In simple terms, AC is easier to manage than DC.
The use and development of electricity can be divided into three stages
1. DC transmission phase: power generation, transmission and power consumption are all DC
Advocating the use of DC transmission: Edison, Kelvin
Advocating the use of AC transmission: Westinghouse, Ferranti
The 57km built in Germany in 1882 to the Munich International Exhibition is powered by a direct current transmission line (2kV, 1.5kW).
2. AC transmission phase: power generation, transmission and power consumption are all AC
Reason: Long-distance power transmission → reduce the loss of electrical energy in the transmission line → change the voltage → AC transmission In 1888, the large-scale AC power station on the Thames River in London designed by Ferranti began to transmit electricity. With the rapid development of three-phase alternators, induction motors and transformers, the field of power generation and power consumption was quickly replaced by alternating current. At the same time, the transformer can easily change the AC voltage, so that the AC transmission and AC grid are rapidly developed and quickly dominated.
3, AC and DC transmission coexistence stage: power generation and electricity for AC transmission is DC
It is not simply the kind of DC transmission that was restored to the Edison era. The electricity generated by the power station and the electricity used by the user are still AC, but in the long-distance transmission, the converter is used to convert the AC high voltage into DC high voltage. Purpose: In order to solve the problems of AC transmission, seek a more reasonable transmission method.
Current Status of China's DC Transmission
1 Early in the early 1950s, people were sent to study the design and manufacture of high-pressure mercury arc valves in the Soviet Union. 1978 Shanghai put into operation a 31kV, 150A, power transmission cable length of 9km HVDC test line, total running 2 300h. 2 Zhoushan HVDC transmission project, passed the national appraisal on September 1, 1989, and officially put into operation. In October 1984, the state approved the construction of Gezhouba to Shanghai DC transmission project, which was put into operation in 1989. The 4-day-wide 500kV DC transmission project was put into operation at the end of December 2000 and was put into operation on June 26, 2001.
5 The Three Gorges to Changzhou ±500kV DC transmission project starts from Yichang Longquan in the west and reaches Zhengzhou in Changzhou in the east. It is 890km long. It was put into operation in 2002 and was put into operation in 2003. The line adopts ASCR-720/50 four-split wire, which is the largest cross-section wire in China. An OPGW composite ground cable is installed along the line. 6 Seven DC transmission projects were arranged during the “10th Five-Year Plan†period. In addition to the Three Gorges to Changzhou, Jingzhou-Huizhou Boluo Xiangshui Town, Anshun-Zhaoqing ±500kV DC transmission project will be put into operation in 2005; later, the Three Gorges to Shanghai Liantang ±500kV project will be started; as a regional interconnection The DC back-to-back project will include Shaanxi-Henan Lingbao, Qizhi Xinxiang, and Northeast-North China projects.
7 The State Power Corporation has deployed a policy of “West-to-East Power Transmission, North-South Interconnection, and National Networkingâ€. The basic pattern of the national interconnected power grid is: the Three Gorges power transmission system is the main body, radiating to the east, west, and north, forming the north, south, and south power transmission channels as the main body, the multi-point interconnection between the north and south power grids, and the vertical interconnection of the national interconnected power grid. pattern. In principle, DC back-to-back or conventional DC separation is used between the three major grids in the north, middle and south to control the scale of the AC synchronous grid. With the sound of the development of the western region, it is expected that there will be many DC transmission projects in the next decade.
Type of DC transmission adopted in China
1 Underwater cable of more than 30km. 2 Asynchronous connection between two AC systems. 3 large-capacity long-distance overhead line transmission.
Comparison of AC and DC transmission characteristics:
Operating characteristics of HVDC transmission and its comparison with AC transmission:
1 Power transmission characteristics: AC transmission considers stability problem; DC transmission has no phase and power angle, of course, there is no stability problem, which is an important feature of DC transmission, and it is also a major advantage. 2 Self-protection capability when the line is faulty: For single-phase (or unipolar) instantaneous grounding, which accounts for 80%~90% of the line fault, the DC ratio has a response block, the recovery time is short, and it is not stable. The second restart and step-down operation to create a multi-faceted advantage such as eliminating fault recovery and normal operating conditions.
3 overload capacity. In general, in terms of overload capacity, communication has more flexibility. If DC needs to have greater overload capacity, it must be considered in the selection of equipment, and it is necessary to increase investment. 4 The use of DC transmission regulation can improve the stability of the AC system. 5 power flow and power control. 6 short circuit capacity. 7 scheduling management. 8 line corridor.
Insufficient DC transmission: 1 The cost of DC circuit breaker is high; 2 The voltage level cannot be changed by transformer; 3 The cost of converter equipment is high; 4 Because of the harmonic generation, AC and DC filters are needed, thus increasing the converter station. Cost; 5 control complexity.
Economic comparison of AC and DC transmission:
Ø After the transmission capacity is determined, the scale of the DC converter station will be determined, and the investment will be fixed. The increase of the distance is only related to the line cost. For the AC transmission mode, the transmission distance not only affects the line investment, but also affects the investment in the substation; Ø In terms of the substation and the line, the investment in the DC transmission station accounts for a large proportion, while the transmission line investment in the AC transmission mainly accounts for Composition; Ø DC transmission power loss is much smaller than AC transmission; Ø When the transmission power increases, the DC transmission can adopt the method of increasing the voltage and increasing the cross section of the conductor. The AC transmission often has to increase the number of loops.
The cost of the DC converter station is much higher than that of the AC transmission, and the cost of the DC transmission line is significantly lower than that of the AC transmission line. At the same time, the network loss of DC transmission is much smaller than that of AC. Therefore, with the change of transmission distance, the cost and total cost of the two transmission modes of AC and DC will increase and decrease accordingly.
Under a certain transmission distance, the total cost of the two is equal, and a distance is called the equivalent distance. This is an important preliminary estimate of the project. When this distance is exceeded, DC is advantageous; when it is less than this distance, it is advantageous to use AC.
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