[Larger illustration] (184 kB)

A plan similar to that just outlined was followed at the station of the Independent Electric Light and Power Company, San Francisco, where each of the 550-volt generators is ordinarily connected directly to the primary coils of two transformers that change the current from two-phase to three-phase and then deliver it through oil switches to the high-tension bus-bars at 11,000 volts. To these bus-bars the 11,000-volt feeders for five sub-stations are connected through switches. At this station there is a set of 550-volt bus-bars to which any of the generators may be connected, but to which no generator is connected in ordinary operation. The generators alone have switches connecting with these bars. When it is desirable to operate any particular generator on some pair of transformers other than its own, that generator is disconnected from its own transformers and connected to the 550-volt bus-bars. The generator whose transformers are to be operated by the generator before mentioned next has its switch connected to the 550-volt bus-bars, while the brushes of the contact rings of the former generator are raised. As the leads from each generator to its two switches are permanently joined, the switching operations just named connect the transformers of one generator with the other generator that has its switch closed on the 550-volt bars.

Fig. 63.—Switchboard at Chambly Power-station.

Where it is desired that a single reserve transformer may be readily substituted for any one of a number of transformers in regular use, the connections to each of these latter transformers may be provided with double-pole double-throw knife switches on both the primary and secondary sides, so that when these switches are thrown one way at any transformer in regular use the reserve transformer will be connected in its place.

Fuses and automatic circuit-breakers alike are intended to break connections without the intervention of human agency under certain predetermined conditions. In the fuse the heat generated by a certain current is sufficient to melt or vaporize a short length of special conductor. In the circuit-breaker a certain current gives a magnet or motor sufficient strength to overcome the pressure of a spring, and contact pieces through which the current is passing are pulled apart. The primary object of both the fuse and the circuit-breaker is thus to open connections and stop the flow of energy when more than a certain current passes. When any current passes through a circuit in the reverse of its regular direction the circuit-breaker can be arranged to break the connections, though the fuse cannot. A fuse must carry the current at which it is designed to melt during some seconds before enough heat is developed to destroy it, and the exact number of seconds for any particular case is made a little uncertain by the possibility of loose connections at the fuse tips which develop additional heat and also by the heat-conducting power of its connecting terminals. A circuit-breaker may be set so as to open its connections in one or more seconds after a certain current begins to flow. When connections are broken by a fuse the molten or vaporized metal forms a path that an arc may easily follow. A circuit-breaker with its contacts under oil offers a much smaller opportunity than a fuse for the maintenance of an arc. These qualities of fuses and circuit-breakers form the basis of their general availability and comparative advantages in transmission circuits.

Much variation exists in practice as to the use of fuses and circuit-breakers on transmission circuits. One view often followed is that fuses and circuit-breakers should be entirely omitted from the generator and transmission lines. The argument in favor of this practice is that temporary short circuits due to birds that fly against the lines or to sticks and loose wires that are thrown onto them will interrupt all or a large part of the transmission service if fuses or circuit-breakers that operate instantly are employed. On the other hand, it may be said that if fuses and circuit-breakers are omitted from the generator and transmission circuits a lasting short circuit will make it necessary to shut down an entire plant in some cases until it can be removed. Electric transmission at high voltages became important before magnetic circuit-breakers competent to open overloaded circuits at such voltages were developed. Consequently the early question was whether a transmission line and the generators that fed it should be provided with fuses or be solidly connected from generators to the distribution circuits of sub-stations. The original tendency was strong to use fuses in accord with the practice at low voltages. The great importance of continuous service from transmission systems and the many interruptions caused by temporary short circuits where fuses were used led to their abandonment in some cases. An example of this sort may be seen at the first Niagara station. In 1893, when this station was equipped, no magnetic circuit-breaker was available for circuits of either 11,000 or 2,200 volts, carrying currents of several thousand horse-power, and fuses were employed in lines at both these pressures (A. I. E. E., vol. xviii., pp. 495, 497). The fuses adopted in this case were the same for both the 2,200 and the 11,000-volt lines and were of the explosive type. Each complete fuse consisted of two lignum-vitæ blocks that were hinged together at one end and were secured when closed at the other. In these blocks three parallel grooves for fuses were cut and in each groove a strip of aluminum was laid and connected to suitable terminals at each end. Vents were provided for the grooves in which the aluminum strips were placed so that the expanding gas when a fuse was blown would escape. When these fuse blocks were new and the blocks of lignum vitæ made tight joints the metallic vapor produced when a fuse was blown was forced out at the vents and the connections of the line were thus broken. After a time, however, when the joints between the blocks were no longer tight because of shrinkage, the expanding gas of the fuse would reach the terminals and an arc would continue after the fuse had blown. These aluminum fuses, which were adopted about 1893, were abandoned at the Niagara plant in 1898. Since this later date the 2,200-volt feeders from the No. 1 power-house to the local consumers have had no fuses at the power-house, nor have circuit-breakers been installed there in the place of the fuses that were removed. At the large manufacturing plants supplied through these local Niagara feeders, the feeders formerly terminated in fuses, but these have since been displaced by circuit-breakers. In the second Niagara power-station, completed in 1902, the local 2,200-volt feeders are provided with circuit-breakers, but no fuses. Between the generators and bus-bars of the first Niagara plant the circuits were provided with neither fuses nor automatic circuit-breakers, and this practice continues there to the present time.

