Why is it better to use AC for external power transmission DC or AC power transmission?

It is because (1) AC can directly transform the voltage and DC can not directly change the voltage at present, high-voltage transmission can reduce losses, (2) the alternator can achieve a large capacity, while the DC generator can not achieve a large capacity, and now there are many high-voltage direct injection transmission, and that is also the first to change the electricity (alternating current) sent by the generator into high voltage, and then turn it into direct current, DC transmission now also has many advantages

It is easy to boost and reduce the voltage, and the purpose can be achieved through the transformer, which is simple and practical, and the current loss is reduced by high voltage when transmitting, and then there is the difficulty of DC voltage boosting, especially the power supply is too large, it is difficult to reach hundreds of thousands of kilovolts, if the voltage is low, and the transmission power is large, coupled with the long transmission distance, the current in the wire will reach a large value, so the loss is relatively large.

Because the commutation problem was not solved before, the transmission was carried out in the alternating current method. With the solution of this problem, HVDC transmission has higher advantages. An 800kV DC transmission line is being built in our country. After completion, the line loss will be greatly reduced.

Edison's earliest power plant was DC, but the disadvantage is as mentioned upstairs, and there is that three wires can transmit three phases, and DC can get six lines, saving lines. The disadvantage of AC is that it has a power factor, so DC transmission will still be used in the future.

DC power transmission is good.

HVDC transmission has the following advantages over AC transmission:

1.When the same power is transmitted, the cost of the line is low.

AC transmission overhead lines typically use 3 conductors, while DC only requires 1 (unipolar) or 2 (bipolar) conductors. Therefore, DC transmission can save a lot of transmission materials, and at the same time, it can also reduce a lot of transportation and installation costs.

2.The active power loss of the line is small.

Since only 1 or 2 wires are used in DC overhead lines, the active loss is smaller, and it has"Space charge"effect, its corona loss and radio interference are smaller than that of AC overhead lines.

3.It is suitable for underwater power transmission.

Under the same conditions of non-ferrous metals and insulating materials, the allowable operating voltage at DC is about 3 times higher than that under AC. A DC cable line with 2 cores transmits a much larger power PD than an AC cable line with 3 cores.

In operation, there is no magnetic induction loss, and when used for DC, there is basically only the resistance loss of the core wire, and the aging of the insulation is much slower, and the service life is correspondingly longer.

4.Stability issues with the system.

In an AC transmission system, all synchronous generators connected to the power system must be kept running in sync. If a DC line is used to connect two AC systems, there is no stability problem as described above because the DC line has no reactance, which means that DC transmission is not limited by the transmission distance.

5.It can limit the short-circuit current of the system.

When connecting two AC systems with AC transmission lines, due to the increase in system capacity, the short-circuit current will increase, which may exceed the blocking capacity of the original circuit breaker, which requires the replacement of a large number of equipment and a large amount of investment. In the case of DC transmission, this is not the case with these problems.

6.Fast regulation and reliable operation.

The thyristor inverter enables easy and fast adjustment of active power and power flow reversal.

If a bipolar line is used, when one pole fails, the other pole can still be used as a loop by earth or water to continue to transmit half of the power, which also improves the reliability of operation.

DC transmission is suitable for the following occasions:

long-distance high-power transmission; submarine cable power transmission; communication between AC systems operating at different frequencies or at the same frequency not at the same time; the use of underground cables to supply electricity to large cities; When the AC system is interconnected or the distribution network is expanded, it is used as one of the measures to limit the short-circuit current; Cooperate with the transmission of new energy.

Resources.

Both DC and AC have their own advantages and disadvantages, and generally speaking, the loss of using DC transmission lines at the same voltage level is smaller than that of AC. Direct current and alternating current will make the wire heat up and produce energy loss (belonging to active loss), but when AC transmission, countless capacitances are formed between the wire and the wire, the wire and the earth are formed, resulting in a large amount of reactive power loss, in addition in the case of high voltage, the corona of the convex part of the wire and line fittings also consumes a lot of energy, and the raindrops hang on the wire to form a protrusion after the rain, and corona discharge will also be produced at high voltage (a large number of electromagnetic waves are generated, which is why the TV signal is not very good after the rain), To put it simply, the wire of AC transmission is emitting radio waves into the air all the time and causing energy loss, (which is why DC loss is generally smaller than AC) We know that long-distance transmission When the load power is constant, P=UI hopes that the line current is as small as possible, so it is necessary to increase the voltage, people invented the transformer, AC transformer is relatively light, so that AC transmission can be used on a large scale, but with the increase of voltage level, DC transmission only has two lines (single circuit), AC transmission three (single circuit), and the loss of DC transmission is smaller than AC, in terms of long-term operation, when a certain voltage level and transmission distance are reached, after the budget, if DC transmission is more economical than AC transmission, DC transmission is used.

DC power transmission has less loss than AC power transmission, so DC is better. However, the cost of DC power transmission is much higher than that of AC. Therefore, AC power transmission is currently used.

The upstairs has been said in great detail, and I will add two points:

1) When the cross-section of the wire is large, due to the skin effect of alternating current, the effective cross-section of the wire will be reduced, so the cross-section has to be increased, and the DC does not have this problem, so the material is relatively reduced;

2) AC voltage conversion is convenient, so it is widely used in high, medium and low voltage conversion; DC must first change AC to DC, and then change to AC after arriving at the destination, and the conversion cost is large, so it is only used in ultra-high voltage occasions.

Summary. Transmission usually means that the power company transmits electrical energy to the user's facility, rather than the manufacturer providing electricity directly to the user.

Transmission usually means that the power company transmits electrical energy to the user's facility, rather than the manufacturer providing electricity directly to the user.

I'm still a little confused, can you be more detailed?

Power transmission refers to the transmission of electric energy to the customer in a certain way, and the power supply is used by the customer. Electric power companies are not generators of electricity, they buy electricity from power plants and transmit it to each customer's home through transmission lines. The power company will carry out a long-term power purchase plan according to the needs of customers to ensure that there is enough electric energy** for customers.

There are two main ways of power transmission, namely direct power supply and transformer distribution, and different power supply methods will also affect the stability and safety of customers when using electricity.