Significant source harmonic reduction achieved using direct parallel connection of two 6-pulse converters

The paper shows that direct parallel connection of the DC outputs of two 6-pulse thyristor converters, fed from 30°-displaced transformer-derived electrical supplies, results in dramatically different operation to the traditionally used arrangements of series bridges or parallel bridges with DC-side inter-phase transformers or sharing reactors. By careful specification of the transformer reactance, very low harmonic content may be achieved at a specific operating point, whereas performance at all others is as good, or better, than the traditional bridge circuit arrangements. No additional equipment or circuitry is required to achieve these benefits. By including additional active/passive components within the DC circuit, ultra low harmonic benefits may be achieved for all operating conditions of voltage and current. This ´harmonic compensator´ circuit adjusts automatically to ensure that minimum harmonic performance is achieved over the full voltage and current ranges, including inversion. Modelling results over the full motoring and regeneration speed and load range for these arrangements are presented. Very low propulsion noise and bus electrical distortion is a requirement for propulsion drives of some military and research ships. In particular, they are required to operate quietly under reduced generator capacity. Circuit analysis and circuit modelling results are presented for these ´Silent running´ conditions for a fisheries research vessel (FRV), fitted with 12-pulse direct parallel-bridge converters with the transformer reactance optimised to give minimum harmonic conditions. Practical results from the vessel are presented which validate the modelling.