The "Century War" of AC and DC

We are now in the new era, and the electrical appliances used are not just light bulbs. Our main appliances are already mobile phones. Now humans are like holding mobile phones for a lifetime. Do not need electric lights, refrigerators, or washing machines. The mobile phone handles everything. The internal battery of the mobile phone is charged and discharged by DC power. Many internal appliances are AD/DC, converted to direct current, and then supply power to the semiconductor chip, including lighting. With LEDs, powering with AC is also going to DC.

Recently working on a project involving AC/DC power supply design, AC/DC has caused many troubles due to power increase, and the design complexity is much more than that of DC/DC, so a very basic question is asked: why are semiconductors? With direct current, but the power plant is sending AC power?

The earliest human demand for electricity was from lighting, and at night, a light bulb could light the darkness, instead of lighting it with dim candles like the ancients. So this allows us to be grateful to Edison, who invented the light bulb on earth. He was a brilliant inventor who was famous in the 19th century.

And in his era, God placed a young talent, Tesla, who also had several hundred patents for inventions. Both of them were in the same field and everyone had a large number of invention patents. A century of war, that is, from the AC and DC conflict.

This involved the genius inventors’ position on the earth. At the time, they both took great pains to win praise from the world. Edison represented the DC side, and Tesla represented the AC side. The current debate over alternating current direct current is still quite exciting.

At the time when the electrical appliances began to appear in a large number of years, Edison did not hesitate to use ac power to die a strong cow in order to win everyone's approval. He advised people to stay away from alternating current, but the end result was unexpected. It was that Tesla represented. The AC won Edison's DC. What is the difference between these two kinds of electricity?

Basically, current and direct current are actually the problems of both direction and size. That is to say, direct current is a current that does not change its direction and does not change its current, while alternating current means that it will change the direction and magnitude of the current. Having such a huge gap will enable Tesla's AC to win, which is an inevitable trend for the development of science and technology on Earth.

For the arrival of the age of using electricity, most regions began to send electricity over long distances. Each household's electricity was transported through the power plants over long wires, but during such long-distance transportation, it was always The use of direct current to convey, inconvenient for boosting, if there is a large amount of wasted power in low-voltage transmission, it is not conducive to economical savings. The transmission line will not be able to carry high-power applications because the current is too large.

With more and more appliances, more and more power, more and more distant transmission distance development, DC power must not be comparable to AC power. It is convenient to use AC power, only need to reduce the current at the output end of the power plant, and The direction can be changed in the midway transportation process, and the loss can be completely reduced in long-distance transportation. When the user end is about to be reached, the pressure can be appropriately changed to achieve the desired circuit effect.

Although direct current is currently inferior to alternating current in terms of long-distance transmission of current, it is still able to find a place in semiconductor materials. This is not an alternative to alternating current.

At present, in view of advocating green and low-carbon globally, in the environment-friendly electronic equipment, there is still a place where DC power can be used, enabling many devices to be compatible with such one-way transmission of direct current power transmission.

Whether or not an invention has vitality in the long history depends on its practical value. For example, the zipper is hailed as the greatest invention. The reason is also based on this. The title is actually about the issue of electrical energy. We know that electricity includes the four major processes of power generation, transmission, distribution, and electricity use. 1. Step-up and step-down of power generation and transmission In the process of power generation and transmission, there is a key device that is a transformer. Why use a transformer to change the voltage? We know that the transmission line of electric energy is a resistor, and the power Pline of the power consumed by the transmission line is:

Here I is the current flowing through the wire. It can be seen that in order to reduce the line loss, the current must be reduced. In fact, the electricity consumed by the transmission of electricity accounts for the proportion of the total generated electricity. It is of great significance to reduce the transmission line loss.

