A new control method preventing transformer DC magnetization for voltage source self-commutated converters

A dc component contained in a output of a voltage source self-commutated converter makes a converter transformer to be dc-magnetized, in the worst case causing an overcurrent due to saturation. We experienced the dc-magnetization of the converter transformer of a 50 MVA self-conmutated SVC installed in the Shinshinano Substation of Tokyo Electric Power Company when system disturbances occurred by energizing of an adjacent large capacity transformer. After analyzing the problem, a novel dc magnetization preventing control was developed that makes a fast flux correction according to the voltage dc component detected in the converter output. This method was applied to the 50 MVA self-commutated SVC and produced satisfactory operation results.