Power conversion apparatus and high-voltage dc transmission system

A power conversion apparatus, comprising:

• three single-phase transformers each having at least four single-phase wirings;
• and
• three converter arms each configured by connecting one or plural unit converters each including a switching device and an energy storage element in series with one another,
• wherein a circuit is configured in each of said three converter arms in such a way that either a power source or a load is connected to a first single-phase winding of each of said three single-phase transformers, a second single-phase winding of any one of said three single-phase transformers, and a third single-phase winding of any one of said three single-phase transformers are connected in series with each other so that an electric power is converted between said circuit and either said power source or said load, and a magnitude relationship between a coupling efficient of said first single-phase winding and said second single-phase winding, and a coupling efficient of said first single-phase winding and said third single-phase winding is equal to a magnitude relationship between a coupling efficient of said fourth single-phase winding and said second single-phase winding, and a coupling efficient of said fourth single-phase winding and said third single-phase winding.