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Selection Of Alternating Current Apparatus For A Central Lighting System

( Originally Published Early 1900's )



MY FRIEND and predecessor in the management of The Edison Illuminating Company of Detroit, Mr. C. P. Gilbert, is responsible for the presentation of this paper. I was of a mind that it became a new member to sit quietly and listen to the words of wisdom spoken by his seniors in the Association, but Mr. Gilbert assured me that no new member was fully initiated and entitled to all the privileges of the Association till he had made a proper exhibition of himself in this way; and when I pleaded further that my views on alternating machinery, which he insisted I should place before you, were neither authoritative nor necessarily my own final views, and were likely to provoke adverse criticism and discussion, he declared that that was what the Convention was for anyway. Therefore, if the paper pleases you, you can thank Mr. Gilbert for its presentation; and, if it does not, I beg that you will wait till you get hold of him before fully expressing your opinions.

It fell last spring that I, being in the employ of the City of Detroit, had to make inquiry into the present state of development of alternating machinery, with a view to planning a distribution of light to all the public buildings of the city and to the island park. There is already a lighting system, designed by me, which supplies the public buildings within the downtown business district. The new work had its centers of distribution 17,000 to 25,000 feet from the power house, and the buildings were much scattered. Mr. Gilbert was at the same time engaged in a similar inquiry with reference to proposed extensions of the Edison plant, and also with reference to the Sacramento plant, of which he has since taken charge. Our distribution problems were different; but, so far as apparatus was concerned, we discovered that we could profit by exchange of information and opinions. The opinions I had formed are those now laid before you.

It is understood that the load is to be of the usual central station class; mostly incandescent lighting, with an increasing load of arcs connected to the incandescent mains, and with all the motor load that can be had; that the three services must be taken care of by the one kind of dynamo, and that switchboard and circuits should be as simple as possible and regulation at least equal to that of the average Edison three-wire system.

I may as well say, to begin with, that I look on alternating distribution as having a different field from the Edison low tension system, and not as a possible substitute. I know towns where the entire distribution is alternating, although apparently the conditions would suit three-wire, but in most of these, this is not due to good engineering but to personal or quasi-personal reasons. On the other hand, there are Edison companies supplementing their three-wire systems by an alternating distribution reaching beyond the three-wire limits, and this number is on the increase. To those companies, including my own, which will constitute the said increase, a full discussion of this subject will be of value.

Dynamos in Multiple and Their Governing

It goes without saying that the operation of the plant must be in multiple—that no combination of switches, however ingenious, will be a satisfactory substitute for the multiple running of the dynamos. That alternators can be run in multiple no one will now deny; but it is not universally understood that successful operation depends almost entirely on the governing of the engines, and very little on the electrical conditions. The higher the frequency the more completely does this rule apply; thus, at exceptionally low frequencies, such as are used only for power transmission work, the division of the work between two alternators in multiple can be influenced by the strength of field of either machine; but at the frequencies of 60 periods and upwards which are used for lighting, the division of the load is dependent only on the power applied by the steam. In our direct current work we seldom pay attention to the harmonizing of the engine characteristics, knowing that any small difference can readily be compensated for by the field rheostats of the dynamos. In alternating work, on the contrary, it is essential that the engine characteristics should harmonize throughout their working range—that the percentage of speed variation between no load and full load should be the same in each engine and that at intermediate loads the change of speed should be a function of the change of load. These requirements will give our engine building friends some thought and will necessitate the setting up of a new ideal of perfect governing. The present ideal is that there should be no change of speed at any load. The new one will be that the change of speed from no load to full load, should be some small percentage of the full speed—say 4 per cent—and that for each and every tenth of the load added or subtracted the change of speed should be one-tenth of the total change, say four-tenths of one per cent.

Such governing is difficult to secure in a lot of engines of different sizes and possibly of different types, if the automatic governors are to be depended on entirely; and it is likely that the engine builders will offer us devices whereby the supply of steam can be adjusted by hand while the engines are running. With such arrangements the division of the load in any desired proportion can be easily obtained; an increase of load on any machine requiring a longer cut-off in the cylinder, and a decrease a shorter cut-off. I think the success of multiple running in England is due to the common use in that country of the throttling governor with hand adjustment; and I notice that even the engines fitted with shaft governors on the high pressure valves have hand gear for the other cylinders. The last Willans' design has both throttling and cut-off governing, and either governor can be adjusted while running.

