Convert DC and AC

From: http://www.shenmeshi.com/Science/Science_20090605155956.html

 

Charge is an inherent property of matter. Normally, the number of positive and negative charges in an object is equal. Once the object loses or gets some electrons, it will show negative or positive electricity. The regular movement of the charge generates the current.

What we usually call "What is the current" is actually "What is the current intensity ". The current intensity indicates the current size. The unit is "ampere", or "an", which is expressed by the symbol ".

If the charge flows along a constant direction in a circuit, this is "DC ". Direct Current refers to the constant current in a certain direction. In daily life, the current provided by the "battery" is DC. The battery has polarity. The battery is divided into positive and negative.

DC transmission is welcomed by the power sector for its advantages such as high transmission capacity, good stability, and flexible control and adjustment. It will be applied in China. In the development of DC transmission, Research and Application of Light-duty DC transmission should be emphasized. The development of modern high voltage direct current transmission technology opens up a broad prospect for efficient power transmission.

When the current in the circuit changes cyclically with the direction and intensity, it is called "AC ". It refers to the current with changing directions and sizes. The electric energy produced by modern power plants is alternating current, and household power and industrial power are also alternating current. We usually use alternating current (AC) for electric lights and motors. In practice, the direct current is represented by the symbol "=", and the alternating current is used as the symbol "~ .

High-voltage DC transmission has obvious advantages over high-voltage AC transmission. in history, only due to technical reasons can AC transmission replace DC transmission. next we will first compare the main advantages and disadvantages of AC and DC to illustrate their respective values in application.

The advantages of alternating current are mainly manifested in power generation and distribution: using an alternating current generator based on the electromagnetic induction principle, mechanical energy (water flow, wind energy, etc.) can be economically and conveniently stored ......) Chemical Energy (petroleum, natural gas ......) Other forms of energy can be converted into electric energy. AC power supply and AC Substations are cheaper than DC power supply and DC Converter Stations with the same power. AC power can be easily increased and lowered by the transformer, this greatly facilitates the distribution of electric energy. this is a unique advantage of AC power compared with DC power.

The main advantages of direct current are transmission:

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

DC transmission uses a two-wire system, with the Earth or sea water as the return line. Compared with a three-line three-phase AC transmission, when the transmission line has the same load area and current density, even if the skin effect is not taken into account, the same electric power can be delivered, while the transmission line and insulation material can save 1/3.

If the skin effect and various losses are taken into account (dielectric loss of insulation materials, Eddy Current Loss of magnetic induction, and corona loss of air cables ), the cross-sectional area of the wire used for transmitting the same power AC is greater than or equal to 1.33 times the cross-sectional area of the wire used for DC transmission. therefore, almost half of AC transmission is used for DC transmission. at the same time, the structure of the DC Transmission Tower is simpler than that of the three-phase AC transmission with the same capacity, and the area of the line corridor is also small.

② In a cable transmission line, DC transmission does not produce capacitor current, but the AC Transmission Line has capacitor current, causing loss.

In some special cases, cables must be used for transmission. for example, when a high-voltage transmission line passes through a large city, underground cables are used. When a transmission line passes through a channel, submarine cables are used. as a coaxial capacitor is formed between the core cable and the earth, the no-load capacitor current in the AC high voltage transmission line is very considerable. A 0.2 kV cable with a capacitance of about 103 μF per kilometer needs to provide a charging power of about 3 × kW per kilometer, the annual power consumption is 2.6 × kW · h. in DC transmission, due to small voltage fluctuations, there is basically no capacitor current applied to the cable.

③ During DC transmission, the AC systems on both sides do not need to run synchronously, but the AC transmission must run synchronously. during long-distance AC transmission, the current phase will produce a significant phase difference at both ends of the AC transmission system. Although the AC frequency of each system connected to the grid is defined as 50Hz, it usually fluctuates. these two factors cause the communication system to fail to run synchronously. They need to be adjusted with a complicated and large compensation system and comprehensive technology. Otherwise, a powerful cyclic current may be formed in the device to damage the device, or cause a power failure that is not synchronized. in countries with underdeveloped technologies, the AC transmission distance is generally no more than 300km. When the DC transmission lines are interconnected, the AC power grids at both ends can run at their respective frequencies and phases, synchronization adjustment is not required.

