Time Limit Generators are liable to be subjected to an excessive overload for a short period of time. This sudden variation in load often occurs in the operation of railroad trains, mining machinery, mills and machines of a similar nature. It would be very inconvenient to have breakers open the circuit whenever these sudden variations in load occur, especially as the generators are usually designed to stand an overload of 25 per cent for half an hour and an overload of 50 per cent for one minute. The time limit relay performs the same func tion that a circuit breaker does, namely, opens the circuit whenever an excessive overload occurs. There is this difference, however, be tween the two devices: the operation of a cir cuit breaker is practically instantaneous, whereas a time limit relay has a time element associated with it. This time element is adjustable and may be arranged so that the circuit will be opened after a definite interval of time follow ing the overload. In one form the magnetic action of the relay is opposed by a bellows or a dash-pot, which contains air. When the relay begins to operate, a solenoid draws a plunger down, which in turn compresses air in a re ceiver. The force exerted by the compressed air is lessened by leakage. When the limit is reached, say five seconds, the circuit breaker is automatically opened.
Reverse Current If a generator become disabled and is unable to develop -a pressure equivalent to that generated by ma chines linked in with it, there is a possibility of a large cross-current flow into the disabled generator from the other machines. To prevent power returning into such a machine a device termed a reverse current relay is employed which automatically cuts out the machine from the service.
Lightning Transmission and distributing systems are subject to abnormal voltage strains due to lightning and potential surges. These disturbances tend to weaken or destroy the insulation of the circuit.. While there is nothing that will protect electrical ap paratus from a direct stroke of lightning, still means have been provided to protect the in sulation from the induced potential resulting from lightning discharges occurring in its vi cinity and also from surges which sometimes occur during the operation of the system. It is customary to protect transmission lines by installing one or more overhead ground wires which are frequently grounded at intervals along their course. The object of these ground wires is to shield the transmission wires from the influence of lightning and also to provide a freer path for the lightning discharge to the earth. These ground wires are not, however, perfect protection and some of these potential disturbances travel along the transmission line to the generating or substations. Hence, pro tective apparatus is needed at the point where each line enters the station. As lightning fre quently oscillates with a very high frequency, a reactance coil, commonly called a choke coil, is installed at this point to impede the discharge of lightning and make it tend to seek another path. This auxiliary path is provided by the
lightning arrester which is connected to each line at the point where the transmission cir cuit is connected to the choke coil. The light ning arrester consists of a path of low im pedance which has a means provided for pre venting the normal line current flowing to ground following lightning discharge. The most efficient form of lightning arrester is known as the aluminum cell arrester which con sists of a number of cells formed of aluminum plates upon which are special films which have a remarkable property of allowing only a small leakage current to flow through the cells in normal potential which will allow higher or abnormal voltages to be discharged to ground. This form of lightning arrester is used for both alternating and direct current circuits. An other form of arrester is the graded shunt multighp. This arrester consists of multiple paths containing graded amounts of resistance in parallel with spark gaps. The application of these multigap arresters is confined to lower voltage alternating current circuits. For the protection of direct current circuits, there is, in addition to the aluminum cell, several forms of gap arrester. These consist of gaps in series with a low resistance and a means pro vided for extinguishing the power current which follows the lightning discharge across the gap. The best-known form is the magnetic blow-out type which has a magnetic coil con nected in shunt with a part of the resistance. When the lightning discharges to ground through the gap and resistance, the line current sustains the arc at the gap. This current flow ing through the coil produces a strong mag netic field in the gap which repels the arc through a suitably formed arc chute by means of which it is quickly extinguished and the cir cuit restored to normal conditions of operation.
Central Station Economics.— It is desirable to reduce the initial cost of the production of electric power to a minimum wherever fuel is expensive and natural sources of energy, such as water power, are absent. Such conditions are met with in many of our large cities. This reduction is accomplished by installing engines and generators of the highest efficiency and by employing all methods which will tend to in crease the efficiency of the plant. One of these methods is to directly connect the armature of the generator to the shaft of the engine. This eliminates the friction losses which would occur if belting were employed, but necessitates good speed regulation, as the pressure of a gener ator is directly proportional to the speed with which it rotates. It is essential for good service that the speed he regulated to within 1 per cent, otherwise lights will flicker and their use will be objectionable. Sometimes in large electric lighting systems the voltage of a given part of the system may vary as much as 5 per cent, but this variation is not continuous and is due more to the local loading of the mains and not to the generator at the central station.