Besides the aluminum fuses in the 11,000-volt transmission line at the first Niagara station, there were lead fuses in the 2,200-volt primary circuits of the step-up transformers that supplied these lines. At the other end of these lines, in the Buffalo sub-station, another set of aluminum fuses was inserted before connection was made with the step-down transformers. Between the secondary coils of these transformers and the 550-volt converters there were no fuses, but these converters were connected to the railway bus-bars through direct current circuit-breakers. These lead fuses, which contained much more metal than those of aluminum, when blown set up arcs that lasted until power was cut off by opening a switch, and usually destroyed their terminals. An effort was made to so adjust the sizes of the fuses in this transmission system that in case of a short circuit in distribution lines at Buffalo only the fuses in the sub-station would be blown, leaving those at Niagara entire. This plan did not prove effective, however, and a severe overload on the distribution lines in Buffalo would blow out fuses clear back to the generator bus-bars at the Niagara station.

In order to accomplish the opening of overloaded circuits with greater certainty, to delay such opening where the overload might be of only a momentary nature, and to confine the open circuit to the lines where the overload existed, automatic circuit-breakers were substituted for the fuses named in the Niagara and Buffalo transmission system. This system was also changed from 11,000 to 22,000 volts on the transmission lines, thus rendering the requirements as to circuit-opening devices more severe. These circuit-breakers were fitted with time-limit attachments so that any breaker could be set to open at the end of any number of seconds after the current flowing through it reached a certain amount. A circuit-breaker with such a time-limit attachment will not open until the time for which it is set, after the amperes flowing through it reach a certain figure, has elapsed, no matter how great the current may be. Moreover, if the overload is removed from a line before the number of seconds for which its time-limit circuit-breaker is set have elapsed, the circuit-breaker resets itself automatically and does not open the connections. If a circuit-breaker is set to open a line after an interval of say three seconds from the time when its current reaches the limit, the line will not be opened by a mere momentary overload such as would blow out a fuse. By setting the time-limit relays of circuit-breakers in transmission lines to actuate the opening mechanism after three seconds from the time that an overload comes on, and then leaving the breakers on distribution lines to operate without a time-limit, it seems that the opening of breakers on the distribution lines should free the system from an overload there before the breakers on the transmission lines have time to act. Such a result is very desirable in order that the entire service of a transmission system may not be interrupted every time there is a fault or short circuit on one of its distribution lines. This plan was followed in the Niagara and Buffalo system. In the 22,000-volt lines at the Niagara station the time relays were set to actuate the breakers after three seconds, at the terminal house in Buffalo, where the transformers step down from 22,000 to 11,000 volts, the circuit-breakers in the 11,000 volt lines to sub-stations had their relays set to open in one second. Finally the circuit-breakers in the distribution lines from the several sub-stations were left to operate without any time limit. By these means it was expected that a short circuit in one of the distribution circuits from a sub-station would not cause the connections of the underground cable between that sub-station and the terminal house to be broken, because of the instant action of the circuit-breaker at the sub-station. Furthermore, it was expected that a short circuit in one of the underground cables between the terminal house and a sub-station would be disconnected from the transmission line at that house and would not cause the circuit-breakers at the Niagara station to operate. It is reported that the foregoing arrangement of circuit-breakers with time relays failed of its object because the breakers did not clear their circuits quick enough and that the time-limit attachments on the 22,000 and 11,000 volt lines are no longer in use (A. I. E. E., vol. xviii., p. 500). As the circuits under consideration convey thousands of horse-power at 11,000 and 22,000 volts it may be that time-limit devices with circuit-breakers would give good results under less exacting conditions. Time-limit relays are perhaps an important aid toward reliable operation of transmission systems, but they are subject to the objection that no matter how great the overload they will not open the circuit until the time for which they are set has run. In the case of a short circuit the time-limit relay may lead to a prolonged drop in voltage throughout the system, which is very undesirable for the lighting service and also allows all synchronous apparatus to fall out of step. With a mere momentary drop in voltage the inertia of the rotating parts of synchronous apparatus will keep them in step. For these reasons it is desirable to have circuit-breakers that will act immediately to open a line on which there is a short circuit or very great overload, but will open the line only after an interval of one or more seconds when the overload is not of a very extreme nature. This action on the part of circuit-breakers at the second Niagara power-station was obtained by the attachment of a dash-pot to the tripping plunger of each circuit-breaker (A. I. E. E., vol. xviii., p. 543). With moderate overloads of a very temporary nature this dash-pot so retards the action of a tripping plunger that the circuit-breaker does not open. When a short circuit or great overload comes onto a line the pull on the tripping plunger or the circuit-breaker on that line is so great that the resistance of the dash-pot to the movement is overcome at once and the line is disconnected from the remainder of the system.

The fact that a circuit-breaker may be designed to open the line which it connects, whenever the direction from which the flow of energy takes place is reversed, is taken advantage of at some sub-stations to guard against a flow of energy from a sub-station back toward the generating station. By this means a flow of energy from a sub-station to a short circuit in one of the lines or cables connecting it with the generating plant is prevented.