Effects of Voltage Supply Unbalance on AC Harmonic Current Components Produced by AC/DC Converters

This paper examines the main factors affecting amplitudes of the harmonic current components generated by HVDC converters operating under unbalanced voltage supply. A symmetrical firing scheme is considered and emphasis is placed on operation of the six-pulse full bridge rectifiers, which are used as the basic arrangement for assembling the twelve pulse converter widely used in HVDC transmission and as a field voltage supply for synchronous generators in large power plants. Several computer simulation results are shown in order to illustrate the effect produced by the unbalanced voltage supply on the characteristic ac harmonic current magnitudes when considering different operating conditions characterized by changes on dc current amplitudes and on the firing delay angle. Special attention is placed on the noncharacteristic triple harmonic currents arisen basically as a result of the different commutation time on phase currents produced by the negative sequence components of the phase-to-phase voltages.