SECOND LECTURE GENERAL DISTRIBUTION DIRECT CURRENT DISTRIBUTION HE TYPICAL direct current distribution is the system of feeders and mains, as devised by Edison, and since used in all direct current distributions. It is shown diagrammatically in Fig. 2. The conductors are usually under- T f2a ^120 W^ -^\\ ft. 1 1 /30 i 1 \30 fZOT #1 /ze? /30 1 1 A ^_.. \ \ ItfO \ Fife. 2 24 GENERAL LECTURES ground, as direct current systems are used only in large cities. A system of three-wire conductors, called the "mains" is laid in the streets of the city, shown diagrammatically by the heavily drawn lines. Commonly, conductors of one million circular mil section (that is, a copper section which as solid round conductor would have a diameter of i") are used for the outside conductors, the "positive" and the "negative" con- ductor; and a conductor of half this size for the middle or "neutral" conductor. The latter is usually grounded, as pro- tection against fire risk, etc. Conductors of more than one million circular mils are not used, but when the load exceeds the capacity of such conductors, a second main is laid in the same street. A number of feeders, shown by dotted lines in Fig. 2, radiate from the generating station or converter substations, and tap into the mains at numerous points ; potential wires run back from the mains to the stations, and so allow of measuring, in the station, the voltage at the different points of the distribution system. All the customers are connected to the mains, but none to the feeders. The mains and feeders are arranged so that no appreciable voltage drop takes place in the mains, but all drop of voltage occurs in the feeders ; and as no customers connect to the feeders, the only limit to the voltage drop in the feeders is efficiency of distribution. The voltage at the feeding points into the mains is kept constant by varying the voltage supply to the feeders with the changes of the load on the mains. This is done by having a number of outside bus bars in the station, as shown diagrammatically in Pig. 3, differing from each other in voltage, and connecting feeders over from bus bar to bus bar, with the change of load. For instance, in a 2 x 120 voltage distribution, the station may have, in addition to the neutral bus bar zero, three positive GENERAL DISTRIBUTION 25 bus bars i, i', i", and three negative bus bars 2, 2', 2", differing respectively from the neutral bus by 120, 130 and 140 volts, as shown in Fig. 3. At light load, when the drop of voltage in the feeders is negligible, the feeders connect to the busses I, o, 2 of 120 volts. When the load increases, some of the feeders are shifted over, by transfer bus bars, to the 130 volt busbars i' and 2'; with still further increase of load, more feeders are connected over to 130 volts; then some feeders are connected to the 140 volt bus bars, i" and 2", and so, by varying the voltage supply to the feeders, the voltage at the mains can be maintained constant with an accuracy depending on the number of bus bars. It is obvious that a shift of a feeder from one voltage to another does not mean a corresponding voltage change on the main supplied by it, but rather a shift of load between the feeders, and so a readjustment of the total voltage in the territory near the supply point of the feeder. For instance, if by the potential wires a drop of voltage below 120 volts is registered in the main at the connection point of feeder A in Fig. 2, and this feeder then shifted from the supply 26 GENERAL LECTURES voltage 130 to 140, the current in the main near A, which before flowed towards A as minimum voltage point, reverses in direction, flows away from A, the load on feeder A and there- fore increases, and the drop of voltage in A increases, while the load on the adjacent feeders decreases, and thereby their drop of voltage decreases, with the result of bringing up the voltage in the mains at the feeder A and all adjacent feeders. This inter- linkage of feeders therefore allows a regulation of voltage in the mains, far closer than the number of voltages available in the station. The different bus bars in the station are supplied with their voltage by having different generators or converters in the sta- tion operate at different voltages, and with increasing load on the station, and consequent increasing demand of higher volt- age by the feeders, shift machines from lower to higher voltage bus bars, inversely with decreasing load; or the different bus bars are operated through boosters, or by connection with the storage battery reserve, etc. In addition to feeders and mains, tie feeders usually con- nect the generating station or substation with adjacent stations, so that during periods of light load, or in case of breakdown, a station may be shut down altogether and supplied from adjacent stations by tie feeders. Such tie feeders also permit most stations to operate without storage battery reserve, that is, to concentrate the storage batteries in a few stations, from which in case of a breakdown of the system, the other stations are supplied over the tie feeders. ALTERNATING CURRENT DISTRIBUTION The system of feeders and mains allows the most perfect voltage regulation in the distributing mains. It is however applicable only to direct current distribution in a territory of GENERAL DISTRIBUTION 27 very concentrated load, as in the interior of a large city, since the independent voltage