Objective The 12-pulse rectifier is widely used as the interface circuit between high-power electrical equipment and the power grid, particularly in applications such as subway traction, ship electric propulsion, and metal smelting. However, the strong nonlinearity of the rectifier diodes causes the 12-pulse rectifier to inject a significant amount of harmonics into the power grid, which seriously contaminates the grid. A hybrid pulse-multiplying inter-phase reactor (HPM-IPR) with pulse-tripling capability is proposed to effectively reduce the harmonics generated by the 12-pulse rectifier, resulting in the development of a 36-pulse autotransformer rectifier based on the HPM-IPR. Methods First, the structure of the proposed HPM-IPR was introduced. It consists of an inter-phase reactor with double secondary windings and two sets of single-phase full-wave rectifier bridges. Then, the topology of the 36-pulse autotransformer rectifier with the HPM-IPR on the DC side was presented. Its main circuit employed a 12-pulse autotransformer rectifier, and the conventional inter-phase reactor in the rectifier was replaced by the HPM-IPR. The primary winding of the HPM-IPR was connected to the positive polarity output of the two three-phase rectifier bridges, while the two sets of single-phase full-wave rectifier bridges on the secondary side of the HPM-IPR were connected in series and parallel with the load, respectively. The winding voltage relationship of the P-type phase-shifting transformer was analyzed, and the winding turn ratio was designed based on the 30° phase shift requirement. Based on the polarity of the HPM-IPR input voltage and its relationship with the output voltage, the four auxiliary diodes in the two sets of single-phase full-wave rectifier bridges were operated alternately, resulting in four operating modes of the rectifier. In all working modes, one auxiliary diode was always connected in series with the load, while in two of the four modes, one auxiliary diode was connected in series and one in parallel with the load. The modulation of the four auxiliary diodes increased the output current level of the rectifier bridge to four, and based on the AC-DC side current relationship, the number of steps in the input current increased from 12 to 36. Thereafter, the functional relationship between the turn ratio of the HPM-IPR and the input current and output voltage of the rectifier was established. Based on the definitions of the total harmonic distortion (THD) of the input current and the ripple coefficient of the output voltage, the relationship surfaces between the input current THD, output voltage ripple coefficient, and the turn ratio of the HPM-IPR were obtained. The analysis results indicated that when the HPM-IPR was designed with the optimal turn ratio, the THD of the input current was reduced to one-third of its original value. The capacity of the HPM-IPR and the voltage and current stresses of the four auxiliary diodes under the optimal turn ratio condition were calculated, and the design methods of the HPM-IPR inductance and the selection criteria for the four auxiliary diodes were determined. The calculation results revealed that the capacity of the HPM-IPR was only 3.7% of the output power. The auxiliary diodes connected in series with the load were subjected to voltage and current stresses of 0.03Ud and Id, respectively, while the auxiliary diodes connected in parallel with the load experienced voltage and current stresses of 2Ud and 0.15Id, respectively. The auxiliary diodes in series with the load were selected as high-current, low-voltage diodes, whereas the auxiliary diodes in parallel with the load were selected as high-voltage, low-current diodes. The efficiency of the rectifier before and after adding the HPM-IPR on the DC side was analyzed, and the efficiency slightly decreased after adding the HPM-IPR. Results and Discussions A 1.5 kW experimental prototype was constructed, and the input current THD of the rectifier before and after employing the HPM-IPR was compared and analyzed. The experimental results showed that after implementing the HPM-IPR, the waveform of the input current improved from 12 steps to 36 steps, and the 11th, 13th, 23rd, and 25th harmonics in the input current were almost eliminated. The input current THD decreased from 10.9% to 2.1%, and the input current harmonics were effectively suppressed. The input current THD of the rectifier was tested when the output current was 3 A, 4 A, 5 A, 6 A, and 7 A to clarify the harmonic suppression effect under various load conditions. As the load current increases, the THD of the input current slightly decreases because the filtering effect of the leakage inductance of the phase-shifting transformer becomes stronger, making the input current smoother. After using the HPM-IPR, the input current harmonics of the rectifier were effectively suppressed over a wide load range, and the input current THD was reduced to less than 3%. Conclusions The results demonstrate that the proposed HPM-IPR increases the number of steps in the rectifier's input current from 12 to 36 and reduces the input current THD to less than 3% over a wide load range, achieving a nearly sinusoidal input current and effectively suppressing input current harmonics. The HPM-IPR scheme exhibited good harmonic suppression performance. In addition, the equivalent capacity of the HPM-IPR was only 3.7% of the output power and did not require active devices, indicating that the proposed HPM-IPR scheme possesses advantages of small equivalent capacity, ease of implementation, and low cost, along with substantial application value.
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