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What are the advantages, disadvantages and functions of high-voltage direct current transmission?

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Advantages and disadvantages of high-voltage direct current transmission

High-voltage direct current transmission has the following advantages:

      (1) Direct current transmission overhead lines only require positive and negative conductors, the tower structure is simple, the line cost is low, and the loss is small;

      (2) Direct current cable lines have large transmission capacity, low cost, low loss, are not easy to age, have a long service life, and are not restricted in transmission distance;

      (3) Direct current transmission does not have the stability problem of alternating current transmission, which is conducive to long-distance and large-capacity transmission;

      (4) Using direct current transmission to achieve asynchronous networking between power systems does not increase the short-circuit capacity of the connected power grid;

      (5) The active power transmitted by direct current transmission The reactive power consumed by the converter can be controlled by the control system. This rapid controllability can be used to improve the operating capacity of the AC system; 

      (6) Under the action of DC, only resistance works, and inductance and capacitance do not work. DC transmission uses the earth as a loop, and the DC current flows to the deep layer of the earth with low resistivity, which can make good use of the earth as a good conductor; 

      (7) DC transmission can be easily constructed in stages and capacity expanded, which is conducive to the investment benefits; 

      (8) The active power transmitted by DC transmission and the reactive power consumed by the converter stations at both ends can be quickly controlled manually or automatically, which is conducive to the economic operation and modern management of the power grid.


HVDC transmission also has some disadvantages: 

      (1) DC transmission converter stations have more equipment than AC substations, with complex structures, high construction costs, large losses, high operating costs, and poor reliability; 

      (2) For the AC side, the converter is not only a load or power source, but also a harmonic current source; 

      (3) Thyristor converters consume a lot of reactive power when performing conversion, and each converter station needs to be equipped with reactive power compensation equipment; 

      (4) DC transmission uses the earth (or seawater) as a circuit, which brings some technical problems; 

      (5) Since there is no current zero point to use for DC circuit breakers, the arc extinguishing problem is difficult to solve, which brings difficulties to manufacturing. 

    

 Function of HVDC transmission:

      In a HVDC transmission system, electrical energy is extracted from a point of the three-phase AC grid, converted into DC at the converter station, and transmitted to the receiving point through overhead lines or cables; DC is converted into AC at the converter station on the other side, and then enters the AC grid of the receiver. The rated power of DC transmission is usually greater than 100 MW, and many are between 1000-3000 MW. 

      High-voltage DC transmission is used for long-distance or ultra-long-distance transmission because it is more economical than traditional AC transmission. 

With the application of high-voltage DC transmission system, the power level and direction can be quickly and accurately controlled. This performance can improve the performance and efficiency of the AC power grid it is connected to. DC transmission system has been widely used. 

      High-voltage DC transmission is a transmission method that converts three-phase AC power into DC power through a converter station, and then sends it to another converter station through a DC transmission line to be inverted into three-phase AC power. It basically consists of two converter stations and DC transmission lines. The two converter stations are connected to the AC system at both ends. 

      The cost of DC transmission line is lower than that of AC transmission line, but the cost of converter station is much higher than that of AC substation. It is generally believed that DC transmission is more economical than AC transmission for overhead lines exceeding 600-800km and cable lines exceeding 40-60km. With the development of high voltage and large capacity thyristor and control protection technology, the cost of commutation equipment has gradually decreased. DC transmission has developed rapidly in recent years. The 1100km, ±500kV, and transmission capacity DC transmission project from Gezhouba to Shanghai in my country has been completed and put into operation. In addition, the Xiangjiaba-Shanghai DC transmission project with a total length of more than 2,000 kilometers has also been completed and put into operation on July 8, 2010. This line is currently (as of early 2011) the world's longest high-voltage DC transmission project.

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