regulation of each one of numerous feeders is economically permissible only where each feeder represents a large amount of power; with alternating cur- rent systems, the inductive drop forbids the concentration of such large currents in a single conductor. That is, conductors of one million circular mils cannot be used economically in an alternating current system. The resistance of a conductor is inversely proportional to the size or section of the conductor, hence decreases rapidly with increasing current: a conductor of one million circular mils is one-tenth the resistance of a conductor of 100,000 circular mils, and so can carry ten times the direct current with the same voltage drop. The reactance of a conductor, however, and so the voltage consumed by self-induction, de- creases only very little with the increasing size of a conductor, as seen from the table of resistances and reactances of conductors. A wire No. 000 B & S G is eight times the section of a wire No. 7, and therefore one-eighth (the resistance; but the wire No. 000 has a reactance of .109 ohms per 1000 feet, the wire No. 7 has a reactance of .133 oms, or only 1.22 times as large. Hence, while in the wire No. 7, the reactance, at 60 cycles, is only .266 times the resistance and therefore not of serious importance, in a wire No. 000 the reactance is 1.76 times the resistance, and the latter conductor is likely to give a voltage drop far in excess of the ohmic resistance drop. The ratio of reactance to resistance therefore rapidly increases with increasing size of conductor, and for alternating currents, large conductors cannot therefore be used economically where close voltage regulation is required. With alternating currents it therefore is preferable to use several smaller conductors in multiple : two conductors of 28 GENERAL LECTURES No. I in multiple have the same resistance as one conductor of No. ooo; but the reactance of one conductor No. ooo is .109 ohms, and so 1.88 times as great as the reactance of two con- ductors of No. I in multiple, which latter is half that of one conductor No. i, or .058 ohms, provided that the two con- ductors are used as separate circuits. In alternating current low tension distribution, the size of the conductor and so the current per conductor, is limited by the self -inductive drop, and alternating current low tension networks are therefore of necessity of smaller size than those of direct current distribution. As regards economy of distribution, this is not a serious objection, as the alternating current transformer and primary distribution permits the use of numerous secondary circuits. In alternating current systems, a primary distribution system of 2200 volts is used, feeding step-down transformers. The different arrangements are — a. A separate transformer for each customer. This is necessary in those cases where the customers are so far apart from each other that they cannot be reached by the same low tension or secondary circuit ; every alternating current system therefore has at least a number of instances where individual transformers are used. This is the most uneconomical arrangement. It requires the use of small transformers, which are necessarily less efficient and more expensive per kilowatt, than large trans- formers. The transformer must l3e built to carry, within its overload capacity, all the lamps installed by the customer, since all the lamps may be used occasionally. Usually, however, only a small part of the lamps are in use, and those only for a small part of the day ; so that the average load on the transformer is a very small part of its capacity. GENERAL DISTRIBUTION 29 As the core loss in the transformer continues whether the transformer is loaded or not, but is not paid for by the cus- tomer, the economy of the arrangement is very low ; and so it can be understood that in the early days, where this arrange- ment was generally used, the financial results of most alternat- ing current distributions were very discouraging. Assuming as an instance a connected load of twenty 16 candle power lamps — low efficiency lamps, of 60 watts per lamp, since (the voltage regulation cannot be very perfect — allowing then in cases of all lamps being used, an overload of 100%, which is rather beyond safe limits, and permissible only on the assumption that this load will occur very rarely, and for a short time — the transformer would have 600 watt ratting. Assuming a core loss of 4%, this gives a continuous power consumption of 24 watts. Usually probably only one or two lamps will be burning, and these only a few hours per day, so that the use of two lamps, at an average — summer and winter — of three hours per day, would probably be a fair example of many such cases. Two lamps or 120 watts, for three hours per day, give an average power of 15 watts, which is paid for by the customer, while the continuous loss in the transformer is 24 watts ; so that the all year efficiency, or the ratio of the power paid for by the customer, to the power con- sumed by the transformer, is only ^^ , ^4 or 38%. By connecting several adjacent customers to the same transformer, the conditions immediately become far more favorable. It is extremely improbable that all the customers will burn all their lamps at the same time, the more so, the greater the number of customers is, which are supplied from the same transformer. It therefore becomes unnecessary to 30 