Research On Parallel-connected Multi-pulse Rectification Technology Based On Power Electronic Transformer

The parallel-connected multi-pulse rectifier with advantages of simple structure and high reliability has been widely used in high-power electrical energy conversion occasions.The traditional multi-pulse rectifier often uses the power-frequency transformer as the phase-shifting transformer,whose volume is bulky and power density is quite low.At the same time,the traditional multi-pulse rectifier has a single interface which can only provide a single voltage level dc output.Therefore,this paper takes a 12-pulse rectifier as the basic topology to study the 12-pulse rectifier based on power electronic transformer,including the topology of the power electronic transformer and its control method,the method of improving the symmetry of the input current,the method of improving the power conversion efficiency and the harmonic suppression method.These are of great significance for improving the comprehensive performances of multi-pulse rectifiers and expanding their applications.In order to improve the power density of the phase-shifting transformer and build a theoretical foundation for high frequency multi-pulse rectification technology,a 12-pulse rectifier based on power electronic transformer is studied.The working principle of the power electronic phase-shifting transformer is analyzed.According to the working principle,the control circuit of the power electronic phase-shifting transformer is designed.According to the Kirchhoff's law and the principle of Ampere-turns balance,the expression of the load voltage and input current is calculated based on the switching function method.According to the conduction conditions of the diodes,the working modes of the three-phase diode rectifier bridge are determined,and the "double-line commutation" is proposed.The simulation and experimental results show that the volume of the phase-shifting transformer is reduced by two-thirds and the power density is greatly increased after using the power electronic transformer.For improving the symmetry of the rectifier's input current and the efficiency of power conversion,the 12-pulse rectifier using a three-phase five-column phase-shifting transformer and Si C MOSFETs is studied.According to the magnetic circuit and winding structure of the three-phase five-core phase-shifting transformer,the electromagnetic model is established,which provides a the oretical basis for the manufacture of the phase-shifting transformer.The loss model of the 12-pulse rectifier switching devices and magnetic components is established.Taking the prototype using Si C MOSFETs and the three-phase five-column phase-shifting transformer as an example,the loss distribution of the rectifier is analyzed.The simulation and experimental results show that,after using the three-phase five-core phase shifting transformer and Si C MOSFETs,the balance of the input current is obviously improved,and the power conversion efficiency of the whole machine is about 94.9%,and the loss is small.To improve the harmonic suppression capability of the 12-pulse rectifier based on power electronic phase-shifting transformer,the charge-discharge RCD snubber circuit and the DC-side harmonic suppression method are studied.The working principle of the charge-discharge RCD snubber circuit is analyzed,and the parameter selection method of the snubber circuit is obtained.According to the working modes of the double-tap converter,the harmonic suppression mechanism is analyzed,and the input current after using the double-tap converter is derived.The single passive and dual passive harmonic suppression circuits are applied to the 12-pulse rectifier based on the power electronic transformer,and the corresponding simulation is carried out.The simulation and experimental results show that the charge-discharge RCD buffer circuit can effectively suppress the spike voltage;the double-tap converter,single passive and double passive harmonic suppression circuits can increase the number of steps of the input current and reduce the THD value of the input current.In order to realize the power factor correction and enrich the voltage level interface of the multi-pulse rectifier,the multi-pulse rectifier based on the three-stage power electronic transformer is studied.According to the proposed multi-pulse rectifier topology,the function of the power electronic transformer is analyzed,and the load voltage and the input current of the high-frequency phase-shifting transformer are calculated.For the boost converter,the current average control algorithm is designed.For the voltage-type SPWM inverter,the voltage instantaneous value feedback control algorithm is designed.The results indicate that a multi-pulse rectifier based on the three-stage power electronic transformer can achieve unity PFC,and can provide two DC interfaces and one AC interface.