ABB has supplied a complete integrated power solution, including the critical process rectifiers for Sohar Aluminium, the world’s longest smelter potline. Max Wiestner, industry manager of ABB’s aluminium business, outlines the technical detail behind the project.

Sohar Aluminium Company (SAC), in the Sultanate of Oman, is the first aluminium smelter operating with 1650 V DC, and with 360 pots it is the longest potline in the world.

In June 2008, on schedule and after three years of construction, the plant produced its first ‘hot metal’. When the full six-month start up process is completed, Sohar will have the capability to produce an impressive 360 000 tonnes of aluminium a year.

Sohar, which is a joint venture between the Oman Oil Company, Abu Dhabi Electricity and Water (ADWEA) of the UAE and Alcan, is part of a massive greenfield industrialization project in Oman to establish alternative sources of revenue to oil and gas. In addition to the smelter, the project includes two steel plants and a chemicals plant, all located 12 km inland from the Port of Sohar.

The Sohar smelter is the first in the world to use the ‘best-in-class’ Alcan/Pechiney AP 35 technology that allows a 350 kA DC feed to the reduction cells. It is considered the most advanced, efficient and environmently benign aluminium smelting process in the world.

ABB’s scope of supply for the project included five giant 113 kA/1650 V DC rectifier transformers, otherwise known as ‘rectiformers’ (the most powerful ever built, each one weighing over 300 tonnes), five AC 800PEC high speed control and protection units for the rectifiers, a level-2 SCADA (supervisory control and data acquisition) system for the rectifier station and a gas-insulated (GIS) substation, and the revolutionary fibre optic current sensor (FOCS) technology.

Substation and power plant

The substation for Sohar’s potline consists of five conversion units fed via 220 kV GIS switchgear. The use of gas-insulated switchgear is common practice for aluminium smelters because of a number of factors, with reliability and availability being the most important. Two 12 km power lines connect the smelter to a new dedicated combined-cycle gas turbine (CCGT) power plant located within the Sohar harbour area, and will deliver around 650 MW to the smelter on a continuous basis. The power plant in turn is connected via two underground cables to the local grid.

Rectifier transformers: the key to the process

The rectifier transformers, comprising transformers combined with high current diode rectifiers, are the critical components in the smelter potline. Operating at an ambient temperature of 55 °C, they convert the AC power supplied by the captive generation plant to DC power which is then passed through the molten electrolyte in the pots to form aluminium. If the rectifiers should fail, causing the potline to shut down, the molten aluminium will solidify within hours, at a cost of $100 million or more.

For many years, 1200 V DC was viewed as the voltage limit for a rectifier transformer. Then it increased to 1500 V DC and now Sohar has set a new level for smelters of 1650 V DC. Increasing current has not been a critical factor for some time, but the voltages were limited by the availability of suitable fuses. The Oman grid configuration made it a requirement that in addition ±10 per cent voltage variations was needed to be able to accommodate all possible operating conditions.

A panaromic view of the rectifier transformers installed at Sohar aluminium plant that convert the AC power from a captive CCGT power plant
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In view of these high voltages, ABB decided to conduct a Fuse-Semiconductor (diode) type test to ensure that all the components installed would be fit for intended purpose. The type tests, for the largest rectifier arrangement built to date (103 kA/111 kA at 1650 V DC) were performed in ABB’s underground high power test laboratory in Switzerland.

The type tests proved successful. Even with a significantly higher voltage of 1784 V AC (rms) – representing a DC voltage of 1900 V – the fuses selected passed the most strenuous maximal energy test. They only failed at 1810 V AC (rms).

The results of the type tests demonstrate that the selected fuses and semiconductors for the Sohar project were correctly rated.

The captive CCGT power plant located within the Sohar harbour area
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Since currents of 100 kA per single unit had already been used in other applications, the relays were not an issue for the rectifier. However, the transformer design was checked carefully to ensure the high electromagnetic fields, especially in respect of the tank wall heating, would not be a problem. The design review was carried out using special software to simulate and map the eddy currents within the transformer tank wall.

