Two of Europe’s largest synchronous motors, along with the highest power-rated medium voltage (MV) drives ever used in a reversing roughing mill, have been commissioned by ABB at Tata Steel’s Port Talbot hot strip mill in South Wales.
The replacement motors and drives were the focus of a major £25m reversing roughing upgrade – a key part of the rolling process which transforms a 23 cm thick slab of red-hot steel into sheet, less than 3.5 cm thick, before further reducing it to produce hot rolled strip. The upgrade aims to increase steel production significantly while improving final product quality.
The reversing rougher is a heavy torque application and as such demands two ABB 12.5 megawatt (MW) salient pole synchronous motors, together with two ABB MV drives rated at 36 mega volt-amperes (MVA), 3.1 kilovolts (kV), to power the slab between a set of rolls.
Previously, two 6 MW direct current (DC) motors, installed in 1985, operated some 30% over their intended capacity. The overload regularly stressed the windings, making the motors difficult to maintain and subject to potential catastrophic failure. Although operating in overload the roughing mill was still the bottleneck process for 50% of the time putting a hard limit on plant output.
The new synchronous motors give the response demanded while the ACS6000 MV drives with its motor control platform using direct torque control (DTC), provides unrivalled speed and torque control. As the slab enters the reversing rougher there is no perceived speed drop, as happened with the DC motors. The new motors and drives control the slab precisely, with no over shoot in speed. This results in the slab being processed much faster and reduces unnecessary torque reversals in the mechanical drive train. The slow control of the DC motors caused severe torque swings in the drive train, resulting in fatigue cycling to the mechanical equipment. This no longer occurs, resulting in a much quieter and safer operating environment, with improved production capacity together with increased reliability.
“With the old system the average cycle time was 95 seconds. Today it is 80 seconds,” said Steve Winkley, Tata Steel’s project manager for the roughing mill project. “When we change direction of the bar, it’s much faster than ever before. When we take advantage of the motors maximum top speed, this will result in an average speed of work increase of 30 tonnes per hour more steel; that is a staggering 200,000 tonnes per year. This significant increase in production is almost a side-effect of having to replace the DC machine. We successfully turned what was a potentially catastrophic situation with the overloaded DC motors into an unprecedented success breaking a series of ‘all time’ production records.”
In a reversing rougher reliability is paramount. ABB’s extensive experience of mill applications together with proven design technology and excellent manufacturing know-how combined with thorough quality control and comprehensive testing throughout the production cycle, combined to make Tata Steel’s decision to use the company easier.
Tata Steel meticulously structured the shutdown of the existing facility; a process of design, planning and risk management that began back in 2011. This risk mitigation strategy lead to some 90% of the installation being completed, commissioned and tested before the plant shutdown.
Over a four week period, starting in August 2014, the plant completely closed for what was to be the most intense upgrade in 25 years. The old motors and spindles were removed and the foundation for the new equipment prepared. The pre-assembled new motors were then lifted into position at the same time that the new spindle system was being installed. The two motors were connected with the spindle system and all the pre-installed and commissioned services and control equipment was connected ready to run the new motors.
“It’s fantastic to see the installation progress to schedule and really justifies the approach we took on this project with 90 percent of equipment pre-installed before shut down,” said Winkley. “We are really happy with ABB, in terms of the equipment and the company; both performed absolutely brilliantly.”