THE COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. Vol. CXV. FRIDAY, MAY 10, 1918. No. 2993. Requirements, Operation and Maintenance of Colliery Power Station Switchboards. current circuits, especially when the load supplied is highly inductive, such as transformers and induction motors. Instead of the E.M.F. and current waves "V X A being exactly in phase, for which kw. = iqqq is correct, there is a displacement between them, i.e., the By L. Developments in the direction of electrical power application in and about collieries have been such as to necessitate the erection of large power-controlling switchboards, comparing in size with many municipal installations. Switchboards are, as a rule, designed in a manner which attracts attention, and often greatly adds to the appearance of well-designed power stations. Nothing, however, should be introduced into the lay-out of a power station switchboard which could be conveniently dispensed with, and every piece of apparatus should have a definite object. Some engineers would cut down the meter equipment of switchboaids to just a few meters, operated by a system of plug receptacles arranged on each panel. Apart from the appearance of such an arrangement, it does not lend itself to proper control. The operator cannot readily see the load on different feeder and other panels, arid therefore things are liable to happen which an operator could reasonably be expected to observe on a well-equipped switchboard, but which pass unnoticed with a receptacle and plug arrangement for meters. On sub station switchboards the arrangement referred to would meet requirements, but for a power station switchboard it is insufficient. The purpose of the present article is to review the complete equipment of a power station switchboard, indicating, step by step, the reasons for installing the various apparatus and meters, in order to assist those who cannot devote time to a study of the subject, but who should, nevertheless, have an idea as to the neces- sary requirements of power plant boards. An average example of a power station with a capacity of several thousand kilowatts will be considered, con- sisting of two or more generating sets, high-tension feeders, transformer panels, etc. The switchboard will further be assumed to be mechanically controlled—that is, with switch gear operated by hand and not by elec- trical control, as on some of the larger switchboards. On all switchboards the generating panels are always the most important, and instruments are used which it would be unnecessary to* place on feeder and other panels, with the result that the wiring of generator panels is always more or less of a complicated nature. In order to assist in making the various circuits more easily understood, some are shown separately without any other wiring connections, so that, after studying them, little difficulty will be found in following the various circuits, and understanding their object when repairing to the complete generator and exciter panel wiring shown in fig. 1. In connection with all high-tension switchboards, the instruments are always supplied through current or pressure transformers, as the case may be. Thus the voltage across the various meters or other apparatus is only a fraction of that across the high tension bus bars. Instrument Potential Transformers. Referring to fig. 1, it will be noticed that four wires cross the top of the diagram and are marked A, B, C, and neutral respectively. These wires are common to all panels, i.e., they pass within easy reach of the meters. It will be seen later that in some instances apparatus require a low voltage supply of alternating current. This applies to other panels than those con- trolling generator units. The supply is obtained from the aforesaid wires, which are connected to the secondary side of a small 3-phase potential transformer, which is usually known as the instrument potential transformer (not shown in the figure) the primary side of which is connected directly to the high tension bus bars, so that when these are alive the pressure coils of all instru- ments on the switchboards are also alive. In order to have a check on this, and also to know that the gener- ator switch is closed, a large voltmeter is provided in a conspicuous position on the switchboard, and receives its current from the potential transformer, so that the voltmeter reading is an indication that things are normal and that the required voltage is being maintained. Equipment of Generator Panel. This should consist of the following:—Voltmeter, ammeter, indicating wattmeter, recording wattmeter and watt-hour meter; reverse current inverse time limit relays, with separate trip coil for releasing the generator switch ; overload trip coils operated by inverse timelimit relays ; synchroscope which is common to all generating panels, each of which is provided with plug receptacles to allow the separate units to be synchronised with another already running. Generator Voltmeter.—This meter is connected directly across the generator through its transformer, which is protected by fuses on the high-tension side. No switch is interposed between the voltmeter and the generator, so that immediately the voltage is raised it is indicated, regardless of whether the generator circuit breaker is open or closed. Once the generator is connected to the FOKES. bus bars this voltmeter is no longer necessary owing to the existence of the bus bar voltmeter ; but if this latter were relied on solely and no other voltmeter provided, it would be impossible to ascertain the voltage of the generator when its circuit-breaker opened. Again, when a generator is already running and connected to the bus bars and another generator is to be connected in parallel, it is necessary, amongst other things, that the incoming machine should be at the same voltage as the bus bars before the circuit-breaker is closed. current lags behind the E.M.F. so that they do not pass through