Having dealt with the technical issues relating to the rectifier transformers it was necessary to investigate and solve the problems presented in transporting such large units, with each one weighing over 300 tonnes. Every stage in the route was checked to ensure that no size restrictions would be encountered.

A schematic of the aluminium making process
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The first leg of the journey, from ABB’s power transformer factory in Bad Honnef, Germany, was by barge down the River Rhine. The units were then transhipped onto a ship equipped with its own heavy lifting gear to make the sea voyage to the port of Sohar.

FOCS current measurement technology

In aluminium smelters, current measurement is essential to control the potline current as a method of process control, as well as an indication of the production efficiency. So another key contributor to process reliability at Sohar is ABB’s FOCS technology. This development of a sensor first used in high voltage substations has revolutionized current measurements in large power consuming industrial applications. Alcan had previously tested FOCS at a smelter in France and was impressed with the results.

Fibre optic technology is proven and well established, but ABB’s innovation has been to apply this technology to a current sensing device, in place of the existing Hall effect solutions. The results are simplified installation and commissioning, increased accuracy and a digital signal for greatly enhanced data acquisition and management, and negligible power consumption.

Contained within a lightweight, modular housing, the sensing fibre of the FOCS can be placed in position without the need to open the bus bars. The signal is determined by the enclosed current, independent of any stray magnetic field distribution, and is not influenced by any currents outside the optical fibre loop – so the sensor can be placed anywhere along the bus bar without fear of interference. On-site recalibration after installation is also unnecessary, reducing installation time from up to two weeks for a large Hall effect system to just half a day for FOCS.

Once in position, the FOCS system utilizes the Faraday effect to measure current, with a simple loop of optical fibre around the bus bar, in place of the sophisticated sensor head of conventional transducers.

The Faraday effect is the phenomenon of polarized light waves, in a medium such as glass, travelling at different speeds if a magnetic field is applied along the propagation direction. As a result the waves accumulate a phase difference.

In the FOCS system, right and left circularly polarized light waves travel through the coil of the sensing fibre, are reflected (and their polarization direction swapped) at the end of the fibre, and then retrace their optical path to the sensor electronics. Because the waves travel at different speeds through the sensing fibre if a DC current is flowing, they accumulate a phase difference. This difference is proportional to the line integral of the magnetic field along the sensing fibre, and is therefore a direct and highly accurate measure for the current.

The FOCS electronics contain the light source, optical phase detection circuit and digital signal processor that converts the optical phase difference into a digital signal, which can then easily be transmitted wherever it is required for analysis.

The FOCS sensing head weighs under 15 kg, while the sensor electronics weigh 5 kg and measure just 150 mm x 100 mm x 450 mm. In contrast, the sensor head for a conventional Hall effect system can weigh up to 1800 kg, depending on size. There is also an external electronic panel, which can weigh up to 320 kg and can be up to 1.4 m x 1.6 m.

versatile Control system

An enhanced level 2 SCADA system was implemented for the rectifier station and 220 kV GIS switchgear to provide the capability to handle all possible operational scenarios, and taking into special consideration the 55 °C ambient operating temperature in the Sohar plant.

The control system is based on ABB’s new electric power control system, the AC 800PEC that combines in a single modular system the capability for high speed control required by large rectifiers and the low speed control tasks usually carried out by separate PLC units.

The AC 800PEC controller is configured and programmed using Control Builder M, ABB’s well-established programming tool complying with IEC 61131-3, and MATLAB/ Simulink with Real-Time Workshop. The AC 800PEC interfaces with ABB’s I/O systems via the optical S800 Modulebus and with ABB’s communication modules via Communication Expansion Bus (CEX). It also supports third party AnyBus-S fieldbus modules.

Ground breaking project

A complex integrated power solution with a broad scope covering rectifier transformers, current sensors and control systems has been successfully implemented at Sohar to contribute to the on-time start-up of the world’s largest aluminium potline.

A second potline of the same size and capacity is also planned for construction in the near future. The larger smelters that will surely follow at other sites worldwide will no doubt benefit from Sohar’s ground breaking experience.