Why do we use alternating current, not DC?

2017-09-18 admin

In simple terms, alternating current is more manageable than DC.
The use and development of electricity can be divided into three stages:
1, DC transmission stage: power generation, transmission and electricity use are DC
Advocating the use of DC transmission: Edison, Kelvin
Advocating for the use of AC transmission: WestingHouse and vlant
57km, which was built in Germany in 1882, sent power to the Munich International Fair (2kV, 1.5kW).

2. AC transmission phase: power generation, transmission and electricity use are alternating current
The reason: long distance power transmission reduces the loss of electric energy in transmission lines, change voltage and AC transmission.
In 1888, the large AC power station on the Thames River in London, designed by fabaronti, began to go on transmission. With the rapid development of three phase alternator, induction motor and transformer, the field of electricity generation and power consumption is quickly replaced by alternating current. At the same time, the transformer can easily change the AC voltage, which makes the AC transmission and the AC power grid develop rapidly and quickly occupy the dominant position.

3. AC and DC transmission phase: power generation and power use for AC transmission to DC
It is not simply the kind of DC transmission that has been restored to the Edison era. The electricity generated by the power station and the electricity used by the user are still alternating current. Only in the long distance transmission, the exchange equipment is used to turn the AC high voltage into the DC high voltage.
Objective: in order to solve the problems of AC transmission, to find a more reasonable transmission mode.

Current situation of HVDC transmission in China:
First, in the early 50s, people sent to learn the design and manufacture of the high pressure mercury arc valves in the Soviet Union. In 1978, a DC transmission test line of 31kV, 150A and power cable long 9km was put into operation in Shanghai, and the cumulative operation of the DC transmission line was 2 300h.
(2) Zhoushan direct current transmission project, in September 1, 1989 passed the national identification, and formally put into operation.
In October 1984, the state approved the construction of Gezhouba Dam to Shanghai direct current transmission project and put into operation in 1989.
The 500kV of Tian Guang HVDC project, the end of December 2000 put into production in June 26, 2001 production of unipolar, bipolar.
(5) the Three Gorges to Changzhou + 500kV DC transmission project from the West Yichang Longquan, the east to the Changzhou Zheng Ping, the full length of 890km, in 2002 monopole, 2003 bipolar delivery. The line adopts ASCR-720/50 four split wire, which is the largest cross section wire in China. A OPGW composite ground wire cable is set up along the line.
The "fifteen" period for a 7 HVDC transmission project. In addition to the Three Gorges to Changzhou, Jingzhou to Huizhou Boluo Xiangshui Town, Anshun to Zhaoqing 500kV HVDC project will be put into operation in 2005; later started with the Three Gorges to Shanghai Liantang + 500kV engineering; as the DC interconnected back-to-back project, Shaanxi will have to Henan, Handan, Xinxiang, Lingbao to the Northeast to North project.
The state power company deployed the "west to East and North and South and the national network interconnection" policy. The basic pattern of the national interconnected power grid is: to the Three Gorges power system as the main body, the radiation to the four corners of the world to the north, in the direction of the formation, and South power transmission channel as the main north-south grid more interconnected, longitudinal channel more closely link the national interconnected power grid pattern.

In the north, middle and south, the three large power grids are in principle using DC back to back or conventional DC isolation to control the scale of the AC synchronous grid. With the sound of the western development horn, it is expected that there are a lot of DC transmission projects in the next decade.

The type of DC transmission used in China:
(1) the underwater cable of more than 30km.
(2) the asynchronous connection between the two communication systems.
The large capacity long-distance transmission overhead line.

Operation characteristics of HVDC and its performance compared with AC transmission.
(1) power transmission characteristics: AC transmission considering stability; DC transmission has no phase and power angle. Of course, there is no stability problem. This is an important characteristic of HVDC, and is also a great advantage of HVDC.
Since the protection ability of the fault line when the single-phase fault line for 80% ~ 90% of the total (or pole) instantaneous grounding for AC DC than with response block, short recovery time and no stable control, multiple restart and step-down operation to create the elimination of fault recovery under normal operation and other advantages.
(3) the capacity of overloading. Generally speaking, communication capacity is more flexible in terms of overload capacity. If DC needs greater overload capacity, it must be considered before equipment selection. At the same time, additional investment is needed.
(4) the use of DC transmission regulation can improve the stability of the AC system.
Power flow and power control.
The short circuit capacity.
The scheduling management.
The line corridor.

The shortage of HVDC transmission:
(1) the cost of DC circuit breaker is high;
The transformer can not be used to change the voltage level.
The cost of the exchange equipment is high;
(4) due to the generation of harmonics, AC and DC filters are needed, thus increasing the cost of the converter station.
The control is complicated.

Economic comparison of AC and DC transmission:
(1) after the transmission capacity is determined, the scale of the DC converter station is determined, and its investment is fixed, the increase of distance is only related to the cost of the line. For AC transmission, the transmission distance does not only affect the investment of the line, but also affects the investment of the transformer.
In the two part of power transmission and line, the investment of DC transmission station is a large proportion, and the main component of AC transmission line investment is the investment of AC transmission line.
(3) the power loss of direct current transmission is much smaller than that of AC transmission.
4. When the transmission power is increased, the direct current transmission can increase the voltage and increase the cross section of the wire, and the AC transmission often has to increase the number of circuits.
The cost of DC converter station is much higher than that of AC transmission, and the cost of DC transmission line is obviously lower than that of AC transmission line. At the same time, the loss of direct current transmission is much smaller than that of communication. Therefore, with the change of the transmission distance, the cost and the total cost of the two transmission modes of AC and DC will be changed accordingly.
At a certain transmission distance, the total cost of the two is equal, and the distance is called the equivalent distance. This is an important initial estimate of the project. When the distance exceeds this distance, the use of DC is favorable, and communication is favorable when it is less than this distance.