by economy. Even admitting that at present no definite cases are at hand where direct-current transmission would be an absolute necessity, it would be unwise to state definitely that no such necessity will arise in the future.

All manufacturers seem to agree that the ground will not be used as a return for the circuit, that a full metallic return will be provided, and that the most likely arrangement for direct current will be threewire with grounded neutral. Electrolysis and other insurmountable troubles from stray currents are therefore not to be expected.

Furthermore, manufacturers and scientists agree that for very long distance underground transmission or even for shorter high-voltage underground installations, direct current seems logical from a cable viewpoint. There may even arise cases in which underground direct-current transmission would cost less than overhead alternating current transmission. This, if proved to be true, would be a great incentive to direct-current transmission. However, it is not possible to establish under what circumstances such cost reduction could be brought about because it is impossible to secure a definite price on equipment until a definite case is at hand.

None of those who are engaged in the study of the subject from a manufacturing or other commercial viewpoint are thinking in terms of high-voltage directcurrent generation. The present direct-current highvoltage generators in the laboratory, so spectacular and interesting that they received considerable newspaper notice, are for bombarding of atoms, but not suitable for power purposes. This, however, does not preclude the possibility that even now some inventor has a conception of a practical direct-current highvoltage generator. Neither is direct-current distribution being considered as a general proposition. The prevailing opinion is that the transformer will remain for a long time the best means for voltage transformation, even when direct-current transmission is used. The present activity of changing over the old direct-current distribution systems into alternating current is expected to continue until direct-current distribution is practically eliminated. But there may be cases in crowded streets, in which a number of radial primary direct-current feeders would prove economical or even become a necessity.

Thus the present conception of direct current in a possible large power system is one of transmission only, generation and distribution remaining alternating current.

"Rectifiers" have been known for some time. They are stationary devices without moving parts for changing alternating current into direct current. Such rectifiers of the mercury-vapor type were first used

127611-37- -19

to operate certain types of arc lamps, and are still used in some cities for this purpose. The vacuum tube as a rectifier is known to every radio amateur. There are also some small metallic rectifiers that are used for charging batteries and as convenient small sources of direct current for auxiliary use on power systems. Mercury-vapor rectifiers of considerable sizes have been used to provide direct current for reasonably heavy-duty railway services, such as the Philadelphia subways, the city-owned subways in New York, the suburban service of the Delaware, Lackawanna & Western Railroad, and other cases. But the use of the inverter is of very recent origin The inverter changes direct current to alternating current without the use of rotating machines.

A 33,000-volt direct-current line for connecting a 5,250-kilowatt, 40-cycle alternating-current generating station to a 60-cycle system, 17 miles away is now in experimental operation. It must be emphasized that this is not a connection between two systems but of a single powerhouse to a system.

The sizes quoted here are quite small as compared with the sizes that would be needed for large power purposes.

The advantages claimed variously for direct-current transmission may be summarized here:

1. The possibility of tying two systems of different frequencies without the need of rotating frequency changers.

2. The same conductor can carry more direct-current amperes than alternating-current amperes. Some experts claim that the same insulated cable could carry five times as many direct-current amperes as alternating-current amperes.

3. The same amount of insulation is good for a much higher voltage under direct current than under alternating current.

4. So-called stability problems are practically nonexistent when direct current is used.

5. Corona losses from both overhead and underground conductors are much smaller for direct current than for alternating current. The skin effect does not exist under direct current. Dielectric losses in cable insulation are very low. Ionization in voids in the insulation is not likely to cause trouble. Hence hollow conductors are probably unnecessary on overhead lines and oil or gas unnecessary in underground cable, if direct current is used.

6. Dr. W. M. White, manager and chief engineer, hydraulic department, Allis-Chalmers Manufacturing Co., writes under date of July 31, 1936:

Power transmission by direct instead of alternating current fundamentally benefits its hydroelectric generating plants in that the requirements of exacting speed or cycle control is eliminated. This not only simplifies the governing

equipment of the turbines but permits also of a substantial range of revolutions or cycles when such variation results in improvement of efficiency otherwise sacrificed when the available seasonal head varies to a marked extent and which variation becomes particularly felt with medium and especially with low head developments.

This may make it economical to develop many water-power sites that are now considered uneconomical.

In conclusion, it must be emphasized that the most, important unknown factor is the cost of equipment. The Future Transmission System

Coordination.-No doubt the most important nearfuture development will be greater coordination of existing facilities. There seems to be no immediate need for very long transmission lines as such, but coordination studies indicate that there are many places in which new tie lines would make possible a better utilization of existing equipment. The larger the systems that are to be connected, the less saving may be expected from a reduction of the required peak capacity, even though there are individual cases in which this saving may be considerable. The great

est saving may be expected from a reduction of the required reserve capacity. The expected benefits from better coordination may be summarized briefly as follows: 24

1. Reduction of fixed charges and resulting reduction in rates by improvement in plant factors and utilization factors and a reduction of energy losses.

