Positive and negative bipolar modular multilevel alternating current-alternating current frequency converter

A positive and negative bipolar modular multilevel alternating current (AC)-AC frequency converter, comprising a modular multilevel frequency converter and a three-phase three-winding bipolar transformer, wherein

• the modular multilevel frequency converter has an ABC three-phase structure, a power frequency side is connected to a power frequency power grid, and a low frequency side is connected to a low frequency power grid through the three-phase three-winding bipolar transformer;
• the three-phase three-winding bipolar transformer comprises two Y-connected secondary transformers T1 and T2, T1 is a positive transformer, and T2 is a negative transformer;
• and three-phase ports of the Y-connected secondary transformer T1 are connected to upper ends of three-phase upper bridge arms of the modular multilevel frequency converter, and three-phase ports of the Y-connected secondary transformer T2 are connected to lower ends of three-phase lower bridge arms of the modular multilevel frequency converter;
• and
• in the three-phase three-winding bipolar transformer, a neutral point o of T1 and a neutral point o′ of T2 are connected with each other, a voltage relationship between T1 and T2 transformer ports is that a phase difference between X and x ports is 180°, a phase difference between Y and y ports is 180°, and a phase difference between Z and z ports is 180°.