GENERAL LECTURES allow a transformer capacity capable of operating all the con- nected load. The larger transformer also has a higher effiicency. Assuming therefore as an instance, four customers of 20 lamps connected load each. The average load would be about 8 lamps. Assuming even one customer burning all 20 lamps, it is not probable that the other customers together would at this time burn more than 10 to 15 lamps, and a trans- former carrying 30 to 35 lamps at overload would probably be sufficient. A 1500 watt transformer would therefore be larger than necessary. At 3% coreloss, this gives a constant loss of 45 watts, while an average load of 8 lamps for 3 hours per day gives a useful output of 60 watts, or an all year efficiency of nearly 60%, while a 1000 watt transformer would give an all year efficiency of 67%. This also illustrates that in smaller transformers a low coreloss is of utmost importance, while the rr loss is of very secondary importance, since it is appreciable only at heavy load, and therefore affects the all year efficiency very little. When it becomes possible to connect a large number of customers to a secondary main fed from one large trans- former the connected load ceases to be of moment in the trans- former capacity ; the transformer capacity is determined by the average load, with a safe margin for overloads; in this case, good all year efficiencies can be reached. Economical alternating current distribution therefore re- quires the use of secondary distribution mains of as large an extent as possible, fed by large transformers. The distance, however, to which a transformer can supply secondary current, is rather limited by the inductive drop of voltage ; therefore, for supplying secondary mains, transformers of larger size than 30 kw. are rarely used, but rather several transformers are em- ployed, to feed in the same main at different points. GENERAL DISTRIBUTION 31 Extending the secondary mains still further by the use of several transformers feeding into the same mains, or, as it may be considered, inter-connecting the secondary mains of the different transformers, we arrive at a system somewhat similar to the direct current system : a low tension distribution system of 220 volts three-wire mains, with a system of feeders tapping into it at a number of points, as shown in Fig. 4. These feeders Fife. 4. Alternating Current Distribution with Secondary Mains and Primary Feeders. are primary feeders of 2200 volts, connecting to .the mains through step-down transformers. In such a system, by vary- ing the voltage impressed upon the primary feeders, a voltage regulation of the system similar to that of direct current dis- tribution becomes feasible. Such an arrangement has these advantages over the direct current system: the drop in the feeders is very much lower, due to their higher voltage; and 32 GENERAL LECTURES that the feeder voltage can be regulated by alternating current feeder regulators or compensators, that is, stationary structures similar to the transformer. It has, however, the disadvantage that, due to the self-induction of the mains, each feeding point can supply current over a far shorter distance than with direct current, and the interchange of current between feeders, by which the load can be shifted and apportioned between the feeders, is far less. As a result, it is difficult to reach as good voltage regu- lation with the same attention to the system; and since this arrangement has the disadvantage that any break- down in the secondary system or in a transformer may involve the entire system, this system of inter-connected secondary mains is rarely used for alternating current distribution, but the secondary mains are usually kept separate. That is, as shown diagrammatically in Fig. 5, a number of separate secondary mains are fed by large trans- formers from primary feeders, and usually each primary feeder connects to a number of transformers. Where the distances are considerable, and the voltage drop in the primary feeders appreciable, voltage regulation of the feeders becomes necessary; and in this case, to get good voltage regulation in the system, attention must be given to the arrangements of the feeders and mains. That is, all the transformers on the same feeder should be at about the same distance from the station, so that the voltage drop between the transformers on the same feeder is negligible ; and the nature of the load on the secondary mains fed by the same feeder should be about as nearly the same as feasible, so that all the mains on the same feeder are about equally loaded. It would therefore be undesirable for voltage regulation, to connect, for instance, a main feeding a GENERAL DISTRIBUTION 33 residential section to the same feeder as a main feeding a business district or an office building. f A\ Fife. 5. Typical Altematlnfe Current Distribution. In a well designed alternating current distribution system, that is, a system using secondary distribution mains as far as feasible, the all year efficiency is about the same as with the direct current system. In such an alternating current system, 34 GENERAL LECTURES the efficiency at heavy load is higher, and at light load lower, than in the direct current system ; in this respect the alternating current system has the advantage over the direct current system, since at the time of heavy load the power is more valuable than at light load.