Control of a Variable Speed Drive with a Multilevel Inverter for subsea applications
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- Institutt for elkraftteknikk 
This work deals with analyses of the control for a subsea drive with multilevel inverter. The use of those drives have become the new trend for using of pumps/compressors at minor oil and gas reservoirs located far away from existing platforms.It is developed a general model for analyzing a variable speed drive with a multilevel inverter with the objective to verify the response of two control methods for a specific application.The simulation model is used to identify the performance of both methods in terms of speed response, torque ripple and transient behavior.Simulation models for multilevel inverter, induction machine and both control techniques are developed. A multilevel inverter and an induction machine have been used as prototypes. The design of the controllers has shown that the whole performance of the two control schemes is comparable. Those models are developed using PSIM simulation tool.Most of the results are related to the performance of the speed, torque ripple and transients behavior of both control methods when a 3kW 460V squirrel cage motor is fed from a cascaded H-bridge inverter.The main difference between the two control methods can be noted in the sensitivity of the parameters and the torque ripple. The implementation of them demands accurate information on motor parameters. However, parameters such as rotor and stator resistances may vary during operating conditions due to the temperature. In that sense, modified direct torque control may have a better performance for practical implementations. However for applications where the estimation of the torque is very important, indirect field oriented control may have better results.