Harmonic improvement in 12-pulse series-connected line-commutated rectifiers

An innovative method for reducing current distortion on the ac side of 12-pulse series-connected line-commutated ac/dc rectifiers is presented. Furthermore, the overlap conduction of thyristors is completely eliminated. The methodology involves an accurate shaping of the dc current by using two self-commutated switches (IGBTs). This dc-current shaping is reflected back into the shaping of the ac input currents, which become pure sine waves. Both rectifying and inverting operations are possible with a simple control circuit, which is able to deal with both rapid load variations and failures in the self-commutated switches. HVdc applications of this innovative method are briefly examined, together with experimental results showing both the steady state and the transient behaviour of the technology by using a 380 V 20 kVA 50 Hz laboratory prototype.