Control Strategies Of The Power Electronic Converters Based On MMC For Marine Shaft Generator

In recent years,with the outbreak of global energy crisis,the ecological environment has been getting bad.Carbon emissions and operation costs of medium-voltage large ships during ocean transportation have kept growing.The main engine of ship uses cheap heavy oil as fuel,and the marine shaft generator is driven by the main shaft of the main engine,which could make full use of the surplus power of the marine main engine.It can partially or even completely replace traditional diesel generator to supply power for whole ship.For the marine shaft generator,operation efficiency is higher,and it is more economical than the diesel generator using light oil as fuel.It could save global energy and reduce emission.Modular Multilevel Converter(MMC)has attracted widespread attention because MMC is highly modular and the voltage level is easily extensible since it had been proposed.Therefor,control strategies of the power electronic converters for marine shaft generator based on MMC study of the marine shaft electricity generation system is of practical significance to the global shipping industry.First of all,the paper theoretically derives the related mathematical models of synchronous generator speed control system and excitation voltage control system.For the modulation strategy,the carrier phase-shift modulation strategy is proposed to make the output harmonic distortion rate low and have a higher equivalent switching frequency.For the control strategy,during rectification,double closed-loop control strategy for the voltage outer loop and current inner loop is proposed for the stability of DC side voltage and AC side current.In the double closed-loop control strategy,the voltage outer loop uses conventional PI control strategy and the current inner loop uses deadbeat control strategy.Compared with the conventional PI strategy,the deadbeat strategy has better dynamics.Due to the deadbeat control strategy based on the mathematical model of system,it is sensitive for the system parameters.Via establishing and analyzing the deadbeat model of the voltage type rectifier,the inductance parameters that affect the accuracy of deadbeat control strategy can be effectively identified in real time.For the prediction error of deadbeat control strategy at a low switching frequency,a deadbeat control strategy based on repetitive control strategy is proposed.Compared with the conventional deadbeat strategy,it reduces current prediction error.In order to deal frequency fluctuations that may occur in marine shaft generator and ensure that controller is not affected,a frequency adaptive FIR repetitive prediction control strategy is proposed.Compared with the conventional FIR repetitive predictive control strategy,it reduces the influence of frequency fluctuations.For the circulating current harmonic of MMC,an improved quasi-proportional resonant control strategy is proposed to achieve the purpose of suppressing circulating current.Compared with the conventional quasi-proportional resonance strategy,it has better circulation suppression effect.For the capacitor voltage stability of the sub-modules of each phase,a hierarchical control strategy is proposed.Compared with the equalizing strategy without phase balance,the sub-module equalizing effect is better.During the inverting,double closed-loop control strategy for the voltage outer loop and the current inner loop based on feed-forward decoupling strategy is adopted to output stable three-phase AC current and AC voltage in order to make up complete AC-DC-AC MMC system.The paper adopts seven-level topology which takes AC-DC-AC converter as the research object based on MMC and marine shaft generator as the application background.Then,It uses Matlab/Simulink software as a platform to analyze and explore many key control strategies of the power electronic converters for marine shaft generator based on MMC.Propose improved methods to solve problems and verify validity and scientific nature of the proposed methods.