DC transmission lines can be divided into three types: overhead lines, cable lines, and overhead-cable hybrid lines.
1. Line introduction of DC transmission line
Due to its simple structure, low line cost, high corridor utilization rate, low operation loss, convenient maintenance, and the characteristics of meeting the requirements of large-capacity and long-distance power transmission, DC overhead lines have been used more and more in power grid construction, and are connected to converter stations. Lines that transmit electricity between them with direct current.
It is an important part of the DC transmission system. The main reason why DC transmission is economically competitive is that the economic indicators of DC transmission lines are better than those of AC transmission lines. According to the basic structure of different DC transmission lines can be divided into DC transmission overhead lines. The return circuit can also be seen as a unique component of the DC transmission line.
2. Advantages of DC transmission lines
Different from the AC line, any one-pole line of DC unipolar operation can use the other-pole line wire (one end is grounded or ungrounded) as a loop, referred to as a metal loop, or can use the earth or seawater as a loop, referred to as the earth loop.
The definition of "pole" in the DC transmission line is equivalent to the "phase" in the three-phase AC line. However, from the technical requirements of power transmission, the AC transmission line must be three-phase to facilitate operation; while the pole ( Positive or negative) can work independently, any pole plus a return circuit can transmit power independently. Under the conditions of the same transmission power and transmission distance, the cost of overhead DC transmission lines is about 20% to 30% lower than that of AC transmission lines.
Therefore, when the transmission distance reaches a certain length, the part of the cost saved by the DC transmission line compared with the AC transmission line will offset the increased cost of the DC converter station compared with the AC substation. This transmission distance is called the equivalent of AC and DC transmission. Distance. Since the power cable line is expensive, the withstand voltage performance of the cable insulation in DC is better than that in AC, and the cost of the DC cable is also lower than that of the AC cable, so the equivalent distance of the cable line is higher than the equivalent distance of the overhead line. much shorter. When the length of the transmission line is greater than the equivalent distance, DC transmission is more economical than AC transmission.
3. Differences between DC transmission lines and high-frequency parallel transmission lines
3.1 Different classifications: High-frequency cables can be divided into baseband coaxial cables and broadband coaxial cables. DC transmission is inside the ordinary cable.
3.2 Different types: high-frequency coaxial cables are divided into thin cable RG-58 and thick cable RG-11. Ordinary cables are divided into flame-retardant rubber sheathed cables, nuclear-grade cables, bare wires and bare conductor products, power cables, communication cables and optical fibers, electromagnetic wires (winding wires), and flexible fire-resistant cables.
The high-frequency signal transmission line is a Category 6 super line, and there are certain production process requirements for the line, and the requirements are relatively high. Ordinary wires are only waterproof and insulated.