What is the Difference Between AC and DC Current?

"Direct Current" (DC for short), also known as "Constant Current". Constant current is a type of direct current. It is a direct current with the same size and direction. It was discovered by Edison. In 1747, Franklin in the United States experimentally proposed the law of conservation of charge and defined the terms positive and negative.

Direct current (DC, direct current) is a unidirectional flow or movement of charge, usually electrons. Current density changes over time, but usually the direction of movement is the same at all times. As an adjective, DC can be used as a reference voltage (its polarity never changes).

In a DC circuit, electrons are formed from a cathode, a negative pole, and a negative magnetic pole, and move toward the anode, the positive pole, and a positive magnetic pole. However, physicists define direct current as the movement from positive to negative.

Direct current is generated by electrochemical and photovoltaic cells and batteries. In contrast, in most countries, the current flowing from the device is alternating current (AC). Alternating current can be converted to direct current through a power source consisting of a converter, a rectifier (prevents current from flowing in the opposite direction), and a filter (eliminates the bounce in the current flowing from the rectifier).

Virtually all electronic and computer hardware requires direct current to work. Most solid-state devices require voltages ranging from 1.5 to 13.5 volts. Demand for current ranges from close to zero in electronic watches to more than 100 amps required for wireless communication energy amplifiers. Devices that use vacuum tubes, such as high-energy radio or television broadcast transmitters or cathode ray tube (CRT) displays, require DC power of about 150 volts to several thousand volts.

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

DC transmission adopts two-wire system, using earth or seawater as the return line. Compared with three-wire three-phase AC transmission, under the condition of the same cross-sectional area of the transmission line and the same current density, it can be transmitted without considering the skin effect The same electric power, and power lines and insulation materials 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 taken into consideration, 1.33 times. Therefore, the wire used for DC transmission is almost half that of AC transmission. At the same time, the structure of the DC transmission pole tower is simpler than the three-phase AC transmission of the same capacity, and the line corridor also takes up less space.

2. In the cable transmission line, there is no capacitive current generated by the DC transmission, and the capacitive current exists in the AC transmission line, causing losses.

In some special occasions, power must be transmitted by cable. For example, when high-voltage power lines pass through large cities, underground cables are used; when power lines pass through the straits, submarine cables are used. As a coaxial capacitor is formed between the cable core and the ground, no-load capacitor current is extremely considerable in AC high-voltage transmission lines. A 200kV cable has a capacitance of about 0.2 F per kilometer, and a charging power of about 3 × 10 ³ kw is required per kilometer. On a transmission line per kilometer, it consumes 2.6 × 10 ⁷ kw / h per year. In DC transmission, due to the small voltage fluctuation, basically no capacitor current is added to the cable.

3. During DC transmission, the AC systems on both sides do not need to run synchronously, and AC transmission must run synchronously. During AC long-distance transmission, the phase of the current will produce a significant phase difference at both ends of the AC transmission system. Although the frequency of the AC power of the grid-connected systems is unified to 50HZ, it often fluctuates. These two factors cause the AC system to fail to run synchronously and need to be adjusted with complex and huge compensation systems and highly comprehensive technologies, otherwise a strong circulating current may be formed in the equipment to damage the equipment, or cause a power outage that occurs asynchronously. In less-developed countries, when the AC transmission distance generally does not exceed 300km and the DC transmission lines are interconnected, the AC grids at both ends can operate with their respective frequencies and phases without the need for synchronous adjustment.

4. The loss of DC transmission failure is smaller than that of AC transmission. If the two AC systems are interconnected with AC lines, when a short circuit occurs on one side of the system, the other side must deliver a short-circuit current to the faulty side. Therefore, the ability of the original switches on both sides of the system 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. Basically, short-circuit current is not transmitted to the AC system where the short circuit occurs on the DC transmission line. Same time. Therefore, it is not necessary to replace the original switches and current-carrying equipment on both sides.

