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

A DC component contained in an output of a voltage source self-commutated converter causes 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-commutated SVC installed in the Shinshinano Substation of Tokyo Electric Power Company when system disturbances occurred by energizing of an adjacent large capacity transformer. After analysing the problem, a novel DC magnetization prevention 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.