THE THYRISTOR

Because it is stepped up by what is just about the simplest type of electrical machinery a transformer; and that will work only for AC. The power is then usually transmitted as AC. But after the voltage has been stepped up, it could also be rectified by "valves" that pass current in only one direction, turning the AC (back and forth) into pulsating DC (alternating between peak and zero, but always in the same direction), and that can be "smoothed" into constant DC flowing steadily. At the destination, the current could be chopped up into a stop-go flow, and then turned into regular AC for stepping down to low voltage and distribution to consumers.

What would be the point? DC for long-distance transmission has a number of advantages. It needs only two, not four, conductors, which is a big saving in costs over hundreds of miles, especially since the towers can be smaller and the right-of-way narrower. For the same power transmitted, DC needs less insulation; another saving. Of course, the terminal stations turning AC into DC and back to AC again are an additional cost, and not a small one, so a DC line will pay off only for long distances - usually 400 miles or more.

There are other advantages. In adverse weather (fog, rain, ice) or through damage to an insulator, an AC line can fail, and it usually stays out until the cause of failure is removed. A DC line is also vulnerable to such faults, but often it is only damaged, not put out of action, for it can be operated at a lower voltage, transmitting less power, but still hanging on.

So what stopped DC being used all along? The rectification valves, mainly. They used to be expensive, inefficient and unable to handle much power. After World War II, better types of such valves (mercury arcs) were developed, and in 1954 the first commercial DC line (20 MW capacity, 60 miles long) went into operation in Sweden. By the early 70's, capacities had increased to 1,000 MW, and distances up to 400 miles. The big change came in 1972, when high-power thyristors came into use, enabling higher capacities to be handled with smaller losses.

Thyristors are semiconductor devices that have long been used in computers. They are essentially valves that will or will not let a current flow through them as instructed by a signal (whereas an old-fashioned rectifier always passes current in one direction and never in the other).

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The ability to take outside instructions is what makes tiny thyristors useful in computers; their big oil and water cooled brothers will pass hundreds of amperes of current (when instructed to do so every other half-cycle), and that is what makes them useful as rectifiers.

And yet DC remains controversial. The transmission line is not everything, say the AC loyalists. Think of an expanding system it's much easier if there are no terminals to consider. Reliability? AC networks are now being designed so that if one line goes out, another will take over, not some, but all, of the power. Besides .. (We have room for no more; see IEEE Spectrums August 1976).

But one thought does occur to us. Can the question be left to the experts? Shouldn't it be decided by referendum in which housewives vote on the basis of what rock stars tell them?