2. Better provision for emergencies, such as floods, strikes, wars, or other sudden disturbances.

3. A more elaborate network of transmission, making power available at low rates where it is not now available at all or can only be made available at high cost.

4. A better utilization of the water-power resources of the country and saving of fuel.

5. Decentralization of industry. While it is known that in many industries the cost of power represents only a small portion of the cost of the product, there are industries that cannot be located at the most desirable places because power is not available at all or its cost is prohibitive. Coordination is expected to make power available at such places at reasonable rates, making it possible in turn to provide better and cleaner living conditions for industrial workers, farmers, or those who in the future are expected to be part farmers and part industrial workers.

6. Better utilization for electric transmission of the facilities owned by other utilities, such as railroad rights-of-way, pipe-line rights-of-way, etc.

7. Coordination by utilities with power-houses owned by industrial establishments for better utilization of the power facilities by both.

8. Transmission over greater distances at night or other offpeak, low-load periods for better utilization of water power that would otherwise go to waste.

9. A more rational handling of the peak problem, with the possible provision of generator units at peak-load locations, making it unnecessary to design the whole system for peak load.

New Insulation for Overhead Lines.—In the present transmission line, porcelain or glass insulators are inounted on poles or steel towers. In the future, the insulators may be of unbreakable plastic material and even the whole supporting structure may be built of plastic insulating material.25

Underground Transmission.-Aviation and national defense will force many transmission lines underground. When airplanes become as numerous as automobiles, overhead lines will become almost impossible. Underground lines offer a better protection against attack from the air than do overhead lines. Direct-current transmission and further development in cables will make underground construction economical.

New Substations.-The future will see the elimination of the unsightly and complicated high-voltage substation with its many high-voltage insulators, each

24 Future System Planning and Station Design Rationalized by M. M. Samuels, Electric Light and Power, November 1934.

25 Dreaming of Future Line and Bus Insulation, M. M. Samuels, Electrical World, Feb. 9, 1929.

of which is a source of trouble. The future station will be completely submerged in nonexplosive oil.26

New Switchgear.-Further activity of scientists in the field of switchgear will bring out new circuit breakers, based on new scientific principle and sold at prices that are not prohibitive.27 28

New Transmission Brought About by Decentralization of Generation.-Very few giant powerhouses will be built. Few new steam plants will contain units of over 50,000 kilowatts. The powerhouses will be located at more logical places and interconnected underground. The generation will be at interconnection voltage, say 33,000 volts or 66,000 volts, thus doing away with large transformers, reducing cost, and increasing reliability.

Independent Testing Institution.-There will be a large independent high-voltage testing institution, where small manufacturers as well as customers can have high-voltage equipment tested in a competent and independent manner.

Direct Current Transmission.-There will be a considerable amount of direct-current transmission, mostly underground.

Fuel Transmission.-A considerable portion of energy will be in the form of fuel transmission by pipe lines instead of electric transmission, the power being generated by steam-electric plants at load centers.

Distribution

Introduction.-Distribution can be best explained as the retail business of the electric utility industry. Electric energy is delivered wholesale to a step-down substation or several substations within a community or on the outskirts of a community, from one or more powerhouses, over one or more transmission lines. From the substation or substations, primary distribution circuits at reasonably low voltages are carried through the streets either overhead or underground to supply energy to, domestic, commercial, industrial, and other classes of consumers. In some communities all the generation is done locally and the primary distribution lines emanate directly from powerhouses instead of substations. The voltages of these primary distribution lines, reasonably low as they are, are still too high for the small consumers purposes. The primary distribution voltage is never less than 2,300 volts, and frequently as high as 27,000 volts. Small transformers, generally known as distribution transformers, are, therefore, provided on poles or in manholes to step the energy down to a voltage suit

A Transformer Station of the Future, M. M. Samuels, Electrical World, Dec. 22, 1923.

27 Hindrances to Circuit Breaker Development, M. M. Samuels, Electrical World, Feb. 5, 1927.

High Tension Oil Circuit Breakers (p. 203), by Roy Wilkins and E. A. Crellin, McGraw-Hill Book Co., 1930.

able for consumers, mostly 220 volts for power purposes, and 110 volts for lighting purposes.

Radial and Network Distribution Systems.—In smaller communities the radial system of distribution is generally employed. Individual primary circuits radiate from a substation or a powerhouse, each circuit furnishing energy to a certain district. Individual transformers are supplied from each primary circuit, each transformer individually furnishing energy over individual secondary wires to a group of consumers or to one larger individual consumer. The secondary wires emanating from one transformer are not connected to those emanating from another transformer, even though in some cases special provision is made for one transformer to take over the burden of another in case the latter is incapacitated.

In condensed load areas in large cities, the co-called alternating current network has recently become popular. Cables are run under all the streets, parallel to each other as well as at right angles to each other, and solidly connected together at each intersection. This constitutes the so-called secondary network.

