Suppression Of Second-order Harmonic Voltage For DC Capacitor In Power Electronic Transformer

With the breakthrough development of new power semiconductor switching devices such as Si C and Ga N,as well as the increase in the intelligent requirements of power distribution network equipment,power electronic transformers that replace traditional power frequency transformers to achieve voltage conversion and have rich power grid management capabilities are becoming more and more popular Among them,the threephase cascaded power electronic transformer is one of the current hot topologies.However,due to the structural characteristics of the three-phase separation of the input stage DC capacitors and the widespread existence of single-phase or three-phase unbalanced loads at the output stage,the DC capacitors of cascaded power electronic transformers inevitably have double-frequency voltage fluctuations,thereby reducing power The output power quality of electronic transformers endangers the safe and stable operation of the system.Aiming at the problem of double-frequency fluctuation voltage in DC capacitors in power electronic transformers,this paper proposes two power electronic transformer DC capacitors that combine input stage and isolation stage from the perspective of the double-frequency fluctuation power generated by the input stage and output stage.Suppression strategy of double frequency fluctuation voltage.First of all,this article analyzes the transmission path of DC power and fluctuating power in power electronic transformers in detail;summarizes the source of double frequency fluctuating power,and concludes that the essence of DC capacitor double frequency fluctuating voltage is present in power electronic transformers.The conclusion of the double frequency fluctuating power;the harm of the double frequency fluctuating voltage of the DC capacitor is clarified in detail,and the generation mechanism and harm of the higher-order fluctuating power and fluctuating voltage are promoted accordingly.Secondly,based on the characteristic that the sum of the three-phase power of the input stage is zero,the isolation stage DAB double-frequency fluctuation power feedforward control strategy is proposed,which transmits the double-frequency fluctuation power generated by the three-phase input stage to the low-voltage DC side to offset it.At the same time,the input stage three-time zero-sequence voltage injection method is used to offset the double frequency fluctuation power generated by the AC side fundamental voltage and current,reduce the demand for DAB capacity of the isolation stage DAB double frequency fluctuation power feedforward control,and jointly suppress the high Low-voltage DC capacitor double-frequency fluctuating voltage.Then,in view of the uneven absorption of fluctuating power by high and low voltage DC capacitors,which may cause excessive local DC fluctuation voltage,the isolation level DAB is proposed based on the design principle of equal double frequency voltage fluctuation rate of high and low voltage DC capacitors and reduction of DAB transfer capacity.The high and low voltage DC capacitor double frequency fluctuation voltage sharing control strategy,and the method of selecting the phase of the double frequency fluctuation power transmitted by the isolation stage DAB is given in detail,which realizes the redistribution of the double frequency fluctuation power in the high and low voltage DC capacitor.Finally,this paper builds a three-phase cascaded power electronic transformer model on matlab/simulink to verify the correctness of the control algorithm proposed in this paper.