their maximum values at the same time. This is shown in the vector diagrams in figs. 2 and 3. Fig. 2 shows the current in phase with the E.M.F., while fig. 3 represents a phase difference equal to the angle 0. The product V x A is only apparent watts in the latter case, and, in order to find the true power, it is necessary to multiply the apparent watts by a factor which is less than unity, and which is equal to the cosine of the angle 0 between the current and voltage vectors. The wattmeter is so designed that it takes account of the phase difference and reads in true kilowatts. Finally, it is sometimes necessary to run temporary connections from a generator which is isolated from the bus bars, in order to carry out tests, etc., so that, in such cases, the generator voltmeter must be relied on to indicate the pressure across the terminals of the machine. Three-phase power systems a*e usually balanced, and therefore it is only necessary to employ ore watt- meter which is calibrated to read the total power in the circuit. Fig. 4 shows the diag am of connections for a balanced three-phase circuit. It will be noticed that the wattmeter has four terminals, two of which are connected to the voltage- coil and two connected to the current-coil of the meter. The current transformer is connected in series with one phase, whilst the voltage is taken from the neutral point and the same phase in which the current transformer is con- nected. R.W.M. W.HM SYNC. J.W.M. '.R.CR. .R.C.R. :.r. E. Fig. 1. R.W.M., recording wattmeter ; I.W.M., indicating wattmeter ; W.H.M., watt- hour meter; R.C.R., reverse current relay; P.F.M., power factor meter; I.T.L R., inverse time limit relay; R.T., reverse current trip coil; O.T.C., overload trip coils; G.P.T., generator potential transformer; G.V.M., generator voltmeter ; Sync., synchroscope; S.L., synchronising lamp; I.S., isolating switch; O.S., oil switch; S.P., synchronising plug; B.V., bus bar voltmeter; I.P.B., instrument potential bars; G, generator; E, earth; C.T., current transformers; E, exciter; E.V., exciter voltmeter ; F.A., main field ammeter; M F.R., main field rheostat; G.F., generator field; E.F., exciter field ; E.F.S., exciter field switch ; A.V.R., automatic voltage regulator; A.R.P.T., automatic voltage regulator potential transformer. The power which an instrument measures when connected as above is one-third of the power in the circuit, but the scale of the meter reads three times the actual power measured, in order to take into account the two other phases in which no measurement is made. It is therefore obvious that, if the phases are balanced, such a method of measuring the power is quite in order. Consider a thiee-phase circuit with a voltage of 3,000 volts between the lines and a current of 150amperes being supplied. The power factor will be considered unity and the generator star connected. The total power in the circuit will be— kw v X 0x^3- • 1,000 x 1 3,000 x 150 x 1’73 1,000 = 780 kw. The wattmeter, as aheady stated, measures the power in one phase only. The voltage from the neutral point to one line is equal to the line voltage divided by 3 ; therefore, in the case of the circuit referred to, the voltage to the neutral 3,000 would be —7=^ = 1,734 volts. The V 3 power in one phase, as mea- sured by the wattmeter, will be Ammeter.—On all high-tension circuits, current trans- formers are employed to avoid bringing the high voltage to the instrument. The generator ammeter is placed on the machine side of the circuit-breaker, similar to the potential transformer for the voltmeter. Current transformers are usually provided on all three phases, and are used not only for the ammeter but also for the current coils of wattmeters, reverse current relays and the trip coils of the circuit-breaker. The generator ammeter is necessary to prevent the generator being overloaded. It provides the switchboard attendant with a check on the load that is being carried, and where an indicating wattmeter is provided, the power factor of the circuit can be readily determined unless a power-factor indicator is installed, in which case the power factor can be read direct. Wattmeter.—It may be mentioned here that, in dealing with direct-current circuits, the power that is being supplied may be determined from the voltmeter i n. volts x amps. ml . and ammeter reading, *.e., kw. =--------F000------ itus does not, however, hold in the case of alternating- , 1,734 x 150 ™ , kw. = = 260 kw. approx. Therefore, the total power in the circuit will be 260 x 3 = 780 kw. as found by the formula for total power in the circuit. The wattmeter reading will give the horse power if multiplied by 1’34. Recording Wattmeter.—This is a most useful instrument, as it enables a record to be kept of the loads encountered during each shift. Any unusual occurences will be indicated on the chart, besides the distribution of load over the various shifts, This enables adjustments of load to be made so as to improve the load factor of the station. A chart which shows that the load is fairly constant over the 12 hours or 24 hours as the case may be, indicates that the distribution of load is such as to require the minimum of generating capacity. On the other hand, if severe peaks are shown at certain times during the shift, the working of the station cannot be said to be economical since stand-by plant must be kept in order to deal with the peaks, and thus the cost of generating energy is increased by the unnecessary standing charges. Integrating or Watt-hour Meter.—The watt-hour meter records the number of units supplied by the generator to the system. Such a meter is indispensable where economical running is desired, and the cost per unit generated is to be cut down to the minimum. The reading is taken in the Board of Trade units, each of which is equal to 1,000 watts supplied for one hour. If therefore the indicating wattmeter shows a constant load of 500 kw. for one hour, the watt-hour meter will have