December 24, 1914. THE COLLIERY GUARDIAN. 1323 value considerably, and on the other hand it is desired to cut out faults to earth with a moderate current flowing. This necessitates means for accurate balancing. The balancing difficulty can, however, be completely eliminated by a simple device which has been employed successfully on e.h.t. service. Each feeder is furnished with a relay having two elements, one an operating element, and the other a restraining element, as, for example, in fig. 11. The operating coil is excited from the feeder which is being protected, and the restraining coil by the feeder in parallel with it. The arrangement is biased in favour of the restraining coil to the extent of, say, 10 per cent., by placing extra turns on that coil so as to make it more powerful than the operating coil by 10 per cent. Now unless the balance is upset by more than 10 per cent, the device cannot operate, and yet at normal load the device tends to operate imme- diately the fault current exceeds 10 per cent, of the normal load current. As inter-locked overload relays are proof against over- loads or faults occurring outside the parallel feeders protected, it will be seen that they are particularly suit- able for the protection of a number of stations in series. All the devices may be set to operate instantaneously and at less than the normal load current, and yet none of them will operate except in the faulty section. Inter-Locked Leakage Relays. — Another method of eliminating the balancing problem is by the use of leakage relays inter-locked in the same way as overload relays. These relays, as previously described, operate only on faults to earth, of W’hich the extent can be limited by an earthing resistance. The relay settings will be in the neighbourhood of the maximum fault current, and thus even a 10 per cent, unbalancing is quite negligible. Seeing that most cable faults are faults to earth, and that of the faults between phases most developed first or simultaneously as faults to earth, this arrangement will successfully remove the large majority of feeder faults, and is proof against disturb- ances from other sources. Two feeders can be protected by one relay instead of requiring three or six. It will be . understood that as leakage relays are .inoperative on faults between phases, their employment only reduces disturbances, and the most severe disturb- ances, namely, those due to faults between phases, are left to produce an extensive or complete shut-down. The addition of inter-locked leakage relays to a system otherwise protected by overload relays will improve the conditions, but cannot be considered a real solution of the problem of protection. Inter-Locked Reverse Current Relays. — Reverse current relays are now obtainable which will operate at about 3 per cent, of the normal working voltage and over a wide range of power factor, but below this voltage they are inoperative on currents of any magnitude. On a fault which develops practically instantaneously and reduces the voltage below this limit, the relays will be of no service. The difficulty due to loss of voltage has been dealt with in another way with promising results. One may almost count on having a working margin of voltage for at least a fraction of a cycle, and the problem is to ensure that this determines the operation of the relay. This is effected by making the moving parts unstable at low voltage, so that any movement initiated is com- pleted. These improvements render the reverse relay almost independent of voltage. The application of the inter-locking idea renders them unaffected by current fed back from substation plant and of surges in either direction. Fig. 12 illustrates diagrammatically the inter-locking of a pair of reverse current relays suitable for the control of the remote end of the feeders in fig. 8. Whilst well-designed reverse current relays with potential windings will deal successfully with the majority of faults, it is to be noted that the faults with which they cannot cope are just those most likely to occur as the result of bad design, bad workmanship, defective material, and carelessness, and are also those most disastrous in their results. Workmen omit to remove earthing links, clearances and oil switches are made too small, porcelain is defective, damage is caused by careless handling, and dead short circuits occur. Protection of Inter-Connectors and Ring Mains. As the energy flow in a sound inter-connector or ring main may be in either direction, varying with the distri- bution of load, reversal is no longer a sign of leakage. A leak of sufficient magnitude may so disturb the dis- tribution that energy may flow into the faulty conductor from both ends. This condition is a sure criterion of a heavy fault. Attempts have been made to utilise this feature to obtain selective action. In one of these the tripping circuits are inter-connected by a single pilot wire, and in such a way that the circuit is completed only if both relays operate. The pilot wire may be employed more profitably to take a sample of current from one end of the feeder to the other for use as a standard. The current at the remote end may then be compared with the sample to determine whether the direction or magnitude of flow is the same. The latter is the better course, as the magnitude changes with the smallest leakage, but the direction will only change with a relatively large leakage. This is the basis of the well-known Merz-Price protective system. The input and output of a given feeder will always be equal unless there has been leakage either to earth or between phases. The comparison may be carried out in various ways, but in practice these have been resolved into two systems, namely, the balanced voltage and the circulating current systems. Current transformers are inserted at the two ends of the conductor to be dealt with, and are connected through the pilot wires. In the balanced voltage system diagrammatically illustrated in fig. 13 the current trans- formers are connected in opposition and the relays in series with them. In the circulating current system illustrated in fig. 14 the transformers are so connected as to circulate the current between themselves under normal conditions. The trip coils are connected in shunt between equipotential points, and carry only the difference current corresponding to the leak. The balanced voltage system is best adapted for feeder pro- tection, and the circulating current system for the protection of transformer and generator windings, for all of which applications well-proved designs are avail- able. There is no better method of protecting existing feeders and all generators and transformers. Fig. 15 shows a typical arrangement for the protection of alternators, and figs. 16 and 17 are for transformer pro- tection. Fig. 18 shows a novel combination for the protection of a large generator and a step-up trans- former. The generator has two windings in parallel on account of the heavy current to be dealt with. The output of one winding is compared with the total output of the e.h.t. side of the transformer. Any change in the ratio of these will indicate a breakdown either in the transformer or the machine, and the e.h.t. oil switch will be actuated. General Solution of the Problem of Feeder Protection. Any arrangement capable of dealing with a single inter-connector can be applied to any cable in the system. The classification employed hitherto is con- venient ; but it will be understood that any cable connecting any two points is in fact an inter-connector, and in practice it will now be seen that even if it be one of a pair of parallel feeders it may still have, at least momentarily, the characteristic previously associated with inter-connectors, of having to carry current in either direction whilst sound. The Merz-Price system is applicable to 'inter-con- nectors, and is therefore a general solution of the problem. We can obtain a technically and practically satisfactory combination by the use of inter-locked 0 Lod,d current Ay controlling oil switch b [Cones & primary • winding's J Secondary • winding's i not shown /z O&i \ I OW \\ 5 \O W \ w [Secondary winding Fault current superposed on load current A w Iron core A. Primary I J ^winding Fig. 19.