Innovative control techniques of a modular multilevel converter for improving the efficiency of electrified ships

This thesis work reports the results of three years of activities carried out during the XXXVI cycle of the doctoral course in Electrical and Information Engineering at Bari Polytechnic University. The PhD is funded by Isotta Fraschini Motori S.p.A., a company of Fincantieri S.p.A. The title of the PhD research activity program is ”Hybrid-electric propulsion systems for naval applications”. Indeed, Isotta Fraschini Motori S.p.A. has the aim to develop a naval propulsion system based on innovative technologies to minimize operating costs, fuel consumption, and greenhouse gas emissions into the atmosphere as required by the objectives set by the European Union. Therefore, during the three years of the Ph.D. program, shortterm research activities with a high level of Technology Readiness Level ( TRL) are carried out for company work, and research activities with a low TRL have been developed at the Power Electronics laboratory of the Politecnico di Bari. The activities carried out at the company during the three years of the Ph.D. program include design optimization of a new diesel generator for Medium Voltage Direct Current marine applications, preliminary design of a shipboard hybrid propulsion system, and sizing of a shipboard power generation system consisting of batteries and fuel cells. The latter project is carried out abroad at the Norwegian University of Science and Technology NTNU in Aalesund in collaboration with Vard Electro S.r.l, a company of Fincantieri S.p.A. The main objective of this work is to study the control techniques of Modular Multilevel Converter in DC Medium Voltage for marine applications. Modular Multilevel Converters are becoming increasingly attractive for many high-voltage and high-power applications. However, due to their topology and operation, they present technical challenges in control system implementation, such as balancing submodule capacitor voltages and suppressing circulating currents. The circulating current introduces additional power losses, increases the current stress on power devices, and reduces their lifetime. Specifically, in this thesis work, a novel control technique is proposed for eliminating circulating current by injecting a low-frequency alternating signal into a single submodule of each converter arm, to also achieve a reduction in voltage ripple across capacitors, benefiting their lifetime. The proposed control technique will be described in detail both through the analytical model of the circular interactions governing the operation of the MMC and through simulations and experimental tests on the set-up present at the Power Electronics Laboratory of the Politecnico di Bari. The entire prototype of the threephase modular multilevel converter was designed, implemented, and tested during the research activity carried out at the university. The analytical steady-state model of the Modular Multilevel Converter is intended to describe mathematically the expressions of the harmonics of the electrical quantities present in the converter so that the effects of the injection signal on converter performance can be investigated and guidelines for calibration of the proposed control can be defined. Experimental results performed on a seven-level converter are presented to validate the proposed technique and compare its performance over techniques already present in the literature. The proposed control can suppress the circulating current flowing in the converter and reduce the voltage ripple, improving the efficiency of the converter and reducing the overall power losses. Eventually, the same technique is extended to pursue another control objective, namely, balancing the voltages of the submodule capacitors. The proposed technique has been validated through experimental results showing good performance.