Long-Distance Electricity Distribution Is Expensive

Press, Volume XCIV, Issue 28050, 18 August 1956, Page 8

Long-Distance Electricity Distribution Is Expensive

It was possible to transmit electric power over very long distances, but this could be done only at a price, said Mr W. H. Bowen, acting-head of the electrical engineering department at Canterbury University College. yesterday.

Discussing long-distance transmission of electricity in an interview, he concluded with these words: “It is a truism that the engineer claims that, given enough money, he can do anything. It is quite possible to transmit large blocks of energy over yery long distances —at a price. When considering, therefore, a specific problem, a very careful assessment of its economics is imperative before any comprehensive scheme is initiated; and undoubtedly such careful consideration has been or will be given to the proposal to transmit power from the South Island to the North Island before the work on the project is begun. “Generally speaking, throughout the whole world today there is an everincreasing demand for electricity,” said Mr Bowen. “As we know to our cost in New Zealand, the demand doubles approximately every seven to eight years, and on an average throughout the rest of the world the demand doubles at intervals of 10 to 12 years, and what is more significant, with the demanded in productivity, there appears to be no foreseeable let-up in this growing demand. Sources of Energy “The net result,” said Mr Bowen, “is that in many countries, including our own, the energy sources adjacent to large centres of population are nearly fully exploited, and we have to look much further afield for other alternative sources of energy. Thus in Scandinavia energy has to be transported from beyond the Arctic Circle down to southern Sweden, where most of the heavy industries are located. Again, in France, the industries in eastern France draw a good deal of their power from the Pyrenees, some 400 miles away. Similar problems confront the planners of industrial India, China, and Egypt. “To sum up, therefore, future natural resources of energy have to be developed many hundreds of miles away from the centres of industry or population. The problem confronting the engineer is that of devising an economic method of transporting large blocks of energy, of the order of 300,000 to 1,000,000 kilowatts, distances up to 1000 miles or even further. “In New Zealand, energy is required to be transmitted from the southern end of the South Island to distribution centres in the North Island, a distance of about 300 to 400 miles.” Mr Bowen said. “It can be shown that there is an economic limit to the distances over which energy can be transmitted by conventional electrical means without having to take somewhat complicated means to overcome certain complex phenomena. “Two power distribution systems operating on a three-phase alternating supply voltage, when running interlinked, may be regarded as two large pendulums swinging together in unison. Any slight disturbance or fault on either or both of these two systems disturbs the swinging of these two pendulums, with the result that it is possible for the unison between them to be lost. When this happens to two such electrical systems, protective measures have to be taken, and loss of synchronism and shut-downs are the inevitable outcome.

“The operation of two such large systems in unison is somewhat delicately balanced.” he said, “and the greater the length of the transmission lines linking the power stations into each system and the length of the link joining two such systems, the more delicate becomes the task of running the two systems in unison. “It can be shown that, having due regard to these complex phenomena, already briefly described, the maximum economic length of a high-volt-age alternating-current transmission line is about 350 to 400 miles. When the length of the line exceeds this, certain remedial steps have to be taken to preserve the unison between the two systems.

“One method has been put forward to overcome this difficulty and to eliminate the cost of long transmission lines,’’ Mr Bowen said. “This method proposes that uranium for nuclear reactors should be prepared near the available energy source, and the resultant product transferred by road or rail to nucleaK power stations located near the centres of load. Another system is to use direct current for conveying power in the link between the main power systems in place of the conventional three-phase alternat-ing-current line.

“From an economic aspect, as the length of the line increases and the amount of power required increases also, then the voltage of the line goes up

“Transmission lines are at present operating at 285,000 volts, and certain experimental lines at 400,000 volts By the use of direct current, a line

suitable for. say, 275.000 volts alternating current, could be used for approximately 400.000 volts D.C., thereby increasing the amount of poxver that could be transmitted by this line by at least 45 per cent. This reasoning applies to an overhead transmission line as well as an underground or submarine cable. Rectifiers Necessary

“Unfortunately the power must still be generated by an alternating-current generator of conventional form, the voltage stepped up by means of conventional transformers, and then rectified for transmission purposes,’’ said Mr Bowen. “At the receiving end, the arriving D.C. current must be inverted —that is, converted back to three-phase A.C. power—before being fed into the distribution system. “The rectifiers and inverters are obviously expensive and complex pieces of apparatus, and although a considerable amount of successful research and development work have been carried out, particularly by Swedish engineers, a good deal has still to be done. It is possible, however, at its present state of development, to view’ with optimism the use of such apparatus. “Hand in hand with these developments have gone the development and production of high-voltage cables for transmission of energy under water and across land,’’ Mr Bowen said. “In Canada, underground cables operating at 300.000 volts have been installed by British manufacturers. These cables are approximately three inches in diameter.

“High-tension cables running under water for-some 20 miles are now being installed at Vancouver, and for a number of years such cables have been in operation elsewhere in the world. A good deal of the highvoltage distribution system in and round London is carried out by means of underground cables operating at 132.000 volts.

“It would appear, therefore, that there is no technical difficulty in supplying cable to operate at high voltage, A.C. or D.C., under Cook Strait. It should, however, be remembered that an underground or submarine cable is, very approximately, at least four times as expensive as a conventional three-phase overhead transmission line; but it is also of interest to note that, according to some authorities, such a cable operating on highvoltage D.C. would cost approximately the same as a three-phase overhead line carrying the same power,’’ Mr Bowen concluded.