Which is better, AC or DC?
DC voltage and direction will not change with time, but AC voltage will change with time, and its voltage and amplitude will change periodically!
In fact, DC was the first to invent batteries, such as primary batteries, voltaic batteries, and even Leighton bottles that can store DC. So at 1832, the permanent magnet hand-cranked generator made by Pixii also outputs DC, and then the generators that generate DC and the equipment and motors that use DC begin to spread!
By 1875, the world's first power station has been built in Paris North Station (the first power station is controversial and can be ignored, which will not affect the discussion in this paper), supplying power for lighting in the nearby areas. Of course, you don't have to doubt that they are DC generators. By 19 13, the global power generation has reached 50 billion kwh, which actually looks great, but it is only 5000 kwh.
1875, the first coal-fired power station was built near Paris North Station, France.
After Tesla appeared, there was the first dispute between AC and DC. At that time, the largest electric company in the world was General Motors, which used DC power generation technology. At that time, DC power generation technology was in full swing, but Tesla, a teenager, suggested to Edison to switch to AC power (AC power was first discovered by Faraday, and Tesla improved AC power generation technology). Of course, Edison didn't listen to Tesla's advice.
Later, Tesla left GM to start his own business. Someone invested in his AC power generation technology. Everyone should know the result of the subsequent AC -DC dispute, because so far, apart from the special tributary transmission technology, there is no large-scale DC power grid, all of which are AC power grids. Why did DC lose its power?
What is the difference between DC and AC?
The biggest advantage of direct current is that you can use any electricity generated by the generator. There are no intermediate links, but there can be. We'll talk later. Of course, this is also the biggest disadvantage. Why? Voltage conversion is very difficult, because the power station is far away from users, so the power station must increase the voltage. If calculated according to the ratio of modern current to cross-sectional area, long-distance transmission needs 1A square millimeter (otherwise the line loss is too large and the heat is serious), or even higher!
Therefore, if the power station uses 220V, 1000A to output about 220KW, then two cables with a cross-sectional area of 1000 mm2 are needed. How thick is this? At least 36 mm diameter cable, plus the outer insulation skin, exceeds 50 mm, which means it is thicker than a child's arm!
Moreover, the distance can't be too far, and you have to use DC motor +DC generator to boost the voltage halfway, and then boost it all the way. If there is another voltage at the destination, DC motor +DC generator is needed to transform the voltage, and the cost is outrageous!
If it is replaced by 22,000 V high-voltage alternating current, the cost will be much lower as long as the cable 10 square millimeter and the cable diameter is about 3.4 millimeters. Of course, in order to increase the strength of overhead lines, it is necessary to increase the number of shares to improve their strength.
930KW step-up transformer and its control panel
Although many friends know that DC can also be used for high voltage, DC power transmission and transformation needs DC motor +DC generator to step up and step down, while AC only needs a simple transformer, and the cost of both is less than110, so 1893 Chicago "Columbia World Expo" chose Westinghouse AC (founder George Westinghouse bought Tesla's AC patent).
Three-phase alternating current
However, at that time, the alternating current did not form the specification of 50-60HZ, so the frequency was relatively high and the number of phases was inconsistent with the current three-phase alternating current.
From AC to DC
Alternating current is very convenient for power transmission and transformation, and the loss is small, and the requirements for cables are greatly reduced. The advantages of alternating current are very obvious. In the early days, lighting was also incandescent, so whether it was DC or AC. As long as the effective voltage is the same, there is no difference in brightness, so alternating current has no effect!
However, with the emergence of electronic technology such as radio, the disadvantages of alternating current have emerged. Alternating current cannot be applied to the electron tube, which will also bring noise to the system. Therefore, at the end of the general AC system, there will be a rectifier generator where DC power is needed, that is, AC power is converted into DC power through the generator (or synchronous rectifier motor or vibration rectifier, which is noisy and inefficient). These occasions are not widely used, so alternating current is still competent!
Since then, with the appearance of argon vacuum tube and mercury arc rectifier tube, these electronic devices have been used to replace mechanical rectifiers in many occasions. In the 1970s, mercury arc was still used on a large scale with good performance, and was later replaced by high-power thyristors.
Later, electronic devices such as electron tubes were gradually replaced by solid-state semiconductors, and later power semiconductors were specially developed. Electronic devices gradually disappeared from vacuum tubes, and then medium and high frequency switching power supplies appeared. Even the step-down transformer has been replaced by a tiny ferrite intermediate frequency transformer, and the power supply is getting smaller and smaller, making it more and more convenient to use. The conversion between alternating current and direct current is no longer as troublesome as before.
The difference between IGBT and IGCT
From DC to AC
Alternating current has full advantages, but it has a fatal disadvantage, that is, it cannot be stored! Because there is no battery to store alternating current, at the emergency power supply site, except for the generator, everything else is battery output, and then DC-AC is converted into direct current. Because of the development of modern power electronics, the conversion efficiency between DC and AC is very high, so DC is slowly reviving!
Compared with three-phase AC transmission, DC only needs two wires, and even DC can use the earth as another loop and only need one wire. Of course, this is an extreme case, and the insulation requirements of DC will be relatively reduced (the working voltage of DC is three times that of ordinary oil-immersed cable AC), and the line loss of conductor is small, and there is no inductive reactance, capacitive reactance, skin effect and space charge effect!
Eight-phase AC, eight-phase DC and eight-phase AC, nine-phase DC (special) high-voltage AC/DC transmission backbone networks.
Of course, all these are attributed to the development of IGBT, IGCT and silicon carbide semiconductor components, which are efficient conversion devices between AC and DC. Among many EHV large-capacity power transmission, the proportion of DC power transmission is increasing year by year, so if we focus on the future, we really don't know who will win DC or AC!