AC drives show their muscle in heavy industrial applications

01 September 2006

AC motors and variable speed drives are gradually replacing older technology DC drives and single-speed ac motors in steel making and offshore platforms.

When Corus steelworks in Port Talbot, Wales, added a new continuous slab caster line, their first inclination was to follow tradition and use DC drives since these had been successful in other casting lines.

But, looking to the potential benefits of reduced motor maintenance from a switch to ac, and the reduced downtime that this implies, the project team grasped the opportunity for increased efficiency. They decided to use Control Techniques’ Unidrive SP and purchased about 60 of the ac drives for the project, and totally enclosed ac motors from CT’s sister company in France, Leroy Somer

‘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 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.’

Many of the key benefits have arisen by implanting digital control and highspeed 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.

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 PLCs, one operating in full time production, the other acting as hot standby.

Key to the successful operation of the plant was the PID-based load sharing system, pre-programmed into each of the Unidrive SP drives. The software reduces wear and tear and provides more consistent casting speeds.

‘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. This Master then communicates with all the other strand drives, keeping them digitally synchronised. 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 turnaround times.’

Medium voltage AC drives
A common theme in medium voltage (MV) variable-speed motor control is the matching of the connected load, thereby enabling large energy savings plus more efficient power conversion at higher supply voltage. MV operation allows higher power output or lower current usage at the same power.

One of the most modern oil drilling rigs built in the last few years, named Stena Don, started operation in the
North Sea in early 2002. Deep-water oil and gas drilling platforms represent severe working environments for electrical equipment, as wind and wave action, salt-water spray, and remote locations make reliability and maintenance of equipment critical issues.

One of Stena Don’s most noteworthy features is that it does not require anchors. The entire deck construction rests on two connected pontoons, each as long as a football pitch and 43-m high. Three azimuth thrusters under each pontoon keep the platform on planned position—to within one metre, even under extreme weather conditions—by means of a satellite-based positioning system.

Each thruster is driven by a variablespeed, high-voltage 3.3 MW motor. The vertical design motors are water cooled, three phase ac synchronous 6 pole machines running on 4.16 kV input. Each is fed by a water cooled Siemens Simovert medium voltage variable speed drive, rated at 4 MVA at 4.16 kV input. ‘The six
azimuth thruster drives had to be able to fix the platform in its desired location to the nearest meter without anchorage, and also to manoeuvre the platform from one operating location to another as quickly as possible,’ says Björn Rasch, MV drives product manager at Siemens A&D Large Drives Division. The maximum transit speed is 8 knots. Nine diesel generators provide the drilling platform’s power supply.

Previous drilling platform thruster design used constant speed motors with hydraulic control of the propeller-blade pitch to change the rig’s speed.

Variable-speed control allows matching propeller speed to specific conditions. All six thrusters are run for maximum transit speed in moving between drilling sites or to hold position in severe weather while one or two thrusters can ‘idle’ the platform in calm seas. The medium voltage drives typically save up to 50% the power of constant-speed thrusters, depending on the application.

Enquiry Code S232
—Frank Bartos of Control Engineering was a major contributor to this article

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