—Standard 20,000-volt. 0T square in. Split Conductor. Fig 20.—Split-conductor System—Sound Feeder. Fig. 21.—Split-conductor System—Faulty Feeder. Fig. 22.—Split-conductor System—Fault at one end. Fig. 23.—Split-conductor protection for three phase 6,600-volt. circuit. excess current devices at both ends of the pair of feeders, and without further selective apparatus, if we regard the pair of conductors as the unit of which the distributing system is to be composed. For example, a ring main run in duplicate may be so protected. Seeing, however, that both conductors will be dis- connected when a fault occurs in either, the economical solution is found in the ingenious suggestion of putting the two conductors in one cable and lightly insulating each from the other. This gives us the split conductor protective system and up to the present the best solution of the problem, for not only does this solution give an arrangement applicable to ring mains, inter-connectors, and dupli- cate feeders—in fact, to all systems of distribution—but it gives an arrangement absolutely selective and so sensitive as to perform this function instantaneously at a fraction of the normal load current. The system requires the employment of a cable of special construc- tion, or, in the case of overhead lines, a special arrange- ment of conductors. The construction involves but a slight departure from that commonly employed, and consists in the splitting or separating of each conductor into two parallel portions lightly insulated from one another. Fig. 19 herewith illustrates a standard 0-1 sq. in. cable suitable for 20,000-volt distribution, constructed for use under the new system, and it will be seen that each core is split (hence the name “ Split-conductor protec- tive system ”). The cable is constructed with an oval concentric core, this construction giving the best distribution of potential strain in the insulation. In small sizes this is especially valuable, and dispenses with the necessity for a hemp core within the copper conductor. The principle of the system is illustrated diagrammatically by fig. 20, which shows the connec- tions for one split conductor. The split conductor is connected at each end to the usual switchgear equip- ment, consisting of oil switch, busbars, etc., through a special current transformer. The current transformer core has two primary windings, to which the two halves of the split conductor are connected, and the core carries also a secondary winding connected to a relay controlling the oil switch trip coil. Under normal conditions, as in fig. 20, current entering the feeder at one end divides equally between the two parallel paths, being united again on the remote side of the second current transformer. In each transformer the magnetising effects of the two primary coils are equal and opposite, thus the transformer offers no impedance to the current flow, and the secondary windings and relays are unaffected. In case, however, a fault develops, for example, at the point A in fig. 21, the fault current flowing towards A will upset the balance of current between the two primary windings in each transformer, thus producing a magnetising effect on the secondary windings and exciting the relays. The transformers also serve a second function in case the fault occurs near one end of the split conductor, as at B in fig. 22. In this case the fault current flowing from the left hand end would divide equally between the two halves of the cable, were it not for the impedance of the transformer near B. The fault current flowing to B through the sound conductor traverses the two primary windings at B in the same direction, thus highly magnetising the core. The impedance so offered hinders the flow of current through the sound conductor, and thus upsets the balance at the remote end, causing the simultaneous operation of both relays. As the cores are not magnetised under normal running conditions, there is no core loss. In practice the core is of circular form, and the primary and secondary windings are superposed. Fig. 23 illustrates an arrangement of apparatus suit- able for a 6,000-volt three-phase service, in which the three special transformers, one' for each phase, are mounted in a common case. For higher voltages, and where the apparatus is mounted with a greater distance between its centres, separate transformers may be furnished for three phases. Use of Split-Contact Switches. — On reference to fig. 21 it will be seen that the arrangement is more sensitive at the end nearest the fault, as the whole fault current passes through the adjacent transformer winding in one direction. Advantage is taken of this to make the whole arrangement more sensitive by employing an oil switch with split contacts, which separates the two halves of the conductor on breaking circuit. The switch nearest the fault will operate first, and on separating the conductors the whole fault current is then concentrated on one conductor at the other end, and brings out the switch there also. Application to Overhead Lines. — This system is readily applied to overhead lines. In this case the half- conductors may be run as separate overhead lines on the same posts. As a general solution, however, the twin conductors are carried on common insulators, and but lightly insu- lated one from the other. Spacers of insulating material are inserted at intervals between supports to keep the wires apart. Fig. 24 shows twp parallel feeders with split conductors carried on common posts, and fig. 25 an enlarged detail of the insulators. If a wire is broken and falls to the ground the line will be cut out, even though the fallen line fails to make a good earth. This adds materially to the protection of the public. The simplicity of this arrangement should be noted. There are no pilot wires and no potential transformers. The relays are plain overload devices requiring no special adjustment. The transformers can generally be built with bar primary windings, a construction which gives the maximum of simplicity and safety. The secondary windings are subjected to no forces except on the occur- rence of a fault in the individual feeder, which has to be protected, however severely the system may be dis- turbed. It is difficult to connect up the apparatus incorrectly, but should the connections be crossed by mistake the switch will open immediately any attempt is made to put the feeder into service, and thus the mistake will be discovered.