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alternating current vs direct current

alternating current vs direct current

(Summary description)Compared with the HVDC transmission mode, the HVDC transmission mode has obvious advantages. Historically, it was only due to technical reasons that AC transmission replaced DC transmission. The main advantages and disadvantages of alternating current and direct current are compared first, so as to explain their respective value in application.

alternating current vs direct current

(Summary description)Compared with the HVDC transmission mode, the HVDC transmission mode has obvious advantages. Historically, it was only due to technical reasons that AC transmission replaced DC transmission. The main advantages and disadvantages of alternating current and direct current are compared first, so as to explain their respective value in application.

Information

Compared with the HVDC transmission mode, the HVDC transmission mode has obvious advantages. Historically, it was only due to technical reasons that AC transmission replaced DC transmission. The main advantages and disadvantages of alternating current and direct current are compared first, so as to explain their respective value in application.

The advantages of alternating current are mainly reflected in power generation and distribution: using an alternator based on the principle of electromagnetic induction, mechanical energy (water flow energy, wind energy...), chemical energy (oil, natural gas...), etc. Other forms of energy can be converted into electrical energy; AC power and AC substations are much cheaper than DC power supplies and DC converter stations of the same power; Great convenience. This is the unique advantage of alternating current compared with direct current.

The advantages of direct current are mainly in power transmission:

①When transmitting the same power, the wire used for DC transmission is only 2/3~l/2 of that of AC transmission

Compared with the three-wire three-phase AC transmission, the DC transmission adopts a two-wire system, and the ground or seawater is used as the return line. Under the conditions of the same carrying area and the same current density of the transmission line, even if the skin effect is not considered, it can be transmitted. The same electric power, and the transmission line and insulation material can save 1/3.

If the skin effect and various losses (dielectric loss of insulating materials, eddy current loss of magnetic induction, corona loss of overhead lines, etc.) are considered, the cross-sectional area of ​​the wire used to transmit the same power AC power is greater than or equal to the cross-sectional area of ​​the wire used for DC transmission. 1.33 times. Therefore, the wires used in DC transmission are almost half of those used in AC transmission. At the same time, the structure of the DC transmission tower is simpler than that of the three-phase AC transmission of the same capacity, and the line corridor occupies less space.

② In the cable transmission line, there is no capacitive current in the DC transmission, but there is a capacitive current in the AC transmission line, which causes losses.

In some special occasions, cables must be used for power transmission. For example, when high-voltage transmission lines pass through large cities, underground cables are used; when transmission lines pass through straits, submarine cables are used. Since a coaxial capacitor is formed between the cable core and the ground, the no-load capacitor current is extremely considerable in the AC high-voltage transmission line. For a 200kV cable, the capacitance per kilometer is about 0.2μF, and the charging power is about 3×103kw per kilometer. On the transmission line per kilometer, the annual power consumption is 2.6×107kw·h. In DC transmission, because the voltage fluctuation is very small, basically no capacitive current is added to the cable.

③ In the case of DC transmission, the AC systems on both sides do not need to run synchronously, but the AC transmission must run synchronously. In AC long-distance transmission, the phase of the current will produce a significant phase difference between the two ends of the AC transmission system; although the frequency of the AC power of each grid-connected system is specified as 50HZ, it often fluctuates. These two factors cause the AC system to be unable to run synchronously, which needs to be adjusted with a complex and huge compensation system and a strong comprehensive technology. Otherwise, a strong circulating current may be formed in the equipment to damage the equipment, or cause a power outage accident of asynchronous operation. . In countries with less developed technology, when the AC transmission distance is generally less than 300km and the DC transmission lines are interconnected, the AC power grids at both ends can operate at their respective frequencies and phases without synchronous adjustment.

④ The loss of DC transmission failure is smaller than that of AC transmission. If two AC systems are interconnected by AC lines, when one side of the system is short-circuited, the other side must deliver short-circuit current to the faulty side. Therefore, the ability of the original switches on both sides to cut off the short-circuit current is threatened, and the switches need to be replaced. In DC transmission, due to the use of thyristor devices, the circuit power can be adjusted quickly and easily. The DC transmission line basically does not transmit short-circuit current to the short-circuited AC system, and the short-circuit current of the AC system on the fault side is not interconnected. Same time. Therefore, it is not necessary to replace the original switches and current-carrying equipment on both sides.

In the DC transmission line, all levels are regulated and operated independently, and have no influence on each other. Therefore, when one pole fails, only the faulty pole needs to be shut down, and the other pole can still transmit electric energy with no less than half of the power. But in the AC transmission line, if any phase has a permanent fault, the whole line must be cut off.

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