5. Steady direct current does not generate electromagnetic radiation. Because it only generates an electric field and does not generate an alternating magnetic field, even ultra-high-voltage direct current, it is just that the electric field is strong enough to ionize the air and emit light. At this time, the optical radiation is emitted by air ionization. It is not a wire and does not generate electromagnetic waves. The so-called electromagnetic radiation is the phenomenon that energy is emitted from a radiation source into space in the form of electromagnetic waves. The flow of current in a conductor generates an electric field, and the change of current in a conductor generates a magnetic field. Therefore, it is called electromagnetic waves. ^[1]

The circuit that the direct current passes is called a direct current circuit, which is a closed conductive loop composed of a direct current power source and a resistor. In this DC circuit, a constant electric field is formed. Outside the power supply, the positive charge flows from the high potential to the low potential through the resistor. In the power supply, the non-static force of the power supply overcomes the electrostatic force, and then reaches the high potential from the low potential. This cycle forms a closed circuit. Current line. Therefore, in the DC circuit, the role of the power supply is to provide a constant electromotive force that does not change with time, and to supplement the energy of the Joule heat consumed on the resistance.

In a relatively simple DC circuit, the relationship between the power emf, resistance, current, and any two points of voltage can be derived according to Ohm's law and the definition of emf. Complex DC networks can be solved according to GR Kirchhoff equations. It includes the node current equation and the loop voltage equation. The former points out that for any node (the intersection of 3 or more branches), the algebraic sum of the currents flowing into and out of the node is zero, which is a constant condition. It is required that the latter point out that for any closed loop (grid), the algebraic sum of the voltage drop of each part is zero, which is the result of the electrostatic field loop theorem, and the two constitute a complete system of equations.

Instruments that measure physical quantities such as current, voltage, resistance, power emf in a DC circuit are called DC instruments. Commonly used are sensitive ammeters (G meters), ammeters, voltmeters, bridges, potential difference meters, etc.

DC power supply

In terms of power transmission, since the 1880s, it was not convenient to transfer DC power.

Direct current is mainly used in various electronic instruments.

In the early days, engineers were mainly working on direct current, and the power supply range of power stations was limited, and it was mainly used for lighting and not yet used as industrial power. For example, 1882

DC screen

The common name of the DC screen is the intelligent maintenance-free DC power screen, referred to as the DC screen, and the general model is GZDW. Simply put, a DC screen is a device that provides stable DC power. (When the input has a 380V power supply, it is directly converted to 220V. When there is no input (mains power and backup power), it is directly converted to battery power supplyDC 220V: it can actually be said to be an industrial emergency power supply) . The power operating power sources in power plants and substations are currently using DC power supplies, which provide power for controlling loads and dynamic loads, as well as DC accident lighting loads, etc., and are the basis of contemporary power system control and protection. The DC screen consists of AC power distribution unit, charging module unit, step-down silicon chain unit, DC power feeding unit, power distribution monitoring unit, monitoring module unit and insulation monitoring unit. It is mainly used in small and medium-sized power plants, hydropower stations, various types of substations, and other users using DC equipment (such as petrochemicals, mines, railways, etc.). Electrical protection and fault lighting.

DC screen technical indicators

1) AC measurement accuracy: 1.0% in the range of 220V and 380V ± 15%

2) DC measurement accuracy: control bus voltage: 110% ~ 240V 0.5% combined bus voltage: 286V ~ 198V 0.5% charging voltage: 286V ~ 198V range 0.5% battery voltage: 12.5V ± 10 % Range is 0.5% Controller, charging current: 0.5% in the range of 10% Ie ~ 100% Ie

3) Charging control parameters: output voltage of voltage regulating port (DC): 0 ~ 8.0V controlled (100mA)

4) Temperature detection: 1 battery room temperature -40 ~ 125 109 battery battery temperature inspection -55 ~ 125

5) Online battery detection: 256-channel DC screen schematic

6) On-line insulation detection: 8 ~ 64, can be customized

7) Branch switch status detection: 8 ~ 64

8) Silicon chain control: Level 5

9) Fault records: 64

10) Relay contact: 220V / 2A ^[4]