Individual Drive for Alternators

I have implied in this discussion, so far, that each alternator is to have its own independent engine; and I wish to say explicitly that I consider this independence essential. One condition may warrant a departure from this rule; namely, where a station is obliged to run dynamos of several different types, lightly loaded, during the later hours of the night, or for a small day load; under which condition it is desirable to bunch all the work on one engine in order to secure steam economy. In such a case it is plain that the load on the alternator will be only a part of the load on the engine and that the speed of the engine will not be controlled by the alternator's load to any appreciable extent. It will follow that this alternator must set the speed for all the others; because the others must run faster than this one when they are light and slower when they are loaded, to allow them to go into and out of multiple with it. I think it likely that the "omnibus" engine will have to be fitted with the hand governing attachment, even if the others can run without it.

One alternator on an omnibus engine is all that should be allowed. A second one on the same engine will be specially trouble-some to multiple, unless both are positively coupled mechanically to the engine or to one another; because belts or ropes will not run alike, nor creep alike, and it is only by belt or rope slipping that getting into multiple can be done in the first place, and breaking out of multiple avoided afterwards. I have heard of belted alternators on the same line shaft being tuned, by infinite pains, to run together; but I wish to be excused from such an undertaking.

Proportioning Engine to Alternator

The proportioning of the size of the engine to that of the alternator requires the same study and is governed by the same rules as in direct current work. The most common mistake is to choose the steam cylinders too large. I prefer to have the extreme power of the engine, within the permissible limit of speed, equal to the load which the alternator will stand for 2 or 3 hours; which is of course considerably more than the rated load for constant work. It is also much more than the economical load of the engine. This secures that the engine will work with economy with any load likely to be put on it in practice; and it also secures that an accidental overload, such as a short circuit, will not damage the dynamo but will simply slow down the engine or engines, if more than one are running. The engines, of course, should be so built as to stand this treatment without injury, and purchasers should see to it that they are so. There is little risk that the dynamos will not be mechanically fit; most alternators nowadays are amply strong in shaft and in coil-driving parts to go through a short circuit that will stop the engine. Some indeed are intentionally so designed electrically that a short circuit does not greatly increase the torque; so that they cannot be strained thereby as the engine will not be called on for excessive power, but as noted later, I prefer the type that will increase the torque till it slows the engine.

Layout of Distribution Circuits

In laying out the circuits it will be necessary to decide between working the feeders in multiple on a common system of mains, or of following the more common alternating practice of separate systems of mains, each with its own feeder. The latter method is a necessity if the dynamos are not to be run in multiple; and it is generally most convenient when the alternating system is used to reach scattered and distant lighting. But when a district requires so much copper in its feeder as to make it inadvisable to use only one pair or set of wires, it is well to connect the second set into the mains at a different point—for the better regulation obtained by reduction of the drop in the mains—rather than to separate the mains into two systems and put one on each set of feeders. And if a system of secondary mains is run and fed by transformers at different points, the feeders are just as much in multiple as if they were connected at their ends by a primary main. This last method, with a three-wire or three-phase four-wire secondary, is the nearest approximation to the Edison system and has most of its advantages. I need not say that the voltage on such a secondary should be as high as the lamps will stand economically. In the present state of lamp making, 113 volts on each side is quite practical.

Choice of Polyphase System

It also goes without saying that a system by which power can be supplied is essential; and that in the present state of the art this requirement means the use of a polyphase distribution. I did look into the possibility of using single-phase motors, and particularly into the claims of the Leonard synchronous motor, now being exploited by the Fort Wayne Company. My conclusion as to that device was that it occupied about the same place in the business that the series motor for arc circuits did eight years ago; and that, like its prototype, it might serve for occasional use where a polyphase service would not be warranted by the business in sight; but that for general use, and in any but the smaller sizes, it would not continue in competition with the polyphase motor, because of its cost and of its intricacy, requiring more skill in handling than the common shunt wound direct current motor, which seems to be the most complicated device that the technical education of the ordinary customer will suffice for.

As to the choice among the three polyphase systems now on the market—two-phase, three-phase and monocyclic—there is no general rule to be laid down. In the majority of cases involving Edison stations, I believe the four-wire two-phase system will be preferable; because it allows all the copper put into lines to be worked to its full capacity, while the monocyclic system requires a comparatively useless middle wire; because it requires only two sets of circuits and bus bars in the station, while the three-phase re-quires three sets; because in no case are more than two transformers required for any service; and because, the circuits being interlaced only in the dynamo (and not necessarily even there), there is no complication in metering the power supplied, while such complication exists in both monocylic and three-phase. In the case of a small station, supplying power at flat rates and having a lighting load much more important than its power supply business, the mono-cyclic system is certainly preferable. And in the opposite case of a station designed primarily for power, or where the entire output is to be delivered to rotary transformers for conversion into direct current, the three-phase system is best.

