942 THE COLLIERY GUARDIAN. May 10, 1918. registered 500 units, so that, under a constant load, these two meters can be checked one against the other. Such a check, however, is a rather rough one, owing to the lowest dial on many generator watt-hour meters reading in hundreds. A more reliable check is obtained as follows:—Watt- hour meters usually bear a plate on which is stated the number of revolutions pei’ minute made by the disc or krmature when a given power flows through the meter. • This power may be equivalent to the maximum load of the generator Assuming, for a given meter, that 70 revolutions of the armature are made in one minute when the gene- rator is supplying 1,000 kw., a check on the indicating wattmeter is easily made with a constant load, which may be less than full load. If the watt-hour meter makes 35 revolutions in one minute, the power that is 35 being supplied must be = half full load, or 500 kw.— the number of revolutions being in proportion to the load. Power Factor Meter.—Although not absolutely necessary, it is an advantage to have the power factor constantly recorded, as it enables the limits of this Fig. 2. Poorer . VxC G I V Fig. 3. c V Pourer* VxCxCosfl, P.EM. WM lOOOQOODi N Fig. 5. Fig. 4. P.F.M., power factor meter ; P.T , potential transformer ; C.T., current transformer ; G, generator ; W.M., wattmeter ; P.C., pressure coil; C.C., current coil; N, neutral point. factor to be readily observed. This is essential when running generators in parallel in order that the power factors of each generator should be alike. The con- nections for a power factor indicator are shown in fig. 5. Two current transformers are necessary, and also a supply of the volt coil of the meter. Two of the current have to be reset after operation, and when this is neces- sary a great deal of arcing would occur at- the relay contacts if provision were not made for interrupting the relay circuit by the opening movement of the generator switch. The small switch shown on the right-hand side of the generator oil switch in fig. 1 is the switch referred to. It is mechanically connected to the oil switch, and opens and closes with it, so that no arcing occurs at the relay contacts when being reset—the circuit being already interrupted as soon as the generator switch operates. The relays should have an inverse time-limit element, which is explained below. Overload Trip Coils.—These trip coils are for protect- ing the generator from overload. Inverse time-limit elements should be provided in each case. The form of trip coil which operates instantaneously is Quite unsuit- able for power work. At a colliery there are a large number of factors which may contribute to momentary overloads, some- times very severe. If, in such cases, the generator switch opened immediately, many delays would result. The application of inverse time-limit elements imposes a certain interval of time between an overload and the operation of the trip coil. The trip coils are supplied from current transformers or by a separate direct- current supply, controlled by a relay in the A.C. circuit. When current transformers are used to supply the trip coils, it may be arranged that the relays short- travels in the reverse direction indicated by the word “ slow.” When the machines are in synchronism, the pointer takes up a definite position, and then the switch of the incoming machine may be closed. In addition to the synchroscope, one or more lamps are provided, which flicker continually so long as the machines are out of phase and the pointer of the synchroscope is moving. These lamps may be con- nected so that they are alight or dark at synchronism. Figs. 6 and 7 show the difference in the connections between lamps alight and dark. In fig. 6 it will be noticed that the lamp is connected between phase C of the incoming generator and phase A of the one which is running. When the generators are in phase, the lamp will glow and the switch may be closed. When the lamp is dark, the generators are out of phase. In fig. 7 the lamp is shown connected from phase C of the incoming machine to phase C of the one already running. It will be obvious that no E.M.F. must exist between phase C of one machine and phase C of the other when the incoming generator switch is closed. Therefore, when the machines are in synchronism, the lamp is dark. It may be said that operators seldom synchronise with lamps alone, as a carbon lamp may have consider- able voltage across its terminals without the filament glowing. Also, it is always preferred that the lamps should glow at synchronism and not be dark. The pos- sibility always exists of a lamp failing at the critical moment and causing an accident. Synchronising*. As regards actual synchronising, the voltmeter and synchroscope alone require attention. The voltmeter •indicating when tbe voltage of the incoming generator is equal to that of the busbars, or running machine, and the synchroscope, indicating whether tbe incoming generator is “fast,” “slow,” or in “phase.” operators usually prefer to synchronise when the synchroscope reads “fast” and the pointer is moving very slowly. The generator switch should be closed just as the pointer gets to the position of synchronism. The advantage of this method lies in guarding against a reverse current when closing the switch, because when the incoming generator is running a little fast it imme- diately takes up some load when its switch is closed, so that what current does result from closing the switch will be in the proper direction. Once the generators are switched together the transference of load should be made, having regard to the anmeter and indicating wattmeter. In transferring the load from one machine to another