"Direct Current" (DC), also known as "constant current", is a kind of direct current, which is a direct current of the same size and direction. It was discovered by Edison.
Constant current means that the size (voltage level) and direction (positive and negative) do not change with time (relative range), such as dry batteries. Pulsating DC means that the direction (positive and negative) is constant, but the size changes with time. For example, we get the typical pulsating DC by rectifying the 50 Hz AC through the diode, and the 50 Hz pulsating DC is obtained by half-wave rectification. Full-wave or bridge-type rectification results in 100Hz pulsating DC power, which only becomes smoothed after being filtered (using inductors or capacitors). Of course, there are still pulsating components (called ripple coefficients), and the size depends on the filter circuit. Filtering effect.
The direct current transmission adopts two-wire system and uses earth or seawater as the return line. Compared with the three-wire three-phase alternating current transmission, the transmission line can be transported even under the condition that the cross-sectional area of the transmission line is the same and the current density is the same. The same electric power, while the transmission line and insulation materials can save 1/3.
When DC power transmission, the AC systems on both sides do not need to run synchronously, and the AC transmission must run synchronously. When 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 of each system in the grid is specified to be 50HZ, it often generates fluctuations. These two factors cause the AC system to be unable to operate synchronously. It needs to be adjusted with a complicated and large compensation system and a highly comprehensive technology. Otherwise, it may form a strong circulating current to damage the equipment in the equipment, or cause a power failure accident that does not run synchronously. . In countries with underdeveloped technologies, the AC transmission distance generally does not exceed 300km. When the DC transmission lines are interconnected, the AC grids at both ends can be operated with their respective frequencies and phases without synchronous adjustment.
The direction of direct current does not change over time. Usually divided into pulsating DC and steady current. There is an alternating current component in the pulsating direct current. For example, a voltage of about 300 volts in the power supply circuit in the color TV is that the pulsating direct current component can be removed by the capacitor. The steady current is ideal, and the size and direction are constant.