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As Continuous Caster 3 was a completely new operation, aimed to increase the output of theplant by 25%, the project team had the flexibility to choose eitherAC or DC drives and motors. The previous major contracts with drives supplier Control Techniques had been upgrades, re-utilising existing DCdrives, so the drive of choice was CT’s digital DC drive, Mentor II.
Now, however, looking to the potential benefits of reduced motor maintenance from a switch to AC, and the reduced downtime that thisimplies, the project team saw an opportunity for increased efficiency. They decided in favour of the Unidrive SP AC drive for the key drives on the twin strand caster. This includes the turret drives and the twin tundish car lateral movements, as well as tundish raise and lowercontrol, and the withdrawal drives and roller tables. This amounts to60 AC drives in all.
“This is the first vertical mould caster for Corus,” says Corus Concast Engineer, Roger Morgan, “and our first real experience with Control Techniques’ Unidrive SP AC drives. We’ve found them to be reliable, and easy to use and 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.”
All the motors ensured totally enclosed protection for improved reliability and less downtime. The scope incorporated two new AC Drive Motor Control Centres, employing some 60 AC Unidrive SP variable speed drives and Leroy Somer AC motors, controlling the caster strand withdrawal sections, the auxiliary caster machines and equipment including therun-out tables. The drives provided a much improved load sharing, a more consistent casting speed, shorter rethread times, and a big boostin reliability. All these features, which are essential for reliability and superior speed control, were a pre-requisite for greater productivity and finished steel quality.
Corus’ markets include the tinplate and automotive markets, producing steel with a high surface finish and clean internally. The project integrated the new plant equipment over a one-year phase, and concluded in the successful commissioning, in 2005, of new strand drives, as well as PLC and SCADA hardware and software for the control monitoring of both the casting plant and water treatment plant. The new line is key to Corus’ target of 4.7 million tones output in 2006.
Many of the benefits have arisen by implanting digital control and high-speed data fieldbus communications,as well as a high level of redundancy built into the system design configuration, as a means of future-proofing the plant for many years ofhigh reliability, accurate control, and low-maintenance.
Failure not an option
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 interface is direct to two programmable controllers (PLCs), one operating in full time production, the other acting as hot standby. Furthermore, two communications links are provided between the PLCs and theControl Techniques drive system, and a failure to communicate via oneof the routes automatically initiates a communications link via the second route.
Each of the Unidrive SP drives has a PID-based load sharing system that is pre-programmed on the plug-in application modules. The software, with its benefits of reduced wear and tear and more consistent casting speed, is evident in the motor current trends. The end result is a much more stable drive control system.
New ACmotors were also supplied by Control Techniques, each fitted with digital encoders and brakes on the vertical part of the caster. The drivecontrol included on board intelligence / specific application software, part of which included closed loop to open loop changeover in the event of a feedback device failure, thus ensuring the caster continuesto run.
Advanced testing
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. The verification trials proved invaluable, as the time spent in in-depth testingat Telford significantly reduced the on-site commissioning period.
“The whole system is now more modular,” explains Mr. Morgan. “Theintelligence in the system is now distributed rather than central, andthis means that just one Unidrive SP Inverter is designated as the master and communicates with the plant PLC. This Master then communicates in turn via CT-Net with all the other strand drives, keeping them digitally synchronised. For reasons of dual redundancy, “Automatic Seamless Master Transfer” passes master control to the next drive in line,in the event of failure. This means that, if necessary, the line couldbe run manually. It is now much more flexible and reflects many of the lessons we have learned over many years of steel-making. The evidence of its success is reflected in the 30% increase in strand speed andthe reduced turn-around times.”
“We are very pleased with the results of this major project,” comments Mr. Morgan. “In many ways theapproach has been simplified thanks to new technology of course, and we anticipate much greater plant utilisation because of the inherent reliability of modern electronics.”
Pictures
a) A technician services one of the 60 AC Unidrive SPvariable speed drives installed in the motor control centres to control Continuous Caster 3.
b) The continuous casting process beginshere, when a ladle full of molten steel is released through the tundish, which acts as a funnel to transfer the steel into the vertical caster located on the floor below.
c) On the floor below the ladle, the molten steel exits the vertical caster and proceeds on these horizontal roller tables. The slabs are cut to specific lengths before they exit the building.
d) These motors power the rollers on theroller tables, and are only a few metres from the high temperature steel. They are totally enclosed for protection.A new continuous castingoperation was installed to supply additional production capacity at the Corus steelworks in Port Talbot, South Wales, in line with the new blast furnace that was installed last year.
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