the power factors of the generators must be kept the same, regardless of the load. This can easily be done by an experienced operator. A little practice will enable one to gauge the approximate power factor by simple comparison between the ammeter and watt- meter, always assuming the voltage to be the same. The actual value of the power factor is immaterial, but it is necessary that each generator should e rry a current in proportion to the reading on the wattmeter. This does not depend on the relative speed of the two generators, but upon their individual excitation. Two generators working in parallel may both work at different power factors by varying their excitation. The over-excited machine will have the lower power factor, a feature which is objectionable owing to the higher C2R losses produced under such condition”. Over-excitation is more serious when generators are running in parallel, £nd are working near their full load, Incoming Running Generator. Generator. Fig. 6. ■B B Incoming Generator. Running Generator. Fig. 7. .Isolating Switch. ' oil Switch 7*1 Change Over Switch' (riTierator. JV-Fan Fig. 8. i • Isolating / Switch. -I Oil • S witch Isolating Switch Fig. 9. transformers which supply the trip coils may be utilised instead of putting in special current transformers. Reverse Current Inverse Ttme Limit Relays.—Two of these are usually placed on generator panels. Their object is to prevent the generator from being motored through a reversal of power supplied by other machines working in parallel. This may occur when synhronising if the incoming machine is running too slowly at the time it is switched on to the bus bars. In these cir- cumstances the machine would be dragged into step immediately the switch was closed, and a heavy current would flow from the bus bars to the generator, driving it for the moment as a motor. Again, when machin s are operating together, a failure of the prime mover of one of them would cause it to be motored, and bearing in mind that before this could take place the other machine would have the whole of the load thrown upon it, plus that due to motoring the other generator, it would be useless to rely on the over- load trips of the motered generator to clear the switch, as before this could take place the remaining generator would bring its own switch out and the whole station be shut down. The relays are therefore set to operate at a small percentage of full-load current, so that only a small motoring current is necessary to open the circuit breaker. The reverse-power relays usually operate a separate trip coil on the generator switch quite independent of the overload trip coils. It is shown in fig. 1, energised by the generator voltmeter transformer. Some relays circuit the trip coils in the normal position, thus pre- venting current passing through them. When the relays operate, the short-circuiting connection is broken, current passes through tbe trip coils and the oil-switch is opened. The advantage of inverse time elements lies in their ability to discriminate between a heavy load and a short-circuit. If a heavy load comes on which is within the range of operation of the relay, this commences to close, but should the load be reduced before the local circuit of the relay is brought into operation, it resets itself in the normal position. On the other hand, in case of a short-circuit, the relay responds so quickly as to open the switch almost in stantaneously. The inverse time-limit relays on a generator panel should be set much higher than those on feeder and other circuits. The latter should always be set to operate first, as the opening of a generator switch is very serious, especially when ventilating load is being carried. Synchroscope.—This instrument is common to all generator panels and is connected from one to the other by means of receptacles and plug connections. The synchroscope is provided with a pointer the direction of rotation of which depends on the resultant torque set up by the E.M.F. of the two generators to be synchronised. If the incoming machine is running too fast, the pointer turns in a direction indicated by the word “ fast,” while should the generator be slow, the pointer as the over-excited machine might be damaged through excessive heating of its windings. Exciter Panel; A complete exciter panel forms a part of the diagram of connections in fig. 1. It includes the following:— Main field ammeter, exciter voltmeter, hand-regulated rheostat in exciter field, main field rheostat, automatic voltage regulator with change-over switch for changing from hand to automatic regulation. Main Field Ammeter. — This meter indicates the current flowing in the generator field. It is one of the principal meters on the whole switchboard, and should be continually under observation. Any trouble in the generator field would be indicated by unusual readings corresponding to various loads, with which an operator should always make himself familiar. Since the voltage of the generator at no load is in proportion to the exciting current, the speed, of com se, being constant, it follows that at no load the main field current is a measure of the voltage of the generator. Exciter Voltmeter.—The readings of the main field ammeter should always be considered in relation to the exciter voltmeter reading, as it will be understood that if the voltmeter is connected across the terminals of the exciter, the voltage necessary to maintain a given current in the generator field will depend upon the amount of resistance included in the circuit by means of the main field rheostat. This may usually be more or less in a fixed position; and, in checking the main field current on open circuit for instance, the rheostat