Research On Efficiency Optimization Strategy Of Mixed Frequency Conversion Power Electronic Transformer

Mixed Frequency Conversion Power Electronic Transformer(MF-PET)has significant advantages over traditional cascaded H-bridge(CHB)or modular multilevel converter(MMC)type PET in terms of the number of switching devices,capacitors,and high frequency transformers(HFTs)used,It has great application potential in AC/DC hybrid distribution networks.However,under unipolar PWM modulation,MF-PET has problems such as difficulty in achieving soft switching at low power,high harmonic content on the grid side,high on-state loss,and difficulty in achieving efficient operation in the full power range.This paper studies the zero voltage switching(ZVS)operation mode of MF-PET based on carrier phase shift modulation;Based on the ZVS operating mode,the related current stress optimization strategy was studied.Based on this,strategies for using ZVS optimization,current stress optimization based on ZVS,and current stress optimization considering backflow power in different power bands are proposed.The specific research contents are as follows:Firstly,the soft switching mode of MF-PET based on carrier phase shift modulation is studied,and a high-frequency equivalent model that can achieve all power switches ZVS on the high and low voltage sides of MF-PET is established.The calculation method of high-frequency current and total inductance in the high-frequency equivalent topology is derived.The operating mode of the high-frequency equivalent topology in the soft switching mode is analyzed,and the expression of the inflection point current is given.Based on the inflection point current,the constraint conditions for switching on the switching device ZVS are obtained,and the operating range of the soft switch is analyzed.Simulation analysis shows that compared to the traditional unipolar PWM soft switching mode,using the soft switching mode of MF-PET based on carrier phase shift modulation proposed in this paper can further reduce the harmonic content on the AC power grid side of MF-PET,expand the soft switching range,and have better dynamic response characteristics.Secondly,a current stress optimization strategy based on ZVS is proposed to suppress the current stress in the MF-PET soft switching mode,which is conducive to further improving the efficiency of PET.Under this optimization strategy,the transmission power is proportional to the control amount,reducing the control complexity of the equivalent topology.The operating modes of the high-frequency equivalent topology are divided,and the expressions of inflection point current,current stress,transmission power,and switch ZVS constraints in the new mode are obtained;Taking the current stress in each mode as the objective function,a Lagrange function is constructed to solve the optimal control variable combination for minimizing the current stress.The simulation results show that under the optimal control variable combination,the proposed optimization strategy can achieve minimum current stress within the soft switching range,and can achieve ZVS compared to the unipolar PWM soft switching mode,while effectively reducing the magnitude of current stress in the soft switching mode.Then,considering the current stress and backflow power issues in the MF-PET soft switching mode,a current stress optimization strategy considering backflow power is proposed.According to the current stress and backflow power expressions of the optimization strategy and the current stress optimization strategy based on ZVS,the current stress ratio and backflow power ratio under the two current stress optimization strategies are obtained.It is proved that the current stress optimization strategy considering backflow power can not only suppress the current stress magnitude,but also significantly reduce the backflow power compared to the stress optimization strategy based on ZVS.The simulation results show that the current stress optimization strategy considering backflow power can reduce the backflow power of MF-PET,and even eliminate the backflow power under specific operating conditions.Finally,based on the proposed optimization strategy,which can achieve the power range of ZVS,a multi-objective optimization strategy is proposed to achieve dual optimization of current stress and backflow power while ensuring MF-PET soft switching,making MF-PET have high operating efficiency in the full power range.