Amin Sultan, C. V. Kamath, Aluminium Bahrain (Alba), Bahrain
Aluminium Bahrain (Alba) was officially commissioned in May 1971 as a 120 000 t/year smelter. Today the company produces more than 500 000 t/year, having expanded in 1981, 1990, 1992 and 1997 – making it one of the largest single-site producers of aluminium in the world.
Aluminium production consumes large amounts of power. For the aluminium smelting process, AC power generated by the power station is converted to DC power by the use of diode rectifiers. The smelter’s power requirement is 840 MW and the energy consumed by the auxiliary power system is 54 MW. Power is generated from three power stations and up until 2003, excess power from Alba was exported to the grid.
Power station 1 was built in 1971 and consists of 19 Frame 5 gas turbines. Over time, however, the power system network at Alba was becoming unreliable due to frequent and spurious tripping of its Frame 5 machines at power station 1. Most of these trips were attributed to the failure of the control system components installed on the machines. Availability of the gas turbines is of utmost importance at Alba.
However, although these machines had their generators rewound to improve overall reliability, ultimately reliability was limited by an unreliable control system.
Figure 1. Control system layout
The old control system for these machines (GE SpeedTronic Mark I control system) as shown in Figures 1 and 2 was based on fully analogue control technology – technology which the OEM supplier was unable to support any longer. The systems used hardwired logic relays for protection and elaborate transistorised electronic circuits to perform all control functionality.
Being an old and obsolete system, there were numerous problems e.g. availability of spares, higher downtime from ageing and failure of components. Attempts to repair these cards incurred significant expenses and had limited success due to lack of availability of old components. Further, being a hardwired system, troubleshooting could not be done effectively. The lack of corrective diagnostics such as system alarms, trends and events recording resulted in a high downtime of these machines. All these problems had a direct bearing on plant efficiency and profitability.
Faced with this serious problem, Alba decided to upgrade and retrofit the control system. The technical and financial issues, which needed to be decided prior to implementation of the project, had great impact on the success or failure of the project. The areas evaluated included cost-effectiveness, available spares inventory, availability of integrated configuration tools, extensive diagnostic system, and reduced installation and commissioning time as these machines are connected to the National Grid of Bahrain.
Figure 2. Analogue control panel arrangement
Alba was faced with a choice of selecting between the latest system from an OEM or opting for a system integrator. The OEM offered a network-based system while the solution from the system integrator was a PLC based system. The bid of the system integrator was significantly lower than the OEM. Further, while Alba had several other installations of PLC systems, the latest OEM system would have been its first experience with proprietary technology. In-house technical expertise and lower spares inventory cost were also in favour of the system integrator. Accordingly, Alba’s technical team opted for the system integrator solution.
Figure 3. Redundant PLC configuration for protection
The technical issues to be resolved during the engineering stage included:
- Time scheduling for minimum downtime during retrofit
- Redundancy features
- Reliability and safety of machines
- Interfacing to existing specialised field instruments
- Reporting requirement for easy troubleshooting and diagnostics
- Using this downtime opportunity to replace other ageing components such as cables, generator control components and field sensors.
Time scheduling was resolved by executing the retrofit during the winter, as the power requirement during that time is comparatively low. Also machines were taken off-line in a staggered fashion.
Redundancy on control was deferred, as it was not cost effective as per Alba’s operating philosophy and was specified only for protection signals and power supplies. Redundancy for protection signals was achieved using two different PLC’s checking each other through a watchdog signal.
Further, the exhaust thermocouples were split into two parts with ten thermocouples reporting to the main PLC for control and four reporting to the redundant PLC for trip. The redundancy of the servo valve was taken care of by energizing the two coils of the servo from two different servo controllers. Installing necessary bulk power supplies energized from two different sources and driving the system components through a diode OR-ing circuit, introduced necessary power supply redundancy.
Reliability is achieved via hardwired trip relay logic for safe shutdown of the system. The speed probe configuration was also modified to improve reliability. Out of the existing four speed probes, three were used for speed control with 2003 logic while the fourth was added to trip logic.
Most of the specialised field devices were replaced with conventional transmitters. The other devices such as flame scanners, servo and vibration probes were interfaced using necessary adaptors and converters.
A panel mounted human machine interface (HMI) as shown in Figure 4 was introduced for the operator interface. Enhanced graphic displays and an elaborate alarming system were configured for easy operation. Advanced maintenance features such as historical trends with one-second time base and event logging were installed for ease of debugging and troubleshooting.
This downtime provided an excellent opportunity to replace all field cables from field to control panel and motor control centre since they were causing frequent ground faults. Additional cables were also laid to separate the paralleled common signals. The old trolley based manual synchronizing unit was replaced with a state-of-the-art panel mounted auto synchronizer.
Working extensively with the system integrator on the retrofit provided several eye openers for Alba. First and foremost the system integrator had to be provided with several technical details, as they were not aware of all the specifics of the existing control system. This led to increased workload on Alba’s technical staff.
Figure 4. A panel mounted human machine interface (HMI) was introduced for the operator interface
Secondly the system integrator generally uses third party products that can cause interfacing problems during commissioning. The interfacing problem of existing LVDT with the new servo controllers was a good example in this case. The system integrator also tends to put more emphasis on achieving low cost as opposed to technical excellence when selecting solutions in order to maximise profit.
Proper planning and co-ordination between vendor and various departments within Alba, however, resulted in an efficient installation and commissioning of the new system. Looking into the future, the generator rewinding and control system upgrade has increased the life expectancy of the gas turbines. The reliability of the new control system has also significantly improved the stability of the power station as a whole.