A Series-Connected 18-Pulse Star Rectifier Employing Pulse-Tripling Circuit on DC Link

In this article, a simple series-connected 18-pulse star rectifier is proposed. It consists of a double-star transformer, two sets of three-phase half-wave bridge rectifiers (THBRs), and a pulse-tripling circuit (PTC). The two sets of THBRs adopt the unique connection method of series connection, which provides a midpoint for the PTC. The comodulation of the four diodes in the PTC makes the rectifier have four working modes and triples the pulse number of the rectifier. Due to the modulation of the PTC, the input current total harmonic distortion (THD) of the rectifier is reduced to less than 1/3 of the original. The proposed scheme only needs to add a low rating (10.82% of the output power) PTC on the dc side of the six-pulse star rectifier, then the new series-connected 18-pulse star rectifier can be obtained. Compared to the conventional 18-pulse rectifiers, the proposed 18-pulse rectifier requires fewer diodes and a simpler phase-shifting transformer. Moreover, in the proposed 18-pulse rectifier, the two sets of THBRs are connected in series, which not only avoids the unbalanced output current problem of THBRs but also provides a new topological choice for series-connected multipulse rectifiers. A 1.16 kW experimental prototype was built to verify the correctness of the theoretical analysis.