一种适用于新能源中压直流汇集的无环流零电流软开关三电平谐振式复合全桥变换器

A Novel Circulating Current Free Zero Current Switching Three-Level Resonant Composite Full Bridge Converter for New Energy Medium Voltage DC Collection System

直流变换器是中压直流(MVDC)汇集系统中的关键部分,起着升压、隔离、电能传输等功能。相比传统的中压交流汇集方案中体积巨大的变压器,中压直流方案中的变换器由于采用中高频控制可大幅减少装置体积。为适应新能源发电端口电压越来越高的发展趋势,该文提出一种无环流零电流软开关(ZCS)三电平复合全桥变换器,其由一个半桥三电平电路和全桥电路共同复用2个开关管而构成,从而所有开关管的电压应力只有输入电压一半,不仅有利于该变换器应用于高输入电压场合,同时还保留所有开关管的软开关特性且消除一次侧环流。为进一步降低所有开关管的峰值电流和辅助开关管的关断电流,又引入了LC串联谐振技术,可大幅降低辅助开关管的关断电流,从而使其开关损耗得到进一步降低。该文对变换器中关键参数进行分析设计,通过PLECS进行仿真,并制作了一台150 V/750 V/1 kW的样机进行了实验验证。

DC converter is a key part of a medium-voltage DC(MVDC)convergence system.Its role is to convert DC power into load-required voltage or current-controllable DC power,which plays a boost,isolation,power transmission,and other functions.The research on the flow converter has always been a hot spot in domestic and foreign power electronics technology.Compared with the huge transformer in the traditional medium-voltage AC convergence scheme,the converter in the medium-voltage DC scheme can significantly reduce the size of the device due to medium and high-frequency control.With the increasing output port voltage and capacity of photovoltaic and wind power,higher requirements are placed on bridge converters with the ability to adapt to high input voltages.In order to adapt to the development trend of new energy power generation with higher and higher port voltage,this paper proposes a circulating current-free ZCS(zero-current-switching)three-level composite full-bridge converter.The converter consists of a half-bridge three-level circuit and a full-bridge circuit multiplexed with two switching tubes.The three-level circuit with a resonant capacitor will transmit 90%or more of the total power.In contrast,the auxiliary full-bridge circuit transmits the remaining power so that the voltage stress on all switching tubes is only half of the input voltage.This structure facilitates the converter to high input voltages,preserves the soft-switching characteristics of all switching tubes,and eliminates the primary-side loop current.However,since the current waveform of the converter is triangular,the peak current is high under the same transmission power,and the turnoff current of two multiplexed switch tubes is the maximum,resulting in large shutdown losses.Therefore,LC series resonance technology is introduced in the half-bridge three-level structure unit,forming LC series resonance to transmit energy to the secondary side of the transformer.With the same electrical parameters as the converter ports,the proposed converter can improve the triangular current waveform to a sinusoidal waveform.Accordingly,the peak currents of all the switching devices and the turn-off currents of auxiliary switching tubes can be reduced,which further decreases their switching losses.Through theoretical analysis,when the input voltage Vin and output voltage Vo are known,the transformer turns ratio Ni determines the power of the two transformers.Moreover,the larger N,is,the larger the proportion of the whole power transferred by the main circuit Ptr1 is,and the smaller the power transferred by the auxiliary transformer Ptr2 is.Therefore,for the main transformer Ptr1 to transmit the majority of power,Ni should be taken large.Decreasing the blocking capacitor Ch is beneficial for reducing conduction loss but will increase its voltage peak.Therefore,the selection of Cb should be considered in a compromise.According to the requirement that the resonant frequency of the converter is greater than the switching frequency,the upper limit of the resonant capacitor value can be obtained.At the same time,the lower limit of the resonant capacitor value can be determined based on the rated power of the converter.This paper analyzes and designs the key parameters of the converter,conducts simulation verification through Plecs,and produces a 150 V/750 V/1 kW prototype for experimental verification.