Research On The Double Star Uncontrolled Rectifier Based On Power Electronic Transformer

In order to improve the power density of the double star uncontrolled rectifier,this paper takes the double star uncontrolled rectifier based on power electronic transformer(PET)as the basic topology,and studies its DC side harmonic suppression method and fault tolerance.To improve the power density of the double star transformer,a double star uncontrolled rectifier based on PET is studied.Using AC-DC-AC type power electronic converter and high-frequency phase-shifting transformer to form PET instead of the original line frequency phase-shifting transformer,according to the control algorithm of switching devices in the power electronic converter and the principle of diode conduction,the operating modes of the three-phase half-wave rectifier(THR)are studied.It is analyzed that this topology can improve the effectiveness of the system power density without changing the nature of the circuit.Simulations and experiments show that this topology can replicate the performance of the traditional double star uncontrolled rectifier.When the operating frequency of the transformer is 500 Hz,the volume of the transformer is reduced to 1/3 of the line frequency transformer,which increases its power density.The influence of traditional DC-side passive harmonic suppression method on the double star uncontrolled rectifier based on PET is studied.The operating modes of the diode when the two harmonic suppression methods of the double-tap converter and the single-phase full-wave rectifier(SFR)are applied to the double star transformer are analyzed,the analytical expression of the input line current is deduced,and the turns ratios of the balanced reactor are optimized.The simulation results show that,in the double star uncontrolled rectifier,the use of traditional DC-side passive harmonic suppression circuit will generate zero-sequence circulating current,which cannot improve the input current quality and load voltage quality of the rectifier,and even deteriorate its power qualit y.In order to overcome the limitation of the existing passive harmonic suppression method applied to the double star uncontrolled rectifier,a double star uncontrolled rectifier based on the zigzag junction transformer was studied.A zigzag junction transformer is designed and applied to a double star uncontrolled rectifier to eliminate zero sequence current harmonics;from the perspective of minimum input line current total harmonic distortion(THD),the turn ratio of the tap converter is optimized.Simulations and experiments show that when the zigzag junction transformer is applied to the double star uncontrolled rectifier based on PET,it can effectively eliminate the limitations of the traditional DC-side passive harmonic suppression method.The THD value of the input current is reduced from28.4% to 17.86%.The wave suppression effect is remarkable.For analyzing the fault tolerance of the double star uncontrolled rectifier to the single-phase short-circuit fault on the grid side,the fault characteristics of the double star uncontrolled rectifier based on PET when the single-phase ground short-circuit on the input side are studied.Under the same fault conditions,the double star uncontrolled rectifier based on PET and the double star uncontrolled rectifier based on the zigzag junction transformer are analyzed;according to Kirchhoff’s law of voltage and current,combined with the principle of ampere-turn balance and the modulation principle of THR,the analytical expressions of the voltage and current of the rectifier under the fault condition are derived,the ripple coefficient of the load voltage is calculated.The simulation results show that when a single-phase short-circuit fault occurs on the grid side,the power quality of the double star uncontrolled rectifier based on PET and the double star uncontrolled rectifier based on the zigzag junction transformer will deteriorate;under the same fault conditions,the latter has better fault tolerance.