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Since power stations generate alternating current (AC) electricity, and utilities deliver AC power to consumers, why is it sometimes better to transmit electricity as high-voltage direct current (HVDC)?

It’s an interesting question, because most electrical power transmissions also use three-phase alternating current. So how does DC transmission fit into a modern power network?

AC has been the preferred global platform for electrical transmission to homes and businesses for the past 100 years. And yet high-voltage AC transmission has some limitations, starting with transmission capacity and distance constraints, and the impossibility of directly connecting two AC power networks of different frequencies.

With the dawn of a new energy era and the need to build a smarter grid, HVDC is expected to grow far beyond its traditional position as a supplement to AC transmission. 

HVDC is now the method of choice for subsea electrical transmission and the interconnection of asynchronous AC grids, providing efficient, stable transmission and control capability. HVDC is also the technology of choice for long-distance bulk power transmission, able to send vast amounts of electricity over very long distances with low electrical losses. That makes it a key technology in overcoming a huge problem with renewable generation like wind, solar and hydro – that these resources are seldom located near the population centers that need them. 

The reasons for choosing HVDC instead of AC to transmit power in a specific case are often numerous and complex. Either HVDC is necessary or desirable from a technical point of view, i.e. controllability. Or HVDC results in a lower total investment, including lower losses, and/or is environmentally superior.

In many cases, HVDC links are justified based on a combination of technical, economic and environmental advantages.

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