Research On High Power Density DC-AC Converter And Its Parallel Control

In the aviation, airport and locomotive and other situations, the technical requirements for DC-AC medium frequency power supply is increasing, such as power level, power density, size and weight. In this paper, two 6kVA high power density DC-AC converters are mainly studied. Large capacity and redundancy are realized through parallel control technology. High power density and electrical isolation is achieved by adopting two-stage conversion structure.Firstly, this paper introduces some typical topology of DC-DC converter and DC-AC converter and analyzes their advantages and disadvantages. According to the project requirements, the pre-stage of high power density converter employs phase-shifted full-bridge ZVS converters with IPOS structure, while the output stage employs three-phase four-wire three-level T-Type inverter. The DC-DC stage is isolated by high-frequency transformer which greatly reduced the size and weight of the transformer and filter inductor. The soft-switching feature of the phase-shifted full-bridge increases the efficiency of the converter. Because of the three-level structure, the THD of output voltage is decreased and the electromagnetic interference is reduced. The size and weight of the output filter inductor is reduced as well.Secondly, the electrical and control parameters of the DC-DC stage and DC-AC stage are designed respectively. The pre-stage uses analog control system and each module is controlled independently. With appropriate compensation loop parameters, the system obtains good dynamic response, which decreases the capacity of the DC-link filter capacitor and maintains DC-link voltage constant with inverter load. The output stage employs digital control system and PR controller is adopted according to the features of the middle-frequency inverter. A voltage-current double closed-loop control strategy with load current feed forward is used to achieve good output performance.Finally, this paper introduced the parallel control strategy which is used in the parallel system consisting of two converters. With the advantages of no interconnect, high reliability and good redundancy, the droop control strategy is adopted in the parallel system. The paper analyzed the circulating current of two parallel inverter and droop control characteristics. By introducing the virtual impedance, the coupling of active and reactive power is reduced and the circulating current is suppressed. With proper design of the equivalent output impedance, the power sharing accuracy is improved. Software simulation and experimental results have verified that the proposed structure and control strategy are effective and feasible.