Subsea motor drives with long subsea cable
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- Institutt for elkraftteknikk 
Oil and gas are extracted from the fields by the pumps, which are driven by the electrical motors. With the tendency to increase the distance between the platform and the subsea field, where the motor is installed, the problem of machine start-up becomes more and more urgent. Two biggest problems during the motor start-up are the need to limit the maximum currents through frequency converter and to avoid transformer saturation at the same time. In special cases, due to the increased impedance of the longer cables, there will be no possibility to start-up the motor at all. The oversizing of the system components is required in order to withstand the high stresses at starting.Induction machines were the main choice for the subsea applications since the beginning of the subsea era, but recently they become replaced by the permanent-magnet synchronous machine. Due to their inherited advantages, the use of permanent-magnet motors allows to achieve lower losses and higher efficiency of the system. Both types of machines are analyzed in this master thesis.The system for the power supply of the electric motor is designed and simulated in Matlab/Simulink. Two different topologies are used in simulations: topology with one step-up transformer and topology with an additional subsea transformer. The conventional method of motor start-up is tested in order to show the challenges that can be encountered.Both IM and PM motors are able to start with the designed system. The results show the superior performance of the systems with PM motor in terms of the transformer flux and system currents. The extension of the step out distance brings corresponding increase in the transformer flux, which can reach magnitude of 3 pu for the system with PM machine and 50km cable.A transformer bypass is a new starting method, suggested by SmartMotor AS. It should allow to fully eliminate transformer saturation problem, thus making the motor starting easier. The simulation results indicate that system with implemented transformer bypass can be used for starting of the motors. The usage of bypass in one transformer topology allows to reduce the transformer fluxes to the rated values and avoid oversizing. Additional challenges arise during implementation of the bypass into the system with subsea transformer. The impossibility of bypassing that transformer and necessity of early reconnection results in the higher than nominal fluxes in transformer. The oversizing of the core is thus still required, but at a lower degree in comparison with conventional starting methods for the same system.Keywords: PM, IM, saturation, transformer bypass, frequency converter.