Externally commutated compensation-type converter system and method of

Abstract

An externally commutated compensation-type converter system comprises a control unit which connects a main and an additional bridge, each built around controlled semiconductor rectifiers. Shunt capacitors are coupled in parallel with respective a-c terminals of the main bridge and series capacitors are placed between respective a-c terminals of the main and the additional bridge. A smoothing reactor connects respective d-c terminals of the main and the additional bridge, said d-c terminals being arranged to interconnect one another. Terminals of the series capacitors, the a-c terminals of the main bridge and terminals of two respective shunt capacitors, which interconnect one another, are used to form common a-c terminals coupled to an a-c mains. According to the proposed method providing for forced commutation and blocking of the rectifiers of the converter system, the shunt and series capacitors, after they have been charged, are coupled to the rectifiers being blocked. The control unit is operated to act on the gates of the rectifiers and to transfer the current passing through the smoothing reactor from one rectifier to another in a given sequence. At the beginning of the commutation interval, the current passing through the smoothing reactor is transferred from one of the rectifiers of the main bridge to one of the rectifiers of the additional bridge via a respective series capacitor. At the end of the commutation interval, the current passing through the smoothing reactor is transferred from one of the rectifiers of the additional bridge and from one of the series capacitors to a respective rectifier of the next phase belonging to the main bridge. During the commutation interval, the current passing through the smoothing reactor is allowed to pass through one more of the rectifiers of the additional bridge and through one more of the series capacitors.