Research On Key Technologies Of The Novel Medium/High Unidirectional Cascaded Voltage Converter

Cascaded H-bridge multilevel converters have been widely used in medium/high voltage variable frequency speed regulation system,photovoltaic grid connected system,flexible AC transmission system and so on.However,the cascaded H-bridge converter is composed by a lot of fully-controlled power switches,which reduces the system reliability and increases the implementation costs.In fact,bidirectional power flow is not required in some industrial applications,such as pump motors and wind turbine systems.Under such scenarios,the advantages of the unidirectional cascaded multilevel converter are more prominent.Compared with the bidirectional cascaded H-bridge converter,the unidirectional cascaded multilevel converter requires fewer fully-controlled power switches,which reduces the complexity of the driving circuit,improves the reliability and the overall efficiency of the system,and reduces the implementation costs.However,at present,the researches on the unidirectional cascaded multilevel converter are relatively rare compared with that of the bidirectional cascaded H-bridge converter in the worldwide.Therefore,this thesis studies the novel high-frequency isolated unidirectional cascaded multilevel converter.The novel cascaded multilevel converter consists of unidirectional cascaded rectifier,high-frequency isolated unidirectional DC-DC converter and cascaded H-bridge inverter.Considering that the cascaded H-bridge inverter has been discussed in many literatures,this thesis mainly focuses on the research of unidirectional cascaded rectifier and high-frequency isolated unidirectional DC-DC converter.The main contents of this thesis include:In order to mitigate the input current zero-crossing distortion of the unidirectional rectifier,taking the three-phase bridgeless rectifier as an example,the operating principle and the reasons that cause the input current zero-crossing distortion for the three-phase bridgeless rectifier are analyzed in detail.Besides,a versatile control method using the neutral-point voltage to compensate ac voltage for mitigating the input current zero-crossing distortion phenomenon is proposed for three-phase bridgeless rectifier.The proposed control method is mainly divided into two parts.One part is that all the three-phase input currents don’t flow into the current zero-crossing distortion zone,that is,there is no input current zero-crossing distortion phenomenon,under such scenarios,the traditional control method can be used to control the three-phase bridgeless rectifier;The other part is that one of the three-phase input currents flows into the zero-crossing distortion zone,that is,there is input current zero-crossing distortion phenomenon,under such scenarios,the neutral-point voltage is used to compensate the ac voltage to realize the accurate tracking of the input current.Using the proposed input current zero-crossing distortion mitigation method,the input current zero-crossing distortion of the bridgeless rectifier is mitigated and the reactive power compensation range of bridgeless rectifier is expanded.Based on this,this thesis quantifies the constraint relationships between the degree of the input current distortion compensation and input power factor regulating range.The control method and the analysis method are also applicable to other three-phase unidirectional rectifiers,such as three-phase VIENNA rectifier and three-phase diode-H bridge+Boost rectifier.In order to improve the power fluctuation,input current distortion and dc voltage imbalance problems of the three-phase cascaded bridgeless rectifier under unbalanced input voltages,considering the mathematical model of the cascaded bridgeless rectifier in the αβ-axis coordinate system has the advantages of less computation,without coupling and easy designing,based on the αβ-axis coordinate system,this thesis studies and deduces the mathematical model and the corresponding operating principle of the three-phase cascaded bridgeless rectifier under unbalanced input voltages.Based on this,a three-layer hybrid control method suitable for balanced and unbalanced input voltages is proposed.The proposed control method realizes the power fluctuation suppression for three-phase cascaded bridgeless rectifier based on the orthogonal complex power theory,the input current zero-crossing distortion mitigation by using the neutral-point voltage compensation,and the dc-link capacitor voltage balancing based on the zero-sequence voltage injection,which ensures that the three-phase cascaded bridgeless rectifier features small input and output power fluctuation,unity power factor and dc-link capacitor voltage balancing under unbalanced input voltages.In view of the fact that the frequency modulation method cannot realize the full output voltage regulation,and the pulse width modulation method has the disadvantages of large current stresses on the components and low efficiency for the half-bridge series resonant converter which is selected as the DC-DC isolation stage of the novel high-frequency isolated unidirectional cascaded multilevel converter,a fixed-on time modulation method suitable for the half-bridge series resonant converter is proposed.With the fixed-on time modulation method,the converter features full output voltage regulation,low current stresses,zero-voltage switching turn-on for transistors,zero-current switching turn-off for diodes and high efficiency.The fixed on-time modulation method is also applicable to different half-bridge series resonant converter with the series or parallel connection at the input or output side as the DC-DC isolation stage of the novel high frequency isolated unidirectional cascaded multilevel converter.In addition,the voltage and current stresses of the components of the series resonant tank on the primary side and the secondary side are compared by adopting the fixed-on time modulation method.Based on this,the optimal topology is given according to the different turns ratio ranges.According to the circuit topology discussed in this thesis,the effectiveness and the validity of the proposed input current zero-crossing distortion mitigation method for three-phase bridgeless rectifier,hybrid control method for three-phase cascaded bridgeless rectifier under unbalanced input voltages and the fixed on-time modulation method for half-bridge series resonant converter are conducted in Matlab/Simulink and PSIM environment.Based on this,three-phase cascaded bridgeless rectifier prototype and half-bridge series resonant converter prototype are built for experimental verification.Both simulation results and experimental results have demonstrated the correctness of the theoretical analysis and the superiority of the proposed control method for three-phase cascaded bridgeless rectifier and half-bridge series resonant converter.