Research On Three-phase Four-leg High Frequency Link Matrix Rectifier And Modulation Strategy

As a universal converter,the high frequency and electrical isolation can be realized by the three-phase high frequency link matrix rectifier which has the advantages of few conversion stages,simple control,compact structure and high efficiency.The three-phase voltage type high frequency link matrix rectifier has a three-phase grid voltage imbalance condition during operation.At this time,the operation performance of the three-phase voltage type rectifier is degraded due to the occurrence of non-characteristic harmonics,current imbalance,and increased loss.Therefore,it is necessary to study the topology and control of three-phase high frequency link matrix rectifier under grid imbalance conditions.Firstly,in order to improve the applicability of the three-phase high frequency link matrix rectifier under the unbalanced grid conditions,a three-phase four-leg high frequency link matrix rectifier topology is proposed.It is obtained by introducing the fourth leg on the basis of the voltage source three-phase three-leg high frequency link matrix rectifier.It is designed to provide a zero-sequence current path to the input unbalanced current,thereby improving its ability to be applied to an unbalanced grid conditions.Secondly,in order to improve the utilization of DC voltage and reduce the input current harmonics,a digital implementation of a de-re-coupling Saddle Pulse Width Modulation(SAPWM)method for bidirectional switches is proposed,which algorithm is simple and flexible.Not only dose it have a low input current harmonic content,high voltage utilization,but also a stable DC voltage and maintain a high power factor when operating under balanced or grid voltage one-phase drop conditions.In addition,the electromagnetic interference of the system is analyzed,and it is verified that the digital implementation of the coupled SAPWM modulation strategy will not have a greater impact on the electromagnetic interfrence caused by power frequency interference and carrier frequency interference.Thirdly,in order to demonstrate that the three-phase four-leg high frequency linkmatrix rectifier can be decomposed into three independent single-phase high frequency link matrix rectifiers,a new idea of analyzing the voltage waveform across the AC-side filter inductor is proposed.The idea is simple and clear,and intuitively confirms the decoupling characteristics of the three-phase four-leg high frequency link matrix rectifier.Then,in order to make the three-phase four-leg high frequency link matrix rectifier get better control performance,the double closed-loop control of the three-phase four-leg high frequency link matrix rectifier is analyzed,and the voltage outer loop and current inner loop controller are designed.In addition,since the single-cycle control can realize the decoupling control of the three-phase PWM rectifier,the de-re-coupling idea and saddle wave pulse width modulation are introduced into the single-cycle control,and the feasibility of this method is verified by simulation.Finally,according to the above theoretical analysis,the hardware circuit and software circuit and software system are designed,and the experimental platform of three-phase four-leg high frequency link matrix rectifier is built.The test is debugged based on DSP and CPLD chip.Experiments are carried out under the conditions of grid voltage balance and one-phase drop.The experimental results demonstrate the feasibility and effectiveness of the proposed topology and theoretical analysis.