Research On Three-phase Three-level T-type Mid-frequency Inverter And Its Parallel Control

As a part of the AC power supply system, mid-frequency inverter is widely used in aviation, marine, locomotives and other fields. Compared with the traditional two-level inverter circuit, multi-level technology can reduce the pressure of single device, reduce the switching losses of devices, cut down the size and weight of the output filter and improve power quality, which is a research hotspot of the inverter. The Parallel control technology of inverters can make power supply redundant and high-capacity, which is one of inverter’s development trends. Based on two6KVA115V/400Hz inverters, the waveform quality control and parallel technology for three-phase three-level T-type mid-frequency inverter are investigated in this thesis.Firstly, this paper makes a brief introduction of the three-phase inverter topology and the three-level topology. After analyzing the advantages and disadvantages of these topologies, this paper selects a scheme of three-phase three-level T-type inverter with two independent power sources according to the requirements of the project. On this basis, this paper analyzes the Electrical schematics and control models of the inverter. Considering the feature of the mid-frequency inverter, this paper selects the voltage and current double closed-loop instantaneous control strategy based on PR controller and designs the control system.Secondly, this paper analyzes some commonly used control technology of the inverter. By analyzing the model of PI controller based on the synchronous rotating coordinate system, the paper certifies the relationship with the PR controller in the frequency domain. Meanwhile the paper analyzes the control equivalent model of the PI controller based on the synchronous rotating coordinate system and the PR controller based on stationary coordinate system in the condition of the three-phase voltage imbalance. Then the paper describes the load current feedforward technology from the aspect of the output impedance and load stability, and explains its advantages and disadvantages by comparing with the conventional inductor current feedback technology. In order to deal with the output voltage distortion, this paper studies the low-order harmonics compensation strategy based on resonant controller and designs the compensation controller parameters.Finally, the paper introduces the basic parallel strategies of the inverter. A redundant master-slave paralleled control strategy based on average power control is chosen according to the requirements of the project. In the paper, the overall structure is analyzed and designed for the master-slave paralleled middle-frequency Inverters’ System. A composite transmission strategy is used on synchronizing signal and data signal to optimize interconnect system. Logic control is also added to realize redundant parallel. At the same time mathematical modeling and analysis of the master-slave paralleled system is done to illustrate the feasibility of designed parallel scheme. Resistive virtual impedance is used to enhance the parallel performance without reducing the output performance. The simulation and experimental results verify the feasibility and reliability of the design.