To reduce the current, it is most convenient to use a transformer. If we ignore the various losses of the transformer, such as iron loss, copper loss, etc., then the power on both sides of the transformer is basically equal. Increasing the voltage on one side of the transformer will naturally reduce the current on that side. However, the transformer can only work in AC circuits and cannot work under DC conditions. Let's look at the working principle of the transformer:

Ignoring transformer losses, there are:

Here, U1 and I1 are the voltage and current on the primary side of the transformer, U2 and I2 are the voltage and current on the secondary side of the transformer, and K is the transformation ratio. We see that as long as U2 is increased, I2 is naturally smaller, so we increase the voltage at the beginning of the transmission and distribution line. At the end of the transmission and distribution line, we lower the voltage again. This reduces the line loss. This is where the high-voltage and ultra-high-voltage power on AC transmission lines is used. We see how easy it is to use a transformer to change the level of the AC voltage.

But can DC boost? Of course, but relative to AC boost, it is a lot more troublesome and the cost is much higher. It is not worthwhile to transport electricity over long distances. Obviously, AC power is much more convenient than direct current in power generation and transmission, and the cost is much lower.

However, this is not to say that DC has no advantage. Compared with direct current, AC line resistance has more skin effect and proximity effect. Therefore, with the same wire, the AC line resistance is greater than the DC line resistance. In other words, AC line loss is actually greater than DC line loss. Therefore, the use of long-distance DC transmission is also one of the goals pursued by people. Our country is at the forefront of the world in this respect. China’s world’s first ±800kV DC transmission line is operating smoothly, efficiently, and safely.

2. Problems with power distribution At the end of the power transmission, electrical energy needs to be distributed. This requires the use of electrical distribution equipment. The main components of distribution equipment are various types of disconnectors and circuit breakers. For the switchgear, arc extinguishing is a big problem. The arc will ablate the contact and generate a huge thermal shock, which seriously affects the stable operation of the switchgear.

Compared to alternating current and direct current, each cycle has two zero crossings, and when the zero crossing occurs, the arc also automatically extinguishes, so the arc extinguishing ability of the alternating current appliance is stronger than that of the direct current appliance. There is a parameter that describes the recovery process and recovery strength of the arc medium gas after arc extinguishment, expressed in Ujf, and another parameter that describes the voltage rise process and intensity after the arc is extinguished, expressed in terms of Uhf. The recovery of the medium gas means that the gas returns from the plasma state of the arc to the normal gas state. Obviously, it is related to the time and also related to the nature of the gas. in case:

That is, if the medium recovery strength is greater than the voltage recovery intensity, the arc will not re-ignite.

In the figure above, at time 0, the AC arc crosses zero and goes out. However, after zero crossing, Ujf2 is smaller than Uhf, so the AC arc is reignited; Ujf1>Uhf, so the AC arc is no longer rekindled and is completely extinguished. However, the DC arc may not be so good that it will not cross zero at all. We look at the picture below:

The figure above is a GIS composite switch, generally used for high voltage distribution network. The GIS looks like a single boiler, and the big tube is the switch. The switch is surrounded by sulfur hexafluoride gas. Sulfur hexafluoride gas is characterized by large molecular weight, very stable, and particularly good insulation properties, so it is used to enhance arc extinguishing in high-voltage electrical appliances and improve Ujf. However, sulfur hexafluoride is a greenhouse gas. So people expect to find a gas to replace it. Vacuum is a good way. You can see how important arcing is. In this respect, DC power is much less favourable than AC power. Therefore, the same switching device must be derated before it is used in DC. In order to improve the arc extinguishing capability of DC switchgears, a layer of special film is plated in the arc-ablated material. Under the high-temperature grilling, gases similar to sulfur hexafluoride can be released to increase the recovery strength of the medium. 3. In terms of electricity and electrical appliances, the most typical is the AC squirrel-cage asynchronous motor. Relative to DC motors, it is cheap, reliable, stable, and does not require replacement of carbon brushes. Therefore, the DC motor is completely inferior to the AC motor, convenient, practical and reliable, and the DC motor application surface is also much smaller. It is undeniable that DC motor speed control performance is good, but since the inverter, AC motor speed control problems have also been resolved, the advantages of DC motor is also weakened.

4. In the power conversion, the AC power is converted into DC power, which is very convenient. It is possible to use a rectifier circuit. However, DC power must be replaced with AC, and it is relatively troublesome.

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