Research On High Performance Multi-pulse Rectifier Based On Power Electronic Transformer

Although the 12-pulse rectifier can eliminate the 5th and 7th harmonics in the input current,the input current still contains a large number of harmonics and the THD of the input current is still high.Further subdivision of the output voltage phase of the transformer can increase the pulse number of the rectifier and thus effectively suppress current harmonics,but as the pulse number increases,the design and processing of the phase-shifting transformer becomes more difficult.In addition,the volume of phase shift transformers operating at low frequencies usually accounts for more than 70% of the total rectifier volume and is bulky,limiting its application in industrial applications requiring high power density.To this end,this paper integrates the power electronics based phase shift transformer with the dc-side pulse multiplication technology and focuses on the dc-side pulse multiplication principle and the modulation theory of the power electronics phase-shift transformer.This paper first introduces the operating modes of existing Tapped inter-phase converter,analyzes the transformer ratios when the system input current THD and load voltage pulsations are minimized,and illustrates the effect of T-IPC on rectifier pulsation number multiplication.The analysis results show that the square wave circulating current generated by T-IPC on the DC side completely cancels the n=12h ±1(h is an odd number)second harmonic in the original input current.However,the diodes connected in series with the load circuit taps produce serious additional losses.Thus,the improved tap inter-phase reactor,IT-IPC is proposed and its operating mode and optimum ratio are analyzed.It is demonstrated that the current amplitude flowing through the tapped diode is only 3.4% of the load current at the optimum ratio,and the conduction loss of the tapped diode is reduced by two orders of magnitude.The pulse wave multiplication mechanism of IT-IPC is clarified,and the analysis results show that although both IT-IPC and T-IPC can achieve pulse wave multiplication of rectifiers,they have different effects on some subspecific subharmonic modulations.In order to further reduce the size and weight of the phase shift transformer and improve the power density of the rectifier,a multi-pulse rectifier based on power electronic phase shift transformer is proposed in this paper.By introducing an AC/AC conversion link between the grid input power and the medium-frequency phase shifting transformer,the industrial-frequency input voltage is modulated into medium-frequency AC power,so that the core of the transformer works in the medium-frequency state,effectively reducing the volume and weight of the phase shifting transformer.This paper analyzes the modulation effect of AC/AC converter,introduces the three-phase rectifier bridge commutation process under medium frequency conditions,and calculates the output current-voltage expression of the DC side of the rectifier bridge.The AC/AC converter does not change the rectifier bridge output current-voltage characteristics,based on which the IT-IPC is combined with a power electronic phase shift transformer to form a 24-pulse rectifier with high power density and built a test prototype with an output of 1.3k W to verify the correctness of the theoretical analysis and to give the theoretical analysis and experimental results in case of system faults(input phase break and open diode fault in IT-IPC).The experimental results show that when the phase shift transformer works at500 Hz,the size of the transformer is reduced to about one-third of the original size;the input current harmonics are effectively suppressed,and the input current THD is reduced from 11.4% to 4.2%;the IT-IPC also has a certain harmonic suppression effect when an open circuit fault occurs in the diode.