Multi-source self-adaption STATCOM and line commutated converter (SLCC) technology can overcome the inherent shortcomings of conventional line commutated converter (LCC) based high-voltage direct current (HVDC) transmission technology. In the "embedded" scenario, the topology and control strategy of a symmetrical unipolar SLCC-HVDC is proposed, which possesses significant technical and economic advantages. Then, the operating principles of SLCC are analyzed. During the non-commutation process, the SLCC can directly control the grid current through the static synchronous compensator (STATCOM) supplementing current. During the commutation process, the SLCC need to passively absorb the commutation current from the grid, while the grid current can still be indirectly controlled through changing the commutation angle. Finally, the operation characteristics of the traditional LCC and the SLCC under the symmetrical unipolar topology are compared. Both the commutation angle and the commutation voltage drop of the SLCC-HVDC are smaller than the LCC-HVDC and the reactive power of the SLCC-HVDC can be adjusted smoothly. Under transient condition, the fluctuation of the commutation angle is smaller, taking less risk of commutation failure. For the harmonic characteristics, SLCC-HVDC obviously decreases the harmonics compared with the LCC-HVDC during the unsymmetrical operation, thus is supposed to possess more flexible operation modes.