Control and efficiency improve after Tata Steel blast furnace rebuild 04 December 2014

Tata Steel has confirmed that a major refurbishment of its Blast Furnace No 4 at the Port Talbot works in South Wales has improved the plant's control and efficiency – notably due to the installation of ABB variable speed drives (VSDs).

The drives were the largest low-voltage air-cooled units in the UK, according to ABB, delivering 2.7MW at 690V – used on fume extraction – while others now manage the stoves combustion air and water tower cooling systems.

This blast furnace was rebuilt in 2012 under a £185 million project that included an upgrade to the fan systems supplying combustion air, cooling air and fume extraction. The latter already used VSDs but they needed upgrading.

Chris Court, electrical project engineer for Tata Steel, says the design, manufacture, installation and commissioning of the transformer, drives, motors and control systems for the stove combustion air fans, cooling tower fans and fume extraction fans, was a major part of this project.

"Iconsys [ABB system integrator] had worked with us before," says Court, adding that ABB is known as a quality manufacturer but was also the most competitive bidder.

"We gave them the drive sizes we required – 90kW, 600kW and 2.7MW – and they quantified the best drive set up to fit our requirements, incorporating drives, motors and ABB controllers," he continues.

"We also detailed the ranges of ABB drives we wanted – ACS800s with harmonic reduction. Plant engineers had a preference for ABB and for air-cooled over water-cooled drives, and the 12-pulse ACS800 was the only way we could get the drive power in an air cooled frame."

Court says the furnace was also equipped with ABB controllers, so plant engineers knew integration would be easy.

Meanwhile, Tata Steel also wanted to improve energy efficiency on the stove combustion and cooling tower fans by replacing the existing DOL (direct-on-line) motor starters with low harmonic VSDs and new transformers and motors.

"As well as achieving energy savings by slowing the speed of the fan motors, we also wanted to increase the capacity of the fume extraction system, so larger drives and motors were needed," says Court.

The stove combustion air fans now use three 600kW ACS800 VSDs, with speed determined by duty cycle and air demand. The fans operate as two duty and one standby, although they can also operate together at reduced speed.

Control and speed setpoints are derived from an ABB AC800 PLC over a Profibus DP link.

Each combustion air fan system consists of 3,300V/690V supply transformers, a 600kW ACS800 low harmonic VSD and local operator stations at the drive, transformer and motor.

"The three combustion air fans were previously 3.3kV DOL, with one fan for each stove," explains Court.

"When a stove was lit, the fan was operating at full speed – there was no speed control. But the three new fans can now control fan speed to give a constant air pressure, while saving energy.

"We can also run two fans on a duty/standby system, rather than having a fan individually tied to a stove [which] gives us better availability and greater energy efficiency."

Interestingly, these drives were fitted with internal ducting, which removes over 50kW of dissipated heat.

As for the cooling tower fans, the existing fixed-speed motor controls were replaced with ACS800 low harmonic 90kW VSDs.

Both speed and number of fans driven is now automatically varied to maintain the set delivery water temperature, while control and speed setpoints are derived from an ABB AC800 PLC over a Profibus DP link.

"The two fume extract fans were previously driven by Syncdrive VSDs [which] were obsolete and very problematic. They were 2.4MW six-phase drives and were complicated, difficult to maintain and had a wound rotor with field controller," says Court.

"These have been replaced by new three-phase drives [2.7MW ACS800s] and squirrel cage motors, and are much simpler. The original fume extraction fans were to be retained but the speed increased from 1,000 to 1,130 rpm."

Each of the two fume extraction systems consists of 11,000 V/690 V supply transformers, a 2.7MW motor including bedplate, coupling and guard and local operator stations.

These drives are also housed in am IP54 steel container fitted with forced air to keep the drives cool and dust free.

One of Tata Steel's requirements was a cost-effective way to maintain the G5/4 harmonic standard. To achieve this, Iconsys supplied the 2.7MW drives with active filters, using a diode front end. The smaller drives use passive filters.

"We have been very pleased with the applications since they were installed in February 2013. Any minor issues have always been very quickly resolved by Iconsys' service department," states Court.

Brian Tinham

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