Frequency

As to frequency: For general use, the frequency of 60 periods per second (7,200 alternations), which has been adopted by the leading American manufacturer, is, all things considered, the most satisfactory. It is low enough for power and for practically silent arc lighting; and yet high enough to give good weight efficiency in transformers. I have had fifteen months' experience of operation at 42 periods—5,000 alternations—and found that figure to be very close to the lower limit of satisfactory operation. So close that when (by reason of overload) the engine let the frequency down to 39 periods, the 3 1/2-watt incandescent lamps we used gave a light that strained the eyes and the 3-watt lamps had a visible flicker. Also the arc lamps, which we finally got to operate without rattling at 42 to 43 periods, behaved very badly at 39. Incidentally, I may say that an arc at 42 is not fit for lighting a drafting room or office or any interior other than a hall, or corridor, because of the same effect on the eyes as noted with the incandescent lamps at 39.

Dynamos

In selecting the dynamos there will be found two types of armature construction to choose between; the one having a distributed winding like that of the ironclad armature used in multi-polar direct current machines, the other having the winding concentrated into a few coils disposed on polar projections. The distributed winding gives a slightly higher efficiency, considerably better inherent regulation—that is to say, regulation independent of any compound winding or hand regulation—, and a nearly perfect sine curve of electric induction. The polar winding has large armature reaction, which is inimical to inherent regulation but which acts to limit the current on short circuit. The regulation in this type is always aided by a powerful compound field winding. The curve of induction given by the polar winding is more or less distorted from the ideal sine curve, which is detrimental for long distance trans-mission but is immaterial for the ordinary conditions of central station work.

Of the two types I prefer the distributed winding for the purpose in view, because it admits of hand regulation, without compounding, with satisfactory results, and because by reason of this same quality of inherent regulation, the unbalancing of the two quarter-phase circuits under differing loads is negligible.

I do not believe in compound winding of alternators for multiple running on a central station load. If the load is such that a single small machine is likely to be running for some length of time, that machine may with advantage be compound wound; but to put a lot of such machines in multiple is unnecessary complication. If any compounding for the full load of the station is to be done, it had better be done on the exciter—better so for efficiency as well as for simplicity.

The condition of the one small alternator on the day load can be met without compound winding by a method which I propose to use in my own work. I premise that the object of running the small alternator is to get the small engine which drives it into use instead of the larger one which is connected to a larger alternator; and so to avoid the cylinder condensation losses at short cut-off and the large friction of the large engine. I propose to connect by jaw clutches two independent engines to the alternator which will take my day load; and during the day to run only one engine, so cutting my engine losses in half, while retaining the superior inherent regulation of the large alternator.

Voltage

For primary electromotive force, the tendency today is to a standard of 2,000 volts. I am not satisfied that this is entirely well.

I believe that in many cases a pressure of 1,150 volts would be preferable; that the saving of copper in these cases does not warrant the increased risk of interruption, and the increased cost of insulation and of details, such as fuse boxes, lightning arresters and station switches. The increased cost of insulation is not trifling even with overhead lines, as it will be found that the loss by leakage of current during rain from lines of the usual construction is very evident at 2,000 volts while imperceptible at 1,150; and further that 2,000-volt service wires must be of the best rubber insulation, while weather-proof will serve for the lower pressure. And at 2,000 volts the station switches and fuses must be of special design while the knife switches that will break 550 volts direct will break 1,150 alternating, for 1,150 volts alternating can hardly be coaxed into starting a bad arc at a switch, while 2,000 volts will do it without any apparent provocation. When underground cables are necessary, the quality of insulation required to give absolute reliability at the lower pressure is much less expensive than that needed for the higher; and where, in a distributing system, the sizes of conductors have to be fixed by mechanical conditions rather than by any conductivity required, it will frequently happen that the 1,150-volt network will cost less than the 2,000-volt one. It is obvious that to limit the loss by leakage to the same value at either working pressure, the insulation of the system at 2,000 volts will have to be nearly four times that at 1,150.

