Test installation of 800-kV DC substation in Europe helps China power line.
An electric transmission line scheduled for operation by 2011 in China will push superlatives for such projects beyond records yet achieved. ABB Inc. last month won orders for a 2,000-kilometer direct-current line operating at 800 kV that will deliver up to 6,400 MW of power from the Xiangjiaba Hydropower Project in Sichuan province to Shanghai.
The 2,000-km length will make it the world’s longest transmission line and the 6,400-MW power rating is more than double the most powerful rating in operation today, say Zurich-based ABB officials. And the line’s ultrahigh-voltage DC (UHVDC) technology “is a vital new technology for the efficient use of hydropower generated in remote areas,” said Peter Leupp, head of ABB’s power systems division.
UHVDC uses thyristor valves equipped with newly developed 6-in. thyristors and an advanced control system. Thyristors are power semiconductors that belong to a group of technologies called “flexible AC transmission systems,” or FACTS, developed by the Electric Power Research Institute, Palo Alto, Calif.
“A thyristor is like a microelectronic device, a transistor, and operates at high power or current,” says Clark Gellings, EPRI vice president of technology. It chops the alternating current from the generator into tiny pieces and assembles it as DC for transmission, reassembling it into AC at the other end, he explains.
The major components of thyristors are a valve, a control system and a refrigeration system, says Gellings. “All three have been problematic” because “valve performance has been less than optimal, the cooling system has given a lot of trouble over the years and the control system must be customized,” he says. ABB adapts control systems from industrial systems to control the valves. EPRI developed and now licenses FACTS technology. But for this, “ABB did it on their own,” he says.
The State Grid Corp. of China and other partners placed orders worth $440 million with ABB for the UHVDC technology for the line. Gellings supports ABB’s claim that the power rating of 6,400 MW is the largest in the world. He says ABB’s claim that transmission losses of less than 7% are significantly less than the losses from conventional 500-kV high-voltage DC (HVDC) systems also are true. “Higher voltage means lower losses,” he says. He calculates the losses to be expected for a 2,000-km HVDC line at 10%. A 500-kV AC line of that length would experience 12% transmission loss, he adds.