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Readers are encouraged to share their views on issues affecting the electric power engineering profession. Send your letters to Mel Olken, editor in chief, Letters may be edited for publication.

Mercury-Arc Valves

Deepak Tiku’s “History” article on early HVdc power transmission in the March/April 2014 issue of IEEE Power & Energy Magazine (“DC Power Transmission: Mercury-Arc to Thyristor HVdc Valves,” vol. 12, no. 2,
pp. 76–96, March/April 2014) was very informative. I, for one, had no idea that so many early installations were made in Europe and elsewhere. In 1932, the General Electric Company (GE) installed such a pioneering transmission line between the GE plant in Schenectady, New York, and an 1898 hydroelectric station on the Champlain Canal in Mechanicville, New York.

Mercury-arc valves were used, and the line reportedly operated at 20 kV. At the Schenectady plant, the dc was inverted to standard 60-Hz ac. However, the Mechanicville station operated at 40 Hz, a frequency introduced by GE during the 1890s in a failed attempt to replace the evolving dual frequency standard of 25 and 60 Hz. Today, the Mechanicville plant is restored and still operating at 40 Hz (with frequency changers to 60 Hz), but the 1932 dc transmission line operated only up to the end of World War II.

This information appeared in an article by James A. Besha, “The Historic Mechanicville Hydroelectric Station— Part 3” in IEEE Industry Applications Magazine, vol. 13, pp. 9–11, May/June 2007.

– Thomas J. Blalock

Author’s Response

I express thanks to Thomas J. Blalock for having gone through my article “DC Power Transmission: Mercury-Arc to Thyristor HVdc Valves.” There were many more experimental dc systems that existed and were operational from early in the 20th century to 1945 than those that were discussed in some detail in the article. As rightly pointed out by Mr. Blalock, one of the important dc links using mercury-arc valves was Mechanicville–Schenectady (United States), a pilot project jointly owned by New York Power and Light Corporation and GE and was reported to be commissioned in 1937. This dc link transmitted 4.7 MW of power over a distance of 17 mi (27.4 km) at a voltage of 27 kV (pole to pole). The dc link was used to connect ac systems that were operating at different frequencies of 40 and 60 Hz.

Before that, in 1935 at Schenectady, GE had established an experimental dc system operated in a “back-to-back” mode with a power-handling capacity 100 kW at 15 kV. After the successful operation of the Mechanicville–Schenectady pilot project, in 1941 GE was asked to undertake the supply of one of the largest mercury-arc-valve-based frequency converters (2 × 10 MW) at the Edgar Thomson works of Carnegie-Illinois Steel Corporation. It was meant for power exchange between Carnegie-Illinois Steel Corporation’s two power systems operating at 44 kV, 25 Hz and 69 kV, 60 Hz connected to a public utility. The frequency converter was commissioned in stages, first a 10-MW unit in December 1943 and a second unit in February 1944.

As always, there is a space constraint. I had the option of breaking up the article into two parts or reducing the size of the article by almost half. I opted for the latter. It was a difficult task, and I tried to include the major milestones that shaped the development of the valves for dc transmission. In doing so, many other experimental links that were equally important could not be discussed. The author regrets the same.

– Deepak Tiku

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