Control system for power conversion system

A control system for a power conversion system composed of a plurality of unit converters and a transformer for connecting AC outputs of said unit converters in series, each of said unit converters composed of a plurality of bridge connected self-turn-off devices for converting DC power into AC power, each of AC sides of said unit converters connected to one of secondary windings of said transformer, respectively, DC sides of said unit converters connected in parallel to each other, respectively, and a primary winding of said transformer connected in series for each phase to take out an output of said power conversion system and adapted for connecting to an AC power system, said control system comprising:

• means for detecting an AC system voltage at said series connected primary winding of said transformer;
• means for generating a command voltage vector of a voltage to be output from said power conversion system at said primary winding of said transformer based on said AC system voltage;
• actual voltage vector selecting means for preparing a plurality of actual voltage vectors of voltages which can be actually output from said power conversion system at said primary winding of said transformer, preparing a plane in which start points of said actual voltage vectors are commonly taken as an origin and said actual voltage vectors are expressed at respective end points of said actual voltage vectors, dividing said plane into a plurality of regions, each including one of said end points of said actual voltage vectors, respectively, and when an end point of said command voltage vector is included in one of said regions, selecting said actual voltage vector included in said one of said regions as a selected actual voltage vector;
• unit converter voltage vector determining means for determining a plurality of unit converter voltage vectors based on said selected actual voltage vector, each of said unit converter voltage vectors allocated to one of said unit converters, respectively, and output from one of said unit converters, respectively;
• means for generating a plurality of ON-OFF pattern signals for said self-turn-off devices, each determined by one of said unit converter voltage vectors, respectively;
• and
• means for generating gate pulses to said self-turn-off devices of said unit converters based on said ON-OFF pattern signals.