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posted by takyon on Tuesday March 05 2019, @05:20AM   Printer-friendly
from the terawatts-gigadollars dept.

Submitted via IRC for Bytram

This is a long read, yet quite instructive of where China has been and where it plans to be when it comes to Ultra High Voltage (UHV) transmission lines, where and why it uses AC vs DC, how it converts between the two, and what some of the challenges are that lay ahead.

China's Ambitious Plan to Build the World's Biggest Supergrid

Wind rips across an isolated utility station in northwestern China's desolate Gansu Corridor. More than 2,000 years ago, Silk Road traders from Central Asia and Europe crossed this arid, narrow plain, threading between forbidding mountains to the south and the Gobi Desert to the north, bearing precious cargo bound for Imperial Beijing. Today the corridor carries a distinctly modern commodity: gigawatts of electricity destined for the megacities of eastern China. One waypoint on that journey is this ultrahigh-voltage (UHV) converter station outside the city of Jiuquan, in Gansu province.

Electricity from the region's wind turbines, solar farms, and coal-fired power plants arrives at the station as alternating current. Two dozen 500-metric-ton transformers feed the AC into a cavernous hall, where AC-DC converter circuits hang from the 28-meter-high ceiling, emitting a penetrating, incessant buzz. Within each circuit, solid-state switches known as thyristors chew up the AC and spit it out as DC flowing at 800 kilovolts.

From here, the transmission line traverses three more provinces before terminating at a sister station in Hunan province, more than 2,300 kilometers away. There, the DC is converted back to AC, to be fed onto the regional power grid. Since it opened in mid-2017, the 26.2 billion yuan (US $3.9 billion) Gansu–Hunan transmission line has moved about 24 terawatt-hours.

The sheer scale of the new line and the advanced grid technology that's been developed to support it dwarf anything going on in pretty much any other country. And yet, here in China, it's just one of 22 such ultrahigh-voltage megaprojects that grid operators have built over the past decade. In the northwestern region of Xinjiang, China recently switched on its largest UHV link: a 1,100-kV DC circuit that cost over 40.7 billion yuan. The new line's taller transmission towers and beefier wires parallel the Gansu–Hunan line through the Gansu Corridor, before diverting to Anhui province in the east.

The result of all this effort is an emerging nationwide supergrid that will interconnect China's six regional grids and rectify the huge geographic mismatch between where China produces its cleanest power (in the north and west) and where power is consumed (in the densely populated east). By using higher voltages of direct current, which flows through conductors more uniformly than does alternating current, the new transmission lines dramatically reduce the amount of power that's lost along the way.


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  • (Score: 1, Informative) by Anonymous Coward on Tuesday March 05 2019, @12:26PM

    by Anonymous Coward on Tuesday March 05 2019, @12:26PM (#810215)

    I thought we use AC because it is more efficient than DC. (Edison's plan for DC substations every block.)

    We use AC because, in Edison's day, voltage conversion in an AC power system is simpler, cheaper, and more efficient (simple transformers). This allows for easy high voltage bulk transmission (reducing resistive losses in the power lines, which occur with the square of the current) with relatively safe low voltage consumption at the consumer site. Modern semiconductor AC/DC convertors did not exist in Edison's day, so the choice was passive transformer or motor-generator (https://en.wikipedia.org/wiki/Motor%E2%80%93generator) sets, and the losses in motor-generator sets are huge compared to transformers.

    But because of the "skin effect" (https://en.wikipedia.org/wiki/Skin_effect) an AC transmission line sees effectively more resistance than an equivalent voltage and line size DC system, so if one could transmit high voltage DC, then convert to AC for use, one could reduce resistive losses in the same size and voltage cable even more. Or (and this is more likely) the same power capacity line could use smaller conductors, saving production and install costs. Or, the same size wire as for AC could instead carry much more total power for the same losses as when carrying AC.

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