Modern ac drives again feature on the latest continuous slab caster at the Corus steelworks in Port Talbot, controlling all of the critical operations at the head of the line. This means that the total output of the plant, anticipated to reach 4.7 million tonnes this year, is dependent on these drives
Corus has installed a continuous caster (CC3) at its plant in Port Talbot to cope with the output from a new blast furnace, installed only last year. The CC3 is a completely new operation, intended to increase the output of the plant by 25%, so the project team had an opportunity to exercise flexibility and choose either the ac or dc motor control route. Previous major contracts had involved upgrades that re-utilising existing dc drives, so dc seemed the most appropriate route to take. However, on examining the potential benefits of low-maintenance ac motors, the project team decided to switch to ac for the key drives on this twin strand caster. These include the turret drives and the twin tundish car lateral movements, as well as the tundish raise and lower control, the withdrawal drives and roller tables - a total of around 60 drives in all.
"This was the first vertical mould caster for Corus," recalls Corus Concast engineer, Roger Morgan. "And it was our first real experience with Control Techniques' Unidrive SP ac drives [Corus had previously used CT's Mentor II dc drives]. We've found them to be very reliable, easy to use and easy to configure. The whole approach to this project has been a move forward in technology, with no analogue to digital interfaces. It's all digital with greatly reduced wiring; in most instances just two wires instead of hundreds."
The proposal included the latest ac drive and motor technology from Emerson Industrial Automation companies, Control Techniques and Leroy Somer. The scope of the project extended to two new ac drive motor control centres (Form 4 MCCs), comprising 60 Unidrive SP variable speed drives and Leroy Somer fully enclosed ac motors. The drives provided a much improved load sharing, a more consistent casting speed, shorter rethread times, and a big boost in reliability. All these features were a pre-requisite for greater productivity and finished steel quality.
Corus' markets include the very demanding tinplate and automotive markets, producing 'clean' steel with a high surface finish. The project successfully integrated new plant over the period of one year, and concluded in the successful commissioning, in 2005, of the new strand drives, as well as the PLC hardware and SCADA software needed to control and monitor both the casting plant and associated water treatment plant. The new line is key to Corus' target of reaching a total 4.7 million tonnes output in 2006.
The need for system redundancy
The implications of a total system failure on a continuous caster are extremely serious, so that back-up redundancy in all areas was a key part of the design. In conjunction with Corus, a dual redundant control and communications system was implemented. The plant interfaces directly to two PLCs, one operating in full time production, the other acting as hot standby. Furthermore, two communications links are provided between the PLCs and the drive system, and a failure to communicate via one of the routes automatically initiates a communications link via the second route.
A key factor in the successful operation of the plant is the PID-based load sharing system, pre-programmed into each of the Unidrive SP drives, and making full use of the platform's plug-in application modules. The success of this software, with its benefits of reduced wear and tear and more consistent casting speed, is very evident in the motor current trends. Rethread times are also much faster, and the drives/electrics on the strands are no longer the limiting factor. The end result is a much more stable drive control system, which is reflected in improved product quality.
The new ac motors are each fitted with digital encoders and brakes on the vertical part of the caster. The drive application control software includes closed-loop to open-loop changeover in the event of failure of any of these encoders, ensuring that the Caster continues to run.
The drives suites were manufactured and tested at Control Techniques' UK Drive Centre in Telford, and additional in-house trials were conducted using Corus' system PLCs and the new motors to replicate plant operation and fault modes. These verification trials proved invaluable, as the time spent in in-depth testing at Telford significantly reduced the on-site commissioning period.
"The whole system is now more modular," explains Mr Morgan. "The intelligence in the system is now distributed rather than central, and this means that just one Unidrive SP Inverter is designated as the master and communicates with the plant PLC. The Master then communicates, in turn, via CT-Net with all the other strand drives, keeping them digitally synchronised. For reasons of dual redundancy, an 'automatic seamless master transfer' passes Master control to the next drive in line, in the event of failure. So, should it become necessary, the line can be run manually. It's now much more flexible and reflects many of the lessons we have learnt over many years of steel-making. The evidence of its success is reflected in the 30% increase in strand speed and the reduced turn-around times."
"We are very pleased with the results of this major project," Mr Morgan continues. "In many ways the approach has been simplified thanks to new technology, of course, and we anticipate much greater plant utilisation because of the inherent reliability of modern electronics. The after sales support we receive from Control Techniques has been excellent and indeed our long standing experience of good support over several major schemes was a factor in Control Techniques being chosen for this project."
The Principal Contractor responsible for the overall Project was the Stockton office of VAI UK, with Control Techniques acting sub-contract to VAI, undertaking the electrical project management and control system design and engineering.