1. Advantages of DC transmission line
The re-emergence and rapid development of DC transmission show that it does have advantages in the field of power transmission technology that cannot be replaced by AC transmission. It is especially advantageous in the following situations.
(1) Long-distance high-power transmission. DC transmission is not subject to the constraints of synchronous operation stability problems, to ensure the stable operation of both ends of the AC grid plays a big role.
(2) Submarine cable power transmission is one of the main uses of DC transmission. To transmit the same power, DC cable not only costs less than AC but also due to the existence of a larger capacitive current than AC cable.
When the length of submarine cable exceeds 40km, it is more reasonable to use DC transmission both economically and technically.
(3) The use of DC transmission can realize the non-synchronous interconnection between domestic district networks or international.
Split the large system into several AC systems that can obtain both networking benefits and relative independence. It avoids the problems caused by the AC power system with too large a total capacity.
(4) DC transmission can be used as a measure to limit the short-circuit current when interconnecting AC power systems or increasing the capacity of distribution networks. This is because its control system has the characteristics of fast regulation and good control performance, which can effectively limit the short-circuit current and keep it basically stable.
(5) To supply electricity to large cities with intensive use of electricity, it is more economical to use high-voltage DC cables when the supply distance reaches a certain level. At the same time, DC transmission can also be used as a measure to limit the increase of short-circuit current in urban power supply networks.
2. Technical comparison between DC transmission and AC transmission
2.1 Advantages of DC transmission
(1) DC transmission does not have the stability problem of synchronous operation between the two ends of the AC system. Its transmission energy and distance are not limited by the stability of synchronous operation.
(2) Networking with DC transmission facilitates zonal dispatch management, which is conducive to rapid emergency support between AC systems in case of failure and limits accident expansion.
(3) DC transmission control system has fast response, precise regulation, easy operation, and can realize multi-objective control.
(4) Smooth voltage distribution along the DC transmission line, no capacitive current, no need for shunt reactance compensation.
(5) Two-terminal DC transmission is convenient for graded phased construction and capacity expansion, which is conducive to early benefits.
2.2 Disadvantages of DC transmission
(1) The converter needs to consume more reactive power when working.
(2) The overload energy of the SCR element is low.
(3) DC transmission in the earth or seawater for the return circuit, the ground along the underground or seawater in the metal facilities caused by corrosion, but also to bring interference to communications and navigation.
(4) DC current does not have a current waveform like AC current over the zero point, so it is more difficult to extinguish the arc.
3. DC transmission and AC transmission of economic comparison
(1) DC overhead line investment is more economical.
DC transmission generally uses a bipolar neutral grounding method, DC lines need only two wires, and three-phase AC lines need three wires, but the power transmitted by the two is almost equal, so you can reduce the load of the tower, Therefore, the load of the tower can be reduced, reducing the width of the line corridor and floor space. Under the conditions of conveying the same power and distance, the investment in DC overhead lines is generally two-thirds of the investment in AC overhead lines.
(2) DC cable line investment is less.
The same cable insulation for DC allows the working voltage to be twice as high as for AC, so at the same voltage, the cost of DC cable is much lower than AC cable.
(3) The investment in converter stations is larger than that in substations.
Converter station equipment is more complex than the AC substation, it must have in addition to the converter transformer, but also the current price is more expensive silicon controlled converter, as well as a converter of other ancillary equipment, so the investment in the converter station is higher than the same capacity and the corresponding voltage of the AC substation.
(4) In the same comparable conditions, when the transmission line length is greater than the equivalent distance, the construction costs are required to use DC transmission than AC transmission savings.
(5) The operating cost is less.
According to foreign operating experience, the annual depreciation and maintenance costs of lines and station equipment as a percentage of the construction costs, AC and DC are roughly similar. But DC transmission power loss in the wire cross-section of the same, the transmission of active power under equal conditions, is two-thirds of the AC transmission.
Many countries have successfully manufactured DC circuit breakers and load switches, and are studying the use of this switchgear combined with the control technology of DC transmission to achieve multi-terminal DC transmission.
The current research on high-temperature superconductivity is flourishing, and the possibility of its application in strong power is increasing day by day. The use of superconductivity for DC transmission is more advantageous than for AC transmission, and it is expected that in the near future, superconductivity will lead to epoch-making changes in the transmission of electrical energy and further advance the development of DC transmission.
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