Power factor improvement of line-commutated graetz converters by increasing their number of pulses: modeling and experimental results

The alternating current (AC) of line commutated converters is nonsinusoidal, generating, therefore, harmonics in the electric system. Six-pulse graetz line-commutated converters are widely employed in industry. Twelve-pulse converter can be achieved by associating two six-pulse converters in series or in parallel. However it is necessary to use supply transformers. These transformers are connected in such a way that the secondary voltage phase angles are displaced thirty degrees in relation to each other. Lower order characteristic harmonics are eliminated in this case (5° , 7°), rendering these transformers as adequate for the proposed application. A very good harmonic mitigation can be obtained when 24 or 48-pulse multiconverters are employed, by associating four or eight graetz six-pulse converters, respectively. When the converter pulse number is increased there will be also, a system power factor improvement. Experimental results are presented and discussed.