Mitigation of current harmonics for multi-pulse diode front end rectifier systems

18 pulse rectifier systems have been shown to be a cost effective harmonic solution in many industries where IEEE 519 is specified. However, multi-pulse rectifiers are susceptible to phase unbalance of the source voltage and preexisting harmonic voltages. A small amount of voltage unbalance can cause a large amount of current harmonic injected back to the input source. Consequently, a stricter preexisting voltage THD condition has to be specified for an 18 pulse rectifier to meet the IEEE 519 current harmonics requirement. This paper introduces a DC link passive filtering method to solve this problem. This method greatly reduces the current harmonics caused by the phase unbalance, the 5^th, and the 7^th order voltage harmonics by adding several series LC resonant loops into the DC link. As a result, the proposed 18-pulse rectifier can still meet IEEE 519 under abnormal source voltage conditions of the AC line. Theory analysis, simulation and experimental result are provided in this paper to verify the effectiveness of this method.