Medium voltage 12-pulse converter: ac side compensation using a shunt active power filter in a novel front end transformer configuration

In this study, a shunt active power filter (APF), with predictive current control is coupled to the 12-pulse converter in two different configurations where the same performance is achieved. The proposed configuration (the second configuration) utilises a three-winding transformer (star/tapped-star/delta) with an APF connected to the secondary taps. This transformer configuration reduces the filter side voltage avoiding the need of a high bandwidth step-down transformer. This reduces the voltage rating of the APF switches, consequently, increases the switching frequency limit. Moreover, the APF-injected compensation current is minimised, as it utilises the natural mitigation of the current harmonic components generated by each 6-pulse converter, because of the phase shift property of the front-end transformer. The APF is controlled to compensate system reactive power, distortion and the oscillating power at the converter ac side. The compensated sinusoidal input current has acceptable total harmonic distortion (THD) and near unity power factor. The converter operates under a specific power locus that compromises between a symmetrical firing power locus and asymmetrical firing loci. This locus provides minimum input current THD when the reactive power is less than 0.5 pu. The simulation results for compensating a 3.3-kV, medium voltage, MV, 12-pulse converter system are experimentally verified using a scaled prototype 12-pulse converter with a shunt APF.