacement system must satisfy the response time required by the Reypak main rack; the solution must offer sensible conversion cost and low cost of ownership; and the replacement system had to provide an enhanced level of diagnostics and be able to meet British Energy's potential future needs.
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British Energy decided that it required nonbespoke proprietary equipment from a major player in the global PLC marketplace and, from a shortlist of potential vendors offered through system integrators, selected Siemens.
Ivor Yule, Projects Engineer at British Energy says: "Before considering the various PLCs we had drawn up a list of requirements including, physical fit (space was limited), system diagnostics, system response time and system support".
"One of the most significant issues was the ability to make a safety case for use of the selected PLC, which was judged on the pedigree of the product and the quality assurance arrangements supporting the product".
"In our view Siemens satisfied all of the requirements".
The project represented the first undertaken by Siemens on behalf of British Energy.
Scomagg was officially appointed system integrator.
The solution is based on Siemens Simatic S7-300 digital I/O mounted inside of the existing racks (which had to remain in place) and a Simatic S7-400 main rack incorporating a 414H CPU that interfaces with the S7-300 racks and the Reypak main rack.
The 414H CPU was specifically chosen as it has been designed specifically for applications with high fail-safety requirements.
The S7-300 I/O modules are mounted in the existing racks along with interface modules to provide the isolation and polarity insensitivity required (the existing Reypak inputs are isolated from each other and are not polarity sensitive, while the outputs are totally isolated relay contacts).
The distributed I/O racks communicate via Profibus linked in "daisychain" fashion to the S7-400 main rack, and this in turn is connected to the Reypak main rack via CP440 communications modules and RS422 serial lines.
In this application Profibus DP is deployed, which provides rapid update times for the process I/O image.
The S7-400H CPU models the Reypak communications protocol, passing outputs from, and input data to the Reypak main rack on demand.
Overall the total installation across two sites incorporates: 17 S7-400 main racks; 91 S7-300 I/O racks; and approximately 400 32bit input modules (SM321) and 170 32bit output modules (SM322).
The reason for deploying 32bit modules is primarily due to space constraints.
In most cases the 16bit Reypak I/O cards are evenly distributed within the racks, hence two Reypak cards can be replaced by a single Siemens 32bit module.
In a few isolated instances the Reypak has an I/O or O/I card pairing, requiring use of a few mixed (16-input/16-output) Siemens modules (SM323).
In terms of the Siemens S7-400 main rack, the processor scans the Siemens I/O through the interface modules in the remote racks, maintaining a process I/O map.
The communications card in the Siemens main rack responds to polling requests from the Reypak main rack communications card, requests the plant I/O data message from the CPU, and then relays this back to the Reypak main rack.
Due to the noninvasive nature of the change and preparation the installation and testing of each I/ O rack only took three hours.
The implementation of the system at Torness was complete much sooner than originally expected and without the need for any significant plant outages, allowing British Energy to move the Heysham II project forward.
The fact that the existing applications software was retained unaltered is thought to have reduced the requirement for a future time-consuming project, even though the PLC had been selected with future migration of the Reypak application software in mind.
"We are very pleased with the outcome of the project", concludes Yule.
"Through this project we have demonstrated that we can, with careful thought and appropriate support, manage our C and I issues".
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