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July 2021
HVDC transmission – in 1904!

HVDC transmission – in 1904!

16 July 2021

Author: Keith Wilson

It’s a popular belief that by the mid 1890s the ‘war of the currents’ had been settled with AC power transmission scoring a decisive victory over DC. This belief has recently been reinforced by the release of the movie The Current War. Of course, very few victories settle things forever and, at the present time, DC transmission is making a strong comeback, particularly in applications such as interconnects between the power systems of different countries.

But was the war of the currents ever really won? Did the 19th century proponents of DC transmission surrender meekly and give up on their ideas? It seems not, because the November 1904 issue of The Engineering Review carries an interesting account of a rear-guard action by the DC camp in Switzerland. This account which, according to the journal, is based on an item published by Electrical Review, New York, describes “new experiments with high-voltage distribution on the directcurrent system, which have been carried out at Geneva under the direction of M Réné Thury.”*

Thury, a noted Swiss engineer, was an enthusiastic supporter of DC power transmission, and the article reports that his ideas had already been proven by the successful operation of the St Maurice-Lausanne hightension line. This line, which was likely to have been about 40 km (25 miles) long, operated at 23 kV DC, with “a constant current of 150 A”. Thury had developed dynamos that would operate at this voltage, but he had bigger ambitions – he wanted to put in place a transmission system operating at 70 kV! He couldn’t produce a machine that would generate such a high voltage, but he had a simple solution, which was to connect three dynamos in series.

The Engineering Review article describes the tests he carried out to prove the feasibility of working with such high voltages and, in particular, “to find out how the standard forms of line insulator would behave toward 70 000 volts direct current, and especially to compare the latter with alternating current of high voltage.” It’s possibly not too surprising, given Thury’s commitment to DC transmission, that he concludes, “direct current proves much superior to alternating current in regard to porcelain insulators and in general to different kinds of insulating material.”

HVDC_ETarticle_2.jpg

Above: Thury Series Power Transmission, Isoverda and Genoa

Thury was also concerned with transmission losses, but his method for presenting these is a little different from the approach that would be used today. He notes that “allowing a loss of 10 per cent in the line and a weight of copper of 30 kg per electric horsepower received from the line, an initial tension of 4200 V will be needed for [transmission over] 10 km (6 miles) and 42 000 V for 100 km (60 miles)”. He comments that the use of the earth as a conductor doubles these distances for the same voltages, then goes on to say: “On the contrary, the use of alternating current reduces these distances, owing to secondary phenomena which increase the loss in the line.” Nice words, but interestingly he doesn’t provide any numbers.

So are Thury’s conclusions valid, or is he just making a rather partisan case in favour of DC? Well, his conclusions were sufficient to underpin the building and operation of the Lyon-Moutiers DC transmission scheme. According to the Wikipedia entry for this, the line was 200 km long, with 190 km run overhead and 10 km of paper-insulated underground cable.

The operating voltage was 75 kV to ground, which gave a total voltage between the two conductors of 150 kV. The Wikipedia entry makes the somewhat startling comment that the cable was rated at 75 A but was later run at 150 A. Clearly it had been rather generously engineered! The power handling capacity of the line is quoted as 30 MW, but the above figures suggest that it was nearer 22.5 MW, which is still impressive for 1906, the year that the scheme went into operation. It operated for 30 years, until it was dismantled in 1936.

It’s easy to see that DC transmission never really disappeared and a quick Internet search will reveal that new schemes were implemented in virtually every decade of the twentieth and twenty-first centuries. So, in backing AC, Westinghouse and Tesla won an important – and very profitable – victory in the battle of the currents, but by no means did they win the war.

*This picture appears by kind permission of the Institution of Engineering and Technology Archives