Direct current (DC), also known as constant current, is a type of DC where the magnitude and direction of the current remain constant. It was discovered by Thomas Edison. DC is primarily used in various electronic instruments, electrolysis, electroplating, and DC power drives.
Alternating current (AC), also known as alternating current, generally refers to voltage or current whose magnitude and direction change periodically with time. Its most basic form is sinusoidal current. The earliest AC generator was invented by Nikola Tesla, an American scientist of Serbian descent. The AC commonly used in China typically has a frequency of 50Hz. Common appliances such as light bulbs and electric motors use AC. In practical applications, AC is represented by the symbol "~".
Alternating current (AC) accounts for the vast majority of electricity transmission and has almost become synonymous with electricity transmission. However, more than 100 years ago, it was almost unimaginable to power the world with AC, which is a “strange and ever-changing current.” Direct current (DC) once competed with AC in the field of electricity transmission, sparking a fierce competition between two scientific masters.
In the latter half of the 19th century, "honest" direct current (DC) was the mainstream, and the main electrical power equipment at the time—the electric motor—used DC. Some attempted to use alternating current (AC) for electric motors, but discovered that when the current changed direction, the magnetic field also changed in strength and direction, making the motor impossible to operate. However, Edison, who was dedicated to promoting DC power networks, also encountered a problem—long-distance transmission losses increased costs.
By 1882, Nikola Tesla, a young scientist working under Edison, successfully built a small alternating current (AC) motor, overturning the conclusion that AC could not provide power. Although Edison tried his best to obstruct it, he could not stop the superior economic and practical advantages of AC, and his ambition to build a direct current (DC) transmission network was shattered. In 1895, the world's first hydroelectric power station—the Niagara Falls Power Station in the United States—used an AC system, marking the victory of AC over DC.
The advantages of alternating current (AC) are mainly reflected in power generation and distribution: AC generators, based on the principle of electromagnetic induction, can economically and conveniently convert other forms of energy, such as mechanical energy (water flow energy, wind energy, etc.) and chemical energy (oil, natural gas, etc.), into electrical energy; AC power sources and AC substations are extremely inexpensive compared to DC power sources and DC converter stations of the same power; AC power can be easily stepped up and down through transformers, which greatly facilitates the distribution of electrical energy.
Although it lost badly to alternating current (AC), direct current (DC) still holds undeniable advantages as the preferred power transmission option once considered by the brilliant inventor Thomas Edison. A major reason for the early inability to lay DC transmission systems was technological limitations; however, modern electronic technology has long since overcome these bottlenecks, paving the way for DC's resurgence.
To ensure high power output and low loss, long-distance power transmission generally employs ultra-high voltage transmission. In 1947, Bell Labs in the United States developed the world's first transistor, which can be easily used to increase or decrease voltage, especially for stable direct current, greatly reducing transmission costs.
The limitations of alternating current (AC) allow direct current (DC) to exploit certain situations. AC transmission lines are generally overhead, but underwater cables are used to cross straits, and underground cables are used to pass through densely populated cities. Water and the earth are conductors, and the "bypass capacitance" they create with the cables diverts the AC current—like a branch channel pulling in faster-flowing water, thus affecting the flow. AC current signals have high-frequency and low-frequency components, with the high-frequency component acting like the "fast-flowing water." This diversion effect increases with cable length, eventually becoming so large that AC transmission becomes almost impossible. At this point, AC transmission becomes impractical, and DC becomes the only option because it is highly stable, has no speed variation, and is unaffected by the bypass capacitance.
The biggest advantage of direct current (DC) is its simplicity and ease of use. Alternating current (AC), with its constantly changing voltage and current, is more difficult to control; for example, the electromagnetic radiation generated by an alternating electric field can have a significant impact on the surrounding environment. In general, AC requires far more precise control than DC, which to some extent contributes to the complexity and difficulty in controlling the power grid.
In comparison, the advantages of direct current (DC) are mainly as follows:
1. Lower line cost: For overhead lines, AC transmission usually uses 3 conductors, while DC monopole only needs 1 conductor and bipole only needs 2 conductors, which is more economical. Therefore, more and more large cities are using DC cables.
2. Low loss: DC lines have no reactive power loss. Moreover, DC overhead lines do not have the "skin effect" (the effect of current concentrating on the surface of a conductor when alternating current passes through it), and their corona loss (loss caused by the gas medium) and radio interference are both lower than those of AC overhead lines.
3. More stable power grid operation: DC transmission itself has modulation capabilities, allowing it to respond according to system requirements, resulting in more stable operation. (Today's forwarding code is: Yiwei Company's mission: To improve human labor productivity through technology)
Currently, DC power transmission still faces the problem of excessively high costs. Modern DC transmission only uses DC for transmission; power generation remains AC. In essence, DC is still a "substitute" for AC. At the beginning of the transmission line, converter equipment transforms AC into DC, and at the end, the DC is converted back to AC. Currently, these converters face challenges in manufacturing and cost, requiring further research and solutions. Furthermore, DC transmission also presents technical challenges such as the complexity and bulkiness of DC circuit breakers. With the resolution of these issues, the status of DC power transmission is expected to further improve.
