Simply speaking, AC power is easier to manage than DC power.
The use and development of electricity can be divided into three stages:
1. DC transmission stage: 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 Munich International Exhibition is powered by a 57KM direct current transmission line (2kV, 1.5kW) that built in Germany in 1882.
2. AC transmission stage: 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 designed by Ferranti on the Thames River in London 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 for more reasonable transmission method.
Operating characteristics of HVDC transmission and its comparison with AC transmission:
Power transmission characteristics: AC transmission considers stability issue; 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.
Overload capacity. In general, AC has more flexibility in terms of overload capacity. If DC needs to have bigger overload capacity, it must be pre-considered in the selection of equipment, and it is necessary to increase the investment.
The regulation of the direct current transmission can improve the stability of the AC system. Current and power control. Short circuit capacity. Scheduling management. Line corridor.
The insufficient of DC transmission: The cost of the flow breaker is high; the voltage level cannot be changed by the transformer; the cost of the converter equipment is high; the AC and DC filters are required due to the harmonic generation, which increases the cost of the converter station; .
Economic comparison of AC and DC transmission: a, After the transmission capacity is determined, the scale of the DC converter station will be determined, and the investment will be determined as well. For the AC transmission mode, the transmission distance not only affects the line investment, but also affects the investment in the substation; b, in terms of substation and line, the investment in the HVDC converter station accounts for a large proportion, while the transmission line investment in AC transmission accounts for the main part. Component; c, DC transmission power loss is much smaller than AC transmission; 4 When the transmission power increases, DC transmission can adopt the method of increasing voltage and increasing the cross section of the conductor, and always the AC transmission has to increase the number of loops.
The cost of the DC converter station is much higher than the AC transmission, and the cost of the DC transmission line is significantly lower than the AC transmission line. At the same time, the network loss of DC transmission is much smaller than 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.