Influence of Interphase Transformers and AC Line Reactances on the Thyristor Bridges’ Current Sharing in 18-Pulse Rectifiers

Abstract

In this paper, computer simulation models of two different topologies of 18-pulse rectifiers are presented. Each rectifier topology consists of three parallel, three-phase six-pulse full-bridge thyristor converters connected via interphase transformers. The influence of the interphase transformer’s layout and AC line reactances variations on the particular thyristor converter current sharing is analysed. Various configurations of 18-pulse rectifiers are examined by simulating operation with symmetric input voltage supply, as well as in the cases of different levels of voltage supply asymmetry, at unequal input conductors impedance, and at different thyristor voltage drops. According to computer simulation results, an optimal 18-pulse rectifier circuit topology is recommended. The optimal topology has three AC line reactances with two input current-balancing interphase transformers. These added AC reactances are also involved with the current commutation process between thyristors. Application of the recommended optimal topology of an 18-pulse rectifier could enable the reduction of the AC line reactances number from nine to three, while the dissipated power of the AC/DC converter’s most loaded thyristor could be reduced by approximately 11 %. During the simulation of the nominally loaded 18-pulse rectifier, in all considered cases the effective value of the output signal ripple below one promil of total current and voltage values has been achieved.