AC transmission is used to transmit electrical power with minimum losses. But sometimes 2 AC system are not properly synchronized.

HVDC back to back system is used whenever 2 AC systems are to be synchronized properly.

Process :
1) Power from two different AC systems are first converted to DC with the help of Power Rectifiers and they are summed up.
2) The resulted power is again converted to AC with the help of Power Inverters and supplied to different grids.

HDVC back to back linking is more easy than frequency matching of two or more asynchronous system by frequency converter.