Because DC transmission only needs 3.5mm X 1.35mm DC Power Cable to point from negative to positive. Form a closed loop. After the converter inverter into AC after the use of three-phase AC.
When using 3.5mm X 1.35mm DC Power Cable, when the DC power supply is connected with the external circuit, the external power supply (external circuit), due to the electric field force, the formation of the positive electrode to the negative electrode current. Inside the power supply (the inner circuit), the non-electrostatic force causes the current to flow from the negative to the positive, thus making the charge flow in a closed loop.
The converter station of DC transmission is more complex than the substation of AC system, the cost is higher, and the operation management requirement is higher. Secondly, the converter (rectifier and inverter) operation needs a lot of reactive power compensation, normal operation can reach 40 ~ 60% of the DC transmission power; The converter will produce harmonics in the AC side and the DC side in operation, so the filter should be installed.
When the DC transmission circuit is made of earth or seawater, it will cause corrosion of metal components along the way, so protective measures are needed. High voltage DC circuit breaker is needed to develop multi - terminal DC transmission.