④ The failure loss of DC transmission is smaller than that of AC transmission. if the two communication systems are connected by AC lines, when one side of the system is short-circuited, the other side must transport short-circuit current to the faulty side. therefore, the capability of the system on both sides to cut off the short-circuit current is threatened and the switch needs to be replaced. in DC transmission, due to the use of a thyristor device, the circuit power can be quickly and conveniently adjusted, the DC Transmission Line basically does not send short-circuit current to the AC system that has a short circuit, the short-circuit current of the faulty-side AC system is the same as that of the non-interconnect system. therefore, you do not need to replace the original switches and streaming devices on both sides.

In a DC transmission line, all levels are independently regulated and operated, without affecting each other. therefore, when one pole fails, only the faulty pole needs to be stopped, and the other pole can still deliver less than half of the power. however, in an AC transmission line, if any phase encounters a permanent fault, the entire line of power failure is required. Note: In the DC transmission system, only the direct current transmission link is used, and the power generation system and power supply system are still AC.

 

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From: http://www.baixueshan.com/post/73.html

AC

"Communication" for short ". It generally refers to the voltage or current that changes periodically at any time in both the size and direction. The most basic form is sine current. The standard frequency of AC power supply in China is 50Hz. Alternating current may change in a variety of forms over time. Different forms of alternating current have different application scopes and effects. Sine alternating current is the most widely used, and other non-sine alternating current can be converted into the superposition of Sine Alternating Current after mathematical processing. Sine current (also called harmonic current) is a simple harmonic function of time.

I = imsin (ω T + φ0)

When the coil rotates at a constant speed in the magnetic field, the size and direction of the coil are periodically changed.

Currently, the alternating current (AC) is generally used to change the direction and intensity by 52Hz per second.

We usually use alternating current (AC) for electric lights and motors. In practice, the alternating current (AC) symbol "~ .

Direct Current)

The size and direction of the current do not change with time. It is also called constant current. The circuit is called a DC circuit, which is a closed conductive circuit consisting of a DC source and a resistor. In this circuit, a constant electric field is formed. Outside the power supply, the positive charge flows from the high potential to the low potential. In the power supply, it depends on the non-static power of the power supply, overcome the power of static power, and then reach the high potential from the low potential, such a cycle, constitute a closed current line. Therefore, in the DC circuit, the power supply is used to provide constant EMR that does not change with time, and to supplement energy consumed by the thermal resistance.

In a relatively simple DC circuit, the relationship between the source EMR, the resistance, the current and any two-point voltage can be obtained according to Ohm's law and the definition of the EMR. Complex DC networks can be solved according to the G. R. kikhov equations. It includes the node current equation and the circuit voltage equation. The former points out that for any node (intersection of three or more branches ), the algebra and zero of each current at the inbound and outbound nodes are constant conditions, and the latter points out that for any closed loop (GRID), the algebra and zero of the voltage drops of each part, this is the result of the loop theorem of the electrostatic field. The two constitute a complete equations.

A dc instrument is used to measure the physical quantities such as current, voltage, resistance, and power source voltage in a DC circuit. Commonly used: Ampersand, ampersand, electric bridge, and electric trend.

DC power supply includes chemical battery, fuel battery, temperature difference battery, solar battery, DC Generator, etc. DC is mainly used in a variety of electronic instruments, electrolysis, electroplating, DC Electric Power drag and other aspects.

In terms of power transmission, after 1880s, due to the inability to easily upgrade the low voltage of DC to high voltage for long-distance transmission, DC transmission was placed at AC transmission. Since 1960s, due to the use of high-voltage and high-power converters to convert DC to AC power, the DC Transmission System has been paid more attention and gained new development.

 

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From: http://zhidao.baidu.com/question/10921801.html

DC and AC general concept of DC: refers to the size and direction of the current does not change with time. Many electrical appliances, such as radios, speakers, and many other electrical appliances without inductance components, are powered by DC. The transformer cannot be used to change the voltage, which is used in low-voltage electrical appliances. Electric Appliances with dry batteries belong to this type. AC: Alternating Current refers to the current with periodic changes in both the size and direction over time. The normal AC changes according to the sine or cosine law, and the current first changes from zero to the maximum, then, it is changed from the maximum to zero. Then, the reverse direction is changed from zero to the maximum, and then from the maximum to zero to complete a cycle. Later, it is the next cycle, which changes repeatedly. AC has many advantages. In addition to some special electrical appliances, such as electric motors, it has special significance for electric transmission, especially long-distance transmission.

When the AC power is positive, the other pole is negative, and the power supply changes constantly (the household power supply changes 60 times per second). The AC power can change the voltage using the transformer, high-pressure transportation of tens of thousands of volts during long-distance transportation can reduce losses caused by resistance. Household appliances, such as refrigerators, televisions, and air conditioners, all use AC power.