Research On Three-Phase Medium-Frequency High Power Factor Rectifier For Aviation Applications

As the import secondary electric power conversion equipments, the aviation static converters have been widely researched by scholars and experts all over the world with the development of more/all electrical aircraft technology. As one kind of aviation static converters, the aviation three-phase rectifier is the key equipment which is used to convert AC power to DC power. Using the controllable devices, not only the output DC voltage can be regulated, but the input power factor correction can be also realized with the developing high performance three-phase PWM(Pulse Width Modulation) rectifier. So, it is beneficial to reduce harmonics pollution in the aviation electric power system. In this paper, the systematic study is carried out on the three-phase three-level VIENNA rectifier based on one-cycle controlled(OCC). The thesis mainly focuses on the system characteristics and control for the VIENNA rectifier. In addition, the fault diagnostics of power MOSFETs which is as the all-control devices in the rectifier has been done a tentative exploration to improve the reliability of the rectifier.Firstly, the technical features of aviation three-phase rectifier and its main control scheme are presented in the paper. Based above analysis, the three-phase three-level VIENNA rectifier with its control strategies is discussed in detail, and the research hotspot and the latest progress of VIENNA rectifier based on OCC is also analyzed. One of the most important methods to improve the reliability of the aviation static converter is to realize the fault diagnostics on the key components of converter. So, the research status of fault diagnostics technology of power electronics circuit is presented, especially in the high failure rate aspect, such as open circuit fault diagnostics on power switch.The method for improving the utilization of DC bus voltage in three-phase VIENNA rectifier based on OCC is studied. In the conventional OCC strategy, the input currents are as the modulation signals, and the PWM signals which control the switch are generated by comparing them with the triangular carriers. The input and output characteristics of rectifier are analyzed under this modulation strategy. The result shows that the output DC voltage of conventional OCC based three-phase VIENNA rectifier should be greater than twice the peak of the AC input voltage for avoiding to produce the input current distortion, which is caused by the over modulation in the control system with low utilization rate of DC bus voltage. The SAPWM(Saddle Pulse Width Modulation) that can be obtained by injecting third harmonic current into three-phase input currents is proposed to introduce in the conventional OCC strategy on this paper. The realization of extract of third harmonic current is discussed in detail, and the input and output characteristic of rectifier based on proposed OCC strategy are also analyzed. Finally, the validity of the theoretical analysis and feasibility of the proposed OCC strategy are verified by system simulation and experimental results based on VIENNA prototype.The fluctuation mechanism of neutral-point-potential(NPP) is researched in the paper. The ideal zero-sequence exists in the bridge switch function of rectifier is deduced for eliminated the ac fluctuation in the NPP. Based on the SAPWM modulation in the OCC strategy, the influence of third harmonic current injection to NPP is also analyzed. Moreover, the neutral-point voltage loop is added in the OCC scheme, and the neutral-point voltage controller is employed for controlling the unbalance dc voltage in the NPP which caused by the non-uniformity of parameters of components. The small signal models of rectifier including the neutral-point voltage loop are built, and the design of main power circuit parameters and control parameters is given. The theoretical analysis is verified by system simulation and experimental results.In the conditions of medium-frequency of input source and medium / high power occasion, the phase angle of input currents of rectifier would lag the corresponding input phase voltage under conventional OCC strategy, which results in the decline of the input power factor. Based on the analysis on the conventional OCC strategy, the modified OCC strategy which considered the voltage drop across the input inductor is proposed. The control equation is deduced and the realized methods are also given. The system stability analysis based modified OCC strategy is present. The phase angles of modulation signals can be adjusted automatically according to the change of the load power. The phase displacement angle between the input current and corresponding line voltage can be reduced, and the rectifier’s total power factor can be also improved under the different load power conditions. The viability of the proposed scheme has been ascertained by performing extensive experimental studies.The operation mechanism of three-phase VIENNA rectifier is analyzed in conditions of unbalanced input voltages. The results show that the output DC voltage of rectifier would contain low frequency ripple whose frequency is twice that of the rectifier’s input source voltage. This ripple signal would cause the input current contains a large amplitude of three-order harmonic component under the OCC control strategy and the THD(Total Harmonic Distortion) of input currents would be increased. In this case, the resonant controller is proposed to introduce in the main voltage loop of conventional OCC control strategy for restraining the low frequency ripple whose frequency is twice of the rectifier’s input frequency, and the influence of unbalance input voltage on the input current can be also reduced obviously. The design of resonant controller and the system stability after introducing the resonant controller are analyzed. Finally, the validity of the theoretical analysis and feasibility of the proposed OCC strategy are verified by system simulation and experimental resultsFor improving the reliability of rectifier system, a preliminary study of the power devices structural fault diagnostics has been done and the results are presented in the last chapter of paper. The failure mode of power device and the influence of open / short circuit fault of power devices on the topology of VIENNA rectifier are analyzed. The failure modes of rectifier are given, and the degree of hazard of power devices is determined. In order to prevent more serious damage to rectifier which is caused by the high degree hazard of power devices, the self-fault diagnostics of power devices are proposed before rectifier PWM operation. In addition, based on the analysis about the failure characteristics of rectifier, the fault classification of power switches based on neutral net work is shown in the paper. The fault modes of rectifier can be distinguished and fault diagnostics of power devices in one phase bridge of rectifier can be also realized.The theory and technology basis for the control strategy and design optimization of the three-phase VIENNA rectifier are provided in this dissertation. In addition, the work of this paper plays a positive role in promoting the application of PWM rectifiers in field of aviation.