Abstract：Multi-pulse rectifiers (MPRs) are converters that the output voltage has more than 6 pulses per power cycle. According to the connection of the rectifier bridges, MPRs are divided into parallel-type and series-type. Compared to the parallel-type MPRs, the series-type MPRs have higher output voltage level. Thus the series-type MPRs are widely served as interfaces between AC generator and DC bus such as aviation power system and ship power system. However, the nonlinearity of the rectifier devices leading to lots of harmonic pollution to the power grid. To improve the power quality and reliability of series-connected MPRs, a passive pulse doubling strategy is proposed. Through installing a set of ancillary circuit at DC link to inject three-step voltage whose frequency is six times of the supply frequency between the two rectifier bridges to suppress the low order harmonics of the rectifier, and the level of the three-step voltage is determined by the turn ratio of the injection transformer. The ancillary circuit is composed of a single-phase injection transformer and two diodes. According to the topology of the converter, the operation modes of the rectifier are analyzed and the operating waveforms of the rectifier are researched. Based on the operation mode, the input voltage and input current of the rectifier are analyzed, then the relationship between the input voltage and the turn ratio of the injection transformer is obtained. When the turn ratio of the injection transformer is 3 : 2 : 6, the THD value of the input voltage is the lowest, and each operation mode has the same duration, the lowest theorical THD value of the input voltage is about 7.74%. To verify the above theoretical analysis, the laboratorial platform is built, and the circuits are debugged. The input phase voltage of the rectifier is about 220 V and the power level of the converter is about 2 kV·A. The results of experiment shows that the input voltage of the rectifier with pulse doubling strategy is modulated from 12-step wave to 24-step wave, the input voltage and input current of the rectifier are closer to sine wave, the experimental results of the THD value of input voltage decrease from 8.6% to 4.4%, and the experimental results of the THD value of input current decrease from 6.5% to 2.6%, which up to the standard of IEEE 519—1992. The load voltage and current remain constant; The capacity of the injection transformer is about 2% of the load power, which is reduced by 15% to 35% compared to the previous research. The proposed pulse doubling strategy has considerable harmonic suppression ability with low cost. The proposed rectifier has the advantages of simple structure, high reliability, low cost, and good harmonic suppression ability, so that it can be served as a general interface of AC and DC bus. Besides, according to the previous research, the more steps of the injection voltage, the better the harmonic suppression ability, the proposed pulse doubling strategy can serve as the basic unit for other harmonic suppression strategy with better harmonic suppression ability.
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2023, Vol. 38 