Energy is supplied to any consumer from the nearest point of any of the cables that make up the network or grid. Primary circuits are run under the streets from substations or powerhouses. Comparatively large transformers are located at suitable points under the streets, receiving their energy from the primary circuits and in turn supplying energy into the network, at points where two or more cables cross and are interconnected.

The radial system of distribution may be compared to a set of individual brooks, each supplying water to a few consumers, independent of any other brook. The secondary network can be compared to a lake occupying the whole area under a city or a section of a city. Anyone in that area can take water out of that lake. Water is supplied to the lake from various brooks at various convenient places. In the first case, when one brook becomes dry, those depending upon it will immediately be without water, whereas in the second case, even if several brooks cease to supply water to the lake, there are many others that continue to do so, and the consumers of water may be altogether unaware of the fact that one or more brooks are dried up.

Similarly, in a radial distribution system, if a short circuit occurs on an individual circuit, those who depend on it will be at once without electric energy, whereas in the case of a network system, even if one or more supply circuits or one or more transformers become incapacitated, the other circuits and transformers continue to supply energy to the network and the consumers continue to take that energy without even knowing that anything happened.

The advantages of the network to the consumer are thus self-evident, but for economic reasons such networks are only used at present in congested load

areas.

Lack of Planning in the Past.-Electric distribution grew as America grew. No one could have possibly predicted the rapid growth of American cities or American industries. During that rapid period of growth, any systematic planning of distribution for several years ahead became out of date on short order. Furthermore, during the rapid growth of the industry, when millions upon millions were spent on generation and transmission, practically all the attention of engineers and manufacturers was concentrated on these two fields. The engineers were so wrapped up in generation and transmission that many of them considered distribution as too lowbrow to merit their attention. tribution was frequently left to the lineman, purchasing agent, and storekeeper, and many distribution systems grew up to give the impression of crazy quilts, without any apparent logic either in circuit sizes, voltages, or locations, and sizes of transformers. It was not possible to secure any data on the number of kilowatt-hours got out of a kilovolt-ampere of installed transformer capacity or a pound of copper. Such information on the degree of utilization of distribution equipment is still scarce.29

Dis

In many cases there was very little coordination in the planning of electric distribution with other utility services, such as telephone, telegraph, gas, street car, water, steam, police and fire signals, sewers, etc. Even now in many communities, three or more poles can be seen close together, each taking care of a different utility service.

In larger cities, the same lack of coordination is evident underground. Figure 45 is an illustration of the congestion of various utilities on the surface and under the street of a reasonably large city, and there are many cases that represent even more complicated mazes. It has not been uncommon for two or more utilities to open and close a street in the same location immediately after one another for the purpose of repair or extension.

Overhead Coordination.-The first serious attempt at coordination of design by various utilities was the joint use of poles for electric distribution, telephone, street cars, etc. Much congestion and unsightly construction was eliminated and is still being eliminated, and considerable economy brought about by such joint use of poles. For many years the telephone companies did not permit electric utilities to run electric circuits of over 5,000 volts on telephone poles. Very

29 See Valid Cost Comparisons in Distribution, by M. M. Samuels, Electrical World, Aug. 25, 1934.

recently, after carefully studying possible hazards and various kinds of interferences, agreements have been reached by telephone companies and some electric utilities to carry higher voltages on jointly owned or jointly used poles.

Underground Coordination.-In many cities there have recently been decided indications of coordination of underground utility services. But here, in congested population centers, the difficulty of finding space under the street may become an even greater problem than the one of traffic on the surface. Cities of the future may find themselves forced to provide continuous tunnels under the main streets, in which all utility services would be arranged systematically and accessible for inspection and repair. It would then be unnecessary to break up the streets. Such tunnels could also be used for truck traffic.

Greater Coverage to Include Rural and Farm Areas.-Much is still left to be done toward rationalization of distribution planning. Considering the fact that distribution must be a monopoly in most cases if for no other reason than the physical impossibility of finding space in city streets for competing services, it is assumed to be the duty of a distribution system to render service to anyone who needs it and at fair rates. Recently, the idea has been expressed that for the same reason distribution systems should be expected to extend their services to the surrounding and adjoining rural and farm areas, spreading the cost over the whole area and making it possible for farmers to receive electric service at reasonably low rates. Coordination of electric distribution with highway lighting, possibly using the same primary circuits on the same poles, and further coordination with telephone service, may in the near future reduce the cost of rendering service to outlying districts. Mr. Cruse, in his chapter, shows that automobile accidents have been on the decrease in the daytime, but on the increase at night, indicating the great need for good highway lighting which would make it possible to drive at night without the use of headlights.

Science and Development.-In distribution, too, science and engineering development played as important a part as invention, and possibly even more so. Here, too, study and development have very recently been carried on by both engineers and manufacturers to such an extent that electric distribution quite recently emerged as an important and respected part of electrical engineering.

New Distribution Transformers.-The humble distribution transformer has received an unprecedented amount of attention as to efficiency, reliability, safety, and cost. Transformer efficiencies are so high now that they are approaching 100 percent. All manufacturers are able to furnish transformers that for