Converter and control method, sst system and control method

A converter, comprising:

• a first conversion module, comprising a first alternating current (AC) port and a second AC port, and configured to convert a first AC into a second AC, or to convert the second AC into the first AC;
• a multi-port module, comprising a first port and a plurality of second ports, and configured to transmit the second AC;
• wherein the first port is electrically connected to the second AC port;
• and
• a second conversion module, comprising a plurality of second conversion subunits, wherein each of the second conversion subunits comprises one third AC port and one first direct current (DC) port, and the plurality of third AC ports are electrically connected to the plurality of second ports in a one-to-one correspondence;
• wherein power transmitted by the second AC port comprises power of a second harmonic component of a frequency of the first AC.