It is likely that 2,000 volts will be selected in most cases, because the reason for using an alternating system at all is to obtain a high pressure for transmission; and my protest is only against the inconsiderate use of a higher voltage than is necessary, simply be-cause the high voltage seems to be a standard.

In those cases where the transmission requires step-up trans-formers at the station and double reduction at the receiving end, the raised E. M. F. should be as high as 5,000 volts, at least. Such a transmission line, without branches or service wires and with no apparatus except at the ends, is easily insulated and easily maintained at its proper standard of insulation; moreover, such a line may be operated three-phase with advantage both in regulation and in efficiency, the transformers having their high pressure coils connected three-phase while the low pressure coils are connected for two-phase working.

The exciting plant requires more study than is usually given to it. Its capacity is not great, the watts for excitation of a good alternator being about 3 per cent of the rated output; but it is in operation 24 hours a day, and its commercial efficiency is, therefore, of considerable importance. And its reliability is of supreme importance. I prefer to have two independently driven exciting sets, either of which can supply the entire plant; the dynamos to be compounded enough to hold constant potential at any load, with the engine speed available at that load; the engine to be direct coupled to the dynamos and to be of a type adapted to long continuous running and requiring a minimum of attendance. These latter requirements are of greater importance than steam economy if the economy means a more complex engine. A station with which I am acquainted uses Westinghouse noncondensing engines, direct coupled, for driving its exciters and habitually runs each exciting set in turn continuously night and day for a week. The steam leaving the engine exhaust goes to a heater, where it gives up its heat to the feed water, which, even at light load, is sufficient to absorb all the heat of vaporization; so that the final fuel economy is quite good. And, in a later plan, a highly efficient condensing engine is coupled to the exciter, and provision is made to keep an economical load on the exciting circuit at all times by supplying current to the station lights and to lights in some adjoining buildings during the hours when there is the least call for exciting current. Another possible way of securing economy during the light load period is to have a small belt-driven exciter connected to the same engine that carries the alternator used at that time—to the omnibus engine, if there is one. This has the objection that it makes the field susceptible to the variations of driving speed, thus multiplying the effect on the voltage of such variations.

The exciting dynamos should, of course, be put in multiple with one another at the moment of changing over; but the general practice is to avoid multiple running of exciters. These small compound wound machines should run together nicely, and I suppose the practice is a survival of the fashion of having a separate exciter for each alternator.

Switches

For station switches of circuits of 2,000 volts or higher, the plug pattern drawing the flash through a tube, which has been worked out by the Westinghouse Company, is very much the best on the market. I have had several unpleasant personal experiences with switches of the knife pattern on 2,000 volts, and counsel you to avoid their use. They are pretty things on a board till you want to use them to break a persistent short circuit, and then they are very likely to develop an additional circuit right around your fingers. Remember that with alternators in multiple, you cannot shut down a machine that is in trouble by pulling its field switch—you have to pull its main switch first, or shut down the plant. Station fuses, too, should be of the covered type, and the Westinghouse designs of this class are also very practical. They have, for large currents, a removable fuse holder of wood with rubber mountings, with an aluminium fuse clamped between two blocks of lignum-vitae; which wooden device (the underwriters to the contrary notwithstanding) will stand more bad usage than the more generally accepted porcelain holders. The heat, or the explosion, wrecks the porcelain too frequently. And in all switchboard details, avoid the most remote possibility of an arc or flash. A 2,000-volt arc on a switchboard, with a large alternator back of it, is about as manageable as a cyclone.

Summary

To sum up these recommendations:

1. Alternators for central station use should be each coupled to an independent engine, amply strong structurally but having cylinders not too large.

2. The engines should govern proportionately to load rather than to a constant speed.

3. And the requirements as to engines being met, the alternators should be run in multiple.

4. Feeders should be run in multiple on common mains, when so doing will assist the regulation.

5. Lamp E. M. F. should be as high as in direct current work; and large transformers on secondary mains should be preferred to isolated transformers.

6. The alternators should be polyphase.

7. They should have good inherent regulation, to avoid the necessity of composite or compound winding, and to make negligible the effect of unbalancing of load on different phases.

8. A frequency of 60 is best for central station use.

9. A primary voltage of 2,000 will be most generally suitable; but the local conditions should be studied before deciding this, as operation at lower voltage is more reliable.

10. Auxiliary apparatus should be studied in detail; the stuff which the builder of the alternators is in the habit of "throwing in" with his dynamos not being always as good as the dynamos.



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