Performance assessment of 18-pulse 3-phase rectifiers using harmonic reducing auto-transformers

The operation of parallel-connected 18-pulse 3-phase rectifiers are described that use non-isolating multi-winding input auto-transformers. Configurations are examined that use both ac and dc inter-phase blocking reactors for limiting the circulating currents between the parallel diode rectifiers. The work presented in this paper represents ongoing research to accurately simulate the operation of multi-pulse rectifiers. Numerous performance curves are presented, obtained using SPICE simulations, that monitor the harmonic cancelling and current sharing features of the various rectifier configurations against the per-unit size of the magnetic components used. The effect on the rectifier performance due to imbalances associated with the transformer output voltages and 3-phase inductances are highlighted. The per-unit current imbalances between the parallel connected rectifier bridges are linked to the degree of voltage and inductance imbalances present