Research On Control Strategy Of Marine Intermediate Voltage Three-lever Converter

In recent years, many countries around the world are striving to develop the marineelectric propulsion technology. According to incomplete statistics, the proportion ofthe newly-built ships which adopted electric propulsion system is constantly growing.However, problems of high-power marine equipment such as reactive power loss andharmonic pollution are increasingly prominent. In this context, the development ofhigh-voltage and large-capacity three-level inverter technology has been promoted aswell as its application in electric propulsion system. This technology can effectivelyimprove the power factor and reduce the harmonic pollution. In addition, it is alsooutstanding in energy conservation, bi-direction energy transformation andcontrollability. Thus, it has been widely used in high-voltage and large-capacity ACdrive system and other industrial control fields.The main object of this study is the variable-frequency speed adjusting system inmarine electric propulsion system, which employs dual PWM three-level invertertechnology to control frequency and speed. The first step is to study the componentsof three-level inverter respectively including rectifier and inverter. Next step, the twoparts are reseached as a whole and the power flow is considered to analyze the controlstrategy. The main work of the research is summarized as follows:Firstly, the working principle of the three-level Space Vector Pulse WidthModulation (SVPWM) algorithm is presented and the implementation process isanalyzed. Furthermore, the influence factor of three-level neutral point potentialfluctuation is analyzed and the algorithm is improved.Secondly, the three-level voltage type rectifier with main circuit topology ofdiode clamped type is analyzed, and the working principle of the rectifying andinverting is introduced. The switching function method is adopted to describe themathematical model of the three-level rectifier under different coordinate system. Thedirect power control principle is analyzed, which adopts virtual flux linkage controlmethod without using voltage sensor. The switching frequency in the rectifier is fixed effectively by SVPWM control technology, so the control method is called fixedfrequency VF-DPC control strategy. The simulation system is established inMATLAB/Simulink and some simulation experiments are designed to validate thecontrol strategy.Thirdly, the physical structure of asynchronous motor and its mathematicalmodel expression in different coordinate system are analyzed. The vector controlmethod can achieve asynchronous motor speed control with a fine accuracy like DCmotor. The asynchronous motor control system is based on rotor field oriented controlmethod. The simulation model with SVPWM control strategy is built inMATLAB/Simulink and four-quadrant operation simulation experiments aredesigned.Fourthly, on the premise of the analysis above, the energy delay responseproblem in dual PWM three-level inverter system is analyzed. The coordinatedcontrol strategy is put forward, which adopts load power’s feed-forward method tofulfill coordination control between rectifier and inverter side to realize theminimization of DC side’s capacitor capacity, to make the asynchronous motorchange fast and smoothly between motor and regenerative braking conditions, toimprove system reliability and dynamic response. The date analysis is made aftercorresponding simulation experiments.