A Study of Deep Sea Mining Electrical Power System Topologies
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- Institutt for elkraftteknikk 
Deep sea mining (DSM) is an emerging technology. The ever-increasing world-wide need for minerals in growing markets and industries of developing countries has made DSM financially reasonable. As there are few known system concepts for DSM it is important to investigate the different possibilities of system design and operation of the future DSM production systems. This thesis has, by investigating R&D and system concepts that are under development, presented different solutions for the electrical distribution system of a DSM production system capable of operating in the Norwegian Sea. In this thesis is it assumed that the DSM production system will consist of three subsea mining machines (referred to as seafloor production tools (SPTs)) designed to excavate, process and collect ore at the seafloor. The electrical power system is estimated to operate at depths up to 3,500m, where the production system will mine seafloor massive sulfide deposits formed through hydrothermal activity. A positive displacement pump, powered by pressurized seawater from the topside installation, is used for all the proposed system topologies. Firstly, simplified models where made in order to investigate three different system topologies of powering the SPTs. In the first model the different components of the SPTs where to be powered individually, thus reducing the number of subsea components by placing the frequency converter of the components at the TS. The simulation proved that powering the SPT components individually would result in excessive power losses, thus ruling out this system topology. In addition the machines of the SPTs perform better when the converters are installed on board the SPTs. Two other system topologies where proposed. The first is powering each of the three SPTs individually from the topside installation. The second is having a combined power distribution to a subsea distribution station which distributes the power to the SPTs. Both these system topologies where deemed suitable for powering a DSM system. Therefore, four simulation models where made in order to investigate the possibilities of power distribution to a DSM production system. The simulation software PowerFactory is used. Two models utilizes AC distribution to the subsea components of the production system, and the two other models use DC distribution. The AC models have a distribution voltage of 6.6kV operating at 60Hz and the DC models operate at +/- 3kV. The voltage levels are chosen in order to obtain a realistic comparison between the two. The simulations performed in this thesis proves that the distribution losses will be lower when powering the subsea mining machines through a subsea distribution station compared to the individual power distribution. In addition will the losses be even lower if DC distribution is utilized. By reducing the power losses the production system will be less expensive to operate due to lower fuel consumption. Harmonic content will also be an issue for the proposed AC system topologies, and measures should be done in order to reduce the influence of such power quality pollution. Although the combined distribution to a subsea station will have the lowest power loss, this system will have a large investment cost as the number of subsea components are increased. Advanced analysis will therefor be necessary to evaluate which system topology should be used for powering a DSM power system. However, due to the large power requirement of possible DSM in the Norwegian Sea, a combined distribution to a subsea distribution station will most likely be recommended for DSM operations in the Norwegian Sea.