Harmonics mitigation in high current multipulse controlled transformer-rectifiers

In this paper a general method to determine the current on AC lines of high current multipulse transformer-rectifier is presented. This method is based on the Ampere´s law application to the magnetic circuit of power transformer. It consider the current restrictions given by the rectifier topology. An analysis to determine AC currents is developed for a 12 pulses rectifier, which is formed by two 6 pulses rectifier connected in parallel, modified zig-zag +22,5° and -7,5° (Group A). Results are compared with same AC lines current measured in a 36 DC load kA DC load, real case power rectifier of similar configuration. This Group A was a part of a 24 pulses power rectifier of a copper plant and it was connected in parallel with a 12 pulses 7,5° -22,5° (Group B). The method was also applied for an emergency connection that was used in the copper plant right after a transformer -22,5° failure, resulting a 24 nonsymmetric pulses controlled rectifier, 4 paralleled groups +22,5°, -7,5°, +7,5° and +15°. Results of the analysis are completely in agreement with real measurement.